Anti-basement membrane autoantibodies in patients with anti-epiligrin cicatricial pemphigoid bind the α subunit of laminin 5

Recent studies have identified a group of cicatricial pemphigoid patients who have IgG anti-basement membrane autoantibodies that recognize epiligrin, a set of disulfide-linked polypeptides closely related if not identical to laminin 5 (formerly called kalinin, nicein, or BM600). To further understand the pathophysiology of blister formation in these patients, we have sought to identify the specific polypeptide(s) targeted by their autoantibodies. Comparative studies show that sera from these patients (nine of nine), P1E1 monoclonal anti-epiligrin antibody, and polyclonal as well as monoclonal anti-laminin 5 antibodies immunoprecipitate the same set of disulfide-linked polypeptides from media of biosynthetically radiolabeled human keratinocytes. Moreover, sera from eight of nine patients with anti-epiligrin cicatricial pemphigoid immunoblot the α subunit of laminin 5 but show no reactivity to its β or γ subunits. In addition, circulating IgG from a representative patient was affinity-purified against the α subunit of laminin 5 and shown to bind the dermal side of 1 M NaCl split skin in the same manner as autoantibodies from all patients with anti-epiligrin cicatricial pemphigoid. Sera from patients with bullous pemphigoid (n = 5), other forms of cicatricial pemphigoid (n = 5), epidermolysis bullosa acquisita (n = 4), or bullous systemic lupus erythematosus (n = 1) show no reactivity against any subunit of this laminin isoform in immunoprecipitation or immunoblot experiments. These findings correlate with prior reports showing that a monoclonal antibody directed against the α subunit of laminin 5 (i.e., laminin subunit α3) induces detachment of human keratinocytes from extracellular matrix in vitro as well as epidermis from human skin in situ. Together, these studies suggest that laminin subunit α3 mediates attachment of basal keratinocytes to epidermal basement membrane and that autoantibodies directed against it may be pathogenic.