Macrophages in Tissue Repair, Regeneration, and Fibrosis

Renal fibrosis, particularly tubulointerstitial fibrosis, is the common final outcome of almost all progressive chronic kidney diseases. Renal fibrosis is also a reliable predictor of prognosis and a major determinant of renal insufficiency. Irrespective of the initial causes, renal fibrogenesis is a dynamic and converging process that consists of four overlapping phases: priming, activation, execution and progression. Nonresolving inflammation after a sustained injury sets up the fibrogenic stage (priming) and triggers the activation and expansion of matrix-producing cells from multiple sources through diverse mechanisms, including activation of interstitial fibroblasts and pericytes, phenotypic conversion of tubular epithelial and endothelial cells and recruitment of circulating fibrocytes. Upon activation, matrix-producing cells assemble a multicomponent, integrin-associated protein complex that integrates input from various fibrogenic signals and orchestrates the production of matrix components and their extracellular assembly. Multiple cellular and molecular events, such as tubular atrophy, microvascular rarefaction and tissue hypoxia, promote scar formation and ensure a vicious progression to end-stage kidney failure. This Review outlines our current understanding of the cellular and molecular mechanisms of renal fibrosis, which could offer novel insights into the development of new therapeutic strategies.

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Cellular and molecular mechanisms of renal fibrosis

Many types of kidney injury induce inflammation as a protective response. However, unresolved inflammation promotes progressive renal fibrosis, which can culminate in end-stage renal disease. Kidney inflammation involves cells of the immune system as well as activation of intrinsic renal cells, with the consequent production and release of profibrotic cytokines and growth factors that drive the fibrotic process. In glomerular diseases, the development of glomerular inflammation precedes interstitial fibrosis; although the mechanisms linking these events are poorly understood, an important role for tubular epithelial cells in mediating this link is gaining support. Data have implicated macrophages in promoting both glomerular and interstitial fibrosis, whereas limited evidence suggests that CD4(+) T cells and mast cells are involved in interstitial fibrosis. However, macrophages can also promote renal repair when the cause of renal injury can be resolved, highlighting their plasticity. Understanding the mechanisms by which inflammation drives renal fibrosis is necessary to facilitate the development of therapeutics to halt the progression of chronic kidney disease.

Inflammatory processes in renal fibrosis.

Acute respiratory distress syndrome (ARDS) is a devastating disease with distinct pathological stages. Fundamental to ARDS is the acute onset of lung inflammation as a part of the body's immune response to a variety of local and systemic stimuli. In patients surviving the inflammatory and subsequent fibroproliferative stages, transition from injury to resolution and recovery is an active process dependent on a series of highly coordinated events regulated by the immune system. Experimental animal models of acute lung injury (ALI) reproduce key components of the injury and resolution phases of human ARDS and provide a methodology to explore mechanisms and potential new therapies. Macrophages are essential to innate immunity and host defense, playing a featured role in the lung and alveolar space. Key aspects of their biological response, including differentiation, phenotype, function, and cellular interactions, are determined in large part by the presence, severity, and chronicity of local inflammation. Studies support the importance of macrophages to initiate and maintain the inflammatory response, as well as a determinant of resolution of lung inflammation and repair. We will discuss distinct roles for lung macrophages during early inflammatory and late resolution phases of ARDS using experimental animal models. In addition, each section will highlight human studies that relate to the diverse role of macrophages in initiation and resolution of ALI and ARDS.

Diverse macrophage populations mediate acute lung inflammation and resolution

Macrophages regulate tissue regeneration following injury. They can worsen tissue injury by producing reactive oxygen species and other toxic mediators that disrupt cell metabolism, induce apoptosis, and exacerbate ischemic injury. However, they also produce a variety of growth factors, such as IGF-1, VEGF-α, TGF-β, and Wnt proteins that regulate epithelial and endothelial cell proliferation, myofibroblast activation, stem and tissue progenitor cell differentiation, and angiogenesis. Proresolving macrophages in turn restore tissue homeostasis by functioning as anti-inflammatory cells, and macrophage-derived matrix metalloproteinases regulate fibrin and collagen turnover. However, dysregulated macrophage function impairs wound healing and contributes to the development of fibrosis. Consequently, the mechanisms that regulate these different macrophage activation states have become active areas of research. In this review, we discuss the common and unique mechanisms by which macrophages instruct tissue repai...

