Alpha 1-microglobulin glycopeptides inhibit antigen-specific stimulation of human peripheral blood leucocytes

Glycopeptides were prepared from proteolytically digested human and guinea pig alpha 1-microglobulin (alpha 1-m) by gel chromatography on Sephadex G-100 and affinity chromatography on concanavalin A-Sepharose. Amino acid analysis showed that the average glycopeptide was a tripeptide containing the amino acids aspartic acid/asparagine, isoleucine, and serine and/or threonine. It has earlier been shown that human alpha 1-m carries two or three N-glycosidically linked oligosaccharides of the high-mannose type. These preparations of glycopeptides inhibited the proliferative response of peripheral human blood leucocytes to the antigen purified protein derivative. The dose-response relationship of the inhibitions showed a greater specific activity of the glycopeptides than of whole alpha 1-m. Mild alkali treatment of human alpha 1-m did not affect its specific inhibitory activity, suggesting that the peptide backbone is of little importance for the immunosuppressive effect of alpha 1-m. No difference in inhibitory activity was seen between human and guinea pig native alpha 1-m or alpha 1-m glycopeptides. Human asialo alpha 1-m exerted a suppressive effect on the antigen-specific leucocyte proliferation similar to that of native alpha 1-m.