c-FLIP Expression in Foxp3-Expressing Cells Is Essential for Survival of Regulatory T Cells and Prevention of Autoimmunity

Targeting caspase-8: a new strategy for combating hepatocellular carcinoma

Hepatocellular carcinoma (HCC) represents the most prevalent form of primary liver cancer and has a high mortality rate. Caspase-8 plays a pivotal role in an array of cellular signaling pathways and is essential for the governance of programmed cell death mechanisms, inflammatory responses, and the dynamics of the tumor microenvironment. Dysregulation of caspase-8 is intricately linked to the complex biological underpinnings of HCC. In this manuscript, we provide a comprehensive review of the regulatory roles of caspase-8 in apoptosis, necroptosis, pyroptosis, and PANoptosis, as well as its impact on inflammatory reactions and the intricate interplay with critical immune cells within the tumor microenvironment, such as tumor-associated macrophages, T cells, natural killer cells, and dendritic cells. Furthermore, we emphasize how caspase-8 plays pivotal roles in the development, progression, and drug resistance observed in HCC, and explore the potential of targeting caspase-8 as a promising strategy for HCC treatment.

Nano-Pulse Treatment Overcomes the Immunosuppressive Tumor Microenvironment to Elicit In Situ Vaccination Protection against Breast Cancer

We previously reported that nano-pulse treatment (NPT), a pulsed power technology, resulted in 4T1-luc mammary tumor elimination and a strong in situ vaccination, thereby completely protecting tumor-free animals against a second live tumor challenge. The mechanism whereby NPT mounts effective antitumor immune responses in the 4T1 breast cancer predominantly immunosuppressive tumor microenvironment (TME) remains unanswered. In this study, orthotopic 4T1 mouse breast tumors were treated with NPT (100 ns, 50 kV/cm, 1000 pulses, 3 Hz). Blood, spleen, draining lymph nodes, and tumors were harvested at 4-h, 8-h, 1-day, 3-day, 7-day, and 3-month post-treatment intervals for the analysis of frequencies, death, and functional markers of various immune cells in addition to the suppressor function of regulatory T cells (Tregs). NPT was verified to elicit strong in situ vaccination (ISV) against breast cancer and promote both acute and long-term T cell memory. NPT abolished immunosuppressive dominance systemically and in the TME by substantially reducing Tregs, myeloid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs). NPT induced apoptosis in Tregs and TAMs. It also functionally diminished the Treg suppression capacity, explained by the downregulation of activation markers, particularly 4-1BB and TGFβ, and a phenotypic shift from predominantly activated (CD44+CD62L-) to naïve (CD44-CD62L+) Tregs. Importantly, NPT selectively induced apoptosis in activated Tregs and spared effector CD4+ and CD8+ T cells. These changes were followed by a concomitant rise in CD8+CD103+ tissue-resident memory T cells and TAM M1 polarization. These findings indicate that NPT effectively switches the TME and secondary lymphatic systems from an immunosuppressive to an immunostimulatory state, allowing cytotoxic T cell function and immune memory formation to eliminate cancer cells and account for the NPT in situ vaccination.

Apoptosis, a Metabolic "Head-to-Head" between Tumor and T Cells: Implications for Immunotherapy

Induction of apoptosis represents a promising therapeutic approach to drive tumor cells to death. However, this poses challenges due to the intricate nature of cancer biology and the mechanisms employed by cancer cells to survive and escape immune surveillance. Furthermore, molecules released from apoptotic cells and phagocytes in the tumor microenvironment (TME) can facilitate cancer progression and immune evasion. Apoptosis is also a pivotal mechanism in modulating the strength and duration of anti-tumor T-cell responses. Combined strategies including molecular targeting of apoptosis, promoting immunogenic cell death, modulating immunosuppressive cells, and affecting energy pathways can potentially overcome resistance and enhance therapeutic outcomes. Thus, an effective approach for targeting apoptosis within the TME should delicately balance the selective induction of apoptosis in tumor cells, while safeguarding survival, metabolic changes, and functionality of T cells targeting crucial molecular pathways involved in T-cell apoptosis regulation. Enhancing the persistence and effectiveness of T cells may bolster a more resilient and enduring anti-tumor immune response, ultimately advancing therapeutic outcomes in cancer treatment. This review delves into the pivotal topics of this multifaceted issue and suggests drugs and druggable targets for possible combined therapies.

