Although the clinical relevance of endothelial cell-monocyte (E-M)2 antigens has been demonstrated in organ graft transplantation, very limited data exist describing the nature of these antigens. The current study presents biochemical characterization of three different surface antigens of endothelial cells and monocytes that are defined by murine monoclonals produced against gamma-interferon-induced human umbilical vein endothelial cells. The antigens gp150, gp48, and gp24 have molecular weights of 150,000, 48,000, and 24,000 respectively, under reducing conditions. The antibody binding sites of gp150 and gp48 are destroyed by prononase and chymotrypsin, indicating that the molecules are at least partly protein in nature. The inability to label the gp48 molecule with 125I using lactoperoxidase suggests that there is little protein structure exposed to the cell surface or that the molecule lacks sufficient cell surface tyrosine residues to enable detection. Immunoprecipitation of the gp24 molecule under nonreducing conditions shows that a molecule with a higher molecular weight ranging from 40,000-70,000 is detected. Although it is possible that this higher-molecular-weight species is a multimer of the 24,000 Mr species, it is also possible that there is another molecule(s) bound to the 24,000 Mr molecule. All three E-M antigens have some carbohydrate nature as evidence by lectin-binding studies. The possible relevance of these antigens in the rejection of transplanted organ grafts is discussed.
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