Meeting Abstract
To gain insight into the evolutionary relationship between vertebrate and invertebrate hematopoietic system, we have characterized the immune system and cell populations of the colonial tunicate Botryllus schlosseri. B. schlosseri belongs to a group considered the closest living invertebrate relative of vertebrates, it has bidirectional blood cell flow through an interconnected vasculature. To isolate and characterize the Botryllus cell populations we adapted Fluorescence-Activated Cell Sorting (FACS). We used Cytof Mass Cytometry to scan 50 diverse antibodies. Antibodies that differentially bind to B. schlosseri cells, in combination with lectins and fluorescent reagents activated by enzymes, were used to isolate live B. schlosseri cell types. Additionally, we used mouse serum against the Botryllus Histocompatibility Factor and analysis of cell size, granularity and auto fluorescence to isolate 34 cell populations. We prepared libraries from these populations for RNAseq, and analyzed their gene expression. This analysis revealed cell population homolog to mammalian hematopoietic stem cells, which upon transplantation, migrated to stem cell niche and differentiated into other cell lineages. Interestingly, we have shown that this niche is homolog to mammalian bone marrow stromal cells. Using functional immunological assays for cytotoxicity and phagocytosis we characterized 3 different phagocytic cell-types. One of these demonstrated transcriptional and functional features resembling myeloid cells in vertebrates. Furthermore, we identified a B. schlosseri cytotoxic cell population originating from large granular lymphocyte-like cells. Our data suggests that the common ancestor of tunicates and vertebrates had a true hematopoietic myeloid lineage, while the cytotoxic cells may result from a convergent evolutionary mechanism.