Single-cell analysis revealed a potential role of T-cell exhaustion in colorectal cancer with liver metastasis

Liver metastasis (LM) is an important factor leading to colorectal cancer (CRC) mortality. However, the effect of T-cell exhaustion on LM in CRC is unclear. Single-cell sequencing data derived from the Gene Expression Omnibus database. Data were normalized using the Seurat package and subsequently clustered and annotated into different cell clusters. The differentiation trajectories of epithelial cells and T cells were characterized based on pseudo-time analysis. Single-sample gene set enrichment analysis (ssGSEA) was used to calculate enrichment scores for different cell clusters and to identify enriched biological pathways. Finally, cell communication analysis was performed. Nine cell subpopulations were identified from CRC samples with LM. The proportion of T cells increased in LM. T cells can be subdivided into NK/T cells, regulatory T cells (Treg) and exhausted T cells (Tex). In LM, cell adhesion and proliferation activity of Tex were promoted. Epithelial cells can be categorized into six subpopulations. The transformation of primary CRC into LM involved two evolutionary branches of Tex cells. Epithelial cells two were at the beginning of the trajectory in CRC but at the end of the trajectory in CRC with LM. The receptor ligands CEACAM5 and ADGRE5-CD55 played critical roles in the interactions between Tex and Treg cell-epithelial cell, which may promote the epithelial-mesenchymal transition process in CRC. Tex cells are able to promote the process of LM in CRC, which in turn promotes tumour development. This provides a new perspective on the treatment and diagnosis of CRC.

Aberrant RNA sensing in regulatory T cells causes systemic autoimmunity

Chronic and aberrant nucleic acid sensing causes type I IFN-driven autoimmune diseases, designated type I interferonopathies. We found a significant reduction of regulatory T cells (Tregs) in patients with type I interferonopathies caused by mutations in ADAR1 or IFIH1 (encoding MDA5). We analyzed the underlying mechanisms using murine models and found that Treg-specific deletion of Adar1 caused peripheral Treg loss and scurfy-like lethal autoimmune disorders. Similarly, knock-in mice with Treg-specific expression of an MDA5 gain-of-function mutant caused apoptosis of peripheral Tregs and severe autoimmunity. Moreover, the impact of ADAR1 deficiency on Tregs is multifaceted, involving both MDA5 and PKR sensing. Together, our results highlight the dysregulation of Treg homeostasis by intrinsic aberrant RNA sensing as a potential determinant for type I interferonopathies.

Integrated BATF transcriptional network regulates suppressive intratumoral regulatory T cells

Human regulatory T cells (Tregs) are crucial regulators of tissue repair, autoimmune diseases, and cancer. However, it is challenging to inhibit the suppressive function of Tregs for cancer therapy without affecting immune homeostasis. Identifying pathways that may distinguish tumor-restricted Tregs is important, yet the transcriptional programs that control intratumoral Treg gene expression, and that are distinct from Tregs in healthy tissues, remain largely unknown. We profiled single-cell transcriptomes of CD4+ T cells in tumors and peripheral blood from patients with head and neck squamous cell carcinomas (HNSCC) and those in nontumor tonsil tissues and peripheral blood from healthy donors. We identified a subpopulation of activated Tregs expressing multiple tumor necrosis factor receptor (TNFR) genes (TNFR+ Tregs) that is highly enriched in the tumor microenvironment (TME) compared with nontumor tissue and the periphery. TNFR+ Tregs are associated with worse prognosis in HNSCC and across multiple solid tumor types. Mechanistically, the transcription factor BATF is a central component of a gene regulatory network that governs key aspects of TNFR+ Tregs. CRISPR-Cas9-mediated BATF knockout in human activated Tregs in conjunction with bulk RNA sequencing, immunophenotyping, and in vitro functional assays corroborated the central role of BATF in limiting excessive activation and promoting the survival of human activated Tregs. Last, we identified a suite of surface molecules reflective of the BATF-driven transcriptional network on intratumoral Tregs in patients with HNSCC. These findings uncover a primary transcriptional regulator of highly suppressive intratumoral Tregs, highlighting potential opportunities for therapeutic intervention in cancer without affecting immune homeostasis.

Heparin Prowess: Favorable Vascular-Immune Reprogramming in Pancreatic Cancer

Several approaches for overcoming immunotherapy resistance in pancreatic and colorectal cancer syngeneic models were assessed using heparin and immunotherapy. Beneficial responses were attributed to heparin-induced vascular normalization, ensuing CD8+ T-cell infiltration, and M1 macrophage polarization, suggesting the potential for heparin-anchored therapies in cold tumors such as pancreatic cancer. See related article by Wei et al., p. 2525.

FAIM Enhances the Efficacy of Mesenchymal Stem Cell Transplantation by Inhibiting JNK-Induced c-FLIP Ubiquitination and Degradation

Background

The poor survival rates of transplanted mesenchymal stem cells (MSCs) in harsh microenvironments impair the efficacy of MSCs transplantation in myocardial infarction (MI). Extrinsic apoptosis pathways play an important role in the apoptosis of transplanted MSCs, and Fas apoptosis inhibitory molecule (FAIM) is involved in regulation of the extrinsic apoptosis pathway. Thus, we aimed to explore whether FAIM augmentation protects MSCs against stress-induced apoptosis and thereby improves the therapeutic efficacy of MSCs.

