Identification and single cell analysis of embryonic lymphoid progenitors that generate neonatal innate T cells

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Project Summary Certain infections during pregnancy are linked to developmental and behavioral abnormalities in the offspring. Whether overt inflammatory responses in the mother can have a lasting impact on the development of immune system of the offspring is unknown. This gap in knowledge is directly linked to a lack of detailed insights into how lymphocytes normally develop in the fetus. How animals generate multiple immunocyte subtypes from fetal to aged stages remains an active area of research with many unresolved fundamental questions. In particular, it is unknown whether the immune system is a) one dimensional, a collective of diverse cell types generated from a single stem cell or is b) multi-layered, with each layer made of functionally specialized cell systems tailored to the distinct developmental age of an animal. We discovered that skin lymphocytes (immune sentinels) essential to prevent dermatitis originate from progenitors with dedicated gene programs that only develop in embryos. Importantly, data further suggest that the unique genetic networks of immune sentinels are active in fetal tissues prior to the emergence of a single hematopoietic stem cell (HSC) in the fetal liver. It has been assumed that fetal HSCs are the primary stem cell for all lymphocytes. Our results thus suggest the existence of undiscovered embryonic innate lymphoid progenitors (eILPs) distinct from classical HSCs or their immediate daughter cells primed toward the lymphoid lineage. We plan to identify and characterize eILP subtypes by employing a spectrum of molecular beacons, each embedded in the genome and reporting the activity of predicted gene network hubs of eILPs. Rare cells in the fetus with a specified combination of beacons will be captured and these candidate eILPs will be analyzed molecularly at a single cell level and transplanted into animals to determine their generative potential. Candidate eILPs are predicted to preferentially generate mucosal immune sentinels in fetal and neonatal animals, and once these sentinels are made they persist long term, well into adulthood. Absence or alterations of these innate sentinels results in aberrant tissue homeostasis and inflammatory disorders. Once the embryonic hematopoietic lineage tree is constructed how immune perturbations in pregnancy impact the development of innate lymphocytes can be systematically assessed.