Islet Resident Macrophages Research Articles

Tipping-point transition from transient to persistent inflammation in pancreatic islets

Type 2 diabetes (T2D) is associated with a systemic increase in the pro-inflammatory cytokine IL-1β. While transient exposure to low IL-1β concentrations improves insulin secretion and β-cell proliferation in pancreatic islets, prolonged exposure leads to impaired insulin secretion and collective β-cell death. IL-1 is secreted locally by islet-resident macrophages and β-cells; however, it is unknown if and how the two opposing modes may emerge at single islet level. We investigated the duality of IL-1β with a quantitative in silico model of the IL-1 regulatory network in pancreatic islets. We find that the network can produce either transient or persistent IL-1 responses when induced by pro-inflammatory and metabolic cues. This suggests that the duality of IL-1 may be regulated at the single islet level. We use two core feedbacks in the IL-1 regulation to explain both modes: First, a fast positive feedback in which IL-1 induces its own production through the IL-1R/IKK/NF-κB pathway. Second, a slow negative feedback where NF-κB upregulates inhibitors acting at different levels along the IL-1R/IKK/NF-κB pathway—IL-1 receptor antagonist and A20, among others. A transient response ensues when the two feedbacks are balanced. When the positive feedback dominates over the negative, islets transit into the persistent inflammation mode. Consistent with several observations, where the size of islets was implicated in its inflammatory state, we find that large islets and islets with high density of IL-1β amplifying cells are more prone to transit into persistent IL-1β mode. Our results are likely not limited to IL-1β but are general for the combined effect of multiple pro-inflammatory cytokines and chemokines. Generalizing complex regulations in terms of two feedback mechanisms of opposing nature and acting on different time scales provides a number of testable predictions. Taking islet architecture and cellular heterogeneity into consideration, further dynamic monitoring and experimental validation in actual islet samples will be crucial to verify the model predictions and enhance its utility in clinical applications.

Scavengers in islets fuel diabetic autoimmunity

Autoreactive lymphocytes that infiltrate the pancreatic islet environment and target β cells are primary drivers of type 1 diabetes. In this issue of Immunity, Srivastava etal.1 examine the role of the islet microenvironment in autoimmunity and find that the scavenging receptor CXCL16 on islet-resident macrophages uptakes oxidized low-density lipoproteins and promotes the differentiation and survival of infiltrating pathogenic CD8+ Tcells.

CXCL16-dependent scavenging of oxidized lipids by islet macrophages promotes differentiation of pathogenic CD8+ T cells in diabetic autoimmunity

The pancreatic islet microenvironment is highly oxidative, rendering β cells vulnerable to autoinflammatory insults. Here, we examined the role of islet resident macrophages in the autoimmune attack that initiates type 1 diabetes. Islet macrophages highly expressed CXCL16, a chemokine and scavenger receptor for oxidized low-density lipoproteins (OxLDLs), regardless of autoimmune predisposition. Deletion of Cxcl16 in nonobese diabetic (NOD) mice suppressed the development of autoimmune diabetes. Mechanistically, Cxcl16 deficiency impaired clearance of OxLDL by islet macrophages, leading to OxLDL accumulation in pancreatic islets and a substantial reduction in intra-islet transitory (Texint) CD8+ Tcells displaying proliferative and effector signatures. Texint cells were vulnerable to oxidative stress and diminished by ferroptosis; PD-1 blockade rescued this population and reversed diabetes resistance in NOD.Cxcl16-/- mice. Thus, OxLDL scavenging in pancreatic islets inadvertently promotes differentiation of pathogenic CD8+ Tcells, presenting a paradigm wherein tissue homeostasis processes can facilitate autoimmune pathogenesis in predisposed individuals.

Islet-resident macrophage-derived miR-155 promotes β cell decompensation via targeting PDX1

Chronic inflammation is critical for the initiation and progression of type 2 diabetes mellitus via causing both insulin resistance and pancreatic β cell dysfunction. miR-155, highly expressed in macrophages, is a master regulator of chronic inflammation. Here we show that blocking a macrophage-derived exosomal miR-155 (MDE-miR-155) mitigates the insulin resistances and glucose intolerances in high-fat-diet (HFD) feeding and type-2 diabetic db/db mice. Lentivirus-based miR-155 sponge decreases the level of miR-155 in the pancreas and improves glucose-stimulated insulin secretion (GSIS) ability of β cells, thus leading to improvements of insulin sensitivities in the liver and adipose tissues. Mechanistically, miR-155 increases its expression in HFD and db/db islets and is released as exosomes by islet-resident macrophages under metabolic stressed conditions. MDE-miR-155 enters β cells and causes defects in GSIS function and insulin biosynthesis via the miR-155-PDX1 axis. Our findings offer a treatment strategy for inflammation-associated diabetes via targeting miR-155.

