Fibrocytes: A Critical Review and Practical Guide

Cancer-associated fibroblasts (CAFs) and plaque-associated fibroblasts (PAFs): Unraveling the cellular crossroads of atherosclerosis and cancer

Atherosclerosis is a complex process involving various cells and molecules within the atherosclerotic plaque. Recent evidence suggests that plaque-associated fibroblasts (PAFs), also known as atherosclerosis-associated fibroblasts (AAFs), might play a significant role in the development and progression of the disease. The microenvironment of the atherosclerotic plaque, resembling the tumor microenvironment (TME), includes various cellular populations like plaque-associated macrophages (PAMs), plaque-associated neutrophils (PANs), vascular smooth muscle cells (VSMCs), myeloid-derived suppressor cells (MDSCs), and PAFs. Similar to cancer-associated fibroblasts (CAFs) in tumors, PAFs exhibits a wide range of characteristics and functions. Their interactions with endothelial cells, smooth muscle cells, and other stromal cells, including adventitial fibroblast precursors, significantly influence atherosclerosis progression. Moreover, the ability of PAFs to express various markers such as alpha-SMA, Desmin, VEGF, and GFAP, highlights their diverse origins from different precursor cells, including vascular smooth muscle cells, endothelial cells, glial cells of the enteric nervous system, adventitial fibroblast precursors, as well as resident and circulating fibrocytes. This article explores the molecular interactions between PAFs, cells associated with atherosclerosis, and other stromal cells. It further examines the role of PAFs in the development and progression of atherosclerosis, and compares their features with those of CAFs. The research suggests that studying tumor-associated fibroblasts can help understand fibroblast subpopulations in atherosclerotic plaque. Identifying specific subpopulations could provide new insight into atherosclerosis complexity and lead to the development of innovative drugs for medical intervention.

CD45+/ Col I+ Fibrocytes: Major source of collagen in the fibrotic lung, but not in passaged fibroblast cultures

The role of cells of the hematopoietic lineage in fibrosis is controversial. Here we evaluate the contribution of Col I+/CD45+ cells (fibrocytes) to lung fibrosis. Systemic bleomycin treatment was used to induce fibrosis in a bone marrow transplant and two transgenic mouse models. Lung cells from these mice were analyzed by flow cytometry, both immediately upon release from the tissue or following growth on tissue-culture plastic. Fibrotic and control human lung tissue were also used. Fibroblasts and fibrocytes derived from a transgenic mouse model were compared in terms of their morphology, growth, and adhesion to fibronectin. Single cell RNAseq was performed with the analysis focusing on CD45-/Col I+ "fibroblasts" and CD45+/Col I+ "fibrocytes" in control and fibrotic mouse lung tissue. Finally, we inhibited fibrosis in mice using a novel, water-soluble version of caveolin scaffolding domain (CSD) called WCSD. In both mouse and human lung tissue, we observed by flow cytometry a large increase in fibrocyte number and Col I expression associated with fibrosis. In contrast, fibroblast number was not significantly increased. A large increase (>50-fold) in fibrocyte number associated with fibrosis was also observed by single cell RNAseq. In this case, fibroblasts increased 5-fold. Single cell RNAseq also revealed that myofibroblast markers in fibrotic tissue are associated with a cluster containing a similar number of fibrocytes and fibroblasts, not with a resident fibroblast cluster. Some investigators claim that fibrocytes are not present among primary fibroblasts. However, we found that fibrocytes were the predominant cell type present in these cultures prior to passage. Fewer fibrocytes were present after one passage, and almost none after two passages. Our experiments suggest that fibrocytes are crowded out of cultures during passage because fibroblasts have a larger footprint than fibrocytes, even though fibrocytes bind more efficiently to fibronectin. Finally, we observed by flow cytometry that in mice treated with bleomycin and WCSD compared to bleomycin alone, there was a large decrease in the number of fibrocytes present but not in the number of fibroblasts. In summary, fibrocytes are a major collagen-producing cell type that is increased in number in association with fibrosis as well as a major source of myofibroblasts. The common observation that collagen-producing spindle-shaped cells associated with fibrosis are CD45- may be an artifact of passage in cell culture.

