PD-1 expression within memory and activated memory subsets. PBMC samples from a subset of HIV+ and HIV- donors were assessed for PD-1 expression by flow cytometry. PD-1 expression in total memory (CD45RA-) and activated (CD38+HLA-DR+) CD4+ T cells in HIV+ donors (open symbols), and HIV- donors (filled symbols) are depicted. Box and whisker plots (in grey) extend from the 5th to the 95th percentile, and the black line depicts the mean. Unpaired 2-tailed t-tests were conducted to compare PD-1 levels between HIV+ and HIV- populations, and Mann–Whitney tests used to compare levels within HIV+ or HIV- populations.