Introduction
Kinetoplast parasites are a group of flagellated protozoans defined by possession of the kinetoplast organelle. As a group, these organisms infect plants and animals (Refs Reference Frolov and Karpov1, Reference Stuart2, 3, Reference Severo4, 5). However, three kinetoplastid species – Trypanosoma cruzi (T.c.), Trypanosoma brucei (T.b.) and Leishmania spp. – cause human diseases recognized by the World Health Organization (WHO) as Neglected Tropical Diseases (NTDs) (Ref. 6). These include Chagas disease (CD), human African trypanosomiasis (HAT) and leishmaniasis, respectively. Each of the three human kinetoplastid diseases has a life cycle involving a blood-feeding arthropod vector as well as a human host and express variable sex-bias in their incidence, morbidity and mortality rates. In this review, we will share evidence of such sex-biases in these diseases as well as explore potential mechanisms and areas requiring further research especially surrounding the immune response, sex-hormone impacts and genetic concerns.
Etiology and epidemiology
Table 1 summarizes the transmission, epidemiology, clinical manifestations, current treatments and prevention strategies for each of the three diseases that will be explored in this paper. This is a general overview and does not cover fine details of each of these categories. The cellular immune responses to each of these disease processes are later detailed in Figures 1, 2, 3, and 4.
Etiology and Epidemiology of Leishmaniasis, Chagas disease and Human African Trypanosomiasis. Overview of the modes of transmission, areas of endemicity, rates of disease, clinical manifestations and current treatment and prevention strategies
Abbreviations: T.c.: Trypanosoma cruzi; CD: Chagas disease; T.b: Trypanosoma brucei; HAT: Human African Trypanosomiasis; NECT: nifurtimox–eflornithine combination therapy.
Hormone differences and immune cell effects between XY males and XX females. While hormone levels vary from person to person, the centre of this figure depicts from left (male) to right (female) typical relative levels of sex hormones between males and females in sexually mature individuals. The left sided text describes typical immune cell effects of increased male sex hormones, and the right sided text describes typical immune cell effects of increased female sex hormone levels. Some immune effects of sex hormones have more complex non-linear relationships between hormone concentration and immune response, which are not fully depicted in this figure.
Immune response against Trypanosoma brucei. T. brucei is transmitted by the bite of many species of tsetse flies belonging to the genus Glossina. Thus, initial immune response happens in the skin, where first responder cells such as (1) neutrophils (purple) and (2) NK cells (light blue) elicit inflammatory cytokines which will later favour macrophage activation towards M1 (Refs Reference Alfituri97, Reference Alfituri98). At early stages of infection, (3) macrophages (beige) play an important role phagocyting T. brucei (Ref. Reference Lopez, Tanowitz and Garg99). (4) Macrophage polarization towards M1 (orange) controls parasitaemia in the early stages of the infection, mainly through the production of TNF-α and NO. (5) Nevertheless chronic inflammation becomes harmful for the host and can cause collateral tissue damage, anaemia, cachexia and even death (Refs Reference Baetselier100, Reference Brown and Meyerhoff101). (6) Therefore, macrophage polarization shift from M1 to M2 is necessary for host survival (Refs Reference Baetselier100, Reference Brown and Meyerhoff101). Although an exacerbated M2 prevalence leads to increased parasitic burden. (7) M1 and M2 cells sustain and are favoured by the Th1 and Th2 cells (green), respectively. (8) Moreover, IL-4 from Th2 cells, (9) together with IL-12, IL-6 and IFN-γ produced by the dendritic cells (blue), induce the antibody production by plasma B cells (pink) (Refs Reference Alfituri97, Reference Alfituri98). 10) Finally, due to parasite antigenic variability and its capacity to survive extracellularly, multiple successive waves of anti-T.b. antibodies are generated by plasma B cells (pink) (Ref. Reference Brown and Meyerhoff101). Therefore, those antibodies can direct phagocytosis, necessary for long-term parasitaemia control (Ref. Reference Brown and Meyerhoff101).
Immune response against Trypanosoma cruzi. Trigonoscuta cruzi is usually transmitted by the bite of a triatomine vector (also known as kissing bug). The infection is then divided in acute and chronic stages (Ref. Reference Junqueira112). During the acute phase (1) macrophages (beige) are recruited to the site of infection and are capable of phagocyting the parasite, releasing reactive nitrogen intermediates (RNI) and promoting inflammasome formation and IL-1β and IL-18 cytokines that favours the parasite elimination (Ref. Reference Junqueira112). (2) Similarly, dendritic cells (blue) also phagocytize the parasite and migrate to the draining lymph node for antigen presentation to the T and B cells. (3) T cell (green) Th1 phenotype is sustained by cytokine production of activated macrophages and dendritic cells, such as IL-12 and TNF-α (Refs Reference Jiménez113, Reference Macaluso114). 4) Th1 cells are IFN-γ producers which sustains (4) classical activation of the macrophages, (5) and promotes NK cells’ (light blue) trypanocidal effector activity and macrophage activation (Ref. Reference Junqueira112). Moreover, (6) IFN-γ stimulates IgG2a production by plasma B cells (pink) (Refs Reference Junqueira112, Reference Macaluso114). (7) Antibodies opsonize the parasite and favour one of the three occurring complement cascades (red), resulting in parasite elimination (Ref. Reference Jiménez113). (8) During the chronic asymptomatic phase of the disease, cytotoxic CD8+ T cells (yellow) are critical for parasitaemia control via cytokine secretion and infected cells elimination (Ref. Reference Jiménez113).
General immune response against leishmaniasis. Leishmania spp. can be transmitted by different species of phlebotomine sandflies. Although Leishmania infection immune response differs among the different species (Ref. Reference Costa-da-Silva132), in general (1) neutrophils (purple) are the first immune cells to arrive to the sandfly bite site. There, neutrophils will release granules and NETs which favour parasite elimination (Refs Reference Rochael133, Reference Guimarães-Costa134). Nevertheless, they can also act like Trojan horses, internalizing parasites and helping them to reach their definitive host, the macrophage (beige) (Refs Reference Regli135, Reference van Zandbergen136). (2) Macrophages then phagocytize the infected neutrophils and become infected themselves (Ref. Reference van Zandbergen136). The immune polarization of the macrophage will determine the course of the infection (Ref. Reference Elmahallawy, Alkhaldi and Saleh137). 3) Inflammatory macrophages (Ref. Reference Rasmi, Jalali and Khalid138) (orange), characterized by higher TLR4 expression as well as by the production of nitric oxide (NO) and inflammatory cytokines as TNF-α, IL-1β and IL-6 will reduce parasite burden and increase antigen presentation. Additionally, M1 macrophages will support Th1 response later, overall resulting to parasite elimination. On the other hand, (4) anti-inflammatory macrophages (M2) (brown) show a reduction in ROS and produce IL-10 and TGF-β, inhibiting inflammatory response and permitting parasite survival, moreover, such cells become long-term host for Leishmania (Ref. Reference Costa-da-Silva132). (5) At the site of infection, dendritic cells (DCs) (blue) will phagocytize the parasite and take it to the lymph node to be presented to T lymphocytes (green). T cells can differentiate into (6) Th1 through the secretion of IL-12 by the DC which leads to the production of TNF-α, IFN-γ and IL-2 which supports Th1 cells proliferation and boost M1 activation, leading to parasite elimination. (7) In absence of IL-12, T cells may also differentiate into Th2 which supports M2 activation though the cytokines IL-4, IL-13 and IL-10, promoting parasite survival (Ref. Reference Elmahallawy, Alkhaldi and Saleh137). Whereas effector Th1 and Th2 cells play a direct role in parasite elimination or persistence, long term immunity is sustained by memory T cells.
