Novel Approaches in Pulmonary Fibrosis: Beyond the Fibroblast

Attempts to develop antifibrotic treatment strategies have largely focused on targeting fibrosis initiation events (fibroblast proliferation, fibroblast-to-myofibroblast differentiation, and extracellular matrix generation). One potential explanation for the relative limited efficacy of current antifibrotic approaches is that for most patients, fibrosis is well-established at the time of diagnosis. Recent work in our lab suggests that targeting age-associated pathological dysfunction to promote fibrosis resolution (myofibroblast dedifferentiation or apoptotic clearance of senescent cells) may be more effective than strategies that block fibrosis development.

Aging processes such as cellular senescence may make a “root cause” contribution to chronic diseases. Senescent cells (SC) accumulate with aging and at sites of etiology of many chronic diseases. SC are resistant to apoptosis. SC can release factors that are pro-apoptotic, inflammatory, pro-fibrotic, cause stem cell dysfunction, and spread senescence, the senescence-associated secretory phenotype (SASP). Transplanting small numbers of SC into young mice, so that only 1/10,000 cells in recipients are transplanted SC, is sufficient to cause frailty, accelerated age-related disease onset, and early mortality.

We developed senolytics — drugs that selectively clear SC by inhibiting survival pathways that protect SC from apoptosis due to their SASP. Intermittent senolytic administration to mice alleviated bleomycininduced pulmonary fibrosis and many other conditions, including age- or diet-related cardiovascular dysfunction, liver fat and fibrosis in steatosis, radiation damage, cognitive dysfunction, and osteoporosis.

Senolytics delayed frailty, chronic diseases, and early death caused by transplanting SC. In old mice, senolytics improved physical function, delayed age-related diseases, and extended remaining lifespan 36%. In early clinical trials, senolytics reduced fat tissue SC in patients with diabetic kidney disease, decreased circulating SASP factors, and alleviated physical dysfunction in patients with idiopathic pulmonary fibrosis. Senolytics may hold promise for delaying, preventing, or treating age- and chronic disease-related disorders.

Idiopathic Pulmonary Fibrosis (IPF) is a lethal chronic age-related lung disease characterized by progressive scarring of the lung. Age-related perturbations are increasingly found in epithelial cells and fibroblasts from IPF lungs and are believed to play a critical role in the predisposition to lung injury, disrepair, and fibrosis. Our studies show that mitochondrial dysfunction and metabolic distress potentiate maladaptation to stress and susceptibility to lung fibrosis. We will discuss novel mechanistic insights of mitochondrial alterations in the pathogenesis of IPF, and potential therapeutic approaches that target aging processes may be beneficial for halting the progression of the disease.

Idiopathic pulmonary fibrosis (IPF) is a common form of interstitial lung disease (ILD) resulting in alveolar remodeling and progressive loss of pulmonary function due to chronic alveolar injury and failure to regenerate the respiratory epithelium. Histologically, fibrotic lesions and honeycomb structures expressing atypical proximal airway epithelial markers replace alveolar structures, the latter normally lined by alveolar type 1 (AT1) and AT2 cells. Bronchial epithelial stem cells (BESCs) can give rise to AT2 and AT1 cells or honeycomb cysts following bleomycin-mediated lung injury. However, little is known about what controls this binary decision or whether this decision can be reversed. Here we report that inactivation of Fgfr2b in BESCs impairs their contribution to both alveolar epithelial regeneration and honeycomb cysts after bleomycin injury. By contrast overexpression of Fgf10 in BESCs enhances fibrosis resolution by favoring the more desirable outcome of alveolar epithelial regeneration over the development of pathologic honeycomb cysts.

