Chen Varol

Intestinal Lamina Propria Dendritic Cell Subsets Have Different Origin and Functions

The intestinal immune system discriminates between tolerance toward the commensal microflora and robust responses to pathogens. Maintenance of this critical balance is attributed to mucosal dendritic cells (DCs) residing in organized lymphoid tissue and dispersed in the subepithelial lamina propria. In situ parameters of lamina propria DCs (lpDCs) remain poorly understood. Here, we combined conditional cell ablation and precursor-mediated in vivo reconstitution to establish that lpDC subsets have distinct origins and functions. CD103(+) CX(3)CR1(-) lpDCs arose from macrophage-DC precursors (MDPs) via DC-committed intermediates (pre-cDCs) through a Flt3L growth-factor-mediated pathway. CD11b(+) CD14(+) CX(3)CR1(+) lpDCs were derived from grafted Ly6C(hi) but not Ly6C(lo) monocytes under the control of GM-CSF. Mice reconstituted exclusively with CX(3)CR1(+) lpDCs when challenged in an innate colitis model developed severe intestinal inflammation that was driven by graft-derived TNF-alpha-secreting CX(3)CR1(+) lpDCs. Our results highlight the critical importance of the lpDC subset balance for robust gut homeostasis.

Infiltrating Blood-Derived Macrophages Are Vital Cells Playing an Anti-inflammatory Role in Recovery from Spinal Cord Injury in Mice

Using a mouse model of spinal injury, Michal Schwartz and colleagues tested the effect of macrophages on the recovery process and demonstrate an important anti-inflammatory role for a subset of infiltrating monocyte-derived macrophages that is dependent upon their expression of interleukin 10.

Distinct differentiation potential of blood monocyte subsets in the lung.

Peripheral blood monocytes are a population of circulating mononuclear phagocytes that harbor potential to differentiate into macrophages and dendritic cells. As in humans, monocytes in the mouse comprise two phenotypically distinct subsets that are Gr1highCX3CR1int and Gr1lowCX3CR1high, respectively. The question remains whether these populations contribute differentially to the generation of peripheral mononuclear phagocytes. In this study, we track the fate of adoptively transferred, fractionated monocyte subsets in the lung of recipient mice. We show that under inflammatory and noninflammatory conditions, both monocyte subsets give rise to pulmonary dendritic cells. In contrast, under the conditions studied, only Gr1lowCX3CR1high monocytes, but not Gr1highCX3CR1int cells, had the potential to differentiate into lung macrophages. However, Gr1highCX3CR1int monocytes could acquire this potential upon conversion into Gr1lowCX3CR1high cells. Our results therefore indicate an intrinsic dichotomy in the differentiation potential of the two main blood monocyte subsets.

Macrophage-Restricted Interleukin-10 Receptor Deficiency, but Not IL-10 Deficiency, Causes Severe Spontaneous Colitis.

Interleukin-10 (IL-10) is a pleiotropic anti-inflammatory cytokine produced and sensed by most hematopoietic cells. Genome-wide association studies and experimental animal models point at a central role of the IL-10 axis in inflammatory bowel diseases. Here we investigated the importance of intestinal macrophage production of IL-10 and their IL-10 exposure, as well as the existence of an IL-10-based autocrine regulatory loop in the gut. Specifically, we generated mice harboring IL-10 or IL-10 receptor (IL-10Rα) mutations in intestinal lamina propria-resident chemokine receptor CX3CR1-expressing macrophages. We found macrophage-derived IL-10 dispensable for gut homeostasis and maintenance of colonic T regulatory cells. In contrast, loss of IL-10 receptor expression impaired the critical conditioning of these monocyte-derived macrophages and resulted in spontaneous development of severe colitis. Collectively, our results highlight IL-10 as a critical homeostatic macrophage-conditioning agent in the colon and define intestinal CX3CR1(hi) macrophages as a decisive factor that determines gut health or inflammation.

Macrophages: Development and Tissue Specialization

Macrophages are myeloid immune cells that are strategically positioned throughout the body tissues, where they ingest and degrade dead cells, debris, and foreign material and orchestrate inflammatory processes. Here we review two major recent paradigm shifts in our understanding of tissue macrophage biology. The first is the realization that most tissue-resident macrophages are established prenatally and maintained through adulthood by longevity and self-renewal. Their generation and maintenance are thus independent from ongoing hematopoiesis, although the cells can be complemented by adult monocyte-derived macrophages. Second, aside from being immune sentinels, tissue macrophages form integral components of their host tissue. This entails their specialization in response to local environmental cues to contribute to the development and specific function of their tissue of residence. Factors that govern tissue macrophage specialization are emerging. Moreover, tissue specialization is reflected in discrete gene expression profiles of macrophages, as well as epigenetic signatures reporting actual and potential enhancer usage.

