Genetic deletion of HVEM in a leukemia B cell line promotes a preferential increase of PD-1- stem cell-like T cells over PD-1+ T cells curbing tumor progression

INTRODUCTION

A high frequency of mutations affecting the gene encoding Herpes Virus Entry Mediator (HVEM, TNFRSF14) is a common clinical finding in a wide variety of human tumors, including those of hematological origin.

METHODS

We have addressed how HVEM expression on A20 leukemia cells influences tumor survival and its involvement in the modulation of the anti-tumor immune responses in a parental into F1 mouse tumor model of hybrid resistance by knocking-out HVEM expression. HVEM WT or HVEM KO leukemia cells were then injected intravenously into semiallogeneic F1 recipients and the extent of tumor dissemination was evaluated.

RESULTS

The loss of HVEM expression on A20 leukemia cells led to a significant increase of lymphoid and myeloid tumor cell infiltration curbing tumor progression. NK cells and to a lesser extent NKT cells and monocytes were the predominant innate populations contributing to the global increase of immune infiltrates in HVEM KO tumors compared to that present in HVEM KO tumors. In the overall increase of the adaptive T cell immune infiltrates, the stem cell-like PD-1- T cells progenitors and the effector T cell populations derived from them were more prominently present than terminally differentiated PD-1+ T cells.

CONCLUSIONS

These results suggest that the PD-1- T cell subpopulation is likely to be a more relevant contributor to tumor rejection than the PD-1+ T cell subpopulation. These findings highlight the role of co-inhibitory signals delivered by HVEM upon engagement of BTLA on T cells and NK cells, placing HVEM/BTLA interaction in the spotlight as a novel immune checkpoint for the reinforcement of the anti-tumor responses in malignancies of hematopoietic origin.

Differential Engraftment of Parental A20 PD-L1 WT and PD-L1 KO Leukemia Cells in Semiallogeneic Recipients in the Context of PD-L1/PD-1 Interaction and NK Cell-Mediated Hybrid Resistance

The contribution of natural killer (NK) cells to tumor rejection in the context of programmed death-ligand 1/programmed death 1 (PD-L1/PD-1) blockade is a matter of intense debate. To elucidate the role of PD-L1 expression on tumor cells and the functional consequences of engaging PD-1 receptor on cytotoxic cells, PD-L1 expression was genetically inactivated and WT or PD-L1-deficient parental tumor cells were adoptively transferred intravenously into F1 recipients. The engraftment of PD-L1-deficient A20 tumor cells in the spleen and liver of F1 recipients was impaired compared with A20 PD-L1 WT tumor counterparts. To elucidate the mechanism responsible for this differential tumor engraftment and determine the relevance of the role of the PD-L1/PD-1 pathway in the interplay of tumor cells/NK cells, a short-term competitive tumor implantation assay in the peritoneal cavity of semiallogeneic F1 recipients was designed. The results presented herein showed that NK cells killed target tumor cells with similar efficiency regardless of PD-L1 expression, whereas PD-L1 expression on A20 tumor cells conferred significant tumor protection against rejection by CD8 T cells confirming the role of the co-inhibitory receptor PD-1 in the modulation of their cytotoxic activity. In summary, PD-L1 expression on A20 leukemia tumor cells modulates CD8 T-cell-mediated responses to tumor-specific antigens but does not contribute to inhibit NK cell-mediated hybrid resistance, which correlates with the inability to detect PD-1 expression on NK cells neither under steady-state conditions nor under inflammatory conditions.

Immunotherapeutic targeting of LIGHT/LTβR/HVEM pathway fully recapitulates the reduced cytotoxic phenotype of LIGHT-deficient T cells

Tumor necrosis factor (TNF)/TNF receptor (TNFR) superfamily members play essential roles in the development of the different phases of the immune response. Mouse LIGHT (TNFSF14) is a type II transmembrane protein with a C-terminus extracellular TNF homology domain (THD) that assembles in homotrimers and regulates the course of the immune responses by signaling through 2 receptors, the herpes virus entry mediator (HVEM, TNFRSF14) and the lymphotoxin β receptor (LTβR, TNFRSF3). LIGHT is a membrane-bound protein transiently expressed on activated T cells, natural killer (NK) cells and immature dendritic cells that can be proteolytically cleaved by a metalloprotease and released to the extracellular milieu. The immunotherapeutic potential of LIGHT blockade was evaluated in vivo. Administration of an antagonist of LIGHT interaction with its receptors attenuated the course of graft-versus-host reaction and recapitulated the reduced cytotoxic activity of LIGHT-deficient T cells adoptively transferred into non-irradiated semiallogeneic recipients. The lack of LIGHT expression on donor T cells or blockade of LIGHT interaction with its receptors slowed down the rate of T cell proliferation and decreased the frequency of precursor alloreactive T cells, retarding T cell differentiation toward effector T cells. The blockade of LIGHT/LTβR/HVEM pathway was associated with delayed downregulation of interleukin-7Rα and delayed upregulation of inducible costimulatory molecule expression on donor alloreactive CD8 T cells that are typical features of impaired T cell differentiation. These results expose the relevance of LIGHT/LTβR/HVEM interaction for the potential therapeutic control of the allogeneic immune responses mediated by alloreactive CD8 T cells that can contribute to prolong allograft survival.