Sex bias: epidemiology and clinical presentation
The precise global incidence and morbidity rates of neglected tropical diseases are difficult to quantify given that inherent constraints in diagnosis and clinical management cause many cases to remain undiagnosed (Ref. Reference Pradhan7). For infectious diseases in general however, sex is known to alter the rate of infection, disease course and severity – often presenting with higher rates of morbidity and mortality in males than in females (Ref. Reference Muenchhoff and Goulder8). Based on current evidence, Leishmania, T.c. and T.b. exhibit differences in the distribution of disease between sexes despite each parasite’s evolutionary relatedness.
Some parasitic infectious diseases exhibit clear male predominance in disease severity, such as Entamoeba histolytica (Ref. Reference Er-Lukowiak, Hansen, Lotter, Klein and Roberts9), while in other parasitic infections, evidence of sex bias effects remains inconclusive or is limited to one stage of infection (Ref. Reference Bernin and Lotter10. Sex-bias in infection and clinical manifestations is related to several factors including genetics, hormones and gender-specific behaviours. A variance in sex-bias has been observed across age in certain infectious diseases, primarily associated with known age-related fluctuations in sex hormone levels, which depicts the interplay of hormones and the immune system in the clinical presentation of sex bias in infectious diseases (Ref. Reference Muenchhoff and Goulder8). Sex biases with female predominance are present in non-infectious autoimmune conditions, including multiple sclerosis (MS) (Ref. Reference Ysrraelit and Correale11), systemic sclerosis (Ref. Reference Medsger12), Hashimoto’s thyroiditis (Ref. 13), myasthenia gravis (MG) (Ref. Reference Pekmezović14), rheumatoid arthritis (RA) (Ref. Reference Maranini15), systemic lupus erythematous (SLE) (Ref. Reference Duarte-García16), among others. In addition to having a female bias, many of these autoimmune conditions, such as MS, MG, SLE and RA, exhibit changes in symptoms and severity with variations in the hormonal state of the host such as puberty, pregnancy and menopause (Ref. Reference Merz17). This further emphasizes the role that sex differences such as hormones play in the immune response.
Sex biases following patterns seen with other infectious diseases have been observed in infection by kinetoplastid parasites. As covered in the following sections, evidence suggests these kinetoplastid diseases demonstrate greater morbidity and mortality among infected males, though inconsistencies exist between reports. We will examine the current data on sex differences in incidence, progression and severity for each disease individually.
Leishmaniasis
Leishmaniasis has been established as exhibiting sex-bias with higher incidence and worse disease outcomes in males by both survey data and medical records, with the male-to-female ratio of disease increasing with increased age (Refs Reference Jeronimo18, 19, 20, Reference Burza21, Reference Jahn, Lelmett and Diesfeld22, Reference Guerra-Silveira and Abad-Franch23, Reference Jervis24, Reference Bruhn25, Reference Sarkari, Hatam and Ghatee26). A local study in Israel, a country with endemic Old-World cutaneous leishmaniasis (CL), found female incidence of leishmaniasis to be about 3/4 of that in males. Furthermore, another study reported a relative risk of 1.27 of developing visceral leishmaniasis (VL) when comparing males to females (Ref. Reference Cloots27). Clinical variations between the sexes in instances of infection have been well-defined but are variable between specific Leishmania spp. among other factors (Ref. Reference Bernin and Lotter10). Previous studies show differences in infectivity dependent on the level of sex hormones in the host (Ref. Reference Bernin and Lotter10), species and strain of the model (Ref. Reference Mock and Nacy28), parasite strain (Ref. Reference Mock and Nacy28) and route of inoculation (Ref. Reference Travi29). Thus, the age, host and parasite are important factors to consider when exploring the differences between sexes associated with leishmaniasis infection.
Cutaneous leishmaniasis in animal models is more severe in male organisms compared to females, with males experiencing larger lesion sizes (Ref. Reference Travi29), higher parasite burdens (Refs Reference Mock and Nacy28, Reference Travi29) and less inflammatory responses compared to females (Ref. Reference Bernin and Lotter10). Specifically, these observations were made with L. major, L. mexicana, L. panamensis and L. guyanenis rodent infection models (Refs Reference Bernin and Lotter10, Reference Mock and Nacy28, Reference Travi29). Male mice infected with L. major have been shown to have a significantly greater parasite burden in the liver (Ref. Reference Mock and Nacy28) and lymph nodes (Ref. Reference Travi29) compared to females. Nevertheless, no significant differences exist between male and female hamsters for neither Leishmania-specific antibody titres nor in the delayed-type hypersensitivity responses (Ref. Reference Travi29). Differences in disease-promoting cytokines present at the infection site have been noted, with lesions in males having significantly increased interleukin (IL)-10, IL-4 and transforming growth factor (TGF)-β cytokine levels which are known to be associated with larger and more severe lesions (Ref. Reference Travi29). Similarly, as in the animal models, differences in leishmaniasis severity are being observed in humans by gender also.
In humans, data shows higher rates of CL reported amongst males compared to females, with males having a higher rate of subsequent progression to mucocutaneous leishmaniasis (MCL). Although studies have been limited, evidence has shown male patients are more likely to suffer from CL caused by L. major, L tropica and L guyanensis (Refs Reference Gandacu30, Reference Spotin, Rouhani and Parvizi31, Reference Collis32, Reference Soares, Abad-Franch and Ferraz33). Other single region studies found CL caused by L. tropica and L. major had no significant sex-bias (Refs Reference Reithinger34, Reference Bettaieb35) partially in contrast to the animal models described above. Moreover, an Afghanistan-based study found that L. tropica infection caused increased CL lesions and scarring in females than in males (Ref. Reference Reithinger34). Likewise in VL caused by Labrus donovani and L. infantum, the male sex has been identified as a risk factor for infection (Refs Reference Bruhn25, Reference Dahal36, Reference Jervis37, Reference Wondimeneh38, Reference Harizanov39, Reference Rodríguez40, Reference Rijal41), although most studies investigating disease severity in humans have been limited by a single-location study design or by a low number of cases (Ref. Reference Jervis24). Studies have also found experience worse severity of VL infection, although when stratified by age, this trend is present only in sexually mature males over the age of 15 (Refs Reference Guerra-Silveira and Abad-Franch23, Reference Cloots27, Reference Cloots42). Younger pre-pubescent females, below age 15, exhibited higher rates of VL-related mortality compared to males under 15 (Ref. Reference Jervis24). Notably, trends surrounding disease severity of VL do not universally correlate with sex. One analysis of suspected cases of VL in India found male-dominant VL disease incidence, but no significant difference in mortality rates between males and females (Ref. Reference Jervis24). This finding may be due to increased healthcare access and thus disease reporting in males, with reporting limited to severe VL cases in females, or due to female cases more commonly being asymptomatic and thus unreported, as was discussed in the study (Ref. Reference Jervis24). These contrasting results indicate a need for further research into the variance of disease severity of VL between males and females using larger patient groups, testing and inclusion of asymptomatic individuals and records of disease progression for a better understanding of sex-bias in symptomatic VL.
Chagas disease
Some reports suggest Chagas disease may exhibit higher incidence in males compared to females (Refs Reference Barretto43, Reference Ibáñez-Cervantes44, Reference Gómez-Ochoa45), while others have found the difference to be insignificant (Ref. Reference Ligonio46), indicating a need for better diagnosis, documentation and reporting of cases to better understand the presence or lack of sex-bias in disease incidence of CD. CD-related cardiac disease and irregularities are significantly more prevalent in male patients compared to females (Ref. Reference Barretto43). However, information regarding clinical sex differences in T.c. infection is complicated by the fact that many cases remain in the intermediate, or asymptomatic, stage and clinical sequelae of CD, such as cardiomyopathies, are not unique to this disease and thus may not be attributed to CD if the underlying infection is not detected (Refs Reference Nunes47, Reference Edwards, Stimpert and Montgomery48). While detecting differences in CD severity in humans is challenging, animal models also support this observation of increased disease severity and complications of CD in males compared to females. Mouse models demonstrate greater susceptibility to infection, as demonstrated by higher parasitaemia levels, and a higher rate of mortality in males compared to females (Refs Reference Schuster and Schaub49, Reference Micucci50).
