Reverse engineering B. Schlosseri developmental-regulated cell death programs for cancer immunotherapy

Abstract

Abstract B. schlosseri, a colonial marine species, offers a unique platform to study mechanisms underlying cellular recognition. We used adoptive transfer studies, FACs analysis, and RNA-sequencing to study the progression by which fused-colonies eliminate chimeric partners. We show the principle cell type mediating elimination of partners in a chimera is a cytotoxic Morula cell (MC), which utilizes developmental-regulated programmed cell death pathways, initiated during the ‘takeover’ phase of colony life, to eliminate partners. Adoptive transfer of FACs purified cytotoxic MCs from third party donors into recipients recapitulates events of chimeric partner elimination. Transfer of 1×10^5 allogeneic MCs eliminated 33 of 78 (42%) of recipient palleal buds and 20 of 76 (20.5%) adult zooids by day 14 post-transfer. Transfer of phagocytic populations had only minimal effects on recipient colonies. We identified 275 genes (FDR < 0.05) from global RNA-seq expression profiling that increased by expression of at least 50 fold associated with elimination of chimeric partners. These include an IL-17 family member (IL-17C), the phagocytic receptors (MerTK, AXL), and programmed cell removal factors (CALR, MFGE-8). To probe the role of specific receptors we used in vitro studies. Recombinant Il-17c promoted allogeneic cytotoxicity while MFGE-8, a phosphatidylserine binding protein, induced engulfment of apoptotic cells. Studies using chimeric phosphatidylserine binding-cytokine fusion proteins are underway. Preliminary data suggests these fusion proteins promote inflammatory turnover of apoptotic cells. These studies provide new insights by which reverse engineering cell death programs can be utilized to enhance anti-tumor immunity.