In vivo depletion and genetic targeting of mouse intestinal CX3CR1(+) mononuclear phagocytes

CSF1R inhibition with PLX5622 affects multiple immune cell compartments and induces tissue-specific metabolic effects in lean mice

AIMS/HYPOTHESIS

Colony stimulating factor 1 (CSF1) promotes the proliferation, differentiation and survival of macrophages, which have been implicated in both beneficial and detrimental effects on glucose metabolism. However, the physiological role of CSF1 signalling in glucose homeostasis and the potential therapeutic implications of modulating this pathway are not known. We aimed to study the composition of tissue macrophages (and other immune cells) following CSF1 receptor (CSF1R) inhibition and elucidate the metabolic consequences of CSF1R inhibition.

METHODS

We assessed immune cell populations in various organs by flow cytometry, and tissue-specific metabolic effects by hyperinsulinaemic-euglycaemic clamps and insulin secretion assays in mice fed a chow diet containing PLX5622 (a CSF1R inhibitor) or a control diet.

RESULTS

CSF1R inhibition depleted macrophages in multiple tissues while simultaneously increasing eosinophils and group 2 innate lymphoid cells. These immunological changes were consistent across different organs and were sex independent and reversible after cessation of the PLX5622. CSF1R inhibition improved hepatic insulin sensitivity but concomitantly impaired insulin secretion. In healthy islets, we found a high frequency of IL-1β+ islet macrophages. Their depletion by CSF1R inhibition led to downregulation of macrophage-related pathways and mediators of cytokine activity, including Nlrp3, suggesting IL-1β as a candidate insulin secretagogue. Partial restoration of physiological insulin secretion was achieved by injecting recombinant IL-1β prior to glucose stimulation in mice lacking macrophages.

CONCLUSIONS/INTERPRETATION

Macrophages and macrophage-derived factors, such as IL-1β, play an important role in physiological insulin secretion. A better understanding of the tissue-specific effects of CSF1R inhibition on immune cells and glucose homeostasis is crucial for the development of targeted immune-modulatory treatments in metabolic disease.

DATA AVAILABILITY

The RNA-Seq dataset is available in the Gene Expression Omnibus (GEO) under the accession number GSE189434 ( http://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE189434 ).

The Role of Intestinal Macrophages in Gastrointestinal Homeostasis: Heterogeneity and Implications in Disease

Intestinal macrophages play a key role in the gut immune system and the regulation of gastrointestinal physiology, including gut motility and secretion. Their ability to keep the gut from chronic inflammation despite constantly facing foreign antigens has been an important focus in gastrointestinal research. However, the heterogeneity of intestinal macrophages has impeded our understanding of their specific roles. It is now becoming clear that subsets of intestinal macrophages play diverse roles in various gastrointestinal diseases. This occurs through a complex interplay between cytokine production and enteric nervous system activation that differs for each pathologic condition. Key diseases and disorders in which intestinal macrophages play a role include postoperative ileus, inflammatory bowel disease, necrotizing enterocolitis, as well as gastrointestinal disorders associated with human immunodeficiency virus and Parkinson's disease. Here, we review the identification of intestinal macrophage subsets based on their origins and functions, how specific subsets regulate gut physiology, and the potential for these heterogeneous subpopulations to contribute to disease states. Furthermore, we outline the potential for these subpopulations to provide unique targets for the development of novel therapies for these disorders.

