Single cell sequencing identifies clonally expanded synovial CD4+ TPH cells expressing GPR56 in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease affecting synovial joints where different CD4+ T cell subsets may contribute to pathology. Here, we perform single cell sequencing on synovial CD4+ T cells from anti-citrullinated protein antibodies (ACPA)+ and ACPA- RA patients and identify two peripheral helper T cell (TPH) states and a cytotoxic CD4+ T cell subset. We show that the adhesion G-protein coupled receptor 56 (GPR56) delineates synovial CXCL13high TPH CD4+ T cells expressing LAG-3 and the tissue-resident memory receptors CXCR6 and CD69. In ACPA- SF, TPH cells display lower levels of GPR56 and LAG-3. Further, most expanded T cell clones in the joint are within CXCL13high TPH CD4+ T cells. Finally, RNA-velocity analyses suggest a common differentiation pathway between the two TPH clusters and effector CD4+ T cells. Our study provides comprehensive immunoprofiling of the synovial CD4+ T cell subsets in ACPA+ and ACPA- RA.