Methods

We ligated the left anterior descending coronary artery (LAD) in the mouse heart to generate an MI model and then injected FAIM-overexpressing MSCs (MSCs_FAIM) into the peri-infarction area in vivo. Moreover, FAIM-overexpressing MSCs were challenged with oxygen, serum, and glucose deprivation (OGD) in vitro, which mimicked the harsh microenvironment that occurs in cardiac infarction.

Results

FAIM was markedly downregulated under OGD conditions, and FAIM overexpression protected MSCs against OGD-induced apoptosis. MSCs_FAIM transplantation improved cell retention, strengthened angiogenesis, and ameliorated heart function. The antiapoptotic effect of FAIM was mediated by cellular-FLICE inhibitory protein (c-FLIP), and FAIM augmentation improved the protein expression of c-FLIP by reducing ubiquitin–proteasome-dependent c-FLIP degradation. Furthermore, FAIM inhibited the activation of JNK, and treatment with the JNK inhibitor SP600125 abrogated the reduction in c-FLIP protein expression caused by FAIM silencing.

Conclusions

Overall, these results indicated that FAIM curbed the JNK-mediated, ubiquitination–proteasome-dependent degradation of c-FLIP, thereby improving the survival of transplanted MSCs and enhancing their efficacy in MI. This study may provide a novel approach to strengthen the therapeutic effect of MSC-based therapy.

Tumour Derived Extracellular Vesicles: Challenging Target to Blunt Tumour Immune Evasion

Simple Summary

Tumour onset and development occur because of specific immune support. The immune system, which is originally able to perceive and eliminate incipient cancer cells, becomes suppressed and hijacked by cancer. For these purposes, tumour cells use extracellular vesicles (TEVs). Specific molecular composition allows TEVs to reprogram immune cells towards tumour tolerance. Circulating TEVs move from their site of origin to other organs, preparing “a fertile soil” for metastasis formation. This implies that TEV molecular content can provide a valuable tool for cancer biomarker discovery and potential targets to reshape the immune system into tumour recognition and eradication.

Abstract

Control of the immune response is crucial for tumour onset and progression. Tumour cells handle the immune reaction by means of secreted factors and extracellular vesicles (EV). Tumour-derived extracellular vesicles (TEV) play key roles in immune reprogramming by delivering their cargo to different immune cells. Tumour-surrounding tissues also contribute to tumour immune editing and evasion, tumour progression, and drug resistance via locally released TEV. Moreover, the increase in circulating TEV has suggested their underpinning role in tumour dissemination. This review brings together data referring to TEV-driven immune regulation and antitumour immune suppression. Attention was also dedicated to TEV-mediated drug resistance.

Caspase-8 has dual roles in regulatory T cell homeostasis balancing immunity to infection and collateral inflammatory damage

Targeting the potent immunosuppressive properties of FOXP3⁺ regulatory T cells (T_regs) has substantial therapeutic potential for treating autoimmune and inflammatory diseases. Yet, the molecular mechanisms controlling T_reg homeostasis, particularly during inflammation, remain unclear. We report that caspase-8 is a central regulator of T_reg homeostasis in a context-specific manner that is decisive during immune responses. In mouse genetic models, targeting caspase-8 in T_regs led to accumulation of effector T_regs resistant to apoptotic cell death. Conversely, inflammation induced the MLKL-dependent necroptosis of caspase-8-deficient lymphoid and tissue T_regs, which enhanced immunity to a variety of chronic infections to promote clearance of viral or parasitic pathogens. However, improved immunity came at the risk of lethal inflammation in overwhelming infections. Caspase-8 inhibition using a clinical-stage compound revealed that human T_regs have heightened sensitivity to necroptosis compared with conventional T cells. These findings reveal a fundamental mechanism in T_regs that could be targeted to manipulate the balance between immune tolerance versus response for therapeutic benefit.

Interaction between the Hepatitis B Virus and Cellular FLIP Variants in Viral Replication and the Innate Immune System

During viral evolution and adaptation, many viruses have utilized host cellular factors and machinery as their partners. HBx, as a multifunctional viral protein encoded by the hepatitis B virus (HBV), promotes HBV replication and greatly contributes to the development of HBV-associated hepatocellular carcinoma (HCC). HBx interacts with several host factors in order to regulate HBV replication and evolve carcinogenesis. The cellular FADD-like IL-1β-converting enzyme (FLICE)-like inhibitory protein (c-FLIP) is a major factor that functions in a variety of cellular pathways and specifically in apoptosis. It has been shown that the interaction between HBx and c-FLIP determines HBV fate. In this review, we provide a comprehensive and detailed overview of the interplay between c-FLIP and HBV in various environmental circumstances. We describe strategies adapted by HBV to establish its chronic infection. We also summarize the conventional roles of c-FLIP and highlight the functional outcome of the interaction between c-FLIP and HBV or other viruses in viral replication and the innate immune system.

