Impaired immune responsiveness in Plasmodium berghei immune mice

Mice immunized against Plasmodium berghei parasites by drug-controlled infection exhibited decreased immunoresponsiveness against rabbit red blood cells (RRBC). Increasing RRBC antigen dose increased responsiveness, but agglutinating anti-RRBC antibodies of the IgG class remained undetectable. Clearance of colloidal carbon from the bloodstream of malaria-immunized mice was not different from controls. Removal of all the persistent parasites from immune mice did not restore responsiveness until 140 days after treatment, suggesting that the parasite per se did not influence responsiveness directly. Because of this, and because of the fact that priming of mice with RRBC before P. berghei immunization was not more effective than priming after immunization, it was concluded that antigen uptake and subsequent presentation were not impaired in P. berghei immune mice, in contrast to infected mice. Anti-RRBC antibodies were detected in serum of P. berghei immune mice, but regulation of responsiveness to RRBC by transfer of such immune mouse serum was not found. Immunoglobulin levels, especially of the IgG2 and IgG3 subclass were elevated in sera of P. berghei immune mice, which indicated an LPS-like polyclonal activation. The results also suggest that during drug-controlled infection, which leads to immunity against infection, a state of B-cell tolerance is induced.