Study of original B cells producing pathogenic IgG and IgA autoantibodies in anti-BP180-type mucous membrane pemphigoid

Excerpt

Anti-BP180-type mucous membrane pemphigoid (MMP) is a rare type of autoimmune bullous disease clinically with erosive lesions on various mucosae and immunologically with IgG and IgA anti-BP180 autoantibodies, reactive primarily with BP180 C-terminal domain [1‐4]. Different classes of immunoglobulins are produced from an original B cell via class switch recombination (CSR). CSR is defined as upstream removal of the IgH chain (Fc region) gene while the variable portion of the gene is retained. Thus, the CSR produces other classes of immunoglobulin that recognize the same antigen epitope [5]. Anti-BP180-type MMP is a useful candidate disease for the examination of CSR, because this disease frequently presents both IgG and IgA autoantibodies. This study was approved by the ethics committees of Osaka Metropolitan University (No. 2022-018) and adhered to the principles of the Declaration of Helsinki. We used sera from 16 anti-BP180-type MMP patients with IgG and/or IgA antibodies reactive with the full-length BP180 C-terminal domain, which were stored at the Departments of Dermatology of both Osaka Metropolitan University and Kurume University. We used the same patient sera in all experiments. We prepared three recombinant proteins (RPs) (Parts 1–3) covering the three parts of the BP180 C-terminal domain (Fig. 1a, b), which were used for immunoblotting for IgG and IgA antibodies in the anti-BP180-type MMP patient sera. All results of the immunoblotting studies of the various RPs for IgG, IgA and the IgG/IgA subclasses are summarized in Table 1. Part 1 was positive for IgG in patient 4 (Fig. 1c). Part 2 was positive for IgA in patient 5 and IgG in patient 6 (Fig. 1d). Part 3 was positive for IgG in sera from patients 1, 4, and 6 and for both IgG and IgA in patients 2 and 5 (Fig. 1e). Patient sera with IgG and IgA antibodies positive for any of the RPs were further examined for subclasses of IgG and IgA (i.e., IgG1-IgG4 and IgA1/IgA2). Patient 2 was negative for all IgG subclasses to Part 3 but positive for IgA1 and IgA2 to Part 3 (Fig. 2b). Patient 4 was positive for IgG1 and IgG2 to Part 1 (Fig. 2c) and for IgG1 to Part 3 (Fig. 2d). Patient 5 was negative for both IgA1 and IgA2 to Part 2 (Fig. 2e) but positive for IgG1 and IgA2 to Part 3 (Fig. 2f). Patient 6 was positive for IgG1 to Part 2 (Fig. 2g) and IgG2 and IgG4 to Part 3 (Fig. 2h). These findings indicate that different classes of immunoglobulins react with different parts and, thus, different epitopes. For example, patient 4 had IgG antibodies to Parts 1 and 3, patient 5 had IgA to Parts 2 and 3, and patient 6 had IgG antibodies to Parts 2 and 3. Thus, the results in our study suggested that autoantibodies in anti-BP180-type MMP patient sera were produced by plasma cells originating from different B cells rather than by plasma cells originating from a single B cell via CSR.

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