CD4 and rheumatoid arthritis: Instead, their co-receptor expression pattern with higher CD4 than CD8 expression, and the fact that CD4hiCD8lo are the most prominent DP T cell subpopulation, as well as the generally presumed pivotal role of CD4+ MHC class II restricted T lymphocytes in RA, suggests that those cells were originally CD4 SP T cells, which gained expression of the second co-receptor CD8.