Abstract
Human CD4 (HuCD4) is the principal receptor for human immunodeficiency virus type 1 (HIV-1) in human cell infection. Susceptibility of rabbit cell lines to infection with HIV-1 raised questions concerning whether a CD4 homolog serves as HIV-1 receptor on rabbit cells. Sequence comparisons of rabbit CD4 (RbCD4) cloned from a rabbit thymus cDNA library showed that 6 of the 18 residues implicated in HIV-1 binding by CD4 differ between the human and rabbit proteins. No correlation between RbCD4 expression by rabbit cell lines and their ability to support HIV-1 infection was seen. Transfection of RbCD4-negative, HTLV-I-transformed cell lines with HuCD4 significantly enhanced HIV-1 infectivity, suggesting that these lines lack a receptor present on other RbCD4-negative lines that produce high levels of p24 in their native state. Inhibition of HIV-1 infection with soluble HuCD4 was demonstrated for all rabbit lines tested, but complete inhibition was obtained only with a rabbit T-cell line expressing RbCD4 and with HuCD4 transfectants. The results suggest that HIV-1 infection of the RbCD4-positive line proceeds through a receptor similar to HuCD4 but that an additional receptor or receptors may serve this purpose in RbCD4-negative lines.