Tissue Homeostasis and Inflammation.

There is a growing interest in understanding tissue organization, homeostasis, and inflammation. However, despite an abundance of data, the organizing principles of tissue biology remain poorly defined. Here, we present a perspective on tissue organization based on the relationships between cell types and the functions that they perform. We provide a formal definition of tissue homeostasis as a collection of circuits that regulate specific variables within the tissue environment, and we describe how the functional organization of tissues allows for the maintenance of both tissue and systemic homeostasis. This leads to a natural definition of inflammation as a response to deviations from homeostasis that cannot be reversed by homeostatic mechanisms alone. We describe how inflammatory signals act on the same cellular functions involved in normal tissue organization and homeostasis in order to coordinate emergency responses to perturbations and ultimately return the system to a homeostatic state. Finally, we consider the hierarchy of homeostatic and inflammatory circuits and the implications for the development of inflammatory diseases.

Adipose-tissue Treg cells restrain differentiation of stromal adipocyte precursors to promote insulin sensitivity and metabolic homeostasis.

Regulatory T (Treg) cells in epidydimal visceral adipose tissue (eVAT) of lean mice and humans regulate metabolic homeostasis. We found that constitutive or punctual depletion of eVAT-Treg cells reined in the differentiation of stromal adipocyte precursors. Co-culture of these precursors with conditional medium from eVAT-Treg cells limited their differentiation in vitro, suggesting a direct effect. Transcriptional comparison of adipocyte precursors, matured in the presence or absence of the eVAT-Treg-conditioned medium, identified the oncostatin-M (OSM) signaling pathway as a key distinction. Addition of OSM to in vitro cultures blocked the differentiation of adipocyte precursors, while co-addition of anti-OSM antibodies reversed the ability of the eVAT-Treg-conditioned medium to inhibit in vitro adipogenesis. Genetic depletion of OSM (specifically in Treg) cells or of the OSM receptor (specifically on stromal cells) strongly impaired insulin sensitivity and related metabolic indices. Thus, Treg-cell-mediated control of local progenitor cells maintains adipose tissue and metabolic homeostasis, a regulatory axis seemingly conserved in humans.

Circuit Design Features of a Stable Two-Cell System.

Cell communication within tissues is mediated by multiple paracrine signals including growth factors, which control cell survival and proliferation. Cells and the growth factors they produce and receive constitute a circuit with specific properties that ensure homeostasis. Here, we used computational and experimental approaches to characterize the features of cell circuits based on growth factor exchange between macrophages and fibroblasts, two cell types found in most mammalian tissues. We found that the macrophage-fibroblast cell circuit is stable and robust to perturbations. Analytical screening of all possible two-cell circuit topologies revealed the circuit features sufficient for stability, including environmental constraint and negative-feedback regulation. Moreover, we found that cell-cell contact is essential for the stability of the macrophage-fibroblast circuit. These findings illustrate principles of cell circuit design and provide a quantitative perspective on cell interactions.

Tumor-associated macrophages expressing the transcription factor IRF8 promote T cell exhaustion in cancer.

Tumors are populated by antigen-presenting cells (APCs) including macrophage subsets with distinct origins and functions. Here, we examined how cancer impacts mononuclear phagocytic APCs in a murine model of breast cancer. Tumors induced the expansion of monocyte-derived tumor-associated macrophages (TAMs) and the activation of type 1 dendritic cells (DC1s), both of which expressed and required the transcription factor interferon regulatory factor-8 (IRF8). Although DC1s mediated cytotoxic T lymphocyte (CTL) priming in tumor-draining lymph nodes, TAMs promoted CTL exhaustion in the tumor, and IRF8 was required for TAMs' ability to present cancer cell antigens. TAM-specific IRF8 deletion prevented exhaustion of cancer-cell-reactive CTLs and suppressed tumor growth. Tumors from patients with immune-infiltrated renal cell carcinoma had abundant TAMs that expressed IRF8 and were enriched for an IRF8 gene expression signature. Furthermore, the TAM-IRF8 signature co-segregated with CTL exhaustion signatures across multiple cancer types. Thus, CTL exhaustion is promoted by TAMs via IRF8.

Cancer Immunosurveillance by Tissue-Resident Innate Lymphoid Cells and Innate-like T Cells.

