CCR5 and HIV-1 infection: Memory CD4 T cells are major targets of HIV-1 infection, and early depletion of memory CD4 T cells in the gastrointestinal tract by the CCR5 (R5)-utilizing T-tropic virus is a major cause of chronic immune activation in patients (Brenchley et al., 2004, 2006; Veazey et al., 1998).