IgA Autoimmune Disorders: Development of a Passive Transfer Mouse Model (1)
IgA in autoimmune disorders of the skin
The IgA dermatoses are listed. Tissue-specific IgA antibodies in the serum may be identified. This is the case in linear IgA bullous dermatosis (LABD) and IgA pemphigus. In dermatitis herpetiformis (DH), IgA vasculitis, and bullous lupus erythematosus, however, no tissue-specific antibodies have been identified in the circulation.
Contrast with IgG dermatoses
Pemphigus vulgaris (PV) and pemphigus foliaceus (PF) demonstrate cell surface deposition of IgG on epithelial cells. These antibodies bind the cadherins desmoglein 3 and desmoglein 1, respectively. IgA pemphigus antibodies only bind infrequently to desmogleins, however, but more often bind to another class of cadherins termed desmocollins. They do not seem to stimulate acantholysis by their physical presence, but require the infiltration of neutrophils for acantholysis and vesiculation. The pathogenic process is clearly different for IgG and IgA dermatoses.
Bullous pemphigoid (BP) and the lamina lucida type of LABD both demonstrate lamina lucida deposition of immunoglobulin and are associated with BMZ vesiculation. Pathogenic epitopes for the IgG immune response are believed to be in the MCW-1 region of the NC16 domain of BP Ag2 (BP180), whereas the epitopes that have stimulated an IgA immune response are toward the carboxy terminus of the same molecule.
Dermatitis Herpetiformis
Dermatitus herpetiformis (DH) patients have a unique HLA type (HLA DQ2 A1*501B1*201). This is present in greater than 90% of patients and appears to be central to the antigen recognition process in which gliadin initiates a systemic immune response.
An Animal Model of IgA Dermotoses
The reactivity of IgA sera would be similar, but the studies identified predominant reactivity with a 97-kDa protein found in epidermal extract. A monoclonal antibody that reacted with a 120-kDa antigen from keratinocyte culture and a 97-kDa antigen from human skin extracts, but not BP180. This antigen is the same as BP180; however, the lack of immunoreactivity of sera with the intact BP180 antigen on immunoblot was not yet understood. The 97-kDa antigen is not expressed in mouse skin but is expressed in human skin. One mg of each purified monoclonal Ab was injected subcutaneously into the grafted athymic mice at 0 and 48 hours. The combination injection involved 1 mg of each antibody. All injections produced titers of 1:640 in the serum of the mice at 48 h, and titers could be maintained at >1:160 for 21 days. At 96 h, biopsies revealed intense deposition of IgG and complement at the BMZ and a basement membrane blister in some, but not all, animals.
After evaluating the IgG hybridomas to the 97-kDa antigen, IgA switch variants were isolated after exposing the hybridomas to the mutagen acridine orange (ICR 191). IgA hybridoma cells were then isolated either by fluorescein-activated cell sorting (FACS) or by the ELISA spot assay. Briefly, the IgG hybridomas were exposed to ICR 191 at the level of 1 μg/ml for 24 h to achieve a 30%–40% death rate. The conditions necessary to maintain serum titers for the IgA antibodies were different from those established for the IgG antibody. IgA antibodies were cleared from the circulation rapidly, so the animals required repeated dosing. Minimal neutrophil infiltration was noted in 9 of 32 animals at baseline, but no animals demonstrated significant neutrophil infiltration.
Following the administration of IgA alone, 6 of 12 animals demonstrated significant neutrophil infiltration, with infiltration of dermal papillae. Significant BMZ separation, however, was noted in test animals after administration of IgA or IgA plus GMCSF.
