Enhancement of SIV infection with soluble receptor molecules

The CD4 receptor on human T cells has been shown to play an integral part in the human immunodeficiency virus type 1 (HIV-1) infection process. Recombinant soluble human CD4 (rCD4) was tested for its ability to inhibit SIVagm, an HIV-like virus that naturally infects African green monkeys, in order to define T cell surface receptors critical for SIVagm infection. The rCD4 was found to enhance SIVagm infection of a human T cell line by as much as 18-fold, whereas HIV-1 infection was blocked by rCD4. Induction of syncytium formation and de novo protein synthesis were observed within the first 24 hours after SIVagm infection, whereas this process took 4 to 6 days in the absence of rCD4. This enhancing effect could be inhibited by monoclonal antibodies directed to rCD4. The enhancing effect could be abrogated with antibodies from naturally infected African green monkeys with inhibitory titers of from 1:2,000 to 1:10,000; these antibodies did not neutralize SIVagm infection in the absence of rCD4. Viral enhancement of SIVagm infection by rCD4 may result from the modulation of the viral membrane through gp120-CD4 binding, thus facilitating secondary events involved in viral fusion and penetration.