The Inflammasome Adaptor ASC Intrinsically Limits CD4+ T-Cell Proliferation to Help Maintain Intestinal Homeostasis

Swaying the advantage: multifaceted functions of inflammasomes in adaptive immunity

Eukaryotic cells are equipped with cytoplasmic sensors that recognize diverse pathogen- or danger-associated molecular patterns. In cells of the myeloid lineage, activation of these sensors leads to the assembly of a multimeric protein complex, called the inflammasome, that culminates in the production of inflammatory cytokines and pyroptosis. Recently, investigation of the inflammasomes in lymphocytes led to the discovery of functional pathways that were initially believed to be confined to the innate arm of the immune system. Thus, the adapter protein apoptosis-associated speck-like protein containing a CARD (ASC) was documented to play a critical role in antigen uptake by dendritic cells, and regulation of T- and B-cell motility at several stages, and absent in melanoma 2 (AIM2) was found to act as a modulator of regulatory T-cell differentiation. Remarkably, NLRP3 was demonstrated to act as a transcription factor that controls Th2 cell polarization, and as a negative regulator of regulatory T-cell differentiation by limiting Foxp3 expression. In B lymphocytes, NLRP3 plays a role in the transcriptional network that regulates B-cell development and homing, and its activation is essential for germinal center formation and maturation of high-affinity antibody responses. Such recently discovered inflammasome-mediated functions in T and B lymphocytes offer multiple cross-talk opportunities for the innate and adaptive arms of the immune system. A better understanding of the dialog between inflammasomes and intracellular components could be beneficial for therapeutic purposes in restoring immune homeostasis and mitigating inflammation in a wide range of disorders.

Metabolism-inflammasome crosstalk shapes innate and adaptive immunity

Inflammasomes are a central component of innate immunity and play a vital role in regulating innate immune response. Activation of inflammasomes is also indispensable for adaptive immunity, modulating the development and response of adaptive immunity. Recently, increasing studies have shown that metabolic alterations and adaptations strongly influence and regulate the differentiation and function of the immune system. In this review, we will take a holistic view of how inflammasomes bridge innate and adaptive (especially T cell) immunity and how inflammasomes crosstalk with metabolic signals during the immune responses. And, special attention will be paid to the metabolic control of inflammasome-mediated interactions between innate and adaptive immunity in disease. Understanding the metabolic regulatory functions of inflammasomes would provide new insights into future research directions in this area and may help to identify potential targets for inflammasome-associated diseases and broaden therapeutic avenues.

Inflammasomes cross-talk with lymphocytes to connect the innate and adaptive immune response

BACKGROUND

Innate and adaptive immunity are two different parts of the immune system that have different characteristics and work together to provide immune protection. Inflammasomes are a major part of the innate immune system that are expressed widely in myeloid cells and are responsible for inflammatory responses. Recent studies have shown that inflammasomes are also expressed and activated in lymphocytes, especially in T and B cells, to regulate the adaptive immune response. Activation of inflammasomes is also under the control of lymphocytes. Therefore, we propose that inflammasomes act as a bridge and they provide crosstalk between the innate and adaptive immune systems to obtain a fine balance in immune responses.

AIM OF REVIEW

This review systematially summarizes the interaction between inflammasomes and lymphocytes and describes the crosstalk between the innate and adaptive immune systems induced by inflammasomes, with the aim of providing new directions and important areas for further research.

KEY SCIENTIFIC CONCEPTS OF REVIEW

When considering the novel function of inflammasomes in various lymphocytes, attention should be given to the activity of specific inflammasomes in studies of lymphocyte function. Moreover, research on the function of various inflammasomes in lymphocytes will help advance knowledge on the mechanisms and treatment of various diseases, including autoimmune diseases and tumors. In addition, when studying inflammatory responses, inflammasomes in both lymphocytes and myeloid cells need to be considered.

