The Role of the Anti-Inflammatory Cytokine Interleukin-10 in Tissue Fibrosis

Abstract

Significance: Fibrosis is the endpoint of chronic disease in multiple organs, including the skin, heart, lungs, intestine, liver, and kidneys. Pathologic accumulation of fibrotic tissue results in a loss of structural integrity and function, with resultant increases in morbidity and mortality. Understanding the pathways governing fibrosis and identifying therapeutic targets within those pathways is necessary to develop novel antifibrotic therapies for fibrotic disease. Recent Advances: Given the connection between inflammation and fibrogenesis, Interleukin-10 (IL-10) has been a focus of potential antifibrotic therapies because of its well-known role as an anti-inflammatory mediator. Despite the apparent dissimilarity of diseases associated with fibrotic progression, pathways involving IL-10 appear to be a conserved molecular theme. More recently, many groups have worked to develop novel delivery tools for recombinant IL-10, such as hydrogels, and cell-based therapies, such as ex vivo activated macrophages, to directly or indirectly modulate IL-10 signaling. Critical Issues: Some efforts in this area, however, have been stymied by IL-10's pleiotropic and sometimes conflicting effects. A deeper, contextual understanding of IL-10 signaling and its interaction with effector cells, particularly immune cells, will be critical to future studies in the field. Future Directions: IL-10 is clearly a gatekeeper of fibrotic/antifibrotic signaling. The development of novel therapeutics and cell-based therapies that capitalize on targets within the IL-10 signaling pathway could have far-reaching implications for patients suffering from the consequences of organ fibrosis.

Keywords: cell biology; extracellular matrix; fibrosis; hyaluronan; interleukin-10.

Sundeep G. Keswani, MD, FACS, FAAP

Figure 1.

Wound healing and fibrogenesis. Wound healing progresses in a well-defined series of steps. After tissue injury (1) coagulation begins, ultimately resulting in the formation of a fibrin clot (2). Damaged tissue or detected pathogens spur the release of local cytokine and growth factor release, beginning the inflammatory phase of wound healing. These “danger signals” result in the recruitment of local and circulating innate immune cells, neutrophils (3) and macrophages (4) being predominant initially. These phagocytes begin antigen presentation and thereby recruit and activate effector cells of the adaptive immune system (5). IL-10 is broadly expressed by immune cells, but the predominant cellular sources are macrophages and T cell subsets (i.e., T helper 2 and regulatory T cells).^, Pathogen and damaged tissue clearance is the ultimate result of the inflammatory phase, staging the wound bed for regrowth. The proliferative phase is characterized by the activities of the fibroblast (6), which secretes the ECM components that provide the scaffolding for regenerated tissue. Granulation tissue, composed of immature blood vessels (8) and loose connective tissue fibers, begins to fill the wound, providing a structure within which fibroblasts can act and upon which epithelial cells migrate (9). Fibroblasts are induced by various cytokines and growth factors to differentiate into myofibroblasts (7), strengthening the wound by depositing collagen fibers, glycosaminoglycans, and other structural macromolecules. The myofibroblast phenotype is also contractile, acting to hasten wound closure. At this point, the remodeling phase begins, as newly created structures undergo maturation and strengthening or are pruned away. This stage can last from months to years. The initially deposited collagen III is replaced by collagen I, and collagen bundling and crosslinking (10) serves to further increase the tensile strength of the wound, while also resulting in what we recognize as scar tissue formation. Inflammatory cells and fibroblasts are no longer recruited, and many of those present in the wound bed undergo apoptosis. This gradual quiescence concludes the wound healing process and prevents the continued production of scar tissue, which could lead to tissue dysfunction. ECM, extracellular matrix; IL-10, interleukin-10.

Figure 2.

Macrophage polarization. Monocytes are recruited to the site of injury and therein undergo a process of differentiation into M1 (classically-activated) or M2 (alternatively activated) species depending on the microenvironmental cues encountered. The M1 phenotype is induced by immunostimulant molecules such as IFN-γ and bacterial LPS, while the M2 phenotype is induced following exposure to cytokines such as IL-10, IL-13, and TGF-β. M1 macrophage activation and expression of MHC class II antigens propagate further inflammatory signaling and cytokine release, which, despite promoting pathogen killing and immunity, may eventually lead to injury site fibrosis and scarring. M2 macrophages are generally thought to attenuating organ injury by suppressing inflammation and promoting beneficial matrix remodeling and repair, partially via their release of regenerative and anti-inflammatory factors such as IL-10. GC, glucocorticoids; IFN, interferon; LPS, lipopolysaccharide; MHC, major histocompatibility class; NO, nitric oxide; ROS, reactive oxygen species; TGF, transforming growth factor; TNF, tumor necrosis factor.

Figure 3.

(A–H) Representative histology of uninjured murine skin and murine wounds treated with LV IL-10 and controls at 28 days postwounding. Images show H&E staining (A, C, E, G) (4 × objective) and Masson's trichrome staining (B, D, F, H) (40 × objective) of the wounded/repaired tissue. LV-IL-10 overexpression in murine wounds results in regenerative wound healing (G) that is indistinguishable from the surrounding skin, compared to scar formation (flattened epidermis, lack of dermal appendages) in PBS (C) and LV-GFP controls (E) at 28 days. As shown in (H), Masson's trichrome staining demonstrates that the addition of IL-10 to the wound results not in a scar, but in the restoration of normal ECM architecture (H) (basketweave collagen packing in blue), compared to thick parallel bundles of collagen in LV-GFP and PBS-treated wounds (D). GFP, green fluorescent protein; H&E, hematoxylin and eosin; LV, lentiviral; PBS, phosphate-buffered saline. Figure reprinted with permission of the author(s) and journal.

Figure 4.

(A–D) Representative histology of bleomycin-challenged murine lungs, with and without intranasal HH-10 hydrogel treatment. Intranasal administration of bleomycin induces pulmonary fibrosis in mice and is an established experimental model of human IPF. Compared to PBS/control-treated animals (A, C), 7 days of intranasal HH-10 (200 ng/mL IL-10 in a hyaluronan-based hydrogel) treatment (B, D) decreases the size and severity of fibrotic lesions in the lungs of bleomycin-challenged mice. Note also that the extent of perivascular fibrosis, as demonstrated by the extent of blue staining (red boxes) was also reduced by treatment with HH-10. Images show trichrome staining (top row: 4 × objective; bottom row: 40 × objective) of lung tissue sections of bleomycin-challenged treatment cohorts. IPF, Idiopathic pulmonary fibrosis. Sample images courtesy of S. Balaji and V. de Jesus Perez, with results as described in Shamskhou et al.

Figure 5.

(A–F) Representative histology of control/normal and 14 days post-UUO murine kidney cortices, with and without LV IL-10 treatment. Images show PAS staining (A–C) (40 × objective) and trichrome staining (D–F) of control, untreated UUO, and IL-10-treated UUO kidney cortices. 14 days of ureteral obstruction results in significant kidney injury (B, E) marked by glomerular involution, tubular dropout, and increased interstitial spaces filled with collagen (blue/purple). With the addition of LV IL-10 (C, F) injected into the renal parenchymal 3 days before injury, the inflammatory damage and fibrosis levied by UUO is markedly reduced. PAS, periodic acid-Schiff; UUO, unilateral ureteral obstruction. Data and figures courtesy of X. Wang; article under review for publication.