A role for the granzyme B inhibitor serine protease inhibitor 6 in CD8+ memory cell homeostasis

Diagnostic and therapeutic value of human serpin family proteins

SERPIN (serine proteinase inhibitors) is an acronym for the superfamily of structurally similar proteins found in animals, plants, bacteria, viruses, and archaea. Over 1500 SERPINs are known in nature, while only 37 SERPINs are found in humans, which participate in inflammation, coagulation, angiogenesis, cell viability, and other pathophysiological processes. Both qualitative or quantitative deficiencies or overexpression and/or abnormal accumulation of SERPIN can lead to diseases commonly referred to as "serpinopathies". Hence, strategies involving SERPIN supplementation, elimination, or correction are utilized and/or under consideration. In this review, we discuss relationships between certain SERPINs and diseases as well as putative strategies for the clinical explorations of SERPINs.

Peripheral blood T-cell subset and its clinical significance in lupus nephritis patients

Objectives

Lupus nephritis (LN) is a common and severe manifestation of SLE. Memory T (TM) cells have been implicated in the pathogenesis of SLE. This study aimed to investigate the clinical significance of T-cell subsets in a cohort of patients with LN.

Method

The peripheral blood T cells of 24 patients with LN and 13 patients with idiopathic membranous nephropathy (iMN) were analysed by flow cytometry. SLE disease activity was evaluated by SLE Disease Activity Index-2000 (SLEDAI-2K). Patients with LN were followed up for >6 months.

Results

Patients with LN presented lower frequency of CD4⁺ cells and higher percentage of CD8⁺ cells than patients with iMN, which was primarily due to lower CD4⁺ cell count. Interestingly, patients with LN under immunosuppressants had lower CD8⁺CD45RO⁺ TM frequency (p=0.007), fewer regulatory CD4⁺ T cells (p=0.04) than those without immunosuppressants. Most CD4⁺ and CD8⁺ TM cells in patients with LN showed an effector memory (CD45RO⁺CCR7⁺) phenotype. The frequency of CD8⁺CD45RO⁺ TM cells among the CD8⁺ T cells was negatively correlated with white blood cell count, haemoglobin, platelet and serum levels of complements C3 and C4, but was positively correlated with serum IgG, erythrocyte sedimentation rate and SLEDAI score (p<0.05 each). Consistently, the frequency of CD8⁺CD45RO⁺ TM cells was higher in patients with LN with positive antidouble-stranded DNA antibody, active renal disease, extrarenal manifestations and with sclerotic glomeruli or moderate-to-severe mesangial hypercellularity in renal pathology (p<0.05). Additionally, CD8⁺CD45RO⁺ TM cell frequency was significantly lower in patients with LN with renal complete remission than that in non-remission LN (18.7% vs 31.2%, p<0.05). None of these significant correlations was observed in CD4⁺ TM cells.

Conclusion

The frequency of CD8⁺ TM cells correlates with disease activity and treatment response to immunosuppressant in patients with LN. CD8⁺ TM monitoring in patients with LN could provide more helpful indices for the monitoring and management of this disease.

Vitexin-2-O-rhamnoside improves immunosuppression, oxidative stress, and phosphorylation of PI3K/Akt signal pathway in cyclophosphamide treated mice

Vitexin-2-O-rhamnoside (VR) is an important active substance in hawthorn, which is widely used as a food or functional food raw material; however, its immunomodulatory activities have not been extensively studied. In this study, BALB/c mice immunocompromised by cyclophosphamide (CY) were used as models to explore the effects of VR on the immunity and antioxidant capacity of mice. The results revealed that VR can restore weight to the immunosuppressed mice to varying degrees, improve spleen and thymus injury, and restore peripheral blood levels. Furthermore, it can effectively promote the proliferation of T and B lymphocytes, natural killer (NK) and cytotoxic T lymphocyte (CTL) cell activities, and the secretion and mRNA expression of cytokines IFN-γ, IL-2, IL-6, and IL-12 to 0.36, 0.34, 50.25%, 45.74%, 28.36 pg/mL or 0.68, 31.81 pg/mL or 0.74, 20.40 pg/mL or 0.75, and 19.81 pg/mL or 0.55, respectively. Moreover, it can upregulate the phosphorylation level of PI3K/Akt signaling pathway in mice immunosuppressed by CY, increase the activities of glutathione peroxidase (GSH-Px), chloramphenicol acetyltransferase (CAT), superoxide dismutase (SOD), and total antioxidant capacity (T-AOC), and decrease the level of malondialdehyde (MDA). This study provides a theoretical and experimental basis for the research and development of health products with targeted efficacy, and the development of diversified products in the hawthorn deep-processing industry.

CCR6 and CXCR6 Identify the Th17 Cells With Cytotoxicity in Experimental Autoimmune Encephalomyelitis

Due to the plasticity of IL-17-producing CD4 T cells (Th17 cells), a long-standing challenge in studying Th17-driven autoimmune is the lack of specific surface marker to identify the pathogenic Th17 cells in vivo. Recently, we discovered that pathogenic CD4 T cells were CXCR6 positive in experimental autoimmune encephalomyelitis (EAE), a commonly used Th17-driven autoimmune model. Herein, we further revealed that peripheral CXCR6⁺CD4 T cells contain a functionally distinct subpopulation, which is CCR6 positive and enriched for conventional Th17 molecules (IL-23R and RORγt) and cytotoxic signatures. Additionally, spinal cord-infiltrating CD4 T cells were highly cytotoxic by expressing Granzyme(s) along with IFNγ and GM-CSF. Collectively, this study suggested that peripheral CCR6⁺CXCR6⁺CD4 T cells were Th17 cells with cytotoxic property in EAE model, and highlighted the cytotoxic granzymes for EAE pathology.

The Serpin Superfamily and Their Role in the Regulation and Dysfunction of Serine Protease Activity in COPD and Other Chronic Lung Diseases

Chronic obstructive pulmonary disease (COPD) is a debilitating heterogeneous disease characterised by unregulated proteolytic destruction of lung tissue mediated via a protease-antiprotease imbalance. In COPD, the relationship between the neutrophil serine protease, neutrophil elastase, and its endogenous inhibitor, alpha-1-antitrypsin (AAT) is the best characterised. AAT belongs to a superfamily of serine protease inhibitors known as serpins. Advances in screening technologies have, however, resulted in many members of the serpin superfamily being identified as having differential expression across a multitude of chronic lung diseases compared to healthy individuals. Serpins exhibit a unique suicide-substrate mechanism of inhibition during which they undergo a dramatic conformational change to a more stable form. A limitation is that this also renders them susceptible to disease-causing mutations. Identification of the extent of their physiological/pathological role in the airways would allow further expansion of knowledge regarding the complexity of protease regulation in the lung and may provide wider opportunity for their use as therapeutics to aid the management of COPD and other chronic airways diseases.

