CXCR4 and rheumatoid arthritis: When CXCR4 expression on CD4+ T cell subsets was combined with conventional immunophenotyping, we were able to detect a significant difference between HD and SE + RA by PCA, indicating that CXCR4 expression on CD4+ T cells is a key feature discriminating HD and SE + RA (Figs 1D and 2D).