Dual interaction of α-defensins with the HIV-1 envelope and the cellular receptors. α-defensins 1 and 2 were preincubated for 20 minutes either with effector cells (expressing the HIV-1 envelope) or with the target cells (expressing the CD4 and CXCR4 receptors) in the absence of serum. An equimolar mixture of α-defensins 1 and 2 was used (total concentration, 11.7 μM). The cells were washed twice prior to the initiation of the fusion reaction, and residual levels of α-defensins were determined by ELISA to be below the limit of detection of the assay. As a control, α-defensins were regularly added at the time of cell mixing and kept in the wells for the entire duration of the test. The fusion assay was performed using as effectors SupT1 cells chronically infected with HIV-1_IIIB and as targets NIH-3T3 cells stably expressing human CD4 and CXCR4. Error bars indicate SD of mean values obtained from 3 replicate experiments.