Interleukin-4 receptor expression on AIDS-associated Kaposi's sarcoma cells and their targeting by a chimeric protein comprised of circularly permuted interleukin-4 and Pseudomonas exotoxin

Background: AIDS-associated Kaposi's sarcoma (AIDS-KS) represents one of the most common malignancies associated with human immunodeficiency virus infection. To target effective therapeutic agents to AIDS-KS, we have identified a new target in the form of interleukin-4 receptors (IL-4R). Materials and Methods: The expression of IL-4R on AIDS-KS cells and their subunit structure was determined by radioligand receptor binding, cross-linking, and Northern and RT-PCR analyses. The in vitro effect of IL-4 and recombinant fusion protein made up of circularly permuted IL-4 and a mutated form of Pseudomonas exotoxin, IL-4(38-37)-PH38KDEL, was examined by clonogenic and protein synthesis inhibition assays. Results: Five AIDS-KS cell lines expressed high-affinity IL-4R with a K-d of 23.5-219 pM. IL-4 appeared to cross-link to one major protein corresponding to 140 kDa and a broad band corresponding to 60-70 kDa. Both cross-linked proteins were immunoprecipitated with an antibody to human IL-4R beta chain. AIDS-KS cells exhibited IL-4R beta-specific mRNA. IL-4 caused a modest inhibition (31-34%) of colony formation in two AIDS-KS cell lines tested. IL-4(38-37)-PE38KDEL was found to be highly effective in inhibiting the protein synthesis in all five AIDS-KS examined. The IC50 ranged from 32 to 1225 pM. The cytotoxic action of IL-4 toxin was blocked by an excess of IL-4, exhibiting the specificity of IL-4(38-37)-PE38KDEL. The cytotoxicity of IL-4 toxin observed by a donogenic assay corroborated well with the IC50 obtained by protein synthesis inhibition assay. Normal human endothelial cells expressed a negligible number of IL-4R (<50 sites/cell) and were less sensitive or not sensitive to IL-4(38-37)-PE38KDEL. Conclusion: The presence of a new plasma membrane protein in the form of IL-4R on AIDS-KS cells may be targeted by IL-4(38-37)-PE38KDEL for its potential implication in the treatment of AIDS-KS.