Is there an interleukin 2 inhibitor in human serum?

Normal mouse serum has been shown to contain an inhibitor of interleukin 2 (IL-2). Here we report that a molecule with similar activity cannot be found in normal human serum (NHS). Although NHS inhibited the IL-2-dependent proliferation of mouse CTLL cells, as expected of an IL-2 inhibitor, it also had inhibitory activity on IL-3-dependent cells and was cytolytic to IL-2-independent mouse cells as measured by a 51Cr release assay, indicating a nonspecific effect. In addition, NHS had no effect on the IL-2-dependent proliferation of human peripheral blood T-cell blasts. Fractionation of NHS by size exclusion HPLC failed to separate cytolytic activity from any putative true IL-2 inhibitor activity. The cytolytic component was not related to immunoglobulin since it had a molecular weight of 50,000 to 60,000 and was not bound by protein-A-Sepharose. However, its molecular weight, heat lability, and trypsin sensitivity suggest it to be a protein.