Maria Koulmanda

Tissue expression of PD-L1 mediates peripheral T cell tolerance

Programmed death 1 (PD-1), an inhibitory receptor expressed on activated lymphocytes, regulates tolerance and autoimmunity. PD-1 has two ligands: PD-1 ligand 1 (PD-L1), which is expressed broadly on hematopoietic and parenchymal cells, including pancreatic islet cells; and PD-L2, which is restricted to macrophages and dendritic cells. To investigate whether PD-L1 and PD-L2 have synergistic or unique roles in regulating T cell activation and tolerance, we generated mice lacking PD-L1 and PD-L2 (PD-L1/PD-L2−/− mice) and compared them to mice lacking either PD-L. PD-L1 and PD-L2 have overlapping functions in inhibiting interleukin-2 and interferon-γ production during T cell activation. However, PD-L1 has a unique and critical role in controlling self-reactive T cells in the pancreas. Our studies with bone marrow chimeras demonstrate that PD-L1/PD-L2 expression only on antigen-presenting cells is insufficient to prevent the early onset diabetes that develops in PD-L1/PD-L2−/− non-obese diabetic mice. PD-L1 expression in islets protects against immunopathology after transplantation of syngeneic islets into diabetic recipients. PD-L1 inhibits pathogenic self-reactive CD4+ T cell–mediated tissue destruction and effector cytokine production. These data provide evidence that PD-L1 expression on parenchymal cells rather than hematopoietic cells protects against autoimmune diabetes and point to a novel role for PD-1–PD-L1 interactions in mediating tissue tolerance.

Curative and β cell regenerative effects of α1-antitrypsin treatment in autoimmune diabetic NOD mice

Invasive insulitis is a destructive T cell-dependent autoimmune process directed against insulin-producing β cells that is central to the pathogenesis of type 1 diabetes mellitus (T1DM) in humans and the clinically relevant nonobese diabetic (NOD) mouse model. Few therapies have succeeded in restoring long-term, drug-free euglycemia and immune tolerance to β cells in overtly diabetic NOD mice, and none have demonstrably enabled enlargement of the functional β cell mass. Recent studies have emphasized the impact of inflammatory cytokines on the commitment of antigen-activated T cells to various effector or regulatory T cell phenotypes and insulin resistance and defective insulin signaling. Hence, we tested the hypothesis that inflammatory mechanisms trigger insulitis, insulin resistance, faulty insulin signaling, and the loss of immune tolerance to islets. We demonstrate that treatment with α1-antitrypsin (AAT), an agent that dampens inflammation, does not directly inhibit T cell activation, ablates invasive insulitis, and restores euglycemia, immune tolerance to β cells, normal insulin signaling, and insulin responsiveness in NOD mice with recent-onset T1DM through favorable changes in the inflammation milieu. Indeed, the functional mass of β cells expands in AAT-treated diabetic NOD mice.

Expression of CD39 by Human Peripheral Blood CD4+CD25+ T Cells Denotes a Regulatory Memory Phenotype

We have shown that CD39 and CD73 are coexpressed on the surface of murine CD4+Foxp3+ regulatory T cells (Treg) and generate extracellular adenosine, contributing to Treg immunosuppressive activity. We now describe that CD39, independently of CD73, is expressed by a subset of blood‐derived human CD4+CD25+CD127lo Treg, defined by robust expression of Foxp3. A further distinct population of CD4+CD39+ T lymphocytes can be identified, which do not express CD25 and FoxP3 and exhibit the memory effector cellular phenotype. Differential expression of CD25 and CD39 on circulating CD4+ T cells distinguishes between Treg and pathogenic cellular populations that secrete proinflammatory cytokines such as IFNγ and IL‐17. These latter cell populations are increased, with a concomitant decrease in the CD4+CD25+CD39+ Tregs, in the peripheral blood of patients with renal allograft rejection. We conclude that the ectonucleotidase CD39 is a useful and dynamic lymphocytes surface marker that can be used to identify different peripheral blood T cell‐populations to allow tracking of these in health and disease, as in renal allograft rejection.

