As observed in our healthy donors, the proportion of ICOS+ CD4+ memory cells varied directly with the proportion of CD4+ T cells that were CXCR5-PD-1high Tph, an effect evident in both RA patients (Pearson ρ = 0.64, P = 0.00040) and controls (Pearson ρ = 0.60, P = 0.0089) (Fig. 4C, Supplementary Fig. 8B, Supplementary Fig. 9). Here, ICOS is linked to rheumatoid arthritis.