Functional screening and rational design of compounds targeting GPR132 to treat diabetes.

Chronic inflammation due to islet-residing macrophages plays key roles in the development of type 2 diabetes mellitus. By systematically profiling intra-islet lipid-transmembrane receptor signalling in islet-resident macrophages, we identified endogenous 9(S)-hydroxy-10,12-octadecadienoic acid-G-protein-coupled receptor 132 (GPR132)-Gi signalling as a significant contributor to islet macrophage reprogramming and found that GPR132 deficiency in macrophages reversed metabolic disorders in mice fed a high-fat diet. The cryo-electron microscopy structures of GPR132 bound with two endogenous agonists, N-palmitoylglycine and 9(S)-hydroxy-10,12-octadecadienoic acid, enabled us to rationally design both GPR132 agonists and antagonists with high potency and selectivity through stepwise translational approaches. We ultimately identified a selective GPR132 antagonist, NOX-6-18, that modulates macrophage reprogramming within pancreatic islets, decreases weight gain and enhances glucose metabolism in mice fed a high-fat diet. Our study not only illustrates that intra-islet lipid signalling contributes to islet macrophage reprogramming but also provides a broadly applicable strategy for the identification of important G-protein-coupled receptor targets in pathophysiological processes, followed by the rational design of therapeutic leads for refractory diseases such as diabetes.

Beta cell expression of ST8Sia6 protects from autoimmune diabetes in the murine NOD model

Abstract In this study, we leveraged immunomodulatory Siglec-sialic acid interactions to protect beta-cells against autoimmune rejection in Type 1 Diabetes (T1D). Our lab previously demonstrated that the sialyltransferase ST8Sia6 generates ligands that bind the Siglec-E receptor expressed on murine islet-resident macrophages. Here, mice were engineered to express ST8Sia6 in beta cells (βST mice), backcrossed to the non-obese diabetic (NOD) mouse model of T1D, and followed for disease. 60% of female littermates spontaneously developed disease by 300 days, compared to only 6% in female βST mice. Histologic analysis of pancreas sections from non-diabetic βST and littermate mice suggested a similar degree of immune infiltration in early life, but drastically reduced insulitis only in βST mice at later timepoints. This was not due to an absence of autoreactive T cells, as an anti-PD-L1 challenge resulted in similarly rapid induction of disease in both βST and littermate mice. Our genetic model allows for temporal control of ST8Sia6 expression in βST mice using the tet-off system. Treating our mice with doxycycline at 20 weeks of age resulted in persistent protection compared to age matched littermates, while the same treatment at 8 weeks of age led to similar incidence of diabetes between βST mice and littermate controls. Flow cytometric analysis of islet infiltrating immune cells from euglycemic mice at 14 weeks of age demonstrated reductions in short-lived effector CD8 T cells and increased thymically derived Tregs in βST mice compared to littermate controls. These results highlight the therapeutic potential of sialyltransferase mediated immune regulation and present a potential path toward improved curative approaches for T1D. Supported by grants from NIH (5R21AI135858-02, 1F30DK127564)

Extracellular Vesicles Derived from Three-Dimensional-Cultured Human Umbilical Cord Blood Mesenchymal Stem Cells Prevent Inflammation and Dedifferentiation in Pancreatic Islets.

