The TGF-β response to Leishmania chagasi in the absence of IL-12

Cure of leishmaniasis requires a type 1 immune response characterized by IFN-gamma production. Leishmania major infection leads to a type 2 response suppressing cure of susceptible BALB/c mice, and L. major causes an exacerbated type 2 response in mouse strains with a gene knockout (KO) such that they lack IL-12p40 (IL-12KO mice). In contrast, type 1 responses are inhibited by TGF-beta without Th2 cell expansion in BALB/c mice infected with L. chagasi. We questioned whether the type 2 or the TGF-beta response would dominate during L. chagasi infection of IL-12KO mice. C57BL/6 mice developed self-resolving L. chagasi infection with abundant IFN-gamma. In contrast, L. chagasi disease was exacerbated and IFN-gamma was low in IL-12KO mice. Total TGF-beta was significantly higher in IL-12KO than control C57BL/6 mice, but IL-4 and IL-10 levels were similar. TGF-beta was further augmented in IL-12/IFN-gamma double-KO mice. Thus, in contrast to L. major, the TGF-beta response was exacerbated whereas type 2 cells were not expanded during L. chagasi infection of IL-12KO mice. We conclude that L. chagasi has an inherent propensity to elicit a prominent TGF-beta response that either suppresses, or is suppressed by, a type 1 response. We propose this be termed a "type 3" immune response, which can antagonize a type 1 response.