Humoral inhibitors of the immune response in uremia. II. Further characterization of an immunosuppressive factor in uremic serum.

The serum of Lewis rats with chronic renal insufficiency (induced by subtotal nephrectomy) contains a nondialyzable inhibitor of the mixed lymphocyte reaction. Fractionation of uremic serum by gel filtration on Sephadex G-200 shows that the inhibitory activity elutes with an apparent molecular weight of greater than 200,000 daltons. To establish the possible relationship of the uremic inhibitor to known immunosuppressive components of normal serum, uremic serum was further fractionated on a DEAE cellulose column. The inhibitory activity elutes in 10 mM sodium phosphate at pH 8.0. This fraction contains both alpha-macroglobulin and IgG. The inhibitory activity of this fraction is completely inactivated by treatment with 2-mercaptoethanol, indicating that the inhibitor is a protein. The inhibitory activity is partially inactivated by periodate treatment, suggesting that it may be a glycoprotein. To determine whether or not the inhibitory factor is an immunoregulatory alpha-macroglobulin, or an immune complex, the uremic serum was fractionated by affinity chromatography procedures, which do not induce artifactual inhibitory properties in control serum. The alpha-macroglobulin was removed by affinity chromatography on a column of Con A-Sepharose; its removal had no effect on the inhibitory activity of serum in the mixed lymphocyte reaction. To examine the possibility that immune complexes may be the uremic inhibitor, the serum was fractionated by affinity chromatography on Protein A-Sepharose or by adsorption to a suspension of Staphylococcus aureus, cowan I. Neither of the two latter procedures had any effect on the inhibitory activity of uremic serum. So far all of our findings indicate that the immunosuppressive factor of uremic serum is distinct from two major immunoregulatory factors, alpha-macroglobulin and immune complexes.