Selective autophagy of the adaptor protein Bcl10 modulates T cell receptor activation of NF-κB

Autophagy and cellular immune responses

Autophagy constitutes a mechanism for the sequestration and lysosomal degradation of various cytoplasmic structures, including damaged organelles and invading microorganisms. Autophagy not only represents an essential cell-intrinsic mechanism to protect against internal and external stress conditions but also shapes cellular immunity. Recent evidence indicates that autophagic responses in antigen-donor cells affect the release of several cytokines and "danger signals." Thus, especially when it precedes cell death, autophagy alerts innate immune effectors to elicit cognate immune responses. Autophagy is also important for the differentiation, survival, and activation of myeloid and lymphoid cells. Accordingly, inherited mutations in autophagy-relevant genes are associated with immune diseases, whereas oncogenesis-associated autophagic defects promote the escape of developing tumors from immunosurveillance. Here, we discuss the regulation of autophagy in the course of cellular immune responses and emphasize its impact on the immunogenicity of antigen-donor cells and on the activity of antigen-presenting cells and T lymphocytes.

When ubiquitin meets NF-κB: a trove for anti-cancer drug development

During the last two decades, the studies on ubiquitination in regulating transcription factor NF-κB activation have elucidated the expanding role of ubiquitination in modulating cellular events by non-proteolytic mechanisms, as well as by proteasomal degradation. The significance of ubiquitination has also been recognized in regulating gene transcription, epigenetic modifications, kinase activation, DNA repair and subcellular translocation. This progress has been translated into novel strategies for developing anti-cancer therapeutics, exemplified by the success of the first FDA-approved proteasome inhibitor drug Bortezomib. Here we discuss the current understanding of the ubiquitin-proteasome system and how it is involved in regulating NF-κB signaling pathways in response to a variety of stimuli. We also focus on the recent progress of anti-cancer drug development targeting various steps of ubiquitination process, and the potential of these drugs in cancer treatment as related to their impact on NF-κB activation.

Immune synapse: conductor of orchestrated organelle movement

To ensure proper cell function, intracellular organelles are not randomly distributed within the cell, but polarized and highly constrained by the cytoskeleton and associated adaptor proteins. This relationship between distribution and function was originally found in neurons and epithelial cells; however, recent evidence suggests that it is a general phenomenon occurring in many highly specialized cells including T lymphocytes. Recent studies reveal that the orchestrated redistribution of organelles is dependent on antigen-specific activation of and immune synapse (IS) formation by T cells. This review highlights the functional implications of organelle polarization in early T cell activation and examines recent findings on how the IS sets the rhythm of organelle motion and the spread of the activation signal to the nucleus.

Autophagy, a novel pathway to regulate calcium mobilization in T lymphocytes

The T lymphocyte response initiates with the recognition of MHC/peptides on antigen presenting cells by the T cell receptor (TCR). After the TCR engagement, the proximal signaling pathways are activated for downstream cellular events. Among these pathways, the calcium-signaling flux is activated through the depletion of endoplasmic reticulum (ER) calcium stores and plays pivotal roles in T cell proliferation, cell survival, and apoptosis. In studying the roles of macroautophagy (hereafter referred to as autophagy) in T cell function, we found that a pathway for intracellular degradation, autophagy, regulates calcium signaling by developmentally maintaining the homeostasis of the ER. Using mouse genetic models with specific deletion of autophagy-related genes in T lymphocytes, we found that the calcium influx is defective and the calcium efflux is increased in autophagy-deficient T cells. The abnormal calcium flux is related to the expansion of the ER and higher calcium stores in the ER. Because of this, treatment with the ER sarco/ER Ca²⁺-ATPase pump inhibitor, thapsigargin, rescues the calcium influx defect in autophagy-deficient T cells. Therefore, autophagy regulates calcium mobilization in T lymphocytes through ER homeostasis.

