CD4 and HIV-1 infection: During very early untreated primary HIV-1 infection (PHI; <22 days after the onset of symptoms), Zaunders et al. detected a 10- to 20-fold increase in the proportion of bulk CD4 T cells that were highly activated (CD38+++), proliferating (Ki-67+), and expressed the HIV-1 co-receptor CCR5 as well as GMP-17/TIA-1, perforin, and GzmB (117).