CARBOHYDRATE-COMPOSITION AND IMMUNOMODULATORY ACTIVITY OF DIFFERENT GLYCOFORMS OF ALPHA(1)-ACID GLYCOPROTEIN

The acute phase protein, alpha(1)-acid glycoprotein (AGP), is a normal constituent of human blood (0.2-1 mg ml(-1)) and its glycosylation an d concentration in the blood change during inflammation. In this revie w of our recent work, we discuss the immunomodulatory properties of AG P in connection with the structure of its carbohydrate chains. AGP sam ples prepared from normal donor serum (nAGP), serum obtained during ab ortion (fAGP), serum of cancer patients (cAGP), and ascitic fluid of p atients with stomach cancer (sAGP) were subjected to analysis. All the samples except for fAGP had five N-linked chains of the 'complex' typ e, however, the numbers of bi-, tri-, and tetra-antennary chains, as w ell as glycan structures terminating these chains, were different. fAG P had three N-linked chains of the lactosamine and polylactosamine typ e and three O-chains which were not present in AGP isolated from the o ther sources. The glycoforms of nAGP and sAGP that were isolated using a ConA affinity column were similar in respect to their branching, bu t differed in their terminal oligosaccharides. sAGP was enriched in un its ending in Le(x) and asialoagalacto (GlcNAc-terminating) forms. Imm unomodulatory activity of different AGP preparations was tested in vit ro by measuring their effect on the proliferative response of human ly mphocytes stimulated by PHA, and by determining their influence on the production of IL-l, IL-2, IL-6, and TNF in the stimulated cells. nAGP was less active compared to cancer or fetal AGP in the proliferation test, but more active in affecting cytokine production. Some AGP glyco forms had opposite immunomodulatory effects. A new approach was develo ped in order to clarify the role of carbohydrate chains in the biologi cal activity of AGP. A pool of N-linked oligosaccharide chains were at tached to a soluble polyacrylamide matrix. This 'pseudoglycoprotein' w as similar to AGP in its molecular weight; in its relative amounts of tetra-, tri-, and bi-antennary chains; and in the content of mono-, di -, tri-, and tetra-sialylated-oligosaccharides. This pseudo-AGP displa yed a similar activity to its parent AGP in the biological tests. Anal ytical flow cytometry of leukocyte subpopulation from human peripheral blood showed that monocytes and granulocytes but not lymphocytes were the main targets for the binding of AGP and pseudo-AGP. This binding was inhibited by synthetic glycoconjugates containing mannose or siali c acid. The binding curve data suggested that there are two monocyte a nd granulocyte populations. These may have different carbohydrate spec ificities. All the evidence provided by these studies indicate that it is the carbohydrate chains on AGP that are important in modulating th e immune system and not the AGP molecule itself.