SLC29A3 supports phagosomal antibacterial signaling, and promotes autophagy by activating TRPML1 in mouse dendritic cells

Solute carrier (SLC)29A3 exports nucleosides from lysosomes into the cytosol, maintaining solute homeostasis and providing metabolic intermediates for cellular processes. Loss-of-function mutations in SLC29A3 cause H syndrome, characterized by histiocytosis, hyperinflammation and immunodeficiency. While dysfunctions in several cell types contribute to H syndrome and to SLC29A3 deficiency in mice, the mechanisms driving hyperinflammation and immunodeficiency are incompletely understood. Notably, the possible role played by dendritic cells (DCs), the most efficient antigen presenting cells and the main link between innate and adaptive immune responses, remains unknown. We show that, in murine DCs, SLC29A3 is recruited to phagosomes after bacterial capture, maintains phagosomal pH homeostasis, and ensures optimal phagosomal signaling to the production of IL-6, IL-12, and CCL-22. In addition, SLC29A3 promotes Ag presentation on MHC-II molecules to initiate adaptive immune responses. Notably, SLC29A3 supports the activity of the lysosomal calcium channel TRPML1, promoting transcription factor TFEB nuclear translocation and inducing autophagy, a major anti-inflammatory mechanism. Overexpression of human SLC29A3, but not the transport mutant G437R, in SLC29A3-deficient murine DCs restores cytokine production in response to bacteria phagocytosis, suggesting that SLC29A3 transport activity is required to drive anti-bacterial phagosomal signaling. Our data suggest that SLC29A3 plays a dual role in supporting immune function by promoting effective anti-microbial signaling and Ag presentation in DCs and inducing autophagy to control inflammation. Our findings also uncover a novel TRPML1-dependent mechanism by which SLC29A3 activates TFEB and suggest that defects in phagosomal signaling, TFEB activation and autophagy may contribute to immunodeficiency and hyperinflammation in SLC29A3 disorders.