Islet Allograft Rejection in Nonobese Diabetic Mice Involves the Common γ-Chain and CD28/CD154-Dependent and -Independent Mechanisms

Human stem cell derived beta-like cells engineered to present PD-L1 improve transplant survival in NOD mice carrying human HLA class I

There is a critical need for therapeutic approaches that combine renewable sources of replacement beta cells with localized immunomodulation to counter recurrence of autoimmunity in type 1 diabetes (T1D). However, there are few examples of animal models to study such approaches that incorporate spontaneous autoimmunity directed against human beta cells rather than allogenic rejection. Here, we address this critical limitation by demonstrating rejection and survival of transplanted human stem cell-derived beta-like cells clusters (sBCs) in a fully immune competent mouse model with matching human HLA class I and spontaneous diabetes development. We engineered localized immune tolerance toward transplanted sBCs via inducible cell surface overexpression of PD-L1 (iP-sBCs) with and without deletion of all HLA class I surface molecules via beta-2 microglobulin knockout (iP-BKO sBCs). NOD.HLA-A2.1 mice, which lack classical murine MHC I and instead express human HLA-A*02:01, underwent transplantation of 1,000 human HLA-A*02:01 sBCs under the kidney capsule and were separated into HLA-A2 positive iP-sBC and HLA-class I negative iP-BKO sBC groups, each with +/- doxycycline (DOX) induced PD-L1 expression. IVIS imaging showed significantly improved graft survival in mice transplanted with PD-L1 expressing iP-sBC at day 3 post transplantation compared to controls. However, luciferase signal dropped below in vivo detection limits by day 14 for all groups in this aggressive immune competent diabetes model. Nonetheless, histological examination revealed significant numbers of surviving insulin⁺/PD-L1⁺ sBCs cells for DOX-treated mice at day 16 post-transplant despite extensive infiltration with high numbers of CD3⁺ and CD45⁺ immune cells. These results show that T cells rapidly infiltrate and attack sBC grafts in this model but that significant numbers of PD-L1 expressing sBCs manage to survive in this harsh immunological environment. This investigation represents one of the first in vivo studies recapitulating key aspects of human autoimmune diabetes to test immune tolerance approaches with renewable sources of beta cells.

Interleukin-15 in autoimmunity

Interleukin-15 (IL-15) is a member of the IL-2 family of cytokines, which use receptor complexes containing the common gamma (γ_c) chain for signaling. IL-15 plays important roles in innate and adaptative immune responses and is implicated in the pathogenesis of several immune diseases. The IL-15 receptor consists of 3 subunits namely, the ligand-binding IL-15Rα chain, the β chain (also used by IL-2) and the γ_c chain. IL-15 uses a unique signaling pathway whereby IL-15 associates with IL-15Rα during biosynthesis, and this complex is 'trans-presented' to responder cells that expresses the IL-2/15Rβγ_c receptor complex. IL-15 is subject to post-transcriptional and post-translational regulation, and evidence also suggests that IL-15 cis-signaling can occur under certain conditions. IL-15 has been implicated in the pathology of various autoimmune diseases such as rheumatoid arthritis, autoimmune diabetes, inflammatory bowel disease, coeliac disease and psoriasis. Studies with pre-clinical models have shown the beneficial effects of targeting IL-15 signaling in autoimmunity. Unlike therapies targeting other cytokines, anti-IL-15 therapies have not yet been successful in humans. We discuss the complexities of IL-15 signaling in autoimmunity and explore potential immunotherapeutic approaches to target the IL-15 signaling pathway.

Immunological aspects of allogeneic pancreatic islet transplantation: a comparison between mouse and human

Pancreatic islet allotransplantation is a treatment for patients with severe forms of type 1 diabetes. As long-term graft function and survival are not yet optimal, additional studies are warranted in order to continue improving transplant outcomes. The mechanisms of islet graft loss and tolerance induction are often studied in murine diabetes models. Despite numerous islet transplantation studies successfully performed over recent years, translation from experimental mouse models to human clinical application remains elusive. This review aims at critically discussing the strengths and limitations of current mouse models of diabetes and experimental islet transplantation. In particular, we will analyze the causes leading to diabetes and compare the immunological mechanisms responsible for rejection between mouse and human. A better understanding of the experimental mouse models should facilitate translation to human clinical application.