Securing the immune tightrope: mononuclear phagocytes in the intestinal lamina propria

The intestinal landscape comprises the hosts own tissue and immune cells, as well as a diverse intestinal microbiota. Intricate regulatory mechanisms have evolved to maintain peaceful coexistence at this site, the breakdown of which can result in devastating inflammatory bowel diseases (IBDs). Mononuclear phagocytes promote both innate and adaptive immune responses in the gut and, as such, are essential for the maintenance of intestinal homeostasis. Here, we review the origins and functions of the mononuclear phagocytes found in the intestinal lamina propria, highlighting the problems that have arisen from their classification. Understanding these cells in their physiological context will be important for developing new therapies for IBDs.

Infiltrating Monocyte-Derived Macrophages and Resident Kupffer Cells Display Different Ontogeny and Functions in Acute Liver Injury

The liver has a remarkable capacity to regenerate after injury; yet, the role of macrophages (MF) in this process remains controversial mainly due to difficulties in distinguishing between different MF subsets. In this study, we used a murine model of acute liver injury induced by overdose of N-acetyl-p-aminophenol (APAP) and defined three distinct MF subsets that populate the liver following injury. Accordingly, resident Kupffer cells (KC) were significantly reduced upon APAP challenge and started recovering by self-renewal at resolution phase without contribution of circulating Ly6Chi monocytes. The latter were recruited in a CCR2- and M-CSF–mediated pathway at the necroinflammatory phase and differentiated into ephemeral Ly6Clo MF subset at resolution phase. Moreover, their inducible ablation resulted in impaired recovery. Microarray-based molecular profiling uncovered high similarity between steady-state KC and those recovered at the resolution phase. In contrast, KC and monocyte-derived MF displayed distinct prorestorative genetic signature at the resolution phase. Finally, we show that infiltrating monocytes acquire a prorestorative polarization manifested by unique expression of proangiogenesis mediators and genes involved with inhibition of neutrophil activity and recruitment and promotion of their clearance. Collectively, our results present a novel phenotypic, ontogenic, and molecular definition of liver-MF compartment following acute injury.

Development and characterization of high affinity leptins and leptin antagonists.

Leptin is a pleiotropic hormone acting both centrally and peripherally. It participates in a variety of biological processes, including energy metabolism, reproduction, and modulation of the immune response. So far, structural elements affecting leptin binding to its receptor remain unknown. We employed random mutagenesis of leptin, followed by selection of high affinity mutants by yeast surface display and discovered that replacing residue Asp-23 with a non-negatively charged amino acid leads to dramatically enhanced affinity of leptin for its soluble receptor. Rational mutagenesis of Asp-23 revealed the D23L substitution to be most effective. Coupling the Asp-23 mutation with alanine mutagenesis of three amino acids (L39A/D40A/F41A) previously reported to convert leptin into antagonist resulted in potent antagonistic activity. These novel superactive mouse and human leptin antagonists (D23L/L39A/D40A/F41A), termed SMLA and SHLA, respectively, exhibited over 60-fold increased binding to leptin receptor and 14-fold higher antagonistic activity in vitro relative to the L39A/D40A/F41A mutants. To prolong and enhance in vivo activity, SMLA and SHLA were monopegylated mainly at the N terminus. Administration of the pegylated SMLA to mice resulted in a remarkably rapid, significant, and reversible 27-fold more potent increase in body weight (as compared with pegylated mouse leptin antagonist), because of increased food consumption. Thus, recognition and mutagenesis of Asp-23 enabled construction of novel compounds that induce potent and reversible central and peripheral leptin deficiency. In addition to enhancing our understanding of leptin interactions with its receptor, these antagonists enable in vivo study of the role of leptin in metabolic and immune processes and hold potential for future therapeutic use in disease pathologies involving leptin.

Origins and tissue-context-dependent fates of blood monocytes.

Peripheral blood monocytes play a central role in the mononuclear phagocyte system by providing a critical link between the bone marrow (BM), as major site of adult hematopoiesis, and peripheral, terminally differentiated mononuclear phagocyte populations, as represented macrophages and dendritic cells. Moreover, recent experimental evidence highlights the plasticity of these ephemeral mobile cells and their direct involvement in the establishment and resolution of inflammatory reactions. Here we summarize the recent advance in our understanding of monocyte origins, subset dynamics and monocyte fates. In particular, we will focus on emerging evidence for monocyte recirculation to the BM and discuss its potential implications in health and disease.

Tumor macrophages are pivotal constructors of tumor collagenous matrix

Tumor-associated macrophages are pivotal constructors of the tumoral ECM structure and molecular composition. In particular, they orchestrate the buildup of the tumorigenic collagenous ECM niche.