Investigations into the role of sex in the progression of CD in humans have largely focused on cardiomyopathy in Trigonoscuta cruzi-seropositive individuals. Cardiac damage in infected patients can occur while patients remain asymptomatic for Chagas disease (Ref. Reference Nunes47). Both the onset and progression of Chagas’ cardiomyopathy are found to be associated with the male sex (Refs Reference Nogueira51, Reference Assunção52). Males demonstrated greater electrocardiogram (ECG) abnormalities and abnormal echocardiograms when infected, they showed significantly lower left ventricular ejection fraction, greater fibrosis and more prevalent transmural involvement when compared to females with Chagas’ cardiomyopathy (Refs Reference Assunção52, Reference Sánchez-Montalvá53). Elevated seroprevalence of T.c. was noted in children and women of reproductive age in Veracruz (Ref. Reference Ligonio46), but age was not associated with changes in the clinical course or severity of Chagas disease (Ref. Reference Añez54). Contradictory results, particularly those in non-endemic countries, indicate the need for improved surveillance and profiling of Chagas disease (Ref. Reference Muñoz55).
Human African Trypanosomiasis
HAT shows variable differences in incidence between sexes with some reports suggesting no difference (Ref. Reference Elenga56), other surveys showing male predominance (Refs Reference Zoller57, Reference Franco58) while others found HAT incidence to be higher among women (Refs Reference Mahoudeau59, Reference Pépin, Mpia and Iloasebe60). The overall incidence of HAT is much lower than CD and leishmaniasis and is recognized to likely have underreported case numbers resulting in inaccurate measurement of disease distribution. Undetected T.b. infections may be due to a suboptimal system of case detection utilizing sentinel sites. This lack of detection results in “silent transmission” of disease (Ref. Reference Njitchouang61). In addition to challenges to surveillance and reporting, the variability of evidence of sex bias in incidence could be attributed to differing sex-specific behaviours in different study areas (Ref. Reference Franco58).
Currently, there is limited published data available regarding animal models demonstrating sex differences in infection or in disease progression of HAT. Yet, one study in mice found that male mice were more susceptible to infection compared to females who demonstrated increased resistance and lower parasite burden (Ref. Reference Greenblatt and Rosenstreich62). Otherwise, mouse HAT models looking at severity and progression did not note sex to be implicated in disease processes. These studies did identify inflammatory cytokines and the role they may play in exacerbating CNS damage during infection, which, although not explored in these studies, are known to be influenced by sex hormones and sex-specific genetics (Refs Reference Bernin and Lotter10, Reference Ysrraelit and Correale11, Reference Travi29, Reference Schurz63, Reference Markle and Fish64, Reference Lipoldová and Demant65, Reference Verthelyi66, Reference Soudan67, Reference Kovats68).
In clinical studies of human disease, a 2002 study investigating differences between the genders in cases of human HAT cases conducted in Nioki, Democratic Republic of Congo (DRC) noted adult men were more likely than women to have an abnormal cerebrospinal fluid (CSF) white cell count, indicating increased progression to stage 2 infection involving encephalopathy in males compared to females (Ref. Reference Pépin, Mpia and Iloasebe60). This difference was not observed between infected males and females 0–15 years of age (Ref. Reference Pépin and Méda69). One study found that in Nioki, DRC, males had elevated numbers of trypanosomes and white blood cells in the CSF compared to female patients (Ref. Reference Pépin, Mpia and Iloasebe60). However, the study also concluded that male sex is not a risk factor for increased mortality or encephalopathy rates (Ref. Reference Pépin, Mpia and Iloasebe60). Another study found that males were more likely to experience relapses of HAT after treatment, related to males having a higher parasite burden in their CSF (Ref. Reference Pépin70).
These various sex-based trends in human diseases arising from Leishmania, T.c. and T.b. infections are summarized (Table 2). While research is ongoing, in some regions, issues arise from diagnostic limitations and low case numbers present challenges for accurate quantification.
Sex differences summary – incidence and severity. Summary of the major clinical and epidemiological sex differences for Leishmaniasis Chagas Disease and Human African Trypanosomiasis. Some conflicting reports exist for HAT, so no well-established differences between sexes are reported.
Effects of sex hormones on immune response
Sex-related hormones are recently appreciated for their involvement on immune responses which may drive sexual dimorphism in the severity of human Kinetoplastida diseases (Figure 1) (Refs Reference Ysrraelit and Correale11, Reference Verthelyi66, Reference Taneja71, Reference Cutolo72). The sex hormones of greatest interest are testosterone, progesterone, prolactin and oestrogen. Oestrogen, prolactin and progesterone are most concentrated in females, whereas testosterone is most concentrated in males, and each are especially linked to certain reproductive events (Ref. Reference Vodo73, Reference Freeman74, Reference Druckmann and Druckmann75, Reference Khan and Ahmed76). While each hormone binds to unique receptors, structurally, progesterone, testosterone and oestrogen are related and can be inter-converted via enzymatic reactions and intermediate products (Ref. Reference Miller77). These hormones have numerous effects throughout the body of all humans (Figure 1), however, it has been generally found that testosterone and progesterone are anti-inflammatory signals, whereas prolactin and oestrogen have pro-inflammatory effects (summarized in Table 3) (Refs Reference Taneja71, Reference Freeman74, Reference Druckmann and Druckmann75, Reference Malkin78, Reference Szekeres-Bartho79, Reference Oettel and Mukhopadhyay80). These immunomodulatory effects can be seen in relevant cell types to Kinetoplastida infections such as innate lymphoid cells (ILCs) and dendritic cells of the skin, among others (Refs Reference Kovats68, Reference Laffont, Seillet and Guéry81, Reference Mackern-Oberti, Jara, Riedel and Kalergis82, Reference Blanquart, Laffont and Guéry83). Therefore, with their varying concentrations and immune impacts, there is the potential for sex hormones to affect the sex-biased incidence and disease progression of human Kinetoplastida diseases.
Summary of immune response and sex hormones in Humans.
Abbreviations: NK: natural killer cell; IFN: interferon, IL: interleukin, TNF: tumor necrosis factor.
Testosterone and immunity during human kinetoplastida-caused diseases
Immune modulation plays a critical role in the severity of many diseases; particularly, during kinetoplastid-caused human diseases (summarized in Figures 2, 3 and 4). For a general example, testosterone’s immunosuppressive effects on innate immune cells are evident in DCs and ILCs - cells commonly present in the skin and serve as the first line of defence for diseases spread by insect bites. Clinical and experimental studies demonstrate that testosterone suppresses DC activation and pro-inflammatory cytokine production of IL-1β, IL-6, tumour necrosis factor (TNF)-α, while reducing major histocompatibility complex (MHC) class II and co-stimulatory molecule expression (Refs. Reference Jaillon, Berthenet and Garlanda84, Reference Trigunaite, Dimo and Jørgensen85). This is evidenced by enhanced DC responses in hypogonadal men and castrated mice, suggesting testosterone restrains DC-mediated immunity (Ref. Reference Trigunaite, Dimo and Jørgensen85). In ILC populations, in vivo studies of humans and mice show testosterone exhibits significant suppressive effects on ILC2s, which are found in higher numbers in women with asthma compared to men (Ref. Reference Cephus, Stier and Fuseini86). Testosterone regulates ILC2 function through direct decrease of ILC2 counts and suppression of ILC2 IL-5/IL-13 production, while also reducing tissue production of ILC2-stimulating cytokines (IL-33 and TSLP). Additionally, testosterone negatively regulates IL-2 receptor-mediated ILC proliferation (Ref. Reference Cephus, Stier and Fuseini86). These effects on innate immune cell populations could influence early immune responses during Kinetoplastida infections, where rapid cytokine production and immune cell recruitment are crucial for parasite control. The immunosuppressive effects of testosterone on these innate immune populations could help explain the generally worse disease outcomes observed in males during Kinetoplastida infections, particularly through delayed or diminished early immune responses that allow for enhanced parasite establishment.