Direct Tumor Killing and Immunotherapy through Anti-SerpinB9 Therapy

Cancer therapies kill tumors either directly or indirectly by evoking immune responses and have been combined with varying levels of success. Here, we describe a paradigm to control cancer growth that is based on both direct tumor killing and the triggering of protective immunity. Genetic ablation of serine protease inhibitor SerpinB9 (Sb9) results in the death of tumor cells in a granzyme B (GrB)-dependent manner. Sb9-deficient mice exhibited protective T cell-based host immunity to tumors in association with a decline in GrB-expressing immunosuppressive cells within the tumor microenvironment (TME). Maximal protection against tumor development was observed when the tumor and host were deficient in Sb9. The therapeutic utility of Sb9 inhibition was demonstrated by the control of tumor growth, resulting in increased survival times in mice. Our studies describe a molecular target that permits a combination of tumor ablation, interference within the TME, and immunotherapy in one potential modality.

Anti-cancer Immunotoxins, Challenges, and Approaches

The development of recombinant immunotoxins (RITs) as a novel therapeutic strategy has made a revolution in the treatment of cancer. RITs result from the fusion of antibodies to toxin proteins for targeting and eliminating cancerous cells by inhibiting protein synthesis. Despite indisputable outcomes of RITs regarding inhibition of multiple cancer types, high immunogenicity has been known as the main obstacle in the clinical use of RITs. Various strategies have been proposed to overcome these limitations, including immunosuppressive therapy, humanization of the antibody fragment moiety, generation of immunotoxins originated from endogenous human cytotoxic enzymes, and modification of the toxin moiety to escape the immune system. This paper is devoted to review recent advances in the design of immunotoxins with lower immunogenicity.

Protective effects of polysaccharides from Cordyceps gunnii mycelia against cyclophosphamide-induced immunosuppression to TLR4/TRAF6/NF-κB signalling in BALB/c mice

Polysaccharides are closely associated with immune regulation. In this study, the aim was to investigate the effect of polysaccharides from Cordyceps gunnii mycelia (PPS) in cyclophosphamide (CTX)-induced immunodeficient mice. Compared with the CTX-induced immunosuppressed mice, the spleen and thymus indexes in mice with orally administered PPS were significantly increased, body weight loss was alleviated, and the natural killer (NK) cytotoxicity and proliferative activities of the lymphocytes were elevated. The recovery of peripheral white blood cells, red blood cells, hemoglobins and platelets was accelerated. Furthermore, the results from ELISA showed that PPS could up-regulate the serum levels of IL-2, IL-12, IFN-γ and IgG, and reduce the level of TGF-β. Histopathological analysis of the spleen revealed the protective effect of PPS against CTX-induced immunosuppression. Western blotting results showed that PPS possessed immunomodulatory activity via TLR4/TRAF6/NF-κB signalling pathways. Finally, the intestinal absorption of PPS was poor, as detected in the Caco-2 transwell system. Taken together, these findings suggest that PPS plays a crucial role in protection against immunosuppression in cyclophosphamide-treated mice and could be a potential candidate for use in immune therapy regimens.

SerpinB1 controls encephalitogenic T helper cells in neuroinflammation

SerpinB1, a protease inhibitor and neutrophil survival factor, was recently linked with IL-17-expressing T cells. Here, we show that serpinB1 (Sb1) is dramatically induced in a subset of effector CD4 cells in experimental autoimmune encephalomyelitis (EAE). Despite normal T cell priming, Sb1 ^-/- mice are resistant to EAE with a paucity of T helper (T_H) cells that produce two or more of the cytokines, IFNγ, GM-CSF, and IL-17. These multiple cytokine-producing CD4 cells proliferate extremely rapidly; highly express the cytolytic granule proteins perforin-A, granzyme C (GzmC), and GzmA and surface receptors IL-23R, IL-7Rα, and IL-1R1; and can be identified by the surface marker CXCR6. In Sb1 ^-/- mice, CXCR6⁺ T_H cells are generated but fail to expand due to enhanced granule protease-mediated mitochondrial damage leading to suicidal cell death. Finally, anti-CXCR6 antibody treatment, like Sb1 deletion, dramatically reverts EAE, strongly indicating that the CXCR6⁺ T cells are the drivers of encephalitis.

Water-soluble polysaccharides from Grifola Frondosa fruiting bodies protect against immunosuppression in cyclophosphamide-induced mice via JAK2/STAT3/SOCS signal transduction pathways

Grifola Frondosa, the king of mushrooms, is one of the most valued traditional medicines and has been used as a health food for a long time in China, Japan, and other Asian countries. The present study was designed to evaluate the immune-modulating effects of water-soluble polysaccharides from the Grifola Frondosa fruiting body (GFP) by using mouse peritoneal macrophage and cytoxan (CTX) induced immunosuppression models. Compared with CTX-induced immunosuppressive mice, the spleen and thymus indexes in mice with GFP orally administrated were significantly increased, body weight loss was alleviated, and the natural killer (NK) cytotoxicity and the proliferative activities of lymphocytes were elevated. Furthermore, levels of interleukin-2 (IL-2), interferon-6 (IL-6) and tumor necrosis factor-α (TNF-α) were notably reduced by CTX, while GFP abolished these effects. GFP also effectively increased total antioxidant capacity and superoxidase dismutase, catalase and glutathione peroxidase activities, and inhibited an increase in the malondialdehyde level. Histopathological analysis of spleens revealed the protective effect of GFP against CTX-induced immunosuppression. Western blotting results showed that GFP possessed immunomodulatory activity by up-regulating transcription factors (p-JAK2/JAK2, p-STAT3/STAT3 and SOCS3) in JAK2/STAT3/SOCS signaling pathways. This study suggested that GFP may provide an alternative strategy for lessening chemotherapy-induced immunosuppression.

