Naturally occurring soluble CD4 in patients with human immunodeficiency virus infection.

To investigate its use as a marker of disease severity, serum soluble CD4 (sCD4) was measured by ELISA in patients with human immunodeficiency virus (HIV) infection. Levels of sCD4 were higher in patients than in controls (P less than .001) but did not increase with disease severity. sCD4 release per CD4 lymphocyte showed a linear increase with disease severity and performed as well as beta 2-microglobulin, a widely used marker. To study the role of sCD4 in the pathogenesis of HIV infection, an ELISA to detect sCD4 complexed with glycoprotein 120 (gp120) HIV envelope protein was developed. Preformed sCD4-gp120 complexes were not detectable in patient serum, but addition of recombinant gp120 showed that circulating sCD4 is capable of binding HIV envelope proteins. This study indicates that the sCD4-to-CD4 lymphocyte ratio increases linearly with disease severity and may be a useful marker of CD4 lymphocyte damage. In addition, serum sCD4 can bind viral particles, which may have implications for the use of recombinant sCD4 as a therapy in HIV infection.