While the immune modulatory power of sex hormones is becoming evident, there are some distinctions with respect to their effect on disease outcomes within kinetoplastid diseases. For example, in T.b. infections of all subspecies, it has been observed in rats (Ref. Reference Soudan67), dogs (Ref. Reference Obi87), boars (Ref. Reference Otesile, Fagbemi and Adeyemo88), rams (Refs Reference Boersma89, Reference Petzke90) and humans (Refs Reference Kimata91, Reference Hublart92) that serum testosterone declines over the course of the disease and recovery of serum testosterone can be slow even after treatment (Refs. Reference Petzke90, Reference Kimata91). This observation of declining serum testosterone has been made in both men and women (Ref. Reference Boersma89). Although in males, parasitic invasion of the testes has been seen which may contribute to Leydig cell dysfunction and therefore lower testosterone production, the exact mechanism of this disruption is still unknown (Refs Reference Soudan67, Reference Boersma89, Reference Hublart92). Notably, inoculations of male rats with trypanolytic T.b. pellet component fractions recapitulated the same loss of testosterone when the fractions included a unique trypanosomal endoprotease as well as undetermined secreted trypanosomal proteins and glycoproteins produced by the parasite (Refs Reference Soudan93, Reference Hublart94). Regardless of the exact cause of the testosterone loss, in the case of T.b. infection, testosterone may be protective for patients with HAT as it promotes humoral immune responses via IL-10 and IL-4 stimulation (Figure 2). Antibody responses can kill T.b. by complement and other mechanisms in vitro, thus accelerating the humoral response development may limit T.b.’s ability to establish infection (Figure 2) (Refs Reference Takayanagi95, Reference McLintock, Turner and Vickerman96). Hence, testosterone loss could have a pronounced impact in males on the course of T.b. infection.
Meanwhile, testosterone has contradictory actions in T.c. infection. On one hand, in rats and mice testosterone declines during T.c. infection (Refs Reference Tavares102, Reference Moreira103, Reference Silva104, Reference Golizeh105, Reference Schuster and Schaub106), but a mechanistic cause has not yet been established. In contrast, there is evidence that T.c. infection may be supported by testosterone presence. For example, T.c. has endogenous enzymes that can produce testosterone when provided with precursors (Ref. Reference Vacchina107). Meanwhile, in mice with T.c. infection, testosterone was also found to be advantageous to the parasite as it is necessary for disrupting thymocyte differentiation (Ref. Reference Mucci108). Specifically, thymus epithelial cells expressing androgen receptors (AR) mediate TNF-α and Caspase3-dependent apoptosis of immature thymocytes, a decline of IL-12 levels, and an increase of TNF-α (Ref. Reference Filipin109). Furthermore, in large vesper mice (Calomys callosus), testosterone has been shown to be required to achieve high parasitaemia in male mice as testicular surgical removal provides resistance to T.c. infection, which was negated when testosterone was supplemented (Refs Reference do110, Reference Pinto111). Mechanistically, this may relate to observations that the castrated (low testosterone) vesper mice had greater percentages of lytic antibodies, more splenocyte proliferation and more nitric oxide (NO) production. All of these events would be protective immune processes (Figure 3) (Ref. Reference Pinto111). Consequently, gender through variation in testosterone levels may be another potential source to explain sexual dimorphism in T.c. disease pathologies. Further research is warranted to determine the impact of testosterone addition or loss during mid-late infection in animal models to determine whether this would be a fruitful pathway for investigation to reduce disease burden in people also.
Lastly, testosterone has been studied and reviewed for its role in leishmaniasis (Refs Reference Roberts, Walker and Alexander115, Reference Snider116, Reference Lockard, Wilson and Rodríguez117, Reference Bernin and Lotter118). Clinically, during CL and VL with unreported Leishmania species, patients showed declines in testosterone levels (Refs Reference Esfandiari119, Reference Verde120). However, patients who developed post-kala-azar dermal leishmaniasis (PKDL) had testosterone increase correlated with IgG up-regulation which combined may impact the disease severity (Refs Reference Mukhopadhyay121, Reference Mukhopadhyay122). In these PKDL patients, treatment with miltefosine caused a decline in testosterone which may have aided in restoring normal immune function to clear the disease, but the mechanism of this change is not yet clear (Ref. Reference Mukhopadhyay121). Also, in vivo and in vitro studies have shown that males and females with dihydrotestosterone (DHT) or testosterone treatment all had worse parasitic outcomes during L. mexicana (Ref. Reference Snider116), L. major (Ref. Reference Mock and Nacy123), and Labrus donovani infections (Refs Reference Anuradha, Pal and Katiyar124, Reference Yin125, Reference Qiao126, Reference Liu127, Reference Zhang128, Reference Liu129). Moreover, L. mexicana promastigote pre-treatment with DHT also stimulated parasite growth, boosted macrophage infection rate and made Leishmania more resistant to killing in vitro which combined to exacerbate the disease severity of BALB/c mice in vivo (Ref. Reference Sánchez-García130). Furthermore, L. panamensis infection of hamsters treated with testosterone resulted in increased lesion size and increased levels of detrimental anti-inflammatory cytokines (Figure 4), indicating a distinct immune response to L. panamensis when testosterone was added (Ref. Reference Travi131). Thus, regardless of the exact Leishmania species, this data reveals that testosterone is a powerful immune modulator that drives disease outcomes and needs further consideration.
Prolactin and immunity during human kinetoplastida-caused diseases
Prolactin functions as both a hormone and a cytokine in the immune system, impacting dendritic cells (DCs) and other innate immune cells. Prolactin and its receptors are expressed by multiple immune cell types, including DCs, macrophages and lymphocytes, promoting both endocrine and autocrine signaling (Refs Reference Rasmi, Jalali and Khalid138, Reference Legorreta-Haquet, Santana-Sánchez, Chávez-Sánchez and Chávez-Rueda139). In DCs, prolactin promotes maturation and differentiation by stimulating IL-6 and IL-23 secretion, potentially enhancing their antigen-presenting capacity and stimulating Th1 inflammatory response (Ref. Reference Wu, Sun and Zhang140). The enhancement of innate immune function may be relevant during early responses to Kinetoplastida parasites, where effective DC and macrophage activation is crucial for pathogen control. Additionally, prolactin’s ability to inhibit immune cell apoptosis and promote pro-inflammatory cytokine production could support sustained immune responses during chronic infection phases, though this requires further investigation in the context of Kinetoplastida diseases (Ref. Reference Legorreta-Haquet, Santana-Sánchez, Chávez-Sánchez and Chávez-Rueda139).
In the context of HAT, Prolactin has not been thoroughly investigated. The few existing studies show that while prolactin release in healthy adults coincides with sleeping hours and with cortisol release, in patients with advanced HAT, circadian rhythm is severely disrupted as is evident through their sleeping patterns and sporadic spikes in cortisol and prolactin release (Refs Reference Stijlemans141, Reference Radomski142, Reference Buguet143). Among these patients with severe sleeping sickness symptoms, interferon (IFN)-γ was 7–12 times higher than in healthy patients (Ref. Reference Buguet143), possibly suggesting a role in circadian rhythm in regulating inflammatory signalling and suggesting that such IFN-γ release is insufficient to control or counterproductive for T.b. late-stage infection, unlike the other Kinetoplastida parasites discussed in this review. However, the relationship between circadian prolactin release and immune functions needs further investigation to link and explain these observations more clearly in human patients. Furthermore, the impact of sex differences on prolactin production and impact on disease state has not been explored for HAT.