Comparative proteomic analysis reveals different responses in porcine lymph nodes to virulent and attenuated homologous African swine fever virus strains

African swine fever (ASF) is a pathology of pigs against which there is no treatment or vaccine. Understanding the equilibrium between innate and adaptive protective responses and immune pathology might contribute to the development of strategies against ASFV. Here we compare, using a proteomic approach, the course of the in vivo infection caused by two homologous strains: the virulent E75 and the attenuated E75CV1. Our results show a progressive loss of proteins by day 7 post-infection (pi) with E75, reflecting tissue destruction. Many signal pathways were affected by both infections but in different ways and extensions. Cytoskeletal remodelling and clathrin-endocytosis were affected by both isolates, while a greater number of proteins involved on inflammatory and immunological pathways were altered by E75CV1. 14-3-3 mediated signalling, related to immunity and apoptosis, was inhibited by both isolates. The implication of the Rho GTPases by E75CV1 throughout infection is also evident. Early events reflected the lack of E75 recognition by the immune system, an evasion strategy acquired by the virulent strains, and significant changes at 7 days post-infection (dpi), coinciding with the peak of infection and the time of death. The protein signature at day 31 pi with E75CV1 seems to reflect events observed at 1 dpi, including the upregulation of proteosomal subunits and molecules described as autoantigens (vimentin, HSPB1, enolase and lymphocyte cytosolic protein 1), which allow the speculation that auto-antibodies could contribute to chronic ASFV infections. Therefore, the use of proteomics could help understand ASFV pathogenesis and immune protection, opening new avenues for future research.

Host-Derived Serine Protease Inhibitor 6 Provides Granzyme B-Independent Protection of Intestinal Epithelial Cells in Murine Graft-versus-Host Disease

Graft-versus-host disease (GVHD) is a serious complication after allogeneic hematopoietic cell transplantation (allo-HCT) that limits the therapeutic potential of this treatment. Host antigen-presenting cells (APCs) play a vital role in activating donor T cells that subsequently use granzyme B (GzmB) and other cytotoxic molecules to damage host normal tissues. Serine protease inhibitor 6 (Spi6), known as the sole endogenous inhibitor of GzmB, has been implicated in protecting T cells and APCs against GzmB-inflicted damage. In this study we used murine models to examine the previously unknown role of host-derived Spi6 in GVHD pathogenesis. Our results indicated that host Spi6 deficiency exacerbated GVHD as evidenced by significantly increased lethality and clinical and histopathologic scores. Using bone marrow chimera system, we found that Spi6 in nonhematopoietic tissue played a dominant role in protecting against GVHD and was significantly upregulated in intestinal epithelial cells after allo-HCT, whereas Spi6 in hematopoietic APCs surprisingly suppressed alloreactive T cell response. Interestingly, the protective effect of Spi6 and its expression in intestinal epithelial cells appeared to be independent of donor-derived GzmB. We used in silico modeling to explore potential targets of Spi6. Interaction tested in silico demonstrated that Spi6 could inhibit caspase-3 and caspase-8 with the same functional loop that inhibits GzmB but was not capable of forming stable interaction with caspase-1 or granzyme A. Using an in vitro co-culture system, we further identified that donor T cell-derived IFN-γ was important for inducing Spi6 expression in an intestinal epithelial cell line. Altogether, our data indicate that host Spi6 plays a novel, GzmB-independent role in regulating alloreactive T cell response and protecting intestinal epithelial cells. Therefore, enhancing host-derived Spi6 function has the potential to reduce GVHD.

Human Granzyme B Based Targeted Cytolytic Fusion Proteins

Cancer immunotherapy aims to selectively target and kill tumor cells whilst limiting the damage to healthy tissues. Controlled delivery of plant, bacterial and human toxins or enzymes has been shown to promote the induction of apoptosis in cancerous cells. The 4th generation of targeted effectors are being designed to be as humanized as possible—a solution to the problem of immunogenicity encountered with existing generations. Granzymes are serine proteases which naturally function in humans as integral cytolytic effectors during the programmed cell death of cancerous and pathogen-infected cells. Secreted predominantly by cytotoxic T lymphocytes and natural killer cells, granzymes function mechanistically by caspase-dependent or caspase-independent pathways. These natural characteristics make granzymes one of the most promising human enzymes for use in the development of fusion protein-based targeted therapeutic strategies for various cancers. In this review, we explore research involving the use of granzymes as cytolytic effectors fused to antibody fragments as selective binding domains.

Serine protease inhibitor 6 protects alloreactive T cells from Granzyme B-mediated mitochondrial damage without affecting graft-versus-tumor effect

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative therapy for hematologic malignancies. Donor T cells are able to eliminate residual tumor cells after allo-HCT, producing the beneficial graft-versus-tumor (GVT) effect, but can also cause graft-versus-host disease (GVHD) when attacking host normal tissues. We previously reported that granzyme B (GzmB) is involved in activation-induced cell death (AICD) of donor T cells and exerts differential impacts on GVHD and GVT effect. Serine protease inhibitor 6 (Spi6) is the sole endogenous inhibitor of GzmB that can protect immune and tissue cells against GzmB-mediated damage. This study is aimed to delineate the mechanism by which the GzmB-Spi6 axis regulates allogeneic T cell response. Using multiple clinically relevant murine allo-HCT models, we have found that Spi6 is concentrated in mitochondria during allogeneic T cell activation, while Spi6^-/- T cells exhibit abnormal mitochondrial membrane potential, mass, reactive oxygen species (ROS) production and increased GzmB-dependent AICD mainly in the form of fratricide. Compared with WT T cells, Spi6^-/- T cells exhibit decreased expansion in the host and cause significantly reduced GVHD. Notably, however, Spi6^-/- T cells demonstrate the same level of GVT activity as WT T cells, which were confirmed by two independent tumor models. In summary, our findings demonstrate that Spi6 plays a novel and critical role in maintaining the integrity of T cell mitochondrial function during allogeneic response, and suggest that disabling Spi6 in donor T cells may represent a novel strategy that can alleviate GVHD without sacrificing the beneficial GVT effect.

