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, anti-human
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.