Meanwhile for American Trypanosomiasis, some have hypothesized that due to prolactin’s role in cardiomyopathies and its role as an autoimmune pro-inflammatory factor, it may contribute to CD (Ref. Reference Pearson144). Since then, it has been observed that during T.c. infection, glucocorticoid endocrine signalling increases, meanwhile prolactin production declines (Refs Reference Savino145, Reference Roggero146, Reference Lepletier147, Reference Corrêa-de-Santana148). Many immune functions are impacted by loss of prolactin. This is evident as prolactin administration to T.c.-infected mice results in elevated lymphocyte proliferation and activation in multiple cell types (CD4+, CD8+, natural killer (NK) and B cells), macrophage activation and NO production, meanwhile, parasite burden declined – all of which are favourable to fight the infection (Figure 3) (Refs Reference Filipin149, Reference Del Vecchio Filipin150). More specifically, T.c. under normal conditions induces apoptosis of immature T cells in the thymus, but prolactin is both necessary and sufficient to stop this parasite effect (Ref. Reference Lepletier151). Thus, prolactin is a vital immune mediator and a potential area for immunotherapy development for Chagas disease.
Finally, in leishmaniasis prolactin has been understudied and yielded conflicting findings. It has been hypothesized that prolactin may be crucial in Leishmania immunity due to its correlation to levels of inflammatory chemokines such as macrophage inflammatory protein (MIP-1a) (Ref. Reference van Zandbergen136) and monocyte chemotactic protein (MCP-1) (Ref. Reference Pearson152). Meanwhile, a study on the levels of serum hormones during localized cutaneous leishmaniasis in Brazil (likely due to either L. braziliensis or L. amazonensis) documented a reduction in serum prolactin levels for both male and female localized CL patients (Ref. Reference Baccan153). Notably, this study found excessive prolactin levels correlated inversely to IFN-γ production and correlated positively to increased lesion diameter (Ref. Reference Baccan153). Finally, prolactin likely is pivotal in L. major immunity as the prolactin-inducible protein (PIP) is necessary for optimal CD4+ T-cell proliferation, Th1 differentiation, IFN-γ production, NO production in macrophages and ultimately L. major killing (Figure 4) (Ref. Reference Li154). Moreover, PIP-deficient macrophages were more susceptible to Leishmania even when IFN-γ and lipopolysaccharide (LPS) stimulated (Ref. Reference Li154). Thus, the role of prolactin in leishmaniasis immunity especially over various disease presentations and parasite species requires further investigation.
Progesterone and immunity during human kinetoplastida-caused diseases
Progesterone has been demonstrated to play a role in regulation of innate immune cells. It has a known impact on DCs, as they contain progesterone receptors, however exact mechanisms are still under investigation (Ref. Reference Xu, He and Li155). Evidence suggests progesterone modulates DC function by promoting DC differentiation and increasing costimulatory factors such as MHC-II, CD40, CD54 and CD86. Also, progesterone can induce a shift towards an anti-inflammatory cytokine profile with enhanced IL-10 and reduced TNF-α production, although this may be less prominent in mature DCs (Ref. Reference Motomura, Miller, Galaz, Liu, Romero and Gomez-Lopez156). For innate lymphoid cells, progesterone’s effects, studied primarily in pregnant women, show tissue-specific variation. In reproductive tissues, high progesterone promotes ILC2-mediated responses (Ref. Reference Collins, McCutcheon and Petroff157) while in the lung, progesterone enhances ILC2 responsiveness to IL-33, leading to enhanced IL-13 and chemokine release (Ref. Reference Trivedi, Deering-Rice and Aamodt158). However, the systemic effects of progesterone on ILCs in the context of parasitic infections, warrants further investigation.
Progesterone has not been explored in the context of T.b. human infection. But, during T.c.-infection of humans and mice with cardiomyopathy, progesterone has been seen to alter gene expression of hub genes which has cascading effects on innate immunity and elsewhere throughout the body (Ref. Reference Wu159). This mechanism may relate to the observed reduction of parasite burden during progesterone supplementation for ovariectomized Calomys callosus infected with T.c. (Ref. Reference Prado júnior JC160). Also, T.c.-infected rats treated with melatonin saw reduced progesterone levels which may have contributed to reduction of IL-1β and TGF-β levels (Ref. Reference Brazão161). Finally, mice infected with T.c. of distinct genotypes revealed that distinct immune responses are generated regardless of equivalent serum progesterone levels, indicating the strong role parasite genotype also plays in potential patient outcomes (Ref. Reference Solana162).
Also, limited investigations into progesterone during leishmaniasis have been conducted. Mechanistically, one study demonstrated that progesterone acts through the glucocorticoid and progesterone receptors on macrophages to reduce IL-12 and NO production, thus limiting killing of Labrus donovani in vitro (Figure 4) (Ref. Reference Jones163). Moreover, clinical observations of un-spayed, nonpregnant dogs with natural VL infection showed that dogs with high serum progesterone were more likely to have chronically high parasite and inflammatory infiltration into the mammary glands (Ref. Reference Bertolo164). Further evidence specific to progesterone involvement in leishmaniasis is lacking, however some prior studies regarding the impact of pregnancy (which requires high progesterone) on Leishmania infections have been reviewed (Refs Reference Roberts, Walker and Alexander115, Reference Snider116). Briefly, pregnant mice were found to have elevated burdens of L. major and elevated Th2 immune responses compared to non-pregnant mice (Refs Reference Krishnan165, Reference Arinola166, Reference Arinola167), however during hamster L. panamensis CL infection, pregnant mice had lower parasitaemia and improved macrophage NO production (Ref. Reference Osorio168). Thus, progesterone requires more detailed investigation to fully understand its role in leishmaniasis for pregnant and nonpregnant patients with various Leishmania spp.
Estrogen and immunity during human kinetoplastida-caused diseases
Estrogen is primarily produced by the ovaries and has a wide range of impacts throughout the body as nuclear oestrogen receptors (ERs) are expressed by a range of cell types (Refs Reference Benjamin and Delgado169, Reference Thomas and Potter170). There are four identified human oestrogens that act on oestrogen receptors with varying affinities. Oestradiol (E2) has the highest affinity for all ER types and is produced most in women of reproductive age (Ref. Reference Arinola166). Estrone (E1) is created primarily in adipose tissue and adrenal glands and is the prominent oestrogen subtype in post-menopausal women (Refs Reference Kovats68, Reference Thomas and Potter170, 171). Estriol (E3) is usually only seen during pregnancy as it is secreted by the placenta (Refs Reference Kovats68, Reference Chen, Li and Ou-Yang172). Finally, oestetrol (E4) is a foetal oestrogen only detectable during pregnancy (Refs Reference Kovats68, Reference Thomas and Potter170, 171). The impact of oestrogen on each cell type is dependent on the intracellular response cascade and has different effects depending on the receptor and cell type. In the immune system, oestrogen exhibits a variety of impacts on both innate and adaptive immunity. Oestrogen acts on B cells, T cells (cytotoxic and helper), dendritic cells, natural killer cells, monocytes and haematopoietic stem cells through three subtypes of ERs: ERa, ERb and G protein-coupled ER1 (GPER1) (Refs Reference Kovats68, 171, Reference Chen, Li and Ou-Yang172, 173, 174). The overall effect of these ERs impacts immune cell activation and function such as the promotion of an increase in total serum antibodies (Refs Reference Kovats68, Reference Harding and Heaton175). Overall, oestrogens can have a wide range of effects depending on the reproductive status of the woman and oestrogen concentration, the microenvironment, the cell and tissue types involved and the ER expression levels in the involved cells (Refs Reference Harding and Heaton175, Reference Fuentes and Silveyra176, Reference Yu177, Reference Salem178).
