CD28 and rheumatoid arthritis: Finally, the expression levels of CXCR2 were decreased in cultures with abatacept compared with mock cultures (P = 0.001 and 0.08 for RA and controls, respectively), whereas the expression of CXCR2 was also decreased in cultures with CD28-Ig compared with mock cultures (P < 0.001 and P = 0.03 for RA and controls, respectively), indicating that the downregulation of CXCR2 was not specific to exposure to abatacept.