Single-cell atlas of grass carp (<i>Ctenopharyngodon idella</i>) peripheral blood IgM<sup>+</sup> B cells provides insights into B cell-mediated immune responses in teleost fish

Teleost peripheral blood contains a remarkably high proportion of B cells, accounting for 15%-50% of circulating lymphocytes. However, their immune responses to bacterial infection are yet to be elucidated. In the present study, 10× Genomics single-cell RNA sequencing (scRNA-seq) was employed to characterize the transcriptomic landscape of peripheral blood IgM ⁺ B cells in grass carp ( Ctenopharyngodon idella) following challenge with Aeromonas hydrophila, a major aquatic pathogen. Six transcriptionally distinct IgM ⁺ B cell subpopulations were identified, including (im)mature B cells, innate B cells, proliferating B cells, IgD ^high B cells, and two infection-induced subsets denoted as infection Ⅰ and Ⅱ B cells. Bacterial infection altered the cellular heterogeneity of IgM ⁺ B cells, triggered metabolic reprogramming in (im)mature and innate B cell subpopulations, and enhanced the immunological activation of circulating B cells. Notably, infection Ⅰ B cells demonstrated robust induction of interferon φ1 ( IFNφ1), a type I IFN, following A. hydrophila exposure. This induction was further validated through in vitro bacterial stimulation, indicating that teleost B cells actively contribute to innate antibacterial responses through IFN signaling. Additionally, the IgD ^high B cell subpopulation remained consistently present in peripheral blood across both infected and uninfected states, pointing to a constitutive and likely mature phenotype. These findings significantly advance our understanding of the heterogeneity of peripheral blood IgM ⁺ B cells and provide new insights into IgM ⁺ B cell-mediated immune responses in teleost fish.