CD4 and rheumatoid arthritis: Healthy donor CD4+CD25− T cells cross-linked with αCD3/αPD-L1 showed a significant reduction in S6 phosphorylation compared to αCD3 and αCD3/αCD28 (Fig 6C), whereas in RA patients, cross-linking with αCD3/αPD-L1 resulted in similar levels of S6 phosphorylation to those seen in response to αCD3 stimulation alone (Fig 6D).