The Immune Microenvironment: New Therapeutic Implications in Organ Fibrosis

Abstract

Fibrosis, characterized by abnormal deposition of structural proteins, is a major cause of tissue dysfunction in chronic diseases. The disease burden associated with progressive fibrosis is substantial, and currently approved drugs are unable to effectively reverse it. Immune cells are increasingly recognized as crucial regulators in the pathological process of fibrosis by releasing effector molecules, such as cytokines, chemokines, extracellular vesicles, metabolites, proteases, or intercellular contact. Therefore, targeting the immune microenvironment can be a potential strategy for fibrosis reduction and reversion. This review summarizes the recent advances in the understanding of the immune microenvironment in fibrosis including phenotypic and functional transformations of immune cells and the interaction of immune cells with other cells. The novel opportunities for the discovery and development of drugs for immune microenvironment remodeling and their associated challenges are also discussed.

Keywords: immune microenvironment; organ fibrosis; therapeutic targets.

Figure 1

Common chronic human diseases associated with fibrosis are shown. Created with BioRender https://www.biorender.com/.

Figure 2

The dual role of macrophages in fibrosis: pro‐fibrotic drivers and resolution mediators. A) Tissue‐resident macrophages originate from the yolk sac and fetal liver precursors during embryogenesis. Bone marrow‐derived monocytes become the postnatal origin of macrophages. In response to environmental stimuli, macrophages undergo the phenotypic and functional transition to exert pro‐fibrotic or pro‐resolution characters. Macrophage‐released effector molecules including cytokines, MMPs, microRNA, and circRNA regulate epithelial/endothelial/fibroblast‐to‐myofibroblast transition. Macrophage‐produced MMPs (MMP2, MMP8, MMP9, MMP12, and MMP28) and TGFBI promote extracellular matrix overdeposition. Moreover, MMP12 production triggers endothelial dysfunction and failure of trans‐endothelial surveillance. On the contrary, pro‐resolution macrophage‐derived MMPs (MMP10, MMP12, and MMP13) induce collagen degradation, and AREG regulates Treg differentiation and stem cell identity to promote organ regeneration. In addition, macrophage‐released IL‐10 and VEGF mediate angiogenesis upon tissue injury. B) Macrophage‐secreted cargo‐loading exosomes participate in fibrosis. Created with BioRender https://www.biorender.com/.

Figure 3

A) Neutrophils in fibrosis: a double‐edged sword for therapeutic targeting. Circulating neutrophils infiltrated in tissues perform pro‐fibrotic (Siglec‐F⁺/MMP9⁺/CXCR4^hiCD62L^low/CSFR3^hiCCR2^hi) or pro‐repair (CSFR3^hiCXCR2^hiCCR1^hi) functions in response to different environmental cues. Neutrophils release various cytokines to regulate fibroblasts and macrophage activation, such as TGFβ, TNFα, IL‐1β, and CCL2. The production of ROS and neutrophil elastase promotes fibroblast differentiation. The activation of NLRP3 inflammasome in neutrophils further facilitates immune cell infiltration. Pro‐fibrotic neutrophil‐generated NET causes monocyte, macrophage, epithelial, and fibroblast differentiation; while, pro‐repair neutrophils produce NGAL and miR‐233 to promote macrophage polarization to restorative phenotype. In addition, neutrophil‐derived MMPs mediate ECM degradation to prevent overdeposition. B) Dendritic cells in fibrosis. Dendritic cells release pro‐fibrotic factors such as IL‐6, IL‐12, and IL‐18 to promote fibroblast activation and anti‐fibrotic factors such as IL‐10 and MMP9 to inhibit fibroblast activation and ECM deposition. In addition to inducing T cell activation by presenting antigen peptides through major histocompatibility complex (MHC) molecules, dendritic cells can also produce cytokines (IL‐1β, IL‐17F, IL‐21, IL‐22, and IL‐23) that promote the differentiation of helper T cells. C) Basophils, eosinophils, and mast cells in fibrosis, when epithelial damage, basophil, eosinophil, and mast cells can produce IL‐4 and IL‐13 to induce myofibroblast activation and Th2 cell induction. In addition to continuous coactivation through heterotypic cell connections, mast cells can also release TGFβ, histamine, and granule contents to activate fibroblasts. Mast cell‐secreted IL‐2 induces ILC2 survival, which produces IL‐9 to continuously activate Th9 cells and mast cells. Further, mast cells‐released IL‐9 triggers Th9 activation in turn. Basophils‐released IL‐6 promotes Th17 cell induction. For eosinophils, they can produce IL‐4 and mEar1 to regulate inflammatory cell infiltration and fibroblast activation. Fibroblast‐secreted GM‐CSF induces eosinophil activation and prolonged survival, which result in various pro‐fibrotic cytokine productions, such as TGFβ, IL‐1β, and IFN‐γ. Created with BioRender https://www.biorender.com/.

