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

Disrupted regulation of serpinB9 in circulating T cells is associated with an increased risk for post-transplant skin cancer

Cutaneous squamous cell carcinoma (cSCC) is a serious complication after organ transplantation and patients benefit from an early risk assessment. We hypothesized that functional differences in circulating T cells may represent risk factors for post‐transplant cSCC development. Here, we analysed genome‐wide DNA methylation of circulating T cells of kidney transplant recipients before the clinical onset of cSCC, to identify differences associated with post‐transplant cSCC development. This analysis identified higher DNA methylation of SERPINB9, which is an intracellular inhibitor of granzyme B, a protein that induces apoptosis in target cells. High DNA methylation of SERPINB9 in circulating T cells was confirmed in a second patient cohort during recurrent cSCC, indicating that high SERPINB9 methylation represents a persistent risk factor for cSCC development. At the functional level, the inverse correlation between DNA methylation and messenger RNA expression present in non‐cSCC patients was absent in the cSCC patients. Also, a significant difference in serpinB9 protein expression between cSCC patients and non‐cSCC patients was observed. It was concluded that disturbed regulation of serpinB9 in circulating T cells represents a novel risk factor for post‐transplant cSCC in kidney transplant recipients.

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.

Testing the Efficacy of Contrast-Enhanced Ultrasound in Detecting Transplant Rejection Using a Murine Model of Heart Transplantation

One of the key unmet needs to improve long-term outcomes of heart transplantation is to develop accurate, noninvasive, and practical diagnostic tools to detect transplant rejection. Early intragraft inflammation and endothelial cell injuries occur prior to advanced transplant rejection. We developed a novel diagnostic imaging platform to detect early declines in microvascular perfusion (MP) of cardiac transplants using contrast-enhanced ultrasonography (CEUS). The efficacy of CEUS in detecting transplant rejection was tested in a murine model of heart transplants, a standard preclinical model of solid organ transplant. As compared to the syngeneic groups, a progressive decline in MP was demonstrated in the allografts undergoing acute transplant rejection (40%, 64%, and 92% on days 4, 6, and 8 posttransplantation, respectively) and chronic rejection (33%, 33%, and 92% on days 5, 14, and 30 posttransplantation, respectively). Our perfusion studies showed restoration of MP following antirejection therapy, highlighting its potential to help monitor efficacy of antirejection therapy. Our data suggest that early endothelial cell injury and platelet aggregation contributed to the early MP decline observed in the allografts. High-resolution MP mapping may allow for noninvasive detection of heart transplant rejection. The data presented have the potential to help in the development of next-generation imaging approaches to diagnose transplant rejection.

Targeted Delivery of Immunomodulators to Lymph Nodes

Active-targeted delivery to lymph nodes represents a major advance toward more effective treatment of immune-mediated disease. The MECA79 antibody recognizes peripheral node addressin molecules expressed by high endothelial venules of lymph nodes. By mimicking lymphocyte trafficking to the lymph nodes, we have engineered MECA79-coated microparticles containing an immunosuppressive medication, tacrolimus. Following intravenous administration, MECA79-bearing particles showed marked accumulation in the draining lymph nodes of transplanted animals. Using an allograft heart transplant model, we show that targeted lymph node delivery of microparticles containing tacrolimus can prolong heart allograft survival with negligible changes in tacrolimus serum level. Using MECA79 conjugation, we have demonstrated targeted delivery of tacrolimus to the lymph nodes following systemic administration, with the capacity for immune modulation in vivo.