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, we have discovered that AIDS-KS cells express high-affinity receptors for interleukin-4 (IL-4), a pleiotropic immune regulatory cytokine. Molecular studies have revealed that AIDS-KS cells express type II IL-4 receptors, in which IL-4 forms a productive complex with primary IL-4 binding protein (IL-4R beta, also known as IL-4R alpha) and a shared subunit between IL-4 and IL-13R systems (IL-13R alpha', also known as IL-13R alpha 1). A recombinant fusion protein composed of IL-4 and a mutated form of a powerful bacterial toxin called Pseudomonas exotoxin (PE)--the fusion protein is termed IL4(3837)-PE38KDEL or cpIL4-PE--was found to be highly and specifically cytotoxic to AIDS-KS cells in vitro. Normal human immune cells (e.g., resting T and B cells and monocytes) or endothelial cells, a possible precursor of AIDS-KS, expressed low numbers of IL-4R and showed little or no sensitivity to cpIL4-PE. Administration of cpIL4-PE in nude mice with established subcutaneously growing AIDS-KS tumors produced remarkable antitumor activity in a dose-dependent manner with the highest dose exhibiting complete responses without any visible toxicity. KS tumors produced metabolic changes including cachexia, hypoglycemia and lymphopenia, all of which were prevented by cpIL4-PE treatment. These studies indicate that cpIL4-PE is a promising experimental therapeutic agent for treatment of AIDS-KS.