T Cell-Derived CD70 Delivers an Immune Checkpoint Function in Inflammatory T Cell Responses.

The CD27-CD70 pathway is known to provide a costimulatory signal, with CD70 expressed on APCs and CD27 functions on T cells. Although CD70 is also expressed on activated T cells, it remains unclear how T cell-derived CD70 affects T cell function. Therefore, we have assessed the role of T cell-derived CD70 using adoptive-transfer models, including autoimmune inflammatory bowel disease and allogeneic graft-versus-host disease. Surprisingly, compared with wild-type T cells, CD70-/- T cells caused more severe inflammatory bowel disease and graft-versus-host disease and produced higher levels of inflammatory cytokines. Mechanistic analyses reveal that IFN-γ induces CD70 expression in T cells, and CD70 limits T cell expansion via a regulatory T cell-independent mechanism that involves caspase-dependent T cell apoptosis and upregulation of inhibitory immune checkpoint molecules. Notably, T cell-intrinsic CD70 signaling contributes, as least in part, to the inhibitory checkpoint function. Overall, our findings demonstrate for the first time, to our knowledge, that T cell-derived CD70 plays a novel immune checkpoint role in inhibiting inflammatory T cell responses. This study suggests that T cell-derived CD70 performs a critical negative feedback function to downregulate inflammatory T cell responses.

Host-Derived CD70 Suppresses Murine Graft-versus-Host Disease by Limiting Donor T Cell Expansion and Effector Function.

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative treatment for hematologic and immunologic diseases. However, graft-versus-host disease (GVHD) may develop when donor-derived T cells recognize and damage genetically distinct normal host tissues. In addition to TCR signaling, costimulatory pathways are involved in T cell activation. CD27 is a TNFR family member expressed on T cells, and its ligand, CD70, is expressed on APCs. The CD27/CD70 costimulatory pathway was shown to be critical for T cell function and survival in viral infection models. However, the role of this pathway in allo-HCT is previously unknown. In this study, we have examined its contribution in GVHD pathogenesis. Surprisingly, Ab blockade of CD70 after allo-HCT significantly increases GVHD. Interestingly, whereas donor T cell- or bone marrow-derived CD70 plays no role in GVHD, host-derived CD70 inhibits GVHD as CD70-/- hosts show significantly increased GVHD. This is evidenced by reduced survival, more severe weight loss, and increased histopathologic damage compared with wild-type hosts. In addition, CD70-/- hosts have higher levels of proinflammatory cytokines TNF-α, IFN-γ, IL-2, and IL-17. Moreover, accumulation of donor CD4+ and CD8+ effector T cells is increased in CD70-/- versus wild-type hosts. Mechanistic analyses suggest that CD70 expressed by host hematopoietic cells is involved in the control of alloreactive T cell apoptosis and expansion. Together, our findings demonstrate that host CD70 serves as a unique negative regulator of allogeneic T cell response by contributing to donor T cell apoptosis and inhibiting expansion of donor effector T cells.

Granzyme B-Mediated Activation-Induced Death of CD4+ T Cells Inhibits Murine Acute Graft-versus-Host Disease.

Granzyme B (GzmB) has previously been shown to be critical for CD8(+) T cell-mediated graft-versus-host disease (GVHD) but dispensable for GVHD mediated by CD4(+) T cells. However, previous studies used high doses of CD4(+) T cells in MHC-mismatched models that caused rapid and lethal GVHD. Because of the hyperacute lethality, it is possible that the role of GzmB was concealed by the system. Therefore, in this study, we have titrated down the T cell dose to precisely determine the contribution of GzmB in GVHD mediated by CD4(+)CD25(-) T cells. Surprisingly, we have found that GzmB(-/-)CD4(+)CD25(-) T cells cause more severe GVHD compared with wild-type CD4(+)CD25(-) T cells in both MHC-matched and mismatched models. Mechanistic analyses reveal that although GzmB does not affect donor T cell engraftment, proliferation or tissue-specific migration, GzmB(-/-) CD4(+)CD25(-) T cells exhibit significantly enhanced expansion because of GzmB-mediated activation-induced cell death of wild-type CD4(+)CD25(-) T cells. As a result of enhanced expansion, GzmB(-/-) T cells produced higher amounts of proinflammatory cytokines (e.g., TNF-α and IFN-γ) that may contribute to the exacerbated GVHD. These results reveal that GzmB diminishes the ability of CD4(+) T cells to cause acute GVHD, which contradicts its established role in CD8(+) T cells. The differential roles suggest that targeting GzmB in selected T cell subsets may provide a strategy to control GVHD.

