CD4 and infection: HIV luciferase reporter viruses were incubated with a range of concentrations of human monoclonal Abs (mAbs), soluble CD4 (sCD4; Immunodiagnostics, Inc., Woburn, MA), or a broadly neutralizing HIV-1 serum (HIV-1 neutralizing serum (serum 2; [25]) obtained from L. Vujcic through the AIDS Research and Reference Reagent Program) 1 h prior to infection of Cf2 cells transiently expressing CD4 and CCR5.