It is unclear whether extracellular vesicles (EVs) from mesenchymal stem cells (MSCs) have a direct protective effect on pancreatic islets. In addition, whether culturing MSCs in three dimensions (3D) instead of a monolayer (2D) can induce changes in the cargo of EVs that facilitate the polarization of macrophages into an M2 phenotype has not been investigated. We sought to determine whether EVs from MSCs cultured in 3D can prevent inflammation and dedifferentiation in pancreatic islets and, if so, whether the protective effect is superior to that of EVs from 2D MSCs. Human umbilical cord blood- (hUCB-) MSCs cultured in 3D were optimized according to cell density, exposure to hypoxia, and cytokine treatment based on the ability of the hUCB-MSC-derived EVs to induce the M2 polarization of macrophages. Islets isolated from human islet amyloid polypeptide (hIAPP) heterozygote transgenic mice were cultured in serum-deprived conditions with hUCB-MSC-derived EVs. EVs derived from 3D hUCB-MSCs had more abundant microRNAs involved in M2 polarization of macrophages and had an enhanced M2 polarization ability on macrophages, which was optimized when the 3D culture condition was 2.5 × 104 cells per spheroid without preconditioning with hypoxia and cytokine exposure. When islets isolated from hIAPP heterozygote transgenic mice were cultured in serum-deprived conditions with hUCB-MSC-derived EVs, the EVs derived from 3D hUCB-MSCs suppressed the expression of proinflammatory cytokines and caspase-1 in pancreatic islets and increased the proportion of M2-polarized islet-resident macrophages. They improved glucose-stimulated insulin secretion, reduced the expression of Oct4 and NGN3, and induced the expression of Pdx1 and FoxO1. The greater suppression of IL-1β, NLRP3 inflammasome, caspase-1, and Oct4 and induction of Pdx1 and FoxO1 were found in islets cultured with the EVs derived from 3D hUCB-MSCs. In conclusion, EVs derived from 3D hUCB-MSCs optimized for M2 polarization attenuated nonspecific inflammation and preserved β-cell identity of pancreatic islets.

Turnover Kinetics of Pancreatic Macrophages in Lean and Obese Diabetic Mice.

Pancreatic resident macrophages, a heterogeneous family of cells with distinct origins and phenotypes, are the main myeloid cells in exocrine and endocrine tissues. Adult exocrine F4/80hi macrophages consist of three different subsets based on the embryonic marker Tim-4 and MHC II expression. Their frequencies shift during aging and obesity with the Tim-4-MHCII+ fraction becoming the predominant subpopulation in the inter acinar stroma. Endocrine resident F4/80hi macrophages are more homogenous and represent the prevalent leukocyte fraction residing within the islets in both lean and obese mice. We used an adult fate mapping mouse model to characterize turnover kinetics within the pancreatic resident macrophages under normal homeostasis and obese diabetic conditions. We demonstrate that islet resident macrophages show unique replenishment kinetics, with embryonic macrophages being gradually replaced by bone marrow-derived monocytes with increasing age. Their replenishment was independent of the CCL2/CCR2 axis. Furthermore, we confirmed that both exocrine Tim-4+MHCIIlow and Tim-4+MHCII+ fractions are long-lived and primarily independent from bone marrow-derived monocytes. In contrast, exocrine Tim-4-MHCII+ macrophages are gradually replaced through a CCR2-dependent influx of bone marrow-derived monocytes in aging. Moreover, we show that obesity and type 2 diabetes do not affect the turnover kinetics of any macrophage subpopulation residing in the pancreas. Our study uncovers new insights on pancreatic macrophage biology in aging and obesity.

Fetuin-A secretion from β-cells leads to accumulation of macrophages in islets, aggravates inflammation and impairs insulin secretion.

Elevated fetuin-A levels, chemokines and islet-resident macrophages are crucial factors associated with obesity-mediated type 2 diabetes (T2D). Here, the aim of the study was to investigate the effect of MIN6 (a mouse insulinoma cell line)-derived fetuin-A (also known as AHSG) in macrophage polarization and decipher the effect of M1 type pro-inflammatory macrophages in commanding over insulin secretion. MIN6 and islet-derived fetuin-A induced expression of the M1 type macrophage markers Emr1 (also known as Adgre1), Cd68 and CD11c (Itgax) (∼1.8 fold) along with increased cytokine secretion. Interestingly, suppression of fetuin-A in MIN6 successfully reduced M1 markers by ∼1.5 fold. MIN6-derived fetuin-A also induced chemotaxis of macrophages in a Boyden chamber chemotaxis assay. Furthermore, high-fat feeding in mice showed elevated cytokine and fetuin-A content in serum and islets, and also migration and polarization of macrophages to the islets, while β-cells failed to meet the increased insulin demand. Moreover, in MIN6 culture, M1 macrophages sharply decreased insulin secretion by ∼2.8 fold. Altogether our results support an association of fetuin-A with islet inflammation and β-cell dysfunction, owing to its role as a key chemoattractant and macrophage polarizing factor.

