Dying to protect: cell death and the control of T-cell homeostasis

When inflammatory stressors dramatically change, disease phenotypes may transform between autoimmune hematopoietic failure and myeloid neoplasms

Aplastic anemia (AA) and hypoplastic myelodysplastic syndrome are paradigms of autoimmune hematopoietic failure (AHF). Myelodysplastic syndrome and acute myeloid leukemia are unequivocal myeloid neoplasms (MNs). Currently, AA is also known to be a clonal hematological disease. Genetic aberrations typically observed in MNs are detected in approximately one-third of AA patients. In AA patients harboring MN-related genetic aberrations, a poor response to immunosuppressive therapy (IST) and an increased risk of transformation to MNs occurring either naturally or after IST are predicted. Approximately 10%-15% of patients with severe AA transform the disease phenotype to MNs following IST, and in some patients, leukemic transformation emerges during or shortly after IST. Phenotypic transformations between AHF and MNs can occur reciprocally. A fraction of advanced MN patients experience an aplastic crisis during which leukemic blasts are repressed. The switch that shapes the disease phenotype is a change in the strength of extramedullary inflammation. Both AHF and MNs have an immune-active bone marrow (BM) environment (BME). In AHF patients, an inflamed BME can be evoked by infiltrated immune cells targeting neoplastic molecules, which contributes to the BM-specific autoimmune impairment. Autoimmune responses in AHF may represent an antileukemic mechanism, and inflammatory stressors strengthen antileukemic immunity, at least in a significant proportion of patients who have MN-related genetic aberrations. During active inflammatory episodes, normal and leukemic hematopoieses are suppressed, which leads to the occurrence of aplastic cytopenia and leukemic cell regression. The successful treatment of underlying infections mitigates inflammatory stress-related antileukemic activities and promotes the penetration of leukemic hematopoiesis. The effect of IST is similar to that of treating underlying infections. Investigating inflammatory stress-powered antileukemic immunity is highly important in theoretical studies and clinical practice, especially given the wide application of immune-activating agents and immune checkpoint inhibitors in the treatment of hematological neoplasms.

Effect of Med1 on T cell development and CD4+ T cell differentiation in immune response

OBJECTIVES

The differentiation of CD4+ T cells is regulated by a complex and fine signaling pathway composed of many molecules during immune response, and the molecular mechanism for regulating T-bet expression is unclear. Mediator complex subunit 1 (Med1) can combine with a variety of co-factors to regulate gene transcription, promote cell proliferation and survival, and affect invariant natural killer T cell (iNKT) development. This study aims to investigate the effect of Med1 on T cell development and CD4+ T cell differentiation in immune response.

METHODS

Mice with T cell-specific knockout of Med1 gene (Med1F/FCD4cre+, KO) were constructed and verified. The percentage and number of CD4+ and CD8+ T cells in thymus, spleen, and lymph nodes of KO mice and control (Con) mice (Med1F/FCD4cre-) were detected by flow cytometry. After 8 days of infection with lymphocytic choriomeningitis virus (LCMV), the percentage and number of CD4+ T cells or antigen-specific (GP66+) CD4+ T cells, the percentage and number of Th1 cells (Ly6c+PSGL1+) in CD4+ T cells or antigen-specific CD4+ T cells were examined in the spleen of mice. Moreover, the fluorescence intensity of T-bet in CD4+ T cells or antigen-specific CD4+ T cells was analyzed.

RESULTS

Compared with the Con group, the percentage and number of CD4+ T cells and CD8+ T cells in the thymus, CD4+ T cells in the spleen and lymph nodes of the KO group showed no significant differences (all P>0.05), but the percentage and number of CD8+ T cells in the spleen and lymph nodes of the KO group were diminished significantly (all P<0.05). After 8 days of infection with LCMV, there was no significant difference in the percentage and number of CD4+ T cells or antigen-specific CD4+ T cells in the spleen between the KO group and the Con group (all P>0.05), while in comparison with the Con group, the percentage and number of Th1 cells in CD4+ T cells or antigen-specific CD4+ T cells, and the expression of T-bet in CD4+ T cells or antigen-specific CD4+ T cells were significantly reduced in the spleen of the KO group (all P<0.05).

