The role of cytotoxic macrophages in non-obese diabetic mice: cytotoxicity against murine mastocytoma and beta-cell lines

Redox-Sensitive Innate Immune Pathways During Macrophage Activation in Type 1 Diabetes

SIGNIFICANCE

Type 1 diabetes (T1D) is an autoimmune disease resulting in β-cell destruction mediated by islet-infiltrating leukocytes. The role of oxidative stress in human and murine models of T1D is highly significant as these noxious molecules contribute to diabetic complications and β-cell lysis, but their direct impact on dysregulated autoimmune responses is highly understudied. Pro-inflammatory macrophages play a vital role in the initiation and effector phases of T1D by producing free radicals and pro-inflammatory cytokines to facilitate β-cell destruction and to present antigen to autoreactive T cells. Recent Advances: Redox modulation of macrophage functions may play critical roles in autoimmunity. These include enhancing pro-inflammatory innate immune signaling pathways in response to environmental triggers, enforcing an M1 macrophage differentiation program, controlling antigen processing, and altering peptide recognition by oxidative post-translational modification. Therefore, an oxidative environment may act on multiple macrophage functions to orchestrate T1D pathogenesis.

CRITICAL ISSUES

Mechanisms involved in the initiation of T1D remain unclear, making preventive and early therapeutics difficult to develop. Although many of these advances in the redox regulation of macrophages are in their infancy, they provide insight into how oxidative stress aids in the precipitating event of autoimmune activation.

FUTURE DIRECTIONS

Future studies should be aimed at mechanistically determining which redox-regulated macrophage functions are pertinent in T1D pathogenesis, as well as at investigating potential targetable therapeutics to halt and/or dampen innate immune activation in T1D.

Hyperplastic islets observed in "reversed" NOD mice treated without hematopoietic cells

At the onset of type 1 diabetes, most of the insulin-producing pancreatic beta cells are destroyed by effector cells, and therefore, the following two factors, at a minimum, are necessary for "reversing" hyperglycemia in autoimmune diabetes; depletion of effector cells and enhancement of beta cell regeneration. In this study, we tried a novel approach for "reversing" autoimmune diabetes in a murine model. Here we show that remission could be achieved with a combination therapy of a single injection of complete Freund's adjuvant (CFA) and a single intraperitoneal injection of a pancreatic beta cell line, MIN6N-9a, in recent-onset diabetic NOD (non-obese diabetic) mice. Five out of seven mice (71%) receiving MIN6N-9a and CFA became normoglycemic within 120 days after treatment, whereas only two of nine (22%) receiving vehicle instead of MIN6N-9a achieved remission. Histological examination of pancreatic specimens from "reversed" mice showed decreased islet number, but each islet was markedly hyperplastic; being about six times larger than those from controls. Although it has been reported that hematopoietic cells such as splenocytes differentiate into insulin-producing cells and play a key role, our data indicate that they are not an absolute requirement for the "reversal" of autoimmune diabetes.

Protective role for cytosolic phospholipase A2alpha in autoimmune diabetes of mice

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) plays an important role in arachidonate pathway. To investigate the contribution of cPLA(2)alpha to autoimmune diabetes, we established non-obese diabetic (NOD) mouse, an excellent model for human type 1 diabetes, deficient in cPLA(2)alpha. These mice showed severe insulitis and a higher incidence of diabetes. In their macrophages, decreased prostaglandin E(2) (PGE(2)) induced by cPLA(2)alpha deficiency, and the increase in production of tumor necrosis factor (TNF)-alpha were observed. These results suggested that cPLA(2)alpha plays a protective role in progression of insulitis and development of autoimmune diabetes by suppression of TNF-alpha production from macrophages.

Prolongation of islet allograft survival following ex vivo transduction with adenovirus encoding a soluble type 1 TNF receptor–Ig fusion decoy

Islet transplantation is a viable long-term therapeutic alternative to daily insulin replacement for type I diabetes. The allogeneic nature of the transplants poses immunological challenges for routine clinical utility. Gene transfer of immunoregulatory molecules and those that improve insulin release kinetics provides rational approaches to facilitate allogeneic islet transplantation as a potential therapy. We have examined the efficacy of a soluble type 1 tumor necrosis factor receptor (TNFR) immunoglobulin-Fc fusion transgene (TNFR-Ig) to protect human islets from cytokine-induced apoptosis in culture, as well as in facilitating allogeneic islet transplants in diabetic mice. Cultured human islets were transduced with an adenoviral vector encoding human TNFR-Ig (Ad-TNFR-Ig). TNFR-Ig protein was secreted by cultured islets, as well as by transduced mouse islet transplants recovered from mouse recipients. Glucose-induced insulin release kinetics were comparable among untransduced, Ad-TNFR-Ig-infected human islets and vector-transduced islets exposed to cytokines. In parallel, Ad-TNFR-Ig-infected islets were protected from cytokine-induced apoptosis activation. Finally, diabetic mice transplanted with allogeneic islets expressing TNFR-Ig returned to and maintained normoglycemia significantly longer than untransduced islet recipients. These data support the potential utility of TNFR-Ig gene transfer to islets as a means of facilitating allogeneic islet transplantation.