A Central Role for Atg5 in Microbiota-Dependent Foxp3+ RORγt+ Treg Cell Preservation to Maintain Intestinal Immune Homeostasis

Autophagy is an evolutionary conserved catabolic pathway that ensures the degradation of intracellular components. The autophagic pathway is regulated by autophagy-related (Atg) proteins that govern formation of double-membraned vesicles called autophagosomes. Autophagy deficiency in regulatory T (Treg) cells leads to increased apoptosis of these cells and to the development of autoimmune disorders, predominantly characterized by intestinal inflammation. Recently, RORγt-expressing Treg cells have been identified as key regulators of gut homeostasis, preventing intestinal immunopathology. To study the role of autophagy in RORγt⁺ Foxp3⁺ Treg cells, we generated mice lacking the essential component of the core autophagy machinery Atg5 in Foxp3⁺ cells. Atg5 deficiency in Treg cells led to a predominant intestinal inflammation. While Atg5-deficient Treg cells were reduced in peripheral lymphoid organs, the intestinal RORγt⁺ Foxp3⁺ subpopulation of Treg cells was most severely affected. Our data indicated that autophagy is essential to maintain the intestinal RORγt⁺ Foxp3⁺ Treg population, thereby protecting the mice from gut inflammatory disorders.

Functional heterogeneity of CD4+ T cells in liver inflammation

CD4⁺ T cells play an essential role in orchestrating adequate immunity, but their overactivity has been associated with the development of immune-mediated inflammatory diseases, including liver inflammatory diseases. These cells can be subclassified according to their maturation stage, cytokine profile, and pro or anti-inflammatory functions, i.e., functional heterogeneity. In this review, we summarize what has been discovered so far regarding the role of the different CD4⁺ T cell polarization states in the progression of two prominent and still different liver inflammatory diseases: non-alcoholic steatohepatitis (NASH) and autoimmune hepatitis (AIH). Finally, the potential of CD4⁺ T cells as a therapeutic target in both NASH and AIH is discussed.

Oxidative stress response in regulatory and conventional T cells: a comparison between patients with chronic coronary syndrome and healthy subjects

Background

Inflammation and oxidative stress form a vicious circle in atherosclerosis. Oxidative stress can have detrimental effects on T cells. A unique subset of CD4⁺ T cells, known as regulatory T (T_reg) cells, has been associated with atheroprotective effects. Reduced numbers of T_reg cells is a consistent finding in patients with chronic coronary syndrome (CCS). However, it is unclear to what extent these cells are sensitive to oxidative stress. In this pilot study, we tested the hypothesis that oxidative stress might be a potential contributor to the T_reg cell deficit in CCS patients.

Methods

Thirty patients with CCS and 24 healthy controls were included. T_reg (CD4⁺CD25⁺CD127⁻) and conventional T (CD4⁺CD25⁻, T_conv) cells were isolated and treated with increasing doses of H₂O₂. Intracellular ROS levels and cell death were measured after 2 and 18 h, respectively. The expression of antioxidant genes was measured in freshly isolated T_reg and T_conv cells. Also, total antioxidant capacity (TAC) was measured in fresh peripheral blood mononuclear cells, and oxidized (ox) LDL/LDL ratios were determined in plasma.

Results

At all doses of H₂O_2, T_reg cells accumulated more ROS and exhibited higher rates of death than their T_conv counterparts, p < 0.0001. T_reg cells also expressed higher levels of antioxidant genes, including thioredoxin and thioredoxin reductase-1 (p < 0.0001), though without any differences between CCS patients and controls. T_conv cells from CCS patients were, on the other hand, more sensitive to oxidative stress ex vivo and expressed more thioredoxin reductase-1 than T_conv cells from controls, p < 0.05. Also, TAC levels were lower in patients, 0.97 vs 1.53 UAE/100 µg, p = 0.001, while oxLDL/LDL ratios were higher, 29 vs 22, p = 0.006.

Conclusion

T_reg cells isolated from either CCS patients or healthy controls were all highly sensitive to oxidative stress ex vivo. There were signs of oxidant-antioxidant imbalance in CCS patients and we thus assume that oxidative stress may play a role in the reduction of T_reg cells in vivo.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12967-021-02906-2.

