DC2 (CD1c+; Supplementary Figure S11)—the most abundant dendritic cell subset and considered to be the most potent APC (12)—had higher MFIs of the co-stimulatory molecules CD86 (Figure 1G, p = 0.0184) and CD275 (ICOS-L; Figure 1H, p = 0.0046), consistent with an increased capacity of RA DC2 to provide co-stimulation to T cells. Here, ICOSLG is linked to rheumatoid arthritis.