CONCLUSIONS

Specific knockout of Med1 in T cells does not affect the development of CD4+ and CD8+ T cells in the thymus, but does affect the maintenance of peripheral CD8+ T cells. In the immune response, Med1 gene deletion affects the expression of transcription factor T-bet, which in turn to reduce Th1 cell differentiation.

Anti-PD1 does not improve pyroptosis induced by γδ T cells but promotes tumor regression in a pleural mesothelioma mouse model

Introduction

Mesothelioma is an aggressive tumor in the pleural cavity that is difficult to treat. Diagnosis is usually late with minimal treatment options available for the patients and with unfavorable outcomes. However, recent advances in immunotherapy using γδ T cells may have potential against mesothelioma, given its ample tumoricidal and tumor-migratory properties could allow its infiltration to the widespread tumor mass. Thus, we hypothesize that Vδ2 T cells can perform cytotoxic activities against mesothelioma especially when combined with immune checkpoint blocker against PD-1.

Methods

Human Vδ2 T cells were expanded from peripheral blood mononuclear cells using Tetrakis-pivaloyloxymethyl 2-(thiazole-2-ylamino) ethylidene-1,1-bisphosphonate (PTA) plus IL-2 for 13 days, before used to test for cytotoxicity against mesothelioma cell lines. Mesothelioma-bearing mice was established by Intrapleural administration of mesothelioma cell lines to test for the efficacy of Vδ2 T cells plus anti-PD-1 antibody combination treatment. Pyroptosis was evaluated by cell morphology, western blot analysis, and ELISA experiments. Flow cytometry was used to examine expression of BTN2A1, BTN3A1, PD-L1, PD-L2 on mesothelioma cell lines. Immunofluorescence staining was performed to detect Vδ2 T cells post adoptive transfer and characteristics of pyroptosis in ex vivo mesothelioma tissue sections.

Results

Indeed, our data demonstrated that Vδ2 T cells killing mesothelioma can be enhanced by anti-PD-1 antibody in vitro, especially for high PD-1 expressing cells, and in vivo in the intrapleural mesothelioma mice model established by us. Adoptive transfer of Vδ2 T cells into these mice leads to tumor regression by 30-40% compared to control. Immunofluorescence of the tumor section confirmed infiltration of Vδ2 T cells into the tumor, especially to cells with BTN2A1 expression (a Vδ2 T cell activating molecule) despite PD-L1 co-localization. Interestingly, these cells co-expressed cleaved gasdermin D, suggesting that pyroptosis was induced by Vδ2 T cells. This was verified by Vδ2 T/mesothelioma co-culture experiments demonstrating membrane ballooning morphology, increased cleaved caspase-3 and gasdermin E, and upregulated IL-1β and IL-18.

Discussion

Vδ2 T cells plus anti-PD1 exhibited cytotoxicity against mesothelioma in vivo. However, we found no advantage for anti-PD-1 against PD-1 high expressing Vδ2 T cells in promoting pyroptosis. Taken together, our work demonstrated that Vδ2 T cells combined with anti-PD-1 antibody can be developed as a potential combination immunotherapy for mesothelioma.

ROS fine-tunes the function and fate of immune cells

The redox state is essential to the process of cell life, which determines cell fate. As an important signaling molecule of the redox state, reactive oxygen species (ROS) are crucial for the homeostasis of immune cells and participate in the pathological processes of different diseases. We discuss the underlying mechanisms and possible signaling pathways of ROS to fine-tune the proliferation, differentiation, polarization and function of immune cells, including T cells, B cells, neutrophils, macrophages, myeloid-derived inhibitory cells (MDSCs) and dendritic cells (DCs). We further emphasize how excessive ROS lead to programmed immune cell death such as apoptosis, ferroptosis, pyroptosis, NETosis and necroptosis, providing valuable insights for future therapeutic strategies in human diseases.

