Reasons why DBA/2 mice are resistant to malarial infection:: expansion of CD3int B220+ γδ T cells with double-negative CD4- CD8- phenotype in the liver

DBA/2 (H-2(d)) mice are known to be more resistant than C57BL/6 (B6, H-2(b)) mice to the non-lethal 17XNL strain of Plasmodium yoelii. This is a very strange phenomenon because the functions of conventional T cells, especially CD8(+) T cells, are known to be somewhat lower in DBA/2 mice than in other strains of mice. We examined herein how immune responses differed between DBA/2 mice and 136 mice during malarial infection. DBA/2 mice and (DBA/2 x B6)F-1 (BDF1, H-2(b/d)) mice were found to have milder parasitaemia and to recover more quickly from malarial infection than 136 mice. These DBA/2 and BDF1 mice were also found to experience a marked expansion of interleukin (IL)-2R beta(+) CD3(int) cells and gamma delta T cells in the liver, especially in the recovery phase. The expansion of unconventional T cells (i.e. B220(+) T cells) was also marked in DBA/2 and BDF1 mice. The majority of B220(+) T cells were gamma delta T cells and these T cells were double-negative CD4(-) CD8(-). More importantly, the production of immunoglobulin M (IgM)-type anti-DNA autoantibody was also higher in DBA/2 and BDF1 mice than in B6 mice. In conjunction with data on cytokine production, these results indicate that primitive T and B cells, namely autoreactive extrathymic T cells and autoantibody-producing B cells, may be much more activated in DBA/2 mice and therefore resistant to the non-lethal 17XNL strain of P. yoelii.