Malignancy can be suppressed by the immune system in a process termed immunosurveillance. However, to what extent immunosurveillance occurs in spontaneous cancers and the composition of participating cell types remains obscure. Here, we show that cell transformation triggers a tissue-resident lymphocyte response in oncogene-induced murine cancer models. Non-circulating cytotoxic lymphocytes, derived from innate, T cell receptor (TCR)αß, and TCRγδ lineages, expand in early tumors. Characterized by high expression of NK1.1, CD49a, and CD103, these cells share a gene-expression signature distinct from those of conventional NK cells, T cells, and invariant NKT cells. Generation of these lymphocytes is dependent on the cytokine IL-15, but not the transcription factor Nfil3 that is required for the differentiation of tumor-infiltrating NK cells, and IL-15 deficiency, but not Nfil3 deficiency, results in accelerated tumor growth. These findings reveal a tumor-elicited immunosurveillance mechanism that engages unconventional type-1-like innate lymphoid cells and type 1 innate-like T cells.

Macrophage phenotypes and functions: resolving inflammation and restoring homeostasis.

Inflammation must be tightly regulated to both defend against pathogens and restore tissue homeostasis. The resolution of inflammatory responses is a dynamic process orchestrated by cells of the immune system. Macrophages, tissue-resident innate immune cells, are key players in modulating inflammation. Here, we review recent work highlighting the importance of macrophages in tissue resolution and the return to homeostasis. We propose that enhancing macrophage pro-resolution functions represents a novel and widely applicable therapeutic strategy to dampen inflammation, promote repair, and restore tissue integrity and function.

Microenvironmental sensing by fibroblasts controls macrophage population size.

Animal tissues comprise diverse cell types. However, the mechanisms controlling the number of each cell type within tissue compartments remain poorly understood. Here, we report that different cell types utilize distinct strategies to control population numbers. Proliferation of fibroblasts, stromal cells important for tissue integrity, is limited by space availability. In contrast, proliferation of macrophages, innate immune cells involved in defense, repair, and homeostasis, is constrained by growth factor availability. Examination of density-dependent gene expression in fibroblasts revealed that Hippo and TGF-ß target genes are both regulated by cell density. We found YAP1, the transcriptional coactivator of the Hippo signaling pathway, directly regulates expression of Csf1, the lineage-specific growth factor for macrophages, through an enhancer of Csf1 that is specifically active in fibroblasts. Activation of YAP1 in fibroblasts elevates Csf1 expression and is sufficient to increase the number of macrophages at steady state. Our data also suggest that expression programs in fibroblasts that change with density may result from sensing of mechanical force through actin-dependent mechanisms. Altogether, we demonstrate that two different modes of population control are connected and coordinated to regulate cell numbers of distinct cell types. Sensing of the tissue environment may serve as a general strategy to control tissue composition.

Fibroblasts and macrophages: Collaborators in tissue homeostasis.

Fibroblasts and macrophages are universal cell types across all mammalian tissues. These cells differ in many ways including their cellular origins; dynamics of renewal, recruitment, and motility within tissues; roles in tissue structure and secretion of signaling molecules; and contributions to the activation and progression of immune responses. However, many of the features that make these two cell types unique are not opposing, but instead complementary. This review will present cell-cell communication in this context and discuss how complementarity makes fibroblasts and macrophages highly compatible partners in the maintenance of tissue homeostasis.

Endocytosis as a stabilizing mechanism for tissue homeostasis.

Cells in tissues communicate by secreted growth factors (GF) and other signals. An important function of cell circuits is tissue homeostasis: maintaining proper balance between the amounts of different cell types. Homeostasis requires negative feedback on the GFs, to avoid a runaway situation in which cells stimulate each other and grow without control. Feedback can be obtained in at least two ways: endocytosis in which a cell removes its cognate GF by internalization and cross-inhibition in which a GF down-regulates the production of another GF. Here we ask whether there are design principles for cell circuits to achieve tissue homeostasis. We develop an analytically solvable framework for circuits with multiple cell types and find that feedback by endocytosis is far more robust to parameter variation and has faster responses than cross-inhibition. Endocytosis, which is found ubiquitously across tissues, can even provide homeostasis to three and four communicating cell types. These design principles form a conceptual basis for how tissues maintain a healthy balance of cell types and how balance may be disrupted in diseases such as degeneration and fibrosis.

T cell surveillance of oncogene-induced prostate cancer is impeded by T cell-derived TGF-ß1 cytokine.

