Natural killer inhibitory substance produced by the peritoneal cells of patients with ovarian cancer.

Peritoneal cells obtained from 8 patients with minimal residual ovarian cancer produced a substance during in vitro culture that markedly inhibited the expression of natural killer (NK) cell-mediated lysis. Its molecular weight was less than 2,000, the same size as the NK-inhibiting substance (NK-IS), a similar NK-suppressive molecule produced by the peritoneal cells of rats. Human NK-IS suppressed the expression of antibody-dependent cell cytotoxicity as well as NK lysis, but it had no effect on erythrocyte-rosette formation and was not cytotoxic to peripheral blood lymphocytes or cell fractions enriched for large granular lymphocytes. NK-IS inhibited lysis mediated by interferon-activated lymphocytes and completely prevented NK activation when used in a preincubation. During intraperitoneal immunotherapy with Corynebacterium parvum, an agent that can activate peritoneal cytotoxic effectors, the production of NK-IS by peritoneal cells decreased considerably. Human peritoneal cells produce an NK-IS similar to the peritoneal cells of rats, and this material may create an environment within the peritoneal cavity that is permissive to the growth of NK-sensitive tumor cells.