M1 macrophage-derived exosomes impair beta cell insulin secretion via miR-212-5p by targeting SIRT2 and inhibiting Akt/GSK-3β/β-catenin pathway in mice.

Macrophage levels are elevated in pancreatic islets, and the resulting inflammatory response is a major contributor to beta cell failure during obesity and type 2 diabetes mellitus. Previous studies by us and others have reported that exosomes released by macrophages play important roles in mediating cell-to-cell communication, and represent a class of inflammatory factors involved in the inflammatory process associated with type 2 diabetes mellitus. However, to date, no reports have demonstrated the effect of macrophage-derived exosomes on beta cells, and little is known regarding their underlying mechanisms in beta cell injury. Thus, we aimed to study the impact of macrophage-derived exosomes on islet beta cell injury in vitro and in vivo. The phenotypic profiles of islet-resident macrophages were analysed in C57BL/6J mice fed a high-fat diet (HFD). Exosomes were collected from the medium of cultured bone marrow-derived macrophages (BMDMs) and from isolated islet-resident macrophages of HFD-fed mice (HFD-Exos). The role of exosomes secreted by inflammatory M1 phenotype BMDMs (M1-Exos) and HFD-Exos on beta cell function was assessed. An miRNA microarray and quantitative real-time PCR (qPCR) were conducted to test the level of M1-Exos-derived miR-212-5p in beta cells. Then, miR-212-5p was overexpressed or inhibited in M1-Exos or beta cells to determine its molecular and functional impact. M1-polarised macrophages were enriched in the islets of obese mice. M1 macrophages and islet-resident macrophages of HFD-fed mice impaired beta cell insulin secretion in an exosome-dependent manner. miR-212-5p was notably upregulated in M1-Exos and HFD-Exos. Enhancing the expression of miR-212-5p impaired beta cell insulin secretion. Blocking miR-212-5p elicited a significant improvement in M1-Exos-mediated beta cell insulin secretion during injury. Mechanistically, M1-Exos mediated an intercellular transfer of the miR-212-5p, targeting the sirtuin 2 gene and regulating the Akt/GSK-3β/β-catenin pathway in recipient beta cells to restrict insulin secretion. A novel exosome-modulated mechanism was delineated for macrophage-beta cell crosstalk that drove beta cell dysfunction and should be explored for its therapeutic utility.

Phagocytosis and Efferocytosis by Resident Macrophages in the Mouse Pancreas.

The tissue microenvironment in the mouse pancreas has been shown to promote very different polarizations of resident macrophages with islet-resident macrophages displaying an inflammatory “M1” profile and macrophages in the exocrine tissue mostly displaying an alternatively activated “M2” profile. The impact of this polarization on tissue homeostasis and diabetes development is unclear. In this study, the ability of pancreas-resident macrophages to phagocyte bacterial and endogenous debris was investigated. Mouse endocrine and exocrine tissues were separated, and tissue-resident macrophages were isolated by magnetic immunolabeling. Isolated macrophages were subjected to flow cytometry for polarization markers and qPCR for phagocytosis-related genes. Functional in vitro investigations included phagocytosis and efferocytosis assays using pH-sensitive fluorescent bacterial particles and dead fluorescent neutrophils, respectively. Intravital confocal imaging of in situ phagocytosis and efferocytosis in the pancreas was used to confirm findings in vivo. Gene expression analysis revealed no significant overall difference in expression of most phagocytosis-related genes in islet-resident vs. exocrine-resident macrophages included in the analysis. In this study, pancreas-resident macrophages were shown to differ in their ability to phagocyte bacterial and endogenous debris depending on their microenvironment. This difference in abilities may be one of the factors polarizing islet-resident macrophages to an inflammatory state since phagocytosis has been found to imprint macrophage heterogeneity. It remains unclear if this difference has any implications in the development of islet dysfunction or autoimmunity.

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

Abstract 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.

Islet-Resident Dendritic Cells and Macrophages in Type 1 Diabetes: In Search of Bigfoot's Print.