Mechanisms of Organ Injury and Repair by Macrophages.

IL-10/TGF-beta-modified macrophages, a subset of activated macrophages, produce anti-inflammatory cytokines, suggesting that they may protect against inflammation-mediated injury. Here, macrophages modified ex vivo by IL-10/TGF-beta (IL-10/TGF-beta Mu2) significantly attenuated renal inflammation, structural injury, and functional decline in murine adriamycin nephrosis (AN). These cells deactivated effector macrophages and inhibited CD4+ T cell proliferation. IL-10/TGF-beta Mu2 expressed high levels of the regulatory co-stimulatory molecule B7-H4, induced regulatory T cells from CD4+CD25- T cells in vitro, and increased the number of regulatory T cells in lymph nodes draining the kidneys in AN. The phenotype of IL-10/TGF-beta Mu2 did not switch to that of effector macrophages in the inflamed kidney, and these cells did not promote fibrosis. Taken together, these data demonstrate that IL-10/TGF-beta-modified macrophages effectively protect against renal injury in AN and may become part of a therapeutic strategy for chronic inflammatory disease.

IL-10/TGF-beta-modified macrophages induce regulatory T cells and protect against adriamycin nephrosis.

Background. Blockade of CD40-CD40 ligand (CD154) interactions protects against renal injury in adriamycin nephropathy (AN) in immunocompetent mice. To investigate whether this protection relied on adaptive or cognate immunity, we tested the effect of CD40-CD154 blockade in severe combined immunodeficient (SCID) mice. Methods. SCID mice were divided into three groups: normal, AN + hamster IgG (ADR+IgG group) and AN + anti-CD154 antibody (MR1) (ADR+MR1 group). AN was induced by tail vein injection of 5.2 mg/kg of adriamycin (ADR). Hamster IgG (control Ab) or MR1 was administered intraperitoneally on days 5, 7, 9 and 11 after ADR injection. Histological and functional data were collected 4 weeks after ADR injection. In vitro experiments tested the effect of soluble and cell-bound CD 154 co-cultured with CD40-expressing cells [macrophages, mesangial cells and renal tubular epithelial cells (RTEC)]. Results. All experimental animals developed nephropathy. Compared to the ADR+IgG group, ADR+MR1 animals had significantly less histological injury (glomerulosclerosis and tubular atrophy) and functional injury (creatinine clearance). Kidneys of ADR+MR1 animals had significantly less macrophage infiltration than those of ADR +IgG animals. Interestingly, expression of CD40 and CD41 (a platelet-specific marker) was significantly less in ADR+MR1 animals compared to ADR+IgG animals. In vitro, CD 154 blockade significantly attenuated upregulation of CCL2 gene expression by RTEC stimulated by activated macrophage-conditioned medium. In contrast, platelet-induced upregulation of macrophage and mesangial cell proinflammatory cytokine gene expression were not CD154-dependent. Conclusion. CD40-CD154 blockade has a significant innate renoprotective effect in ADR nephrosis. This is potentially due to inhibition of macrophage-derived soluble CD 154.

/pdf/the-cd40-cd154-co-stimulation-pathway-mediates-innate-immune-a5xp7ba5za.pdf

The CD40–CD154 co-stimulation pathway mediates innate immune injury in adriamycin nephrosis

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IL-10/TGF-beta-modified macrophages induce regulatory T cells and protect against adriamycin nephrosis.

The CD40–CD154 co-stimulation pathway mediates innate immune injury in adriamycin nephrosis