Immunoregulatory functions of mTOR inhibition

Abstract

The potent immunosuppressive action of rapamycin is commonly ascribed to inhibition of growth factor-induced T cell proliferation. However, it is now evident that the serine/threonine protein kinase mammalian target of rapamycin (mTOR) has an important role in the modulation of both innate and adaptive immune responses. mTOR regulates diverse functions of professional antigen-presenting cells, such as dendritic cells (DCs), and has important roles in the activation of effector T cells and the function and proliferation of regulatory T cells. In this Review, we discuss our current understanding of the mTOR pathway and the consequences of mTOR inhibition, both in DCs and T cells, including new data on the regulation of forkhead box P3 expression.

Figure 1. mTORC1 (mammalian target of rapamycin complex 1) and mTORC2 signalling pathways

mTORC1 is the direct target of the rapamycin–FK506-binding protein 1A, 12 kDa (FKBP12) complex and regulates cell growth and size by controlling translation, ribosome biogenesis and autophagy. Diverse signals, arising from growth factors, such as insulin and Fms-like tyrosine kinase 3 ligand (FLT3L), various cytokines, ligated costimulatory molecules and antigen receptors, WNT (Wingless and Integrase-1) proteins, and the relative cellular energy and oxygen levels, determine mTORC1 activity as a result of their effects on the tuberous sclerosis complex 1 (TSC1)–TSC2 complex, which is the main negative regulator of mTORC1. Activation of Ras–MAPK (mitogen-activated protein kinase) and phosphatidylinositol-3-kinase (PI3K)–AKT results in inhibitory phosphorylation of TSC2 and removes repression of Rheb (Ras homologue enriched in brain), which is the mTORC1 stimulator. Activation of AKT by PI3K is negatively regulated by phosphatase and tensin homologue (PTEN). Activated mTORC1 promotes translation through stimulating S6K1 (p70 ribosomal protein S6 kinase 1) and inhibiting 4EBP1 (eIF4E binding protein 1). Activated S6K1 can also feed back to negatively regulate input from PI3K-AKT by facilitating the degradation of signaling intermediates between surface receptors (such as the insulin receptor) and PI3K. Low energy and nutrient levels, as well as hypoxic conditions, increase TSC1–TSC2-mediated inhibition of mTORC1 through input from GSK3 (glycogen synthase kinase 3) and AMPK (AMP-activated protein kinase). mTORC2 is not inhibited directly by rapamycin, but long-term rapamycin administration disrupts its assembly in some cells. mTORC2, activated by PI3K, directly phosphorylates AKT. mTORC2 also regulates cytoskeleton dynamics. PDK1,= phosphoinositide-dependent kinase 1.

Figure 2. mTOR and rapamycin regulate APC function

a | Several changes are observed in antigen acquisition and presentation by antigen–presenting cells (APCs) exposed to or differentiated in rapamycin. Rapamycin inhibits endocytosis and phagocytosis, and the expression of antigen uptake receptors and MHC class II molecules, and disrupts dendritic cell aggresome-like structures (DALIS), although cross-presentation of exogenous antigen on MHC class I molecules might not be affected. Rapamycin facilitates autophagy, an evolutionarily conserved process and could thereby influence self-antigen presentation. Thus, mTOR has a role in regulation of immune responses at an early stage, influencing how exogenous and endogenous antigen is acquired, processed and presented. b | Interruption of mTOR signalling in monocytes, macrophages and dendritic cells (DCs) during Toll-like receptor (TLR) ligation results in increased IL-12 production by decreasing IL-10 production and derepressing NF-κB. This indicates that mTOR is a crucial mediator of T-cell polarization and immune responses. c | In plasmacytoid DCs (pDCs), coordinated signalling through TLR9 and phosphatidylinositol-3-kinase (PI3)K–mTOR is needed to drive type-1 IFN-α/β production. S6K phosphorylation by mTORC1 promotes the interaction of MyD88–TLR9–IFN regulatory factor (IRF)7 and the subsequent translocation of phosphorylated IRF7 to the nucleus to initiate transcription of the genes encoding type-1 IFN. mTOR has been reported to interact with MyD88 and positively regulate cytokine production mediated by IRF5 and IRF7. d | Differentiation of DCs in rapamycin generates DCs with weak T-cell stimulatory capacity, but intact/improved homing to CCL21. mTOR is a negative regulator of caspase-1 and treatment with rapamycin therefore promotes IL-1β production by caspase 1. IL-1β subsequently induces expression of ST2, which sequesters MyD88 and negatively regulates TLR4 and TLR9 signalling. Failure of rapamycin to impede AKT signalling might allow/promote CCR7 activity and DC migration to CCL21.

Figure 3. Proposed molecular mechanisms responsible for T-cell susceptibility or resistance to rapamycin

a | In conventional T cells, mTOR integrates the TCR and CD28 signals that are necessary to pass two checkpoints of T-cell activation. The transition from G0 to G1 phase of the cell cycle (checkpoint 1) requires activation of NFAT, MAPKs and NF-κB. The activity of NF-κB is controlled by the PI3K–AKT–mTOR pathway. mTOR works as part of the complexes TORC1 and TORC2, of which the former is susceptible to inhibition by rapamycin. The coordinated activity of NFAT, AP1 and NF-κB regulates multiple genes involved in cell-cycle progression and the expression of IL-2 and its high-affinity receptor. IL-2R signals through the PI3K–AKT-mediated activation of mTOR complexed with survivin and aurora B, which regulates G1- to S-phase progression (checkpoint 2). In addition, mTOR activity is involved in rapamycin-sensitive down-regulation of the transcription factor Kruppel-like factor 2 (KLF2), which controls expression of lymphoid tissue-homing molecules to ensure that activated T cells can exit the lymph nodes. b | The ability of T_Reg cells to proliferate when stimulated in the presence of rapamycin seems to be connected to two effects. First, T_Reg cells do not down-regulate phosphatase and tensin homologue (PTEN) expression after TCR engagement, which impedes the activation of the rapamycin-susceptible PI3K–AKT–mTOR pathway. Second, FOXP3 drives expression of phosphatidylinositol mannoside (PIM)2, reinforced by IL-2- and TCR-mediated activation of signal transducer and activator of transcription (STAT)5, which compensates for AKT inactivity and promotes cell-cycle progression.

Figure 4. Mechanisms of FOXP3 induction in naïve T cells

a | Stimulation of FOXP3⁻ T cells through TCR and CD28 in the presence of TGFβ promotes expression of the FOXP3 gene through the cooperation of NFAT and mothers against decapentaplegic homologue 3 (SMAD3). This process is counteracted by mTOR activation, which explains the increased expression of FOXP3 when stimulation takes place in the presence of rapamycin (which inhibits mTOR activation). b | Limited TCR–CD28 stimulation (<18 hours) promotes a PI3K–AKT–mTOR-mediated re-organization of chromatin that includes increased accessibility to the FOXP3 gene. Prolonged TCR/CD28 stimulation prevents, again through activation of the PI3K–AKT–mTOR pathway, the expression of FOXP3 which would otherwise probably be induced by signal transducer and activator of transcription (STAT)5-activating cytokines generated during the initial stimulation. This two-step model rationalizes the opposing effects of rapamycin administration during T-cell activation that have been observed, which probably depend on the timing of mTOR inhibition.