CD4+ T-Cells With High Common γ Chain Expression and Disturbed Cytokine Production Are Enriched in Children With Type-1 Diabetes

The common gamma chain (γ_c) contributes to the formation of different cytokine receptors [e.g., IL-2 receptor (IL-2R), IL-7R, and IL-15R], which are important for generation of self-reactive T-cells in autoimmune diseases, like in type 1 diabetes (T1D). Whereas, the roles of membrane and soluble IL-2Rα and IL-7Rα variants in T1D disease pathogenesis are well-described, effects of γ_c expression and availability for dependent receptors remain elusive. We investigated expression of the γ_c and dependent receptors on T-cells and soluble γ_c concentrations in serum from patients with T1D (n = 34) and healthy controls (n = 27). Effector T-cell cytokines as well as IL-2, IL-7, and IL-15 induced STAT5 phosphorylation were analyzed to determine functional implications of differential γ_c expression of CD4⁺ T-cell subsets classified by t-distributed Stochastic Neighbor Embedding (t-SNE) analyses. We found increased γ_c and IL-7Rα expression of CD4⁺ T-cells from T1D patients as compared to controls. t-SNE analyses assigned differential expression to subsets of memory T-cells co-expressing γ_c and IL-7Rα. Whereas, γ_c expression was positively correlated with IL-2Rα in memory T-cells from healthy controls, no dependency was found for patients with T1D. Similarly, the effector T-cell cytokine, IL-21, correlated inversely with γ_c expression in healthy controls, but not in T1D patients. Finally, T1D patients with high γ_c expression had increased proportions of IL-2 sensitive pSTAT5⁺ effector T-cells. These results indicated aberrantly high γ_c expression of T-cells from T1D patients with implications on dependent cytokine receptor signaling and effector T-cell cytokine production.

Antibody combination therapy targeting CD25, CD70 and CD8 reduces islet inflammation and improves glycaemia in diabetic mice

Destruction of pancreatic islets in type 1 diabetes is caused by infiltrating, primed and activated T cells. In a clinical setting this autoimmune process is already in an advanced stage before intervention therapy can be administered. Therefore, an effective intervention needs to reduce islet inflammation and preserve any remaining islet function. In this study we have investigated the role of targeting activated T cells in reversing autoimmune diabetes. A combination therapy consisting of CD25-, CD70- and CD8-specific monoclonal antibodies was administered to non-obese diabetic (NOD) mice with either new-onset diabetes or with advanced diabetes. In NOD mice with new-onset diabetes antibody combination treatment reversed hyperglycaemia and achieved long-term protection from diabetes (blood glucose <13·9 mmol/l) in >50% of mice. In contrast, in the control, untreated group blood glucose levels continued to increase and none of the mice were protected from diabetes (P < 0·0001). Starting therapy early when hyperglycaemia was relatively mild proved critical, as the mice with advanced diabetes showed less efficient control of blood glucose and shorter life span. Histological analysis (insulitis score) showed islet preservation and reduced immune infiltration in all treated groups, compared to their controls. In conclusion, antibody combination therapy that targets CD25, CD70 and CD8 results in decreased islet infiltration and improved blood glucose levels in NOD mice with established diabetes.

Promoting long-term survival of insulin-producing cell grafts that differentiate from adipose tissue-derived stem cells to cure type 1 diabetes

BACKGROUND

Insulin-producing cell clusters (IPCCs) have recently been generated in vitro from adipose tissue-derived stem cells (ASCs) to circumvent islet shortage. However, it is unknown how long they can survive upon transplantation, whether they are eventually rejected by recipients, and how their long-term survival can be induced to permanently cure type 1 diabetes. IPCC graft survival is critical for their clinical application and this issue must be systematically addressed prior to their in-depth clinical trials.

METHODOLOGY/PRINCIPAL FINDINGS

Here we found that IPCC grafts that differentiated from murine ASCs in vitro, unlike their freshly isolated islet counterparts, did not survive long-term in syngeneic mice, suggesting that ASC-derived IPCCs have intrinsic survival disadvantage over freshly isolated islets. Indeed, β cells retrieved from IPCC syngrafts underwent faster apoptosis than their islet counterparts. However, blocking both Fas and TNF receptor death pathways inhibited their apoptosis and restored their long-term survival in syngeneic recipients. Furthermore, blocking CD40-CD154 costimulation and Fas/TNF signaling induced long-term IPCC allograft survival in overwhelming majority of recipients. Importantly, Fas-deficient IPCC allografts exhibited certain immune privilege and enjoyed long-term survival in diabetic NOD mice in the presence of CD28/CD40 joint blockade while their islet counterparts failed to do so.

