Regulation of CD8+ T memory and exhaustion by the mTOR signals

Abstract

CD8⁺ T cells are the key executioners of the adaptive immune arm, which mediates antitumor and antiviral immunity. Naïve CD8⁺ T cells develop in the thymus and are quickly activated in the periphery after encountering a cognate antigen, which induces these cells to proliferate and differentiate into effector cells that fight the initial infection. Simultaneously, a fraction of these cells become long-lived memory CD8⁺ T cells that combat future infections. Notably, the generation and maintenance of memory cells is profoundly affected by various in vivo conditions, such as the mode of primary activation (e.g., acute vs. chronic immunization) or fluctuations in host metabolic, inflammatory, or aging factors. Therefore, many T cells may be lost or become exhausted and no longer functional. Complicated intracellular signaling pathways, transcription factors, epigenetic modifications, and metabolic processes are involved in this process. Therefore, understanding the cellular and molecular basis for the generation and fate of memory and exhausted CD8⁺ cells is central for harnessing cellular immunity. In this review, we focus on mammalian target of rapamycin (mTOR), particularly signaling mediated by mTOR complex (mTORC) 2 in memory and exhausted CD8⁺ T cells at the molecular level.

Keywords: CD8+ T cell; Sin1; T-cell exhaustion; T-cell memory; mTOR.

Fig. 1

mTORC1/mTORC2 integrate diverse extracellular cues. Upon stimulation by amino acids, GATOR2 inhibits GATOR1, leading to a conformational change in Regulator. This change affects the GTP/GDP state of RagA/RagC and facilitates the recruitment of mTORC1 to the lysosome surface, where it becomes activated. Metabolic stress, such as low glucose, can inhibit mTORC1 through Rag-GTPase-dependent or AMPK-dependent mechanisms. Active AMPK directly phosphorylates Raptor or indirectly phosphorylates and activates TSC2 to inhibit mTORC1. Once mTORC1 is activated, downstream kinases S6K and translation initiation factor 4E-BP1 are phosphorylated, collectively promoting protein synthesis. To prevent futile metabolism, the autophagy activator ULK1 is phosphorylated by active mTORC1, thereby inhibiting autophagy initiation. On the other hand, mTORC2 has been primarily recognized as downstream integrators of insulin stimulation. Various immune-related signaling pathways, including TCR-signaling, CD28-mediated co-stimulatory signaling, and cytokines, have been identified to modulate mTORC2 activity. AGC kinase family members serve as the primary effectors for mTORC2. As a multifunctional kinase, active Akt phosphorylated by mTORC2 can also mediate mTORC1 activity by either blocking the inhibitory effects of the raptor-binding protein PRAS40 on mTORC1 or dissociating the TSC complex from the lysosomal surface, thereby enabling Rheb-mediated mTORC1 activation

Fig. 2

The role of the mTOR signaling pathway in the differentiation of memory/exhaustion T-cell subsets. A During acute infection, virus-specific naïve CD8⁺ T cells differentiate into SLECs that display potent cytotoxicity and MPECs that have a greater capacity to form memory cells following viral clearance. After the antigen has been eliminated, a minority of SLECs manage to survive and transform into LLECs. On the other hand, MPECs develop into various types of memory T cells, including Tem, Tcm, and Trm. mTORC1 activity instructs SLEC and Tem differentiation. Inhibition of mTOR, either by Rapamycin treatment or through siRNA-mediated knockdown, promotes MPEC and memory formation, particularly for Tcm and human Tscm. However, mTOR activation in T cells promotes their differentiation into Trm cells and enhances their survival in peripheral tissues. B During chronic infection or cancer, virus-specific naïve CD8⁺ T cells are activated and segregate into early effector cells and precursor cells. The precursor cells develop into Progenitor Tex, which further differentiate into terminally exhausted cells and effector-like cells marked by CX₃CR1 expression. Inhibition of mTOR activity enhances Progenitor Tex formation at both the early and late stages. However, Progenitor Tex cells retain the ability to activate the mTOR pathway in response to antigen receptor signals, and mTOR is required for the transition of Progenitor Tex cells into exhausted and effector cells in the chronic phase. MPEC memory precursor effector cells, SLEC short-lived effector cells, LLEC long-lived effector cells, Tem effector memory T-cell, Tcm central memory T cells, Tscm T memory stem cells, Trm resident memory T cells, Progenitor Tex progenitor exhausted T-cell