Housing Temperature-Induced Stress Is Suppressing Murine Graft-versus-Host Disease through ß2-Adrenergic Receptor Signaling.

Graft-versus-host disease (GVHD) is the major complication of allogeneic hematopoietic cell transplantation, a potentially curative therapy for hematologic diseases. It has long been thought that murine bone marrow-derived T cells do not mediate severe GVHD because of their quantity and/or phenotype. During the course of experiments testing the impact of housing temperatures on GVHD, we discovered that this apparent resistance is a function of the relatively cool ambient housing temperature. Murine bone marrow-derived T cells have the ability to mediate severe GVHD in mice housed at a thermoneutral temperature. Specifically, mice housed at Institutional Animal Care and Use Committee-mandated, cool standard temperatures (∼ 22°C) are more resistant to developing GVHD than are mice housed at thermoneutral temperatures (∼ 30°C). We learned that the mechanism underlying this housing-dependent immunosuppression is associated with increased norepinephrine production and excessive signaling through ß-adrenergic receptor signaling, which is increased when mice are cold stressed. Treatment of mice housed at 22°C with a ß2-adrenergic antagonist reverses the norepinephrine-driven suppression of GVHD and yields similar disease to mice housed at 30°C. Conversely, administering a ß2-adrenergic agonist decreases GVHD in mice housed at 30°C. In further mechanistic studies using ß2-adrenergic receptor-deficient (ß2-AR(-/-)) mice, we found that it is host cell ß2-AR signaling that is essential for decreasing GVHD. These data reveal how baseline levels of ß-adrenergic receptor signaling can influence murine GVHD and point to the feasibility of manipulation of ß2-AR signaling to ameliorate GVHD in the clinical setting.

Co-stimulatory and co-inhibitory pathways in cancer immunotherapy.

Cancer remains the leading cause of death worldwide. Traditional treatments such as surgery, radiation, and chemotherapy have had limited efficacy, especially with late stage cancers. Cancer immunotherapy and targeted therapy have revolutionized how cancer is treated, especially in patients with late stage disease. In 2013 cancer immunotherapy was named the breakthrough of the year, partially due to the established efficacy of blockade of CTLA-4 and PD-1, both T cell co-inhibitory molecules involved in tumor-induced immunosuppression. Though early trials promised success, toxicity and tolerance to immunotherapy have hindered long-term successes. Optimizing the use of co-stimulatory and co-inhibitory pathways has the potential to increase the effectiveness of T cell-mediated antitumor immune response, leading to increased efficacy of cancer immunotherapy. This review will address major T cell co-stimulatory and co-inhibitory pathways and the role they play in regulating immune responses during cancer development and treatment.

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.

Granzyme B Contributes to the Optimal Graft-Versus-Tumor Effect Mediated by Conventional CD4+ T Cells.

Granzyme B (GzmB) is a key cytotoxic molecule utilized by T cells to kill pathogen-infected cells or transformed tumor cells. Previous studies using allogeneic hematopoietic cell transplantation (allo-HCT) murine models showed that GzmB is required for CD8+ T cells to cause graft-versus-host disease (GVHD). However, our recent study demonstrated that GzmB-mediated damage of CD8+ T cells diminished their graft-versus-tumor (GVT) activity. In this study, we examined the role of GzmB in GVT effect mediated by conventional CD4+CD25- T cells (CD4+ Tcon). GzmB-/-CD4+ Tcon cells exhibited decreased GVT activity compared to wild-type (WT) CD4+ Tcon cells, suggesting that GzmB is required for the optimal GVT activity of CD4+ Tcon cells. On the other hand, GzmB-/- CD4+CD25+ regulatory T cells were as suppressive as WT regulatory T cells in suppressing GVT activity, which is consistent with our previous report showing that GzmB is not required for regulatory T cell-mediated suppression of GVHD. These results demonstrate that GzmB causes opposite impacts on GVT effect mediated by CD4+CD25- versus CD8+ T cells. Interestingly, GzmB-/- total T cells exhibited GVT activity equivalent to that of WT total T cells, suggesting that the opposite impacts of GzmB on the GVT effect of CD4+CD25- versus CD8+ T cells may neutralize each other, which can only be observed when an individual T cell subset is examined. Importantly, these differential roles suggest that targeting GzmB in selective T cell subsets may have the potential to enhance the beneficial GVT effect.