Characterization of the effects of Interferon gamma protein-10 (CXCL10) against the protozoan parasite Leishmania amazonesis

Leishmania amazonensis causes progressive disease in most inbred strains of mice. We have previously shown that L. amazonensis-infected C57BL/6 mice have profound impairments in expression of pro-inflammatory cytokines, chemokines and in activation of antigen-specific CD4+ T cells. These impairments are independent of IL-4. The precise mechanism of pathogenesis associated with L. amazonensis infection remains largely unresolved. Since chemokines are essential mediators of leukocyte recruitment and effector cell function, we hypothesized that these molecules are important for the initiation of early responses locally and the eventual control of the infection. In this study, we found that CXCL10-treated bone marrow-derived macrophages from both BALB/c and C57BL/6 mice showed decreased numbers of L. amazonensis parasites, which was partially due to increased nitric oxide production, as well as elevated production of pro-inflammatory chemokines. When susceptible C57BL/6 mice were locally injected with CXCL10 following L. amazonensis infection, there was a significant delay in lesion development and reduction in parasite burdens, accompanied by a 7- and 3.5-fold increase in IFN-γ and IL-12 secretion, respectively, in re-stimulated lymph node cells. This study confirms that CXCL10 assists in the reduction of intracellular parasites. To address the mechanism underlying this enhanced immunity we utilized stationary promastigotes to infect bone marrow-derived DCs of C57BL/6 mice and assessed the activation of DC subsets and the capacity of these DC subsets in priming CD4+ T cells in vitro. We found that CXCL10 induced IL-12p40, but reduced IL-10 production in DCs. Yet, L. amazonensis-infected DCs produced elevated levels of IL-10, despite CXCL10 treatment. Elimination of endogenous IL-10 led to increased responsiveness to CXCL10 treatment, as judged by increased IL-12 production in DCs, as well as increased proliferation and IFN-γ production by CD4+ T cells. In addition, CXCL10-treated CD4+ T cells became more responsive to IL-12 via increased expression of the IL-12Rβ2 chain and produced elevated IFN-γ. This study indicates the interplay between CXCL10 and IL-10 in the generation of Th1-favored, pro-inflammatory responses and further highlights the utility of CXCL10 as a potential therapeutic for the control of non-healing cutaneous leishmaniasis.