The cytokines, IL-17A and TGF-β, in Programmed Death-1+ CD4+ T cells have been implicated in numerous chronic diseases, such as pulmonary fibrosis.  The contribution of female gonadotrophic hormones to the observed sex disparities in outcome of interstitial lung disease has not been defined. Programmed Death-1 (PD-1)+CD4+ T cells, inducing pSTAT3 signaling to produce IL-17A and TGF-β1, have been implicated in lung pathogenesis. Here we show that female gonadotrophic hormones augment PD-1 and IL-6 expression while simultaneously inhibiting pSTAT3 activation in CD4+ T cells. This results in a shift from IL-6-JAK-pSTAT3-IL-17A signaling to enhanced IL-6-JAK-ERK-TGF-β1 expression. Murine and human females with fibrotic lung disease demonstrate increased TGF-β1, whereas males possess increased IL-17A. This shift from proinflammatory to immunosuppressive lung microenvironments improves female mortality. In vivo PD-1 pathway blockade following bleomycin administration revealed mortality benefit in female but not male mice. Thus, enhanced survival in premenopausal females with chronic lung diseases may be due to hormone restrictions on pSTAT3 signaling in PD-1+CD4+ T cells. These findings support sex-specific therapeutic approaches in fibrotic lung disease.

Idiopathic pulmonary fibrosis is progressive lung disease disorder characterized by excessive collagen deposition. While other studies examine structural cells in the setting of IPF, our research focuses on the role of myeloid cells. Our data demonstrate that interleukin-1 receptor associated kinase (IRAK)-M, a negative regulator of Toll-like receptor signaling, was elevated in macrophages and monocytes from human IPF patients and murine macrophages after experimental fibrosis. In a murine model of bleomycin-induced pulmonary fibrosis, upregulation of IRAK-M contributes to the generation of a profibrotic macrophage. Recently, new work has determined that macrophage origin regulates their activation phenotype. In this study we investigated the role of IRAK-M in regulating monocyte trafficking and macrophage differentiation following bleomycin challenge. Using flow cytometry, we measured lung leukocytes after bleomycin challenge. There was no difference in the number of tissue resident alveolar macrophages between groups; however, there were increased monocyte–derived macrophages and inflammatory Ly6chi monocytes in the lungs of WT, but not IRAK-M^−/− mice after bleomycin. Our data indicate that expression of IRAK-M regulates monocyte trafficking through the blood into the lungs resulting in more profibrotic macrophages. Understanding the mechanism by which monocytes traffic to the lungs is essential for the development of novel therapeutics.

Coauthors: Rose Viguna, Thomas Becket, Brenda Reader, David Weimar, Kristina Luikart, and John Christman, The Ohio State University Wexner College of Medicine.

Tissues lose integrity upon injury. To rapidly restore mechanical stability, a variety of different cell types are activated to become myofibroblasts. Hallmarks of the myofibroblast are secretion of extracellular matrix (ECM), development of adhesion structures with the ECM, and formation of contractile stress fiber bundles.

Rapid repair comes at the cost of tissue contracture due to the inability of the myofibroblast to regenerate tissue. When contracture and ECM remodeling become progressive and manifest as organ fibrosis, stiff scar tissue obstructs and ultimately destroys organ function. Pivotal for the formation and persistence of myofibroblasts are mechanical stimuli arising during tissue repair and chronic persistence of inflammatory cells.

I will give an overview on our current projects that address how mechanical factors orchestrate the development of myofibroblasts by mediating direct and far/ranging communication between myofibroblasts and macrophages. We show that contracting fibroblasts generate dynamic deformation fields in fibrillar collagen matrix that initiate and direct macrophage migration to the contraction source. In contrast, collagen condensation and fiber alignment resulting from fibroblast remodelling activities or chemotactic signals are neither required nor sufficient to guide macrophage migration. Our data support that macrophages mechanosense the velocity of local displacements of their substrate, allowing contractile fibroblasts to attract macrophages over distances that exceed the range of chemotactic gradients.

By understanding and manipulating myofibroblast and macrophage mechanoperception we will be able to devise better therapies to reduce scarring and support normal wound healing.