CD4 and HIV-1 infection: IL-21 inhibited both R5- and X4-tropic HIV-1 infection (Fig. 1 and Supplementary Fig. 2a), and did not alter surface expression of the HIV entry coreceptors CD4, CCR5 or CXCR4 on CD4 T cells (Supplementary Fig. 4a), suggesting that the antiviral mechanism of IL-21 likely involved post-entry events.