Reprogramming of mitochondrial metabolism by innate immunity.

The reprogramming of cellular metabolism has emerged as a major aspect of innate immune cell activation. Mitochondria, which are well known for their critical functions in cellular bioenergetics and metabolism, also serve innate immune purposes by providing specific signaling platforms. Latest advances in our understanding of innate immune receptor-mediated metabolic reprogramming have unraveled specific immune functions of mitochondrial metabolites that place mitochondrial metabolism and particularly the mitochondrial respiratory chain at the center of innate immunity. This review highlights some recent studies that support mitochondrial metabolism as major immune signaling rheostat upon microbe recognition by innate immune cells.

How Mitochondrial Metabolism Contributes to Macrophage Phenotype and Functions.

Metabolic reprogramming of cells from the innate immune system is one of the most noteworthy topics in immunological research nowadays. Upon infection or tissue damage, innate immune cells, such as macrophages, mobilize various immune and metabolic signals to mount a response best suited to eradicate the threat. Current data indicate that both the immune and metabolic responses are closely interconnected. On account of its peculiar position in regulating both of these processes, the mitochondrion has emerged as a critical organelle that orchestrates the coordinated metabolic and immune adaptations in macrophages. Significant effort is now underway to understand how metabolic features of differentiated macrophages regulate their immune specificities with the eventual goal to manipulate cellular metabolism to control immunity. In this review, we highlight some of the recent work that place cellular and mitochondrial metabolism in a central position in the macrophage differentiation program.

Priming of dendritic cells by DNA-containing extracellular vesicles from activated T cells through antigen-driven contacts.

Interaction of T cell with antigen-bearing dendritic cells (DC) results in T cell activation, but whether this interaction has physiological consequences on DC function is largely unexplored. Here we show that when antigen-bearing DCs contact T cells, DCs initiate anti-pathogenic programs. Signals of this interaction are transmitted from the T cell to the DC, through extracellular vesicles (EV) that contain genomic and mitochondrial DNA, to induce antiviral responses via the cGAS/STING cytosolic DNA-sensing pathway and expression of IRF3-dependent interferon regulated genes. Moreover, EV-treated DCs are more resistant to subsequent viral infections. In summary, our results show that T cells prime DCs through the transfer of exosomal DNA, supporting a specific role for antigen-dependent contacts in conferring protection to DCs against pathogen infection. The reciprocal communication between innate and adaptive immune cells thus allow efficacious responses to unknown threats.

Mitochondrial Complex I activity signals antioxidant response through ERK5.

Oxidative phosphorylation (OXPHOS) generates ROS as a byproduct of mitochondrial complex I activity. ROS-detoxifying enzymes are made available through the activation of their antioxidant response elements (ARE) in their gene promoters. NRF2 binds to AREs and induces this anti-oxidant response. We show that cells from multiple origins performing OXPHOS induced NRF2 expression and its transcriptional activity. The NRF2 promoter contains MEF2 binding sites and the MAPK ERK5 induced MEF2-dependent NRF2 expression. Blocking OXPHOS in a mouse model decreased Erk5 and Nrf2 expression. Furthermore, fibroblasts derived from patients with mitochondrial disorders also showed low expression of ERK5 and NRF2 mRNAs. Notably, in cells lacking functional mitochondrial complex I activity OXPHOS did not induce ERK5 expression and failed to generate this anti-oxidant response. Complex I activity induces ERK5 expression through fumarate accumulation. Eukaryotic cells have evolved a genetic program to prevent oxidative stress directly linked to OXPHOS and not requiring ROS.

The mitochondrial respiratory chain: A metabolic rheostat of innate immune cell-mediated antibacterial responses.

Upon microbial infection, cells of the innate immune system undergo profound metabolic reprogramming in order to eradicate pathogens, promote inflammation, and eventually restore tissue homeostasis. Mitochondria are at the core of these adaptations, given their dual role as metabolic hubs and innate immune signaling platforms. The mitochondrial electron transport chain (ETC) is very well characterized at the genetic, molecular, structural, and biochemical level. In contrast, the role for mitochondrial ETC and metabolites beyond fulfilling cellular ATP synthesis in innate immune cell biology was not understood until recently. Here we discuss the latest advances in our understanding of immune functions of mitochondria and particularly the mitochondrial respiratory chain.