CONCLUSIONS/SIGNIFICANCE

Long-term survival of ASC-derived IPCC syngeneic grafts requires blocking Fas and TNF death pathways, whereas blocking both death pathways and CD28/CD40 costimulation is needed for long-term IPCC allograft survival in diabetic NOD mice. Our studies have important clinical implications for treating type 1 diabetes via ASC-derived IPCC transplantation.

Contribution of astrocyte-derived IL-15 to CD8 T cell effector functions in multiple sclerosis

The contribution of local factors to the activation of immune cells infiltrating the CNS of patients with multiple sclerosis (MS) remains to be defined. The cytokine IL-15 is pivotal in the maintenance and activation of CD8 T lymphocytes, a prominent lymphocyte population found in MS lesions. We investigated whether astrocytes are a functional source of IL-15 sufficient to enhance CD8 T lymphocyte responses and whether they provide IL-15 in the inflamed CNS of patients with MS. We observed that human astrocytes in primary cultures increased surface IL-15 levels upon activation with combinations of proinflammatory cytokines. Expanded human myelin autoreactive CD8 T lymphocytes cultured with such activated astrocytes displayed elevated lytic enzyme content, NKG2D expression, and Ag-specific cytotoxicity. These functional enhancements were abrogated by anti-IL-15-blocking Abs. Immunohistochemical analysis of brain tissue sections obtained from patients with MS demonstrated colocalization for IL-15 and the astrocyte marker glial fibrillary acidic protein within white matter lesions. The majority of astrocytes (80-90%) present in demyelinating MS lesions expressed IL-15, whereas few astrocytes in normal control brain sections had detectable IL-15. IL-15 could be detected in the majority of Iba-1-expressing microglia in the control sections, albeit in lower numbers when compared with microglia/macrophages in MS lesions. Furthermore, infiltrating CD8 T lymphocytes in MS lesions were in close proximity to IL-15-expressing cells. Astrocyte production of IL-15 resulting in the activation of CD8 T lymphocytes ascribes a role for these cells as contributors to the exacerbation of tissue damage during MS pathogenesis.

Molecular mechanism and function of CD40/CD40L engagement in the immune system

SUMMARY

During the generation of a successful adaptive immune response, multiple molecular signals are required. A primary signal is the binding of cognate antigen to an antigen receptor expressed by T and B lymphocytes. Multiple secondary signals involve the engagement of costimulatory molecules expressed by T and B lymphocytes with their respective ligands. Because of its essential role in immunity, one of the best characterized of the costimulatory molecules is the receptor CD40. This receptor, a member of the tumor necrosis factor receptor family, is expressed by B cells, professional antigen-presenting cells, as well as non-immune cells and tumors. CD40 binds its ligand CD40L, which is transiently expressed on T cells and other non-immune cells under inflammatory conditions. A wide spectrum of molecular and cellular processes is regulated by CD40 engagement including the initiation and progression of cellular and humoral adaptive immunity. In this review, we describe the downstream signaling pathways initiated by CD40 and overview how CD40 engagement or antagonism modulates humoral and cellular immunity. Lastly, we discuss the role of CD40 as a target in harnessing anti-tumor immunity. This review underscores the essential role CD40 plays in adaptive immunity.

Islet transplantation in patients with autoimmune diabetes induces homeostatic cytokines that expand autoreactive memory T cells

Successful transplantation requires the prevention of allograft rejection and, in the case of transplantation to treat autoimmune disease, the suppression of autoimmune responses. The standard immunosuppressive treatment regimen given to patients with autoimmune type 1 diabetes who have received an islet transplant results in the loss of T cells. In many other situations, the immune system responds to T cell loss through cytokine-dependant homeostatic proliferation of any remaining T cells. Here we show that T cell loss after islet transplantation in patients with autoimmune type 1 diabetes was associated with both increased serum concentrations of IL-7 and IL-15 and in vivo proliferation of memory CD45RO(+) T cells, highly enriched in autoreactive glutamic acid decarboxylase 65-specific T cell clones. Immunosuppression with FK506 and rapamycin after transplantation resulted in a chronic homeostatic expansion of T cells, which acquired effector function after immunosuppression was removed. In contrast, the cytostatic drug mycophenolate mofetil efficiently blocked homeostatic T cell expansion. We propose that the increased production of cytokines that induce homeostatic expansion could contribute to recurrent autoimmunity in transplanted patients with autoimmune disease and that therapy that prevents the expansion of autoreactive T cells will improve the outcome of islet transplantation.