ITIM-dependent negative signaling pathways for the control of cell-mediated xenogeneic immune responses

Xenotransplantation is an innovative field of research with the potential to provide us with an alternative source of organs to face the severe shortage of human organ donors. For several reasons, pigs have been chosen as the most suitable source of organs and tissues for transplantation in humans. However, porcine xenografts undergo cellular immune responses representing a major barrier to their acceptance and normal functioning. Innate and adaptive xenogeneic immunity is mediated by both the recognition of xenogeneic tissue antigens and the lack of inhibition due to molecular cross-species incompatibilities of regulatory pathways. Therefore, the delivery of immunoreceptor tyrosine-based inhibitory motif (ITIM)-dependent and related negative signals to control innate (NK cells, macrophages) and adaptive T and B cells might overcome cell-mediated xenogeneic immunity. The proof of this concept has already been achieved in vitro by the transgenic overexpression of human ligands of several inhibitory receptors in porcine cells resulting in their resistance against xenoreactivity. Consequently, several transgenic pigs expressing tissue-specific human ligands of inhibitory coreceptors (HLA-E, CD47) or soluble competitors of costimulation (belatacept) have already been generated. The development of these robust and innovative approaches to modulate human anti-pig cellular immune responses, complementary to conventional immunosuppression, will help to achieve long-term xenograft survival. In this review, we will focus on the current strategies to enhance negative signaling pathways for the regulation of undesirable cell-mediated xenoreactive immune responses.

Selective blockade of herpesvirus entry mediator-B and T lymphocyte attenuator pathway ameliorates acute graft-versus-host reaction

The cosignaling network mediated by the herpesvirus entry mediator (HVEM; TNFRSF14) functions as a dual directional system that involves proinflammatory ligand, lymphotoxin that exhibits inducible expression and competes with HSV glycoprotein D for HVEM, a receptor expressed by T lymphocytes (LIGHT; TNFSF14), and the inhibitory Ig family member B and T lymphocyte attenuator (BTLA). To dissect the differential contributions of HVEM/BTLA and HVEM/LIGHT interactions, topographically-specific, competitive, and nonblocking anti-HVEM Abs that inhibit BTLA binding, but not LIGHT, were developed. We demonstrate that a BTLA-specific competitor attenuated the course of acute graft-versus-host reaction in a murine F(1) transfer semiallogeneic model. Selective HVEM/BTLA blockade did not inhibit donor T cell infiltration into graft-versus-host reaction target organs, but decreased the functional activity of the alloreactive T cells. These results highlight the critical role of HVEM/BTLA pathway in the control of the allogeneic immune response and identify a new therapeutic target for transplantation and autoimmune diseases.

B- and T-lymphocyte attenuator targeting protects against the acute phase of graft versus host reaction by inhibiting donor anti-host cytotoxicity

BACKGROUND

B- and T-lymphocyte attenuator (BTLA) functions as a coinhibitory/costimulatory molecule that belongs to the immunoglobulin superfamily and exhibits a pattern of expression restricted to the hematopoietic compartment. Engagement of BTLA by its ligand, herpes virus entry mediator (HVEM), delivers negative signals to T cells, whereas engagement of HVEM receptor on T cells by surface BTLA expressed on other immune cells costimulates T activation. Previous work has reported that parental donor BTLA knock-out or HVEM knock-out T cells adoptively transferred into nonirradiated F1 recipient mice survived poorly, and the rejection of host hematopoietic cells was attenuated compared with F1 recipients receiving wild-type T cells.

METHODS

Parent into nonirradiated immunocompetent F1 murine model of acute graft versus host reaction, which is induced with the adoptive transfer of splenocytes from donor B6 mice (H-2(b)) into F1 recipients (BALB/c×B6, H-2(d/b)), was used as an experimental approach to test the therapeutic effect of targeting BTLA during the course of an allogeneic immune response.

RESULTS

We herein provide evidence that administration of an anti-BTLA monoclonal antibody leads to significant reduction of donor anti-host allogeneic immune response against bone marrow and thymus during the acute phase of graft versus host reaction in a parent into nonirradiated F1 murine model of alloreactivity. Anti-BTLA protection against donor anti-host hematopoietic cell rejection correlated with impaired anti-host cytotoxic T-lymphocyte activity than reduction in T-cell number infiltrating host tissues.

