The granzyme B inhibitor proteinase inhibitor 9 (PI9) is expressed by human mast cells

Beyond target cell death - Granzyme serine proteases in health and disease

Granzymes are a family of small (∼32 kDa) serine proteases with a range of substrate specificities that are stored in, and released from, the cytoplasmic secretory vesicles ('granules') of cytotoxic T lymphocytes and natural killer cells. Granzymes are not digestive proteases but finely tuned processing enzymes that target their substrates in specific ways to activate various signalling pathways, or to inactivate viral proteins and other targets. Great emphasis has been placed on studying the pro-apoptotic functions of granzymes, which largely depend on their synergy with the pore-forming protein perforin, on which they rely for penetration into the target cell cytosol to access their substrates. While a critical role for granzyme B in target cell apoptosis is undisputed, both it and the remaining granzymes also influence a variety of other biological processes (including important immunoregulatory functions), which are discussed in this review. This includes the targeting of many extracellular as well as intracellular substrates, and can also lead to deleterious outcomes for the host if granzyme expression or function are dysregulated or abrogated. A final important consideration is that granzyme repertoire, biochemistry and function vary considerably across species, probably resulting from the pressures applied by viruses and other pathogens across evolutionary time. This has implications for the interpretation of granzyme function in preclinical models of disease.

SERPINB9 is commonly amplified and high expression in cancer cells correlates with poor immune checkpoint blockade response

Immunotherapies, in particular immune checkpoint blockade (ICB), have improved the clinical outcome of cancer patients, although many fail to mount a durable response. Several resistance mechanisms have been identified, but our understanding of the requirements for a robust ICB response is incomplete. We have engineered an MHC I/antigen: TCR-matched panel of human NSCLC cancer and T cells to identify tumor cell-intrinsic T cell resistance mechanisms. The top differentially expressed gene in resistant tumor cells was SERPINB9. This serine protease inhibitor of the effector T cell-derived molecule granzyme B prevents caspase-mediated tumor apoptosis. Concordantly, we show that genetic ablation of SERPINB9 reverts T cell resistance of NSCLC cell lines, whereas its overexpression reduces T cell sensitivity. SERPINB9 expression in NSCLC strongly correlates with a mesenchymal phenotype. We also find that SERPINB9 is commonly amplified in cancer, particularly melanoma in which it is indicative of poor prognosis. Single-cell RNA sequencing of ICB-treated melanomas revealed that SERPINB9 expression is elevated not only in cells from post- versus pre-treatment cancers, but also in ICB-refractory cancers. In NSCLC we commonly observed rare SERPINB9-positive cancer cells, possibly accounting for reservoirs of ICB-resistant cells. While underscoring SERPINB9 as a potential target to combat immunotherapy resistance, these results suggest its potential to serve as a prognostic and predictive biomarker.

Granzyme B as a therapeutic target: an update in 2022

INTRODUCTION

Granzyme B is a serine protease extensively studied for its implication in cytotoxic lymphocyte-mediated apoptosis. In recent years, the paradigm that the role of granzyme B is restricted to immune cell-mediated killing has been challenged as extracellular roles for the protease have emerged. While mostly absent from healthy tissues, granzyme B levels are elevated in several autoimmune and/or chronic inflammatory conditions. In the skin, its accumulation significantly impairs proper wound healing.

AREAS COVERED

After an overview of the current knowledge on granzyme B, a description of newly identified functions will be presented, focussing on granzyme B ability to promote cell-cell and dermal-epidermal junction disruption, extracellular matrix degradation, vascular permeabilization, and epithelial barrier dysfunction. Progress in granzyme B inhibition, as well as the use of granzyme B inhibitors for the treatment of tissue damage, will be discussed.

EXPERT OPINION

The absence of endogenous extracellular inhibitors renders extracellular granzyme B accumulation deleterious for the proper healing of chronic wounds due to sustained proteolytic activity. Consequently, specific granzyme B inhibitors have been developed as new therapeutic approaches. Beyond applications in wound healing, other autoimmune and/or chronic inflammatory conditions related to exacerbated granzyme B activity may also benefit from the development of these inhibitors.