New insights into Microalgal astaxanthin's effect on Lambda-cyhalothrin-induced lymphocytes immunotoxicity in Cyprinus carpio: Involving miRNA-194-5p-FoxO1-mediated-mitophagy and pyroptosis

Lambda-cyhalothrin (LC), a pyrethroid insecticide widely used in agriculture, causes immunotoxicity to aquatic organisms in the aquatic environment. Microalgal astaxanthin (MA) is a natural carotenoid that enhances viability of a variety of fish. To investigate the immunotoxicity of LC and the improvement effect of MA in lymphocytes (Cyprinus carpio), lymphocytes were treated with LC (80 M) and/or MA (50 M) for 24 h. Firstly, CCK8 combined with PI staining results showed that MA significantly attenuated the LC-induced lymphocyte death rate. Secondly, LC exposure resulted in excessively damaged mitochondrial and mtROS, diminished mitochondrial membrane potential and ATP content, which could be improved by MA. Thirdly, MA upregulated the levels of mitophagy-related regulatory factors (Beclin1, LC3, ATG5, Tom20 and Lamp2) induced by LC. Importantly, MA decreased the levels of pyroptosis-related genes treated with LC, including NLRP3, Cas-4, GSDMD and active Cas-1. Further study indicated that LC treatment caused excessive miRNA-194-5p and reduced levels of FoxO1, PINK1 and Parkin, which was inhibited by MA treatment. Overall, we concluded that MA could enhance damaged mitochondrial elimination by promoting the miRNA-194-5p-FoxO1-PINK1/Parkin-mitophagy in lymphocytes, which reduced mtROS accumulation and alleviated pyroptosis. It offers insights into the importance of MA application in aquaculture as well as the defense of farmed fish against agrobiological hazards in fish under LC.

Molecular, metabolic, and functional CD4 T cell paralysis in the lymph node impedes tumor control

CD4 T cells are central effectors of anti-cancer immunity and immunotherapy, yet the regulation of CD4 tumor-specific T (TTS) cells is unclear. We demonstrate that CD4 TTS cells are quickly primed and begin to divide following tumor initiation. However, unlike CD8 TTS cells or exhaustion programming, CD4 TTS cell proliferation is rapidly frozen in place by a functional interplay of regulatory T cells and CTLA4. Together these mechanisms paralyze CD4 TTS cell differentiation, redirecting metabolic circuits, and reducing their accumulation in the tumor. The paralyzed state is actively maintained throughout cancer progression and CD4 TTS cells rapidly resume proliferation and functional differentiation when the suppressive constraints are alleviated. Overcoming their paralysis established long-term tumor control, demonstrating the importance of rapidly crippling CD4 TTS cells for tumor progression and their potential restoration as therapeutic targets.

Molecular, metabolic and functional CD4 T cell paralysis impedes tumor control

CD4 T cells are important effectors of anti-tumor immunity, yet the regulation of CD4 tumor-specific T (T TS ) cells during cancer development is still unclear. We demonstrate that CD4 T TS cells are initially primed in the tumor draining lymph node and begin to divide following tumor initiation. Distinct from CD8 T TS cells and previously defined exhaustion programs, CD4 T TS cell proliferation is rapidly frozen in place and differentiation stunted by a functional interplay of T regulatory cells and both intrinsic and extrinsic CTLA4 signaling. Together these mechanisms paralyze CD4 T TS cell differentiation, redirecting metabolic and cytokine production circuits, and reducing CD4 T TS cell accumulation in the tumor. Paralysis is actively maintained throughout cancer progression and CD4 T TS cells rapidly resume proliferation and functional differentiation when both suppressive reactions are alleviated. Strikingly, Treg depletion alone reciprocally induced CD4 T TS cells to themselves become tumor-specific Tregs, whereas CTLA4 blockade alone failed to promote T helper differentiation. Overcoming their paralysis established long-term tumor control, demonstrating a novel immune evasion mechanism that specifically cripples CD4 T TS cells to favor tumor progression.

The regulation of self-tolerance and the role of inflammasome molecules

Inflammasome molecules make up a family of receptors that typically function to initiate a proinflammatory response upon infection by microbial pathogens. Dysregulation of inflammasome activity has been linked to unwanted chronic inflammation, which has also been implicated in certain autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, type 1 diabetes, systemic lupus erythematosus, and related animal models. Classical inflammasome activation-dependent events have intrinsic and extrinsic effects on both innate and adaptive immune effectors, as well as resident cells in the target tissue, which all can contribute to an autoimmune response. Recently, inflammasome molecules have also been found to regulate the differentiation and function of immune effector cells independent of classical inflammasome-activated inflammation. These alternative functions for inflammasome molecules shape the nature of the adaptive immune response, that in turn can either promote or suppress the progression of autoimmunity. In this review we will summarize the roles of inflammasome molecules in regulating self-tolerance and the development of autoimmunity.