Anti-pyroptotic function of TGF-β is suppressed by a synthetic dsRNA analogue in triple negative breast cancer cells

Development of innovative therapeutic modalities would address an unmet clinical need in the treatment of triple negative breast cancer (TNBC). Activation of retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) such as melanoma differentiation-associated gene 5 (MDA5) and RIG-I in cancer cells is suggested to suppress tumor progression by inducing cell death. Transfection of polyI:C, a conventionally used synthetic double-stranded RNA (dsRNA) analogue that activates RLRs, has been evaluated in clinical trials. However, detailed mechanisms of tumor suppression by RLRs, especially interactions with other signaling pathways, remain elusive. Here, we showed that transfection of polyI:C suppressed transforming growth factor-β (TGF-β) signaling in a MDA5- and RIG-I-dependent manner. We found that suppression of TGF-β signaling by polyI:C promoted cancer cell death, which was attenuated by forced expression of constitutively active Smad3. More detailed analysis suggested that cell death by polyI:C transfection exhibited characteristics of pyroptosis, which is distinct from apoptosis. Therapeutic efficacy of polyI:C transfection was also demonstrated using a mouse model. These results indicated that intratumor administration of polyI:C and related dsRNA analogues may be promising treatments for TNBC through inhibition of the anti-pyroptotic function of TGF-β.

c-FLIP regulates pyroptosis in retinal neurons following oxygen-glucose deprivation/recovery via a GSDMD-mediated pathway

Cellular FLICE-inhibitory protein (c-FLIP), an anti-apoptotic regulator, shows remarkable similarities to caspase-8, which plays a key role in the cleavage of gasdermin D (GSDMD). It has been reported that the oxygen-glucose deprivation/recovery (OGD/R) model and lipopolysaccharide (LPS)/adenosine triphosphate (ATP) treatment could induce inflammation and pyroptosis. However, the regulatory role of c-FLIP in the pyroptotic death of retinal neurons is unclear. In this study, we hypothesized that c-FLIP might regulate retinal neuronal pyroptosis by GSDMD cleavage. To investigate this hypothesis, we induced retinal neuronal damage in vitro (OGD/R and LPS/ATP) and in vivo (acute high intraocular pressure [aHIOP]). Our results demonstrated that the three injuries triggered the up-regulation of pyroptosis-related proteins, and c-FLIP could cleave GSDMD to generate a functional N-terminal (NT) domain of GSDMD, causing retinal neuronal pyroptosis. In addition, c-FLIP knockdown in vivo ameliorated the already established visual impairment mediated by acute IOP elevation. Taken together, these findings revealed that decreased c-FLIP expression protected against pyroptotic death of retinal neurons possibly by inhibiting GSDMD-NT generation. Therefore, c-FLIP might provide new insights into the pathogenesis of pyroptosis-related diseases and help to elucidate new therapeutic targets and potential treatment strategies.

Influenza A virus-induced thymus atrophy differentially affects dynamics of conventional and regulatory T-cell development in mice

Foxp3⁺ Treg cells, which are crucial for maintenance of self-tolerance, mainly develop within the thymus, where they arise from CD25⁺ Foxp3^- or CD25^- Foxp3⁺ Treg cell precursors. Although it is known that infections can cause transient thymic involution, the impact of infection-induced thymus atrophy on thymic Treg (tTreg) cell development is unknown. Here, we infected mice with influenza A virus (IAV) and studied thymocyte population dynamics post infection. IAV infection caused a massive, but transient thymic involution, dominated by a loss of CD4⁺ CD8⁺ double-positive (DP) thymocytes, which was accompanied by a significant increase in the frequency of CD25⁺ Foxp3⁺ tTreg cells. Differential apoptosis susceptibility could be experimentally excluded as a reason for the relative tTreg cell increase, and mathematical modeling suggested that enhanced tTreg cell generation cannot explain the increased frequency of tTreg cells. Yet, an increased death of DP thymocytes and augmented exit of single-positive (SP) thymocytes was suggested to be causative. Interestingly, IAV-induced thymus atrophy resulted in a significantly reduced T-cell receptor (TCR) repertoire diversity of newly produced tTreg cells. Taken together, IAV-induced thymus atrophy is substantially altering the dynamics of major thymocyte populations, finally resulting in a relative increase of tTreg cells with an altered TCR repertoire.

