Cellular and molecular waypoints along the path of T cell exhaustion

Abstract

Thirty years of foundational research investigating molecular and cellular mechanisms promoting T cell exhaustion are now enabling rational design of T cell-based therapies for the treatment of chronic infections and cancer. Once described as a static cell fate, it is now well appreciated that the developmental path toward exhaustion is composed of a heterogeneous pool of cells with varying degrees of effector potential that ultimately converge on a terminally differentiated state. Recent description of the developmental stages along the differentiation trajectory of T cell exhaustion has provided insight into past immunotherapeutic success and future opportunities. Here, we discuss the hallmarks of distinct developmental stages occurring along the path to T cell dysfunction and the impact of these discrete CD8⁺ T cell fates on cancer immunotherapy.

Figure 1.. A progressive model depicting the stages of CD8⁺ T exhaustion.

CD8⁺ T cell differentiation post-activation is a process whereby T cells acquire specialized functions that are adapted to the duration of antigen (Ag) exposure through the transitioning of a continuum of cellular states. The first major bifurcation in CD8⁺ T cell developmental path is to become a terminal effector, a committed cell fate with little differentiation potential (1), versus becoming a precursor T cell that retains the developmental plasticity to further differentiate into long-live memory T cell (Tmem) (during acute Ag exposure) or exhausted progenitor T cell (Tpex) (during chronic Ag exposure) (2). The adaptation of CD8⁺ T cells to chronic stimulation starts from Tpex by stably expressing Tox and PD-1 promoting cell survival while enforcing progeny T cells with dampened proliferative and killing capacity. In contrast, the progenies of Tmem have heightened effector potential. The fates of Teff-like cells derived from Tpex at later chronic infection stages are determined by the presence of CD4⁺ T cells. In the absence of CD4⁺ T cells, Tpex-derived Teff-like subset (Tim3⁺CD101⁻) is a transitory state between Tpex and terminal Tex (3). However, CD4⁺ T cells can divert effector-like (CX3CR1⁺) T cells to a terminally differentiated state with superior cytotoxicity (4). Both Teff-like subsets serve as the source of cytotoxicity of the exhausted T cell pool. This overview of CD8⁺ T cell differentiation provides a rationale for therapeutically targeting T cell exhaustion in chronic infection and cancer, among which major directions are diverting T cells away from exhaustion lineage into more potent effectors, preserving Tpex and/or transitory effectors to delay T cell terminal exhaustion, and reprogramming exhausted T cells after fate commitment.

Figure 2.. Anatomical partitioning of exhausted T cell subsets.

In chronic viral infection, Tpex are maintained in the white pulp of spleen by cDC1 and redirected through CXCR3 and CXCL9/10 signaling to the red pulp where they undergo further differentiation into transitory effectors and ultimately terminal exhausted T cells. The distribution of distinct exhausted subsets also varies across tissues. CX3CR1⁺ effector-like subsets are mostly found in the lung, spleen and blood while terminal exhausted subsets (CX3CR1⁻CXCR6⁺) are enriched in the liver, where the phenotypes of antigen-specific T cells are the most homogeneous. In the setting of tumors, Tpex are generated and maintained in tumor-draining lymph nodes, which serve as a reservoir that sustains intratumoral Tpex. A critical entry gate to extravasate into tumors is via tumor-associated high endothelial venules (TA-HEV). Once arriving at tumor sites, CD8⁺ T cells interact with a variety of antigen-presenting cells and tumor-associated macrophages (TAM). Specifically, APC-T cell zones can be found in tumors and are critical for the maintenance of tumor-infiltrating Tpex. CCR7⁺DCs are indispensable for the survival of CXCR6⁺ effectors in tumors. Exhausted T cells can secret chemokines such as CCL3/4/5 to actively recruit monocytes and shape their differentiation into suppressive TAM, promoting the terminal exhaustion of CD8⁺ T cells. CXCR4⁺ Tpex can exit tumor via tumor-associated lymphatic vessels (TA-LV) and traffic to tumor-draining lymph nodes.

Figure 3.. Translating epigenetic programming into cancer immunotherapies.

During the terminal differentiation of naïve or stem-like progenitor into effectors, CD8⁺ T cells acquire de novo epigenetic programs where stemness-associated loci are repressed through DNA methylation and chromatin remodeling. Similarly, effector-associated loci are suppressed during the development of exhaustion. Blocking the acquisition of the epigenetic programs limiting T cell stemness and effector potential is a major therapeutic opportunity. In adoptive cell therapy approaches, autologous or allogenic T cells can be genetically engineered or modified using chemicals that disrupt the catalytic function of epigenetic enzymes. Examples of ‘epigenetic blockade’ of exhaustion include deletion of Dnmt3a or Tet2 to maintain progenitor T cells, which results in heightened tumor control. LSD1 deletion in LCMV-specific CD8⁺ T cells also promotes enrichment of Tpex and a better response to anti-PD-1. Inhibiting Suv39h1 activity or its deletion enhances effector capacity and tumor killing. cBAF components, Arid1a and Smarcd2 also contribute to exhausted CD8⁺ T cell differentiation and could represent therapeutic targets.