Differential survival of cytotoxic T cells and memory cell precursors

The biological function of Serpinb9 and Serpinb9-based therapy

Recent breakthroughs in discovering novel immune signaling pathways have revolutionized different disease treatments. SERPINB9 (Sb9), also known as Proteinase Inhibitor 9 (PI-9), is a well-known endogenous inhibitor of Granzyme B (GzmB). GzmB is a potent cytotoxic molecule secreted by cytotoxic T lymphocytes and natural killer cells, which plays a crucial role in inducing apoptosis in target cells during immune responses. Sb9 acts as a protective mechanism against the potentially harmful effects of GzmB within the cells of the immune system itself. On the other hand, overexpression of Sb9 is an important mechanism of immune evasion in diseases like cancers and viral infections. The intricate functions of Sb9 in different cell types represent a fine-tuned regulatory mechanism for preventing immunopathology, protection against autoimmune diseases, and the regulation of cell death, all of which are essential for maintaining health and responding effectively to disease challenges. Dysregulation of the Sb9 will disrupt human normal physiological condition, potentially leading to a range of diseases, including cancers, inflammatory conditions, viral infections or other pathological disorders. Deepening our understanding of the role of Sb9 will aid in the discovery of innovative and effective treatments for various medical conditions. Therefore, the objective of this review is to consolidate current knowledge regarding the biological role of Sb9. It aims to offer insights into its discovery, structure, functions, distribution, its association with various diseases, and the potential of nanoparticle-based therapies targeting Sb9.

Features of Effective T Cell-Inducing Vaccines against Chronic Viral Infections

For many years, the focus of prophylactic vaccines was to elicit neutralizing antibodies, but it has become increasingly evident that T cell-mediated immunity plays a central role in controlling persistent viral infections such as with human immunodeficiency virus, cytomegalovirus, and hepatitis C virus. Currently, various promising prophylactic vaccines, capable of inducing substantial vaccine-specific T cell responses, are investigated in preclinical and clinical studies. There is compelling evidence that protection by T cells is related to the magnitude and breadth of the T cell response, the type and homing properties of the memory T cell subsets, and their cytokine polyfunctionality and metabolic fitness. In this review, we evaluated these key factors that determine the qualitative and quantitative properties of CD4+ and CD8+ T cell responses in the context of chronic viral disease and prophylactic vaccine development. Elucidation of the mechanisms underlying T cell-mediated protection against chronic viral pathogens will facilitate the development of more potent, durable and safe prophylactic T cell-based vaccines.

NK1.1+ CD8+ T cells escape TGF-β control and contribute to early microbial pathogen response

Following microbial pathogen invasion, one of the main challenges for the host is to rapidly control pathogen spreading to avoid vital tissue damage. Here we report that an effector CD8(+) T-cell population that expresses the marker NK1.1 undergoes delayed contraction and sustains early anti-microbial protection. NK1.1(+) CD8(+) T cells are derived from CD8(+) T cells during priming, and their differentiation is inhibited by transforming growth factor-β signalling. After their own contraction phase, they form a distinct pool of KLRG1 CD127 double-positive memory T cells and rapidly produce both interferon-γ and granzyme B, providing significant pathogen protection in an antigen-independent manner within only a few hours. Thus, by prolonging the CD8(+) T-cell response at the effector stage and by expressing exacerbated innate-like features at the memory stage, NK1.1(+) cells represent a distinct subset of CD8(+) T cell that contributes to the early control of microbial pathogen re-infections.

Spi6 protects alloreactive CD4(+) but not CD8 (+) memory T cell from granzyme B attack by double-negative T regulatory cell

Memory T (Tm) cells pose a major barrier to long-term transplant survival. Whether regulatory T cells (Tregs)can control them remains poorly defined. Previously,we established that double-negative (DN) Tregs suppress effector T (Teff) cells. Here, we demonstrate that DNTregs effectively suppress CD4+/CD8+Teff and CD8+Tm but not CD4+Tm cells, whereas the suppression on CD8+Tm is abrogated by perforin (PFN) deficiency in DNTregs. Consistently, in a BALB/c to B6-Rag1-/-skin transplantation, transfer of DN Tregs suppressed the rejection mediated by CD4þ/CD8+Teff and CD8+Tmcells (76.0±4.9, 87.5±5.0 and 63.0±4.7 days, respectively)but not CD4þTmcells (25.3±1.4 days). Both CD8þ effector memory T and central memory T compartments significantly reduced after DN Treg transfer. CD4+Tm highly expresses granzyme B (GzmB) inhibitor serine protease inhibitor-6 (Spi6). Spi6 deficiency renders CD4þTm susceptible to DN Treg suppression. In addition,transfer of WT DN Tregs, but not PFN-/-DN Tregs,inhibited the skin allograft rejection mediated by Spi6-/-CD4þTm(75.5±7.9 days). In conclusion, CD4+ and CD8+Tm cells differentially respond toDNTregs’ suppression.The GzmB resistance conferred by Spi6 in CD4þTm cells might hint at the physiological significance of Tmpersistence