Moonlighting Proteins Are Important Players in Cancer Immunology

Plasticity and adaptation to environmental stress are the main features that tumor and immune system share. Except for intrinsic and high-defined properties, cancer and immune cells need to overcome the opponent's defenses by activating more effective signaling networks, based on common elements such as transcriptional factors, protein-based complexes and receptors. Interestingly, growing evidence point to an increasing number of proteins capable of performing diverse and unpredictable functions. These multifunctional proteins are defined as moonlighting proteins. During cancer progression, several moonlighting proteins are involved in promoting an immunosuppressive microenvironment by reprogramming immune cells to support tumor growth and metastatic spread. Conversely, other moonlighting proteins support tumor antigen presentation and lymphocytes activation, leading to several anti-cancer immunological responses. In this light, moonlighting proteins could be used as promising new potential targets for improving current cancer therapies. In this review, we describe in details 12 unprecedented moonlighting proteins that during cancer progression play a decisive role in guiding cancer-associated immunomodulation by shaping innate or adaptive immune response.

RelB regulates the homeostatic proliferation but not the function of Tregs

Background

RelB, a member of the NF-κB family, plays a critical role in the development of T cells. However, the role of RelB in Foxp3⁺ regulatory T cells (Tregs) remains controversial.

Results

Using a bone marrow chimeric mouse model, we demonstrated that the expansion of Foxp3⁺ Tregs in vivo could be mediated by extrinsic mechanisms. RelB plays an important role in inhibiting the homeostatic proliferation of Tregs, but not their survival. Even with the heightened expansion, RelB^−/− Treg cells displayed normal suppressive function in vitro. Among the expanded populations of Treg cells, most were nTreg cells; however, the population of iTregs did not increase. Mechanistically, RelB seems to regulate Treg proliferation independently of the signal transducer and activator of transcription 5 (STAT5) pathway.

Conclusions

These data suggest that RelB regulates Treg proliferation independently of the STAT5 pathway, but does not alter the function of Tregs. Further studies are warranted to uncover such mechanisms.

Post-mortem multiple sclerosis lesion pathology is influenced by single nucleotide polymorphisms

Over the last few decades, several common single nucleotide polymorphisms (SNPs) have been identified that correlate with clinical outcome in multiple sclerosis (MS), but the pathogenic mechanisms underlying the clinical effects of these SNPs are unknown. This is in part because of the difficulty in the functional translation of genotype into disease-relevant mechanisms. Building on our recent work showing the association of clinical disease course with post-mortem MS lesion characteristics, we hypothesized that SNPs that correlate with clinical disease course would also correlate with specific MS lesion characteristics in autopsy tissue. To test this hypothesis, 179 MS brain donors from the Netherlands Brain Bank MS autopsy cohort were genotyped for 102 SNPs, selected based on their reported associations with clinical outcome or their associations with genes that show differential gene expression in MS lesions. Three SNPs linked to MS clinical severity showed a significant association between the genotype and either the proportion of active lesions (rs2234978/FAS and rs11957313/KCNIP1) or the proportion of mixed active/inactive lesions (rs8056098/CLEC16A). Three SNPs linked to MS pathology-associated genes showed a significant association with either proportion of active lesions (rs3130253/MOG), incidence of cortical gray matter lesions (rs1064395/NCAN) or the proportion of remyelinated lesions (rs5742909/CTLA4). In addition, rs2234978/FAS T-allele carriers showed increased FAS gene expression levels in perivascular T cells and perilesional oligodendrocytes, cell types that have been implicated in MS lesion formation. Thus, by combining pathological characterization of MS brain autopsy tissue with genetics, we now start to translate genotypes linked to clinical outcomes in MS into mechanisms involved in MS lesion pathogenesis.

Activation-induced cell death of self-reactive regulatory T cells drives autoimmunity

Activation of self-reactive T cells is a major driver to autoimmunity and is suppressed by mechanisms of regulation. In a humanized model of autoimmune thyroiditis, we investigated the mechanism underlying break of tolerance. Here, we found that a human TCR specific for the self-antigen thyroid peroxidase (TPO) is positively selected in the thymus of RAG KO mice on both T effector (T_eff) and T regulatory (T_reg) CD4⁺Foxp3⁺ cells. In vivo T_eff are present in all immune organs, whereas the TPO-specific T_reg are present in all lymphoid organs with the exception of the thyroid-draining lymph nodes. We suggest that the presence of TPO in the thyroid draining lymph nodes induces the activation of T_eff and the depletion of T_reg via activation-induced cell death (AICD). Our findings provide insights on the failure of the mechanisms of immune tolerance, with potential implications in designing immunotherapeutic strategies.

