ALPHA-2-MACROGLOBULIN ON HUMAN VASCULAR ENDOTHELIUM

.alpha.2-Macroglobulin (.alpha.2M) was identified on the luminal surface of endothelial cells in sections of normal human arteries, veins and lymphatics by the indirect immunofluorescent technique. The specificity of the immunofluorescent reaction was confirmed by immunoabsorption studies. Prior absorption of the anti-.alpha.2M antiserum by purified .alpha.2M at equivalence completely inhibited endothelial surface and hepatic parenchymal cell staining. Endothelial cells in blood vessels were not stained when sections were treated with rabbit antisera toward .alpha.1-antitrypsin, antithrombin III, IgG [immunoglobulin G] IgA, IgM, complement component C3, or fibrinogen. The location of .alpha.2M at the surface of the vessel wall suggests that this protease inhibitor may protect the vascular endothelium from potentially injurious intravascular proteases.