Resistance to Malarial Infection Is Achieved by the Cooperation of NK1.1+ and NK1.1− Subsets of Intermediate TCR Cells Which Are Constituents of Innate Immunity

We previously reported that the major expanding lymphocytes were intermediate TCR (TCRint) cells (mainly NK1.1−) during malarial infection in mice. Cell transfer experiments of TCRint cells indicated that these T cells mediated resistance to malaria. However, TCRint cells always contain NK1.1+TCRint cells (i.e., NKT cells) and controversial results (NKT cells were effective or not for resistance to malaria) have been reported by different investigators. In this study, we used CD1d(−/−) mice, which almost completely lack NKT cells in the liver and other immune organs. Parasitemia was prolonged in the blood of CD1d(−/−) mice and the expansion of lymphocytes in the liver of these mice was more prominent after an injection of Plasmodium yoelii-infected erythrocytes. However, these mice finally recovered from malaria. In contrast to B6 mice, CD4−8− NKT cells as well as NK1.1−CD3int cells expanded in CD1d(−/−) mice after malarial infection, instead of CD4+ (and CD8+) NKT cells. These newly generated CD4−8−NKT cells in CD1d(−/−) mice did not use an invariant chain of Vα14Jα281 for TCRα. Other evidence was that severe thymic atrophy and autoantibody production were accompanied by malarial infection, irrespective of the mice used. These results suggest that both NK1.1− and NK1.1+ subsets of TCRint cells (i.e., constituents of innate immunity) are associated with resistance to malaria and that an autoimmune-like state is induced during malarial infection.