Significance A large body of evidence has indicated that recognition of self-nucleic acids by endosomal toll-like receptors (TLRs) is central to the pathogenesis of lupus-like systemic autoimmunity in spontaneous mouse models, and the solute carrier SLC15A4 is required for this recognition. Here we describe a mechanism in which SLC15A4 is a major contributor to the proper trafficking of TLRs and their ligands to endolysosomes, wherein recognition and signaling is initiated. This finding supports ongoing efforts to identify pharmacologic inhibitors for this carrier as a means to treat lupus and other inflammatory disorders.