Stemness of T cells and the hematopoietic stem cells: fate, memory, niche, cytokines

Stem cells are able to generate both cells that differentiate and cells that remain undifferentiated but potentially have the same developmental program. The prolonged duration of the protective immune memory for infectious diseases such as polio, small pox, and measles, suggested that memory T cells may have stem cell properties. Understanding the molecular basis for the life-long persistence of memory T cells may be useful to project targeted therapies for immune deficiencies and infectious diseases and to formulate vaccines. In the last decade evidence from different laboratories shows that memory T cells may share self-renewal pathways with bone marrow hematopoietic stem cells. In stem cells the intrinsic self-renewal activity, which depends on gene expression, is known to be modulated by extrinsic signals from the environment that may be tissue specific. These extrinsic signals for stemness of memory T cells include cytokines such as IL-7 and IL-15 and there are other cytokine signals for maintaining the cytokine signature (TH1, TH2, etc.) of memory T cells. Intrinsic and extrinsic pathways that might be common to bone marrow hematopoietic stem cells and memory T lymphocytes are discussed and related to self-renewal functions.

An emerging role for Serine Protease Inhibitors in T lymphocyte immunity and beyond

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmo-regulation, tissue re-modeling and immunity to infection. T lymphocytes are required for adaptive cell mediated immunity and serine proteases are not only important for effector function but also homeostatic regulation of cell numbers. Serine Protease Inhibitors (Serpins) are the physiological regulators of serine proteases activity. In this review, I will discuss the role of serpins in controlling the recognition of antigen, effector function and homeostatic control of T lymphocytes through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This will be discussed in the context of the T lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. The potent anti-apoptotic properties of serpins can also work against adaptive cell immunity by protecting viruses and tumors from eradication by cytotoxic T cells (CTL). Recent insights from knock-out mouse models demonstrate that these serpins also are required for hematological progenitor cells and so are critical for the development of lineages other than T lymphocytes. Given the emerging role of serpins in multiple aspects of lymphocyte immunity and blood development I will review the progress to date in developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Cathepsin B controls the persistence of memory CD8+ T lymphocytes

The persistence of memory T lymphocytes confers lifelong protection from pathogens. Memory T cells survive and undergo homeostatic proliferation (HSP) in the absence of Ag, although the cell-intrinsic mechanisms by which cytokines drive the HSP of memory T cells are not well understood. In this study we report that lysosome stability limits the long-term maintenance of memory CD8(+) T cell populations. Serine protease inhibitor (Spi) 2A, an anti-apoptotic cytosolic cathepsin inhibitor, is induced by both IL-15 and IL-7. Mice deficient in Spi2A developed fewer memory phenotype CD44(hi)CD8(+) T cells with age, which underwent reduced HSP in the bone marrow. Spi2A was also required for the maintenance of central memory CD8(+) T cell populations after acute infection with lymphocytic choriomeningitis virus. Spi2A-deficient Ag-specific CD8(+) T cell populations declined more than wild-type competitors after viral infection, and they were eroded further after successive infections. Spi2A protected memory cells from lysosomal breakdown by inhibiting cathepsin B. The impaired maintenance of Spi2A-deficient memory CD8(+) T cells was rescued by concomitant cathepsin B deficiency, demonstrating that cathepsin B was a physiological target of Spi2A in memory CD8(+) T cell survival. Our findings support a model in which protection from lysosomal rupture through cytokine-induced expression of Spi2A determines the long-term persistence of memory CD8(+) T cells.

Serine protease inhibitor 6 is required to protect dendritic cells from the kiss of death

How dendritic cells (DC) present Ag to cytotoxic T cells (CTL) without themselves being killed through contact-mediated cytotoxicity (so-called kiss of death) has proved to be controversial. Using mice deficient in serine protease inhibitor 6 (Spi6), we show that Spi6 protects DC from the kiss of death by inhibiting granzyme B (GrB) delivered by CTL. Infection of Spi6 knockout mice with lymphocytic choriomeningitis virus revealed impaired survival of CD8α DC. The impaired survival of Spi6 knockout CD8α DC resulted in impaired priming and expansion of both primary and memory lymphocytic choriomeningitis virus-specific CTL, which could be corrected by GrB deficiency. The rescue in the clonal burst obtained by GrB elimination demonstrated that GrB was the physiological target through which Spi6 protected DC from CTL. We conclude that the negative regulation of DC priming of CD8 T lymphocyte immunity by CTL killing is mitigated by the physiological inhibition of GrB by Spi6.

