CD4 and HIV-1 infection: To further examine the impact of donor age on the susceptibility of CD4+ T cells to HIV-1 infection and replication, we used X4 and R5 HIV-1 NL4-3 reporter viruses co-expressing the enhanced version of the green fluorescent protein (eGFP) and Nef via an internal ribosome entry site (IRES) [25, 26].