Measurement and partial characterization of an interleukin-2 inhibitor (IL-2-IN) in human urine

We observed a human urine-derived protein complex (IL-2-IN) which competitively inhibits interleukin-2 (IL-2) dependent murine lymphocyte proliferation. Measurements of urinary IL-2-IN have been used to stratify the immune response of patients to bacteria in the bladder. Partial characterization of IL-2-IN indicates that it is a heat-stable, 75 kDa complex comprised of interleukin-2 bound to another protein(s). Although the IL-2-IN complex is stable in physiologic buffers, the complex can be disrupted using acidic or low-ionic strength buffers, thereby liberating IL-2. IL-2-IN activity is susceptible to bacterial and endogenous urinary proteolysis. The IL-2 bound in the IL-2-IN complex cannot be detected using a double monoclonal antibody radioimmunoassay for IL-2. Unlike other IL-2 binding proteins, the IL-2 binding protein of the IL-2-IN complex is not a soluble interleukin-2 receptor. A modification of the bioassay for interleukin-2 activity is the method of choice for the detection and quantification of urinary IL-2-IN.