Ann Friedman

Blockade of individual Notch ligands and receptors controls graft-versus-host disease

Graft-versus-host disease (GVHD) is the main complication of allogeneic bone marrow transplantation. Current strategies to control GVHD rely on global immunosuppression. These strategies are incompletely effective and decrease the anticancer activity of the allogeneic graft. We previously identified Notch signaling in T cells as a new therapeutic target for preventing GVHD. Notch-deprived T cells showed markedly decreased production of inflammatory cytokines, but normal in vivo proliferation, increased accumulation of regulatory T cells, and preserved anticancer effects. Here, we report that γ-secretase inhibitors can block all Notch signals in alloreactive T cells, but lead to severe on-target intestinal toxicity. Using newly developed humanized antibodies and conditional genetic models, we demonstrate that Notch1/Notch2 receptors and the Notch ligands Delta-like1/4 mediate all the effects of Notch signaling in T cells during GVHD, with dominant roles for Notch1 and Delta-like4. Notch1 inhibition controlled GVHD, but led to treatment-limiting toxicity. In contrast, Delta-like1/4 inhibition blocked GVHD without limiting adverse effects while preserving substantial anticancer activity. Transient blockade in the peritransplant period provided durable protection. These findings open new perspectives for selective and safe targeting of individual Notch pathway components in GVHD and other T cell-mediated human disorders.

Notch signaling is a critical regulator of allogeneic CD4+ T-cell responses mediating graft-versus-host disease

Graft-versus-host disease (GVHD) remains the major barrier to the success of allogeneic hematopoietic stem cell transplantation (HSCT). GVHD is caused by donor T cells that mediate host tissue injury through multiple inflammatory mechanisms. Blockade of individual effector molecules has limited efficacy in controlling GVHD. Here, we report that Notch signaling is a potent regulator of T-cell activation, differentiation, and function during acute GVHD. Inhibition of canonical Notch signaling in donor T cells markedly reduced GVHD severity and mortality in mouse models of allogeneic HSCT. Although Notch-deprived T cells proliferated and expanded in response to alloantigens in vivo, their ability to produce interleukin-2 and inflammatory cytokines was defective, and both CD4(+) and CD8(+) T cells failed to up-regulate selected effector molecules. Notch inhibition decreased the accumulation of alloreactive T cells in the intestine, a key GVHD target organ. However, Notch-deprived alloreactive CD4(+) T cells retained significant cytotoxic potential and antileukemic activity, leading to improved overall survival of the recipients. These results identify Notch as a novel essential regulator of pathogenic CD4(+) T-cell responses during acute GVHD and suggest that Notch signaling in T cells should be investigated as a therapeutic target after allogeneic HSCT.

T Cell–Specific Notch Inhibition Blocks Graft-versus-Host Disease by Inducing a Hyporesponsive Program in Alloreactive CD4+ and CD8+ T Cells

Graft-versus-host disease (GVHD) induced by donor-derived T cells remains the major limitation of allogeneic bone marrow transplantation (allo-BMT). We previously reported that the pan-Notch inhibitor dominant-negative form of Mastermind-like 1 (DNMAML) markedly decreased the severity and mortality of acute GVHD mediated by CD4+ T cells in mice. To elucidate the mechanisms of Notch action in GVHD and its role in CD8+ T cells, we studied the effects of Notch inhibition in alloreactive CD4+ and CD8+ T cells using mouse models of allo-BMT. DNMAML blocked GVHD induced by either CD4+ or CD8+ T cells. Both CD4+ and CD8+ Notch-deprived T cells had preserved expansion in lymphoid organs of recipients, but profoundly decreased IFN-γ production despite normal T-bet and enhanced Eomesodermin expression. Alloreactive DNMAML T cells exhibited decreased Ras/MAPK and NF-κB activity upon ex vivo restimulation through the TCR. In addition, alloreactive T cells primed in the absence of Notch signaling had increased expression of several negative regulators of T cell activation, including Dgka, Cblb, and Pdcd1. DNMAML expression had modest effects on in vivo proliferation but preserved overall alloreactive T cell expansion while enhancing accumulation of pre-existing natural regulatory T cells. Overall, DNMAML T cells acquired a hyporesponsive phenotype that blocked cytokine production but maintained their expansion in irradiated allo-BMT recipients, as well as their in vivo and ex vivo cytotoxic potential. Our results reveal parallel roles for Notch signaling in alloreactive CD4+ and CD8+ T cells that differ from past reports of Notch action and highlight the therapeutic potential of Notch inhibition in GVHD.

