PhD student Cornell Univ. Col. of Vet. Med. Ithaca, New York, United States
Disclosure(s):
Isabel M. Forlastro: No financial relationships to disclose
Introduction/Rationale: CD8+ T cells produced in early life exhibit distinct phenotypic and functional characteristics compared to those generated in adulthood. In neonates, CD8+ T cells have a propensity to differentiate into virtual memory (VM) cells, which allows for rapid, innate-effector responses to immune challenges. However, the mechanisms that bias neonatal CD8+ T cells towards a more innate-like lineage remains poorly understood.
Methods: We hypothesized that innate-like programming of CD8+ T cells in early life could be due to changes in thymic selection, TCR usage, thymic environment or production from alternative hematopoietic progenitors. To determine how each factor contributes to changes in CD8+ T development at various stages of life, we used a multitude of cellular approaches (flow cytometry, intrathymic injections, artificial thymic organoids) and sequencing assays (paired single cell TCR/RNA sequencing, methyl-seq) with thymocytes from different aged mice.
Results: We identified a distinct wave of CD8⁺ thymocytes with high Eomes expression in the neonatal thymus that was absent in adults. Single-cell RNA/TCR sequencing showed these Eomes⁺ cells formed a unique cluster with effector-like gene signatures and TCRs enriched for germline-encoded features. Intrathymic injections of fetal progenitors showed that CD8 thymocytes maintained Eomes expression even in an adult thymic environment. Lin28b expression in adult thymocytes cell-autonomously induced Eomes. Finally, multiple fetal progenitor populations, but not adult HSCs, generated VM cells, which corresponded to developmental-related changes Lin28b expression (master regulator of lymphopoiesis) and methylation of innate genes.
Conclusion: Our findings demonstrate that the bias for VM T cell development in early life is intrinsically programmed within fetal hematopoietic progenitors. This program is established during a specific developmental window and is driven by Eomes, which is sufficient to cell-autonomously promote the innate-like T cell fate.