Loss of RPA1 Impairs Peripheral T Cell Homeostasis and Exacerbates Inflammatory Damage through Triggering T Cell Necroptosis

The peripheral T cell pool is maintained at dynamic homeostasis through fine-tuning of thymic output and self-renewal of naïve T cells. Lymphopenia or reduced lymphocyte number is implicated in autoimmune diseases, yet little is known about the homeostatic mechanisms. Here, it is reported that the replication protein A1 (RPA1) plays a critical role in T cell homeostasis. Utilizing T cell-specific Rpa1-deficient (Rpa1^fl/fl Cd4-cre) mice, loss of Rpa1 results in lymphopenia through restraining peripheral T cell population and limiting TCR repertoire diversity. Moreover, Rpa1^fl/fl Cd4-cre mice exhibit increased susceptibility to inflammatory diseases, including colitis and hepatitis. Clinical analysis reveals that the expression of RPA1 is reduced in patients with ulcerative colitis or other autoinflammatory diseases. Mechanistically, depletion of RPA1 activates ZBP1-RIPK3 signaling through triggering the genomic DNA leakage into cytosol, consequently resulting in T cell necroptosis. This necroptotic T cell death induced by RPA1 deficiency allows the release of damage-associated molecular patterns (DAMPs), which in turn recruits leukocytes and exacerbates inflammatory response. Reciprocally, chemical or genetic inhibition of necroptosis signaling can ameliorate the Rpa1 deficiency-induced inflammatory damage. The studies thus uncover the importance of RPA1-ZBP1-RIPK3 axis in T cell homeostasis and provide a promising strategy for autoinflammatory disease treatment.

A Beginner's Guide to T Cell Development

T lymphocytes (T cells) are essential components of the adaptive immune system; they serve multiple functions in responses to pathogens and to ensure immune homeostasis. Written for readers first entering this field of study, this chapter is a brief overview of the development of T cells in the thymus, from the entry of thymus-settling bone marrow-derived precursors to the egress of mature T cells. Surveyed topics include the differentiation and expansion of early precursors, the generation of the T cell antigen receptor repertoire, the selection of αβ T cell precursors, and their acquisition of functional competency.

Restoring tolerance to β-cells in Type 1 diabetes: Current and emerging strategies

Type 1 diabetes (T1D) results from insulin insufficiency due to islet death and dysfunction following T cell-mediated autoimmune attack. The technical feasibility of durable, functional autologous islet restoration is progressing such that it presents the most likely long-term cure for T1D but cannot succeed without the necessary counterpart of clinically effective therapeutic strategies that prevent grafted islets' destruction by pre-existing anti-islet T cells. While advances have been made in broad immunosuppression to lower off-target effects, the risk of opportunistic infections and cancers remains a concern, especially for well-managed T1D patients. Current immunomodulatory strategies in development focus on autologous Treg expansion, treatments to decrease antigen presentation and T effector (Teff) activation, and broad depletion of T cells with or without hematopoietic stem cell transplants. Emerging strategies harnessing the intensified DNA damage response present in expanding T cells, exacerbating their already high sensitivity to apoptosis to abate autoreactive Teff cells.