Presence of Protease Inhibitor 9 and Granzyme B in Healthy and Pathological Human Corneas

Simple Summary

Detailed knowledge of the structure and properties of the human cornea is a prerequisite not only for the treatment of various corneal diseases but also for successful corneal transplantation and its long-term survival after grafting. Using various cell and molecular biology approaches, we found in cornea the protease inhibitor 9. This protein, known to be present in other human tissues but not yet reported in cornea, is directly involved in the immune response after transplantation. Together with its inhibitor (granzyme B), we localized this protein, especially in the superficial and inner cornea layers. This localization indicates that protease inhibitor 9 protein may be involved in protecting the cornea from external damage, but also in protection against immune cells inducing corneal graft rejection. Furthermore, we have shown on pathological corneal samples from corneal melting and herpes virus keratitis that the increased expression of both proteins is linked to these diseases. These experiments and their results represent an important contribution to the basic research of cornea biological properties with direct overlap into clinical practice.

Abstract

The aim of this study was to find out whether protease inhibitor 9 (PI-9) and granzyme B (GrB) molecules that contribute to immune response and the immunological privilege of various tissues are expressed in healthy and pathological human corneas. Using cryosections, cell imprints of control corneoscleral discs, we showed that PI-9 was expressed particularly in the endothelium, the superficial and suprabasal epithelium of healthy corneas, limbus, and conjunctiva. GrB was localized in healthy corneal and conjunctival epithelium, while the endothelium showed weak immunostaining. The expression of PI-6 and GrB was confirmed by qRT-PCR. Increased expression levels of the PI-9 and GrB genes were determined when the corneas were cultured with proinflammatory cytokines. Fluorescent and enzymatic immunohistochemistry of pathological corneal explants (corneal melting and herpes virus keratitis) showed pronounced PI-9, GrB, human leucocyte antigen (HLA)-DR, and leukocyte-common antigen (CD45) signals localized in multicellular stromal infiltrates and inflammatory cells scattered in the corneal stroma. We conclude that increased expression of the PI-9 and GrB proteins under pathological conditions and their upregulation in an inflammatory environment indicate their participation in immune response of the cornea during the inflammatory process.

Granule Leakage Induces Cell-Intrinsic, Granzyme B-Mediated Apoptosis in Mast Cells

Mast cells are multifunctional immune cells scattered in tissues near blood vessels and mucosal surfaces where they mediate important reactions against parasites and contribute to the pathogenesis of allergic reactions. Serine proteases released from secretory granules upon mast cell activation contribute to these functions by modulating cytokine activity, platelet activation and proteolytic neutralization of toxins. The forced release of granule proteases into the cytosol of mast cells to induce cell suicide has recently been proposed as a therapeutic approach to reduce mast cell numbers in allergic diseases, but the molecular pathways involved in granule-mediated mast cell suicide are incompletely defined. To identify intrinsic granule proteases that can cause mast cell death, we used mice deficient in cytosolic serine protease inhibitors and their respective target proteases. We found that deficiency in Serpinb1a, Serpinb6a, and Serpinb9a or in their target proteases did not alter the kinetics of apoptosis induced by growth factor deprivation in vitro or the number of peritoneal mast cells in vivo. The serine protease cathepsin G induced marginal cell death upon mast cell granule permeabilization only when its inhibitors Serpinb1a or Serpinb6a were deleted. In contrast, the serine protease granzyme B was essential for driving apoptosis in mast cells. On granule permeabilization, granzyme B was required for caspase-3 processing and cell death. Moreover, cytosolic granzyme B inhibitor Serpinb9a prevented caspase-3 processing and mast cell death in a granzyme B-dependent manner. Together, our findings demonstrate that cytosolic serpins provide an inhibitory shield preventing granule protease-induced mast cell apoptosis, and that the granzyme B-Serpinb9a-caspase-3 axis is critical in mast cell survival and could be targeted in the context of allergic diseases.