Subretinal fibrosis secondary to neovascular age-related macular degeneration: mechanisms and potential therapeutic targets

Subretinal fibrosis is the end-stage sequelae of neovascular age-related macular degeneration. It causes local damage to photoreceptors, retinal pigment epithelium, and choroidal vessels, which leads to permanent central vision loss of patients with neovascular age-related macular degeneration. The pathogenesis of subretinal fibrosis is complex, and the underlying mechanisms are largely unknown. Therefore, there are no effective treatment options. A thorough understanding of the pathogenesis of subretinal fibrosis and its related mechanisms is important to elucidate its complications and explore potential treatments. The current article reviews several aspects of subretinal fibrosis, including the current understanding on the relationship between neovascular age-related macular degeneration and subretinal fibrosis; multimodal imaging techniques for subretinal fibrosis; animal models for studying subretinal fibrosis; cellular and non-cellular constituents of subretinal fibrosis; pathophysiological mechanisms involved in subretinal fibrosis, such as aging, infiltration of macrophages, different sources of mesenchymal transition to myofibroblast, and activation of complement system and immune cells; and several key molecules and signaling pathways participating in the pathogenesis of subretinal fibrosis, such as vascular endothelial growth factor, connective tissue growth factor, fibroblast growth factor 2, platelet-derived growth factor and platelet-derived growth factor receptor-β, transforming growth factor-β signaling pathway, Wnt signaling pathway, and the axis of heat shock protein 70-Toll-like receptors 2/4-interleukin-10. This review will improve the understanding of the pathogenesis of subretinal fibrosis, allow the discovery of molecular targets, and explore potential treatments for the management of subretinal fibrosis.

Fibrocyte enrichment and myofibroblastic adaptation causes nucleus pulposus fibrosis and associates with disc degeneration severity

Fibrotic remodeling of nucleus pulposus (NP) leads to structural and mechanical anomalies of intervertebral discs that prone to degeneration, leading to low back pain incidence and disability. Emergence of fibroblastic cells in disc degeneration has been reported, yet their nature and origin remain elusive. In this study, we performed an integrative analysis of multiple single-cell RNA sequencing datasets to interrogate the cellular heterogeneity and fibroblast-like entities in degenerative human NP specimens. We found that disc degeneration severity is associated with an enrichment of fibrocyte phenotype, characterized by CD45 and collagen I dual positivity, and expression of myofibroblast marker α-smooth muscle actin. Refined clustering and classification distinguished the fibrocyte-like populations as subtypes in the NP cells - and immunocytes-clusters, expressing disc degeneration markers HTRA1 and ANGPTL4 and genes related to response to TGF-β. In injury-induced mouse disc degeneration model, fibrocytes were found recruited into the NP undergoing fibrosis and adopted a myofibroblast phenotype. Depleting the fibrocytes in CD11b-DTR mice in which myeloid-derived lineages were ablated by diphtheria toxin could markedly attenuate fibrous modeling and myofibroblast formation in the NP of the degenerative discs, and prevent disc height loss and histomorphological abnormalities. Marker analysis supports that disc degeneration progression is dependent on a function of CD45+COL1A1+ and αSMA+ cells. Our findings reveal that myeloid-derived fibrocytes play a pivotal role in NP fibrosis and may therefore be a target for modifying disc degeneration and promoting its repair.

Fibrocytes in tumor microenvironment: Identification of their fraction and novel therapeutic strategy

Fibrocytes were identified as bone marrow-derived myeloid cells that also have fibroblast-like phenotypes, such as ECM production and differentiation to myofibroblasts. Although fibrocytes are known to contribute to various types of tissue fibrosis, their functions in the tumor microenvironment are unclear. We focused on fibrocytes as pivotal regulators of tumor progression. Our previous studies have indicated that fibrocytes induce angiogenesis and cancer stem cell-like phenotypes by secreting various growth factors. In contrast, immune checkpoint inhibitor (ICI)-treated fibrocytes demonstrated antigen-presenting capacity and enhanced antitumor T cell proliferation. Taken together, these findings indicate that fibrocytes have multiple effects on tumor progression. However, the detailed phenotypes of fibrocytes have not been fully elucidated because the isolation of distinct fibrocyte clusters has not been achieved without culturing in ECM-coated conditions or intracellular staining of ECM. The development of single-cell analyses partially resolves these problems. Single-cell RNA sequences in CD45+ immune cells from tumor tissue identified ECM-expressing myeloid-like cells as distinct fibrocyte clusters. In addition, these findings enabled the isolation of tumor-infiltrating fibrocytes as CD45+CD34+ cells. These tumor-infiltrating fibrocytes demonstrated both antigen-presenting ability and differentiation into myofibroblast-like cancer-associated fibroblasts. Considering these functions of fibrocytes in tumor progression, molecular-targeting agents for the migration, activity, and differentiation of fibrocytes are promising therapeutic strategies. Furthermore, identification of specific cell surface markers and master regulators of fibrocytes will advance novel fibrocyte-targeting therapies. In this review, we discuss the multiple roles of tumor-infiltrating fibrocytes and novel cancer therapeutic strategies.

