Adipocytic Progenitor Cells Give Rise to Pathogenic Myofibroblasts: Adipocyte-to-Mesenchymal Transition and Its Emerging Role in Fibrosis in Multiple Organs.

PURPOSE OF REVIEW: Adipocytes have recently been shown to be able to reprogram to a myofibroblastic phenotype in a process termed adipocyte mesenchymal transition (AMT). This review seeks to discuss the relevance of this process to disease and explore its mechanisms. RECENT FINDINGS: AMT occurs in multiple organs and diseases, transdifferentiation goes through a precursor cell and there is a reversible process that can be influenced by metabolic stress, myeloid cells, immune dysregulation, and pharmacological intervention. AMT is a newly appreciated and highly relevant process in multiple forms of fibrosis. Targeting AMT may serve as a novel method of treating fibrosis.

The roles of dermal white adipose tissue loss in scleroderma skin fibrosis.

PURPOSE OF REVIEW: Dermal white adipose tissue (DWAT) is distinct from subcutaneous white adipose tissue and is lost in scleroderma skin fibrosis. The roles of DWAT loss in scleroderma skin fibrosis have not been well understood, and here we discuss recent findings that begin to provide insight into the multiple mechanisms involved. RECENT FINDINGS: The DWAT loss in part reflects the direct contribution of DWAT cells to the fibrotic tissue, with the reprogramming of adipocytes to myofibroblasts. The DWAT contains reparative adipose-derived stromal cells and expresses antifibrotic cytokines such as adiponectin, and the loss of these skin-protective mechanisms with DWAT loss further contributes to skin fibrosis and injury. SUMMARY: Potentially, halting or reversing the transdifferentiation of adipocytes to myofibroblasts along with improving survival of reparative adipose-derived stromal cells (ADSCs) and expression of antifibrotic cytokines may be effective therapeutic avenues.

Animal models of scleroderma: recent progress.

PURPOSE OF REVIEW: We discuss recent advances in evaluating and optimizing animal models of systemic sclerosis (SSc). Such models could be of value for illuminating etiopathogenesis using hypothesis-testing experimental approaches, for developing effective disease-modifying therapies, and for uncovering clinically relevant biomarkers. RECENT FINDINGS: We describe recent advances in previously reported and novel animal models of SSc. The limitations of each animal model and their ability to recapitulate the pathophysiology of recognized molecular subsets of SSc are discussed. We highlight attrition of dermal white adipose tissue as a consistent pathological feature of dermal fibrosis in mouse models, and its relevance to SSc-associated cutaneous fibrosis. SUMMARY: Several animal models potentially useful for studying SSc pathogenesis have been described. Recent studies highlight particular strengths and weaknesses of selected models in recapitulating distinct features of the human disease. When used in the appropriate experimental setting, and in combination, these models singly and together provide a powerful set of in-vivo tools to define underlying mechanisms of disease and to develop and evaluate effective antifibrotic therapies.

Systemic sclerosis and silica exposure: a rare association in a large Brazilian cohort.

The objective of this study is to describe the characteristics of patients with Erasmus syndrome (ES) in a large SSc Brazilian cohort. Nine hundred and forty-seven SSc patients attended at the Scleroderma Outpatient Clinic at two academic medical centers in Brazil and classified as SSc according to the ACR/EULAR criteria were retrospectively studied. Information on demographics, clinical, and laboratory features was obtained by chart review. ES patients had their HLA class II characterized by PCR-SSO method as available. Among the 947 SSc patients studied, nine (0.9 %) had ES. These ES patients were predominantly male (78 %) and smokers (68 %) and presented diffuse SSc (67 %). Mean time of occupational exposure to silica was 13.7 years, with mean age at onset of 47 years. Previous history of tuberculosis was referred by 33 % of the ES patients. All the ES patients presented Raynaud's phenomenon, esophageal involvement, and interstitial lung disease (ILD). Antinuclear antibodies were present in all the ES patients, while anti-topoisomerase I was positive in 44 % and no patient had anticentromere antibody. Three different HLA-DQB alleles (0506, 0305, and 0303) were observed. Compared to non-ES cases, patients with ES were associated with male gender (p < 0.001), diffuse SSc (p < 0.05), ILD (p < 0.05), positive anti-topoisomerase I antibodies (p < 0.05), and death (p < 0.05). Multivariate analysis did not confirm that silicosis is an independent risk factor for SSc. To conclude, ES was rare in this large SSc cohort, although associated with a bad prognosis.

