Soluble CD4-PE40 Is Cytotoxic for a Transfected Mammalian Cell Line Stably Expressing the Envelope Protein of Human Immunodeficiency Virus (HIV-1), and Cytotoxicity Is Variably Inhibited by the Sera of HIV-1-Infected Patients

Sera were obtained from 50 individuals infected with human immunodeficiency virus type 1 or from HIV-1-uninfected individuals before or after vaccination with recombinant gp160. These sera were evaluated for activity antagonistic to the cell-killing activity of the chimeric Pseudomonas exotoxin hybrid protein, sCD4-PE40. For these studies, Chinese hamster ovary (CHO) cells were transfected with a chimeric plasmid encoding the tat, rev, and envelope genes of HIV-1 and a cell line was selected for stable expression of the envelope glycoproteins at the cell surface (CHO-env). Cytotoxicity of sCD4-PE40 for CHO-env in the presence or absence of added human serum was quantitated spectrophometrically following enzymatic reduction of a tetrazolium bromide within the mitochondria of viable cells (MTT assay). Several HIV+ sera inhibited the cytotoxic activity of sCD4-PE40; the antagonist had properties consistent with those of immunoglobulins in that it was heat stable, absorbed by protein A, and reversible by incr...