CD4 and infection: Indeed, infection of CD4+ T cells purified from PBMCs with VSV-G pseudotyped NL4.3 demonstrated that cells infected with virus lacking either (dNef, dVpu) or both (dVpu dNef) viral proteins had significantly higher mean levels of CD28, compared to cells infected with virus encoding both Nef and Vpu (NL4.3) (Fig. 2d, e).