major infection and have a strong Th2 response (3). We were surprised to find that L. mexicana-infected B6 IL-12p40 KO mice had no change in their chronic, but CHIR-99021 datasheet nonprogressive disease picture (1). Lesion progression, parasite burdens, as well as IFN-γ and IL-4 responses were indistinguishable from infected B6 mice (1). It appears that the IL-12 pathway is suppressed by IL-10 in L. mexicana infection as blockade of IL-12 in vivo does prevent healing in IL-10 KO mice and suppresses the IFN-γ response, which would otherwise

resolve L. mexicana lesions (4). This leaves us with a pathway by which IL-12, and the related cytokine IL-23 (which shares the IL-12p40 subunit) are not required for the partial control of L. mexicana infection, but STAT4, known primarily for its role in the IL-12 signalling pathway, is absolutely required.

Thus, there is an IL-12-independent, but STAT4-dependent IFN-γ pathway responsible for preventing progressive disease in L. mexicana infection. We decided to investigate the role of type I IFNs in L. mexicana infection because there is evidence that IFN-α and β can signal through STAT4. Type I IFNs (IFN-α and β) play an important role in viral infections such as vesicular stomatitis virus, Semliki forest virus Bortezomib nmr and vaccinia virus (5). IFN-α/βR signalling was shown to phosphorylate STAT4 directly and lead to IFN-γ in lymphocytic choriomeningitis acetylcholine virus infection (6). Type I IFNs are also important in Gram-negative bacterial infections through a STAT4 pathway, with IFN-α/β inducing IL-12-independent STAT4 phosphorylation in mouse splenocytes from several mouse strains (7). Plasmacytoid DCs, but not myeloid DCs or macrophages, make IFN-α/β in response to various Leishmania species (L. major, L. braziliensis, and L. infantum) (8). L. major can inhibit the release of IFN-α/β from myeloid DCs and macrophages induced by poly I:C

(9), perhaps explaining why these cells do not secrete type I IFNs to the extent that plasmacytoid DCs do. In vivo, a congenic strain of mice that was a low producer of type I IFNs had more severe L. major disease than the WT mice, but healed nonetheless, demonstrating an early protective role of type I IFNs, albeit a nonessential one, in resistance to L. major (10). In those same studies, it was found that IFN-α was able to synergize with low levels of lipopolysaccharide to induce nitric oxide and enhance leishmanial killing by macrophages. Blockade of IFN-α/βin vivo in 129/B6 mice decreased NK cell cytotoxicity and IFN-γ early in L. major infection, perhaps explaining this early role of IFN-α/β (11). Also, exogenous IFN-β was able to protect highly susceptible BALB/c mice from L. major infection and induced increased phosphorylation (activation) of STAT4. IFN-γ enhancement was also shown to be STAT4-dependent (12).