Innate Immune Function of Mitochondrial Metabolism.

Sensing of microbe-associated molecular patterns or danger signals by innate immune receptors drives a complex exchange of information. Innate receptor signaling not only triggers transcriptional events but also induces profound changes in metabolic fluxes, redox balance, and metabolite abundance thereby influencing immune cell function. Mitochondria are at the core of metabolic adaptation to the changing environment. The close interaction between mitochondrial metabolism and immune signaling has emerged as a central regulator of innate sensing. Metabolic processes generate a constant flow of electrons that eventually end up in the mitochondrial electron transport chain (ETC). Two electron carriers and four respiratory complexes that can assemble as larger molecular supercomplexes compose the ETC in the mitochondrial inner membrane. While the meaning and biological relevance of such structural organization is a matter of passionate debates, recent data support that innate stimuli remodel the ETC. We will review the function of mitochondrial metabolism and ETC dynamics as innate rheostats that regulate signaling, transcription, and epigenetics to orchestrate innate immune responses.

Mitochondrial respiratory-chain adaptations in macrophages contribute to antibacterial host defense.

Macrophages tightly scale their core metabolism after being activated, but the precise regulation of the mitochondrial electron-transport chain (ETC) and its functional implications are currently unknown. Here we found that recognition of live bacteria by macrophages transiently decreased assembly of the ETC complex I (CI) and CI-containing super-complexes and switched the relative contributions of CI and CII to mitochondrial respiration. This was mediated by phagosomal NADPH oxidase and the reactive oxygen species (ROS)-dependent tyrosine kinase Fgr. It required Toll-like receptor signaling and the NLRP3 inflammasome, which were both connected to bacterial viability-specific immune responses. Inhibition of CII during infection with Escherichia coli normalized serum concentrations of interleukin 1ß (IL-1ß) and IL-10 to those in mice treated with dead bacteria and impaired control of bacteria. We have thus identified ETC adaptations as an early immunological-metabolic checkpoint that adjusts innate immune responses to bacterial infection.

IFNα signaling through PKC-θ is essential for antitumor NK cell function.

We have previously shown that the development of a major histocompatibility complex class I (MHC-I)-deficient tumor was favored in protein kinase C-θ knockout (PKC-θ-/-) mice compared to that occurring in wild-type mice. This phenomenon was associated with scarce recruitment of natural killer (NK) cells to the tumor site, as well as impaired NK cell activation and reduced cytotoxicity ex vivo. Poly-inosinic:cytidylic acid (poly I:C) treatment activated PKC-θ in NK cells depending on the presence of a soluble factor produced by a different splenocyte subset. In the present work, we sought to analyze whether interleukin-15 (IL-15) and/or interferon-α (IFNα) mediate PKC-θ-dependent antitumor NK cell function. We found that IL-15 improves NK cell viability, granzyme B expression, degranulation capacity and interferon-γ (IFNγ) secretion independently of PKC-θ. In contrast, we found that IFNα improves the degranulation capability of NK cells against target cancer cells in a PKC-θ-dependent fashion both ex vivo and in vivo. Furthermore, IFNα induces PKC-θ auto-phosphorylation in NK cells, in a signal transduction pathway involving both phosphatidylinositol-3-kinase (PI3K) and phospholipase-C (PLC) activation. PKC-θ dependence was further implicated in IFNα-induced transcriptional upregulation of chemokine (C-X-C motif) ligand 10 (CXCL10), a signal transducer and activator of transcription-1 (STAT-1)-dependent target of IFNα. The absence of PKC-θ did not affect IFNα-induced STAT-1 Tyr701 phosphorylation but affected the increase in STAT-1 phosphorylation on Ser727, attenuating CXCL10 secretion. This connection between IFNα and PKC-θ in NK cells may be exploited in NK cell-based tumor immunotherapy.

