CD4 and rheumatoid arthritis: The antigen presentation was reflected by numerous HLA genes including the strongest known RA-risk gene HLA-DRB1 as well as β2-microglobulin (B2M), ATP-binding cassette transporter TAP1, and invariant chain CD74/CLIP responsible for the unidirectional translocation of antigen peptides and loading into the HLA groove for presentation to CD4+T cells (Figure 3I).