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 (T PH ) states and a cytotoxic CD4 + T cell subset. We show that the adhesion G-protein coupled receptor 56 (GPR56) delineates synovial CXCL13 high T PH CD4 + T cells expressing LAG-3 and the tissue-resident memory receptors CXCR6 and CD69. In ACPA- SF, T PH cells display lower levels of GPR56 and LAG-3. Further, most expanded T cell clones in the joint are within CXCL13 high T PH CD4 + T cells. Finally, RNA-velocity analyses suggest a common differentiation pathway between the two T PH clusters and effector CD4 + T cells. Our study provides comprehensive immunoprofiling of the synovial CD4 + T cell subsets in ACPA+ and ACPA- RA. Antibodies against citrullinated proteins (ACPA) can divide rheumatoid arthritis patients but the difference in immune phenotype is not clear. Here the authors characterise T peripheral helper cells in arthritic joints comparing ACPA+ and ACPA- patients showing changes in immune cell phenotype and surface molecules.