CONCLUSIONS

These findings place BTLA receptor as a potential immunoregulatory target for the modulation of cytotoxic T-lymphocyte-mediated alloresponses.

Flt3L-mobilized dendritic cells bearing H2-Kbm1 apoptotic cells do not induce cross-tolerance to CD8+ T cells across a class I MHC mismatched barrier

Tolerization of allogeneic CD8(+) T cells is still a pending issue in the field of transplantation research to achieve long-term survival. To test whether dendritic cells (DC) bearing allogeneic major histocompatibility complex (MHC) class I mismatched apoptotic cells could induce cross-tolerance to alloreactive CD8(+) T cells, the following experimental strategy was devised. Rag2/γ(c) KO B6 mice were treated with Fms-like tyrosine kinase 3 ligand (Flt3L)-transduced B16 melanoma cells to drive a rapid expansion and mobilization of DC in vivo. Of all DC populations expanded, splenic CD11c(+) CD103(+) CD8α(+) DC were selectively involved in the process of antigen clearance of X-ray irradiated apoptotic thymocytes in vivo. Considering that CD11c(+) CD103(+) CD8α(+) DC selectively take up apoptotic cells and that they are highly specialized in cross-presenting antigen to CD8(+) T cells, we investigated whether B6 mice adoptively transferred with Flt3L-derived DC loaded with donor-derived apoptotic thymocytes could induce tolerance to bm1 skin allografts. Our findings on host anti-donor alloresponse, as revealed by skin allograft survival and cytotoxic T lymphocyte assays, indicated that the administration of syngeneic DC presenting K(bm1) donor-derived allopeptides through the indirect pathway of antigen presentation was not sufficient to induce cross-tolerance to alloreactive CD8(+) T cells responding to bm1 alloantigens in a murine model of skin allograft transplantation across an MHC class I mismatched barrier.

Development and functional specialization of CD103+ dendritic cells

CD103 (alpha(E)) integrin expression distinguishes a population of dendritic cells (DCs) that can be found in many if not all lymphoid and non-lymphoid organs. CD103(+) DCs display distinct functional activities. Migratory CD103(+) DCs derived from skin, lung, and intestine efficiently present exogenous antigens in their corresponding draining lymph nodes to specific CD8(+) T cells through a mechanism known as cross-presentation. On the T cells they prime, intestinal CD103(+) DCs can drive the induction of the chemokine receptor CCR9 and alpha(4)beta(7) integrin, both known as gut-homing receptors. CD103(+) DCs also contribute to control inflammatory responses and intestinal homeostasis by fostering the conversion of naive T cells into induced Foxp3(+) regulatory T cells, a mechanism that relies on transforming growth factor-beta and retinoic acid signaling. This review discusses recent findings that identify murine CD103(+) DCs as important regulators of the immune response.

CX3CR1+ c-kit+ bone marrow cells give rise to CD103+ and CD103- dendritic cells with distinct functional properties

Dendritic cells (DC) represent a rather heterogeneous cell population with regard to morphology, phenotype, and function and, like most cells of the immune system, are subjected to a continuous renewal process. CD103(+) (integrin alpha(E)) DC have been identified as a major mucosal DC subset involved in the induction of tissue-specific homing molecules on T cells, but little is known about progenitors able to replenish this DC subset. Herein we report that lineage (lin)(-)CX(3)CR1(+)c-kit(+) (GFP(+)c-kit(+)) bone marrow cells can differentiate to either CD11c(+)CD103(-) or CD11c(+)CD103(+) DC in vitro and in vivo. Gene expression as well as functional assays reveal distinct phenotypical and functional properties of both subsets generated in vitro. CD103(-) DC exhibit enhanced phagocytosis and respond to LPS stimulation by secreting proinflammatory cytokines, whereas CD103(+) DC express high levels of costimulatory molecules and efficiently induce allogeneic T cell proliferation. Following adoptive transfer of GFP(+)c-kit(+) bone marrow cells to irradiated recipients undergoing allergic lung inflammation, we identified donor-derived CD103(+) DC in lung and the lung-draining bronchial lymph node. Collectively, these data indicate that GFP(+)c-kit(+) cells contribute to the replenishment of CD103(+) DC in lymphoid and nonlymphoid organs.

PD-1/PD-L1, PD-1/PD-L2, and other co-inhibitory signaling pathways in transplantation

Transplantation of cells, tissues and vascularized solid organs is a successful therapeutic intervention for many end-stage chronic diseases. The combination of co-stimulatory blockade with the delivery of negative signals to T cells through co-inhibitory receptors would provide a robust approach to modulating T-cell receptor signaling and improving alloantigen-specific control of transplant rejection. This approach based on fundamental knowledge of APC/T-cell interactions may complement conventional therapies in the near future to reinforce long-term allograft survival, and permit minimal immunosuppression. The focus of this review was primarily on two major co-inhibitory signaling pathways, namely PD-1/PD-L1/PD-L2 and BTLA/CD160/HVEM/LIGHT that have been thoroughly characterized in murine models of transplantation using genetically modified mice, specific monoclonal antibodies and fusion proteins.

