Characterization of IgM mediated humoral immune memory responses in Megalobrama amblycephala upon reinfection with Aeromonas hydrophila
Teleost fish represent the earliest vertebrates possessing adaptive immunity mediated by immunoglobulins; however, whether they can mount a true memory-like humoral response, especially a rapid and enhanced antibody reaction upon re-encounter with a pathogen, remains controversial. Here, we addressed this question by establishing primary and secondary infection models in blunt snout bream (Megalobrama amblycephala) using Aeromonas hydrophila, a widespread freshwater pathogen. Transcriptomic data showed that secondary infection uniquely enriched antigen processing pathways (phagosome, proteasome, lysosome) and B cell activation markers, contrasting with the innate inflammatory signature of primary infection. At the protein level, rechallenge induced significantly faster and higher serum total and specific IgM responses, with elevation as early as 3 days post infection. Concurrently, splenic IgM+ B cells expanded markedly, correlating with enhanced bacterial clearance, reduced tissue damage, and improved survival. These results demonstrate that blunt snout bream possess a genuine IgM mediated memory response, characterized by accelerated kinetics and amplified magnitude upon secondary challenge. Our findings not only provide crucial evidence for the evolutionary conservation of adaptive immune memory in vertebrates but also offer theoretical support for vaccine design and protective immunity evaluation in aquaculture.