Immune responses to infectious diseases in bivalves

Many species of bivalve mollusks (phylum Mollusca, class Bivalvia) are important in fisheries and aquaculture, whilst others are critical to ecosystem structure and function. These crucial roles mean that considerable attention has been paid to the immune responses of bivalves such as oysters, clams and mussels against infectious diseases that can threaten the viability of entire populations. As with many invertebrates, bivalves have a comprehensive repertoire of immune cells, genes and proteins. Hemocytes represent the backbone of the bivalve immune system. However, it is clear that mucosal tissues at the interface with the environment also play a critical role in host defense. Bivalve immune cells express a range of pattern recognition receptors and are highly responsive to the recognition of microbe-associated molecular patterns. Their responses to infection include chemotaxis, phagolysosomal activity, encapsulation, complex intracellular signaling and transcriptional activity, apoptosis, and the induction of anti-viral states. Bivalves also express a range of inducible extracellular recognition and effector proteins, such as lectins, peptidoglycan-recognition proteins, thioester bearing proteins, lipopolysaccharide and β1,3-glucan-binding proteins, fibrinogen-related proteins (FREPs) and antimicrobial proteins. The identification of FREPs and other highly diversified gene families in bivalves leaves open the possibility that some of their responses to infection may involve a high degree of pathogen specificity and immune priming. The current review article provides a comprehensive, but not exhaustive, description of these factors and how they are regulated by infectious agents. It concludes that one of the remaining challenges is to use new "omics" technologies to understand how this diverse array of factors is integrated and controlled during infection.

Serine protease inhibitor-6 differentially affects the survival of effector and memory alloreactive CD8-T cells

The clonal expansion of effector T cells and subsequent generation of memory T cells are critical in determining the outcome of transplantation. While cytotoxic T lymphocytes induce direct cytolysis of target cells through secretion of Granzyme-B (GrB), they also express cytoplasmic serine protease inhibitor-6 (Spi6) to protect themselves from GrB that has leaked from granules. Here, we studied the role of GrB/Spi6 axis in determining clonal expansion of alloreactive CD8-T cells and subsequent generation of memory CD8-T cells in transplantation. CD8-T cells from Spi6(-/-) mice underwent more GrB mediated apoptosis upon alloantigen stimulation in vitro and in vivo following adoptive transfer into an allogeneic host. Interestingly, while OT1.Spi6(-/-) CD8 T cells showed significantly lower clonal expansion following skin transplants from OVA mice, there was no difference in the size of the effector memory CD8-T cells long after transplantation. Furthermore, lack of Spi6 resulted in a decrease of short-lived-effector-CD8-cells but did not impact the pool of memory-precursor-effector-CD8-cells. Similar results were found in heart transplant models. Our findings suggest that the final alloreactive CD8-memory-pool-size is independent from the initial clonal-proliferation as memory precursors express low levels of GrB and therefore are independent of Spi6 for survival. These data advance our understanding of memory T cells generation in transplantation and provide basis for Spi6 based strategies to target effector T cells.

Transcriptional changes in Manila clam (Ruditapes philippinarum) in response to Brown Ring Disease

Brown Ring Disease (BRD) is a bacterial infection affecting the economically-important clam Ruditapes philippinarum. The disease is caused by a bacterium, Vibrio tapetis, that colonizes the edge of the mantle, altering the biomineralization process and normal shell growth. Altered organic shell matrices accumulate on the inner face of the shell leading to the formation of the typical brown ring in the extrapallial space (between the mantle and the shell). Even though structural and functional changes have been described in solid (mantle) and fluid (hemolymph and extrapallial fluids) tissues from infected clams, the underlying molecular alterations and responses remain largely unknown. This study was designed to gather information on clam molecular responses to the disease and to compare focal responses at the site of the infection (mantle and extrapallial fluid) with systemic (hemolymph) responses. To do so, we designed and produced a Manila clam expression oligoarray (15K Agilent) using transcriptomic data available in public databases and used this platform to comparatively assess transcriptomic changes in mantle, hemolymph and extrapallial fluid of infected clams. Results showed significant regulation in diseased clams of molecules involved in pathogen recognition (e.g. lectins, C1q domain-containing proteins) and killing (defensin), apoptosis regulation (death-associated protein, bcl-2) and in biomineralization (shell matrix proteins, perlucin, galaxin, chitin- and calcium-binding proteins). While most changes in response to the disease were tissue-specific, systemic alterations included co-regulation in all 3 tested tissues of molecules involved in microbe recognition and killing (complement-related factors, defensin). These results provide a first glance at molecular alterations and responses caused by BRD and identify targets for future functional investigations.

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

Asymptomatic memory CD8+ T cells: from development and regulation to consideration for human vaccines and immunotherapeutics

Generation and maintenance of high quantity and quality memory CD8(+) T cells determine the level of protection from viral, bacterial, and parasitic re-infections, and hence constitutes a primary goal for T cell epitope-based human vaccines and immunotherapeutics. Phenotypically and functionally characterizing memory CD8(+) T cells that provide protection against herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) infections, which cause blinding ocular herpes, genital herpes, and oro-facial herpes, is critical for better vaccine design. We have recently categorized 2 new major sub-populations of memory symptomatic and asymptomatic CD8(+) T cells based on their phenotype, protective vs. pathogenic function, and anatomical locations. In this report we are discussing a new direction in developing T cell-based human herpes vaccines and immunotherapeutics based on the emerging new concept of "symptomatic and asymptomatic memory CD8(+) T cells."

Memory cytolytic T-lymphocytes: induction, regulation and implications for vaccine design

The design of vaccines that protect against intracellular infections or cancer remains a challenge. In many cases, immunity depends on the development of antigen-specific memory CD8+ T-cells that can express cytokines and kill antigen-bearing cells when they encounter the pathogen or tumor. Here, the authors review current understanding of the signals and cells that lead to memory CD8+ T-cell differentiation, the relationship between the primary CD8+ T-cell response and the memory response and the regulation of memory CD8+ T-cell survival and function. The implications of this new knowledge for vaccine design are discussed, and recent progress in the development of lipidated peptide vaccines as a promising approach for vaccination against intracellular infections and cancer is reviewed.

The impact of KLF2 modulation on the transcriptional program and function of CD8 T cells

Krüppel-like factor 2 (KLF2) is a transcription factor that is highly expressed in quiescent T lymphocytes and downregulated in effector T cells. We now show that antigen receptor engagement downregulates KLF2 expression in a graded response determined by the affinity of T cell antigen receptor (TCR) ligand and the integrated activation of protein kinase B and the MAP kinases ERK1/2. The present study explores the importance of KLF2 downregulation and reveals that the loss of KLF2 controls a select portion of the CD8 effector T cell transcriptional program. In particular, KLF2 loss is required for CD8 T cells to express the inflammatory chemokine receptor CXCR3 and for maximum clonal expansion of T cells. KLF2 thus negatively controls the ability of CD8 T cells to respond to the CXCR3 ligand CXCL10. Strikingly, the KLF2 threshold for restraining expression of CXCR3 is very low and quite distinct to the KLF2 threshold for restraining T cell proliferation. KLF2 is thus an analogue (tunable) not a digital (on/off) cellular switch where the magnitude of KLF2 expression differentially modifies the T cell responses.