[Control of innate immune system for cancer therapy]. / Mobiliser l'immunité innée dans le traitement des cancers.

Immunotherapies are particularly attractive when one wants to fight cancer since they aim at controlling the immune system in order to eradicate cancer cells and prevent tumor dissemination. While the clinical use of specific ligands for the innate immune receptors is promising, it is still facing several drawbacks when entering clinical trials. Nevertheless, recent advances in our understanding of the biology of these receptors constrain us to re-assess existing strategies and let foresee new approaches in the pipeline of therapies that would overcome current limitations.

From tumor cell metabolism to tumor immune escape.

Tumorigenesis implies adaptation of tumor cells to an adverse environment. First, developing tumors must acquire nutrients to ensure their rapid growth. Second, they must escape the attack from the host immune system. Recent studies suggest that these phenomena could be related and that tumor cell metabolism may propel tumor immune escape. Tumor cell metabolism tends to avoid mitochondrial activity and oxidative phosphorylation (OXPHOS), and largely relies on glycolysis to produce energy. This specific metabolism helps tumor cells to avoid the immune attack from the host by blocking or avoiding the immune attack. By changing their metabolism, tumor cells produce or sequester a variety of amino acids, lipids and chemical compounds that directly alter immune function therefore promoting immune evasion. A second group of metabolism-related modification targets the major histocompatibility complex-I (MHC-I) and related molecules. Tumor MHC-I presents tumor-associated antigens (TAAs) to cytotoxic T-cells (CTLs) and hence, sensitizes cancer cells to the cytolytic actions of the anti-tumor adaptive immune response. Blocking tumor mitochondrial activity decreases expression of MHC-I molecules at the tumor cell surface. And peroxynitrite (PNT), produced by tumor-infiltrating myeloid cells, chemically modifies MHC-I avoiding TAA expression in the plasma membrane. These evidences on the role of tumor cell metabolism on tumor immune escape open the possibility of combining drugs designed to control tumor cell metabolism with new procedures of anti-tumor immunotherapy. This article is part of a Directed Issue entitled: Bioenergetic dysfunction, adaptation and therapy.

"Flagellated" cancer cells propel anti-tumor immunity.

The use of innate immune receptor agonists in cancer therapies has suffered from many drawbacks. Our recent observations suggest that some of these hurdles can be overcome by introducing flagellin into tumor cells to promote tumor antigen presentation by dendritic cells (DCs) and simultaneously trigger two types of pattern recognition receptors (PRRs).

The protooncogene Vav1 regulates murine leukemia virus-induced T-cell leukemogenesis.

Vav1 is expressed exclusively in hematopoietic cells and is required for T cell development and activation. Vav1-deficient mice show thymic hypocellularity due to a partial block during thymocyte development at the DN3 stage and between the double positive (DP) and single positive (SP) transition. Vav1 has been shown to play a significant role in several non-hematopoietic tumors but its role in leukemogenesis is unknown. To address this question, we investigated the role of Vav1 in retrovirus-induced T cell leukemogenesis. Infection of Vav1-deficient mice with the Moloney strain of murine leukemia virus (M-MuLV) significantly affected tumor phenotype without modulating tumor incidence or latency. M-MuLV-infected Vav1-deficient mice showed reduced splenomegaly, higher hematocrit levels and hypertrophic thymi. Notably, Vav1-deficient mice with M-MuLV leukemias presented with markedly lower TCRß/CD3 levels, indicating that transformation occurred at an earlier stage of T cell development than in WT mice. Thus, impaired T cell development modulates the outcome of retrovirus-induced T cell leukemias, demonstrating a link between T cell development and T cell leukemogenesis.

Protein Kinase C-θ (PKC-θ) in Natural Killer Cell Function and Anti-Tumor Immunity.