Human LT-alpha-mediated resistance to autoimmune diabetes is induced in NOD, but not NOD-scid, mice and abrogated by IL-12

Systemic administration of human lymphotoxin-alpha (hLT-alpha) made NOD mice resistant not only to spontaneous autoimmune type 1 diabetes mellitus but also to cyclophosphamide (CY)-induced diabetes and diabetes transfer by diabetic NOD spleen cells (triple resistance). In this study we analyzed the mechanisms of hLT-alpha-induced resistance, focusing on (1) hLT-alpha-induced resistance in the pancreatic beta cell, (2) CY-resistant suppressor cells, (3) suppression of induction or function of effector cells for beta cell destruction, or (4) others. To examine the first possibility in vitro, a NOD-derived beta cell line (MIN6N) was pretreated with hLT-alpha and then mixed with diabetic NOD spleen cells and MIN6N cell viability was measured. Treatment with hLT-alpha did not protect MIN6N cells but rather enhanced cytotoxicity. Next NOD-scid mice were pretreated with hLT-alpha and then transferred with diabetic NOD spleen. All the recipients developed diabetes. These results excluded the first possibility. The second possibility was also excluded by a cotransfer experiment, in which diabetic NOD spleen cells were cotransferred to NOD-scid mice with nontreated or hLT-alpha-treated nondiabetic NOD spleens. There was no significant difference in diabetes incidence between the two groups. To observe the third possibility, spleen cells of hLT-alpha-treated triple-resistant NOD mice were transferred to NOD-scid mice. Diabetes developed in the recipients, although the onset of diabetes was slightly delayed. Finally, hLT-alpha-treated triple-resistant NOD mice developed diabetes 1 week after daily IL-12 treatment. In summary, hLT-alpha administration made NOD mice resistant to effector cells for beta cell destruction. This resistance was induced in NOD, but not in NOD-scid, mice, indicating that lymphocytes were obligatory for the resistance. However, it was not mediated by transferable suppressor cells. Because effector cells were present in hLT-alpha-treated NOD spleen and the resistance was abrogated by IL-12 treatment, it is speculated that hLT-alpha treatment may have changed a local cytokine balance protective from beta cell destruction.

Protection of human islets from the effects of interleukin-1beta by adenoviral gene transfer of an Ikappa B repressor

Interleukin-1beta (IL-1beta) is a pro-inflammatory cytokine that inhibits beta cell function and promotes Fas-triggered apoptosis. IL-1beta is thought to act early in the initiation of the autoimmune destruction of pancreatic beta cells in type I diabetes. IL-1beta promotes beta cell impairment, in part, by activating NF-kappaB transcription factor-dependent signaling pathways. We have examined whether beta cells could be protected from the effects of IL-1beta by overexpressing an inhibitor of NF-kappaB activity, IkappaB, by adenoviral gene transfer to intact human islets in culture. Infection of islets with an adenoviral vector encoding a non-phosphorylatable, non-degradable variant of IkappaBalpha resulted in normal insulin responses to glucose in the presence of IL-1beta. Furthermore, nitric oxide production was prevented and, more importantly, Fas-triggered apoptosis was inhibited following IkappaBalpha gene transfer. These results suggest that blocking the NF-kappaB pathway might prevent cytokine-induced beta cell impairment as a means of facilitating islet transplantation.

Controlling mature CD4+ T cell responses

Immune responses are by necessity highly regulated to achieve the appropriate balance of aggression and restraint. Among the many factors involved in maintaining this balance are the interactions between accessory molecule receptors expressed on T cells and their ligands on antigen-presenting cells. Our studies during the past several years have focused on defining how particular accessory molecule interactions influence the activation of naïve CD4+ T cells and the subsequent development of effector function. In this article, we discuss our findings on the effects of distinct accessory molecules with particular attention to the unique roles of LFA-1 and CD28 during different phases of the naïve CD4+ cell response.