Serine protease inhibitors and cytotoxic T lymphocytes

Serine proteases control a wide variety of physiological and pathological processes in multi-cellular organisms, including blood clotting, cancer, cell death, osmoregulation, tissue remodeling, and immunity to infection. Cytotoxic T lymphocytes (CTLs) are required for adaptive cell-mediated immunity to intracellular pathogens by killing infected cells and through the development of memory T cells. Serine proteases not only allow a CTL to kill but also impose homeostatic control on CTL number. Serine protease inhibitors (serpins) are the physiological regulators of serine proteases' activity. In this review, I discuss the role of serpins in controlling the recognition of antigen, effector function, and homeostatic control of CTLs through the inhibition of physiological serine protease targets. An emerging view of serpins is that they are important promoters of cellular viability through their inhibition of executioner proteases. This view is discussed in the context of the T-lymphocyte survival during effector responses and the development and persistence of long-lived memory T cells. Given the important role serpins play in CTL immunity, I discuss the potential for developing new immunotherapeutic approaches based directly on serpins or knowledge gained from identifying their physiologically relevant protease targets.

Serine protease inhibitor 6 protects iNKT cells from self-inflicted damage

The role played by apoptosis in the homeostasis of effector cells of the innate immune system is unclear. Serine protease inhibitor 6 (Spi6) is an inhibitor of granzyme B (GrB) that protects cytotoxic T lymphocytes of the adaptive immune system from apoptosis. To determine whether Spi6 also protects cells of the innate immune system from self-inflicted damage we have examined invariant NKT (iNKT) cells. Spi6-deficient iNKT cells harbored increased levels of GrB after TCR stimulation with the PBS-57 glycolipid Ag and were susceptible to apoptosis. The increased apoptosis of Spi6 knock-out (KO) iNKT cells lead to a complete loss in the production of IL-4 and IFN-gamma by Spi6 KO iNKT cells after PBS-57 challenge. The increased activation-induced apoptosis resulted in impaired survival and a decreased clonal burst size of Spi6 KO iNKT cells, which could be corrected by GrB deficiency. However, the clonal burst of Spi6 KO iNKT cells after TCR-independent activation with lymphocytic choriomeningitis virus was not affected. Our findings demonstrate that Spi6 protects cytotoxic cells of the innate immune system from GrB-mediated self-inflicted triggered by the recognition of Ag.

Intrinsic and extrinsic control of effector T cell survival and memory T cell development

Following infection or vaccination T cells expand exponentially and differentiate into effector T cells in order to control infection and coordinate the multiple effector arms of the immune system. Soon after this expansion, the majority of antigen-specific T cells die to reattain homeostasis and a small pool of memory T cells forms to provide long-term immunity to subsequent re-infection. Our understanding of how this process is controlled has improved considerably over the recent years, but many questions remain outstanding. This review focuses on the recent advancements in this area with an emphasis on how the contraction of activated T cells is coordinately regulated by a combination of factors extrinsic and intrinsic to the activated T cells.

Transcriptional repressor Blimp-1 promotes CD8(+) T cell terminal differentiation and represses the acquisition of central memory T cell properties

During acute infections, a small population of effector CD8(+) T cells evades terminal differentiation and survives as long-lived memory T cells. We demonstrate that the transcriptional repressor Blimp-1 enhanced the formation of terminally differentiated CD8(+) T cells during lymphocytic choriomeningitis virus (LCMV) infection, and Blimp-1 deficiency promoted the acquisition of memory cell properties by effector cells. Blimp-1 expression was preferentially increased in terminally differentiated effector and "effector memory" (Tem) CD8(+) T cells, and gradually decayed after infection as central memory (Tcm) cells developed. Blimp-1-deficient effector CD8(+) T cells showed some reduction in effector molecule expression, but primarily developed into memory precursor cells that survived better and more rapidly acquired several Tcm cell attributes, including CD62L and IL-2 expression and enhanced proliferative responses. These results reveal a critical role for Blimp-1 in controlling terminal differentiation and suppressing memory cell developmental potential in effector CD8(+) T cells during viral infection.

Apoptotic signal transduction and T cell tolerance

The healthy immune system makes use of a variety of surveillance mechanisms at different stages of lymphoid development to prevent the occurrence and expansion of potentially harmful autoreactive T cell clones. Disruption of these mechanisms may lead to inappropriate activation of T cells and the development of autoimmune and lymphoproliferative diseases [such as multiple sclerosis, rheumatoid arthritis, lupus erythematosus, diabetes and autoimmune lymphoproliferative syndrome (ALPS)]. Clonal deletion of T cells with high affinities for self-peptide-MHC via programmed cell death (apoptosis) is an essential mechanism leading to self-tolerance. Referred to as negative selection, central tolerance in the thymus serves as the first checkpoint for the developing T cell repertoire and involves the apoptotic elimination of potentially autoreactive T cells clones bearing high affinity T cell receptors (TCR) that recognize autoantigens presented by thymic epithelial cells. Autoreactive T cells that escape negative selection are held in check in the periphery by either functional inactivation ("anergy") or extrathymic clonal deletion, both of which are dependent on the strength and frequency of the TCR signal and the costimulatory context, or by regulatory T cells. This review provides an overview of the different molecular executioners of cell death programs that are vital to intrathymic or extrathymic clonal deletion of T cells. Further, the potential involvement of various apoptotic signaling paradigms are discussed with respect to the genesis and pathophysiology of autoimmune disease.