Multilevel mechanism of immune checkpoint inhibitor action in solid tumors: History, present issues and future development (Review)

Immunotherapy with checkpoint inhibitors (antibodies that target and block immune checkpoints in the tumor microenvironment) is included in the standard of care for patients with different types of malignancy, such as melanoma, renal cell and urothelial carcinoma, lung cancer etc. The introduction of this new immunotherapy has altered the view on potential targets for treatment of solid tumors from tumor cells themselves to their immune microenvironment; this has led to a reconsideration of the mechanisms of tumor-associated immunity. However, only a subset of patients benefit from immunotherapy and patient response is often unpredictable, even with known initial levels of prognostic markers; the biomarkers for favorable response are still being investigated. Mechanisms of immune checkpoint inhibitors efficiency, as well as the origins of treatment failure, require further investigation. From a clinical standpoint, discrepancies between the theoretical explanation of inhibitors of immune checkpoint actions at the cellular level and their deployment at a tissue/organ level impede the effective clinical implementation of novel immune therapy. The present review assessed existing experimental and clinical data on functional activity of inhibitors of immune checkpoints to provide a more comprehensive picture of their mechanisms of action on a cellular and higher levels of biological organization.

Mechanisms of Immunosuppressive Tumor Evasion: Focus on Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is a malignancy with high heterogeneity in its biological features and treatments. Although the overall survival (OS) of patients with ALL has recently improved considerably, owing to the application of conventional chemo-therapeutic agents, approximately 20% of the pediatric cases and 40-50% of the adult patients relapse during and after the treatment period. The potential mechanisms that cause relapse involve clonal evolution, innate and acquired chemoresistance, and the ability of ALL cells to escape the immune-suppressive tumor response. Currently, immunotherapy in combination with conventional treatment is used to enhance the immune response against tumor cells, thereby significantly improving the OS in patients with ALL. Therefore, understanding the mechanisms of immune evasion by leukemia cells could be useful for developing novel therapeutic strategies.

Canonical Secretomes, Innate Immune Caspase-1-, 4/11-Gasdermin D Non-Canonical Secretomes and Exosomes May Contribute to Maintain Treg-Ness for Treg Immunosuppression, Tissue Repair and Modulate Anti-Tumor Immunity via ROS Pathways

We performed a transcriptomic analyses using the strategies we pioneered and made the following findings: 1) Normal lymphoid Tregs, diseased kidney Tregs, splenic Tregs from mice with injured muscle have 3, 17 and 3 specific (S-) pathways, respectively; 2) Tumor splenic Tregs share 12 pathways with tumor Tregs; tumor splenic Tregs and tumor Tregs have 11 and 8 S-pathways, respectively; 3) Normal and non-tumor disease Tregs upregulate some of novel 2641 canonical secretomic genes (SGs) with 24 pathways, and tumor Tregs upregulate canonical secretomes with 17 pathways; 4) Normal and non-tumor disease tissue Tregs upregulate some of novel 6560 exosome SGs with 56 exosome SG pathways (ESP), tumor Treg ESP are more focused than other Tregs; 5) Normal, non-tumor diseased Treg and tumor Tregs upregulate some of novel 961 innate immune caspase-1 SGs and 1223 innate immune caspase-4 SGs to fulfill their tissue/SG-specific and shared functions; 6) Most tissue Treg transcriptomes are controlled by Foxp3; and Tumor Tregs had increased Foxp3 non-collaboration genes with ROS and 17 other pathways; 7) Immune checkpoint receptor PD-1 does, but CTLA-4 does not, play significant roles in promoting Treg upregulated genes in normal and non-tumor disease tissue Tregs; and tumor splenic and tumor Tregs have certain CTLA-4-, and PD-1-, non-collaboration transcriptomic changes with innate immune dominant pathways; 8) Tumor Tregs downregulate more immunometabolic and innate immune memory (trained immunity) genes than Tregs from other groups; and 11) ROS significantly regulate Treg transcriptomes; and ROS-suppressed genes are downregulated more in tumor Tregs than Tregs from other groups. Our results have provided novel insights on the roles of Tregs in normal, injuries, regeneration, tumor conditions and some of canonical and innate immune non-canonical secretomes via ROS-regulatory mechanisms and new therapeutic targets for immunosuppression, tissue repair, cardiovascular diseases, chronic kidney disease, autoimmune diseases, transplantation, and cancers.