Menin regulates the function of hematopoietic stem cells and lymphoid progenitors

Men1 is a tumor suppressor gene mutated in endocrine neoplasms. Besides its endocrine role, the Men1 gene product menin interacts with the mixed lineage leukemia (MLL) protein, a histone H3 lysine 4 methyltransferase. Although menin and MLL fusion proteins cooperate to activate Homeobox (Hox) gene expression during transformation, little is known about the normal hematopoietic functions of menin. Here, we studied hematopoiesis after Men1 ablation. Menin loss modestly impaired blood neutrophil, lymphocyte, and platelet counts. Without hematopoietic stress, multilineage and myelo-erythroid bone marrow progenitor numbers were preserved, while B lymphoid progenitors were decreased. In contrast, competitive transplantation revealed a marked functional defect of long-term hematopoietic stem cells (HSC) in the absence of menin, despite normal initial homing of progenitors to the bone marrow. HoxA9 gene expression was only modestly decreased in menin-deficient HSCs. These observations reveal a novel and essential role for menin in HSC homeostasis that was most apparent during situations of hematopoietic recovery, suggesting that menin regulates molecular pathways that are essential during the adaptive HSC response to stress.

Ikaros-Notch axis in host hematopoietic cells regulates experimental graft-versus-host disease.

Host hematopoietically derived APCs play a vital role in the initiation of GVH responses. However, the APC autonomous molecular mechanisms that are critical for the induction of GVHD are not known. We report here that the Ikaros-Notch axis in host hematopoietically derived APCs regulates the severity of acute GVHD across multiple clinically relevant murine models of experimental bone marrow transplantation. In the present study, Ikaros deficiency (Ik(-/-)) limited to host hematopoietically derived APCs enhanced donor T-cell expansion and intensified acute GVHD, as determined by survival and other GVHD-specific parameters. The Ik(-/-) conventional CD8(+) and CD8(-)CD11c(+) dendritic cells (DCs), the most potent APCs, showed no increase in the expression of activation markers or in response to TLR stimulation compared with wild-type controls. However, Ik(-/-) DCs demonstrated an enhanced stimulation of allogeneic T cells. Deficiency of Ikaros in the conventional CD8(+) and CD8(-)CD11c(+) DCs was associated with an increase in Notch signaling, the blockade of which mitigated the enhanced in vitro and in vivo allostimulatory capacity. Therefore, the Ikaros-Notch axis is a novel pathway that modulates DC biology in general, and targeting this pathway in host hematopoietically derived APCs may reduce GVHD.

Ash1l controls quiescence and self-renewal potential in hematopoietic stem cells

Rapidly cycling fetal and neonatal hematopoietic stem cells (HSCs) generate a pool of quiescent adult HSCs after establishing hematopoiesis in the bone marrow. We report an essential role for the trithorax group gene absent, small, or homeotic 1-like (Ash1l) at this developmental transition. Emergence and expansion of Ash1l-deficient fetal/neonatal HSCs were preserved; however, in young adult animals, HSCs were profoundly depleted. Ash1l-deficient adult HSCs had markedly decreased quiescence and reduced cyclin-dependent kinase inhibitor 1b/c (Cdkn1b/1c) expression and failed to establish long-term trilineage bone marrow hematopoiesis after transplantation to irradiated recipients. Wild-type HSCs could efficiently engraft when transferred to unirradiated, Ash1l-deficient recipients, indicating increased availability of functional HSC niches in these mice. Ash1l deficiency also decreased expression of multiple Hox genes in hematopoietic progenitors. Ash1l cooperated functionally with mixed-lineage leukemia 1 (Mll1), as combined loss of Ash1l and Mll1, but not isolated Ash1l or Mll1 deficiency, induced overt hematopoietic failure. Our results uncover a trithorax group gene network that controls quiescence, niche occupancy, and self-renewal potential in adult HSCs.

Transient Blockade of Delta-like Notch Ligands Prevents Allograft Rejection Mediated by Cellular and Humoral Mechanisms in a Mouse Model of Heart Transplantation

Rejection remains a major clinical challenge limiting allograft survival after solid organ transplantation. Both cellular and humoral immunity contribute to this complication, with increased recognition of Ab-mediated damage during acute and chronic rejection. Using a mouse model of MHC-mismatched heart transplantation, we report markedly protective effects of Notch inhibition, dampening both T cell and Ab-driven rejection. T cell–specific pan-Notch blockade prolonged heart allograft survival and decreased IFN-γ and IL-4 production by alloreactive T cells, especially when combined with depletion of recipient CD8+ T cells. These effects were associated with decreased infiltration by conventional T cells and an increased proportion of regulatory T cells in the graft. Transient administration of neutralizing Abs specific for delta-like (Dll)1/4 Notch ligands in the peritransplant period led to prolonged acceptance of allogeneic hearts, with superior outcome over Notch inhibition only in T cells. Systemic Dll1/4 inhibition decreased T cell cytokines and graft infiltration, germinal center B cell and plasmablast numbers, as well as production of donor-specific alloantibodies and complement deposition in the transplanted hearts. Dll1 or Dll4 inhibition alone provided partial protection. Thus, pathogenic signals delivered by Dll1/4 Notch ligands early after transplantation promote organ rejection through several complementary mechanisms. Transient interruption of these signals represents an attractive new therapeutic strategy to enhance long-term allograft survival.

