Role of CD28 in Polyclonal and Specific T and B Cell Responses Required for Protection against Blood Stage Malaria

The role of B7/CD28 costimulatory pathway in the polyclonal and specific lymphocyte activation induced by blood stages of Plasmodium chabaudi AS was investigated in CD28 gene knockout (CD28(-/-)) and C57BL/6 (CD28(+/+)) mice. Analysis of the spleen during the acute infection revealed a similar increase in T and B cell populations in both groups of mice. Moreover, CD28(-/-) mice were able to develop a polyclonal IgM response to P. chabaudi. On the contrary, the polyclonal IgG2a response was markedly reduced in the absence of CD28. Production of IFN-gamma; up-regulation of CD69, CD40L, CD95 (Fas), and CD95L (Fas ligand); and induction of apoptosis were also affected by the lack of CD28. Interestingly, the ability to control the first parasitemia peak was not compromised in acutely infected CD28(-/-) mice, but CD28(-/-) mice failed to eradicate the parasites that persisted in the blood for >3 mo after infection. In addition, drug-cured CD28(-/-) mice were unable to generate memory T cells, develop an anamnesic IgG response, or eliminate the parasites from a secondary challenge. The incapacity of CD28(-/-) mice to acquire a full protective immunity to P. chabaudi correlated with an impaired production of specific IgG2a. Moreover, reinfected CD28(-/-) mice were protected by the adoptive transfer of serum from reinfected CD28(+/+) mice containing specific IgG2a. Our results demonstrate that the polyclonal lymphocyte response is only partially affected by the absence of CD28, but this coreceptor is essential to generate specific T and B cell responses required for complete protection against P. chabaudi malaria.