Suppression of granzyme B activity and caspase-3 activation in leukaemia cells constitutively expressing the protease inhibitor 9

Immune surveillance against malignant cells is mediated by cytotoxic T-lymphocytes and NK-cells (CTL/NK) that induce apoptosis through the granzyme-B-dependent pathway. The serine protease inhibitor serpinB9/protease inhibitor-9 (PI-9) is a known inhibitor of granzyme B. Ectopic expression of PI-9 in tumour cells has been reported. However, the impact of PI-9 on granzyme-B-induced apoptosis in tumour cells remains unclear. The aim of this study was to investigate the influence of constitutive PI-9 expression in leukaemia cell lines on the activity of granzyme B and apoptosis induction. PI-9 negative (lymphoblastic Jurkat cells; myeloblastic U937 cells) and PI-9-expressing cell lines (myeloblastic K562 cells, EBV-transformed LCL-1 and LCL-2 B-cells, lymphoblastic Daudi cells, AML-R cells f leukaemia and the U937 subclone U937PI-9(+)). For accurate granzyme B activity determination a quantitative substrate (Ac-IEPD-pNA) cleavage assay was established and caspase-3 activation measured for apoptosis assessment. Cells were treated with a cytotoxic granule isolate that has previously been shown to induce apoptosis through granzyme B signalling. We found a robust correlation between constitutive PI-9 expression levels and the suppression of granzyme B activity. Further, inhibition of granzyme B translated into reduced caspase-3 activation. We conclude, suppression of granzyme B initiated apoptosis in PI-9-expressing cells could contribute to immune evasion and the measurement of granzyme B activity with our assay might be a useful predictive marker in immune-therapeutic approaches against cancer.

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.

SerpinB9 expression in human renal tubular epithelial cells is induced by triggering of the viral dsRNA sensors TLR3, MDA5 and RIG-I

BACKGROUND

Serine protease inhibitor B9 (serpinB9) protects against granzyme B-mediated apoptosis and could help to reduce tubular damage under inflammatory conditions like interstitial nephritis. Previously, we found that tubular serpinB9 expression was increased during subclinical rejection. Here, we studied the regulation of serpinB9 expression in tubular epithelial cells (TECs) under inflammatory conditions.

METHODS

SerpinB9 expression was analysed on messenger RNA (mRNA), and protein levels in primary human TECs were stimulated with various cytokines and pattern recognition receptor ligands and in kidney transplant biopsies obtained during different types of viral infection.

RESULTS

Of the inflammatory stimuli tested, only the double-stranded RNA (dsRNA) analogue poly(I:C) promoted serpinB9 mRNA and protein expression. We found that TECs express the viral dsRNA receptors Toll-like receptor 3 (TLR3), melanoma differentiation-associated gene 5 (MDA5) and retinoic acid-inducible gene-I (RIG-I). dsRNA receptor ligands enhanced serpinB9 expression, which involved nuclear factor-kappaB (NF-κB) activation, did not require Type I interferon production and was a direct result of dsRNA receptor-induced gene transcription. In kidney transplants, serpinB9 transcription was increased during infection with cytomegalovirus, Epstein-Barr virus or BK virus compared to stable grafts. Immunohistochemistry showed that tubuli and lymphocytes expressed the inhibitor.

CONCLUSION

SerpinB9 expression in human TECs is induced by triggering of the viral dsRNA sensors TLR3, MDA5 and RIG-I. Viral dsRNA may increase the threshold for granzyme B-mediated apoptosis in TECs via serpinB9 upregulation and thus help to protect the kidney against cytotoxic insults during viral infection.

The role of mast cells and angiogenesis in benign and malignant neoplasms of the uterus

In this study, we looked for a relationship between the extent of angiogenesis and mast cell density (MCD) in human leiomyomas and endometrial carcinomas (EC), and investigated the clinicopathological relevance of mast cells (MCs) in EC. Specimens of 15 control, 20 leiomyoma, and 23 EC patients were investigated immunohistochemically using anti-CD31 and anti-tryptase antibodies. In EC, both stromal and myometrial expressions of CD31 were significantly higher than in the controls (p<0.01 and p=0.013; respectively). Stromal tryptase expression was not significantly lower than that of leiomyoma. In addition, in the leiomyoma group, CD31 and tryptase expressions were not much different compared to the controls. Moreover, a correlation was detected between cancer histological grade and both stromal and myometrial expressions of CD31 (p=0.017 and p=0.005; respectively). The findings show that high grade EC has a higher degree of vascularization than EC of lower grade, but MCD does not increase in parallel with the histological grade. This study has demonstrated that MCD does not correlate with angiogenesis and progression of grade in EC. Moreover, MCD in EC was found to be lower than in benign lesions of the uterus. In conclusion, MCs may not account for the angiogenic process which facilitates tumor growth.

