Macrophages and Fibrosis Resolution. Harnessing Wnt/β-Catenin Signaling as the Way and the Means

In this issue of the Journal, Sennello and colleagues (pp. 191–201) tackle the complexity of pulmonary fibrosis resolution mediated by macrophages (1). Idiopathic pulmonary fibrosis (IPF) is a progressive, fibrosing interstitial lung disease in which macrophages serve as producers of profibrotic signaling mediators, including transforming growth factor-β (2). Moreover, there is now direct experimental evidence that either the loss of macrophage and monocyte populations or an alteration in their phenotype during the fibrotic response is capable of inducing fibrosis resolution (1, 3–6). Macrophages are highly plastic cells and can be categorized into subtypes according to origin, tissue location, and recruitment (2, 7). The individual roles and regulation of these distinct populations in the balance of pro- and antifibrotic activities remain unclear.

This article poses a fundamental question concerning the mechanism by which macrophages become antifibrotic and promote resolution through loss of Wnt/β-catenin signaling (1). Although we often focus on Wnt/β-catenin singling in stromal cells (e.g., epithelium), tissue resident alveolar macrophages and recruited monocyte-derived alveolar macrophages also have active Wnt/β-catenin signaling during fibrosis. When tissue resident alveolar macrophages and monocyte-derived alveolar macrophages were targeted to lose Wnt/β-catenin signaling using CD11c^cre;β-catein^flox/flox mice, bleomycin-induced fibrosis was similar at 3 weeks, but resolution was accelerated compared with wild-type mice. Similarly, mice with a whole-body deficiency in Lrp5 (an essential Wnt coreceptor) had accelerated fibrosis resolution after both bleomycin and asbestos exposure. Importantly, loss of Lrp5 after bleomycin reduced infiltrating monocyte-derived macrophages and induced a gene signature in the lungs that was enriched for genes involved in matrix processing, including Mmp13 and lysyl oxidase (Lox).

A current struggle in the field of macrophage biology is the ability to accurately identify and describe macrophage populations, especially during disease (7). The identification of antifibrotic/resolving macrophages becomes increasingly complex as researchers look to induce a phenotypic switch in profibrotic macrophages present in the lungs during nonresolving fibrotic disease (8). This becomes a fundamental challenge of inducing antifibrotic behavior and kick-starting a “normal” resolution phase during ongoing extracellular matrix deposition and fibrosis. To this end, it becomes important to discern macrophages on the basis of their function and aim to either recruit a specific antifibrotic monocyte-derived macrophage to the lungs or therapeutically target profibrotic macrophages to become antifibrotic/resolving in function.

The authors worked within the confines of our current lineage-targeting technology for targeting alveolar macrophages, using the CD11c^cre driver mice. However, CD11c is not specific, and it is possible that the loss of Wnt/β-catenin signaling in other cells, including dendritic cells and Ly6C^lo monocytes, could have contributed to the more rapid resolution of fibrosis. Work in the lung and liver by Gibbons and Ramachandran, respectively, demonstrate distinctive roles for the Ly6C^lo and Ly6C^hi monocyte populations in facilitating fibrosis resolution (3, 6). Depletion of Ly6C^hi monocytes during the fibrotic phase after bleomycin accelerated pulmonary fibrosis resolution (3), whereas depletion of the Ly6C^lo monocytes caused a failure of fibrosis resolution in the CCl₄ model of liver fibrosis, indicating that subpopulations of monocytes may play complex roles in the maintenance and resolution of fibrotic disease (6).

In this work, Sennello and colleagues further show that the loss of Wnt/β-catenin signaling in the lungs of Lrp-deficient mice reduces the differentiation of monocyte-derived macrophages into tissue resident alveolar macrophages, suggesting that a reduction of this population contributes to a proresolving pulmonary environment (1). The next challenges will be to examine the monocyte-derived alveolar macrophage and tissue resident macrophage populations in the CD11c^cre;β-catein^flox/flox mice and to determine why the reduced differentiation phenotype was only seen in the Lrp-deficient mice after bleomycin and not after asbestos, although accelerated resolution was observed in both models. There is further work to be done to parse out the role of Wnt/β-catenin signaling in these macrophage/monocyte subpopulations, and how it may be driving a resolution phenotype.

Despite the need for continued work, it is clear that active Wnt/β-catenin signaling in macrophages during pulmonary fibrosis may be important for their profibrotic phenotype. How targeting the Wnt/β-catenin pathway will lend itself to therapeutic intervention in fibrotic disease is a question that remains unanswered. Wnt/β-catenin signaling is complex and has far-reaching effects on development and homeostasis. Dysregulation, therefore, should be playing a critical role in many diseases, including fibrosis of the lung, liver, kidney, skin, and heart (9–13). For diseases such as IPF, in which treatment options are still limited and are not specifically designed to alter the macrophage-programming signature, further understanding of how targeting specific pathways within the immune system is necessary. This becomes acutely apparent when dealing with the complexities of Wnt signaling during fibrotic disease (9, 12, 14). The importance of Wnt/β-catenin signaling in pulmonary fibrosis has been demonstrated by an increase in Wnt gene expression in patients with IPF compared with normal lung tissue (15). Immunostaining in IPF tissue for Wnt target genes as a marker for increased functional Wnt/β-catenin signaling also revealed increased expression in fibroblastic foci and alveolar epithelium compared with normal lung tissue, but activation in macrophages was not examined (12, 14). In addition, therapeutic intervention with Wnt/β-catenin pathway inhibitors during bleomycin-induced fibrosis augmented the resolution process (16, 17). Again, the caveat remains that these studies are examining and treating the lungs as a homogenous organ. Targeting Wnt/β-catenin signaling selectively in macrophages may be difficult without also causing unwanted effects in fibroblasts or epithelium (14, 18). For example, the specific ablation of Wnt/β-catenin signaling in alveolar type II cells under the SPC promoter augmented bleomycin-induced fibrosis, rather than inducing its resolution (19).

Finding the balance in targeting Wnt/β-catenin signaling will be critical in pursuing therapeutic intervention for patients with IPF. It is clear that macrophages present in fibrotic lungs promote the continuation of fibrotic disease. Although others have elegantly shown that a loss of specific macrophage populations or an alteration in their phenotype is capable of attenuating their profibrotic activities, this article highlights the signaling mechanism by which this may be occurring. It remains to be seen whether we can harness the power of the antifibrotic macrophage as a therapeutic strategy, but capitalizing on this important immune cell may be a direction therapeutic interventions should attempt to target.