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.

Probing CD8 T cell responses with Listeria monocytogenes infection

CD8 T cells play a critical role in the control and eradication of intracellular pathogens. Increased understanding of CD8 T cell biology provides insight that can be translated into improved vaccination strategies. The intracellular bacterium, Listeria monocytogenes, has been used as a model organism to study every phase of the CD8 T cell response to intracellular bacterial infection. Infection of laboratory mice with L. monocytogenes has provided insight into the factors that are involved in primary T cell responses, memory CD8 T cell generation, maintenance, functionality, and diversification following repeated pathogenic challenges. In this review, we will focus on work from our laboratories utilizing the murine model of L. monocytogenes to investigate the characteristics of CD8 T cell responses to infection. This model has profoundly advanced our understanding of the CD8 T cell response to infection and is likely to continue to provide invaluable basic insights that can be translated into the development of effective vaccination strategies to protect against pathogens.

Granzyme B regulates antiviral CD8+ T cell responses

CTLs and NK cells use the perforin/granzyme cytotoxic pathway to kill virally infected cells and tumors. Human regulatory T cells also express functional granzymes and perforin and can induce autologous target cell death in vitro. Perforin-deficient mice die of excessive immune responses after viral challenges, implicating a potential role for this pathway in immune regulation. To further investigate the role of granzyme B in immune regulation in response to viral infections, we characterized the immune response in wild-type, granzyme B-deficient, and perforin-deficient mice infected with Sendai virus. Interestingly, granzyme B-deficient mice, and to a lesser extent perforin-deficient mice, exhibited a significant increase in the number of Ag-specific CD8(+) T cells in the lungs and draining lymph nodes of virally infected animals. This increase was not the result of failure in viral clearance because viral titers in granzyme B-deficient mice were similar to wild-type mice and significantly less than perforin-deficient mice. Regulatory T cells from WT mice expressed high levels of granzyme B in response to infection, and depletion of regulatory T cells from these mice resulted in an increase in the number of Ag-specific CD8(+) T cells, similar to that observed in granzyme B-deficient mice. Furthermore, granzyme B-deficient regulatory T cells displayed defective suppression of CD8(+) T cell proliferation in vitro. Taken together, these results suggest a role for granzyme B in the regulatory T cell compartment in immune regulation to viral infections.

SerpinB1 protects the mature neutrophil reserve in the bone marrow

SerpinB1 is among the most efficient inhibitors of neutrophil serine proteases--NE, CG, and PR-3--and we investigated here its role in neutrophil development and homeostasis. We found that serpinB1 is expressed in all human bone marrow leukocytes, including stem and progenitor cells. Expression levels were highest in the neutrophil lineage and peaked at the promyelocyte stage, coincident with the production and packaging of the target proteases. Neutrophil numbers were decreased substantially in the bone marrow of serpinB1(-/-) mice. This cellular deficit was associated with an increase in serum G-CSF levels. On induction of acute pulmonary injury, neutrophils were recruited to the lungs, causing the bone marrow reserve pool to be completely exhausted in serpinB1(-/-) mice. Numbers of myeloid progenitors were normal in serpinB1(-/-) bone marrow, coincident with the absence of target protease expression at these developmental stages. Maturation arrest of serpinB1(-/-) neutrophils was excluded by the normal CFU-G growth in vitro and the normal expression in mature neutrophils of early and late differentiation markers. Normal absolute numbers of proliferating neutrophils and pulse-chase kinetic studies in vivo showed that the bone marrow deficit in serpinB1(-/-) mice was largely restricted to mature, postmitotic neutrophils. Finally, upon overnight culture, apoptosis and necrosis were greater in purified bone marrow neutrophils from serpinB1(-/-) compared with WT mice. Collectively, these findings demonstrate that serpinB1 sustains a healthy neutrophil reserve that is required in acute immune responses.

Mesenchymal stem cells express serine protease inhibitor to evade the host immune response

Clinical trials using mesenchymal stem cells (MSCs) have been initiated worldwide. An improved understanding of the mechanisms by which allogeneic MSCs evade host immune responses is paramount to regulating their survival after administration. This study has focused on the novel role of serine protease inhibitor (SPI) in the escape of MSCs from host immunosurveillance through the inhibition of granzyme B (GrB). Our data indicate bone marrow-derived murine MSCs express SPI6 constitutively. MSCs from mice deficient for SPI6 (SPI6(-/-)) exhibited a 4-fold higher death rate by primed allogeneic cytotoxic T cells than did wild-type MSCs. A GrB inhibitor rescued SPI6(-/-) MSCs from cytotoxic T-cell killing. Transduction of wild-type MSCs with MigR1-SPI6 also protected MSCs from cytotoxic T cell-mediated death in vitro. In addition, SPI6(-/-) MSCs displayed a shorter lifespan than wild-type MSCs when injected into an allogeneic host. We conclude that SPI6 protects MSCs from GrB-mediated killing and plays a pivotal role in their survival in vivo. Our data could serve as a basis for future SPI-based strategies to regulate the survival and function of MSCs after administration and to enhance the efficacy of MSC-based therapy for diseases.

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.

Hematopoietic stem cell function and survival depend on c-Myc and N-Myc activity

Myc activity is emerging as a key element in acquisition and maintenance of stem cell properties. We have previously shown that c-Myc deficiency results in accumulation of defective hematopoietic stem cells (HSCs) due to niche-dependent differentiation defects. Here we report that immature HSCs coexpress c-myc and N-myc mRNA at similar levels. Although conditional deletion of N-myc in the bone marrow does not affect hematopoiesis, combined deficiency of c-Myc and N-Myc (dKO) results in pancytopenia and rapid lethality. Interestingly, proliferation of HSCs depends on both myc genes during homeostasis, but is c-Myc/N-Myc independent during bone marrow repair after injury. Strikingly, while most dKO hematopoietic cells undergo apoptosis, only self-renewing HSCs accumulate the cytotoxic molecule Granzyme B, normally employed by the innate immune system, thereby revealing an unexpected mechanism of stem cell apoptosis. Collectively, Myc activity (c-Myc and N-Myc) controls crucial aspects of HSC function including proliferation, differentiation, and survival.