The immune landscape of murine skeletal muscle regeneration and aging

Age-related alterations in the immune system are starting to emerge as key contributors to impairments found in aged organs. A decline in regenerative capacity is a hallmark of tissue aging; however, the contribution of immune aging to regenerative failure is just starting to be explored. Here, we apply a strategy combining single-cell RNA sequencing with flow cytometry, histological analysis, and functional assays to perform a complete analysis of the immune environment of the aged regenerating skeletal muscle on a time course following injury with single-cell resolution. Our results reveal an unanticipated complexity and functional heterogeneity in immune populations within the skeletal muscle that have been regarded as homogeneous. Furthermore, we uncover a profound remodeling of both myeloid and lymphoid compartments in aging. These discoveries challenge established notions on immune regulation of skeletal muscle regeneration, providing a set of potential targets to improve skeletal muscle health and regenerative capacity in aging.

Fibrocyte: A missing piece in the pathogenesis of fibrous epulis

OBJECTIVES

To explore the role of fibrocytes in the recurrence and calcification of fibrous epulides.

METHODS

Different subtypes of fibrous epulides and normal gingival tissue specimens were first collected for histological and immunofluorescence analyses to see if fibrocytes were present and whether they differentiated into myofibroblasts and osteoblasts upon stimulated by transforming growth factor-β1 (TGF-β1). Electron microscopy and elemental analysis were used to characterize the extracellular microenvironment in different subtypes of fibrous epulides. Human peripheral blood mononuclear cells (PBMCs) were subsequently isolated from in vitro models to mimic the microenvironment in fibrous epulides to identify whether TGF-β1 as well as the calcium and phosphorus ion concentration in the extracellular matrix (ECM) of a fibrous epulis trigger fibrocyte differentiation.

RESULTS

Fibrous epulides contain fibrocytes that accumulate in the local inflammatory environment and have the ability to differentiate into myofibroblasts or osteoblasts. TGF-β1 promotes fibrocytes differentiation into myofibroblasts in a concentration-dependent manner, while TGF-β1 stimulates the fibrocytes to differentiate into osteoblasts when combined with a high calcium and phosphorus environment.

CONCLUSIONS

Our study revealed fibrocytes play an important role in the fibrogenesis and osteogenesis in fibrous epulis, and might serve as a therapeutic target for the inhibition of recurrence of fibrous epulides.

The possible pathogenesis of liver fibrosis: therapeutic potential of natural polyphenols

Liver fibrosis is the formation of a fibrous scar resulting from chronic liver injury, independently from etiology. Although many of the mechanical details remain unknown, activation of hepatic stellate cells (HSCs) is a central driver of liver fibrosis. Extracellular mechanisms such as apoptotic bodies, paracrine stimuli, inflammation, and oxidative stress are critical in activating HSCs. The potential for liver fibrosis to reverse after removing the causative agent has heightened interest in developing antifibrotic therapies. Polyphenols, the secondary plant metabolites, have gained attention because of their health-beneficial properties, including well-recognized antioxidant and anti-inflammatory activities, in the setting of liver fibrosis. In this review, we present an overview of the mechanisms underlying liver fibrosis with a specific focus on the activation of resident HSCs. We highlight the therapeutic potential and promising role of natural polyphenols to mitigate liver fibrosis pathogenesis, focusing on HSCs activation. We also discuss the translational gap from preclinical findings to clinical treatments involved in natural polyphenols in liver fibrosis.