Figure 4

A) Innate lymphoid cell: friends or foes? Upon tissue injury, ILC1 not only releases MMP9 to inhibit ECM deposition but also secretes IFN‐γ to induce the activation of pro‐inflammatory macrophages and fibroblasts. Stromal IL‐33 or TGF can induce ILC2 to adopt a pro‐fibrotic phenotype, releasing IL‐12 to activate fibroblasts and secreting IL‐5 to trigger eosinophilia. On the contrary, Stromal IL‐25 can induce ILC2 to adopt an anti‐fibrotic phenotype, releasing IL‐10 to inhibit fibroblast activation. For ILC3, matrix‐derived IL‐23 and IL‐1β induce the production of IL‐17, IL‐22, and IL‐13, which activate fibroblasts, as well as GM‐CSF to promote the polarization of M1 macrophages. In addition to releasing cytoplasmic granules containing TRAIL, perforin, and granzyme, the killing process of NK cells against HSCs depends on NKG2D, NKp46, Siglec‐7, and E‐prostanoid 3 receptors expressed by NK cells and NCR1 ligands expressed by HSCs. B) Unconventional T cells as pleiotropic orchestrators in fibrotic pathogenesis. After CD1‐expressing DCs activate NKT cells, NKT cells can produce IL‐10, IL‐4, and other factors to promote cardiomyocyte hypertrophy and fibroblast activation to contribute to fibrosis. When damaged, γδ T cells‐secreted proinflammatory cytokine IL‐17A directly activates CD3⁺ T cells to produce chemokine RANTES; and then, recruits macrophages/T cells to promote inflammatory response and fibrosis. Moreover, IL‐17A can induce hepatocytes to produce chemokines (CCL1/CXCL1/CXCL2), which facilitate proinflammatory cell infiltration. γδ T cells also perform pro‐repair functions by producing IL‐22 and IFN‐γ to inhibit T cell infiltration and differentiation or inducing HSC apoptosis via the Fas‐FasL pathway. MAIT cells can produce perforin and granzyme B to trigger epithelial cell necrosis. Further, MAIT cell regulates proinflammatory macrophage phenotypic transition, which releases IL‐6 and IL‐8 to promote chronic inflammation. IL‐17 and TNF secreted from MAIT cells contribute to fibrosis by triggering myofibroblast proliferation. Created with BioRender https://www.biorender.com/.

Figure 5

Immune microenvironment alteration‐driven regulation of ECM‐producing cells during tissue fibrosis. Upon tissue injury, site‐specific stimuli trigger leukocyte infiltration and proliferation of tissue‐resident immune cells. During this process, in response to dynamic microenvironmental alterations, immune cells undergo phenotypic and functional transformations and release diverse pro‐fibrotic effector molecules. These mediators contribute to scar formation by activating fibroblasts and inducing the transdifferentiation of epithelial cells, pericytes, endothelial cells, and circulating CD45‐positive hematopoietic lineage cells (including fibrocytes and macrophages) into ECM‐producing myofibroblasts. Created with BioRender https://www.biorender.com/.

Figure 6

Screening therapeutic targets for fibrosis using multi‐model and multi‐omics single‐cell approaches. The pathogenesis of fibrosis is complex, involving multiple cell types within the tissue as well as cells that migrate from circulation to the tissue. Integrating single‐cell data across various temporal stages, tissues/organs, and expression levels is essential for constructing a high‐resolution spatiotemporal multi‐omics integrative atlas. This comprehensive approach facilitates the assessment of immune cell states, ontogeny, and the transitions of phenotypes and functions during human fibrotic diseases. It aims to elucidate immune cell subsets and microenvironment characteristics, as well as key biomarkers associated with the fibrotic phenotype. Further, functional validation and exploration of molecular mechanisms can be conducted using various animal models. In addition, employing precision cut tissue slices and organoid systems to develop 3D in vitro culture models can better simulate the pathological processes of fibrosis and facilitate drug screening. The novel biological insights derived from these integrative methodologies are expected to identify potential therapeutic targets. hiPSC, human induced pluripotent stem cell. Created with BioRender https://www.biorender.com/.