Lessons on autoimmune diabetes from animal models

T1DM (Type I diabetes mellitus) results from selective destruction of the insulin-producing beta-cells of the pancreas by the immune system, and is characterized by hyperglycaemia and vascular complications arising from suboptimal control of blood glucose levels. The discovery of animal models of T1DM in the late 1970s and early 1980s, particularly the NOD (non-obese diabetic) mouse and the BB (BioBreeding) diabetes-prone rat, had a fundamental impact on our ability to understand the genetics, aetiology and pathogenesis of this disease. NOD and BB diabetes-prone rats spontaneously develop a form of diabetes that closely resembles the human counterpart. Early studies of these animals quickly led to the realization that T1DM is caused by autoreactive T-lymphocytes and revealed that the development of T1DM is controlled by numerous polymorphic genetic elements that are scattered throughout the genome. The development of transgenic and gene-targeting technologies during the 1980s allowed the generation of models of T1DM of reduced genetic and pathogenic complexity, and a more detailed understanding of the immunogenetics of T1DM. In this review, we summarize the contribution of studies in animal models of T1DM to our current understanding of four fundamental aspects of T1DM: (i) the nature of genetic elements affording T1DM susceptibility or resistance; (ii) the mechanisms underlying the development and recruitment of pathogenic autoreactive T-cells; (iii) the identity of islet antigens that contribute to the initiation and/or progression of islet inflammation and beta-cell destruction; and (iv) the design of avenues for therapeutic intervention that are rooted in the knowledge gained from studies of animal models. Development of new animal models will ensure continued progress in these four areas.

Islet allograft survival induced by costimulation blockade in NOD mice is controlled by allelic variants of Idd3

NOD mice develop type 1 autoimmune diabetes and exhibit genetically dominant resistance to transplantation tolerance induction. These two phenotypes are genetically separable. Costimulation blockade fails to prolong skin allograft survival in (NOD x C57BL/6)F1 mice and in NOD-related strains made diabetes-resistant by congenic introduction of protective major histocompatibility complex (MHC) or non-MHC Idd region genes. Here, we tested the hypothesis that the genetic basis for the resistance of NOD mice to skin allograft tolerance also applies to islet allografts. Surprisingly, costimulation blockade induced permanent islet allograft survival in (NOD x C57BL/6)F1 mice but not in NOD mice. After costimulation blockade, islet allograft survival was prolonged in diabetes-resistant NOD.B6 Idd3 mice and shortened in diabetes-free C57BL/6 mice congenic for the NOD Idd3 variant. Islet allograft tolerance could not be induced in diabetes-resistant NOD.B10 Idd5 and NOD.B10 Idd9 mice. The data demonstrate that 1) NOD mice resist islet allograft tolerance induction; 2) unlike skin allografts, resistance to islet allograft tolerance is a genetically recessive trait; 3) an Idd3 region gene(s) is an important determinant of islet allograft tolerance induction; and 4) there may be overlap in the mechanism by which the Idd3 resistance locus improves self-tolerance and the induction of allotolerance.

Memory T cells in transplantation: generation, function, and potential role in rejection

The adaptive immune system is endowed with long-lived memory to recall previous antigen encounters and respond more effectively to them. Memory immune responses are mediated by antigen-specific memory T lymphocytes that exhibit enhanced function compared with naïve T cells that have never encountered antigen. While the generation of memory T cells specific for pathogens is beneficial in providing protective immunity, memory T cells specific for alloantigens can be deleterious to the recipient of a transplanted organ. In graft rejection, memory T cells mediate accelerated, "second-set" rejection and their presence has been associated with increased propensity for early rejection. Recent findings have demonstrated that alloreactive memory T cells can be generated via exposure to alloantigens, as well as stimuli that are cross-reactive with alloantigens, and are therefore likely present in "naïve" individuals. This review focuses on the characteristics of memory T cells which make them of special interest to the transplant community, including differential activation requirements, broad homing properties, and resistance to tolerance induction. The multiple ways in which memory T cells can contribute to early and late graft rejection are discussed, as well as potential targets for combating alloreactive memory to be considered in the future design of tolerance induction strategies.