Softwood knots, i.e. the branch base encased in the tree stem, belonging to the genera Pinus and Picea, contain exceptionally large amounts of bioactive phenolic compounds⁽Reference Willför, Ahotupa, Hemming, Reunanen, Eklund, Sjöholm, Eckerman, Pohjamo and Holmbom¹⁾, which can be 50–100 times greater than in adjacent stemwood. They constitute a valuable resource with a potential for use as technical antioxidants, biological antioxidants in foodstuffs, functional food ingredients and natural biocides⁽Reference Willför, Ahotupa, Hemming, Reunanen, Eklund, Sjöholm, Eckerman, Pohjamo and Holmbom¹⁾. Previously it has been reported that in human Jurkat T cells knot extracts from Pinus banksiana lamb. (Jack pine, Saskatchewan, Canada) are more cytotoxic than Picea sitchensis (bong.) Carr. (Sitka spruce, Co. Mayo, Republic of Ireland), with half maximal inhibitory concentration values of 153.0 μg/ml and 376.1 μg/ml respectively⁽Reference Phelan, Aherne, Wong and OBrien²⁾. Thus, the aim of the present study was to further assess the potential bioactivity of these wood knot samples by investigating their effect on cell membrane integrity, antioxidant enzyme activity and cytokine production in human Jurkat T cells. In addition, their potential protective effects against oxidant-induced cell injury and DNA damage were determined.

Cell membrane integrity was determined using the lactate dehydrogenase-release assay kit (BioGenesis, Poole, Dorset, UK). Catalase activity was determined using a Calbiochem colorimetric kit (Merck Chemicals Ltd, Nottingham, UK). Supplementation with non-toxic doses of the Jack pine and Sitka spruce knot extracts (10 and 30 μg/ml) for 24 h had no effect on Jurkat cell membrane integrity or catalase activity.

To assess potential immunomodulatory effects Jurkat cells were treated with concanavalin A (Con A; 25 μg/ml) in the presence or absence of softwood knot extracts (10 and 30 μg/ml) for 24 h. IL-2 production was determined by an eBioscience ELISA kit (Insight Biotechnology Ltd, Wembley, Middlesex, UK). Jack pine knot extract at a concentration of 30 μg/ml was the only sample to significantly suppress (P<0.05) ConA-induced IL-2 production.

Cytoprotection was assessed by measuring the viability of Jurkat cells supplemented with Sitka spruce and Jack pine knot extracts (10 and 30 μg/ml) for 24 h followed by treatment with H₂O₂ (125 and 250 μm) for 1 h. Cell viability was determined by the MTT assay II kit (Roche Diagnostics, Burgess Hill, West Sussex, UK). Pre-treatment with Sitka spruce (10 and 30 μg/ml) and Jack pine (10 μg/ml) protected (P<0.01) against cell injury induced by the presence of 125 μm-H₂O₂. Both softwood extracts significantly enhanced (P<0.01) cell viability following exposure to 250 μm-H₂O₂ compared with cells treated with the oxidant alone. Caco-2 cells were pre-incubated with the wood knot extracts for 24 h followed by exposure to H₂O₂ (50 μm for 30 min). DNA damage was assessed using the comet assay by the method of Tice et al.⁽Reference Tice, Andrews, Hirai, Singh and Witmer³⁾ None of the extracts at the concentrations tested protected the cells against H₂O₂-induced DNA damage.

From the findings it is evident that there are interesting opportunities for phytochemicals found in tree species that need to be further evaluated and researched. In particular, further investigation into the immunomodulatory and cytoprotective effects of softwood knot extracts is warranted.