scRNA-Seq profiling of stickleback fish splenocytes: Expansion of myeloid and B cells on immunization

Abstract Threespine stickleback is characterized by wide phenotypic variations attributed to repeated evolutions that took place when colonizing new environments. This fish species is hence deemed a powerful model candidate for comparative immunology to gain new insights into the function and evolution of vertebrate immune system, given also fish small size, wide distribution, and easiness of husbandry. Harnessing single-cell RNA sequencing potential to dissect tissue cellular heterogeneity at high resolution, here we employed that technique to profile naïve, and immunized stickleback spleens identifying the different immune cell types present, and reveal the transcriptional changes driven by immunization. Analysis of the compiled splenic data revealed 18 cell clusters, including major leukocyte types (e.g., B cells, macrophages, and neutrophils), and erythrocytes. Surprisingly, none of the clusters could be recognized or presumed as a T cell population based on the genes expressed in each cluster. Fish immunization led to total cell expansion and interestingly to the emergence of distinct myeloid and B cell populations. The myeloid population lacks the expression of MHCII genes, but was found enriched in the expression of GRN2, S100A10, NCCRP, BPIFC, and PRDX1 genes, suggesting that additional cytotoxic roles are possibly played by that population. Despite the B cell population being enriched in the expression of proteolysis associated genes, i.e., TRY and PRSS59, it also, unlike other B cell populations, lacks the expression of MHCII genes. This study creates a comprehensive atlas of threespine stickleback spleen at single-cell resolution and raises new questions about the immune response evolving in such vertebrate species. Supported by a grants from NIH (Grant number: NIH R01 AI146168)