Immunosuppressive Properties of α1-Microglobulin

α₁-Microglobulin (α₁m), a serum glycoprotein (26,000 d). was found to impede the proliferative response of human lymphocytes to purified protein derivative (PPD) and tetanus toxoid. The data suggest that, α₁m operates through an unstable suppressor mechanism, which no longer can function after 24 h of preculturing. This effect of α₁m on antigen stimulation did not seem to be due to binding of α₁m to PPD or cells, to altered kinetics of the PPD response, or to non-specific cytotoxicity. In contrast, PPD-induced leucocyte migration inhibition was not reversed by α₁m. α₁m did not cause significant inhibition in experiments in which lymphocytes were stimulated by the mitogens phytohaemagglutinin or concanavalin A. Finally, α₁m had its own leucocyte migration inhibitory effect.