Inflammatory Dendritic Cells, Regulated by IL-4 Receptor Alpha Signaling, Control Replication, and Dissemination of Leishmania major in Mice

Leishmaniasis is a vector-borne disease caused by Leishmania parasites. Macrophages are considered the primary parasite host cell, but dendritic cells (DCs) play a critical role in initiating adaptive immunity and controlling Leishmania infection. Accordingly, our previous study in CD11c ^creIL-4Rα ^−/lox mice, which have impaired IL-4 receptor alpha (IL-4Rα) expression on CD11c ⁺ cells including DCs, confirmed a protective role for IL-4/IL-13-responsive DCs in replication and dissemination of parasites during cutaneous leishmaniasis. However, it was unclear which DC subset/s was executing this function. To investigate this, we infected CD11c ^creIL-4Rα ^−/lox and control mice with L. major GFP ⁺ parasites and identified subsets of infected DCs by flow cytometry. Three days after infection, CD11b ⁺ DCs and CD103 ⁺ DCs were the main infected DC subsets in the footpad and draining lymph node, respectively and by 4 weeks post-infection, Ly6C ⁺ and Ly6C ⁻ CD11b ⁺ DCs were the main infected DC populations in both the lymph nodes and footpads. Interestingly, Ly6C ⁺CD11b ⁺ inflammatory monocyte-derived DCs but not Ly6C ⁻CD11b ⁺ DCs hosted parasites in the spleen. Importantly, intracellular parasitism was significantly higher in IL-4Rα-deficient DCs. In terms of DC effector function, we found no change in the expression of pattern-recognition receptors (TLR4 and TLR9) nor in expression of the co-stimulatory marker, CD80, but MHCII expression was lower in CD11c ^creIL-4Rα ^−/lox mice at later time-points compared to the controls. Interestingly, in CD11c ^creIL-4Rα ^−/lox mice, which have reduced Th1 responses, CD11b ⁺ DCs had impaired iNOS production, suggesting that DC IL-4Rα expression and NO production is important for controlling parasite numbers and preventing dissemination. Expression of the alternative activation marker arginase was unchanged in CD11b ⁺ DCs in CD11 ^creIL-4Rα ^−/lox mice compared to littermate controls, but RELM-α was upregulated, suggesting IL-4Rα-independent alternative activation. In summary, L. major parasites may use Ly6C ⁺CD11b ⁺ inflammatory DCs derived from monocytes recruited to infection as “Trojan horses” to migrate to secondary lymphoid organs and peripheral sites, and DC IL-4Rα expression is important for controlling infection.