B cell selection sites in the nurse shark spleen may represent evolutionary precursors of mammalian germinal centers

Abstract

Abstract Sharks are members of the oldest extant taxonomic group (Chondrichthyes) to have immunoglobulin (Ig)-based adaptive immunity. While research has demonstrated that sharks can produce antigen-specific antibody responses, affinity mature these antibodies, and establish immunological memory, histological examination has so far failed to find evidence of germinal centers (GCs). To explore this apparent discrepancy, we immunized several nurse sharks (Ginglymostoma cirratum) with phycoerythrin and visualized the subsequent splenic response using a combination of single nuclei (sn)RNA sequencing and fluorescent microscopy. Our snRNA data reveal the presence of both centrocyte-like and centroblast-like B cells in sharks, as well as a T cell subset expressing several markers found in mammalian T follicular helper cells. RNA fluorescent in situ hybridization experiments show distinct regions of AID expression, polarization of CXCR4 and CXCR5 expression, and association of T cell aggregates with splenic B cell follicles. Fluorescent microscopy clearly demonstrates presentation of nondegraded antigen in the center of B cell follicles. Finally, analysis of Ig variable regions amplified from such B cell follicles reveals evidence of positive selection of clonotypes. Together, our data supports the presence of B cell selection sites in the cartilaginous fish spleen that functionally support the selection of antigen-specific B cells and affinity maturation of the shark Ig repertoire. Further, these sites potentially represent the primordial structures that gave rise to mammalian GCs. Supported by grants from NIH T32 AI095190, F31 AI147532