The protein kinase C-θ (PKCθ), which is essential for T cell function and survival, is also required for efficient anti-tumor immune surveillance. Natural killer (NK) cells, which express PKCθ, play a prominent role in this process, mainly by elimination of tumor cells with reduced or absent major histocompatibility complex class-I (MHC-I) expression. This justifies the increased interest of the use of activated NK cells in anti-tumor immunotherapy in the clinic. The in vivo development of MHC-I-deficient tumors is much favored in PKCθ(-/-) mice compared with wild-type mice. Recent data offer some clues on the mechanism that could explain the important role of PKCθ in NK cell-mediated anti-tumor immune surveillance: some studies show that PKCθ is implicated in signal transduction and anti-tumoral activity of NK cells elicited by interleukin (IL)-12 or IL-15, while others show that it is implicated in NK cell functional activation mediated by certain killer-activating receptors. Alternatively, the possibility that PKCθ is involved in NK cell degranulation is discussed, since recent data indicate that it is implicated in microtubule-organizing center polarization to the immune synapse in CD4(+) T cells. The implication of PKC isoforms in degranulation has been more extensively studied in cytotoxic T lymphocyte, and these studies will be also summarized.

Simultaneous targeting of toll- and nod-like receptors induces effective tumor-specific immune responses.

Toll-like receptor (TLR) ligands are increasingly being used as adjuvants in cancer vaccine trials to harness innate immunity and prime effective antitumor immune responses. Despite some success, enhancing tumor antigen presentation, promoting a protective antitumor response, and overcoming the immunosuppressive tumor microenvironment pose considerable challenges that necessitate further improvements in vaccine design. Here, we show that expression of the TLR ligand flagellin within tumor cells constitutes an effective antitumor vaccination strategy that relies on simultaneous engagement of TLR5 and the Nod-like receptors (NLRs) NLRC4/NAIP5 (neuronal apoptosis inhibitory protein 5) by flagellin along with associative recognition of tumor antigen for optimal antigen presentation to T cells. Although TLR5 signaling was critical for mediating rapid macrophage-dependent clearance of flagellin-expressing tumor cells in vivo, TLR5 and NLRC4/NAIP5 were equally important for priming antitumor CD4(+) and CD8(+) T cells and suppressing tumor growth. Vaccination with irradiated flagellin-expressing tumor cells prevented tumor development, and disrupting flagellin recognition by TLR5 or NLRC4/NAIP5 impaired protective immunization against an existing or subsequent tumor. Our findings delineate a new strategy to induce anticancer immune responses consisting of introducing microbial structures with dual TLR and NLR stimulatory activity into tumor cells. This ensures recognition of tumor-derived antigen within the inflammatory context of microbial recognition and additionally activates both the phagocytic and the cytosolic pathways of innate immune defense against the tumor.

ICOStomizing immunotherapies with T(H)17.

New insights on the role of costimulatory molecules in T helper cell function have yielded exciting alternatives to the development of therapeutic strategies that target T cell costimulatory pathways. Inducible costimulatory molecule (ICOS) signaling is now shown by Paulos and colleagues to support expansion of human T helper 17 (T(H)17) cells that could exert antitumor activity. Here we discuss (i) how these findings aid in our understanding of mechanisms that govern T(H)17 cell functions and (ii) the potential application of these new insights to the development of immunotherapies.

Infection and apoptosis as a combined inflammatory trigger.

While inflammatory phagocytosis of microbial pathogens and non-inflammatory phagocytosis of apoptotic cells have each been studied extensively, the consequences of innate immune recognition of host cells undergoing apoptosis as a direct result of infection are unclear. In this situation, the innate immune system is confronted with mixed signals, those from apoptotic cells and those from the infecting pathogen. Nuclear receptor activation has been implicated downstream of apoptotic cell recognition while Toll-like receptors are the prototypical inflammatory receptors engaged during infection. When the two signals combine, a new set of events takes place beginning with transrepression of a subset of inflammatory-response genes and ending with the induction of a T helper-17 adaptive immune response. This response is best suited for clearing the infecting pathogen and repairing the damage that occurred to the host tissue during infection.

Innate immune recognition of infected apoptotic cells directs T(H)17 cell differentiation.