Hydrogel crosslinking modulates macrophages, fibroblasts, and their communication, during wound healing

Biomaterial wound dressings, such as hydrogels, interact with host cells to regulate tissue repair. This study investigates how crosslinking of gelatin-based hydrogels influences immune and stromal cell behavior and wound healing in female mice. We observe that softer, lightly crosslinked hydrogels promote greater cellular infiltration and result in smaller scars compared to stiffer, heavily crosslinked hydrogels. Using single-cell RNA sequencing, we further show that heavily crosslinked hydrogels increase inflammation and lead to the formation of a distinct macrophage subpopulation exhibiting signs of oxidative activity and cell fusion. Conversely, lightly crosslinked hydrogels are more readily taken up by macrophages and integrated within the tissue. The physical properties differentially affect macrophage and fibroblast interactions, with heavily crosslinked hydrogels promoting pro-fibrotic fibroblast activity that drives macrophage fusion through RANKL signaling. These findings suggest that tuning the physical properties of hydrogels can guide cellular responses and improve healing, offering insights for designing better biomaterials for wound treatment.

Exploring the role of granzyme B in subretinal fibrosis of age-related macular degeneration

Age-related macular degeneration (AMD), a prevalent and progressive degenerative disease of the macula, is the leading cause of blindness in elderly individuals in developed countries. The advanced stages include neovascular AMD (nAMD), characterized by choroidal neovascularization (CNV), leading to subretinal fibrosis and permanent vision loss. Despite the efficacy of anti-vascular endothelial growth factor (VEGF) therapy in stabilizing or improving vision in nAMD, the development of subretinal fibrosis following CNV remains a significant concern. In this review, we explore multifaceted aspects of subretinal fibrosis in nAMD, focusing on its clinical manifestations, risk factors, and underlying pathophysiology. We also outline the potential sources of myofibroblast precursors and inflammatory mechanisms underlying their recruitment and transdifferentiation. Special attention is given to the potential role of mast cells in CNV and subretinal fibrosis, with a focus on putative mast cell mediators, tryptase and granzyme B. We summarize our findings on the role of GzmB in CNV and speculate how GzmB may be involved in the pathological transition from CNV to subretinal fibrosis in nAMD. Finally, we discuss the advantages and drawbacks of animal models of subretinal fibrosis and pinpoint potential therapeutic targets for subretinal fibrosis.

Exosomes: compositions, biogenesis, and mechanisms in diabetic wound healing

Diabetic wounds are characterized by incomplete healing and delayed healing, resulting in a considerable global health care burden. Exosomes are lipid bilayer structures secreted by nearly all cells and express characteristic conserved proteins and parent cell-associated proteins. Exosomes harbor a diverse range of biologically active macromolecules and small molecules that can act as messengers between different cells, triggering functional changes in recipient cells and thus endowing the ability to cure various diseases, including diabetic wounds. Exosomes accelerate diabetic wound healing by regulating cellular function, inhibiting oxidative stress damage, suppressing the inflammatory response, promoting vascular regeneration, accelerating epithelial regeneration, facilitating collagen remodeling, and reducing scarring. Exosomes from different tissues or cells potentially possess functions of varying levels and can promote wound healing. For example, mesenchymal stem cell-derived exosomes (MSC-exos) have favorable potential in the field of healing due to their superior stability, permeability, biocompatibility, and immunomodulatory properties. Exosomes, which are derived from skin cellular components, can modulate inflammation and promote the regeneration of key skin cells, which in turn promotes skin healing. Therefore, this review mainly emphasizes the roles and mechanisms of exosomes from different sources, represented by MSCs and skin sources, in improving diabetic wound healing. A deeper understanding of therapeutic exosomes will yield promising candidates and perspectives for diabetic wound healing management.

Fibrocyte Phenotype of ENTPD1+CD55+ Cells and Its Association with Pain in Osteoarthritic Synovium

Osteoarthritis (OA) is a prevalent degenerative joint disorder characterized by cartilage erosion, structural changes, and inflammation. Synovial fibroblasts play a crucial role in OA pathophysiology, with abnormal fibroblastic cells contributing significantly to joint pathology. Fibrocytes, expressing markers of both hematopoietic and stromal cells, are implicated in inflammation and fibrosis, yet their marker and role in OA remain unclear. ENTPD1, an ectonucleotidase involved in purinergic signaling and expressed in specific fibroblasts in fibrotic conditions, led us to speculate that ENTPD1 plays a role in OA pathology by being expressed in fibrocytes. This study aimed to investigate the phenotype of ENTPD1+CD55+ and ENTPD1-CD55+ synovial fibroblasts in OA patients. Proteomic analysis revealed a distinct molecular profile in ENTPD1+CD55+ cells, including the upregulation of fibrocyte markers and extracellular matrix-related proteins. Pathway analysis suggested shared mechanisms between OA and rheumatoid arthritis. Correlation analysis revealed an association between ENTPD1+CD55+ fibrocytes and resting pain in OA. These findings highlight the potential involvement of ENTPD1 in OA pain and suggest avenues for targeted therapeutic strategies. Further research is needed to elucidate the underlying molecular mechanisms and validate potential therapeutic targets.

