BIOLOGICAL EXPRESSIONS OF LYMPHOCYTE-ACTIVATION .5. CHARACTERIZATION OF A SOLUBLE IMMUNE-RESPONSE SUPPRESSOR (SIRS) PRODUCED BY CONCANAVALIN A-ACTIVATED SPLEEN-CELLS
Supernatant fluids from murine spleen cell cultures incubated with concanavalin A for 48 h contain a factor(s), soluble immune response suppressor (SIRS), which suppresses plaque-forming cell responses to sheep erythrocytes by murine spleen cells in vitro. In the present studies, some of the biochemical and biophsyical properties of SIRS were investigated. SIRS was non-dialyzable; the suppressive activity was stable at 56.degree. C for 30 min, but was destroyed by treatment at 70.degree. C for 30 min, 80.degree. C for 10 min or at pH 2. The suppressive activity was not absorbed by the stimulating antigen, SRBC, or antisera against murine IgG [immunoglobulin G] or .mu.-chain, suggesting that SIRS does not contain Ig determininants. Murine spleen and thymus, but not kidney cells, absorbed SIRS activity. Enzyme treatments revealed that SIRS was resistant to DNase and RNase, but was destroyed by trypsin and chymotrypsin. In gel filtration with Sephadex G-100, SIRS activity eluted in the fraction corresponding to MW in the range 48,000-67,000. With polyacrylamide gel electrophoresis, SIRS activity migrated in the region cathodal to albumin. Isopycinic centrifugation in a CsCl gradient suggested that SIRS is a glycoprotein. These supernatant fluids with SIRS activity contained macrophage migration inhibitory factor (MIF). Using gel filtration, polyacrylamide gel electrophoresis, and isopycnic centrifugation to fractionate supernatant fluids, SIRS and MIF activity were found in the same fractions, and it was not possible to dissociate definitively SIRS activity from MIF activity.