Oestrogen’s effects on innate immunity show complex patterns that vary with oestrogen concentration and cellular environments. In dendritic cells, oestrogen acts on ERα to regulate DC development depending on the cytokine environment. Oestradiol acts on DC progenitor cells at physiologic levels in females, it promotes granulocyte-macrophage colony-stimulating factor (GM-CSF) mediated differentiation of DCs while inhibiting Flt3L-mediated DC differentiation (Ref. Reference Kovats and Carreras179). Furthermore, ERα signalling modulates DC function by influencing cytokine production. DCs from ERα-deficient mice show reduced production of IL-6, IL-23, IL-12 and IL-1β in response to TLR stimulation (Ref. Reference Kovats and Carreras179). However, at high serum oestrogen concentrations such as those during pregnancy, oestrogen can act as an anti-inflammatory modulator by promoting a tolerogenic DC phenotype with increased expression of inhibitory molecules and regulatory cytokines (Ref. Reference Kovats68). Evidence as to the relationship between oestrogen and ILC2s appears more limited, with most tissue-resident ILC2s lacking significant expression of oestrogen receptors. While uterine ILC2s express oestrogen receptors and respond to oestradiol through a mechanism of altered gene expression, this may be related to oestrogen’s effects on the uterine environment rather than direct immune cell regulation. In the lung, where ILC2s play important roles in inflammation, there is little evidence for direct oestrogen regulation of ILC2 numbers or function under normal conditions (Ref. Reference Blanquart, Laffont and Guéry83). These complex regulatory patterns of oestrogen on innate immune cells may help explain the generally enhanced immune responses observed in females during Kinetoplastida infections, while also contributing to increased susceptibility to autoimmune conditions.
Additional immune mediation impacts of oestrogens include shifting Th2:Th1 ratio, elevating antibody levels and modulating expression of cytokines and inflammatory mediators (Refs Reference Harding and Heaton175, Reference Salem178). Of note, these pro-inflammatory actions can be harmful as high affinity autoreactive B cells were found in increasing levels in response to oestrogen, which is a potential mechanism for autoimmune disease in females (Ref. Reference Grimaldi180). Moderately high levels of oestrogen, such as those in reproductive age females, enhance the Th1 response (Figure 1) (Refs Reference Verthelyi66, Reference Khan and Ahmed181). Studies demonstrate that at normal female physiological levels, oestrogen enhances IFN-γ expression by Th1 cells in both humans and mice (Refs Reference Khan and Ahmed181, Reference Grasso and Muscettola182). In the presence of very low, such as those seen in males, and very high oestrogen levels, such as those seen during pregnancy, Th2 differentiation and cytokine expression is favoured (Refs Reference Kovats68, Reference Harding and Heaton175, Reference Salem178, Reference Khan and Ahmed181, Reference Javadian183). One study showed that at very high concentrations, oestrogen induces a shift towards a Th2 signature, inhibiting Th1 cytokines, including IL-12, TNF-α and IFN-γ, and stimulate production Th2 cytokines, including IL-10, IL-4 and TGF-β (Ref. Reference Salem178). This, however, is only supported during especially high levels of oestrogen, seen physiologically during pregnancy (Ref. Reference Salem178). Other studies support the finding that overabundance of oestrogenooestrogens shifts the T-cell profile to favour Th2 differentiation and cytokine production (Refs Reference Cutolo72, Reference Doria184, Reference Doria185). Thus, oestrogen levels in pregnant women exacerbate Th1-mediated diseases and inhibit Th2-mediated autoimmune diseases, but oestrogen levels in reproductive aged non-pregnant females compared to males have a protective effect against Th1 mediated diseases such as viral, bacterial and eukaryotic infections. Given this variability in immune effects, oestrogens also may contribute to the sexual dimorphism seen in HAT, Chagas disease and leishmaniasis; Th1 cytokines – IL-12, IFN-γ TNF-α – are known to be implicated in the immune response and elimination of these pathogens (Figures 2, 3, and 4), while Th2 cytokines – IL-10, IL-4 and TGF-β – are implicated in Leishmania survival (Figure 4).
Oestrogen and its impacts on leishmaniasis have been well studied to date. Oestrogen has been shown to have a protective effect against Leishmania infection and the severity of the disease (Refs Reference Mock and Nacy123, 186, Reference Lockard, Wilson and Rodríguez187, Reference Al-khayat, Agha and Alharmni188, Reference de Araújo Albuquerque189). This is supported by studies where sex-differences are significantly less prominent or absent before puberty (Refs 186, Reference Lockard, Wilson and Rodríguez187). Moreover, adult females exhibit smaller Leishmania skin lesions compared to pre-pubescent juveniles (Refs 186, Reference Lockard, Wilson and Rodríguez187). Overall, adult female infections showed to be less severe and less frequent than those in juvenile females or males related to the reduced Th2 response in adult females (Ref. Reference Bernin and Lotter10). Additionally, in mice, exogenous E2 administration was shown to induce leishmanicidal activity in macrophages through NO modulation (Ref. Reference Lezama-Dávila190). As seen in Figure 4, Leishmania species favour elimination in the presence of Th1 cytokines, M1 cytokines and pro-inflammatory mediators such as NO. Thus, given oestrogen’s ability to enhance these at physiologic levels in non-pregnant females, oestrogen likely works through a variety of these mechanisms to enhance the immune response in females and favour parasite elimination thus contributing to sex-bias seen in leishmaniasis.
Despite systemic mammalian oestrogen having an overall protective effect against leishmaniasis infection, selective oestrogen receptor modulator (SERM) therapy with Tamoxifen, which competitively inhibits oestrogen action, is effective in inducing parasiticidal activity in infection by certain Leishmania spp. (L. amazonensis, L. braziliensis, L. major, L. chagasi and L. donovani) in vitro, in vivo and clinical trials (Refs Reference Lockard, Wilson and Rodríguez187, Reference Al-khayat, Agha and Alharmni188, Reference de Araújo Albuquerque189). Tamoxifen’s therapeutic benefit in leishmaniasis stems from its highly selective interference of oestrogen-mediated sphingolipid production in Leishmania resulting in parasite death due to cell membrane destruction (Refs Reference Trinconi191, Reference Sfogliarini192). Given the selective nature of SERMs, only impacting a very specific subset of ERs, this effect on sphingolipid production in the parasite occurs without impacting all oestrogen receptors of the host, thus limiting off target negative side effects (Refs Reference Trinconi191, Reference Sfogliarini192, Reference Zewdie193). This highlights the complex nature of oestrogens and oestrogen receptors making it difficult to rule the hormone’s effects as either protective from or supportive of infection. Certain mechanisms such as its impact on sphingolipid production is supportive of infection and thus used as a drug target (Refs Reference Lockard, Wilson and Rodríguez187, Reference Al-khayat, Agha and Alharmni188, Reference de Araújo Albuquerque189), while other cytokine and immune modulatory mechanisms of oestrogen such as Th1 promotion, are protective against most forms of leishmaniasis (Refs Reference Snider116, Reference Mock and Nacy123, Reference Lockard, Wilson and Rodríguez187, Reference Al-khayat, Agha and Alharmni188, Reference Lezama-Dávila190).
The relationship of T.c. and T.b. infections to oestrogen is less well documented possibly related to the lack of evidence of sex bias in these diseases resulting in a relative lack of investigation into this topic. As demonstrated in Figure 3, T.c. elimination is supported by reactive nitrogen species, cytokines from macrophages, parasite-specific antigens and Th1 cytokines such as IL-12 (Ref. Reference Galvão Da Silva, Jacysyn and Abrahamsohn194). Thus, oestrogen’s role in shifting the Th cell profile is likely not influential in contributing to any sex differences in Chagas disease. More so, oestrogen likely impacts immune response to T.c. infection through Th1 cytokines IL-12/IFN-γ and antibody production and reactive nitrogen species (RNS) (Refs Reference Galvão Da Silva, Jacysyn and Abrahamsohn194, Reference Llaguno195). Moreover oestrogens play a role in promoting RNS (Ref. Reference Roy196) and the elimination of T.c. in acute phase disease (Ref. Reference de Souza197), along with the role of reactive nitrogen species in T.c. elimination (Ref. Reference Nogueira51). However, there is not any available evidence confirming the direct effects of oestrogens on acute phase RNS during T.c. infection.
