The lysosomal carrier SLC29A3 supports antibacterial signaling, and promotes autophagy by activating TRPML1 in murine dendritic cells

The 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 various cell types contribute to H syndrome and to SLC29A3 deficiency in mice, the mechanisms driving hyperinflammation and immunodeficiency are incompletely understood. Remarkably, the possible role played by dendritic cells (DCs), the most efficient antigen (Ag)-presenting cells and the main cellular link between innate and adaptive immunity, remains unknown. We show that, in murine DCs, SLC29A3 is recruited to phagosomes after bacterial capture, maintains phagosomal pH homeostasis, and ensures optimal antimicrobial phagosomal signaling to the production of IL-6, IL-12, pro-IL-1β, and CCL22. 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 the nuclear translocation of transcription factor TFEB 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 bacterial phagocytosis, suggesting that SLC29A3 transport activity is required to drive phagosomal signaling. Our data suggest that SLC29A3 supports and controls immune function in DCs by promoting effective antimicrobial signaling and Ag presentation, and inducing autophagy. Our findings also uncover a TRPML1-dependent mechanism by which SLC29A3 activates TFEB and suggest that defects in phagosomal antibacterial signaling, TFEB activation, and autophagy may contribute to immunodeficiency and hyperinflammation in SLC29A3 disorders.