Analysis of vaccine- and infection-induced immune responses contributing to suppression of blood-stage malaria in the Plasmodium yoelii rodent model

Immunization with Plasmodium yoelii merozoite surface protein-8 (PyMSP-8) protects mice from lethal malaria but does not prevent infection. Using this MSP-based vaccine model, vaccine- and infection-induced immune responses that contribute to protection were investigated. Analysis of prechallenge sera from rPyMSP-8 immunized C57BL/6 and BALB/c mice revealed high and comparable levels of antigen-specific IgG but differences in isotype profile and specificity for conformational epitopes were noted. As both strains of mice were similarly protected against P. yoelii, vaccine-induced responses did not correlate with protection. However, passive immunization studies suggested that protection resulted from differing immune responses. Studies with cytokine-deficient mice showed that protection was induced by immunization of C57BL/6 mice only when IL-4 and IFN-gamma were both present. In BALB/c mice, the absence of either IL-4 or IFN-gamma led to predictable shifts in the IgG isotype profile but did not reduce the magnitude of the antibody response induced by rPyMSP-8 immunization. Immunized IL-4-/- BALB/c mice were protected against P. yoelii. Surprisingly, immunized IFN-gamma -/- BALB/c mice initially controlled parasite growth but eventually succumbed to infection. Analysis of cytokine production revealed that P. yoelii infection induced two distinct peaks of IFN-gamma in rPyMSP-8 immunized BALB/c mice. Further evaluation determined that it was the early production of IFN-gamma that was necessary for protection in immunized and challenged mice. Maximal parasite growth in rPyMSP-8 immunized mice occurred during a period of sustained TGF-beta production. TGF-beta neutralization did not lead to an uncontrolled, pathologic proinflammatory response but resulted in modestly improved protection in rPyMSP-8 immunized mice. There were no significant differences in antibody or cytokine responses between isotype- and anti-TGF-beta-treated, rPyMSP-8 immunized mice. Interestingly, rPyMSP-8 immunized IFN-gamma -/- BALB/c mice, which normally succumb to lethal P. yoelii malaria, were protected when TGF-beta was neutralized. These data suggest that in rPyMSP-8 immunized mice, TGF-beta is not necessary for down-modulating inflammatory responses and is detrimental to disease outcome in the absence of IFN-gamma. Combined, the data indicate that suppression of lethal P. yoelii in rPyMSP-8 immunized mice requires high titers of PyMSP-8 antibody and infection-induced IFN-gamma which is associated with development of the protective response to additional parasite antigens.