APCs represented by DCs present the RA-related antigens such as citrullinated peptides to T cells with the major phenotypes as CD4+ PD1+CXCR5-, and they are also called peripheral helper T (Tph) cells that generate IL-21 primarily within the synovium.180 Besides, some CD8+GZMK+ T cells also appear to generate IFN-γ.181 Tph cells can assist B cells to differentiate into plasm cells and generate a large number of antibodies along with IL-6 and GM-CSF to attack the tissue in the synovium. Here, CD4 is linked to rheumatoid arthritis.