Inflammasome is a central player in B cell development and homing

Whereas the role of the nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing protein (NLRP) 3 pathway in innate immunity has been extensively studied, little attention has been paid to its contribution to adaptive immunity. Studies in animal models and human subjects have shown the contribution of NLRP3 to the T cell compartment, and its role in B lymphocyte functions has been proposed. Here, we report that ablation of nlrp3 in mice led to altered B cell development in the bone marrow, and distorted expression of B cell subsets that play innate-like functions, that is, marginal zone B cells in the spleen and B-1a cells in the peritoneal cavity. Mechanistically, in the absence of NLRP3 expression, the transcription factor IRF4, previously found to interact with NLRP3 in the nucleus of lymphocytes, was up-regulated. NLRP3 ablation reduced the expression of the chemokine receptors CXCR4 and CCR7 in an IRF4-dependent manner, indicating that the presence of NLRP3 is critical for optimal expression of chemokine receptors on B cells. We conclude that activation of the NLRP3 inflammasome plays a role in B cell development, homing, and retention in lymphoid organs.

T lymphocyte-mediated pyroptosis: A new regulatory mechanism in non-viral liver disease

T lymphocyte-mediated pyroptosis plays an important role in the development of non-viral liver diseases. Pyroptosis as a programmed cell death process, has been a hot topic of research on disease pathogenesis in recent years. As one of the most common immune cells in the body, T cells are the major players in adaptive immunity. An increasing number of studies have shown that T lymphocyte-mediated pyroptosis functions in non-viral liver diseases to regulate immune function, alter the immune microenvironment, and thus influence disease progression. These findings will guide us and provide new ideas for the development of subsequent therapeutic agents for non-viral liver diseases.

Insights into inflammasome regulation: cellular, molecular, and pathogenic control of inflammasome activation

Maintenance of immune homeostasis is an intricate process wherein inflammasomes play a pivotal role by contributing to innate and adaptive immune responses. Inflammasomes are ensembles of adaptor proteins that can trigger a signal following innate sensing of pathogens or non-pathogens eventuating in the inductions of IL-1β and IL-18. These inflammatory cytokines substantially influence the antigen-presenting cell's costimulatory functions and T helper cell differentiation, contributing to adaptive immunity. As acute and chronic disease conditions may accompany parallel tissue damage, we analyze the critical role of extracellular factors such as cytokines, amyloids, cholesterol crystals, etc., intracellular metabolites, and signaling molecules regulating inflammasome activation/inhibition. We develop an operative framework for inflammasome function and regulation by host cell factors and pathogens. While inflammasomes influence the innate and adaptive immune components' interplay modulating the anti-pathogen adaptive immune response, pathogens may target inflammasome inhibition as a survival strategy. As trapped between health and diseases, inflammasomes serve as promising therapeutic targets and their modus operandi serves as a scientific rationale for devising better therapeutic strategies.

Inflammasomes in T cells

The concept of non-self recognition through germ-line encoded pattern recognition receptors (PRRs) has been well-established for professional innate immune cells. However, there is growing evidence that also T cells employ PRRs and associated effector functions in response to certain non-self or damage signals. Inflammasomes constitute a special subgroup of PRRs that is hardwired to a signaling cascade that culminates in the activation of caspase-1. Active caspase-1 processes pro-inflammatory cytokines of the IL-1 family and also triggers a lytic programmed cell death pathway known as pyroptosis. An increasing body of literature suggests that inflammasomes are also functional in T cells. On the one hand, conventional inflammasome signaling cascades have been described that operate similarly to pathways characterized in innate immune cells. On the other hand, unconventional functions have been suggested, in which certain inflammasome components play a role in unrelated processes, such as cell fate decisions and functions of T helper cells. In this review, we discuss our current knowledge on inflammasome functions in T cells and the biological implications of these findings for health and disease.

