Single cell sequencing of MHC-II positive and negative fibroblastic reticular cells

Autoimmunity can be initiated by autoreactive T cells, that escaped central and peripheral tolerance induction. Peripheral tolerance in lymph nodes (LNs) is maintained by fibroblastic reticular cells (FRCs) via self-antigen presentation in major histocompatibility complex (MHC) class-II. FRCs can be divided in various subsets, with specific markers, functions and locations. FRCs located in the T-cell zone (TRCs) can express genes for antigen presentation in MHC class-II, e.g. H2-Ab1 and Cd74, as well as the immune inhibitory ligand Cd200. However, whether this can be linked to MHC class-II protein expression and thus tolerance is unknown. By combining scRNAseq on murine FRCs with protein staining for extracellular MHC class-II, we confirm that murine TRCs have the highest MHC class-II transcript levels, while protein levels are elevated in multiple FRC subsets. Gene expression for MHC class-II, as well as Bst1 and Cd200, gradually increase along the pseudotime trajectory with TRCs representing the end, indicating maturation. Finally, we validated in fresh LN cell suspensions that MHC class-II protein expression is associated with murine BST1+ FRCs, independent of CD200, and human BST1+CD200+ TRCs. The identification of this mature FRC subset that is equipped to maintain peripheral tolerance, could be an interesting target for autoimmune therapies.