The transcription factors GFI1 and GFI1B as modulators of the innate and acquired immune response

GFI1 and GFI1B are small nuclear proteins of 45 and 37kDa, respectively, that have a simple two-domain structure: The first consists of a group of six c-terminal C₂H₂ zinc finger motifs that are almost identical in sequence and bind to very similar, specific DNA sites. The second is an N-terminal 20 amino acid SNAG domain that can bind to the pocket of the histone demethylase KDM1A (LSD1) near its active site. When bound to DNA, both proteins act as bridging factors that bring LSD1 and associated proteins into the vicinity of methylated substrates, in particular histone H3 or TP53. GFI1 can also bring methyl transferases such as PRMT1 together with its substrates that include the DNA repair proteins MRE11 and 53BP1, thereby enabling their methylation and activation. While GFI1B is expressed almost exclusively in the erythroid and megakaryocytic lineage, GFI1 has clear biological roles in the development and differentiation of lymphoid and myeloid immune cells. GFI1 is required for lymphoid/myeloid and monocyte/granulocyte lineage decision as well as the correct nuclear interpretation of a number of important immune-signaling pathways that are initiated by NOTCH1, interleukins such as IL2, IL4, IL5 or IL7, by the pre TCR or -BCR receptors during early lymphoid differentiation or by T and B cell receptors during activation of lymphoid cells. Myeloid cells also depend on GFI1 at both stages of early differentiation as well as later stages in the process of activation of macrophages through Toll-like receptors in response to pathogen-associated molecular patterns. The knowledge gathered on these factors over the last decades puts GFI1 and GFI1B at the center of many biological processes that are critical for both the innate and acquired immune system.

Selecting a Cell Engineering Methodology During Cell Therapy Product Development

When considering the development pathway for a genetically modified cell therapy product, it is critically important that the product is engineered consistent with its intended human use. For scientists looking to develop and commercialize a new technology, the decision to select a genetic modification method depends on several practical considerations. Whichever path is chosen, the developer must understand the key risks and potential mitigations of the cell engineering approach. The developer should also understand the clinical implications: permanent/memory establishment versus transient expression, and clinical manufacturing considerations when dealing with transplantation of genetically engineered cells. This review covers important topics for mapping out a strategy for developers of new cell-based therapeutics. Biological, technological, manufacturing, and clinical considerations are all presented to map out development lanes for the initiation and risk management of new gene-based cell therapeutic products for human use.

Immunologic evaluation and genetic defects of apoptosis in patients with autoimmune lymphoproliferative syndrome (ALPS)

Apoptosis plays an important role in controlling the adaptive immune response and general homeostasis of the immune cells, and impaired apoptosis in the immune system results in autoimmunity and immune dysregulation. In the last 25 years, inherited human diseases of the Fas-FasL pathway have been recognized. Autoimmune lymphoproliferative syndrome (ALPS) is an inborn error of immunity, characterized clinically by nonmalignant and noninfectious lymphoproliferation, autoimmunity, and increased risk of lymphoma due to a defect in lymphocyte apoptosis. The laboratory hallmarks of ALPS are an elevated percentage of T-cell receptor αβ double negative T cells (DNTs), elevated levels of vitamin B12, soluble FasL, IL-10, IL-18 and IgG, and defective in vitro Fas-mediated apoptosis. In order of frequency, the genetic defects associated with ALPS are germinal and somatic ALPS-FAS, ALPS-FASLG, ALPS-CASP10, ALPS-FADD, and ALPS-CASP8. Partial disease penetrance and severity suggest the combination of germline and somatic FAS mutations as well as other risk factor genes. In this report, we summarize human defects of apoptosis leading to ALPS and defects that are known as ALPS-like syndromes that can be clinically similar to, but are genetically distinct from, ALPS. An efficient genetic and immunological diagnostic approach to patients suspected of having ALPS or ALPS-like syndromes is essential because this enables the establishment of specific therapeutic strategies for improving the prognosis and quality of life of patients.

