CD8+ T Cell Exhaustion in Cancer

Abstract

A paradigm shift in the understanding of the exhausted CD8⁺ T cell (T_ex) lineage is underway. Originally thought to be a uniform population that progressively loses effector function in response to persistent antigen, single-cell analysis has now revealed that CD8⁺ T_ex is composed of multiple interconnected subpopulations. The heterogeneity within the CD8⁺ T_ex lineage is comprised of immune checkpoint blockade (ICB) permissive and refractory subsets termed stem-like and terminally differentiated cells, respectively. These populations occupy distinct peripheral and intratumoral niches and are characterized by transcriptional processes that govern transitions between cell states. This review presents key findings in the field to construct an updated view of the spatial, transcriptional, and functional heterogeneity of anti-tumoral CD8⁺ T_ex. These emerging insights broadly call for (re-)focusing cancer immunotherapies to center on the driver mechanism(s) underlying the CD8⁺ T_ex developmental continuum aimed at stabilizing functional subsets.

Keywords: CXCR3; PD-1/PD-L1; T cell exhaustion; T cell trafficking; cancer immunotherapy; co-stimulatory/inhibitory receptors; stem-like CD8+ T cells; tumor immunity.

Figure 1

Antigen load differentially influences CD8⁺ T cell memory and exhaustion fates. CD8⁺ T cell differentiation during acute infection versus chronic infection and cancer. (A) Activation of naïve CD8⁺ T cells during acute infection leads to SLEC and MPEC differentiation. Upon antigen clearance, SLECs undergo apoptosis while MPECs survive and differentiate into long-lived, self-renewing memory CD8⁺ T cells. (B) With chronic infection and cancer, SLEC specific to peptides of high MHC I affinity develop and prematurely die while the MPEC subset does not form. Instead of memory formation, CD8⁺ T cells against peptides of low MHC I affinity expand, exhaust (in a unidirectional PD-1^lo to PD-1^hi transition), and die in a continued stalemate against persistent antigen.

Figure 2

Spatiotemporal organization of early versus late stages of tumor-mediated CD8⁺ T cell dysfunction. (A) Naïve CD8⁺ T cell priming against tumor antigen in peripheral LNs (or intratumoral TLS, not depicted) results in the formation of a stem-like PD-1^loCD8⁺ T cell population with self-renewing properties. (B) This population represents an active reservoir of cells that can give rise to effector-like PD-1^loCD8⁺ T_ex after chemokine-mediated trafficking to and positioning within the TME via CCL5 and CXCL9. (C) However, persistent antigen load in the TME eventually forces continued differentiation of these cells into terminally dysfunctional PD-1^hiCD8⁺ T_ex. The PD-1^hi state is accompanied by heightened co-inhibitory receptor expression (including Tim-3, Lag-3, CD160, 2B4, TIGIT, and CTLA-4) and progressive loss of effector functions. Once CD8⁺ T_ex enter a PD-1^hi state, epigenetic enforcement prevents de-differentiation back to functional stem-like and effector-like PD-1^lo states. Anti-tumoral responses facilitated by ICB (e.g., anti-PD-1) arise from expansion from only lymphoid or intratumoral PD-1^loCD8⁺ T_ex subsets. The functionally inferior, ICB-resistant PD-1^hiCD8⁺ T_ex fate ultimately culminates in apoptosis.

Figure 3

T-bet and Eomes partitioning during CD8⁺ T cell priming and expansion. (A) The orientation and strength of TCR/MHC ligation, co-stimulation (e.g., CD28 interaction with CD80 and CD86), CD4⁺ T cell help (CD40L/CD40 licensing of DCs including up-regulation of MHC I, CD80/CD86, CD70, and third signal cytokines), autocrine IL-2 exposure, and innate inflammatory stimuli (danger- and pathogen-associated molecular patterns) all influence the activation, survival, and differentiation of naïve CD8⁺ T cells. (B) CD8⁺ T cells integrate these input events at priming and during the first division. The uneven partitioning of T-bet and Eomes favors SLEC (effector) versus MPEC (memory) differentiation early after activation, respectively. In contrast, the CD8⁺ T_ex lineage requires both transcription factors and retains some features of memory cells including self-renewal of PD-1^lo subsets and expression of memory-associated transcription factors and survival molecules. The reliance on homeostatic cytokines (predominantly IL-7) versus persistent antigen for development and self-renewal distinguishes memory from PD-1^lo/hi exhaustion lineages, respectively.

Figure 4

The CD8⁺ T cell exhaustion lineage is comprised of a continuum of transcriptionally and epigenetically controlled states. (A) Activation of naïve CD8⁺ T cells for SLEC and MPEC/memory differentiation is optimally driven by transcription factors such as NFAT/AP-1 and sufficient co-stimulation (e.g., CD27). Early development of stem-like CD8⁺ T_ex precursors instead involves partnerless NFAT, lack of co-stimulation/help, constant Nur77 activity, and/or strict dependence on BACH2. These events appear to be stabilized by TCF-1 activity and PD-1 dampening of chronic TCR ligation. (B, C) TCF-1 further supports stemness (ability to survive, self-renew, and proliferate) by promoting Eomes, ID3, c-Myb, Bcl-2, and Bcl-6 expression while antagonizing effector-associated transcription factors including Blimp-1, ID2, RUNX3, and T-bet. Stem-like CD8⁺ T_ex precursors (B) and progenitors (C) are collectively marked by a PD-1^loTim-3⁻Slamf6⁺ surface profile and varied expression of CXCR3 and CXCR5 in specific tumor settings. Although equally stabilized by TCF-1, precursors can be distinguished from progenitors as being quiescent, LN-resident, less reliant on antigen, and having a CD69⁺KLRG-1⁺Ki-67⁻ profile. (C, D) TCF-1 down-regulation coupled with ongoing exposure to persistent antigen drives constant BATF and IRF4 signaling (which positively feedback on partnerless NFAT activity) and T-bet expression. T-bet additionally overrides a TCF-1 memory-like program by supporting Blimp-1 and ID2 activity leading to an effector-like transitory PD-1^loTim-3⁺ state marked by initiation of granzyme B production. (D, E) Continued NFAT activity ultimately leads to TOX upregulation within this subpopulation, which epigenetically enforces terminal exhaustion, inhibits T-bet-mediated effector programming, and promotes heightened PD-1 expression. Transitory cells (D) are discriminated from terminally dysfunctional cells (E) by a PD-1^loTim-3⁺CD101⁻KLRG-1⁺CX3CR1⁺ surface phenotype, remnant IFN-γ and TNF production, and having high proliferative potential. Terminally dysfunctional PD-1^hiTim-3⁺CD8⁺ T_ex co-express multiple co-inhibitory receptors (not depicted), cannot proliferate, have diminished polyfunctionality, but retain granzyme-based cytolytic potential. PD-1^lo precursors, progenitors, and transitory CD8⁺ T_ex subpopulations are amenable to ICB (B–D), whereas terminally dysfunctional PD-1^hiCD8⁺ T_ex are not (E). Precursors and progenitors may interconvert, whereas differentiation into transitory and terminally dysfunctional subsets is unidirectional.