Two-site enzyme immunoassay of CD4 and CD8 molecules on the surface of T lymphocytes from healthy subjects and HIV-1-infected patients.

A highly sensitive two-site enzyme immunoassay (Capcellia) was developed to determine the concentration of CD4 and CD8 molecules expressed on the surface of human T lymphocytes. This assay, performed in one step (20 min), involves the specific immunocapture of T lymphocytes and reaction of the CD4 or CD8 molecules with an enzyme-labeled monoclonal antibody (mAb). The results were expressed as molar concentrations of the T-cell markers on the basis of results obtained with calibrated CD4 and CD8 standards. The assay was sensitive enough to detect 0.4 pmol/L CD4 or 0.8 pmol/L CD8, which corresponded to approximately 20 x 10(6) CD4+ or CD8+ T cells per liter of blood. Mean concentrations in healthy adults were 17.2 pmol/L for CD4 and 22.1 pmol/L for CD8. The CD4 concentration was < 8 pmol/L in 50% of HIV-1-infected patients and in 95% of AIDS patients. Given the epitopic specificity of the mAb to CD4 we used, these values correspond to the concentration of CD4 molecules free of envelope glycoprotein (gp)120.