Infection of immunodeficient mice with a mouse-adapted substrain of the gray strain of Babesia microti.

A mouse-adapted substrain was recovered from a severe combined immunodeficient (SCID) mouse inoculated with the Gray strain of Babesia microti. In contrast to the original Gray strain, infection with this substrain (Gray/mo strain) resulted in a high level of parasitemia for a long period in SCID and nude mice, whereas immunocompetent BALB/c mice controlled the infection. The infection profile with the Gray/mo strain was closer to that of human babesiosis than that with the original Gray strain, suggesting usefulness of this mouse model for immunological studies of babesial infection. No antibody activity was revealed in sera of the infected SCID and nude mice. Although strong antibody activity was shown in the sera of the recovered BALB/c mice, no protective activity was demonstrated by the immune serum. Adaptive transfer of thymocytes from the normal BALB/c mouse resulted in SCID and nude mice being able to control the infection, indicating that cell-mediated immunity played a major role in control of infection with B. microti. However, because the reconstituted mice also produced significant antibody titer, antibody remains a possibility for a supportive role in control of the infection.