Single-cell epigenomic dysregulation of Systemic Sclerosis fibroblasts via CREB1/EGR1 axis in self-assembled human skin equivalents

Systemic sclerosis (SSc) is an autoimmune disease characterized by skin fibrosis, internal organ involvement and vascular dropout. We previously developed and phenotypically characterized an in vitro 3D skin-like tissue model of SSc, and now analyze the transcriptomic (scRNA-seq) and epigenetic (scATAC-seq) characteristics of this model at single-cell resolution. SSc 3D skin-like tissues were fabricated using autologous fibroblasts, macrophages, and plasma from SSc patients or healthy control (HC) donors. SSc tissues displayed increased dermal thickness and contractility, as well as increased α-SMA staining. Single-cell transcriptomic and epigenomic analyses identified keratinocytes, macrophages, and five populations of fibroblasts (labeled FB1 - 5). Notably, FB1 APOE-expressing fibroblasts were 12-fold enriched in SSc tissues and were characterized by high EGR1 motif accessibility. Pseudotime analysis suggests that FB1 fibroblasts differentiate from a TGF-β1-responsive fibroblast population and ligand-receptor analysis indicates that the FB1 fibroblasts are active in macrophage crosstalk via soluble ligands including FGF2 and APP. These findings provide characterization of the 3D skin-like model at single cell resolution and establish that it recapitulates subsets of fibroblasts and macrophage phenotypes observed in skin biopsies.

Deconvolution of synovial myeloid cell subsets across pathotypes and role of COL3A1+ macrophages in rheumatoid arthritis remission

Background

Monocyte/macrophage (Mo/Mp) is a critical cell population involved in immune modulation of rheumatoid synovitis (RA) across different pathotypes. This study aims to investigate the contribution of Mo/Mp clusters to RA activity, and the biological function of particular subtypes in RA remission.

Methods

We integrated single-cell RNA sequencing datasets from 4 published and 1 in-house studies using Liger selected by comparison. We estimated the abundance of Mo/Mp subtypes in bulk RNA-seq data from the 81 patients of the Pathobiology of Early Arthritis Cohort (PEAC) using deconvolution analysis. Correlations between Mo/Mp subtypes and RA clinical metrics were assessed. A particular cell type was identified using multicolor immunofluorescence and flow cytometry in vivo and successfully induced from a cell line in vitro. Potential immune modulation function of it was performed using immunohistochemical staining, adhesion assay, and RT-qPCR.

Results

We identified 8 Mo/Mp clusters. As a particular subtype among them, COL3A1+ Mp (CD68+, COL3A1+, ACTA2-) enriched in myeloid pathotype and negatively correlated with RA severity metrics in all pathotypes. Flow cytometry and multicolor immunofluorescence evidenced the enrichment and M2-like phenotype of COL3A1+ Mp in the myeloid pathotype. Further assays suggested that COL3A1+ Mp potentially attenuates RA severity via expressing anti-inflammatory cytokines, enhancing Mp adhesion, and forming a physical barrier at the synovial lining.

Conclusion

This study reported unexplored associations between different pathologies and myeloid cell subtypes. We also identified a fibroblast-and-M2-like cluster named COL3A1+ Mp, which potentially contributes to synovial immune homeostasis. Targeting the development of COL3A1+ Mp may hold promise for inducing RA remission.

