Role of TLR9 in CD55lo FRCs. Mus musculus

Fibroblastic reticular cells (FRCs) are a subpopulation of stromal cells modulating the immune environments in health and diseases. We have previously shown that activation of TLR9 signaling in FRC of the fat-associated lymphoid cluster (FALC) regulates peritoneal immunity via suppressing immune cell recruitment and peritoneal resident macrophage (PRM) retention. However, FRCs are heterogeneous across tissues and organs. The functions of each FRC subset and the regulation of TLR9 in distinct FRC subsets remain elusive. Here, we confirmed that specific deletion of TLR9 in FRC improved bacterial clearance and survival during peritoneal infection. Furthermore, using single-cell RNA sequencing, we found two subsets of FRCs (CD55hi and CD55lo) in the mesenteric FALC. The CD55hi FRCs were enriched in gene expression related to extracellular matrix formation. The CD55lo FRCs were enriched in gene expression related to immune response. Interestingly, we found that TLR9 is dominantly expressed in the CD55lo subset. Activation of TLR9 signaling suppressed proliferation, cytokine production, and retinoid metabolism in the CD55lo FRC. Furthermore, we identify the CD55hi and CD55lo subsets and confirm the suppressive effect of TLR9 on the proliferation and cytokine production in the CD55lo subsets of human adipose tissue-derived FRC. The regulation of TLR9 in the CD55lo FRCs may be utilized to standardize and improve the therapeutic efficacy of FRC-based therapy for peritonitis.