Leishmania amazonensis infection induces PD-L1 expression on dendritic cells in an mTor-dependent manner and impairs Th1 responses in vitro and in vivo

Leishmania amazonensis is one of the etiological agents of diffuse cutaneous leishmaniasis in South America. In murine models of this infection, dysregulated expansion of effector T cells or exhaustion of Th1 responses are known to be related to pathogenesis, while the induction of regulatory T cells (Tregs) promotes lesion resolution. Recent research has identified several important co-stimulator/receptor pairs, including PD-1/PD-L1, for modulating Th1 responses and Treg induction. In this study, we examined the roles of these molecules in L. amazonensis-infected C57BL/6 mice. We found a significant and selective increase in the expression of PD-1 and PD-L1 (20-fold and 5-fold, respectively), in infected footpad tissues, which correlated with an increased percentage of PD-L1+CD11c+ dendritic cells (DCs) and PD-1+CD4+ T cells in the draining lymph nodes. To evaluate the mechanism of parasite-induced PD-L1 expression, we infected bone marrow-derived DCs (BMDCs) with promastigotes and amastigotes in vitro in the presence of small molecular inhibitors of critical signaling pathways. While L. amazonensis infection decreased PD-L2 expression on the surface of BMDCs, infection-mediated PD-L1 up-regulation was consistently detected, which was dependent on mTOR and partially dependent on STAT3, PI3K, and MAPK. Infected BMDCs also significantly inhibited the expansion of Th1 cells, but were more competent in inducing CD25+FoxP3+ Tregs in vitro than the non-infected BMDCs; such effects were PD-L1-dependent, as BMDCs from PD-L1-/- mice failed to do the same. In vivo experiments revealed that infected PD-L1-/- mouse tissues showed increased Th1 responses and IFN-{gamma} production, as well as reduced lesion sizes and parasite loads, without affecting Tregs cell expansion. Together, these results support a protective role for PD-1/PD-L1 signaling in regulating local immune responses during L. amazonensis infection. This study provides new insights on immune regulation in New World cutaneous leishmaniasis.