Targeting Regulatory T Cells by Addressing Tumor Necrosis Factor and Its Receptors in Allogeneic Hematopoietic Cell Transplantation and Cancer

An intricate network of molecular and cellular actors orchestrates the delicate balance between effector immune responses and immune tolerance. The pleiotropic cytokine tumor necrosis factor-alpha (TNF) proves as a pivotal protagonist promoting but also suppressing immune responses. These opposite actions are accomplished through specialist cell types responding to TNF via TNF receptors TNFR1 and TNFR2. Recent findings highlight the importance of TNFR2 as a key regulator of activated natural FoxP3⁺ regulatory T cells (Tregs) in inflammatory conditions, such as acute graft-vs.-host disease (GvHD) and the tumor microenvironment. Here we review recent advances in our understanding of TNFR2 signaling in T cells and discuss how these can reconcile seemingly conflicting observations when manipulating TNF and TNFRs. As TNFR2 emerges as a new and attractive target we furthermore pinpoint strategies and potential pitfalls for therapeutic targeting of TNFR2 for cancer treatment and immune tolerance after allogeneic hematopoietic cell transplantation.

Apoptosis of tumor-infiltrating T lymphocytes: a new immune checkpoint mechanism

Immunotherapy based on checkpoint inhibitors is providing substantial clinical benefit, but only to a minority of cancer patients. The current priority is to understand why the majority of patients fail to respond. Besides T-cell dysfunction, T-cell apoptosis was reported in several recent studies as a relevant mechanism of tumoral immune resistance. Several death receptors (Fas, DR3, DR4, DR5, TNFR1) can trigger apoptosis when activated by their respective ligands. In this review, we discuss the immunomodulatory role of the main death receptors and how these are shaping the tumor microenvironment, with a focus on Fas and its ligand. Fas-mediated apoptosis of T cells has long been known as a mechanism allowing the contraction of T-cell responses to prevent immunopathology, a phenomenon known as activation-induced cell death, which is triggered by induction of Fas ligand (FasL) expression on T cells themselves and qualifies as an immune checkpoint mechanism. Recent evidence indicates that other cells in the tumor microenvironment can express FasL and trigger apoptosis of tumor-infiltrating lymphocytes (TIL), including endothelial cells and myeloid-derived suppressor cells. The resulting disappearance of TIL prevents anti-tumor immunity and may in fact contribute to the absence of TIL that is typical of "cold" tumors that fail to respond to immunotherapy. Interfering with the Fas-FasL pathway in the tumor microenvironment has the potential to increase the efficacy of cancer immunotherapy.

c-FLIP, a Novel Biomarker for Cancer Prognosis, Immunosuppression, Alzheimer's Disease, Chronic Obstructive Pulmonary Disease (COPD), and a Rationale Therapeutic Target

Dysregulation of c-FLIP (cellular FADD-like IL-1β-converting enzyme inhibitory protein) has been shown in several diseases including cancer, Alzheimer's disease, and chronic obstructive pulmonary disease (COPD). c-FLIP is a critical anti-cell death protein often overexpressed in tumors and hematological malignancies and its increased expression is often associated with a poor prognosis. c-FLIP frequently exists as long (c-FLIPL) and short (c-FLIPS) isoforms, regulates its anti-cell death functions through binding to FADD (FAS associated death domain protein), an adaptor protein known to activate caspases-8 and -10 and links c-FLIP to several cell death regulating complexes including the death-inducing signaling complex (DISC) formed by various death receptors. c-FLIP also plays a critical role in necroptosis and autophagy. Furthermore, c-FLIP is able to activate several pathways involved in cytoprotection, proliferation, and survival of cancer cells through various critical signaling proteins. Additionally, c-FLIP can inhibit cell death induced by several chemotherapeutics, anti-cancer small molecule inhibitors, and ionizing radiation. Moreover, c-FLIP plays major roles in aiding the survival of immunosuppressive tumor-promoting immune cells and functions in inflammation, Alzheimer's disease (AD), and chronic obstructive pulmonary disease (COPD). Therefore, c-FLIP can serve as a versatile biomarker for cancer prognosis, a diagnostic marker for several diseases, and an effective therapeutic target. In this article, we review the functions of c-FLIP as an anti-apoptotic protein and negative prognostic factor in human cancers, and its roles in resistance to anticancer drugs, necroptosis and autophagy, immunosuppression, Alzheimer's disease, and COPD.

Induction of immunosuppressive functions and NF-κB by FLIP in monocytes

Immunosuppression is a hallmark of tumor progression, and treatments that inhibit or deplete monocytic myeloid-derived suppressive cells could promote anti-tumor immunity. c-FLIP is a central regulator of caspase-8-mediated apoptosis and necroptosis. Here we show that low-dose cytotoxic chemotherapy agents cause apoptosis linked to c-FLIP down-regulation selectively in monocytes. Enforced expression of c-FLIP or viral FLIP rescues monocytes from cytotoxicity and concurrently induces potent immunosuppressive activity, in T cell cultures and in vivo models of tumor progression and immunotherapy. FLIP-transduced human blood monocytes can suppress graft versus host disease. Neither expression of FLIP in granulocytes nor expression of other anti-apoptotic genes in monocytes conferred immunosuppression, suggesting that FLIP effects on immunosuppression are specific to monocytic lineage and distinct from death inhibition. Mechanistically, FLIP controls a broad transcriptional program, partially by NF-κB activation. Therefore, modulation of FLIP in monocytes offers a means to elicit or block immunosuppressive myeloid cells.

