Resistance to acute babesiosis is associated with interleukin-12- and gamma interferon-mediated responses and requires macrophages and natural killer cells

Abstract

We examined the role of the cytokines gamma interferon (IFN-gamma) and interleukin-12 (IL-12) in the model of acute babesiosis with the WA1 Babesia. Mice genetically deficient in IFN-gamma-mediated responses (IFNGR2KO mice) and IL-12-mediated responses (Stat4KO mice) were infected with the WA1 Babesia, and observations were made on the course of infection and cytokine responses. Levels of IFN-gamma and IL-12 in serum increased 24 h after parasite inoculation. The augmented susceptibility observed in IFNGR2KO and Stat-4KO mice suggests that the early IL-12- and IFN-gamma-mediated responses are involved in protection against acute babesiosis. Resistance appears to correlate with an increase in nitric oxide (NO) production. In order to assess the contribution of different cell subsets to resistance against the parasite, we also studied mice lacking B cells, CD4+ T cells, NK cells, and macrophages. Mice genetically deficient in B lymphocytes or CD4+ T lymphocytes were able to mount protective responses comparable to those of immunosufficient mice. In contrast, in vivo depletion of macrophages or NK cells resulted in elevated susceptibility to the infection. Our observations suggest that a crucial part of the response that protects from the pathogenic Babesia WA1 is mediated by macrophages and NK cells, probably through early production of IL-12 and IFN-gamma, and induction of macrophage-derived effector molecules like NO.

FIG. 1.

Infection course in IFN-γ-R2-deficient mice. (A) Survival in IFNGR2KO mice and 129 controls. (B) Parasitemia levels in IFNGR2KO mice and 129 controls. (C through E) Levels of cytokines and NO in serum after infection with the WA1 Babesia. Filled bars represent uninfected controls; white bars represent infected animals. Data shown are the averages of 3 to 5 individual plasma samples. (C) Levels of IL-12 24 h after infection. (D) Levels of IFN-γ 24 h after infection. (E) Nitrite levels in serum at day 7 after infection. Data shown are pooled from two replicate experiments.

FIG. 2.

Infection course in Stat4-deficient mice. (A) Survival in Stat-4KO mice and BALB/c controls. (B) Parasitemia levels in Stat-4KO mice and BALB/c controls. (C through E) Levels of cytokines and NO in serum after infection with the WA1 Babesia. Filled bars represent uninfected controls; white bars represent infected animals. Data shown are the averages of 3 to 5 individual plasma samples. (C) Levels of IL-12 24 h after infection. (D) Levels of IFN-γ 24 h after infection. (E) Levels of nitrite in serum at day 7 after infection. Data shown are pooled from two replicate experiments.

FIG. 3.

Immune cells involved in protection to acute babesiosis in mice. Intact C57BL/6 controls as well as C57BL/6 mice lacking B cells (B6 μMT mice), CD4⁺ T cells (B6 CD4^−/− mice), macrophages (B6-Mφ-D mice), and NK cells (B6-NK-D mice) were inoculated with the WA1 Babesia. Each experimental group included 10 to 16 mice, pooled from two independent experiments. (A) Survival during the course of the infection. (B) Parasitemia levels during infection.