Tolerance induction in T cells takes place in most tumors and is thought to account for tumor evasion from immune eradication. Production of the cytokine TGF-ß is implicated in immunosuppression, but the cellular mechanism by which TGF-ß induces T cell dysfunction remains unclear. With a transgenic model of prostate cancer, we showed that tumor development was not suppressed by the adaptive immune system, which was associated with heightened TGF-ß signaling in T cells from the tumor-draining lymph nodes. Blockade of TGF-ß signaling in T cells enhanced tumor antigen-specific T cell responses and inhibited tumor development. Surprisingly, T cell- but not Treg cell-specific ablation of TGF-ß1 was sufficient to augment T cell cytotoxic activity and blocked tumor growth and metastases. These findings reveal that T cell production of TGF-ß1 is an essential requirement for tumors to evade immunosurveillance independent of TGF-ß produced by tumors.

A specialized population of monocyte-derived tracheal macrophages promote airway epithelial regeneration through a CCR2-dependent mechanism.

Macrophages are critical for maintenance and repair of mucosal tissues. While functionally distinct subtypes of macrophage are known to have important roles in injury response and repair in the lungs, little is known about macrophages in the proximal conducting airways. Single-cell RNA sequencing and flow cytometry demonstrated murine tracheal macrophages are largely monocyte-derived and are phenotypically distinct from lung macrophages at homeostasis. Following sterile airway injury, monocyte-derived macrophages are recruited to the trachea and activate a pro-regenerative phenotype associated with wound healing. Animals lacking the chemokine receptor CCR2 have reduced numbers of circulating monocytes and tracheal macrophages, deficient pro-regenerative macrophage activation and defective epithelial repair. Together, these studies indicate that recruitment and activation of monocyte-derived tracheal macrophages is CCR2-dependent and is required for normal airway epithelial regeneration.

Airway injury induces alveolar epithelial and mesenchymal responses mediated by macrophages.

Acute injury in the airways or the lung activates local progenitors and stimulates changes in cell-cell interactions to restore homeostasis, but it is not appreciated how more distant niches are impacted. We utilized mouse models of airway-specific epithelial injury to examine secondary tissue-wide alveolar, immune, and mesenchymal responses. Single-cell transcriptomics and in vivo validation revealed transient, tissue-wide proliferation of alveolar type 2 (AT2) progenitor cells after club cell-specific ablation. The AT2 cell proliferative response was reliant on alveolar macrophages (AMs) via upregulation of Spp1 which encodes the secreted factor Osteopontin. A previously uncharacterized mesenchymal population we termed Mesenchymal Airway/Adventitial Niche Cell 2 (MANC2) also exhibited dynamic changes in abundance and a pro-fibrotic transcriptional signature after club cell ablation in an AM-dependent manner. Overall, these results demonstrate that acute airway damage can trigger distal lung responses including altered cell-cell interactions that may contribute to potential vulnerabilities for further dysregulation and disease.

Macrophages control pathological interferon responses during viral respiratory infection.

Antiviral immune mediators, including interferons and their downstream effectors, are critical for host defense yet can become detrimental when uncontrolled. Here, we identify a macrophage-mediated anti-inflammatory mechanism that limits type I interferon (IFN-I) responses. Specifically, we found that cellular stress and pathogen recognition induce Oncostatin M (OSM) production by macrophages. OSM-deficient mice succumbed to challenge with influenza or a viral mimic due to heightened IFN-I activation. Macrophage-derived OSM restricted excessive IFN-I production by lung epithelial cells following viral stimulation. Furthermore, reconstitution of OSM in the respiratory tract was sufficient to protect mice lacking macrophage-derived OSM against morbidity, indicating the importance of local OSM production. This work reveals a host strategy to dampen inflammation in the lung through the negative regulation of IFN-I by macrophages.

Untangling iNKT Cell Function in Adipose Tissue Homeostasis.

iNKT cells are important regulatory cells in metabolic health and disease. In this issue of Cell Metabolism, LaMarche et al. (2020) clarify the origin and heterogeneity of these unconventional T cells and identify the specific signals in adipose tissues that direct their function.

Principles of Cell Circuits for Tissue Repair and Fibrosis.

