The immune response and immunization studies in avirulent DK and virulent DS strains of Plasmodium chabaudi adami and a synthetic peptide immunization in P. chabaudi AS

In the first part of the present study the immune responses in NIH mice against asexual blood stages of either avirulent DK or virulent DS strains of P. chabaudi adami, in single or mixed infections, were examined using the ELISA test for the detection and measurement of cytokine and antibody production. The present results showed that the profile of the immune response in all the above infections suggests a sequential Thl/Th2 CD4+ T cell response. Previous studies have shown that there is a sequential Thl/Th2 response in Plasmodium chabaudi AS infection which is reflected in the activation of both cell- and antibody-mediated responses. These findings, therefore, indicate that vaccines which induce both arms of the protective immune response against malaria parasites could be most effective. The sequential Th1/Th2 response was supported by detecting early high levels of IFNγ and IgG2a during the acute phase of the infection and later by elevation of IL-4 and IgGl levels during the course of infection compared to controls. The levels of IgG2a were at highest levels at or immediately after the peak parasitaemia while the levels of IgG1 increased in later stages in the course of infection. However, a higher level IFNγ early in the infection indicated a stronger Thl response in the avirulent DK strain infection compared to the virulent DS strain or mixed infections. On the other hand, in the virulent DS infection a stronger Th2 response with higher IL-4 levels compared to the DK strain and mixed infections was observed in mice treated with chloroquine. In the mixed infection, an infective dose consisting of 8x103 pRBCs of the avirulent DK strain and 2x103 pRBCs of the virulent DS strain was used. Despite a relatively low number of pRBCs of the DS strain in the infective dose the peak parasitaemia was significantly higher than that in the single-infection of 1x104 pRBCs of the DK strain. The mixed infection also showed a significantly lower peak parasitaemia compared to that in mice given 1x104 pRBCs of the DS strain single-infection in untreated mice. So, it may be concluded that a higher peak parasitaemia in the mixed infection compared to the DK single-infection is reflected in a higher replication rate of the DS strain compared to the DK strain during the course of the mixed infection