Blockade of the PD-1/PD-1L pathway reverses the protective effect of anti-CD40L therapy in a rat to mouse concordant islet xenotransplantation model

BACKGROUND

We have previously demonstrated that costimulatory blockade with anti-CD40L monoclonal antibody (mAb) prolongs the survival of non-vascularized concordant rat to mouse islet xenografts. Here, we examine whether signaling through the PD-1/PD-1L pathway is required for the anti-CD40L therapy to prolong concordant islet graft survival using a novel anti-murine PD-1 mAb (clone 4F10).

METHODS

C57BL/6 mice received a cellular concordant islet xenograft under the left kidney capsule and four experimental groups were prepared. Group I: untreated control; group II: recipient mice were treated with three doses of 0.5 mg of anti-CD40L mAb (clone MR1) on days 0, 2 and 4; group III: mice were treated with 0.5 mg of anti-PD-1 (CD279) mAb (clone 4F10) every other day for 8 days; and finally group IV: mice received the combined treatment that consisted of anti-CD40L plus anti-PD-1 mAb.

RESULTS

Concordant islet xenografts transplanted in control untreated mice showed a median survival time (MST) of 17 +/- 7.43 days, whereas anti-CD40L treatment led to a significant prolongation of graft survival (MST: 154 +/- 65.56, P < 0.0001). The administration of anti-PD-1 alone significantly accelerated graft rejection compared to non-treated controls (MST: 10 +/- 2.24 vs. MST: 17 +/- 7.43, P < 0.0004). Remarkably, the combined administration of anti-CD40L and anti-PD-1 reversed the protective effect obtained with anti-CD40L alone (anti-CD40L, MST: 154 +/- 65.56 vs. anti-CD40L plus anti-PD-1, MST: 10 +/- 7.72, P < 0.0002).

CONCLUSION

Overall, our data indicate that the PD-1/PD-1L pathway is required for the achievement of prolonged graft survival in anti-CD40L-treated mice in a setting of rat to mouse concordant islet xenotransplantation.

CD103− and CD103+ Bronchial Lymph Node Dendritic Cells Are Specialized in Presenting and Cross-Presenting Innocuous Antigen to CD4+ and CD8+ T Cells

Dendritic cells (DC) are able to capture, process, and present exogenous Ag to CD8(+) T lymphocytes through MHC class I, a process referred to as cross-presentation. In this study, we demonstrate that CD103(+) (CD11c(high)CD11b(low)) and CD103(-) (CD11c(int)CD11b(high)) DC residing in the lung-draining bronchial lymph node (brLN) have evolved to acquire opposing functions in presenting innocuous inhaled Ag. Thus, under tolerogenic conditions, CD103(-) DC are specialized in presenting innocuous Ag to CD4(+) T cells, whereas CD103(+) DC, which do not express CD8alpha, are specialized in presenting Ag exclusively to CD8(+) T cells. In CCR7-deficient but not in plt/plt mice, Ag-carrying CD103(+) DC are largely absent in the brLN, although CD103(+) DC are present in the lung of CCR7-deficient mice. As a consequence, adoptively transferred CD8(+) T cells can be activated under tolerizing conditions in plt/plt but not in CCR7-deficient mice. These data reveal that CD103(+) brLN DC are specialized in cross-presenting innocuous inhaled Ag in vivo. Because these cells are largely absent in CCR7(-/-) mice, our findings strongly suggest that brLN CD103(+) DC are lung-derived and that expression of CCR7 is required for their migration from the lung into its draining lymph node.

Induction of tolerance to innocuous inhaled antigen relies on a CCR7-dependent dendritic cell-mediated antigen transport to the bronchial lymph node

Allergic airway diseases such as asthma are caused by a failure of the immune system to induce tolerance against environmental Ags. The underlying molecular and cellular mechanisms that initiate tolerance are only partly understood. In this study, we demonstrated that a CCR7-dependent migration of both CD103+ and CD103- lung dendritic cells (DC) to the bronchial lymph node (brLN) is indispensable for this process. Although inhaled Ag is amply present in the brLN of CCR7-deficient mice, T cells cannot be tolerized because of the impaired migration of Ag-carrying DC and subsequent transport of Ag from the lung to the draining lymph node. Consequently, the repeated inhalation of Ag protects wild-type but not CCR7-deficient mice from developing allergic airway diseases. Thus, the continuous DC-mediated transport of inhaled Ag to the brLN is critical for the induction of tolerance to innocuous Ags.