Fibrosis: from mechanisms to medicines

Abstract

Fibrosis can affect any organ and is responsible for up to 45% of all deaths in the industrialized world. It has long been thought to be relentlessly progressive and irreversible, but both preclinical models and clinical trials in various organ systems have shown that fibrosis is a highly dynamic process. This has clear implications for therapeutic interventions that are designed to capitalize on this inherent plasticity. However, despite substantial progress in our understanding of the pathobiology of fibrosis, a translational gap remains between the identification of putative antifibrotic targets and conversion of this knowledge into effective treatments in humans. Here we discuss the transformative experimental strategies that are being leveraged to dissect the key cellular and molecular mechanisms that regulate fibrosis, and the translational approaches that are enabling the emergence of precision medicine-based therapies for patients with fibrosis.

Figure 1:. Deconvolving fibrosis using multi-modal single-cell approaches.

Cutting-edge single-cell approaches are transforming our understanding of the complex cellular and molecular mechanisms regulating fibrosis, allowing assessment of the transcriptome, genome, epigenome and proteome at single-cell level, in addition to spatial profiling. Furthermore, combined readouts from the same single cell are now possible (for example, the simultaneous profiling of transcriptome and chromatin accessibilty), and integration of these multi-modal single-cell omics readouts has allowed ever more powerful, comprehensive assessments of cell state, ontogeny, phenotype and function during human fibrotic disease. The new biological insights gained from these integrated approaches should enable the identification of novel and tractable therapeutic targets to treat patients across a broad range of fibrotic diseases.

Figure 2:. Functional fibroblast heterogeneity and plasticity.

Recent studies have uncovered significant functional heterogeneity and plasticity within fibroblast populations during fibrosis. In the context of arthritis, scRNASeq combined with adoptive transfer experiments identified two anatomically distinct fibroblast subsets within the FAPα⁺ population: FAPα⁺ thymus cell antigen (THY1)⁺ immune ‘effector’ fibroblasts located in the synovial sub-lining, and FAPα⁺THY1⁻ ‘destructive’ fibroblasts restricted to the synovial lining layer. Studies of mesenchymal progenitor (MP) populations (Hic1⁺Pdgfrα⁺ LY6A⁺) in heart and skeletal muscle have demonstrated MP hierarchy, diversity in the pathophysiological roles of their progeny, and how fate determination of MP is tissue-dependent. Fibroblasts are also capable of remarkable degrees of plasticity and phenotype switching during progression and regression of fibrosis. Myofibroblasts can revert to a quiescent state in the absence of ongoing injury, or may undergo full lineage switching with adipocytes which is observed during cutaneous wound healing in mice. Furthermore, genetic and pharmacological inactivation of the transcription factor PU.1 can reprogramme fibrotic fibroblasts into resting fibroblasts, resulting in regression of fibrosis in several organs. Finally, viral vector-mediated expression of specific transcription factors in myofibroblasts in the liver has been used to reprogram myofibroblasts into hepatocyte-like cells in fibrotic mouse livers, thereby reducing liver fibrosis and increasing liver function.

Figure 3:. Metabolomic reprogramming of activated fibroblasts.

Profibrotic fibroblasts increase glycolysis via hexokinase leading to ehanced pyruvate and lactate. Lactate decreases extracellular pH and activates latent TGF-β1. Pyruvate also feeds the TCA cycle after conversion to acetyl-CoA increasing succinate. Both mechanisms lead to increase in a-SMA, collagen production and proliferation. Glutaminase activity is increased converting glutamate to glutamine which gets converted into α-KG via the TCA cycle and decreases apoptosis and enhances collagen stabilization. Abbreviations: α-KG: alpha-ketoglutarate; a-SMA: alpha smooth muscle actin; LDH: Lactate hydrogenase; P: Phosphate; R: Receptor; TCA:tricarboxylic acid; TGF: Transforming growth factor.

Figure 4:. Divergent cytokine pathways drive fibrosis.

The Innate acute phase pro-inflammatory cytokines IL-1, IL-6, and TNF, together with TGF beta promote the development of IL-17 secreting cells. IL-17A potentiates neutrophil responses that contribute to tissue injury through ROS production, while increasing TGF-beta receptor expression on fibroblasts that facilitates ECM production in response to TGF-beta. TGF-beta, activated locally through integrin-mediated mechanisms, serves as a key driver of fibrosis. A second and distinct cytokine-mediated pathway that can promote fibrosis independently of TGF-beta is the type-2 cytokine axis. Here, the alarmin cytokines IL-25, IL-35, and TSLP, secreted by epithelial cells and other damaged tissues drives the expansion and activation of type-2 innate lymphoid cells (ILC2) that secrete large amounts of IL-5 and IL-13. IL-5 in turn drives the recruitment and activation of local tissue eosinophils, which provide an additional source of type 2 cytokines and other pro-fibrotic mediators. IL-13, derived from eosinophils, CD4+ Th2 cells, and ILC2s exhibits potent pro-fibrotic activity that is TGF-beta independent. Finally, the cytokine IL-11, produced by activated myofibroblasts has been found to stimulate ECM production by myofibroblasts in response to multiple pro-fibrotic mediators, including TGF beta and IL-13.

Figure 5:

Challenges and solutions in the translation of antifibrotic mechanisms into drugs.