Fibroblastic niches prime T cell alloimmunity through Delta-like Notch ligands

Alloimmune T cell responses induce graft-versus-host disease (GVHD), a serious complication of allogeneic bone marrow transplantation (allo-BMT). Although Notch signaling mediated by Delta-like 1/4 (DLL1/4) Notch ligands has emerged as a major regulator of GVHD pathogenesis, little is known about the timing of essential Notch signals and the cellular source of Notch ligands after allo-BMT. Here, we have shown that critical DLL1/4-mediated Notch signals are delivered to donor T cells during a short 48-hour window after transplantation in a mouse allo-BMT model. Stromal, but not hematopoietic, cells were the essential source of Notch ligands during in vivo priming of alloreactive T cells. GVHD could be prevented by selective inactivation of Dll1 and Dll4 in subsets of fibroblastic stromal cells that were derived from chemokine Ccl19-expressing host cells, including fibroblastic reticular cells and follicular dendritic cells. However, neither T cell recruitment into secondary lymphoid organs nor initial T cell activation was affected by Dll1/4 loss. Thus, we have uncovered a pathogenic function for fibroblastic stromal cells in alloimmune reactivity that can be dissociated from their homeostatic functions. Our results reveal what we believe to be a previously unrecognized Notch-mediated immunopathogenic role for stromal cell niches in secondary lymphoid organs after allo-BMT and define a framework of early cellular and molecular interactions that regulate T cell alloimmunity.

Hematopoietic stem cells are acutely sensitive to Acd shelterin gene inactivation

The shelterin complex plays dual functions in telomere homeostasis by recruiting telomerase and preventing the activation of a DNA damage response at telomeric ends. Somatic stem cells require telomerase activity, as evidenced by progressive stem cell loss leading to bone marrow failure in hereditary dyskeratosis congenita. Recent work demonstrates that dyskeratosis congenita can also arise from mutations in specific shelterin genes, although little is known about shelterin functions in somatic stem cells. We found that mouse hematopoietic stem cells (HSCs) are acutely sensitive to inactivation of the shelterin gene Acd, encoding TPP1. Homozygosity for a hypomorphic acd allele preserved the emergence and expansion of fetal HSCs but led to profoundly defective function in transplantation assays. Upon complete Acd inactivation, HSCs expressed p53 target genes, underwent cell cycle arrest, and were severely depleted within days, leading to hematopoietic failure. TPP1 loss induced increased telomeric fusion events in bone marrow progenitors. However, unlike in epidermal stem cells, p53 deficiency did not rescue TPP1-deficient HSCs, indicating that shelterin dysfunction has unique effects in different stem cell populations. Because the consequences of telomere shortening are progressive and unsynchronized, acute loss of shelterin function represents an attractive alternative for studying telomere crisis in hematopoietic progenitors.

Notch signaling mediated by Delta-like1/4 ligands controls the pathogenesis of chronic graft-versus-host disease in mice.

Chronic graft-versus-host disease (cGVHD) is a major complication of allogeneic hematopoietic cell transplantation (allo-HCT) and remains an area of unmet clinical need with few treatment options available. Notch blockade prevents acute GVHD in multiple mouse models, but the impact of Notch signaling on cGVHD remains unknown. Using genetic and antibody-mediated strategies of Notch inhibition, we investigated the role of Notch signaling in complementary mouse cGVHD models that mimic several aspects of human cGVHD in search of candidate therapeutics. In the B10.D2→BALB/c model of sclerodermatous cGVHD, Delta-like ligand 4 (Dll4)-driven Notch signaling was essential for disease development. Antibody-mediated Dll4 inhibition conferred maximum benefits when pursued early in a preventative fashion, with anti-Dll1 enhancing early protection. Notch-deficient alloantigen-specific T cells showed no early defects in proliferation or helper polarization in vivo but subsequently exhibited markedly decreased cytokine secretion and enhanced accumulation of FoxP3+ regulatory T cells. In the B6→B10.BR major histocompatibility complex-mismatched model with multi-organ system cGVHD and prominent bronchiolitis obliterans (BO), but not skin manifestations, absence of Notch signaling in T cells provided long-lasting disease protection that was replicated by systemic targeting of Dll1, Dll4, or both Notch ligands, even during established disease. Notch inhibition decreased target organ damage and germinal center formation. Moreover, decreased BO-cGVHD was observed upon inactivation of Notch1 and/or Notch2 in T cells. Systemic targeting of Notch2 alone was safe and conferred therapeutic benefits. Altogether, Notch ligands and receptors regulate key pathogenic steps in cGVHD and emerge as novel druggable targets to prevent or treat different forms of cGVHD.