CD28 and rheumatoid arthritis: In diseases like RA, granulomatous polyangiitis, Sjögren's syndrome, and even in insulin-dependent diabetes mellitus CD4+CD28− T cells were detected, which are able to express NKG2D receptors [70] together with other killer cell immunoregulatory inhibitory receptors (KIR), mediating cellular cytotoxicity [71, 72, 83].