Studies of C3d,g in normal human epidermal basement membrane

Recent studies in our laboratory have demonstrated that polyclonal as well as monoclonal antibodies directed against C3d,g, a specific 41,000-Da fragment of the third component of complement, bind normal human epidermal basement membrane in a continuous pattern. No such reactivity is present within dermal microvascular basement membranes. By direct immunofluorescence microscopy, antihuman C3d,g antibodies bind the base of the cleavage plane in 1 M NaCl split human skin. By immunoelectron microscopy, anti-human C3d,g reactivity is found along the base of the lamina densa and in the sublamina densa region. Control antibodies directed against human C3, C3c, C5, IgG, IgA, or IgM do not bind normal human epidermal basement membrane or identify in situ deposits of immune complexes in multiple samples of normal human skin that are all positive for C3d,g. Preabsorption of monoclonal or polyclonal anti-human C3d,g antibodies with purified human C3d completely blocks these reagents' epidermal basement membrane reactivity. Studies of skin samples from a patient with congenital C3 deficiency reveal that anti-human C3d,g antibodies do not bind this subject's epidermal basement membrane. Moreover, in vitro treatment of this patient's skin with normal serum, aged serum containing C3d,g, purified human C3, or zymosan-serum reaction mixtures does not restore epidermal basement membrane anti-human C3d,g binding. Studies of other primate tissues demonstrate that C3d,g is not restricted to basement membranes of stratified squamous epithelia as it is also present within renal tubule and glomerular basement membranes. While a recent study has demonstrated that C3d binds laminin in vitro, our investigations show a difference in both the regional and ultrastructural distribution of laminin and C3d,g in normal human skin. Furthermore, C3d,g is absent from laminin-rich basement membranes of papulonodular basal cell carcinomas. These findings suggest that C3d,g is not passively incorporated within selected epithelial basement membranes but rather is a previously unrecognized normal constituent. Basement membrane-associated C3d,g may play a role in adhesive interactions between leukocytes and matrix proteins. Moreover, a C3d,g binding site(s) in selected epithelial basement membranes may account for the accumulation of C3 containing immune complexes in such tissues.