Unlike human
malaria parasites that induce
persistent infection, some rodent
malaria parasites, like Plasmodium yoelii strain 17XNL (Py17XNL), induce a transient (self-curing)
malaria infection. Cooperation between CD4 T cells and B cells to produce
antibodies is thought to be critical for clearance of Py17XNL parasites from the blood, with major histocompatibility complex (
MHC) class II molecules being required for activation of CD4 T cells. In order to better understand the correspondence between murine
malaria models and human
malaria, and in particular the role of MHC (HLA) class II molecules, we studied the ability of humanized mice expressing human HLA class II molecules to clear Py17XNL
infection. We showed that humanized mice expressing
HLA-DR4 (DR0401) molecules and lacking mouse
MHC class II molecules (EA(0)) have impaired production of specific
antibodies to Py17XNL and cannot cure the
infection. In contrast, mice expressing
HLA-DR4 (DR0402),
HLA-DQ6 (DQ0601),
HLA-DQ8 (DQ0302), or
HLA-DR3 (DR0301) molecules in an EA(0) background were able to elicit specific
antibodies and self-cure the
infection. In a series of experiments, we determined that the inability of humanized DR0401.EA(0) mice to elicit specific
antibodies was due to expansion and activation of regulatory CD4(+) Foxp3(+) T cells (Tregs) that suppressed B cells to secrete
antibodies through cell-cell interactions. Treg depletion allowed the DR0401.EA(0) mice to elicit specific
antibodies and self-cure the
infection. Our results demonstrated a differential role of MHC (HLA) class II molecules in supporting antibody responses to Py17XNL
malaria and revealed a new mechanism by which
malaria parasites stimulate B cell-suppressogenic Tregs that prevent clearance of
infection.