NLRP3 at the crossroads between immune/inflammatory responses and enteric neuroplastic remodelling in a mouse model of diet-induced obesity

BACKGROUND AND PURPOSE

Enteric neurogenic/inflammation contributes to bowel dysmotility in obesity. We examined the role of NLRP3 in colonic neuromuscular dysfunctions in mice with high-fat diet (HFD)-induced obesity.

EXPERIMENTAL APPROACH

Wild-type C57BL/6J and NLRP3-KO (Nlrp3^-/- ) mice were fed with HFD or standard diet for 8 weeks. The activation of inflammasome pathways in colonic tissues from obese mice was assessed. The role of NLRP3 in in vivo colonic transit and in vitro tachykininergic contractions and substance P distribution was evaluated. The effect of substance P on NLRP3 signalling was tested in cultured cells.

KEY RESULTS

HFD mice displayed increased body and epididymal fat weight, cholesterol levels, plasma resistin levels and plasma and colonic IL-1β levels, colonic inflammasome adaptor protein apoptosis-associated speck-like protein containing caspase-recruitment domain (ASC) and caspase-1 mRNA expression and ASC immunopositivity in macrophages. Colonic tachykininergic contractions were enhanced in HFD mice. HFD NLRP3^-/- mice developed lower increase in body and epididymal fat weight, cholesterol levels, systemic and bowel inflammation. In HFD Nlrp3^-/- mice, the functional alterations of tachykinergic pathways and faecal output were normalized. In THP-1 cells, substance P promoted IL-1β release. This effect was inhibited upon incubation with caspase-1 inhibitor or NK₁ antagonist and not observed in ASC^-/- cells.

CONCLUSION AND IMPLICATIONS

In obesity, NLRP3 regulates an interplay between the shaping of enteric immune/inflammatory responses and the activation of substance P/NK₁ pathways underlying the onset of colonic dysmotility. Identifying NLRP3 as a therapeutic target for the treatment of bowel symptoms related to obesity.

Acetylbritannilactone attenuates contrast-induced acute kidney injury through its anti-pyroptosis effects

Contrast-induced acute kidney injury (CI-AKI) is a severe complication caused by intravascular applied radial contrast media (CM). Pyroptosis is a lytic type of cell death inherently associated with inflammation response and the secretion of pro-inflammatory cytokines following caspase-1 activation. The aim of the present study was to investigate the protective effects of acetylbritannilactone (ABL) on iopromide (IOP)-induced acute renal failure and reveal the underlying mechanism. In vivo and in vitro, IOP treatment caused renal damage and elevated the caspase-1 (+) propidium iodide (PI) (+) cell count, interleukin (IL)-1β and IL-18 levels, lactate dehydrogenase (LDH) release, and the relative expression of nucleotide-binding domain, leucine-rich repeat containing protein 3 (NLRP3), apoptosis-associated speck-like protein (ASC), and gasdermin D (GSDMD), suggesting that IOP induces AKI via the activation of pyroptosis. Furthermore, the pretreatment of ABL partly mitigated the CI-AKI, development of pyroptosis, and subsequent kidney inflammation. These data revealed that ABL partially prevents renal dysfunction and reduces pyroptosis in CI-AKI, which may provide a therapeutic target for the treatment of CM-induced AKI.

Downregulation of lung miR-203a-3p expression by high-altitude hypoxia enhances VEGF/Notch signaling

Hypoxia-related microRNAs (miRNAs) are involved in the pathogenesis of various diseases. Because potential variations in miRNA expression mediated by hypoxic lung injury at high altitude remain incompletely characterized, we used a rat model to investigate the biochemical and miRNA changes induced by high-altitude hypoxia. After 24, 48, or 72 h of hypoxic exposure, expression of VEGF/Notch pathway-related proteins were increased in rat lung tissues. Microarray screening of hypoxic lung samples revealed 57 differentially expressed miRNAs, 19 of which were related to the VEGF/Notch signaling pathway. We verified that the top downregulated miRNA (miR-203a-3p) suppresses VEGF-A translation through direct binding and also indirectly reduces Notch1, VEGFR2, and Hes1 levels, which restricts the angiogenic capacity of pulmonary microvascular endothelial cells in vitro. These findings may aid in the development of new therapeutic strategies for the prevention and treatment of hypoxic lung injury at high altitude.