PDCD1 and HIV-1 infection: (2016) found that during untreated HIV-1 infection, T-cells expressing HIV-1 RNA and Gag protein were enriched particularly in T-cells expressing PD-1 and Tfh cell markers (PD-1+CXCR5+), with additional enrichment in cells co-expressing PD-1 with other immune checkpoint markers (TIGIT+ and CTLA-4+) (28).