Aquaporin-5-specific heavy chain VDJ knock-in (A5H) mice reveal molecular mimicry-driven initiation and diversification of autoreactive B-cell responses.

Molecular mimicry, where microbial antigens resemble self-antigens, is implicated in triggering autoimmune responses, yet the early B-cell events leading to autoantibody production remain unclear. Here, we generated A5H mice, a knock-in model in which a heavy chain binding to the aquaporin-5 (AQP5) "E" epitope (AQP5E) preferentially generates B cells recognizing a microbial mimic peptide (PmE-L). A5H mice exhibited normal B-cell development with a phenotypic reduction in the anergy-associated B-cell compartment and an expanded pool of mimic-reactive B cells. At steady state, A5H mice produced anti-PmE-L antibodies, whereas anti-AQP5E autoantibodies were not detectable by ELISA. Upon immunization with PmE-L, mimic-reactive B cells were recruited into germinal center responses, leading to robust production of anti-AQP5E autoantibodies that cross-reacted with homologous AQPs (AQP4 and AQP1). Furthermore, A5H mice generated anti-AQP5E autoantibodies upon immunization with streptavidin-complexed PmE-L, indicating that A5H-derived mimic-reactive B cells can overcome competition with the immunodominant carrier antigen streptavidin. However, evidence of intramolecular epitope spreading to native AQP5 was not observed. Consistently, tissue deposition of autoantibodies and salivary gland pathology remained minimal. Notably, autoantibody responses were stronger in heterozygous (+/A5H) than homozygous (A5H/A5H) mice, indicating that allelic inclusion enhances autoreactive B-cell activation. These findings establish A5H mice as a valuable model to dissect how molecular mimicry initiates and diversifies autoreactive B-cell responses, advancing understanding of autoimmune disease initiation.