P007 IL-21, B cells and CD4-CD8- T cells are key for resistance to Babesia microti infection in immunosuppressed hosts

Introduction Babesiosis is an emerging infectious disease in the Northeast and upper Midwest of the United States. The etiologic agent is Babesia microti, a protozoan parasite that invades red blood cells. In healthy individuals, B. microti infection is typically asymptomatic or evokes a mild flu-like illness that quickly resolves. In immunocompromised patients, particularly those infected with HIV and those treated with rituximab for B cell lymphoma, babesiosis is severe, persistent, and sometimes fatal. To understand the mechanisms by which the host controls and clears B. microti infection, we have developed a mouse model of human babesiosis. Methods Wild-type C57BL/6 mice and C57BL/6 mice that carried a single or a double gene deficiency were infected with B. microti parasitized red blood cells. Parasitemia was assessed by flow cytometry and is defined as the frequency of red blood cells that stain positive for DNA. Babesia specific IgG subclasses were quantified using Babesia infected red blood cells as a source of parasite antigens. Frequencies of interleukin (IL)-21 and/or interferon-gamma (IFNγ) producing T cell subsets were assessed by flow cytometry. Results In wild-type mice, parasitemia was modest and quickly resolved. Lack of mature B cells due to igh6 deficiency did not hinder resistance whereas depletion of CD4+ T cells conferred susceptibility and prevented resolution of infection. Intense, persistent parasitemia was also observed in athymic mice, in rag1−/− mice, and in mice lacking TCRβ or MHC class II. Surprisingly, in cd4−/− mice, parasitemia was intense but resolved. In their spleens, CD8+ T cells and CD4−CD8− (double negative, DN) T cells expanded. Parasitemia resolved in cd4−/− cd8−/− mice, but persisted in cd4−/− igh6−/− mice. Likewise, parasitemia persisted in cd4−/− mice depleted of B cells by repeated administration of anti-CD20. In cd4−/− mice, the rise in circulating Babesia specific IgGs was delayed but concomitant with resolution of parasitemia. Despite the lack of CD4, all subisotypes were produced. IgG1 levels were higher in cd4−/− mice than in wild-type mice whereas IgG3 levels were lower. In cd4−/− mice, DN T cells producing both IL-21 and IFNγ expanded, but not as fast as CD4+ T cells producing both cytokines in wild-type mice. In contrast, DN T cells producing IFNγ alone expanded as fast as their CD4+ T cell counterparts. Furthermore, DN T cells failed to produce IL-21 alone whereas CD4+ T cells producing IL-21 alone quickly expanded. Parasitemia resolved in cd4−/− mice treated with a neutralizing antibody against mouse IFNγ, but persisted in cd4−/− mice that also lacked the IL-21 receptor gene. Despite the requirement for B cells and the production of IgGs, Fcγ receptors were dispensable for resolution of parasitemia in cd4−/− mice. Conclusion Our studies establish that CD4+ T cells are critical for resistance to B. microti infection in C57BL/6 mice and suggest that in the absence of CD4, CD4−CD8− T cells are central for resolution of B. microti parasitemia. In cd4−/− mice, resolution of parasitemia requires B cells and IL-21 but not IFNγ. Given that IgGs are produced but Fcγ receptors are dispensable for host resistance in cd4−/− mice, the role of complement is being investigated.