Regulation of T cell function by the ubiquitin-specific protease USP9X via modulating the Carma1-Bcl10-Malt1 complex

The ubiquitin conjugation system plays an important role in immune regulation; however, the ubiquitin-specific proteases (USPs) that carry out deubiquitination of cellular substrates are poorly understood. Here we show that in vivo knockdown of the deubiquitinating enzyme USP9X attenuates T-cell proliferation. In addition, naïve CD4(+) T cells from USP9X knockdown chimeric mice display decreased cytokine production and T helper cell differentiation in vitro, which we confirmed in vivo by performing adoptive transfer of transgenic T cells and subsequent immunization. USP9X silencing in both a human T-cell line and mouse primary T cells reduced T-cell receptor (TCR) signaling-induced NF-κB activation. Mechanistically, USP9X interacts with Bcl10 of the Carma1-Bcl10-Malt1 (CBM) complex and removes the TCR-induced ubiquitin chain from Bcl10, which facilitates the association of Carma1 with Bcl0-Malt1. These results demonstrate that USP9X is a crucial positive regulator of the TCR signaling pathway and is required for T-cell function through the modulation of CBM complex formation.

Pathogen-induced autophagy signaling in innate immunity

Innate immunity induces rapid responses to fight invading pathogens. To eliminate intracellular bacteria or viruses, innate cellular responses lead to the production of nuclear factor-κB-dependent inflammatory cytokines, inflammasome activation, type I interferon synthesis, and/or eventually death of the infected cells. Autophagy emerged as another component of innate immunity, as it offers an immediate autonomous cell defense mechanism by degrading intracellular pathogens. In addition, autophagy participates in the regulation of immune and inflammatory cell responses. Instead of providing a comprehensive status of the art that has already been addressed elsewhere, we chose to highlight some recent issues brought up in the field.

A new look at T cell receptor signaling to nuclear factor-κB

Antigen stimulation of T cell receptor (TCR) signaling to nuclear factor (NF)-κB is required for T cell proliferation and differentiation of effector cells. The TCR-to-NF-κB pathway is generally viewed as a linear sequence of events in which TCR engagement triggers a cytoplasmic cascade of protein-protein interactions and post-translational modifications, ultimately culminating in the nuclear translocation of NF-κB. However, recent findings suggest a more complex picture in which distinct signalosomes, previously unrecognized proteins, and newly identified regulatory mechanisms play key roles in signal transmission. In this review, we evaluate recent data and suggest areas of future emphasis in the study of this important pathway.

mTOR, linking metabolism and immunity

mTOR is an evolutionarily conserved serine/threonine kinase that plays a critical role in cell growth and metabolism by sensing different environmental cues. There is a growing appreciation of mTOR in immunology for its role in integrating diverse signals from the immune microenvironment and coordinating the functions of immune cells and their metabolism. In CD8 T cells, mTOR has shown to influence cellular commitment to effector versus memory programming; in CD4 T cells, mTOR integrates environmental cues that instruct effector cell differentiation. In this review, we summarize and discuss recent advances in the field, with a focus on the mechanisms through which mTOR regulates cellular and humoral immunity. Further understanding will enable the manipulation of mTOR signaling to direct the biological functions of immune cells, which holds great potential for improving immune therapies and vaccination against infections and cancer.

Kinase-independent feedback of the TAK1/TAB1 complex on BCL10 turnover and NF-κB activation

Antigen receptors activate pathways that control cell survival, proliferation, and differentiation. Two important targets of antigen receptors, NF-κB and Jun N-terminal kinase (JNK), are activated downstream of CARMA1, a scaffolding protein that nucleates a complex including BCL10, MALT1, and other IκB kinase (IKK)-signalosome components. Somatic mutations that constitutively activate CARMA1 occur frequently in diffuse large B cell lymphoma (DLBCL) and mediate essential survival signals. Mechanisms that downregulate this pathway might thus yield important therapeutic targets. Stimulation of antigen receptors induces not only BCL10 activation but also its degradation downstream of CARMA1, thereby ultimately limiting signals to its downstream targets. Here, using lymphocyte cell models, we identify a kinase-independent requirement for TAK1 and its adaptor, TAB1, in antigen receptor-induced BCL10 degradation. We show that TAK1 acts as an adaptor for E3 ubiquitin ligases that target BCL10 for degradation. Functionally, TAK1 overexpression restrains CARMA1-dependent activation of NF-κB by reducing BCL10 levels. TAK1 also promotes counterselection of NF-κB-addicted DLBCL lines by a dual mechanism involving kinase-independent degradation of BCL10 and kinase-dependent activation of JNK. Thus, by directly promoting BCL10 degradation, TAK1 counterbalances NF-κB and JNK signals essential for the activation and survival of lymphocytes and CARMA1-addicted lymphoma types.

