From ancient cold-blooded fishes to mammals, all vertebrates are protected by adaptive immunity, and retain immunological memory. Although immunologists can demonstrate these phenomena in all fish, the responding cells remain elusive, without the tools to study them nor markers to define them. Fundamentally, we posited that it is longevity that defines a memory cell, like how it is antibody production that defines a plasma cell. We infected the common carp with Sphaerospora molnari, a cnidarian parasite which causes seasonal outbreaks to which no vaccine is available. B cells proliferated and expressed gene signatures of differentiation. Despite a half-year gap between EdU labeling and sampling, IgM+ B cells retained the thymidine analogue, suggesting that these are at least six-month-old resting memory cells stemming from proliferating precursors. Additionally, we identified a lymphoid organ-resident population of plasma cells by the exceptional levels of IgM they express. Thus, we demonstrate that a teleost fish produces the lymphocytes key to vaccination success and long-term disease protection, supporting the idea that immunological memory is observable and universal across vertebrates.

Department of Botany and Zoology Faculty of Science Masaryk University Brno Czechia

Faculty of Science University of South Bohemia České Budějovice Czechia

Fish Health Division University of Veterinary Medicine Vienna Austria

Laboratory of Fish Immunology Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czechia

Laboratory of Fish Protistology Institute of Parasitology Biology Centre Czech Academy of Sciences České Budějovice Czechia

South Bohemian Research Centre of Aquaculture and Biodiversity of Hydrocenoses Institute of Aquaculture and Protection of Waters Faculty of Fisheries and Protection of Waters University of South Bohemia České Budějovice Czechia

Working Group Fish Genetics Research Institute for Farm Animal Biology Dummerstorf Germany

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