There is no available literature on oestrogen’s impact on the immune response to T.b., nevertheless, based on what is known of the immune response to T.b. and oestrogen’s immunomodulatory effects, we could hypothesize that oestrogen would favour of the Th1 and M1 response over the Th2 and M2 response. While the M1 and Th1 responses are important in limiting parasitaemia in acute infection, it is key to host survival that the immune response favours an M2 response during an advanced infection to avoid excess inflammatory cytokines that cause host tissue damage and cell death (Figure 2) (Ref. Reference Baetselier198). Studies show a relationship between inflammatory cytokine levels during stage two HAT, indicating CNS infection, and subsequent risk of nervous system damage (Refs Reference Bernin and Lotter10, Reference Ysrraelit and Correale11, Reference Travi29, Reference Schurz63, Reference Markle and Fish64, Reference Lipoldová and Demant65, Reference Verthelyi66, Reference Soudan67, Reference Kovats68). Given this, there is the potential that oestrogen related pro-inflammatory cytokines could exacerbate tissue damage in the CNS during this stage of infection. Although females tended to have lower parasite concentrations (Refs Reference Greenblatt and Rosenstreich62, Reference Pépin and Méda69), pro-inflammatory states may cause adverse disease outcomes in women. Studies have also found that the inability to upregulate alternative macrophage activation and limit classical activation created increased susceptibility to infection and death of infected mice earlier on in the disease (Ref. Reference Baetselier198). Still, further research on how oestrogens impact macrophages, and other innate and adaptive immune cells and regulate gene expression during the immune response to T.b. is yet to be described.
In summary, the sex hormones testosterone, progesterone, prolactin and oestrogen modulate immune responses against Leishmania, Trigonoscuta cruzi and T. brucei parasites through varied and complex mechanisms. While more research is required in many areas, current evidence suggests testosterone tends to exacerbate disease caused by kinetoplastids, although it may be protective in the case of HAT, prolactin is important for controlling Chagas disease, progesterone’s role is unclear, and oestrogen often enhances immune clearance of Leishmania but has less characterized impacts for Chagas disease and HAT.
Genetic and gene expression differences in immune response
Numerous clinical studies show that there are substantial sex-based disparities in the prognosis of several infectious and immune diseases (Refs Reference Garenne199, Reference Sugimoto200, Reference Steinberg201, Reference Haupt202). Females typically have stronger humoral and cellular responses to infection or antigenic stimulation than males (Refs Reference Markle and Fish64, Reference Klein203). This characteristic can provide protection against various infections, although in some cases an exacerbated immune response has been observed as a potential risk factor during immunopathological changes or treatment, such as immune restoration diseases in HIV-TB co-infected patients (Refs Reference Amur, Parekh and Mummaneni204, Reference Verma205). While there are general sex-differences in innate immune response, female and male genetic influences on immune responses during parasite infection are pathogen-specific (Ref. Reference Bernin and Lotter206). Trichomonas vaginalis and Toxoplasma gondii resistance was shown to be higher in males, while leishmaniasis resistance was found to be higher in females (Refs Reference Morales-Montor207, Reference Barletta Ferreira208). Males are more likely to be affected by VL (Ref. Reference Bruhn25, Reference Dahal36) and certain CL infections than females (Refs Reference Gandacu30, Reference Spotin, Rouhani and Parvizi31, Reference Collis32, Reference Soares, Abad-Franch and Ferraz33, Reference Cloots42). While in individual infectious diseases, some differences according to gender have been documented, determining what originates from genetic differences between genders can be thorny. For example, genetic differences between genders could manifest as different rates of beneficial or harmful alleles, different gene expression patterns or different allele numbers, particularly for genes related to the sex chromosomes; yet genetic differences also combine to produce different hormone levels discussed previously which have their own cascading effects. We will distinguish these sources of genetic differences here. For example, the X and Y chromosomes’ genetic material controls the expression of traits with non-hormonal bias toward either sex (Refs Reference Schurz63, Reference Lipoldová and Demant65, Reference Case and Teuscher209). Many immune-related miRNAs are also found on the X chromosome, among immune-related genes such as toll-like receptor (TLR)-7 and interleukin-1 receptor-associated kinase-1 (IRAK-1) (Ref. Reference Schurz63). Some portion of the X chromosome remains active, which may boost sex-specific gene transcription, despite the fact that X-chromosome inactivation (or silence of one X chromosome) in women is supposed to result in dosage compensation of X-linked genes (Refs Reference Balaton and Brown210, Reference Karnam211).
In contrast, Y chromosomes affect infection susceptibility, immunological gene expression and regulation (Ref. Reference Case and Teuscher209). Further studies have also observed more specific gene-pathogen associations that are differentially apparent by gender. This includes the genes Lmr4 (chromosome; Chr-6), Lmr14 (Chr-2), Lmr15 (Chr-2) and Lmr27 (Chr-17) that control anti-L. major responses in male BALB/c mice by limiting parasite load in lymph nodes (Figure 5) (Refs Reference Slapničková212, Reference Kobets213). Similarly, CD4+ T cells’ specific Il4ra gene on Chr-7 promotes the L. mexicana-mediated non-healing phenotype in male BALB/c mice (Ref. Reference Bryson214), and genes Tbbr1 (Chr-3) and Tbbr2 (Chr-12) help in increasing survival in BALB/c females infected with T. b. brucei (Ref. Reference Síma215). Tbbr1 is only evident in females, and Tbbr2 expresses higher in females than males. Loci on Chr-17 and Chr-5 determine susceptibility to T.c., however, the role of Chr-5 in sex differentiation remains unelucidated. Because of its overlap with Lmr22 and interaction with Lmr5, which regulates IL-4 release in the serum of L. major-infected mice, Tbbr2 may also play an important role in regulating L. major (Ref. Reference Kurey216). On a similar note, DBA/2 mice infected with L. mexicana showed two different clinical manifestations: ulcerative lesions in males that did not heal and healing lesions in females (Refs Reference Blackwell, Roberts and Alexander217, Reference Alexander218). Scl2 is the gene has been identified to regulate this (Ref. Reference Alexander218). Due to antigen-specific vulnerability, these various genes across several chromosomes may affect immune responses differently between genders and need more investigation to elucidate their regulation further.
Parasite regulation by differential genetic loci and chromosome. The chromosomal gene loci indicated are directly associated with kinetoplastid disease outcomes and differences in their roles have been observed by sex. Additional genetic products such as proteins, hormones and enzymes also exist which are interdependent in their influence on immune responses, and which may also show differences by sex. Abbreviations: Chr: Chromosome, Lmr: Leishmania major response, Tbbr: Trypanosoma brucei brucei response.
Socio-cultural Influences
Disease-related stigma denies impacted individuals of social acceptance, reduces economic opportunities, allows for discrimination and greatly influences health-seeking behaviour by creating a barrier to care (Ref. Reference Stangl219). Certain diagnoses can lead to stigmatizing behaviours and discrimination including housing refusal, verbal abuse, gossip and self-stigma, creating social devaluation and negative emotions for an individual (Ref. Reference Stangl219). While stigma affects men and women, women have reported avoidance of healthcare services due to fear of stigmatization by their communities or their providers for a multitude of diseases, contributing to worsened health outcomes and underreporting of disease (Ref. Reference Stangl219).
