Humoral inhibitors of the immune response in uremia. V. Induction of suppressor cells in vitro by uremic serum.

The mechanism of inhibition of mixed lymphocyte reaction (MLR) by serum of chronically uremic rats has been studied. The inhibitory activity of the serum has been associated with a discrete subset of very low density lipoproteins (VLDL) of Sf 100-400. The degree of the inhibitory activity of uremic serum correlates with the severity of uremia. Spleen cells from normal rats incubated for 20 hours with uremic serum or its VLDL fraction suppress the response of control syngeneic cells in the MLR. Induction of such suppressor activity does not require cell proliferation because it is not inhibited by mitomycin C. although the exact identity of the induced suppressor cells has not been established, they may be macrophages. The suppressor activity of induced spleen cells can be markedly reduced by filtration of spleen cells on glass wool or on nylon wool columns. Reconstruction experiments show that the adherent cell fraction of spleen cells exposed to uremic serum suppresses the response of the nonadherent fraction of control spleen cells. These results indicate that the immunosuppressive effects of rat uremic serum in vitro involve the induction of suppressor cells.