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.

Control of granzymes by serpins

Although proteolysis mediated by granzymes has an important role in the immune response to infection or tumours, unrestrained granzyme activity may damage normal cells. In this review, we discuss the role of serpins within the immune system, as specific regulators of granzymes. The well-characterised human granzyme B-SERPINB9 interaction highlights the cytoprotective function that serpins have in safeguarding lymphocytes from granzymes that may leak from granules. We also discuss some of the pitfalls inherent in using rodent models of granzyme-serpin interactions and the ways in which our understanding of serpins can help resolve some of the current, contentious issues in granzyme biology.

Primary cutaneous T-cell lymphoma of unspecified type with cytotoxic phenotype: clinicopathological analysis of 27 patients

The objective of our study was to investigate the clinicopathological features of the currently ill-defined subtype of primary cutaneous T-cell lymphoma of unspecified type (CTCLU) with a cytotoxic phenotype and no Epstein-Barr virus (EBV) association. A series of 27 patients with CTCLU (median age 49 years; range 25-87 years; 18 men) was reviewed. Performance status scores above 1 (7%), clinical stages above 2 (15%), B symptoms (26%), extracutaneous involvement (30%), and a fatal course within 1 year of diagnosis (19%) were observed infrequently. The International Prognostic Index was high or high to intermediate in 11%, and the Prognostic Index for Peripheral T-cell Lymphoma unspecified was above group 2 in 22%. Notably, the rates of spontaneous regression and T-cell receptor gene rearrangements by polymerase chain reaction analysis were seen in 26 and 17% of our cases, respectively. Histologically, 22 patients had subcutaneous involvement of whom eight showed a lethal clinical course, and five patients without subcutaneous involvement were all survivors. Immunophenotypical and morphological features allowed us to subclassify our cases according to the following four categories: (1) epidermotropic CD8+ T-cell lymphoma (n=5); (2) cutaneous gamma/delta T-cell lymphoma (n=8); (3) cutaneous alpha/beta pleomorphic T-cell lymphoma (n=8); and (4) cutaneous medium/large pleomorphic T-cell lymphoma, not otherwise specified (n=6). All four of these groups of lymphomas exhibited a relatively favorable clinical course compared to previous reports. However, epidermotropic CD8+ T-cell lymphoma appeared to be unique with a higher ratio (80%) of spontaneous regression, a lower ratio (40%) of subcutaneous involvement, and a more favorable clinical course than the other three subcategories.

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.

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.

Granzyme B and natural killer (NK) cell death

Granzyme B is a unique serine protease, which plays a crucial role for target cell death. Several mechanisms of delivery of granzyme B to target cells have been recently identified. Granzyme B directly activates Bid, a specific substrate for granzyme B, resulting in caspase activation. Granzyme B efficiently cleaves many prominent autoantigens, and the hypothesis that autoantibodies arise when cryptic determinants are revealed to the immune system has been proposed. Some autoantibodies directed against granzyme B-specific neoepitopes are present in serum from patients with autoimmune diseases. In the tissues from autoimmune diseases, granzyme B might play an important role for disease progression (i.e., rheumatoid arthritis synovium) or inhibition (i.e., regulatory T cells). We have identified a novel type of activation-induced cell death (granzyme B leakage-induced cell death). Activation-induced natural killer (NK) cell death is accompanied by the leakage of granzyme B from intracellular granules into the cytoplasm, and it triggers apoptosis by directing Bid to mitochondrial membranes. An excess of "leaked" granzyme B over its inhibitor, serpin proteinase inhibitor 9, is a major determinant of cell death. The role of granzyme B in autoimmunity and its influence on NK cell death are discussed.