Self-eating and self-defense: autophagy controls innate immunity and adaptive immunity

Autophagy (macroautophagy; "self-eating") is a degradation process, in which cytoplasmic content is engulfed and degraded by the lysosome. And, immunity is an important mechanism of the "self-defense" system. Autophagy has long been recognized as a stress response to nutrient deprivation. This will provide energy and anabolic building blocks to maintain cellular bioenergetic homeostasis. Thus, autophagy plays critical roles in regulating a wide variety of pathophysiological processes, including tumorigenesis, embryo development, tissue remodeling, and most recently, immunity. The latter shows that a self-eating (autophagy) process could regulate a self-defense (immune) system. In this review, we summarize the recent findings regarding the regulatory and mechanistic insights of the autophagy pathway in immunity.

Autophagy and intestinal homeostasis

Nutrient absorption is the basic function that drives mammalian intestinal biology. To facilitate nutrient uptake, the host's epithelial barrier is composed of a single layer of cells. This constraint is problematic, as a design of this type can be easily disrupted. The solution during the course of evolution was to add numerous host defense mechanisms that can help prevent local and systemic infection. These mechanisms include specialized epithelial cells that produce a physiochemical barrier overlying the cellular barrier, robust and organized adaptive and innate immune cells, and the ability to mount an inflammatory response that is commensurate with a specific threat level. The autophagy pathway is a critical cellular process that strongly influences all these functions. Therefore, a fundamental understanding of the components of this pathway and their influence on inflammation, immunity, and barrier function will facilitate our understanding of homeostasis in the gastrointestinal tract.

TBK1 kinase addiction in lung cancer cells is mediated via autophagy of Tax1bp1/Ndp52 and non-canonical NF-κB signalling

K-Ras dependent non-small cell lung cancer (NSCLC) cells are 'addicted' to basal autophagy that reprograms cellular metabolism in a lysosomal-sensitive manner. Here we demonstrate that the xenophagy-associated kinase TBK1 drives basal autophagy, consistent with its known requirement in K-Ras-dependent NSCLC proliferation. Furthermore, basal autophagy in this context is characterised by sequestration of the xenophagy cargo receptor Ndp52 and its paralogue Tax1bp1, which we demonstrate here to be a bona fide cargo receptor. Autophagy of these cargo receptors promotes non-canonical NF-κB signalling. We propose that this TBK1-dependent mechanism for NF-κB signalling contributes to autophagy addiction in K-Ras driven NSCLC.

Autophagy and the regulation of the immune response

Autophagy is a highly conserved mechanism of lysosomal-mediated protein degradation that plays a crucial role in maintaining cellular homeostasis by recycling amino acids, reducing the amount of damaged proteins and regulating protein levels in response to extracellular signals. In the last few years specific functions for different forms of autophagy have been identified in many tissues and organs. In the Immune System, autophagy functions range from the elimination infectious agents and the modulation of the inflammatory response, to the selection of antigens for presentation and the regulation of T cell homeostasis and activation. Here, we review the recent advances that have allowed us to better understand why autophagy is a crucial process in the regulation of the innate and adaptive immune responses.

Dampening NF-κB signaling by "self-eating"

Activation of NF-κB transcription factor is crucial for survival, proliferation, and differentiation of T cells. In this issue of Immunity, Paul et al. (2012) demonstrate that autophagy is a pathway by which TCR-activated NF-κB is turned over.