The BALB/c strain of mice is comparatively more resistant to
sporozoite infections of Plasmodium berghei than the C57BL6
strain. Infection with live sporozoites results in the formation of small
hepatic forms in the BALB/c liver that persist for
as long as 6 days. Upon infection with small numbers of sporozoites, some
of the parasites are destroyed in the liver
whereas the rest persist as blocked forms. When larger numbers of sporozoites
are injected the same process occurs but,
in addition, a fraction of the liver-stage parasites complete full development
and give rise to blood forms. Although blocked
liver forms persist until day 6 post-infection they actually develop to
only 24 h of maturity. The nature of these persistent
forms is similar to those obtained from irradiated sporozoite immunization.
There is a stronger cell proliferation to liver-stage antigens by spleen
lymphocytes of irradiated sporozoite-immunized BALB/c mice in comparison
to that of
immunized C57BL6 mice suggesting that a stronger priming to liver-stage
antigens, probably due to the presence of
blocked hepatic forms in the liver for a longer period of time (as compared
to C57BL6), occurs in the BALB/c mice. This
could be a reason for the long-lasting protective memory observed in BALB/c
mice.