CD8+ T cells are critical responders to acute infections, chronic infections, and cancer. Upon specific recognition of an intracellular pathogen or neoantigen, the responsive CD8+ T cells differentiate into a functionally diverse pool. Why a given cell adopts one cell fate or another in response to infection has been a major focus of the field but is incompletely understood. Mechanistic understanding of cell fate decisions could lead to enhanced patient immunity during infections and cancer alike.

Recent work has started to recognize layers of heterogeneity within the antigen-inexperienced CD8+ T cell pool. One such example is the discovery of Virtual Memory (VM) CD8+ T cells, which contrast in phenotype and function with True Naïve (TN) CD8+ T cells. Although both subsets are antigen-inexperienced, VM cells mount a remarkably rapid response to acute infection and in fact protect from pathogens similarly to bona fide memory CD8+ T cells. Why and how VM cells act as first-responders is unknown. In chapter 2, I asked whether VM cells were optimally positioned in the lymph node to respond to acute infection. I discovered that VM cells are recruited to the periphery of the draining LN more rapidly than their TN counterparts and produce robust amounts of IFNg in a CXCR3-dependent manner. This work is the first to link a naïve CD8+ T cell subset with unique lymphoid positioning, and thus challenges our traditional understanding of the lymph node “T cell zone” by introducing spatially organized regions where T cell priming occurs.

An additional source of heterogeneity among antigen-inexperienced CD8+ T cells is the stage of life at which the T cell is made. Our lab has published extensive work on how fetal-derived and neonatal CD8+ T cells respond more quickly to acute infection than their adult-derived counterparts, and preferentially differentiate into highly cytotoxic effectors. However, whether these unique cell subsets differ in their response to chronic stimulation was unknown before my doctoral research. In chapters 3 and 4, I asked how neonatal CD8+ T cells and developmental origin, respectively, contribute to cell fate decisions after chronic infection. I found that neonatal CD8+ T cells resist terminal exhaustion, and that fetal-derived CD8+ T cells may preferentially adopt an effector fate in a tissue-specific manner. This work introduces a new framework to understand the heterogenous cell states that emerge during chronic infection and cancer.

By investigating how cell-intrinsic CD8+ T cell identity influences cell fate decisions, I have contributed a novel perspective to explain diverse patient responses to infections and cancer. Ultimately, my doctoral research contributes to our conceptual understanding of CD8+ T cell behavior, clarifies our understanding of neonatal immunity, and provides inspiration to study how unique CD8+ T cell subsets could be used as novel cell-based immunotherapies.