Adaptive immune responses rely on differentiation of CD4 T helper cells into subsets with distinct effector functions best suited for host defence against the invading pathogen. Interleukin (IL)-17-producing T helper cells (T(H)17) are a recently identified subset, separate from the T helper type 1 (T(H)1) and T helper type 2 (T(H)2) subsets. Synergy between the cytokines transforming growth factor-beta and IL-6 in vitro induces development of T(H)17 cells in mouse and human systems, whereas IL-23 supports expansion of these cells. However, it is not known which conditions in vivo would induce this combination of cytokines. Furthermore, it is enigmatic that a combination of pro-inflammatory and anti-inflammatory cytokines would be required to generate an effector T(H)17 response. Here we show that the relevant physiological stimulus triggering this combination of cytokines is the recognition and phagocytosis of infected apoptotic cells by dendritic cells. Phagocytosis of infected apoptotic cells uniquely triggers the combination of IL-6 and transforming growth factor-beta through recognition of pathogen-associated molecular patterns and phosphatidylserine exposed on apoptotic cells, respectively. Conversely, phagocytosis of apoptotic cells in the absence of microbial signals induces differentiation of the closely related regulatory T cells, which are important for controlling autoimmunity. Blocking apoptosis during infection of the mouse intestinal epithelium with the rodent pathogen Citrobacter rodentium, which models human infections with the attaching and effacing enteropathogenic and enterohaemorrhagic Escherichia coli, impairs the characteristic T(H)17 response in the lamina propria. Our results demonstrate that infected apoptotic cells are a critical component of the innate immune signals instructing T(H)17 differentiation, and point to pathogens particularly adept at triggering apoptosis that might preferentially induce T(H)17-mediated immunity. Because T(H)17 cells have been correlated with autoimmune diseases, investigation of the pathways of innate recognition of infected apoptotic cells might lead to improved understanding of the causative defects in autoimmunity.

ERK5 knockdown generates mouse leukemia cells with low MHC class I levels that activate NK cells and block tumorigenesis.

Tumor cell-based vaccines are currently used in clinical trails, but they are in general poorly immunogenic because they are composed of cell extracts or apoptotic cells. Live tumor cells should be much better Ags provided that they are properly processed by the host immune system. We show herein that stable expression of a small hairpin RNA for ERK5 (shERK5) decreases ERK5 levels in human and mouse leukemic cells and leads to their elimination by NK cells in vivo. The shERK5 cells show down-regulation of MHC class I expression at the plasma membrane. Accordingly, ectopic activation of the ERK5 pathway induces MHC class I gene expression. Coinjection of shERK5-expressing cells into the peritoneum diminishes survival of engrafted wild-type tumor cells. Moreover, s.c. injection of shERK5-expressing cells strongly diminishes tumor development by wild-type cells. Our results show that shERK5 expression in leukemia cells effectively attenuates their tumor activity and allows their use as a tumor cell-based vaccine.

Protein kinase C-theta is required for NK cell activation and in vivo control of tumor progression.

Protein kinase C-theta (PKCtheta) was initially isolated as an important PKC isoform expressed in T cells, although its expression is not restricted to these cells. Despite the central function of PKCtheta in several immune responses, its role in the antitumor response against MHC class I (MHC-I)-negative cells has not been investigated. This is an important issue because most tumor cells growing in vivo down-regulate MHC-I expression to escape the CTL-mediated response. In the present work, we show that in vivo development of a MHC-I-deficient tumor (RMA-S) is much favored in PKCtheta(-/-) mice compared with wild-type mice. This is associated with a reduced recruitment of NK cells to the site of tumor development and a reduced activation status of recruited NK cells. This correlates with a reduced ex vivo and in vivo cytotoxic potential of NK cells isolated from PKCtheta(-/-) mice treated with polyinosinic:polycytidylic acid. Consistently, polinosinic:cytidilic acid treatment induces PKCtheta expression and activation of its enzymatic activity in NK cells in an indirect manner. These observations underline the relevance of PKCtheta as a key molecule in NK cell-mediated antitumor immune surveillance.