IgM, an ancient immunoglobulin that protect grass carp (Ctenopharyngodon idella) offspring from aquatic reovirus

Maternal immunity is a key mechanism of early defense in vertebrates. Among the various maternal molecules, antibodies (Abs) play a crucial role. In teleost fish, immunoglobulin M (IgM) is the earliest discovered and most abundant Ab type. However, the maternal immune function of teleost IgM is still waiting for elucidation. Here, we found that IgM is an important maternal Ab type in grass carp (Ctenopharyngodon idella), distributed both around the egg membrane and within the eggs. The polymeric immunoglobulin receptor (pIgR) can bind to IgM in the eggs, suggesting its potential involvement in the transport of maternal IgM into the eggs. When female broodstock were immunized with genotype Ⅱ grass carp reovirus (GCRV-Ⅱ), GCRV-Ⅱ-specific IgM was found to be maternally transferred. Moreover, egg lysates from immunized females effectively neutralized GCRV-Ⅱ and improved the survival rate of infected larvae. However, when IgM was removed from the egg lysates, the survival rate decreased significantly. Notably, the offspring of female but not male broodstock immunized with GCRV-Ⅱ exhibited stronger resistance to GCRV-Ⅱ and higher survival rates after infection. When IgM was depleted in female broodstock by injection of anti-IgM monoclonal Ab, offspring exhibited drastically reduced survival following GCRV‑Ⅱ infection. In summary, this study systematically revealed the maternal immune function of IgM in grass carp. The findings not only expand our understanding of the immune roles of teleost IgM but also provide strategies for the prevention of pathogens during the early stage of fish development.