Immunocompatibility and biocompatibility of cell delivery systems

Immunoisolation therapy overcomes important disadvantages of implanting free cells. By mechanically blocking immune attacks, synthetic membranes around grafted cells should obviate the need for immunosuppression. The membrane used for encapsulation must be biocompatible and immunocompatible to the recipient and also to the encapsulated graft. The ability of the host to accept the implanted graft depends not only on the material used for encapsulation, but also on the defense reaction of the recipient, which is very individual. Such a reaction usually starts as absorption of cell-adhesive proteins, immunoglobulins, complement components, growth factors and some other proteins on the surface of the device. The absorption of proteins is difficult to avoid, but the amount and specificity of absorbed proteins can be controlled to some extent by selection and modification of the device material. If the adsorption of proteins to the surface of the implanted material is reduced, the overgrowth of the device with fibroblast-like and macrophage-like cells is also reduced. Cell adhesion at the surface of the implanted device is, in addition to the selected polymeric material, greatly influenced by the device content. Xenografts trigger a more vigorous inflammatory reaction than allografts, most probably due to the release of antigenic products from encapsulated deteriorated and dying cells which diffuse through the membrane and activate adhering immune cells. There is an evident effect of autoimmune status on the fate of the encapsulated graft. While encapsulated xenogeneic islets readily reverse streptozotocin-induced diabetes in mice, the same xenografts are short-functioning in NOD autoimmune diabetes-prone mice. Autoantibodies, to which most devices are impermeable, are not involved. Among the cytotoxic factors which are responsible for the limited survival of the encapsulated graft the most important are cytokines and perhaps some other low-molecular-weight factors released by activated macrophages at the surface of the encapsulating membrane.

An immunohistochemical study of macrophage influx and the co-localization of inducible nitric oxide synthase in the pancreas of non-obese diabetic (NOD) mice during disease acceleration with cyclophosphamide

Cyclophosphamide has been used to accelerate and synchronize diabetes in non-obese diabetic (NOD) mice. It was injected to 70-day-old female NOD mice and its effect on the progression of insulitis studied at days 0, 4, 7, 11 and at onset of diabetes. Pancreatic sections were also examined for the influx of CD4 and CD8 T cells and macrophages following immunofluorescence staining. The kinetics of macrophage immunoreactive cells in the exocrine and intra-islet areas were also investigated. Light and confocal microscopy were-employed to examine the expression and co-localization of inducible nitric oxide synthase following dual- and triple-label immunofluorescence histochemistry. After cyclophosphamide administration, the severity of insulitis remained similar from days 0 to 4 but began to rise at day 7 and markedly by day 11 and at onset of diabetes. At these two later stages, the insulitis scores were close to 100% while in age-matched control groups the insulitis scores were considerably lower. Immunohistochemical staining showed increasing numbers of CD4 and CD8 T cell subsets and macrophages within the islets and in exocrine, sinusoidal and peri-vascular regions. At onset of diabetes, several islets contained prominent clusters of macrophage immunoreactive cells. Macrophage influx into the islets increased sharply from day 7 (mean number per islet: 119 +/- 54 SEM), peaked at day 11 (mean number per islet: 228 +/- 42), and then declined at onset of diabetes (mean number per islet: 148 +/- 49). Several cells with immunolabelling for inducible nitric oxide synthase were detectable from day 7 onwards until the onset of diabetes. Dual- and triple-label immunohistochemistry showed that a significant proportion of macrophages and only a few beta cells contained the enzyme. Macrophages positive for the enzyme were located as clusters or occasionally contiguously, in the peri-islet and intra-islet areas but rarely in the exocrine region. Islets with minimal distribution of macrophages in the peri-islet areas were not positive for inducible nitric oxide synthase. Beta cells positive for the enzyme were observed in islets with significant macrophage infiltration in locations close to macrophages. The present results show that cyclophosphamide administration to female NOD mice results in a rapid influx of CD4 and CD8 cells and macrophages. The marked up-regulation of inducible nitric oxide synthase in a selective proportion of macrophages, within the islets, immediately preceding and during the onset of diabetes suggests that nitric oxide released by islet macrophages may be an important molecular mediator of beta cell destruction in this accelerated model of insulin-dependent diabetes mellitus.

Interleukin-1-beta, tumor necrosis factor-alpha, insulin secretion and oral glucose tolerance in non-diabetic siblings of children with IDDM