Granzyme C supports efficient CTL-mediated killing late in primary alloimmune responses

It is well established that granzymes A and B play a role in CTL killing of target cells by the perforin-dependent granule exocytosis pathway. The functions of multiple additional granzymes expressed in CTL are less well defined. In the present studies, CTL generated from mice deficient in dipeptidyl peptidase 1 (DPP1) were used to investigate the contribution of granzyme C to CTL killing of allogeneic target cells. DPP1 is required for activation of granzymes A and B by proteolytic removal of their N-terminal dipeptide prodomains while a significant portion of granzyme C is processed normally in the absence of DPP1. Cytotoxicity of DPP1(-/-) CTL generated in early (5-day) MLC in vitro and in peritoneal exudate cells 5 days after initial allogeneic sensitization in vivo was significantly impaired compared with wild-type CTL. Following 3 days of restimulation with fresh allogeneic stimulators however, cytotoxicity of these DPP1(-/-) effector cells was comparable to that of wild-type CTL. Killing mediated by DPP1(-/-) CTL following restimulation was rapid, perforin dependent, Fas independent and associated with early mitochondrial injury, phosphatidyl serine externalization, and DNA degradation, implicating a granzyme-dependent apoptotic pathway. The increased cytotoxicity of DPP1(-/-) CTL following restimulation coincided with increased expression of granzyme C. Moreover, small interfering RNA inhibition of granzyme C expression during restimulation significantly decreased cytotoxicity of DPP1(-/-) but not wild-type CTL. These results indicate that during late primary alloimmune responses, granzyme C can support CTL-mediated killing by the granule exocytosis pathway in the absence of functional granzymes A or B.

Transplantation survival is maintained by granzyme B+ regulatory cells and adaptive regulatory T cells

Granzyme B (GZB) has been implicated as an effector mechanism in regulatory T cells (T(reg)) suppression. In a model of T(reg)-dependent graft tolerance, it is shown that GZB- deficient mice are unable to establish long-term tolerance. Moreover, mice overexpressing the inhibitor of GZB, serine protease inhibitor 6, are also resistant to tolerization to alloantigen. Graft survival was shorter in bone marrow-mixed chimeras reconstituted with GZB-deficient T(reg) as compared with wild-type T(reg). Whereas there was no difference in graft survival in mixed chimeras reconstituted with wild-type, perforin-deficient, or Fas ligand-deficient T(reg). Finally, data also show that if alloreactive effectors cannot express FoxP3 and be induced to convert in the presence of competent T(reg), then graft tolerance is lost. Our data are the first in vivo data to implicate GZB expression by T(reg) in sustaining long-lived graft survival.

Serpins in T cell immunity

Serine protease inhibitors (serpins) are a family of proteins that are important in the regulation of several biological processes. This mainly involves the inhibition of serine proteases, although some serpins inhibit a different class of proteases or even function without inhibitory activity. In contrast to other protease inhibitor families, serpins inhibit their target proteases by a specific mechanism, which depends on a change in conformation. This review primarily focuses on one subgroup of serpins--ovalbumin (ov)-serpins. Different than most members of the family, this group of serpins lacks secretion signal sequences and therefore, mainly functions intracellularly. In addition to expression in most normal tissues, ov-serpins can be found in multiple different cells of the immune system. Interestingly, expression of ov-serpins in these cells is tightly regulated, indicating a role for these serpins in the regulation of immune responses. The role of serpins in the immune response will be the topic of this review.

Death by a thousand cuts: granzyme pathways of programmed cell death

The granzymes are cell death-inducing enzymes, stored in the cytotoxic granules of cytotoxic T lymphocytes and natural killer cells, that are released during granule exocytosis when a specific virus-infected or transformed target cell is marked for elimination. Recent work suggests that this homologous family of serine esterases can activate at least three distinct pathways of cell death. This redundancy likely evolved to provide protection against pathogens and tumors with diverse strategies for evading cell death. This review discusses what is known about granzyme-mediated pathways of cell death as well as recent studies that implicate granzymes in immune regulation and extracellular proteolytic functions in inflammation.

Correlative analyses of notch signaling with resveratrol-induced differentiation and apoptosis of human medulloblastoma cells

Altered Notch signaling seems linked with medulloblastoma (MB) formation and resveratrol exhibits anti-medulloblastoma effects. However, the influence of resveratrol in Notch signaling of MB cells has not been described. This issue was addressed here by checking Notch1 and Notch2 statuses in three MB cell lines with and without resveratrol treatment. Notch1 and Notch2 were detected in the cytoplasm of three cell lines under normal condition, which were up-regulated by resveratrol along with differentiation, apoptosis and enhanced Hes1 nuclear translocation. Nevertheless, blockage of Notch enzymatic cleavage with gamma-seacretase inhibitors, DAPT and L-685,458, neither interrupted resveratrol-caused cellular events nor affected MB cell growth. These results demonstrate that Notch signaling has little relevance with resveratrol-induced differentiation and apoptosis and may not be a universal critical factor of MB cells.

Intracellular and extracellular serpins modulate lung disease

An imbalance between peptidases and their inhibitors leads to pulmonary disease. Imbalances occur in the adult and the neonate at risk for a specific set of lung pathologies. Serpins (serine peptidase inhibitors) make up the major source of antipeptidase activity in the lung. The purpose of this review is to describe the serpin mechanism of inhibition, their roles in the normal and pathological lung and their potential as therapeutic agents.

Intrahepatic lymphocyte expression of dipeptidyl peptidase I-processed granzyme B and perforin induces hepatocyte expression of serine proteinase inhibitor 6 (Serpinb9/SPI-6)

Human proteinase inhibitor 9 (PI-9/serpinB9) and the murine ortholog, serine proteinase inhibitor 6 (SPI-6/serpinb9) are members of a family of intracellular serine proteinase inhibitors (serpins). PI-9 and SPI-6 expression in immune-privileged cells, APCs, and CTLs protects these cells against the actions of granzyme B, and when expressed in tumor cells or virally infected hepatocytes, confers resistance to killing by CTL and NK cells. The present studies were designed to assess the existence of any correlation between granzyme B activity in intrahepatic lymphocytes and induction of hepatic SPI-6 expression. To this end, SPI-6, PI-9, and serpinB9 homolog expression was examined in response to IFN-alpha treatment and during in vivo adenoviral infection of the liver. SPI-6 mRNA expression increased 10- to 100-fold in the liver after IFN-alpha stimulation and during the course of viral infection, whereas no significant up-regulation of SPI-8 and <5-fold increases in other PI-9/serpinB9 homolog mRNAs was observed. Increased SPI-6 gene expression during viral infection correlated with influxes of NK cells and CTL. Moreover, IFN-alpha-induced up-regulation of hepatocyte SPI-6 mRNA expression was not observed in NK cell-depleted mice. Additional experiments using genetically altered mice either deficient in perforin or unable to process or express granzyme B indicated that SPI-6 is selectively up-regulated in hepatocytes in response to infiltration of the liver by NK cells that express perforin and enzymatically active granzyme B.

