α-defensins inhibit HIV-1 envelope-mediated cell fusion. (A) Dose-dependent inhibition of HIV-1_IIIB (squares), HIV-1_BaL (circles), and HIV-1_MN (triangles) by α-defensins 1 and 2. The fusion assays were performed using as effectors human T cells persistently infected with HIV-1 (PM1 for BaL and MN, SupT1 for IIIB) and infected 18 hours earlier with a recombinant vaccinia vector expressing the lacZ reporter gene under control of the bacteriophage T7 promoter (vCB-21R); the target cells were NIH-3T3 mouse fibroblasts stably expressing human CD4 and either CXCR4 or CCR5 and infected 18 hours earlier with a recombinant vaccinia vector expressing the T7 RNA polymerase (vTF7-3). The fusion reaction was initiated by incubating effector and target cells together for 2 hours at 37°C in the presence or absence of different concentrations of an equimolar mixture of synthetic α-defensins 1 and 2 at the indicated concentrations (total dose). The fusion reaction was conducted in DMEM supplemented with 2.5% FBS (solid symbols) or no FBS (open symbols). (B) Dose-dependent inhibition of HIV-1_MN envelope-mediated fusion by synthetic α-defensin-1 (○), synthetic α-defensin-2 (•), and recombinant α-defensin-1 (□) used individually. The assays were performed as in panel A in the absence of FBS.