FLIP as a therapeutic target in cancer

One of the classic hallmarks of cancer is disruption of cell death signalling. Inhibition of cell death promotes tumour growth and metastasis, causes resistance to chemo- and radiotherapies as well as targeted agents, and is frequently due to overexpression of antiapoptotic proteins rather than loss of pro-apoptotic effectors. FLIP is a major apoptosis-regulatory protein frequently overexpressed in solid and haematological cancers, in which its high expression is often correlated with poor prognosis. FLIP, which is expressed as long (FLIP(L)) and short (FLIP(S)) splice forms, achieves its cell death regulatory functions by binding to FADD, a critical adaptor protein which links FLIP to the apical caspase in the extrinsic apoptotic pathway, caspase-8, in a number of cell death regulating complexes, such as the death-inducing signalling complexes (DISCs) formed by death receptors. FLIP also plays a key role (together with caspase-8) in regulating another form of cell death termed programmed necrosis or 'necroptosis', as well as in other key cellular processes that impact cell survival, including autophagy. In addition, FLIP impacts activation of the intrinsic mitochondrial-mediated apoptotic pathway by regulating caspase-8-mediated activation of the pro-apoptotic Bcl-2 family member Bid. It has been demonstrated that FLIP can not only inhibit death receptor-mediated apoptosis, but also cell death induced by a range of clinically relevant chemotherapeutic and targeted agents as well as ionizing radiation. More recently, key roles for FLIP in promoting the survival of immunosuppressive tumour-promoting immune cells have been discovered. Thus, FLIP is of significant interest as an anticancer therapeutic target. In this article, we review FLIP's biology and potential ways of targeting this important tumour and immune cell death regulator.

Prolonged IKKβ Inhibition Improves Ongoing CTL Antitumor Responses by Incapacitating Regulatory T Cells

Regulatory T cells (Tregs) prevent autoimmunity but limit antitumor immunity. The canonical NF-κB signaling pathway both activates immunity and promotes thymic Treg development. Here, we report that mature Tregs continue to require NF-κB signaling through IκB-kinase β (IKKβ) after thymic egress. Mice lacking IKKβ in mature Tregs developed scurfy-like immunopathology due to death of peripheral FoxP3⁺ Tregs. Also, pharmacological IKKβ inhibition reduced Treg numbers in the circulation by ∼50% and downregulated FoxP3 and CD25 expression and STAT5 phosphorylation. In contrast, activated cytotoxic T lymphocytes (CTLs) were resistant to IKKβ inhibition because other pathways, in particular nuclear factor of activated T cells (NFATc1) signaling, sustained their survival and expansion. In a melanoma mouse model, IKKβ inhibition after CTL cross-priming improved the antitumor response and delayed tumor growth. In conclusion, prolonged IKKβ inhibition decimates circulating Tregs and improves CTL responses when commenced after tumor vaccination, indicating that IKKβ represents a druggable checkpoint.

Role of the NF-κB Family Member RelB in Regulation of Foxp3+ Regulatory T Cells In Vivo

The NF-κB family member RelB is an important transcription factor that is capable of regulating diverse immune and inflammatory responses. However, its role in the regulation of Foxp3⁺ regulatory T cells (Tregs) in vivo is poorly defined. In this study, we demonstrated that germline deletion of Relb resulted in systemic autoimmunity, which is associated with significant accumulation of Foxp3⁺ Tregs in lymphoid and nonlymphoid organs. Foxp3⁺ Tregs from RelB-deficient mice were functional and capable of suppressing T effector cells in vitro and in vivo, but Foxp3^- T effector cells from RelB-deficient mice showed features of hyperactivation and spontaneously produced high levels of IL-2. Surprisingly, mice with conditional deletion of Relb in T cells (Cd4^CreRelb^f/f mice) or specifically in Foxp3⁺ Tregs (Foxp3^CreRelb^f/f mice) did not show signs of autoimmunity and had similar frequencies of Foxp3⁺ Tregs in the periphery as wild-type C57BL/6 controls. Both strains of conditional knockout mice also had a normal conventional T cell compartment. However, reconstituting Rag-1^-/-Relb^-/- hosts with wild-type C57BL/6 bone marrow cells led to hyperactivation of T effector cells, as well as marked expansion of Foxp3⁺ T cells. These data suggest that the autoimmune phenotype in germline RelB-deficient mice is most likely caused by T cell-extrinsic mechanisms, and further studies are warranted to uncover such mechanisms.