Cellular Immunity, but Not Gamma Interferon, Is Essential for Resolution of Babesia microti Infection in BALB/c Mice

A new strain of Babesia microti (KR-1) was isolated from a Connecticut resident with babesiosis by hamster inoculation and adapted to C3H/HeJ and BALB/c mice. To examine the relative importance of humoral and cellular immunity for the control of B. microti infection, we compared the course of disease in wild-type BALB/c mice with that in BALB/c SCID mice, JHD-null (B-cell-deficient) mice, and T-cell receptor αβ (TCRβ(−/−)) or gamma interferon (IFN-γ) (IFN-γ(−/−)) knockout mice following inoculation with the KR-1-strain. SCID mice and TCRαβ knockouts sustained a severe but nonlethal parasitemia averaging 35 to 45% infected erythrocytes. IFN-γ-deficient mice developed a less severe parasitemia but were able to clear the infection. In contrast, in six of eight JHD-null mice, the levels of parasitemia were indistinguishable from those in the wild-type animals. These data indicate that cellular immunity is critical for the clearance of B. microti in BALB/c mice but that disease resolution can occur even in the absence of IFN-γ.