Tissue repair is a protective response after injury, but repetitive or prolonged injury can lead to fibrosis, a pathological state of excessive scarring. To pinpoint the dynamic mechanisms underlying fibrosis, it is important to understand the principles of the cell circuits that carry out tissue repair. In this study, we establish a cell-circuit framework for the myofibroblast-macrophage circuit in wound healing, including the accumulation of scar-forming extracellular matrix. We find that fibrosis results from multistability between three outcomes, which we term "hot fibrosis" characterized by many macrophages, "cold fibrosis" lacking macrophages, and normal wound healing. This framework clarifies several unexplained phenomena including the paradoxical effect of macrophage depletion, the limited time-window in which removing inflammation leads to healing, and why scar maturation takes months. We define key parameters that control the transition from healing to fibrosis, which may serve as potential targets for therapeutic reduction of fibrosis.

Desynchronization of the molecular clock contributes to the heterogeneity of the inflammatory response.

Heterogeneity in the behavior of genetically and developmentally equivalent cells is becoming increasingly appreciated. There are several sources of cellular heterogeneity, including both intrinsic and extrinsic noise. We found that some aspects of heterogeneity in the response of macrophages to bacterial lipopolysaccharide (LPS) were due to intercellular desynchronization of the molecular clock, a cell-intrinsic oscillator. We found that the ratio of the relative expression of two clock genes, Nfil3 and Dbp, expressed in opposite phases of the clock, determined the fraction of cells that produced the cytokine IL-12p40 in response to LPS. The clock can be entrained by various environmental stimuli, making it a mechanism by which population-level heterogeneity and the inflammatory response can be regulated.

Ontogeny of Tumor-associated Macrophages and Its Implication in Cancer Regulation.

Macrophages are innate immune cells with evolutionarily conserved functions in tissue maintenance and host defense. As such, macrophages are among the first hematopoietic cells that seed developing tissues, and respond to inflammatory insults by in situ proliferation or de novo differentiation from monocytes. Recent studies have revealed that monocyte-derived tumor-induced macrophages represent a major tumor-associated macrophage population, which can further expand following their differentiation in tumors. Compared to tissue-resident tumor-associated macrophages, these newly differentiated cells are phenotypically distinct, and likely play a unique role in tissue dysregulation and immune modulation in cancer. These findings imply that tumor growth elicits a specific innate immune response. In this review, we explore the different routes of macrophage seeding and maintenance in tissues during steady state and inflammation and how these principles underlie the responses observed during tumor development. In addition, we highlight the relationship between the origin and function of macrophages in different settings and how this knowledge may be used to create new opportunities for cancer immunotherapy.

Determining Leukocyte Origins Using Parabiosis in the PyMT Breast Tumor Model.

Tumors develop in a complex microenvironment alongside numerous cell types that impact their survival. Immune cells make up a large proportion of these accessory cells and many are known to promote tumor progression. Macrophages, in particular, are associated with poor patient prognosis and are therefore potential candidates for therapeutic targeting in cancer. However, to develop successful strategies to target macrophages, it is important to clarify whether these cells are derived from blood-borne precursors or a tissue-resident population. Parabiosis, or the surgical connection of two mice resulting in a shared blood circulation, allows the distinction between these two cellular sources. Here, we describe the use of parabiosis to define cell ontogeny in a mouse model of breast cancer.

The ontogeny of tumor-associated macrophages: a new understanding of cancer-elicited inflammation.

Clinical and experimental models have identified macrophages as potential targets for cancer therapy, however, the nature of macrophage differentiation and function in the context of malignant disease remain largely uncharacterized. This commentary provides the author's perspective on the recently published article "The cellular and molecular origin of tumor-associated macrophages," which demonstrated that tumor growth elicits a specific macrophage differentiation pathway.

The cellular and molecular origin of tumor-associated macrophages.

Long recognized as an evolutionarily ancient cell type involved in tissue homeostasis and immune defense against pathogens, macrophages are being rediscovered as regulators of several diseases, including cancer. Here we show that in mice, mammary tumor growth induces the accumulation of tumor-associated macrophages (TAMs) that are phenotypically and functionally distinct from mammary tissue macrophages (MTMs). TAMs express the adhesion molecule Vcam1 and proliferate upon their differentiation from inflammatory monocytes, but do not exhibit an "alternatively activated" phenotype. TAM terminal differentiation depends on the transcriptional regulator of Notch signaling, RBPJ; and TAM, but not MTM, depletion restores tumor-infiltrating cytotoxic T cell responses and suppresses tumor growth. These findings reveal the ontogeny of TAMs and a discrete tumor-elicited inflammatory response, which may provide new opportunities for cancer immunotherapy.