An atlas of teleost IgM+ B cells in homeostasis and bacterial infection reveals the unique heterogeneity of B cells in early vertebrates

Teleost B cells are primitive lymphocytes with both innate and adaptive immune functions. However, the heterogeneity and differentiation landscape of teleost B cells remain largely unknown. Here, using grass carp as the research model, we applied single cell RNA sequencing to show a comprehensive transcriptional landscape of head kidney derived B cells in their steady state and during bacterial infection. The results showed that grass carp B cells are mainly comprised of six defined populations Remarkably, a significant percentage of B cells, especially plasma cells can express multiple IgM, indicating that teleost individual B cells can produce multiple antibodies through co-expressing IgM subclass genes. This study not only uncovered the atlas of teleost B cells in homeostasis and bacterial infection, but also added new evidence to the newly discovered one cell multiple antibodies paradigm.