SLC15A4 is a lysosome-resident, proton-coupled amino-acid transporter that moves histidine and oligopeptides from inside the lysosome to the cytosol of eukaryotic cells. SLC15A4 is required for Toll-like receptor 7 (TLR7)- and TLR9-mediated type I interferon (IFN-I) productions in plasmacytoid dendritic cells (pDCs) and is involved in the pathogenesis of certain diseases including lupus-like autoimmunity. How SLC15A4 contributes to diseases is largely unknown. Here we have shown that B cell SLC15A4 was crucial for TLR7-triggered IFN-I and autoantibody productions in a mouse lupus model. SLC15A4 loss disturbed the endolysosomal pH regulation and probably the v-ATPase integrity, and these changes were associated with disruption of the mTOR pathway, leading to failure of the IFN regulatory factor 7 (IRF7)-IFN-I regulatory circuit. Importantly, SLC15A4's transporter activity was necessary for the TLR-triggered cytokine production. Our findings revealed that SLC15A4-mediated optimization of the endolysosomal state is integral to a TLR7-triggered, mTOR-dependent IRF7-IFN-I circuit that leads to autoantibody production.

SLC15A4 is an endolysosome-resident amino acid transporter that regulates innate immune responses, and is genetically associated with inflammatory diseases such as systemic lupus erythematosus (SLE) and colitis. SLC15A4-deficient mice showed the amelioration of symptoms of these model diseases, and thus SLC15A4 is a promising therapeutic target of SLE and colitis. For developing SLC15A4-based therapeutic strategy, understanding human SLC15A4’s property is essential. Here we characterized human SLC15A4 and demonstrated that human SLC15A4 possessed pH- and temperature-dependent activity for the transportation of dipeptide or tripeptide. Human SLC15A4 localized in LAMP1 + compartments and constitutively associated with Raptor and LAMTORs. We also investigated SLC15A4’s role in inflammatory responses using human plasmacytoid dendritic cell line, CAL-1. Knock-down (KD) of SLC15A4 gene in CAL-1 (SLC15A4-KD CAL1) impaired TLR7/8 or TLR9-triggered type I interferon (IFN-I) production and mTORC1 activity, indicating that human SLC15A4 is critical for TLR7/8/9-mediated inflammatory signaling. We also examined SLC15A4’s role in autophagy response since SLC15A4 loss caused the decrease of mTORC1 activity, which greatly influences on autophagy. We found that SLC15A4 was not required for autophagy induction, but was critical for autophagy sustainability. Notably, SLC15A4-KD CAL1 severely decreased mitochondria membrane potential in the starvation condition. Our findings revealed that SLC15A4 plays a key role in mitochondria integrity in human cells, which might benefit immune cells to fulfill their functions in inflammatory milieu.

SLC15A4 regulates secretory-granule biogenesis in mast-cell lysosomes

Macrophages play critical roles in the onset of various diseases and in maintaining homeostasis. There are several functional subsets, of which M1 and M2 macrophages are of particular interest because they are differentially involved in inflammation and its resolution. Here, we investigated the differences in regulatory mechanisms between M1- and M2-polarized macrophages by examining mRNA metabolic machineries such as stress granules (SGs) and processing bodies (P-bodies). Human monocytic leukemia THP-1 cells cultured under M1-polarizing conditions (M1-THPs) had less ability to assemble oxidative-stress-induced SGs than those cultured under M2-polarizing conditions (M2-THPs). In contrast, P-body assembly in response to oxidative stress or TLR4 stimulation was increased in M1-THPs as compared to M2-THPs. These results suggest that mRNA metabolism is controlled differently in M1-THPs and M2-THPs. Interestingly, knocking down EDC4 or Dcp1a, which are components of P-bodies, severely reduced the production of IL-6, but not TNF-α in M1-THPs without decreasing the amount of IL-6 mRNA. This is the first report to demonstrate that the assembly of EDC4 and Dcp1a into P-bodies is critical in the posttranscriptional regulation of IL-6. Thus, improving our understanding of the mechanisms governing mRNA metabolism by examining macrophage subtypes may lead to new therapeutic targets.