C3b receptor in normal human skin

Receptors for C3b in normal skin were studied. C3b was produced by treating normal human serum with cobra venom factor and by partial digestion of purified C3 with trypsin. Cryostat sections of normal human skin were incubated with C3b, followed by a direct immunofluorescent technique using monospecific goat antihuman C3. The histologic localization of C3b fluorescence was ascertained by fixing cryostat sections with glutaraldehyde and staining with hematoxylin and eosin. The following structures showed staining with anti-C3: (1) endothelial cells in capillaries, larger vessels, and arteries, (2) smooth muscle in arrector pilori muscles and artery walls, and (3) myoepithelial cells in the secretory portion of sweat glands. C3b did not bind to the intercellular substance nor to the basement membrane zone in normal human skin. Normal human sera treated with EDTA, EGTA, and heat (56 degrees C for 30 min) were negative, as was purified C3 by itself, thus indicating that native C3 did not bind to these receptors. Specificity for C3/C3b was shown by blocking with both unconjugated rabbit antihuman C3 and purified C3. The endothelial C3b receptor may have an important role in the localization of immune complexes in cutaneous vasculitis.