Serine protease inhibitor 6-deficient mice have increased neutrophil immunity to Pseudomonas aeruginosa

Inflammation is a localized, protective response to trauma or microbial invasion that destroys the injurious agent and the injured tissue. Neutrophil elastase (NE), a serine protease stored in the azurophil granules of polymorphonuclear neutrophils, digests microbes after phagocytosis. NE can also digest microbes extracellularly but is associated with tissue damage and inflammatory disease. In this study, we show that polymorphonuclear neutrophils from mice deficient in serine protease inhibitor 6, a weak intracellular NE inhibitor, had increased susceptibility to self-inflicted lysis because of increased NE activity. The resulting transient increase in local extracellular NE activity was within a narrow range that resulted in the clearance of Pseudomonas aeruginosa but did not damage the lung. Therefore, deficiency in a weak intracellular inhibitor of NE results in an acute inflammatory response that protects from P. aeruginosa but does not cause lung disease.

The role of serpinb9/serine protease inhibitor 6 in preventing granzyme B-dependent hepatotoxicity

Virally infected hepatocytes are resistant to cytotoxic lymphocyte killing by perforin-dependent and granzyme-dependent effector mechanisms. The present studies were designed to examine the role of serine protease inhibitor 6 (SPI-6) in limiting granzyme B-dependent cytotoxic effector mechanisms in the liver. SPI-6-specific small interfering RNA (siRNA) administration to C57Bl/6J (B6) mice elicited transient alanine aminotransferase (ALT) elevations that were not observed in either granzyme B-deficient B6 (B6.gzmb(-/-)) or natural killer (NK) cell-depleted B6 mice. When SPI-6 expression was abolished by siRNA administration at the time of infection with a recombinant, replication-deficient adenovirus [E1-deleted adenovirus encoding beta-galactosidase (AdCMV-LacZ)], earlier and dramatically increased, and earlier ALT elevations were observed in wild-type B6 but not in B6.gzmb(-/-) or NK cell-depleted mice. When a 3-fold higher dose of AdCMV-LacZ was administered to B6 mice, the coadministration of SPI-6 siRNA resulted in the early onset of lethal, acute liver failure. Of note, the accelerated clearance of AdCMV-LacZ was observed in recipients of SPI-6 siRNA.

CONCLUSION

These results indicate that the regulated expression of SPI-6 in hepatocytes during viral infection or following noninfectious causes of liver injury protects hepatocytes against excessively vigorous granzyme B-dependent killing but may also delay immune clearance of virally infected hepatocytes.

Differential survival of cytotoxic T cells and memory cell precursors

It is widely assumed that the development of memory CD8 T cells requires the escape of CTLs from programmed cell death. We show in this study that although serine protease inhibitor 6 (Spi6) is required to protect clonal bursts of CTLs from granzyme B-induced programmed cell death, it is not required for the development of memory cells. This conclusion is reached because memory cell precursors down-regulate both Spi6 and granzyme B, unlike CTLs, and they do not require Spi6 for survival. These findings suggest that memory CD8 T cells are derived from progenitors that are refractory to self-inflicted damage, rather than derived from fully differentiated CTLs.

Expression of high levels of human proteinase inhibitor 9 blocks both perforin/granzyme and Fas/Fas ligand-mediated cytotoxicity

Proteinase inhibitor 9 (PI-9, SerpinB9) is the only known human intracellular granzyme B inhibitor. Whether expression of PI-9 is sufficient to block cytolysis induced by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells remains controversial. To evaluate the roles of PI-9, we isolated and tested three lines of stably transfected HeLa cells expressing wild-type PI-9 and one line expressing an inactive mutant PI-9. Expressions of wild-type PI-9, but not the inactive mutant PI-9, inhibited cytolysis induced by human NK92 and NKL natural killer cells. Expression of high levels of PI-9 is therefore sufficient to protect human cells against NK cell-mediated cell death. Using two assays, we show that expressing wild-type PI-9, but not the inactive mutant PI-9, blocks Fas/Fas ligand (Fas/FasL)-mediated apoptosis. PI-9 expression has no effect on etoposide-induced apoptosis. HeLa cells exhibiting substantial resistance to Fas/FasL-mediated apoptosis contain 2- to 3-fold higher PI-9 levels than HCT116 human colon cancer cells and 2- to 3-fold lower PI-9 levels than MCF7/ERHA breast cancer cells, in which PI-9 is strongly induced by estrogens, and by tamoxifen. Expression of increasing levels of PI-9 in target cells may progressively inhibit immune surveillance by blocking NK and CTL-induced cytotoxicity through the perforin/granzyme pathway and then through the Fas/FasL pathway.

Bcl6 controls granzyme B expression in effector CD8+ T cells

Bcl6, a sequence-specific transcriptional repressor, is important for generation and maintenance of memory CD8(+) T cells. Although memory CD8(+) T cells are generated from effector CD8(+) T cells, a role for Bcl6 in effector CD8(+) T cells is largely unknown. We show here that Bcl6 expression was transiently induced in activated CD8(+) T cells and continuously up-regulated in effector CD8(+) T cells. The amount of granzyme B mRNA among effector molecules produced by effector CD8(+) T cells inversely correlated with the amount of Bcl6 mRNA in CD8(+) T cells. Overexpression of Bcl6 in CD8(+) T cells resulted in lower killing activity at their effector phase, supporting the reduction of granzyme B expression in effector CD8(+) T cells by Bcl6. We identified a putative Bcl6-binding DNA sequence in the promoter region of the granzyme B gene. Binding of Bcl6 to the Bcl6-binding sequence was detected in naive CD8(+) T cells but not in activated CD8(+) T cells by chromatin immunoprecipitation assay. Furthermore, the Bcl6-binding sequence was required for Bcl6 to repress the luciferase reporter gene expression controlled by the granzyme B promoter. Thus, the granzyme B gene is a molecular target of Bcl6 in effector CD8(+) T cells.