Activated neutrophils exert myeloid-derived suppressor cell activity damaging T cells beyond repair

Myeloid-derived suppressor cells (MDSCs) have the capacity to suppress T-cell-mediated immune responses and impact the clinical outcome of cancer, infections, and transplantation settings. Although MDSCs were initially described as bone marrow-derived immature myeloid cells (either monocytic or granulocytic MDSCs), mature neutrophils have been shown to exert MDSC activity toward T cells in ways that remain unclear. In this study, we demonstrated that human neutrophils from both healthy donors and cancer patients do not exert MDSC activity unless they are activated. By using neutrophils with genetically well-defined defects, we found that reactive oxygen species (ROS) and granule-derived constituents are required for MDSC activity after direct CD11b-dependent interactions between neutrophils and T cells. In addition to these cellular interactions, neutrophils are engaged in the uptake of pieces of T-cell membrane, a process called trogocytosis. Together, these interactions led to changes in T-cell morphology, mitochondrial dysfunction, and adenosine triphosphate depletion, as indicated by electron microscopy, mass spectrometry, and metabolic parameters. Our studies characterize the different steps by which activated mature neutrophils induce functional T-cell nonresponsiveness and irreparable cell damage.

T-cell receptor signal strength and epigenetic control of Bim predict memory CD8+ T-cell fate

Most effector CD8+ T cells die, while some persist and become either "effector" (TEM) or "central" (TCM) memory T cells. Paradoxically, effector CD8+ T cells with greater memory potential have higher levels of the pro-apoptotic molecule Bim. Here, we report, using a novel Bim-mCherry knock-in mouse, that cells with high levels of Bim preferentially develop into TCM cells. Bim levels remained stable and were regulated by DNA methylation at the Bim promoter. Notably, high levels of Bcl-2 were required for Bimhi cells to survive. Using Nur77-GFP mice as an indicator of TCR signal strength, Nur77 levels correlated with Bim expression and Nur77hi cells also selectively developed into TCM cells. Altogether, these data show that Bim levels and TCR signal strength are predictive of TEM- vs. TCM-cell fate. Further, given the many other biologic functions of Bim, these mice will have broad utility beyond CD8+ T-cell fate.

Fatty Acid Synthase Contributes to Restimulation-Induced Cell Death of Human CD4 T Cells

Restimulation-induced cell death (RICD) is an apoptotic pathway triggered in activated effector T cells after T cell receptor (TCR) re-engagement. RICD operates at the peak of the immune response to ensure T cell expansion remains in check to maintain immune homeostasis. Understanding the biochemical regulation of RICD sensitivity may provide strategies for tuning the magnitude of an effector T cell response. Metabolic reprogramming in activated T cells is not only critical for T cell differentiation and effector functions, but also influences apoptosis sensitivity. We previously demonstrated that aerobic glycolysis correlates with optimum RICD sensitivity in human effector CD8 T cells. However, metabolic programming in CD4 T cells has not been investigated in this context. We employed a pharmacological approach to explore the effects of fatty acid and glycolytic metabolism on RICD sensitivity in primary human CD4 T cells. Blockade of fatty acid synthase (FASN) with the compound C75 significantly protected CD4 effector T cells from RICD, suggesting that fatty acid biosynthesis contributes to RICD sensitivity. Interestingly, sphingolipid synthesis and fatty acid oxidation (FAO) were dispensable for RICD. Disruption of glycolysis did not protect CD4 T cells from RICD unless glyceraldehyde-3-phosphate dehydrogenase (GAPDH) enzymatic activity was targeted specifically, highlighting important differences in the metabolic control of RICD in effector CD4 vs. CD8 T cell populations. Moreover, C75 treatment protected effector CD4 T cells derived from naïve, effector memory, and central memory T cell subsets. Decreased RICD in C75-treated CD4 T cells correlated with markedly reduced FAS ligand (FASL) induction and a Th2-skewed phenotype, consistent with RICD-resistant CD4 T cells. These findings highlight FASN as a critical metabolic potentiator of RICD in human effector CD4 T cells.