The classical view of type 1 diabetes assumes that the autoimmune mediated targeting of insulin producing ß-cells is caused by an error of the immune system. Malfunction and stress of beta cells added the target tissue at the center of action. The innate immune system, and in particular islet-resident cells of the myeloid lineage, could function as a link between stressed ß-cells and activation and recognition by the adaptive immune system. We survey the role of islet-resident macrophages and dendritic cells in healthy islet homeostasis and pathophysiology of T1D. Knowledge of islet-resident antigen presenting cells in rodents is substantial, but quite scarce in humans, in particular regarding dendritic cells. Differences in blood between healthy and diseased individuals were reported, but it remains elusive to what extend these contribute to T1D onset. Increasing our understanding of the interaction between ß-cells and innate immune cells may provide new insights into disease initiation and development that could ultimately point to future treatment options. Here we review current knowledge of islet-resident macrophages and dendritic cells, place these in context of current clinical trials, and guide future research.

Crosstalk between Macrophages and Pancreatic β-Cells in Islet Development, Homeostasis and Disease.

Macrophages are highly heterogeneous and plastic immune cells with peculiar characteristics dependent on their origin and microenvironment. Following pathogen infection or damage, circulating monocytes can be recruited in different tissues where they differentiate into macrophages. Stimuli present in the surrounding milieu induce the polarisation of macrophages towards a pro-inflammatory or anti-inflammatory profile, mediating inflammatory or homeostatic responses, respectively. However, macrophages can also derive from embryonic hematopoietic precursors and reside in specific tissues, actively participating in the development and the homeostasis in physiological conditions. Pancreatic islet resident macrophages are present from the prenatal stages onwards and show specific surface markers and functions. They localise in close proximity to β-cells, being exquisite sensors of their secretory ability and viability. Over the years, the crucial role of macrophages in β-cell differentiation and homeostasis has been highlighted. In addition, macrophages are emerging as central players in the initiation of autoimmune insulitis in type 1 diabetes and in the low-grade chronic inflammation characteristic of obesity and type 2 diabetes pathogenesis. The present work reviews the current knowledge in the field, with a particular focus on the mechanisms of communication between β-cells and macrophages that have been described so far.

Cutting Edge: ST8Sia6-Generated α-2,8-Disialic Acids Mitigate Hyperglycemia in Multiple Low-Dose Streptozotocin-Induced Diabetes.

The immune system contains a series of checks and balances that maintain tolerance and prevent autoimmunity. Sialic acid-binding Ig-type lectins (Siglecs) are cell surface receptors found on immune cells and inhibit inflammation by recruiting protein tyrosine phosphatases to ITIMs. Islet-resident macrophages express Siglec-E, and Siglec-E expression decreases on islet-resident macrophages as insulitis progresses in the NOD mouse. The sialyltransferase ST8Sia6 generates α-2,8-disialic acids that are ligands for Siglec-E in vivo. We hypothesized that engaging Siglec-E through ST8Sia6-generated ligands may inhibit the development of immune-mediated diabetes. Constitutive overexpression of ST8Sia6 in pancreatic β cells mitigated hyperglycemia in the multiple low-dose streptozotocin model of diabetes, demonstrating that engagement of this immune receptor facilitates tolerance in the setting of inflammation and autoimmune disease.

Critical role of macrophages in checkpoint blockade induced autoimmune diabetes in the NOD mouse

Abstract We confirmed that the PD-1/PD-L1 regulatory pathway is one control mechanism of autoimmune diabetes in NOD mice: administration of anti-PD-1 but not anti-CTLA-4 antibodies rapidly induced diabetes in young mice showing a control phase established very early in the autoimmune process. We confirmed that both CD4 and CD8 T cells and resident macrophages were essential for the PD-1 regulation to take effect. We performed an unbiased single-cell RNA sequencing analysis of islets after anti-PD-1 treatment. We identified a subpopulation of highly activated macrophage showing an IFNγ signature with high level of CXCL9 and PD-L1 and a monocyte signature distinguishable by the markers Ly6c and CCR2. Flow cytometry analysis of islets upon anti-PD-1 treatment identified a predominance of the monocyte derived macrophages compared to the islet resident macrophages. Depleting blood monocytes by clodronate liposomes inhibited the recruitment of the monocyte derived macrophages and prevented the development of acute diabetes induced by anti-PD-1 treatment. To analyze the pathogenicity of this monocyte-derived macrophages, we performed cytotoxicity assays by co-culturing macrophages stimulated with LPS&IFNγ and a β-cell line Min6: the highly activated macrophage killed β-cells in a manner dependent on IFNγ and inducible nitric oxide. In sum, our study shows highly activated macrophage derived from monocytes are cytotoxic for beta cells and promoting diabetes progression. Our findings also indicate that a consequence of checkpoint blockade is the recruitment and activation of cytocidal macrophages derived from monocytes. Targeting monocyte-derived macrophages may prevent some of the checkpoint blockade related immune-adverse effects.