Identification of unique genomic signatures in patients with fibromyalgia and chronic pain

Fibromyalgia (FM) is a chronic pain syndrome characterized by widespread pain. The pathophysiology of fibromyalgia is not clearly understood and there are no specific biomarkers available for accurate diagnosis. Here we define genomic signatures using high throughput RNA sequencing on 96 fibromyalgia and 93 control cases. Our findings revealed three major fibromyalgia-associated expression signatures. The first group included 43 patients with a signature enriched for gene expression associated with extracellular matrix and downregulation of RhoGDI signaling pathway. The second group included 30 patients and showed a profound reduction in the expression of inflammatory mediators with an increased expression of genes involved in the CLEAR signaling pathway. These results suggest defective tissue homeostasis associated with the extra-cellular matrix and cellular program that regulates lysosomal biogenesis and participates in macromolecule clearance in fibromyalgia. The third group of 17 FM patients showed overexpression of pathways that control acute inflammation and dysfunction of the global transcriptional process. The result of this study indicates that FM is a heterogeneous and complex disease. Further elucidation of these pathways will lead to the development of accurate diagnostic markers, and effective therapeutic options for fibromyalgia.

Cellular Transdifferentiation: A Crucial Mechanism of Fibrosis in Systemic Sclerosis

Systemic Sclerosis (SSc) is a systemic autoimmune disease of unknown etiology with a highly complex pathogenesis that despite extensive investigation is not completely understood. The clinical and pathologic manifestations of the disease result from three distinct processes: 1) Severe and frequently progressive tissue fibrosis causing exaggerated and deleterious accumulation of interstitial collagens and other extracellular matrix molecules in the skin and various internal organs; 2) extensive fibroproliferative vascular lesions affecting small arteries and arterioles causing tissue ischemic alterations; and 3) cellular and humoral immunity abnormalities with the production of numerous autoantibodies, some with very high specificity for SSc. The fibrotic process in SSc is one of the main causes of disability and high mortality of the disease. Owing to its essentially universal presence and the severity of its clinical effects, the mechanisms involved in the development and progression of tissue fibrosis have been extensively investigated, however, despite intensive investigation, the precise molecular mechanisms have not been fully elucidated. Several recent studies have suggested that cellular transdifferentiation resulting in the phenotypic conversion of various cell types into activated myofibroblasts may be one important mechanism. Here, we review the potential role that cellular transdifferentiation may play in the development of severe and often progressive tissue fibrosis in SSc.

The miR-183/96/182 cluster is a checkpoint for resident immune cells and shapes the cellular landscape of the cornea

PURPOSE

The conserved miR-183/96/182 cluster (miR-183C) regulates both corneal sensory innervation and corneal resident immune cells (CRICs). This study is to uncover its role in CRICs and in shaping the corneal cellular landscape at a single-cell (sc) level.

METHODS

Corneas of naïve, young adult [2 and 6 months old (mo)], female miR-183C knockout (KO) mice and wild-type (WT) littermates were harvested and dissociated into single cells. Dead cells were removed using a Dead Cell Removal kit. CD45+ CRICs were enriched by Magnetic Activated Cell Sorting (MACS). scRNA libraries were constructed and sequenced followed by comprehensive bioinformatic analyses.

RESULTS

The composition of major cell types of the cornea stays relatively stable in WT mice from 2 to 6 mo, however the compositions of subtypes of corneal cells shift with age. Inactivation of miR-183C disrupts the stability of the major cell-type composition and age-related transcriptomic shifts of subtypes of corneal cells. The diversity of CRICs is enhanced with age. Naïve mouse cornea contains previously-unrecognized resident fibrocytes and neutrophils. Resident macrophages (ResMφ) adopt cornea-specific function by expressing abundant extracellular matrix (ECM) and ECM organization-related genes. Naïve cornea is endowed with partially-differentiated proliferative ResMφ and contains microglia-like Mφ. Resident lymphocytes, including innate lymphoid cells (ILCs), NKT and γδT cells, are the major source of innate IL-17a. miR-183C limits the diversity and polarity of ResMφ.

CONCLUSION

miR-183C serves as a checkpoint for CRICs and imposes a global regulation of the cellular landscape of the cornea.

Macrophage Implication in IPF: Updates on Immune, Epigenetic, and Metabolic Pathways

Idiopathic pulmonary fibrosis (IPF) is a lethal interstitial lung disease of unknown etiology with a poor prognosis. It is a chronic and progressive disease that has a distinct radiological and pathological pattern from common interstitial pneumonia. The use of immunosuppressive medication was shown to be completely ineffective in clinical trials, resulting in years of neglect of the immune component. However, recent developments in fundamental and translational science demonstrate that immune cells play a significant regulatory role in IPF, and macrophages appear to be among the most crucial. These highly plastic cells generate multiple growth factors and mediators that highly affect the initiation and progression of IPF. In this review, we will provide an update on the role of macrophages in IPF through a systemic discussion of various regulatory mechanisms involving immune receptors, cytokines, metabolism, and epigenetics.

