Merozoite surface protein (MSP) 1 is a large protein of ~200 kDa which is proteolytically processed into a number of fragments during schizont rupture and red blood cell invasion. The 19 kDa (MSP119) and 42 kDa (MSP142) carboxyl terminal fragments are promising vaccine candidates against blood stage malaria as they induce protective immunity in animal models. Immunity induced by immunisation with MSP 119 is thought to be mediated predominantly by high titres of specific antibodies present at the time of infection. However, passively transferred MSP 119 - specific antibody alone is insufficient to control infection in immunodeficient mice, suggesting that immunity induced by MSP 119 also depends on a continuing active immune response post-infection. The nature and specificity of this active immune response post-infection were investigated in this study. MSP 119 -immunised B cell knockout mice which were administered MSPli9-specific antibodies were unable to completely control an infection with P yoelii YM, suggesting that effector T cells do not play an important role in the immunity. If CD4+ T cells specific for p24, a helper T cell epitope on MSP 119, were adoptively transferred to naive nude (athymic) mice, which were subsequently immunised with MSP 119, the mice were resistant to challenge infection. Western blots showed that only MSPl19-specific antibodies were produced in these mice. Naive nude mice or mice that received T cells specific for an irrelevant antigen, ovalbumin (OVA), prior to immunisation were not protected. The data indicate that helper T cells specific for MSP 119 can provide help to B cells for the production of MSP 119 -specific antibodies, and that an MSP 119 -specific active immune response post challenge is sufficient for immunity.
While CD4+ T cells are clearly critical for immunity, we found that helper T cells specific for p24, but not OVA-specific T cells, were deleted as a result of P. yoelii infection. A higher percentage of CFSE-labelled p24 T cells bound Annexin and expressed active Caspase-3 in infected mice, compared with uninfected mice, suggesting that apoptosis contributes to the deletion of p24 T cells during infection. This correlated with a higher level of Fas expression on the surface of p24-specific T cells recovered from infected mice, suggesting thati P. yoelii-induced apoptosis is, at least in part, mediated by Fas. As a result of infection, spleen cells recovered from infected p24 T cell-transfused mice demonstrated reduced proliferation in response to specific antigen. This has implications for the development of immunity in the face of endemic parasite exposure.
The potential of a human compatible adjuvant, Montanide ISA 720, to enhance MSPl19-induced immunity was compared with Freund's adjuvant. Groups of BALB/c or C57BL/6 mice were given 3 doses of MSP 119 formulated in Freund's or Montanide ISA720. We found that Freund's adjuvant was more potent in enhancing MSP 119 -induced immunity than Montanide ISA720.
MSP 119 and MSP 142 are the major MSPl-derived vaccine candidates, but little is known about the significance of the shed fragment, MSP 133, a product of secondary processing of MSP 142. Here, T cell epitopes on MSP 133 were identified, and their role in immunity to blood stage malaria was examined. Peptides spanning the length of MSP 133 were analysed for their ability to induce T cell proliferation in BALB/c and C57BL/6 mice. Multiple epitopes were recognised by these two strains of mice. Helper and/or effector functions of the dominant epitopes were then investigated. Peptides Cm 15 and Cm21 were of particular interest as they were able to induce effector T cells capable of delaying growth of lethal Plasmodium yoelii YM following adoptive transfer into immunodeficient mice, without inducing detectable antibody responses. Homologues of such epitopes could be candidates for inclusion in a subunit vaccine.
Taken together, these data could have implications for the development of a blood stage malaria vaccine based on MSPl.