Stem, Effector, and Hybrid States of Memory CD8+ T Cells

Abstract

CD8⁺ T cell immunological memory of past antigen exposure can confer long-lived protection against infections or tumors. The fact that CD8⁺ memory T cells can have features of both naïve and effector cells has forced the field to struggle with several conceptual questions about the developmental origin of the cell and, consequently, the mechanism(s) that contribute to memory development. Here, we discuss recent conceptual advances in our understanding of memory T cell development that incorporate data describing a hybrid stem and/or effector state of differentiation. We theorize that the mechanisms involved in developing these cells could be mediated, in part, through epigenetic programs. Finally, we consider the potential therapeutic implications of inducing and/or utilizing such hybrid cells clinically.

Keywords: CD8+ T cells; TCF1; cancer immunotherapy; effector; exhaustion; memory; naïve.

Figure 1.. Long-lived T cell compartments in physiology and chronic stimulation.

Under healthy homeostatic conditions long-lived memory CD8⁺ T cells are endowed with a strong proliferation potential. Upon secondary encounter with cognate antigen these cells can rapidly re-elicit effector functions that include granzyme (GZM) and cytokine production. Multipotent memory CD8⁺ T cells express molecules that are also expressed in naïve T cells such as CD62L and CCR7. Additionally, they can express TCF1 and LEF1 that serve with BCL6 to enable self-renewal and long-term persistence. Under chronic stimulation (i.e. cancer, chronic infections) progenitor exhausted T (T_EX) cells are generated. These cells have a strong proliferative potential that is fully unleashed upon ICB. TCF1⁺ progenitor T_EX are poised for rapid cytokine production and express a mixed phenotype of effector cells (lack of CCR7 and expression of GZMs) and exhausted cells (mainly expressing PD-1), along with the transcription factor TOX governing their development and survival among the pool of T_EX cells. Progenitor T_EX have also been identified as CXCR5⁺, a homing receptor for the lymph nodes’ B cell zone.

Figure 2.. Epigenetic regulation of CD8⁺ terminal effector (TE) and memory-precursors effector cell (MPEC) differentiation.

Following activation by a professional APC, naïve CD8⁺ T cells undergo epigenetic reprogramming of genes related to stemness such as Tcf7 (TCF1) and Lef1, and tissue homing such as Sell (CD62L) and Il7R. These epigenetic changes include DNA methylation and demethylation events and an array of histone modifications. Inhibition of de novo DNA methylation or histone methylation restricts the progression of early effector cells into terminally differentiated T cells and/or increases the multipotency of MPECs. Deletion of Dnmt3a, a de novo methyltransferase, prior to effector differentiation has resulted in heightened re-expression of naïve-associated genes during the antigen-independent stage of memory CD8⁺ T cell differentiation in mice [67]. Genetic manipulation of H3K9me3 by Suv39h1 knockout (KO) in mice has resulted in increased expression of stem-like genes relative to wild type (WT) murine CD8⁺ T cells [66]. Likewise, manipulation of the polycomb repressive complex 2 (PRC2) has been demonstrated to promote increased skewing of cells towards an MPEC phenotype in mice following acute lymphocytic choriomeningitis virus (LCMV) infection [64]. However, conflicting reports of EZH2 on memory formation do exist [65]. Taken together, these studies highlight the role epigenetic programming plays in regulating the plasticity of CD8⁺ T cells during effector and memory differentiation.

Figure 3.. Potential therapeutic approaches for stem-effector T cell generation in the treatment of chronic diseases such as cancer.

A hypothetical therapeutic approach to treating certain cancers is to utilize DNA demethylating agents to target epigenetic programs that induce a de-differentiated pool of tumor-specific endogenous or adoptively-transferred T cells. These agents could be used to either create a uniform pool of stem-effector hybrid cells or a heterogeneous population of mixed progenitor and terminal effector cells capable of exerting anti-tumor functions after reinfusion into cancer patients.

Figure 4.. Manipulation of epigenetic programming could result in improved T cell therapeutic potential against tumors.

Prolonged exposure of T cells to their cognate antigen during chronic viral infection or cancer promotes the development of terminal exhaustion programming. These programs are reinforced by epigenetic modifications and result in a reduction in the T cell’s proliferative and cytotoxic potential and limit the T cell’s response to immune checkpoint blockade therapy (ICB). Inhibiting these epigenetic programs can preserve an ICB responsive population of T cells expressing TCF1, and has resulted in improved viral control in pre-clinical murine studies [88]. Treatment with the DNA demethylating agent, decitabine (DAC), can partially recapitulate the overall genetic deletion phenotype in mice and result in improved T cell responsiveness to ICB relative to controls [88]. Hypothetically, these strategies might be utilized to provide epigenetically permissible ICB-responsive CD8⁺ T cells that can lead to improved tumoral clearance.