Susan Kaech, PhD

What are the decisive factors that determine which effector cells survive to become long-lived memory cells and which cells die during the contraction phase? We have characterized the transcriptome of effector and memory T cells and identified genetic pathways and several transcription factors that regulate this life or death decision in activated T cells. Our work has helped to outline a model of effector T cell differentiation wherein a small subset of T cells develop into memory precursor cells that are more fit to persist following the first infection than the majority of effector cells. These memory precursor cells develop into long-lived memory T cells that protect against re-infection. Several types of memory T cells, which differ by their phenotypes, functions and anatomical locations, are produced to create a sophisticated, multi-layered defense system. Conceptually, the memory T cells are divided into three subsets: (1) Tissue resident memory T (T_RM) cells, which locally reside in mucosal tissues to provide the front line of defense against pathogens that breach our barriers; (2) Effector memory T (T_EM) cells, which circulate through the blood and tissues and are rapidly recruited to the sites of inflammation upon reinfection; and (3) central memory T (T_CM) cells, which circulate through the blood and lymphoid organs to produce a second wave of effector T cells upon reinfection. T_CM cell populations also contain multipotent “memory stem cells" that self-renew to sustain the memory T cell population over time.

Despite this general knowledge, we lack a deep understanding of how different types of memory T cells are generated during an immune response and persist thereafter to provide protective immunity upon reinfection. Such knowledge will have a significant impact on the development of vaccines and immunotherapies to fight infectious disease, cancer, and autoimmunity. My lab has spent the last decade elucidating regulatory pathways that control whether an effector T cell lives and adopts memory cell fates or terminally differentiates into shorter-lived effector cells and dies. These discoveries have shown that memory T cell fate determination is influenced by environmental cues and a balance of inflammatory and anti-inflammatory cytokines. Currently, my lab is trying to understand how signals in the tissue microenvironment and nutrient availability governs changes in gene expression, epigenetic remodeling and memory T cell metabolism that regulate the types of memory T cells that form and their homeostasis following infection (Figure 1). Work in this area will not only reveal basic principles in the generation and maintenance of memory T cells, but will also enhance our understanding of broad biological principles in tissue and tissue stem cell homeostasis, tumor microenvironments, and control of cellular metabolism by environmental conditions. Currently, we are focusing on several fundamental questions surrounding the development of protective memory T cells to fight infectious disease and cancer.

First, little is known about how tissues specify memory T cell properties and regulate their long-term survival and homeostasis. To develop a multilayered defense system, T_CM, T_EM and T_RM cells must distribute themselves broadly and adopt tissue-specific properties dictated by their environments. We do not understand even basic aspects of how this occurs: for example, what are the relevant tissue-trophic factors and cell types that govern this? We are trying to elucidate the key components of the tissue microenvironment that direct transcriptional and epigenetic changes in memory T cells as they undergo environmental adaptation to different tissues and inflammatory conditions. This work is paramount to developing vaccines with tissue-targeting precision that protect against different routes of pathogen entry (e.g., airways, skin, genitals, and blood).

A second major gap exists in our understanding of the metabolic determinants of memory T cell longevity and self-renewal. We know very little about the metabolic and nutrient-utilization pathways that regulate survival and self-renewal of the different memory T cell subsets in various tissues. One recent breakthrough is our discovery that triglyceride storage is essential for the longevity and self-renewal of memory T cells. We are studying how lipid synthesis and storage are regulated in memory T cells by T cell growth factors and tissue environments. In addition to the relevance to T cell biology, this work will likely reveal conserved paradigms for metabolic control of stem cell longevity and self-renewal in general.

Third, our understanding of the functional and metabolic connections between T cells and tumors is in its infancy. Central goals of our work are to understand how T cells are regulated by the tumor microenvironment and determine if the metabolic rate of the tumor itself modulates that of the infiltrating T cells. We are also addressing the novel concept that nutrient competition between the T cells and tumor cells is a key component of immunosuppression in the tumor microenvironment. We will also examine an unexplored process of tumor “metabolic editing”, to determine if T cell immunosurveillance actually selects for cancer cells with higher metabolic rates and intensifies nutrient competition, paradoxically promoting tumor progression.

Memory CD8 T cell differentiation

Memory CD4 T cell differentiation

Chronic viral infection

Tumor Immunology and Immunotherapy