A single-cell atlas of grass carp (Ctenopharyngodon idella) peripheral blood IgM+ B cells provides insights into B cell mediated immune responses in teleost fish

The concentration of teleost B cells in peripheral blood is very high, accounting for 15~50% of peripheral blood lymphocytes. However, the immune responses of teleost peripheral blood B cells to bacterial infection are still waiting for elucidation. Therefore, in the present study, we employed the 10× Genomics single cell RNA sequencing (scRNA-seq) technology to investigate the responses of grass carp (Ctenopharyngodon idella) peripheral blood IgM+ B cells to infectious Aeromonas hydrophila, an important fish pathogen. The results indicated that grass carp peripheral blood IgM+ B cells consist of six subpopulations: (im)mature B cell, innate B cell, proliferating B cell, IgDhigh B cell, and two infection-induced subpopulations, desiganted infection Ⅰ B cell and infection Ⅱ B cell, respectively. Bacterial infection alters the heterogeneity of IgM+ B cells, drives metabolic reprogramming in specific IgM+ B cell subpopulations such as (im)mature B cell and innate B cell, as well as primes IgM+ B cells in peripheral blood for enhanced functionality. Notably, infection Ⅰ B cells exhibit elevated expression of interferon φ1 (IFNφ1), a type I IFN, which was further vailidated through an in vitro A.hydrophila stimulation assay, suggesting that teleost B cells can contribute to antibacterial defense by secreting interferons. Additionally, the IgDhigh B cell subpopulation is present in grass carp peripheral blood, regardless of health status. Collectively, these findings significantly advance our understanding of the heterogeneity of peripheral blood IgM+ B cells and provided new insights into the IgM+ B cell mediated immune response in early vertebrates.