Macrophages Are Critical for Cross-Protective Immunity Conferred by Babesia microti against Babesia rodhaini Infection in Mice

<jats:title>ABSTRACT</jats:title><jats:p>Although primary infection of mice with<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">Babesia microti</jats:named-content>has been shown to protect mice against subsequent lethal infection by<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">Babesia rodhaini</jats:named-content>, the mechanism behind the cross-protection is unknown. To unravel this mechanism, we investigated the influence of primary infection of mice with nonlethal<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. microti</jats:named-content>using different time courses on the outcome of subsequent lethal<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. rodhaini</jats:named-content>infection. Simultaneous infections of mice with these parasites resulted in rapid increases in parasitemia, with 100% mortality in BALB/c mice, as observed with control mice infected with<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. rodhaini</jats:named-content>alone. In contrast, mice with acute, resolving, and chronic-phase<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. microti</jats:named-content>infections were completely protected against<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. rodhaini</jats:named-content>, resulting in low parasitemia and no mortalities. Mice immunized with dead<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. microti</jats:named-content>were not protected from<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. rodhaini</jats:named-content>infection, although high antibody responses were induced. Interestingly, the protected mice had significantly decreased levels of antibody response, cytokines (including gamma interferon [IFN-γ], interleukin-2 [IL-2], IL-8, IL-10, and IL-12), and nitric oxide levels after infection with<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. rodhaini</jats:named-content>. SCID mice and IFN-γ-deficient mice with chronic<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. microti</jats:named-content>infections demonstrated protective responses comparable to those of immunocompetent mice. Likewise,<jats:italic>in vivo</jats:italic>NK cell depletion did not significantly impair the protective responses. Conversely, macrophage depletion resulted in increased susceptibility to<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">B. rodhaini</jats:named-content>infection associated with changes in their antibody and cytokines profiles, indicating that macrophages contribute to the protection against this challenge infection. We conclude that future development of vaccines against<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">Babesia</jats:named-content>should include a strategy that enhances the appropriate activation of macrophages.</jats:p>