Mechanisms Underlying Blockade of Allograft Acceptance by TLR Ligands

Immune activation via TLRs is known to prevent transplantation tolerance in multiple animal models. To investigate the mechanisms underlying this barrier to tolerance induction, we used complementary murine models of skin and cardiac transplantation in which prolonged allograft acceptance is either spontaneous or pharmacologically induced with anti-CD154 mAb and rapamycin. In each model, we found that prolonged allograft survival requires the presence of natural CD4+Foxp3+ T regulatory cells (Tregs), and that the TLR9 ligand CpG prevents graft acceptance both by interfering with natural Treg function and by promoting the differentiation of Th1 effector T cells in vivo. We further demonstrate that although Th17 cells differentiate from naive alloreactive T cells, these cells do not arise from natural Tregs in either CpG-treated or untreated graft recipients. Finally, we show that CpG impairs natural Treg suppressor capability and prevents Treg-dependent allograft acceptance in an IL-6-independent fashion. Our data therefore suggest that TLR signals do not prevent prolonged graft acceptance by directing natural Tregs into the Th17 lineage or by using other IL-6-dependent mechanisms. Instead, graft destruction results from the ability of CpG to drive Th1 differentiation and interfere with immunoregulation established by alloreactive natural CD4+Foxp3+ Tregs.

Adult porcine islet transplantation in baboons treated with conventional immunosuppression or a non-myeloablative regimen and CD154 blockade

The aim of the present study was to assess the survival of adult porcine islets transplanted into baboons receiving either (1) conventional triple drug immunosuppressive therapy or (2) a non‐myeloablative regimen and an anti‐CD154 monoclonal antibody (mAb) aimed at tolerance‐induction.

The Effect of Low Versus High Dose of Streptozotocin in Cynomolgus Monkeys (Macaca Fascilularis)

Streptozotocin (STZ) is often used to induce diabetes in animal models. However, morbidity associated with STZ and its ability to induce diabetes vary with different dosages among different animal species, including nonhuman primates. To find an optimal dose of STZ that would cause diabetes with minimal toxicity, we compared low and high doses of STZ. Male cynomolgus monkeys (3–6 years old) were given a single dose of 100 mg/kg (high dose, 4 animals) or 55 mg/kg (low dose, 20 animals) of STZ. Blood glucose levels, intravenous glucose tolerance test (IVGTT), pancreatic biopsies, liver function tests (LFTs), liver biopsies, kidney function tests, and kidney biopsies were performed periodically. Animals from both groups developed diabetes within 24 h after administration of STZ. Serum C‐peptide levels in both groups decreased from 2 to 8 ng/mL before STZ to between 0.01 and 0.6 ng/mL after STZ. Animals with the high dose of STZ developed transient vomiting within minutes after injection. During the first week after STZ injection, high‐dose animals developed elevated LFTs, BUN and creatinine. In contrast, low‐dose animals had normal liver and kidney function tests. Histological analysis showed that animals given the high dose of STZ developed marked steatosis of the liver and tubular injury in the kidneys, whereas animals given the low dose of STZ had normal‐looking liver and kidney histology. The pancreatic islets in both groups were indistinguishable by immunoperoxidase staining for insulin, and showed either no insulin‐positive cells or rare insulin‐positive cells. Glucagon staining was normal. Over time, low‐dose diabetic monkeys remained persistently hyperglycemic with negligible C‐peptide stimulation by intravenous glucose. We conclude that low‐dose STZ at 55 mg/mL successfully induces diabetes in cynomolgus monkeys with minimal liver and kidney toxicity.

Evidence that macrophages are required for T-cell infiltration and rejection of fetal pig pancreas xenografts in nonobese diabetic mice.

