Mol. CD4+ T cells. Indeed, it increased HIV uptake in a dose-dependent manner, suggesting functional differences between the specific gp120-targeting CD4-IgG2 agent and nonspecific HIV binding inhibitors. Thus, the inhibition of the specific conversation between gp120 and CD4 protein could be an effective strategy to inhibit HIV binding to CD4+ T cells, and the mechanism by which CD4+ T cells lacking the appropriate coreceptor may be converted in HIV service providers. Human immunodeficiency computer virus type 1 (HIV-1) cell-to-cell transmission implies the polarization of viral production in the infected cell and the viral receptors and coreceptors in the target cell. This polarization prospects to the formation of a functional virological synapse, triggering a rapid and efficient contamination of target cells (19). Moreover, during these cellular contacts, large amounts of HIV particles may be transferred from infected cells to CD4+ T cells in the absence of membrane fusion in a manner that is impartial of coreceptor expression or any later step of HIV computer virus cycle but appears to depend on cellular contacts mediated by the binding of surface (SU; gp120) to CD4. HIV particles taken up by CD4+ T cells may reside in large intracellular vesicles and may be released to extracellular compartments and finally transmitted to a third uninfected cell (9). Productive HIV-1 access into target cells is a process initiated by the binding of the HIV envelope, SU/transmembrane (TM; KIAA0558 gp41), to CD4, triggering conformational changes that enable SU binding to chemokine receptors and, subsequently, TM-mediated membrane fusion. CD4-immunoglobulin G2 (CD4-IgG2; PRO-542) inhibits HIV-1 access, blocking the conversation between the envelope glycoprotein gp120 and CD4 protein. CD4-IgG2 is usually a tetravalent recombinant antibody-like fusion protein wherein the heavy- and light-chain variable domains of human IgG2 were replaced with the V1 and V2 domains of human CD4 (1, 31). CD4-IgG2 binds the HIV-1 SU with nanomolar affinity, inhibits syncytium formation, and has a 90% reduction in computer virus infectivity (90% inhibitory concentration) at 20 g/ml (1, 18). Phase I and phase II clinical studies show antiviral activity of CD4-IgG2 in HIV-1-infected adults, especially in advanced disease (17, 18). Taking into account that binding of SU to CD4 is a necessary step to trigger HIV-1 coreceptor-independent cell-to-cell HIV-1 transmission and that CD4-IgG2 inhibits SU-CD4 7CKA binding, we have analyzed the effect of CD4-IgG2 in coreceptor-independent transmission of HIV. Our results demonstrate that CD4-IgG2 inhibits HIV coreceptor-independent transmission in a dose-dependent manner, with a 50% effective dose (EC50) comparable to its anti-HIV activity. MATERIALS AND METHODS Cells. Peripheral blood mononuclear cells (PBMC) from healthy donors were purified by Ficoll-Hypaque sedimentation. CD4+ T cells were immediately purified ( 95%) from PBMC by unfavorable selection using the CD4+ T cell enrichment kit (StemCell Technologies, Vancouver, Canada). Main cells were used without previous stimulation. Media were supplemented with 10% heat-inactivated fetal calf serum (Invitrogen, Madrid, Spain), 100 U/ml penicillin, 100 g/ml streptomycin. HIV-1 chronically infected MOLT-4/CCR5 cells were generated in the laboratory after contamination of MOLT-4/CCR5 cells with the following viruses: recombinant viruses transporting the HIV-1 envelope (Env) sequences corresponding to the X4 HIV-1 strain NL4-3 or the R5 HIV-1 strain BaL constructed in an HIVHXB2 backbone as explained previously (6); the primary isolate 168.1 (13, 15); and a C34-resistant and T-20-cross-resistant HIV-1 NL4-3 strain (2), NT38, generated by sequential passages of the NL4-3 strain in vitro in MT-4 cells in the presence of increasing 7CKA concentrations of T-20 (3). After contamination peak, persistently infected cultures of MOLT-NL4-3, MOLT-BaL, MOLT-168.1, and MOLT-NT38 cells were grown 7CKA and characterized for Env expression and virus production (7). Uninfected MOLT-4/CCR5 (MOLT-uninfected) cells were used as negative controls in all experiments. Cocultures of infected.