CD4 and HIV-1 infection: To explore this, resting primary CD4+ T cells were treated with IL-7 to make them permissive to HIV-1 infection without T cell activation (33), infected with viruses that cannot down-regulate CD3 (NL4.3, L4 LF, and K2 LF), and subsequently stimulated with anti-CD3/CD28 antibodies to induce TCR triggering and activation (Fig. 5A and SI Appendix, Fig. S9).