Utility of factor D and other alternative complement factors as biomarkers in systemic sclerosis-associated pulmonary arterial hypertension (SSc-PAH).

BACKGROUND: Activation of the complement cascade is thought to play a role in scleroderma vasculopathy. We previously showed that complement factor D was elevated in patients with limited cutaneous SSc and pulmonary arterial hypertension (PAH). In this study, we sought to assess multiple relevant components of the complement cascade to determine if they are altered in SSc-PAH, as well as their potential utility as biomarkers of disease severity and progression. METHODS: Complement components (n = 14) were measured using multiplex assays in 156 patients with SSc-PAH from a multi-site repository and were compared to 33 patients with SSc without PAH, and 40 healthy controls. Data were evaluated for correlations between complement levels, right heart catheterization measures, and clinical endpoints including 6-minute walk distance. To assess complement longitudinally, serum complement levels were assayed at 0, 4, 12, 24, 36 and 48 weeks in 52 SSc-PAH patients who participated in a prior clinical trial. RESULTS: We found that factor D was significantly elevated in SSc-PAH compared to SSc without PAH (p < 0.0001) and was highly sensitive and specific for SSc-PAH (AUC=0.82, p < 0.001). In SSc-PAH patients, alterations in factor H, C4, and factor D were associated with measures of PAH disease severity including right heart catheterization measurements (cardiac output, right atrial pressure, and VO2 max), survival, and 6-minute walk distance. No significant changes in complement levels or clinical associations were seen over time or associated with treatment in the longitudinal clinical trial study. CONCLUSION: Our work confirms prior studies demonstrating a role for complement activation in SSc vascular disease and elevations of factor D in a large SSc-PAH population. Further, factor H and other complement factors are associated with severity of PAH including mortality. Taken together, these findings suggest that the alternative complement pathway plays a role in SSc-PAH pathogenesis and may serve as a biomarker to inform diagnosis and prognosis.

Systemic sclerosis sine scleroderma: distinct features in a large Brazilian cohort.

OBJECTIVE: Systemic sclerosis sine scleroderma (ssSSc) is an infrequent SSc variant characterized by visceral and immunological manifestations of SSc in the absence of clinically detectable skin involvement. We sought to delineate the characteristics of ssSSc in a cohort of Brazilian patients and contrast them with those in the literature. METHODS: SSc patients seen at two academic medical centres in Brazil were retrospectively analysed. Patients were classified as ssSSc if they presented with RP, positive ANAs and at least one visceral involvement typical of SSc in the absence of skin thickening. Demographics, clinical and laboratory data were obtained by chart review. Literature review was performed by searching available original studies up until June 2012. RESULTS: Among the 947 consecutive patients with SSc, 79 (8.3%) were classified as ssSSc. Oesophagus was the most frequently affected organ (83.1%), followed by pulmonary involvement (63.2%). Compared with the limited cutaneous form of SSc, telangiectasia was the only variable significantly different after multivariate logistic regression analyses (odds ratio 0.46; 95% CI 0.27, 0.81). Compared with the diffuse cutaneous form of SSc, multivariate analyses revealed that ssSSc patients were less likely to be male (odds ratio 0.15; 95% CI 0.04, 0.57), have digital ulcers (odds ratio 0.26; 95% CI 0.13, 0.51) or anti-Scl70 antibodies (odds ratio 0.19; 95% CI 0.07, 0.55) and less frequently treated with CYC (odds ratio 0.23; 95% CI 0.12, 0.43). These features were comparable to those in the published literature. CONCLUSION: In this series, patients with ssSSc had a relatively mild disease with good prognosis.

Circulating CTRP9 Is Associated With Severity of Systemic Sclerosis-Associated Interstitial Lung Disease.

