Reversible loss of surface receptors on lymphocytes.

An α-2-glycoprotein from bovine serum (Fraction C) with immunosuppressive properties and anti-proliferative activity has been shown to reversibly inhibit the synthesis of cell surface receptors . The proliferative response of human lymphocytes to PHA and PPD can be inhibited by prolonged preincubation with Fraction C, which also inhibits the ability of lymphoid cells to bind PHA, 3–4 days for 90 per cent inhibition, and anti-immunoglobulin, 10 hours for 90 per cent inhibition. This inhibition is reversible and the binding ability recovers after removal of Fraction C at a similar rate to that at which it declined. Fraction C also rapidly and reversibly inhibits the secretion of antibody by immune spleen cells . The anti-proliferative effect of Fraction C is shown to involve an effect on ribosomal RNA synthesis and it is suggested that the mechanism of this action depends on the inhibition of protein synthesis, probably mediated by an effect on newly synthesized messenger RNA. The demonstration of different turnover rates of cell surface components on different types of cells suggests that it may be possible to use Fraction C to affect responsiveness of one type of lymphocyte while leaving another type unaffected.