Human mast cells produce and release the cytotoxic lymphocyte associated protease granzyme B upon activation

Mast cells are widely distributed throughout the body and express effector functions in allergic reactions, inflammatory diseases, and host defense. Activation of mast cells results in exocytosis of preformed chemical mediators and leads to novel synthesis and secretion of lipid mediators and cytokines. Here, we show that human mast cells also express and release the cytotoxic lymphocyte-associated protease, granzyme B. Granzyme B was active and localized in cytoplasmic granules, morphologically resembling those present in cytotoxic lymphocytes. Expression and release of granzyme B by mast cell-lines HMC-1 and LAD 2 and by cord blood- and mature skin-derived human mast cells depended on the mode of activation of these cells. In mast cell lines and cord blood-derived mast cells, granzyme B expression was mainly induced by non-physiological stimuli (A23187/PMA, Compound 48/80) and substance P. In contrast, mature skin-derived mast cells only produced granzyme B upon IgE-dependent stimulation. We conclude that granzyme B is expressed and released by human mast cells upon physiologic stimulation. This suggests a role for granzyme B as a novel mediator in mast cell biology.

Ectopic expression of the serine protease inhibitor PI9 modulates death receptor-mediated apoptosis

Apoptosis is a highly controlled process, whose triggering is associated with the activation of caspases. Apoptosis can be induced via a subgroup of the tumor necrosis factor (TNF) receptor superfamily, which recruit and activate pro-caspase-8 and -10. Regulation of apoptosis is achieved by several inhibitors, including c-FLICE-inhibitory protein, which prevents apoptosis by inhibiting the pro-apoptotic activation of upstream caspases. Here we show that the human intracellular serine protease inhibitor (serpin), protease inhibitor 9 (PI9), inhibits TNF-, TNF-related apoptosis-inducing ligand- and Fas ligand-mediated apoptosis in certain TNF-sensitive cell lines. The reactive center P1 residue of PI9 was required for this inhibition since PI9 harboring a Glu --> Ala mutation in its reactive center failed to impair death receptor-induced cell death. This suggests a classical serpin-protease interaction. Indeed, PI9 inhibited apoptotic death by directly interacting with the intermediate active forms of caspase-8 and -10. This indicates that PI9 can regulate pro-apoptotic apical caspases.

A Pulmonary Perspective on GASPIDs: Granule-Associated Serine Peptidases of Immune Defense

Airways are protected from pathogens by forces allied with innate and adaptive immunity. Recent investigations establish critical defensive roles for leukocyte and mast cell serine-class peptidases garrisoned in membrane-bound organelles-here termed Granule-Associated Serine Peptidases of Immune Defense, or GASPIDs. Some better characterized GASPIDs include neutrophil elastase and cathepsin G (which defend against bacteria), proteinase-3 (targeted by antineutrophil antibodies in Wegener's vasculitis), mast cell beta-tryptase and chymase (which promote allergic inflammation), granzymes A and B (which launch apoptosis pathways in infected host cells), and factor D (which activates complement's alternative pathway). GASPIDs can defend against pathogens but can harm host cells in the process, and therefore become targets for pharmaceutical inhibition. They vary widely in specificity, yet are phylogenetically similar. Mammalian speciation supported a remarkable flowering of these enzymes as they co-evolved with specialized immune cells, including mast cells, basophils, eosinophils, cytolytic T-cells, natural killer cells, neutrophils, macrophages and dendritic cells. Many GASPIDs continue to evolve rapidly, providing some of the most conspicuous examples of divergent protein evolution. Consequently, students of GASPIDs are rewarded not only with insights into their roles in lung immune defense but also with clues to the origins of cellular specialization in vertebrate immunity.

Cancer prevention with semi-allogeneic ES cell-derived dendritic cells

Dendritic cells (DC) genetically modified to present tumor-associated antigen are a promising means for anti-cancer immunotherapy. By introducing expression vectors into ES cells and subsequently inducing differentiation to DC (ES-DC), we can generate transfectant DC expressing the transgenes. In the future clinical application of this technology, the unavailability of human ES cells genetically identical to the patients will be a problem. However, in most cases, semi-allogeneic ES cells sharing some of HLA alleles with recipients are expected to be available. In the present study, we observed that model tumor antigen (OVA)-expressing mouse ES-DC transferred into semi-allogeneic mice potently primed OVA-reactive CTL and elicited a significant protection against challenge with OVA-expressing tumor. Genetic modification of ES-DC to overexpress SPI-6, the specific inhibitor of granzyme B, further enhanced their capacity to prime antigen-specific CTL in semi-allogeneic recipient mice. These results suggest the potential of ES-DC as a novel means for anti-cancer immunotherapy.