Induced pluripotent and CD34+ stem cell derived myeloid cells display differential responses to particle and dust mite exposure

Myeloid cells form an essential component of initial responses to environmental hazards and toxic exposures. The ability to model these responses in vitro is central to efforts tasked with identifying hazardous materials and understanding mechanisms of injury and disease. Induced pluripotent stem cell (iPSC) derived cells have been suggested as alternatives to more established primary cell testing systems for these purposes. iPSC derived macrophage and dendritic like cells were compared to CD34+ haematopoietic stem cell derived populations using transcriptomic analysis. Using single cell sequencing-based characterisation of iPSC derived myeloid cells, we identified transitional, mature and M2 like macrophages as well as dendritic like antigen presenting cells and fibrocytes. Direct transcriptomic comparisons between iPSC and CD34+ cell derived populations revealed higher expression of myeloid differentiation genes such as MNDA, CSF1R and CSF2RB in CD34+ cells, while iPSC populations had higher fibroblastic and proliferative markers. Exposure of differentiated macrophage populations to nanoparticle alone or in combination with dust mite, resulted in differential gene expression on combination only, with responses markedly absent in iPSC compared to CD34+ derived cells. The lack of responsiveness in iPSC derived cells may be attributable to lower levels of dust mite component receptors CD14, TLR4, CLEC7A and CD36. In summary, iPSC derived myeloid cells display typical characteristics of immune cells but may lack a fully mature phenotype to adequately respond to environmental exposures.

Mammalian organ regeneration in spiny mice

Fibrosis-driven solid organ failure is a major world-wide health burden with few therapeutic options. Spiny mice (genus: Acomys) are terrestrial mammals that regenerate severe skin wounds without fibrotic scars to evade predators. Recent studies have shown that spiny mice also regenerate acute ischemic and traumatic injuries to kidney, heart, spinal cord, and skeletal muscle. A common feature of this evolved wound healing response is a lack of formation of fibrotic scar tissue that degrades organ function, inhibits regeneration, and leads to organ failure. Complex tissue regeneration is an extremely rare property among mammalian species. In this article, we discuss the evidence that Acomys represents an emerging model organism that offers a unique opportunity for the biomedical community to investigate and clinically translate molecular mechanisms of scarless wound healing and regeneration of organ function in a mammalian species.

Fibroblasts are a site of murine cytomegalovirus lytic replication and Stat1-dependent latent persistence in vivo

To date, no herpesvirus has been shown to latently persist in fibroblastic cells. Here, we show that murine cytomegalovirus, a β-herpesvirus, persists for the long term and across organs in PDGFRα-positive fibroblastic cells, with similar or higher genome loads than in the previously known sites of murine cytomegalovirus latency. Whereas murine cytomegalovirus gene transcription in PDGFRα-positive fibroblastic cells is almost completely silenced at 5 months post-infection, these cells give rise to reactivated virus ex vivo, arguing that they support latent murine cytomegalovirus infection. Notably, PDGFRα-positive fibroblastic cells also support productive virus replication during primary murine cytomegalovirus infection. Mechanistically, Stat1-deficiency promotes lytic infection but abolishes latent persistence of murine cytomegalovirus in PDGFRα-positive fibroblastic cells in vivo. In sum, fibroblastic cells have a dual role as a site of lytic murine cytomegalovirus replication and a reservoir of latent murine cytomegalovirus in vivo and STAT1 is required for murine cytomegalovirus latent persistence in vivo.

Old age promotes retinal fibrosis in choroidal neovascularization through circulating fibrocytes and profibrotic macrophages

BACKGROUND

Retinal fibrosis affects 40-70% of neovascular age-related macular degeneration patients. This study investigated the effect of ageing on subretinal fibrosis secondary to choroidal neovascularization and the mechanism of action.