BACKGROUND Host macrophages are abundant within fetal pig pancreas xenografts undergoing rejection, but their role is unknown. Therefore, we examined the effect of host macrophage depletion on xenograft rejection. METHODS Nonobese diabetic (NOD) mice were given clodronate-loaded liposomes intravenously to deplete macrophages. Controls received phosphate-buffered saline (PBS) or PBS-liposomes. General immune status was assessed after 2, 3, and 7 days by (1) fluorescence-activated cell sorter analysis of peripheral blood, spleen, and lymph node cells, (2) immunohistochemistry on spleens, and (3) mixed lymphocyte reaction. Organ-cultured fetal pig pancreas was transplanted under the kidney capsule of NOD mice 3 days after clodronate or PBS injection. Grafts were assessed histologically at 4, 5, 6, and 8 days after transplantation. RESULTS Splenic macrophages and peripheral blood monocytes were depleted 2 days after clodronate treatment but had recovered within 11 days. T cell, B cell, and dendritic cell numbers were normal in spleen, peripheral blood, and lymph nodes of clodronate-treated mice, and T cells and antigen-presenting cells from these mice functioned normally in mixed lymphocyte reaction. Clodronate treatment markedly reduced graft infiltration by macrophages, T cells, and eosinophils at 4, 5, and 6 days after transplantation, and was associated with maintenance of endocrine cell viability and insulin expression. However, all grafts were rejected 8 days after transplantation, concordant with reappearance of splenic macrophages. CONCLUSIONS Short-term, specific depletion of macrophages markedly delayed cellular infiltration and rejection of xenografts. The results provide the first evidence that macrophages promote T-cell infiltration and rejection of fetal pig pancreas xenografts in NOD mice.

Modification of adverse inflammation is required to cure new-onset type 1 diabetic hosts

In nonobese diabetic (NOD) mice with overt new-onset type 1 diabetes mellitus (T1DM), short-term treatment with a “triple-therapy” regimen [rapamycin plus agonist IL-2-related and antagonist-type, mutant IL-15-related Ig fusion proteins (IL-2.Ig and mutIL-15.Ig)] halts autoimmune destruction of insulin-producing beta cells and restores both euglycemia and immune tolerance to beta cells. Increases in the mass of insulin-producing beta cells or circulating insulin levels were not linked to the restoration of euglycemia. Instead, the restoration of euglycemia was linked to relief from an inflammatory state that impaired the hosts response to insulin. Both restoration of immune tolerance to beta cells and relief from the adverse metabolic effects of an inflammatory state in insulin-sensitive tissues appear essential for permanent restoration of normoglycemia in this T1DM model. Thus, this triple-therapy regimen, possessing both tolerance-inducing and select antiinflammatory properties, may represent a prototype for therapies able to restore euglycemia and self-tolerance in T1DM.

Inflammation and the balance of Treg and Th17 cells in transplant rejection and tolerance.

Purpose of reviewInflammation of the allograft, occurring as a consequence of hypoxia and ischemia/reperfusion injury, adversely influences short-term and long-term transplant outcomes. Thus far, imbalance of tissue-protective Treg and tissue-destructive Th17 cells has been confirmed in a number of tissue-inflammatory states, including autoimmune disease. Hence, benefits of tilting Treg–Th17 equilibrium toward dominance of Tregs may promote transplant tolerance. Recent findingsAdverse graft inflammation creates extreme resistance to the induction of donor-specific tolerance. Proinflammatory cytokines, when abundantly expressed within the graft and draining lymph nodes, prevent commitment of donor-activated T cells into graft-protective, T-regulatory phenotype, while fostering generation of donor-reactive Th1, Th2 or Th17 effector subsets. In addition, the inflammatory milieu may destabilize the program of both natural and induced Tregs, converting them into inflammatory, effector-like phenotypes. Therefore permanent, Treg-dependent acceptance of an allograft may not be achieved without limiting adverse tissue inflammation. SummaryBalance of graft-protective regulatory and graft-destructive effector T cells largely depends on the balance of proinflammatory and anti-inflammatory cytokines in the milieu, in which donor-directed T-cell response occurs. In the absence of proinflammatory cytokines, the constitutive expression of TGF-β may guide recipient T cells into a tissue-protective, pro-tolerant mode. Therefore, targeting adverse tissue inflammation may represent a powerful means to tilt antidonor immunity towards tolerance.

Recently Discovered T Cell Subsets Cannot Keep Their Commitments

After activation by antigen/MHC (signal 1) and CD28-dependent co-stimulation (signal 2), resting CD4(+) T cells commit to one of a variety of functionally and molecularly defined phenotypes. Two long established CD4 phenotypes, Th1 and Th2 cells, have been regarded as terminally differentiated formats. Recently, two additional phenotypes, tissue-protective regulatory (Tregs) and tissue-destructive Th17 T cells, have also been discovered, and neither represents a terminally differentiated phenotype. Rather, Tregs and Th17(+) cells respond to cues provided by the inflammatory texture in which these cells reside. We review the important scientific and therapeutic implications of these differences herein.