Caspase-8 restricts antiviral CD8 T cell hyperaccumulation

The magnitude of CD8 T cell responses against viruses is checked by the balance of proliferation and death. Caspase-8 (CASP8) has the potential to influence response characteristics through initiation of apoptosis, suppression of necroptosis, and modulation of cell death-independent signal transduction. Mice deficient in CASP8 and RIPK3 (Casp8 ^-/- Ripk3 ^-/- ) mount enhanced peak CD8 T cell levels against the natural mouse pathogen murine cytomegalovirus (MCMV) or the human pathogen herpes simplex virus-1 compared with littermate control RIPK3-deficient or WT C57BL/6 mice, suggesting an impact of CASP8 on the magnitude of antiviral CD8 T cell expansion and not on contraction. The higher peak response to MCMV in Casp8 ^-/- Ripk3 ^-/- mice resulted from accumulation of greater numbers of terminally differentiated KLRG1^hi effector CD8 T cell subsets. Antiviral Casp8 ^-/- Ripk3 ^-/- T cells exhibited enhanced proliferation when splenocytes were transferred into WT recipient mice. Thus, cell-autonomous CASP8 normally restricts CD8 T cell proliferation following T cell receptor activation in response to foreign antigen. Memory inflation is a hallmark quality of the T cell response to cytomegalovirus infection. Surprisingly, MCMV-specific memory inflation was not sustained long-term in Casp8 ^-/- Ripk3 ^-/- mice even though these mice retained immunity to secondary challenge. In addition, the accumulation of abnormal B220⁺CD3⁺ T cells in these viable CASP8-deficient mice was reduced by chronic MCMV infection. Combined, these data brings to light the cell death-independent role of CASP8 during CD8 T cell expansion in mice lacking the confounding impact of RIPK3-mediated necroptosis.

Caspase-8-dependent control of NK- and T cell responses during cytomegalovirus infection

Caspase-8 (CASP8) impacts antiviral immunity in expected as well as unexpected ways. Mice with combined deficiency in CASP8 and RIPK3 cannot support extrinsic apoptosis or RIPK3-dependent programmed necrosis, enabling studies of CASP8 function without complications of unleashed necroptosis. These extrinsic cell death pathways are naturally targeted by murine cytomegalovirus (MCMV)-encoded cell death suppressors, showing they are key to cell-autonomous host defense. Remarkably, Casp8^-/-Ripk3^-/-, Ripk1^-/-Casp8^-/-Ripk3^-/- and Casp8^-/-Ripk3^K51A/K51A mice mount robust antiviral T cell responses to control MCMV infection. Studies in Casp8^-/-Ripk3^-/- mice show that CASP8 restrains expansion of MCMV-specific natural killer (NK) and CD8 T cells without compromising contraction or immune memory. Infected Casp8^-/-Ripk3^-/- or Casp8^-/-Ripk3^K51A/K51A mice have higher levels of virus-specific NK cells and CD8 T cells compared to matched RIPK3-deficient littermates or WT mice. CASP8, likely acting downstream of Fas death receptor, dampens proliferation of CD8 T cells during expansion. Importantly, contraction proceeds unimpaired in the absence of extrinsic death pathways owing to intact Bim-dependent (intrinsic) apoptosis. CD8 T cell memory develops in Casp8^-/-Ripk3^-/- mice, but memory inflation characteristic of MCMV infection is not sustained in the absence of CASP8 function. Despite this, Casp8^-/-Ripk3^-/- mice are immune to secondary challenge. Interferon (IFN)γ is recognized as a key cytokine for adaptive immune control of MCMV. Ifngr^-/-Casp8^-/-Ripk3^-/- mice exhibit increased lifelong persistence in salivary glands as well as lungs compared to Ifngr^-/- and Casp8^-/-Ripk3^-/- mice. Thus, mice deficient in CASP8 and RIPK3 are more dependent on IFNγ mechanisms for sustained T cell immune control of MCMV. Overall, appropriate NK- and T cell immunity to MCMV is dependent on host CASP8 function independent of RIPK3-regulated pathways.