METHODS

Subretinal fibrosis was induced in young (2.5-month) and aged (15-16-month) C57BL/6J mice using the two-stage laser protocol. Five and 30 days later, eyes were collected and stained for CD45 and collagen-1 and observed by confocal microscopy. Fibrocytes (CD45+collagen-1+) were detected in the bone marrow (BM), blood and fibrotic lesions by flow cytometry and confocal microscopy, respectively. BM-derived macrophages (BMDMs) were cultured from young and aged mice with or without TGF-β1 (10 ng/mL) treatment. The expression of mesenchymal marker αSMA (Acta2), fibronectin (Fn1) and collagen-1 (Col1a1) was examined by qPCR and immunocytochemistry, whereas cytokine/chemokine production was measured using the Luminex multiplex cytokine assay. BM were transplanted from 22-month (Ly5.2) aged mice into 2.5-month (Ly5.1) young mice and vice versa. Six weeks later, subretinal fibrosis was induced in recipient mice and eyes were collected for evaluation of fibrotic lesion size.

RESULTS

Under normal conditions, the number of circulating fibrocytes (CD45+collagen-1+) and the expression levels of Tgfb1, Col1a1, Acta2 and Fn1 in BMDMs were significantly higher in aged mice compared to young mice. Induction of subretinal fibrosis significantly increased the number of circulating fibrocytes, enhanced the expression of Col1a1, Acta2 and Fn1 and the production of soluble urokinase plasminogen activator surface receptor (suPAR) but decreased the production of CXCL10 in BMDMs. BMDMs from aged subretinal fibrosis mice produced significantly higher levels of VEGF, angiopoietin-2 and osteopontin than cells from young subretinal fibrosis mice. The subretinal fibrotic lesion in 15-16-month aged mice was 62% larger than that in 2.5-month young mice. The lesion in aged mice contained a significantly higher number of fibrocytes compared to that in young mice. The number of circulating fibrocytes positively correlated with the size of subretinal fibrotic lesion. Transplantation of BM from aged mice significantly increased subretinal fibrosis in young mice.

CONCLUSIONS

A retina-BM-blood-retina pathway of fibrocyte/macrophage recruitment exists during retinal injury. Ageing promotes subretinal fibrosis through higher numbers of circulating fibrocytes and profibrotic potential of BM-derived macrophages.

Mesenchymal stromal cells in tumor microenvironment remodeling of BCR-ABL negative myeloproliferative diseases

Chronic myeloproliferative neoplasms encompass the BCR-ABL1-negative neoplasms polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF). These are characterized by calreticulin (CALR), myeloproliferative leukemia virus proto-oncogene (MPL) and the tyrosine kinase Janus kinase 2 (JAK2) mutations, eventually establishing a hyperinflammatory tumor microenvironment (TME). Several reports have come to describe how constitutive activation of JAK-STAT and NFκB signaling pathways lead to uncontrolled myeloproliferation and pro-inflammatory cytokines secretion. In such a highly oxidative TME, the balance between Hematopoietic Stem Cells (HSCs) and Mesenchymal Stromal Cells (MSCs) has a crucial role in MPN development. For this reason, we sought to review the current literature concerning the interplay between HSCs and MSCs. The latter have been reported to play an outstanding role in establishing of the typical bone marrow (BM) fibrotic TME as a consequence of the upregulation of different fibrosis-associated genes including PDGF- β upon their exposure to the hyperoxidative TME characterizing MPNs. Therefore, MSCs might turn to be valuable candidates for niche-targeted targeting the synthesis of cytokines and oxidative stress in association with drugs eradicating the hematopoietic clone.

Current Perspectives on Nucleus Pulposus Fibrosis in Disc Degeneration and Repair

A growing body of evidence in humans and animal models indicates an association between intervertebral disc degeneration (IDD) and increased fibrotic elements in the nucleus pulposus (NP). These include enhanced matrix turnover along with the abnormal deposition of collagens and other fibrous matrices, the emergence of fibrosis effector cells, such as macrophages and active fibroblasts, and the upregulation of the fibroinflammatory factors TGF-β1 and IL-1/-13. Studies have suggested a role for NP cells in fibroblastic differentiation through the TGF-βR1-Smad2/3 pathway, inflammatory activation and mechanosensing machineries. Moreover, NP fibrosis is linked to abnormal MMP activity, consistent with the role of matrix proteases in regulating tissue fibrosis. MMP-2 and MMP-12 are the two main profibrogenic markers of myofibroblastic NP cells. This review revisits studies in the literature relevant to NP fibrosis in an attempt to stratify its biochemical features and the molecular identity of fibroblastic cells in the context of IDD. Given the role of fibrosis in tissue healing and diseases, the perspective may provide new insights into the pathomechanism of IDD and its management.