Transcription factor ELF4 controls the proliferation and homing of CD8+ T cells via the Krüppel-like factors KLF4 and KLF2

Transcription factors that regulate the quiescence, proliferation and homing of lymphocytes are critical for effective immune system function. Here we demonstrate that the transcription factor ELF4 directly activated the tumor suppressor KLF4 'downstream' of T cell antigen receptor signaling to induce cell cycle arrest in naive CD8(+) T cells. Elf4- and Klf4-deficient mice accumulated CD8(+)CD44(hi) T cells during steady-state conditions and generated more memory T cells after immunization. The homeostatic population expansion of CD8(+)CD44(hi) T cells in Elf4-null mice resulted in a redistribution of cells to nonlymphoid tissue because of lower expression of the transcription factor KLF2 and the surface proteins CCR7 and CD62L. Our work describes the combinatorial effect of lymphocyte-intrinsic factors on the homeostasis, activation and homing of T cells.

Stem cell-like plasticity of naïve and distinct memory CD8+ T cell subsets

Most models regarding the 'clonal' origin of CD8(+) T cell effector and memory subset diversification suggest that during the first contact of a naïve T cell with the priming antigen-presenting cell major decisions for subsequent differentiation are made. Data using novel single-cell T cell tracking technologies demonstrate that a single naïve CD8(+) T cell can give rise to virtually all different subtypes of effector and memory T cells, and direct major determinants of subset diversification to the time period beyond the first cell division. Thereby, some 'stem cell-like' characteristics typical for naïve T cells are probably still maintained within distinct subsets of memory T cells. These observations have direct consequences for clinical applications like adoptive T cell therapy.

Brief antigenic stimulation generates effector CD8 T cells with low cytotoxic activity and high IL-2 production

It is currently believed that a brief antigenic stimulation is sufficient to induce CD8 T cells to complete their differentiation program, become effector T cells, and subsequently generate memory. Because this concept was derived from studies in which only a single effector function was analyzed (either IFN-gamma production or target cell lysis), we wondered whether monitoring for multiple effector functions might reveal novel characteristics of effector CD8 T cells elicited by brief or prolonged Ag exposure. Using an in vitro system to generate effector T cells and an in vivo adoptive transfer model to track donor CD8 T cells, we found that the differentiation programs acquired by CD8 T cells after brief or prolonged antigenic stimulation were different. Although the frequencies of IFN-gamma and TNF-alpha producers were comparable for both effector CD8 T cell populations, there were major differences in cytotoxic potential and IL-2 production. Whereas prolonged (>24 h) Ag exposure stimulated effector CD8 T cells with high cytotoxic activity and low IL-2 production, brief (<24 h) stimulation generated effector CD8 T cells with low cytotoxic activity and high IL-2 production. The latter effector T cells rapidly converted into central memory-like CD8 T cells, exhibited long-term survival in adoptively transferred hosts, and gave robust recall responses upon Ag challenge. These data suggest that not all functions of effector CD8 T cells are equally inherited after brief or prolonged antigenic stimulation.

Functional and genomic profiling of effector CD8 T cell subsets with distinct memory fates

An important question in memory development is understanding the differences between effector CD8 T cells that die versus effector cells that survive and give rise to memory cells. In this study, we provide a comprehensive phenotypic, functional, and genomic profiling of terminal effectors and memory precursors. Using killer cell lectin-like receptor G1 as a marker to distinguish these effector subsets, we found that despite their diverse cell fates, both subsets possessed remarkably similar gene expression profiles and functioned as equally potent killer cells. However, only the memory precursors were capable of making interleukin (IL) 2, thus defining a novel effector cell that was cytotoxic, expressed granzyme B, and produced inflammatory cytokines in addition to IL-2. This effector population then differentiated into long-lived protective memory T cells capable of self-renewal and rapid recall responses. Experiments to understand the signals that regulate the generation of terminal effectors versus memory precursors showed that cells that continued to receive antigenic stimulation during the later stages of infection were more likely to become terminal effectors. Importantly, curtailing antigenic stimulation toward the tail end of the acute infection enhanced the generation of memory cells. These studies support the decreasing potential model of memory differentiation and show that the duration of antigenic stimulation is a critical regulator of memory formation.