Survival of Naïve T Cells Requires the Expression of Let-7 miRNAs

Maintaining the diversity and constant numbers of naïve T cells throughout the organism's lifetime is necessary for efficient immune responses. Naïve T cell homeostasis, which consists of prolonged survival, occasional proliferation and enforcement of quiescence, is tightly regulated by multiple signaling pathways which are in turn controlled by various transcription factors. However, full understanding of the molecular mechanisms underlying the maintenance of the peripheral T cell pool has not been achieved. In the present study, we demonstrate that T cell-specific deficiency in let-7 miRNAs results in peripheral T cell lymphopenia resembling that of Dicer1 knockout mice. Deletion of let-7 leads to profound T cell apoptosis while overexpression prevents it. We further show that in the absence of let-7, T cells cannot sustain optimal levels of the pro-survival factor Bcl2 in spite of the intact IL-7 signaling, and re-expression of Bcl2 in let-7 deficient T cells completely rescues the survival defect. Thus, we have uncovered a novel let-7-dependent mechanism of post-transcriptional regulation of naïve T cell survival in vivo.

Mitochondria as emerging targets for therapies against T cell acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) comprises a heterogeneous group of hematologic malignancies, arising from diverse genetic alterations in the early lymphocyte development. T-cell subtype of ALL (T-ALL) accounts for about 15% and 25% of ALL in children and adults, respectively. Being less frequent among ALL subtypes, T-ALL represents a high-risk factor for poor prognosis due to its aggressiveness and resistance to common antileukemic drugs. Mitochondria were widely explored recently as a target for anticancer treatment because they are involved in a metabolic reprogramming of a cancer cell and play key roles in reactive oxygen species generation, Ca²⁺ signaling, and cell death induction. Accordingly, a new class of anticancer compounds named mitocans has been developed, which target mitochondria at distinct crucial points to promote their dysfunction and subsequent cell death. The present review analyses the role of mitochondria in malignant reprogramming and emerging therapeutic strategies targeting mitochondria as an "Achilles' heel" in T-ALL, with an emphasis on BH3 mimetics, sequestering pro-survival BCL proteins and voltage-dependent anion channel (VDAC)1-directed drugs, which promote the suppression of aerobic glycolysis, VDAC1 closure, mitochondrial Ca²⁺ overload, stoppage of the oxidative phosphorylation, oxidative stress, and release of proapoptotic factors.

T Cells and Regulated Cell Death: Kill or Be Killed

Cell death plays two major complementary roles in T cell biology: mediating the removal of cells that are targeted by T cells and the removal of T cells themselves. T cells serve as major actors in the adaptive immune response and function by selectively killing cells which are infected or dysfunctional. This feature is highly involved during homeostatic maintenance, and is relied upon and modulated in the context of cancer immunotherapy. The vital recognition and elimination of both autoreactive T cells and cells which are unable to recognize threats is a highly selective and regulated process. Moreover, detection of potential threats will result in the activation and expansion of T cells, which on resolution of the immune response will need to be eliminated. The culling of these T cells can be executed via a multitude of cell death pathways which are used in context-specific manners. Failure of these processes may result in an accumulation of misdirected or dysfunctional T cells, leading to complications such as autoimmunity or cancer. This review will focus on the role of cell death regulation in the maintenance of T cell homeostasis, as well as T cell-mediated elimination of infected or dysfunctional cells, and will summarize and discuss the current knowledge of the cellular mechanisms which are implicated in these processes.