Network approach reveals preferential T-cell and macrophage association with α-linked β-cells in early stage of insulitis in NOD mice

One of the challenges in studying islet inflammation - insulitis - is that it is a transient phenomenon. Traditional reporting of the insulitis progression is based on cumulative, donor-averaged values of leucocyte density in the vicinity of pancreatic islets, that hinders intra- and inter-islet heterogeneity of disease progression. Here, we aimed to understand why insulitis is non-uniform, often with peri-insulitis lesions formed on one side of an islet. To achieve this, we demonstrated applicability of network theory in detangling intra-islet multi-cellular interactions during insulitis. Specifically, we asked the question "what is unique about regions of the islet which interact with immune cells first". This study utilized the non-obese diabetic mouse model of type one diabetes and examined the interplay among α-, β-, T-cells, myeloid cells, and macrophages in pancreatic islets during the progression of insulitis. Disease evolution was tracked based on T/β cell ratio in individual islets. In the early stage, we found that immune cells are preferentially interacting with α-cell-rich regions of an islet. At the islet periphery α-linked β-cells were found to be targeted significantly more compared to those without α-cell neighbors. Additionally, network analysis revealed increased T-myeloid, and T-macrophage interactions with all β-cells.

Islet Lymphocytes Maintain a Stable Regulatory Phenotype Under Homeostatic Conditions and Metabolic Stress

T cells and B cells have been identified in human and murine islets, but the phenotype and role of islet lymphocytes is unknown. Resident immune populations set the stage for responses to inflammation in the islets during homeostasis and diabetes. Thus, we sought to identify the phenotype and effector function of islet lymphocytes to better understand their role in normal islets and in islets under metabolic stress. Lymphocytes were located in the islet parenchyma, and were comprised of a mix of naïve, activated, and memory T cell and B cell subsets, with an enrichment for regulatory B cell subsets. Use of a Nur77 reporter indicated that CD8 T cells and B cells both received local antigen stimulus, indicating that they responded to antigens present in the islets. Analysis of effector function showed that islet T cells and B cells produced the regulatory cytokine IL-10. The regulatory phenotype of islet T cells and B cells and their response to local antigenic stimuli remained stable under conditions of metabolic stress in the diet induced obesity (DIO) model. T cells present in human islets retained a similar activated and memory phenotype in non-diabetic and T2D donors. Under steady-state conditions, islet T cells and B cells have a regulatory phenotype, and thus may play a protective role in maintaining tissue homeostasis.

MERTK on mononuclear phagocytes regulates T cell antigen recognition at autoimmune and tumor sites

Understanding mechanisms of immune regulation is key to developing immunotherapies for autoimmunity and cancer. We examined the role of mononuclear phagocytes during peripheral T cell regulation in type 1 diabetes and melanoma. MERTK expression and activity in mononuclear phagocytes in the pancreatic islets promoted islet T cell regulation, resulting in reduced sensitivity of T cell scanning for cognate antigen in prediabetic islets. MERTK-dependent regulation led to reduced T cell activation and effector function at the disease site in islets and prevented rapid progression of type 1 diabetes. In human islets, MERTK-expressing cells were increased in remaining insulin-containing islets of type 1 diabetic patients, suggesting that MERTK protects islets from autoimmune destruction. MERTK also regulated T cell arrest in melanoma tumors. These data indicate that MERTK signaling in mononuclear phagocytes drives T cell regulation at inflammatory disease sites in peripheral tissues through a mechanism that reduces the sensitivity of scanning for antigen leading to reduced responsiveness to antigen.

Mononuclear phagocytes drive Mertk-dependent T cell regulation at autoimmune and tumor sites

Understanding mechanisms of immune regulation is key to developing effective immunotherapies for autoimmunity and cancer. We examined the role of mononuclear phagocytes (MNPs) in regulating effector T cells in type 1 diabetes and melanoma. MNPs in the islets impaired T cell responsiveness to antigen by preventing T cell arrest. MNPs in the autoimmune lesion express the TAM family receptor tyrosine kinase Mertk which functions in efferocytosis. Inhibition or deficiency of Mertk led to a release from T cell regulation characterized by enhanced T cell arrest in pre-diabetic islets and at the tumor site. T cell arrest was accompanied by increased T cell-antigen presenting cell interactions in the islets. Antigen experience and effector function by T cells was increased with Mertk inhibition in the islets, but not in lymph nodes. Single cell RNA-seq analysis of islet MNPs identified multiple myeloid subsets. Inhibition of Mertk induced the most significant changes in gene expression among the islet resident macrophages including upregulation of inflammatory cytokine and adhesion genes. In the NOD mouse model, inhibition of Mertk-dependent T cell regulation culminated in the rapid acceleration of autoimmune pathology and disease. In human islets, the number of Mertk-expressing cells were increased in remaining insulin-containing islets from type 1 diabetic patients, suggesting that they might have a protective role in human disease. These data indicate that Mertk signaling in MNPs drives T cell regulation that functions specifically at the disease site in peripheral tissues through a mechanism that prevents T cell arrest and hinders response to antigen.