Fig. 3

Composition of mTORC1/2 core subunits and selected substrates. A Main protein domains and phosphorylation sites of mTORC1 core subunits and selected substrates. mTOR and mLST8 are shared subunits of mTORC1/2, while Raptor and Rictor/Sin1 are the defining subunits for mTORC1 and mTORC2, respectively. S6K is phosphorylated by mTORC1 at S371 (TM) and T389 (HM) in the linker region. 4E-BP utilizes the TOS and RAIP motif to interact with Raptor/mTORC1 and is sequentially phosphorylated by mTORC1 at T37/T46 and S65/T70. ULK1 can be phosphorylated on S757. B Main protein domains and phosphorylation sites of mTORC2 core subunits and selected substrates. MLST8 interacts with Sin1 to position its substrate-interacting CRIM domain, providing substrate specificity of mTORC2. Sin1/mTORC2 phosphorylates T450 (turn motif) and S473 (hydrophobic motif) in the C-tail of Akt1. This dual phosphorylation has also been observed in PKC, while for SGK1, S422(HM) is the only known site phosphorylated by mTORC2. HEAT repeat found in Huntingtin, elongation factor 3 (EF3), protein phosphatase 2A (PP2A), and the yeast kinase TOR1, FAT FAK focal adhesion targeting, FRB FKBP-rapamycin-binding, FATC FRAP-ATM-TRRAP-C-terminal, RNC Raptor N-terminal conserved, NTD N-terminal domain, TOS TOR signaling, RAIP Arg-Ala-Ile-Pro motif, ARM Armadillo, HD HEAT-like domain, CD C-terminal domain, CRIM conserved region in the middle, RBD Ras-binding domain, PH Pleckstrin homology, PS pseudosubstrate, C1,C2 membrane targeting module

Fig. 4

The role of the mTOR signaling pathway in metabolic programs and differentiation of CD8 T-cell subsets. A The mTORC2-Akt pathway promotes FOXO1 phosphorylation, resulting in decreased nuclear accumulation of FOXO1. Nuclear FOXO1 promotes memory formation through the Wnt/TCF1 pathways, directly binds and suppresses AP-1 transcription factors that are known to be key regulators of effector programs, induces KLF2 expression to regulate homeostatic trafficking, and sustains PD-1 expression while inducing the terminal differentiation of exhausted CD8⁺ T cells during chronic infection. B Naïve T cells uptake low levels of glucose and amino acids and rely on mitochondrial oxidative phosphorylation (OXPHOS). Upon T-cell activation, effector CD8⁺ T cells require high levels of glucose metabolism to support their rapid proliferation and production of cytokines and cytotoxic molecules. During an immune response, effector cells undergo a metabolic switch from OXPHOS to glycolysis. In contrast, memory CD8⁺ T cells have a more quiescent metabolism and rely more on OXPHOS for energy production. Memory cells also exhibit higher levels of fatty acid oxidation. mTORC1 activity is required to sustain high levels of glycolysis in effector T cells in both acute and chronic infections. Inhibition of mTORC2 activity, on the other hand, enhances the metabolic capacity of CD8⁺ T cells. C Tex have been reported to exhibit metabolic insufficiency with suppressed oxidation and glycolysis. Early progenitor Tex cells exhibit and retain a catabolic metabolism characterized by mitochondrial fatty acid oxidation (FAO) and oxidation