Antibodies to Pathogenic Epitopes on TypeXVII Collagen Cause Skin Fragility in a Complement-Dependent and -Independent Manner (1)
Type XVII collagen (COL17; formerly called BPAG2 or BP180) is a hemidesmosomal component that mediates the adhesion of keratinocytes to the epidermal basement membrane. COL17 is thought to be the main target recognized by autoantibodies in patients with bullous pemphigoid (BP), the most common autoimmune blistering disease. Passive transfer of IgG from BP patients into neonatal COL17-humanized mice leads to skin detachment, reproducing the human BP phenotype.
Five rabbit IgG polyclonal Abs (08012IgG and 10046IgG against NC16A, 08005IgG against ICD1, 08009IgG against ECD3, and 10047IgG against R7) were generated by immunization of rabbits and purification of the antisera.
08012IgG against NC16A produced dermal-epidermal separation, although 08005IgG against ICD1, 08009IgG against ECD3, and normal rabbit IgG did not produce skin detachment. DIF revealed IgG and C3 deposition at the DEJ in the 08012IgG-injected mice.
Injection of the R7-affinity IgG led to dermal–epidermal separation, even though the R4-, R5-, and R8-affinity IgGs did not produce skin fragility in the injected mice. DIF studies demonstrated IgG and C3 deposition at the DEJ in mice injected with R7-affinity IgG.
Mice injected with 08012IgG with immunoadsorption against R7 (08012IgG-R7/ IA) showed less skin fragility than those without immunoadsorption, even though in vivo deposits of IgG and C3 were observed in both groups.
NHKs after treatment with 08012IgG-R7/ IA contained COL17 in amounts comparable with those in NHKs after treatment with normal rabbit IgG.
Notably, 10046 F(ab’)2 produced skin detachment in the injected mice as did 10046IgG. Three of the seven mice injected with human BP F (ab’)2 and all of those injected with human BP IgG showed dermal-epidermal separation.
immunoblotting using cell lysates from cultured NHKs revealed that treatment with human BP NC16A-affinity F(ab’)2 reduced the amount of COL17.
The pathogenesis of BP is not solely explained by sequential steps including complement activation but could also involve impairment of hemidesmosomal formation due to COL17 depletion resulting from circulating Abs against pathogenic epitopes. A previous study of epitope mapping within NC16A reported that most of the IgG Abs from BP sera react with fragments within NC16A, NC16A1 (Glu490 to Arg506), NC16A2 (Arg507 to Glu520 ), and NC16A3 (Lys521to Asp534). R7 peptides, which are strongly involved in skin detachment in our passive transfer model, extend from Asp522 to Glin545. This disparity might be explained by differences between humans and the humanized animal model.