Active and scarred CL lesions create a greater stigma for women than men (Ref. Reference Wenning220). In rural Morocco, Yemen and Afghanistan, CL facial scars cause stigmatization that hinders marriage prospects and harms the self-esteem of young women (Refs Reference Wenning220, Reference Bennis221). Additionally, scarring can decrease education and employment opportunities (Ref. Reference Wenning220). Because women typically perform unpaid labour in these areas, they face a greater financial barrier to care than men (Ref. Reference Bennis221). Furthermore, CL-afflicted women in Colombia and Brazil reported avoiding healthcare clinics due to distrust, while pregnant or nursing women in Ecuador feared seeking treatment due to drug toxicity and potential damage to their child, and Afghani women reported restrictions to care due to the required presence of a male in clinic with the patient, and the requirement that women be treated by a female provider (Ref. Reference Wenning220). Additionally, for every Colombian man who sought treatment, 13 women sought home remedies, leading to decreased reporting and worsened outcomes for women (Ref. Reference Wenning220).
CD infection, despite its lack of association with any physical attributes, is also associated with poverty, stigma and decreased quality of life (Ref. Reference Hofstraat and van Brakel222, Reference Almeida223). Chagas may be used as a reason to fire an individual from work and prevent them from passing pre-hiring physical examinations in Brazil (Ref. Reference Hofstraat and van Brakel222). Employment also acts as a deterrent to seeking treatment for CD in women. In Italy, Bolivian female migrants often work in caregiving roles, regardless of their education, and seeking care for CD required them to take time off from work and was associated with stigmatization by employers that believed CD to be contagious, decreasing access to and seeking of healthcare for women, contributing to underreporting (Ref. Reference Castaldo224). In contrast to some other findings, in Barcarena, Pará and Brazil, gender was not found to be a statistically significant factor in T. c. infection, indicating that in those communities infection negatively impacts employment opportunities and healthcare-seeking for both men and women (Ref. Reference Sousa225).
HAT does not demonstrate differences in stigmatization between men and women, likely due to HAT being endemic in areas where it is studied, allowing communities to become familiar with the disease (Ref. Reference Hofstraat and van Brakel222). However, HAT is associated with stigmatization in both women and men due to potential resultant neurological impairments and performance impediments that could lead to individuals facing gossip, mockery and loss of social status, deterring individuals from seeking care (Ref. Reference Hofstraat and van Brakel222, Reference Kabanga226). Moreover, in the DRC (Ref. Reference Kabanga226), neurological symptoms associated with HAT are believed to be associated with witchcraft by women or dark forces (Ref. Reference Kabanga226). While women and men with HAT are both stigmatized, only women are believed by some communities to be responsible for the disease (Ref. Reference Kabanga226).
While stigma may impact the rate that individuals seek treatment, other behaviours may also impact the rate of disease incidence among genders. Risk-taking behaviours such as risk-taking behaviours could also impact the rates of disease among each gender. Risk-taking behaviour includes unsafe sexual practices, rule breaking, self-injurious behaviours and aggressive violent or behaviours (Ref. Reference Salas-Rodríguez227) and given their associations to various disease states, differences in these behaviours between males and females can contribute to gender differences in disease occurrence and outcomes. Males take part in risk-taking behaviours at a greater rate than females. This trend is attributed to these behaviours providing males with greater reproductive success and enhancement of social status from an evolutionary psychology perspective; males faced stronger reproductive competition than females, creating a riskier sexual selection process for males (Ref. Reference Salas-Rodríguez227). Therefore, males are more likely to perceive the benefits of risky-behaviours, while females are more likely to perceive the associated risks (Ref. Reference Salas-Rodríguez227).
In the context of leishmaniasis, the impact of risk-taking behaviours is beginning to be observed. A study of leishmaniasis in Israel suggests risk-taking behaviours, specifically travel to endemic regions with increased outdoor exposure, led to a high incidence of males contracting Leishmania spp. not known to be endemic in Israel (Ref. Reference Solomon228). Risky behaviours, such as working in a specific area, refusal to seek treatment, travel to endemic areas and participation in outdoor activities leading to increased vector exposure contribute to the differences in infectious disease incidence between men and women such as that seen in leishmaniasis (Ref. Reference Wenning220). Furthermore, in Suriname, risky behaviours such as working in the sectors of gold mining, lumber or construction increased one’s risk of CL and were associated with masculinity (Ref. Reference Wenning220). In Colombia, where risk-taking is associated with responsibility, CL was linked to serving in the army, which predominantly affected males (Ref. Reference Wenning220). In Morocco, however, where women were primarily responsible for farming, handling manure and caring for cattle, they were placed at higher risk of CL infection than men (Ref. Reference Wenning220). These differences may contribute to variable incidence reports and risk calculations of sex in different communities.
Despite areas endemic to CD also having gender roles associated with jobs, there was no significant difference in infection between men and women, indicating there is likely equal exposure to infection (Ref. Reference Sousa225, Reference Hoyos229). However, in Bolivia, men were found to be less likely to seek help when infected to avoid appearing weak and potentially losing their jobs which could mean an underreporting in males and thus underestimation of sex-bias towards male infection for CD (Ref. Reference Ventura-Garcia230, Reference Jimeno231). Women, however, were more likely to utilize healthcare services related to their reported desire to stay alive and healthy to provide care for their children (Ref. Reference Jimeno231). This finding for CD contrasts with that in Colombia for CL. While women in Bolivia were more likely to seek treatment for CD than men, women in Colombia were far more likely to try multiple home remedies first for CL (Ref. Reference Hoyos229, Reference Wenning220). Although not confirmed, this could be attributed to stigmatization associated with CL facial scars in women (Ref. Reference Wenning220). CD, on the other hand, does not have an association with physical attributes and does not create this stigma between men and women (Ref. Reference Hofstraat and van Brakel222).
Research suggests regardless of gender, communities perceived all individuals to be at risk for HAT (Ref. Reference Kabanga226). Some studies reported that activities with the most significant gender differences were not significant predictors of HAT risk (Ref. Reference Zoller57). Other studies reported activities such as cleansing rituals in Uganda and Kenya, as well as circumcision in Uganda, were the most important cultural risk factors for HAT infection due to increased exposure to tsetse flies (Ref. Reference Houweling232). Male-associated behaviours such as poaching, honey-gathering and land clearing have also been associated with increased HAT incidence (Ref. Reference Madanitsa, Chisi and Ngwira233). Thus, strong gender differences in stigmatization and risk-taking behaviour associated with leishmaniasis likely contribute to the gender bias seen in the disease. Meanwhile, less prevalent gender differences seen with CD and HAT may be related to the lesser differences in incidence rates between males and females for these diseases.
Expert and topical summary
Leishmaniasis, Chagas disease and Human African Trypanosomiasis are neglected tropical diseases caused by kinetoplastid parasites that exhibit sex differences in incidence and disease severity. Leishmaniasis shows a clear male bias, with higher incidence, morbidity and mortality in males compared to females. This is influenced by sex hormones, genetics and gender-related behaviours. In contrast, CD and HAT show less consistent or minimal sex differences in outcomes. The reasons for variability in sex bias between the diseases are not fully understood but likely involve differing interactions between the immune response and sex-related factors such as genetics, hormones and behaviours. More research is needed to identify and understand the specific factors driving increased susceptibility and disease severity in males or females for each pathogen, particularly to understand HAT and CD which have little information available in this area. While the differences between males and females in Chagas and HAT disease risk and outcomes are not fully clear, continuing research in this area could have implications for disease management, treatment, epidemiology and control. Thus, the patient’s sex remains an important factor to consider in the context of these neglected tropical diseases.
Acknowledgments
We thank the editors and reviewers of the Cambridge University Press Expert Reviews in Molecular Medicine team.
Author Contribution
Conceptualization: all; writing-original draft preparation: O.B., R.H.H., C.V., T.P.F., S.A.K., A.C.; writing-review and editing: O.B., R.H.H., C.V., T.P.F., S.A.K., A.C., A.R.S; visualization: T.P.F., C.V.; supervision: A.R.S. All authors have read and agreed to the published version of the manuscript.
Funding statement
This research received no external funding.
Competing interest
The authors declare no conflicts of interest.
Disclaimer
Our contributions are an informal communication and represent our own best judgement. These comments do not bind or obligate FDA.
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