Mucous membrane pemphigoid (MMP), which mainly presents as mucosal lesions, is related to autoantibodies against the C-terminus (C-term) of collagen XVII (COL17). The blistering mechanisms for oral lesions in MMP have not been fully understood. MMP patients tended to have reduced inflammatory findings in histological analysis. One of the blistering mechanisms in MMP may be associated with the inhibition of the protein-protein bindings by autoantibodies against the C-term of COL17. We investigated the interaction of COL17 with the extracellular matrix components, COL4.
We firstly performed immunoprecipitation using mAb TS39-3 targeting COL17-NC16A, and mAb C17-C1 targeting C-term of COL17. When COL17 was precipitated with mAb TS39-3, COL4 was co-precipitated both in normal human oral keratinocytes (OMKs) and in normal human epidermal keratinocytes (NHEKs) cell lysates. In contrast, when COL17 was precipitated with mAb C17-C1, COL4 was not co-precepitated in OMKs, suggesting  that the direct bindings of COL17 with COL4 are interrupted by mAb C17-C1 in OMKs but not in NHEKs. For further investigations, we examined the isoform differences of COL4 between NHEKs and OMKs. Six types of COL4 genes (COL4A1, A2, A3, A4, A5, A6) encode the distinct chains of COL4α1 (IV) through α6 (IV), which form the heterotrimeric isoforms. The mRNA expressions of COL4A1 and A2 were mainly observed in both keratinocytes, but those ratio (A1/A2) were slightly different. We reasoned that the different bindings in COL17 and COL4 may attribute to the different expression levels of COL4 genes between NHEKs and OMKs. Finally, with the in vitro binding inhibition assays, the preincubation of COL17 with mAb C17-C1 or MMP-IgGs significantly reduced the amounts of COL17 binding to COL4 plates, confirming the disturbance effects of autoantibodies against C-term of COL17. In summary, we provide the unique mechanisms for the oral lesions in MMP, in which pemphigoid IgG targeting the C-term of COL17 may play a pathogenic role by disturbing protein-protein interactions.

A unique mechanism of epithelial detachment in mucous membrane pemphigoid caused by autoantibodies against the C-terminus of COL17