CD4 and HIV infectious disease: Latent HIV infection is enriched in memory CD4+ T cells that express multiple immune checkpoint (IC) molecules, including programmed cell death protein 1 (PD-1), programmed cell death protein ligand 1 (PD-L1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), T cell immunoreceptor with Ig and ITIM domains, and T cell immunoglobulin domain, and mucin domain-3 (138–140).