The pro-inflammatory responses of innate immune cells to Leishmania RNA virus 2-infected L. major support the survival and proliferation of the parasites

Infection of Leishmania by Leishmania RNA virus (LRV) has been proposed as a pathogenic factor that induces pro-inflammatory responses through the TLR3/TLR4 signaling pathway. We investigated the effect of L. major infection by LRV2 on innate immune cell responses (human neutrophil (HL-60) and macrophage (THP-1) cell lines). The expression levels of pro- and anti-inflammatory cytokine and chemokine genes as well as genes involved in the amino acid metabolism of arginine were then investigated by RT-qPCR. Moreover, the expression of TLR genes and their downstream signaling pathways were compared in THP-1 cells infected with the two isolates. Apoptosis was also evaluated in infected THP-1 and HL-60 cells using the PI/Annexin V flow cytometry assay. In both cell lines, the expression of pro-inflammatory cytokines increased in response to LRV2+ L. major (Lm+), and the expression of chemokines shifted toward macrophage recruitment. In contrast to LRV2- L. major (Lm-), Lm + infected THP-1 cells acquired the M2-like phenotype. The presence of LRV2 increased the gene expression of TLRs and their signaling pathways, especially TLR3 and TLR4, which was proportional to the increase in pro-inflammatory cytokines. In addition, Lm + increased the expression of IL-10 and IFN-β, which contribute to the survival and growth of the parasite in the phagolysosome. Altogether, our results showed that Lm + could stimulate pro-inflammatory responses that promote parasite replication and stabilization in the host.