Organized B cell sites in cartilaginous fishes reveal the evolutionary foundation of germinal centers

The absence of germinal centers (GCs) in cartilaginous fishes lies at odds with data showing that nurse sharks can produce robust antigen-specific responses and affinity mature their B cell repertoires. To investigate this apparent incongruity, we performed RNA sequencing on single nuclei, allowing us to characterize the cell types present in the nurse shark spleen, and RNAscope to provide in situ cellular resolution of key marker gene expression following immunization with R-phycoerythrin (PE). We tracked PE to the splenic follicles where it co-localizes with CXCR5high centrocyte-like B cells and a population of putative T follicular helper (Tfh) cells, surrounded by a peripheral ring of Ki67+ AID+ CXCR4+ centroblast-like B cells. Further, we reveal selection of mutations in B cell clones dissected from these follicles. We propose that the B cell sites iden-tified here represent the evolutionary foundation of GCs, dating back to the jawed vertebrate ancestor.

Automated summary

By studying the spleens of nurse sharks, we found that sharks are capable of producing robust antigen-specific responses and affinity maturation of B cells, despite the absence of germinal centers (GCs) observed in other animals. We also identified immune-related marker gene expression in the spleen cells of sharks and tracked their localization in the splenic follicles. These findings provide insights into the evolutionary foundation of GCs.