The major human and mouse granzymes are structurally and functionally divergent

Approximately 2% of mammalian genes encode proteases. Comparative genomics reveals that those involved in immunity and reproduction show the most interspecies diversity and evidence of positive selection during evolution. This is particularly true of granzymes, the cytotoxic proteases of natural killer cells and CD8+ T cells. There are 5 granzyme genes in humans and 10 in mice, and it is suggested that granzymes evolve to meet species-specific immune challenge through gene duplication and more subtle alterations to substrate specificity. We show that mouse and human granzyme B have distinct structural and functional characteristics. Specifically, mouse granzyme B is 30 times less cytotoxic than human granzyme B and does not require Bid for killing but regains cytotoxicity on engineering of its active site cleft. We also show that mouse granzyme A is considerably more cytotoxic than human granzyme A. These results demonstrate that even "orthologous" granzymes have species-specific functions, having evolved in distinct environments that pose different challenges.

Interplay between the levels of estrogen and estrogen receptor controls the level of the granzyme inhibitor, proteinase inhibitor 9 and susceptibility to immune surveillance by natural killer cells

Estrogens promote cell proliferation and metastases in several human cancers. Here, we describe a different action of estrogens likely to contribute to tumor development-blocking immunosurveillance. In breast cancer cells, increasing concentrations of estrogen induce increasing levels of the granzyme B inhibitor, SerpinB9/proteinase inhibitor 9 (PI-9) and progressively block cell death induced by NK92 natural killer (NK) cells, but do not block killing by a second NK cell line, NKL cells. RNA interference knockdown of PI-9 abolishes estrogen's ability to block NK92 cell-induced cytotoxicity. Expressing elevated levels of estrogen receptor alpha (ERalpha) increases the induced level of PI-9, and makes tamoxifen (TAM), but not raloxifene or ICI 182,780, a potent inducer of PI-9. At elevated levels of ERalpha, induction of PI-9 by estradiol or TAM blocks killing by both NK92 and NKL cells. When the Erk pathway is activated with epidermal growth factor, the concentration of estrogen required to induce a protective level of PI-9 is reduced to 10 pM. Elevated concentrations of estrogen and ER may provide a dual selective advantage to breast cancer cells by controlling PI-9 levels and thereby blocking immunosurveillance. Expressing elevated levels of ERalpha reveals a potentially important difference in the effects of TAM, raloxifene and ICI 182,780 on immunosurveillance in breast cancer.

Induction of cytotoxic granules in human memory CD8+ T cell subsets requires cell cycle progression

Memory CD8(+) T cell responses are thought to be more effective as a result of both a higher frequency of Ag-specific clones and more rapid execution of effector functions such as granule-mediated lysis. Murine models have indicated that memory CD8(+) T cells exhibit constitutive expression of perforin and can lyse targets directly ex vivo. However, the regulated expression of cytotoxic granules in human memory CD8(+) T cell subsets has been underexplored. Using intracellular flow cytometry, we observed that only a minor fraction of CD45RA(-)CD8(+) T cells, or of CD8(+) T cells reactive to EBV-HLA2 tetramer, expressed intracellular granzyme B (GrB). Induction of GrB-containing cytotoxic granules in both CD45RA(+) and CD45RA(-) cells was achieved by stimulation with anti-CD3/anti-CD28 mAb-coated beads, required at least 3 days, occurred after several rounds of cell division, and required cell cycle progression. The strongest GrB induction was seen in the CCR7(+) subpopulations, with poorest proliferation being observed in the CD45RA(-)CCR7(-) effector-memory pool. Our results indicate that, as with naive T cells, induction of cytotoxic granules in human Ag-experienced CD8(+) T cells requires time and cell division, arguing that the main numerical advantage of a memory T cell pool is a larger frequency of CTL precursors. The fact that granule induction can be achieved through TCR and CD28 ligation has implications for restoring lytic effector function in the context of antitumor immunity.

Serine protease inhibitor 6 protects cytotoxic T cells from self-inflicted injury by ensuring the integrity of cytotoxic granules

How cytotoxic T lymphocytes (CTLs) kill intracellular pathogens without killing themselves has been a recurring question ever since their discovery. By using mice deficient in Serine Protease Inhibitor 6 (Spi6), we show that by inhibiting granzyme B (GrB), Spi6 protects CTLs from self-inflicted injury. Infection with either Lymphocytic Choriomeningitis virus (LCMV) or Listeria monocytogenes (LM) revealed increased apoptosis and diminished survival of Spi6 knockout (KO) CTLs, which was cell autonomous and could be corrected by GrB deficiency. Spi6 KO mice in turn were impaired in their ability to clear LCMV infection. Spi6 KO CTLs revealed a breakdown in the integrity of cytotoxic granules, increased cytoplasmic GrB, and ensuing apoptosis. We conclude that Spi6 protects CTLs from suicide caused by GrB-mediated breakdown of cytotoxic granules.

Apoptotic death concurrent with CD3 stimulation in primary human CD8+ T lymphocytes: a role for endogenous granzyme B

We exposed primary CD8(+) T cells to soluble CD3 mAb plus IL-2 and limited numbers of monocytes (3%). These cells were activated but concurrently subjected to ongoing apoptosis ( approximately 25% were apoptotic from day 2 of culture). However, their costimulated CD4(+) counterparts were much less prone to apoptosis. The apoptotic signaling pathway bypassed Fas and TNFRs, and required the activity of cathepsin C, a protease which performs the proteolytic maturation of granzyme (Gr) A and GrB proenzymes within the cytolytic granules. Silencing the GrB gene by RNA interference in activated CD8(+) T cells prevented the activation of procaspase-3 and Bid, and indicated that GrB was the upstream death mediator. A GrB-specific mAb immunoprecipitated a approximately 70-kDa molecular complex from cytolytic extracts of activated CD8(+) (but not resting) T cells, that was specifically recognized by a nucleocytoplasmic protease inhibitor 9 (PI-9) specific mAb. This complex was also detected after reciprocal immunoprecipitation of PI-9. It coexisted in the cytosol with the 32-kDa form of GrB. As neither were detected in the cytosol of CD4(+) bystander T cells (which poorly synthesized GrB), and as silencing the perforin (Pf) gene had no effect in our system, endogenous GrB was likely implicated. Immunoprecipitation experiments failed to reveal Pf in the cytosol of CD8(+) T cells, and only a tiny efflux of granular GrA was detected by ELISA. We propose that some GrB is released from cytolytic granules to the cytosol of CD8(+) T lymphocytes upon CD3/TCR stimulation and escapes PI-9, thereby mediating apoptotic cell death.