TLR7/8 stress response drives histiocytosis in SLC29A3 disorders

SLC29A3, also known as ENT3, is a lysosomal transmembrane protein that transports nucleosides from the lysosomes to the cytoplasm1. Loss-of-function mutations inSLC29A3cause lysosomal nucleoside storage and histiocytosis: phagocyte accumulation in multiple organs2,3. However, little is known about the mechanism through which lysosomal nucleoside storage drives histiocytosis. Herein, histiocytosis inSlc29a3−/−mice was demonstrated to depend on TLR7, which senses a combination of nucleosides and oligoribonucleotides4,5. TLR7 responded to lysosomal nucleoside storage and enhanced proliferation of Ly6ChiCX3CR1lowimmature monocytes and their maturation into Ly6Clowphagocytes inSlc29a3−/−mice. Because accumulated nucleosides primarily originated from cell corpse phagocytosis, TLR7 in immature monocytes recognized nucleoside storage as lysosomal stress and increased phagocyte numbers. This non-inflammatory compensatory response is referred to as the TLR7 stress response where Syk, GSK3β, β-catenin, and mTORC1 serve as downstream signalling molecules. In SLC29A3 disorders, histiocytosis accompanies inflammation6,7. Nucleoside storage failed to induce pro-inflammatory cytokine production inSlc29a3−/−mice, but enhanced ssRNA-dependent pro-inflammatory cytokine production in Ly6Chiclassical monocytes and peripheral macrophages, not proliferating immature monocytes. Patient-derived monocytes harbouring G208RSLC29A3mutation showed higher survival and proliferation in the presence of M-CSF and produced larger amounts of IL-6 upon ssRNA stimulation than did those derived from healthy subjects. A TLR8 antagonist inhibited the survival/proliferation of patient-derived macrophages. These results demonstrated that TLR7/8 responses to lysosomal nucleoside stress drive SLC29A3 disorders.