Patients with minimal change nephrotic syndrome (MCNS) frequently have suppressed in vivo and in vitro immune responsiveness of uncertain etiology. Because increased suppressor cell activity has been associated with this disease, urines from MCNS patients were screened for activity of the lymphokine soluble immune response suppressor (SIRS), a product of concanavalin A- or interferon-activated suppressor T cells. Urines from untreated MCNS patients suppressed polyclonal plaque-forming cell responses of cultured splenocytes. This suppressive activity was identified as human SIRS by the following functional and physical criteria: (a) molecular weight estimated by gel filtration; (b) kinetics of suppression; (c) inhibition of suppression by catalase, levamisole, and 2-mercaptoethanol; (d) abrogation of activity by acid or protease treatment; (e) elution pattern on high performance liquid chromatography; and (f) cross-reactivity with monoclonal antimurine SIRS antibodies. Suppressive activity disappeared from urine after initiation of treatment but before remission of symptoms. Urines were tested from 11 patients with MCNS, all of whom excreted SIRS. In addition, two nephrotic patients with acute glomerulonephritis and three nephrotic patients with membranoproliferative disease excreted SIRS, but other nephrotics and all nonnephrotic patients did not. These results indicate that excretion of SIRS occurs in certain cases of nephrotic syndrome and that the presence of SIRS in the urine is not accounted for solely by the presence of proteinuria or nephrosis. Serum from four nephrotic patients also contained SIRS, whereas neither serum nor urine from six normal subjects contained SIRS activity. The systemic presence of SIRS in these four patients, and the identification of SIRS in urines from a larger group of patients, suggest a possible role for SIRS in the suppressed immune responses often found in nephrotic syndrome.
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