Lymphocytes in autoimmune-resistant mice may protect the islets through IL-10 production

We have identified T cells and B cells in the islets of autoimmune resistant mice, yet the role of these lymphocytes is unknown. In the islets of autoimmune resistant C57BL/6 mice, local antigen receptor signaling was assessed using Nur77-GFP mice. Nur77-GFP expression was increased in islet CD8+ T cells and B cells suggesting that these lymphocyte subsets receive local antigenic stimulus. Islet T cell and B cell populations expressed the regulatory cytokine IL-10, assessed using the B6.TIGER reporter mouse. T cell populations lacked expression of the pro-inflammatory cytokine IFN-γ. These findings indicate that islet T cell populations are likely playing a regulatory role. In addition, cultured whole islets produced IL-10 protein, but not IFN-γ protein. To test the stability of the islet lymphocyte populations under inflammatory conditions, mice were exposed to metabolic stress through a diet-induced obesity (DIO) model. Obesity and type 2 diabetes (T2D) are associated with circulating inflammatory cytokines. In T2D patients, islet lymphocytes have been identified, though the function of these populations is largely unknown. C57BL/6 mice were fed a high fat diet (DIO) or a matched control diet (Control) starting at 5 weeks of age, and then analyzed at 16–25 weeks of age. The number and phenotype of lymphocytes in the islets were similar in DIO and control mice. Notably, under the conditions of obesity-associated systemic inflammation, islet lymphocytes in the DIO mice maintained IL-10 production. From these data, we propose that islet lymphocytes play a regulatory role to protect the islets from systemic inflammation through IL-10 production.

Mononuclear phagocytes drive Mertk-dependent T cell regulation at autoimmune and tumor sites

Understanding mechanisms of immune regulation is key to developing effective immunotherapies for autoimmunity and cancer. We examined the role of mononuclear phagocytes (MNPs) in regulating effector T cells in type 1 diabetes and melanoma. MNPs in the islets impaired T cell responsiveness to antigen by preventing antigen-mediated T cell arrest. MNPs in the autoimmune lesion express the TAM family receptor tyrosine kinase Mertk which functions in efferocytosis. Inhibition or deficiency of Mertk led to a release from T cell regulation characterized by enhanced T cell arrest in pre-diabetic islets and at the tumor site. T cell arrest was accompanied by increased T cell-antigen presenting cell interactions as well as increased antigen experience and effector function by T cells in the islets. Single cell RNA-seq analysis of islet MNPs identified multiple myeloid subsets. Inhibition of Mertk induced the most significant changes in gene expression among the islet resident macrophages including upregulation of inflammatory cytokine and adhesion genes. In the NOD mouse model, inhibition of Mertk-dependent T cell regulation culminated in the rapid acceleration of autoimmune pathology and disease. In human islets, the number of Mertk-expressing cells were increased in remaining insulin-containing islets from type 1 diabetic patients, suggesting that they might have a protective role in human disease. These data indicate that Mertk signaling in MNPs drives T cell regulation that functions specifically at the disease site in peripheral tissues through a mechanism that prevents T cell arrest and response to antigen.

CD11c+ Cells Are Gatekeepers for Lymphocyte Trafficking to Infiltrated Islets During Type 1 Diabetes

Type 1 diabetes (T1D) is a T cell mediated autoimmune disease that affects more than 19 million people with incidence increasing rapidly worldwide. For T cells to effectively drive T1D, they must first traffic to the islets and extravasate through the islet vasculature. Understanding the cues that lead to T cell entry into inflamed islets is important because diagnosed T1D patients already have established immune infiltration of their islets. Here we show that CD11c⁺ cells are a key mediator of T cell trafficking to infiltrated islets in non-obese diabetic (NOD) mice. Using intravital 2-photon islet imaging we show that T cell extravasation into the islets is an extended process, with T cells arresting in the islet vasculature in close proximity to perivascular CD11c⁺ cells. Antigen is not required for T cell trafficking to infiltrated islets, but T cell chemokine receptor signaling is necessary. Using RNAseq, we show that islet CD11c⁺ cells express over 20 different chemokines that bind chemokine receptors expressed on islet T cells. One highly expressed chemokine-receptor pair is CXCL16-CXCR6. However, NOD. CXCR6^-/- mice progressed normally to T1D and CXCR6 deficient T cells trafficked normally to the islets. Even with CXCR3 and CXCR6 dual deficiency, T cells trafficked to infiltrated islets. These data reinforce that chemokine receptor signaling is highly redundant for T cell trafficking to inflamed islets. Importantly, depletion of CD11c⁺ cells strongly inhibited T cell trafficking to infiltrated islets of NOD mice. We suggest that targeted depletion of CD11c⁺ cells associated with the islet vasculature may yield a therapeutic target to inhibit T cell trafficking to inflamed islets to prevent progression of T1D.