Gut-resident CX3CR1hi macrophages induce tertiary lymphoid structures and IgA response in situ

Intestinal mononuclear phagocytes (MPs) are composed of heterogeneous dendritic cell (DC) and macrophage subsets necessary for the initiation of immune response and control of inflammation. Although MPs in the normal intestine have been extensively studied, the heterogeneity and function of inflammatory MPs remain poorly defined. We performed phenotypical, transcriptional, and functional analyses of inflammatory MPs in infectious Salmonella colitis and identified CX3CR1⁺ MPs as the most prevalent inflammatory cell type. CX3CR1⁺ MPs were further divided into three distinct populations, namely, Nos2 ⁺CX3CR1^lo, Ccr7 ⁺CX3CR1^int (lymph migratory), and Cxcl13 ⁺CX3CR1^hi (mucosa resident), all of which were transcriptionally aligned with macrophages and derived from monocytes. In follow-up experiments in vivo, intestinal CX3CR1⁺ macrophages were superior to conventional DC1 (cDC1) and cDC2 in inducing Salmonella-specific mucosal IgA. We next examined spatial organization of the immune response induced by CX3CR1⁺ macrophage subsets and identified mucosa-resident Cxcl13 ⁺CX3CR1^hi macrophages as the antigen-presenting cells responsible for recruitment and activation of CD4⁺ T and B cells to the sites of Salmonella invasion, followed by tertiary lymphoid structure formation and the local pathogen-specific IgA response. Using mice we developed with a floxed Ccr7 allele, we showed that this local IgA response developed independently of migration of the Ccr7 ⁺CX3CR1^int population to the mesenteric lymph nodes and contributed to the total mucosal IgA response to infection. The differential activity of intestinal macrophage subsets in promoting mucosal IgA responses should be considered in the development of vaccines to prevent Salmonella infection and in the design of anti-inflammatory therapies aimed at modulating macrophage function in inflammatory bowel disease.

Proliferation of Resident Macrophages Is Dispensable for Protection during Giardia duodenalis Infections

Infection with the intestinal parasite Giardia duodenalis is one of the most common causes of diarrheal disease in the world. Previous work has demonstrated that the cells and mechanisms of the adaptive immune system are critical for clearance of this parasite. However, the innate system has not been as well studied in the context of Giardia infection. We have previously demonstrated that Giardia infection leads to the accumulation of a population of CD11b⁺, F4/80⁺, ARG1⁺, and NOS2⁺ macrophages in the small intestinal lamina propria. In this report, we sought to identify the accumulation mechanism of duodenal macrophages during Giardia infection and to determine if these cells were essential to the induction of protective Giardia immunity. We show that F4/80⁺, CD11b⁺, CD11c^int, CX3CR1⁺, MHC class II⁺, Ly6C⁻, ARG1⁺, and NOS2⁺ macrophages accumulate in the small intestine during infections in mice. Consistent with this resident macrophage phenotype, macrophage accumulation does not require CCR2, and the macrophages incorporate EdU, indicating in situ proliferation rather than the recruitment of monocytes. Depletion of macrophages using anti-CSF1R did not impact parasite clearance nor development of regulatory T cell or Th17 cellular responses, suggesting that these macrophages are dispensable for protective Giardia immunity.

LAG3+ Regulatory T Cells Restrain Interleukin-23-Producing CX3CR1+ Gut-Resident Macrophages during Group 3 Innate Lymphoid Cell-Driven Colitis

Interleukin-22 (IL-22)-producing group 3 innate lymphoid cells (ILC3) maintains gut homeostasis but can also promote inflammatory bowel disease (IBD). The regulation of ILC3-dependent colitis remains to be elucidated. Here we show that Foxp3+ regulatory T cells (Treg cells) prevented ILC3-mediated colitis in an IL-10-independent manner. Treg cells inhibited IL-23 and IL-1β production from intestinal-resident CX3CR1+ macrophages but not CD103+ dendritic cells. Moreover, Treg cells restrained ILC3 production of IL-22 through suppression of CX3CR1+ macrophage production of IL-23 and IL-1β. This suppression was contact dependent and was mediated by latent activation gene-3 (LAG-3)-an immune checkpoint receptor-expressed on Treg cells. Engagement of LAG-3 on MHC class II drove profound immunosuppression of CX3CR1+ tissue-resident macrophages. Our study reveals that the health of the intestinal mucosa is maintained by an axis driven by Treg cells communication with resident macrophages that withhold inflammatory stimuli required for ILC3 function.