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

Abstract 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.

The role of macrophages in obesity-associated islet inflammation and β-cell abnormalities.

Chronic, unresolved tissue inflammation is a well-described feature of obesity, type 2 diabetes mellitus (T2DM) and other insulin-resistant states. In this context, adipose tissue andliver inflammation have been particularly well studied; however, abundant evidence demonstrates that inflammatory processes are also activated in pancreatic islets from obese animals and humans with obesity and/or T2DM. In this Review, we focus on the characteristics ofimmune cell-mediated inflammation in islets and the consequences of this with respect to β-cell function. In contrast to type 1 diabetes mellitus, the dominant immune cell type causing inflammation in obese and T2DM islets is the macrophage. The increased macrophage accumulation in T2DM islets primarily arises through local proliferation of resident macrophages, which then provide signals (such as platelet-derived growth factor) that drive β-cell hyperplasia (a classic feature of obesity). In addition, islet macrophages also impair the insulin secretory capacity of β-cells. Through these mechanisms, islet-resident macrophages underlie the inflammatory response in obesity and mechanistically participate in the β-cell hyperplasia and dysfunction thatcharacterizes this insulin-resistant state. These findings point to the possibility of therapeutics that target islet inflammation to elicit beneficial effects on β-cell function andglycaemia.

SINGLE-CELL TRANSCRIPTOMICS OF AGING MOUSE ISLET REVEALS AGE-RELATED RECRUITMENT OF ISLET-RESIDENT MACROPHAGE

Type 2 diabetes (T2D) prevalence increases with age. The notion of inevitable progression of T2D has been challenged by reports of remission in some human T2D cases; however, this remission is dependent on islet function reserve. To elucidate the molecular mechanisms driving islet cell dysfunction, it is necessary to understand islet cell composition, diversity, and function throughout the lifespan. We generated a single-cell transcriptomic atlas of healthy islets isolated from young (5 weeks old), middle-aged (12 months old), and older-aged (25 months old) mice. Cell clustering identified 13 initial cell clusters that were further sub-clustered. This single-cell RNAseq profile showed that each cell type/group has different markers and functional characteristics and that age causes a remarkable shift in islet cell composition, diversity, and number. By comparing macrophages from young and old mice, we also found that aged islets contain a higher number of islet-resident macrophages. Overall, this single-cell islet atlas covers nearly all cells in the normal islet and allows a comprehensive exploration of all transcriptional states throughout the lifespan.

Islet macrophages are associated with islet vascular remodeling and compensatory hyperinsulinemia during diabetes.

β-Cells respond to peripheral insulin resistance by first increasing circulating insulin during diabetes. Islet remodeling supports this compensation, but its drivers remain poorly understood. Infiltrating macrophages have been implicated in late-stage type 2 diabetes, but relatively little is known on islet resident macrophages, especially during compensatory hyperinsulinemia. We hypothesized that islet resident macrophages would contribute to islet vascular remodeling and hyperinsulinemia during diabetes, the failure of which results in a rapid progression to frank diabetes. We used chemical (clodronate), genetics (CD169-diphtheria toxin receptor mice), or antibody-mediated (colony-stimulating factor 1 receptor α) macrophage ablation methods in diabetic (db/db) and diet-induced models of compensatory hyperinsulinemia to investigate the role of macrophages in islet remodeling. We transplanted islets devoid of macrophages into naïve diabetic mice and assessed the impact on islet vascularization. With the use of the above methods, we showed that macrophage depletion significantly and consistently compromised islet remodeling in terms of size, vascular density, and insulin secretion capacity. Depletion of islet macrophages reduced VEGF-A secretion in both human and mouse islets ex vivo, and this functionally translated to delayed revascularization upon transplantation in vivo. We revealed that islet resident macrophages were associated with islet remodeling and increased insulin secretion during diabetes. This suggests utility in harnessing islet macrophages during this phase to promote islet vascularization, remodeling, and insulin secretion.