CD28 and rheumatoid arthritis: Furthermore, DP T cells were found to preferentially have lost CD28 expression (mean%: 19,4, n = 13) in ACPA positive RA patients when compared to CD4 SP (mean%: 1.97) T cells, while the frequency of CD28 negative cells was even higher in CD8 SP (mean%: 33) T cells (figure 6B).