Recent reports suggest that the pancreas participates in tumor necrosis factor alpha (TNF-A) production during stress, and that the islets are predominantly responsible for such synthesis. In vitro TNF-A and interleukin 1-beta (IL-1-beta) inhibit insulin release from islet beta-cells. We measured the circulating levels of IL-1-beta, TNF-A and islet cell antibody (ICA) in 30 children with IDDM (10 of them at their first presentation), 30 of their non-diabetic siblings, and 30 normal age-matched children. In the non-diabetic children we investigated the early phase of insulin release after intravenous bolus of glucose and evaluated tolerance to oral glucose (OGTT). IL-1-beta and TNF-A concentrations were significantly higher in IDDM-siblings (31.8 +/- 7.7 pg/ml and 650 +/- 155 pg/ml respectively) versus normal children (21.2 +/- 6.4 pg/ml and 383 +/- 122 pg/ml respectively). IL-1-beta and TNF-A concentrations did not differ significantly between the diabetic children and healthy age-matched controls. ICA were detected in 60% of the recently diagnosed diabetic children vs. 30% of those with longer duration of diabetes (3.1 +/- 1.2 years). Despite the significantly high prevalence of ICA in the recently diagnosed children with IDDM, their IL-1-beta and TNF-A concentrations were lower than those for the normal children. In experimental animals these cytokines can induce round cell infiltration (insulinitis) and inhibit insulin secretion by beta-cell. The presence of significantly higher concentrations of these cytokines in IDDM siblings, with high prevalence of ICA (16%), was associated with normal oral glucose tolerance and normal peak insulin response (60 +/- 10.4 mlU/ml) after i.v. glucose bolus compared to normal children (52.3 +/- 9.5 mlU/ml). However, after 2 years of follow up, one of them developed IDDM and another developed IGT but none of the normal controls developed abnormal glucose tolerance. It appears that the process of autoimmune aggression against beta-cells, and its effect on insulin release and glucose homeostasis, is a slow and chronic process. However, the production of these cytokines and consequently the degree of beta-cell destruction, in a genetically susceptible subject, might be enhanced by several factors including viral infections. In summary, IL-1-beta and TNF-A levels can be used as indicators of continuing autoimmune aggression against beta-cells before the development of extensive beta-cell destruction.

Primary nonfunction of islet grafts in autoimmune diabetic nonobese diabetic mice is prevented by treatment with interleukin-4 and interleukin-10

In isologous islet transplantation in spontaneously diabetic nonobese (NOD) mice, destruction of the islet graft is caused by recurrence of T helper (Th)1-driven insulitis[fnc,1. We established a model of transplantation in which female NOD recipients were rendered diabetic by a single injection of cyclophosphamide (250 mg/kg). Under these conditions, 500 freshly isolated islets from young NOD mice transplanted under the kidney capsule did not lead to normoglycemia within 3 day after transplantation, but underwent immediate impairment of function. This primary nonfunction was seen in > 80% of the recipients. Treatment of the recipients with the Th2-associated cytokine interleukin (IL)-4 alone did not prevent primary nonfunction, whereas treatment of the recipients with a combination of IL-4 and IL-10 restored immediate function of the grafts. Cytokine treatment did not prevent later rejection of grafts. Histological analysis of the grafts revealed less severely infiltrated islets, with well preserved islet architecture, in only normoglycemic animals treated with IL-4 or with IL-4 and IL-10. Staining for lymphocytes, macrophages, and tumor necrosis factor (TNF)-alpha did not show differences between the groups, but IFN-gamma was markedly less expressed in IL-4- and IL-10-treated grafts. Concomitantly, analysis of animals treated for 8 days after injection of cyclophosphamide, with IL-4 and IL-10, revealed a reduction of IL-12 mRNA in the pancreas. We conclude from these data that primary nonfunction of islet grafts is prevented by treatment of the recipients with a combination of IL-4 and IL-10, via downregulation of Th1 cytokines.

Acceleration of diabetes in young NOD mice with peritoneal macrophages

To elucidate the roles of macrophages in the pathogenesis of NOD murine diabetes, peritoneal macrophages from NOD mice were injected into young NOD mice. We used 12 to 20 week-old NOD mice of both sexes as donors, and sex-matched 2-week-old NOD mice as recipients. Cyclophosphamide (CY), 200 mg/kg, was intraperitoneally injected into the donors. Two weeks later, peritoneal exudate cells (PEC) were collected from the diabetic donors. Macrophage-rich fractions (MRF) were collected by adherence. Then PEC(5-8 x 10(6)) or MRF(3-7 x 10(6)) were transferred, intraperitoneally, to the recipients. Two weeks later, some of the recipients were killed in order to perform immunofluorescent analysis of splenocytes and to assess pancreatic histology. Mac 1 positive splenocytes were increased in PEC- and in MRF-injected recipient mice. Insulitis was seen in PEC- and MRF-injected mice, but not in controls. Some of the recipients were injected with CY, 200 mg/kg, intraperitoneally, at two weeks post cell transfer. Two weeks after CY injection, the animals were examined for the presence of diabetes. The incidences of diabetes were 67% in PEC-injected mice, 40% in the MRF-injected group, and 3% in the controls. These results suggest that peritoneal macrophages accelerate the disease process in NOD mice.