OBJECTIVE: While interstitial lung disease (ILD) is the leading cause of morbidity and mortality in systemic sclerosis (SSc), there remains a paucity of predictive markers to assess disease progression. We previously demonstrated that adipose tissue metabolism and adipokine homeostasis is dysregulated in SSc. The present study was undertaken to determine the association and predictive ability of the novel adipokine C1q/tumor necrosis factor-related protein 9 (CTRP9) for SSc-associated ILD. METHODS: We performed a retrospective longitudinal study utilizing the Northwestern Scleroderma Program Patient Registry and Biorepository. Serum levels of CTRP9 were measured in 110 SSc patients at baseline, and demographic, clinical, and pulmonary function test data were collected in 12-month intervals to 48 months. Longitudinal trajectory of forced vital capacity percent predicted (FVC%) was used as a primary outcome measure. We utilized a mixed model to compare trajectories of lung function by CTRP9 groups and performed latent trajectory analysis to accommodate for heterogeneity. RESULTS: In cross-sectional analysis, elevated circulating CTRP9 was associated with significantly lower FVC% at baseline (72% ± 17 versus 80% ± 18; P = 0.02) and 48 months (68 ± 19 versus 84 ± 18; P = 0.001). In mixed model analysis, high CTRP9 was associated with worse lung function but not with a different trajectory (P = 0.23). In contrast, low CTRP9 identified patients with stability of lung disease with reasonable accuracy (sensitivity 73%). Latent trajectory analysis confirmed the association of lower CTRP9 with higher FVC%. CONCLUSION: Higher circulating CTRP9 associated with worse pulmonary function, while low CTRP9 identified patients with lung disease stability over time. These findings suggest that CTRP9 may be a potential biomarker in SSc-associated ILD.

Thy-1 plays a pathogenic role and is a potential biomarker for skin fibrosis in scleroderma.

Thy-1 (CD90) is a well-known marker of fibroblasts implicated in organ fibrosis, but its contribution to skin fibrosis remains unknown. We examined Thy-1 expression in scleroderma skin and its potential role as a biomarker and pathogenic factor in animal models of skin fibrosis. Skin from patients with systemic sclerosis demonstrated markedly elevated Thy-1 expression compared with controls, colocalized with fibroblast activator protein in the deep dermis, and correlated with the severity of skin involvement (modified Rodnan skin score). Serial imaging of skin from Thy-1 yellow fluorescent protein reporter mice by IVIS showed an increase in Thy-1 expression that correlated with onset and progression of fibrosis. In contrast to lung fibrosis, Thy-1-KO mice had attenuated skin fibrosis in both bleomycin and tight skin-1 murine models. Moreover, Thy-1 regulated key pathogenic pathways involved in fibrosis, including inflammation, myofibroblast differentiation, apoptosis, and multiple additional canonical fibrotic pathways. Therefore, although Thy-1 deficiency leads to exacerbated lung fibrosis, in skin it is protective. Moreover, Thy-1 may serve as a longitudinal marker to assess skin fibrosis.

PLG nanoparticles target fibroblasts and MARCO+ monocytes to reverse multiorgan fibrosis.

Systemic sclerosis (SSc) is a chronic, multisystem orphan disease with a highly variable clinical course, high mortality rate, and a poorly understood complex pathogenesis. We have identified an important role for a subpopulation of monocytes and macrophages characterized by surface expression of the scavenger receptor macrophage receptor with collagenous structure (MARCO) in chronic inflammation and fibrosis in SSc and in preclinical disease models. We show that MARCO+ monocytes and macrophages accumulate in lesional skin and lung in topographic proximity to activated myofibroblasts in patients with SSc and in the bleomycin-induced mouse model of SSc. Short-term treatment of mice with a potentially novel nanoparticle, poly(lactic-co-glycolic) acid (PLG), which is composed of a carboxylated, FDA-approved, biodegradable polymer and modulates activation and trafficking of MARCO+ inflammatory monocytes, markedly attenuated bleomycin-induced skin and lung inflammation and fibrosis. Mechanistically, in isolated cells in culture, PLG nanoparticles inhibited TGF-dependent fibrotic responses in vitro. Thus, MARCO+ monocytes are potent effector cells of skin and lung fibrosis and can be therapeutically targeted in SSc using PLG nanoparticles.

Fibroblast A20 governs fibrosis susceptibility and its repression by DREAM promotes fibrosis in multiple organs.

In addition to autoimmune and inflammatory diseases, variants of the TNFAIP3 gene encoding the ubiquitin-editing enzyme A20 are also associated with fibrosis in systemic sclerosis (SSc). However, it remains unclear how genetic factors contribute to SSc pathogenesis, and which cell types drive the disease due to SSc-specific genetic alterations. We therefore characterize the expression, function, and role of A20, and its negative transcriptional regulator DREAM, in patients with SSc and disease models. Levels of A20 are significantly reduced in SSc skin and lungs, while DREAM is elevated. In isolated fibroblasts, A20 mitigates ex vivo profibrotic responses. Mice haploinsufficient for A20, or harboring fibroblasts-specific A20 deletion, recapitulate major pathological features of SSc, whereas DREAM-null mice with elevated A20 expression are protected. In DREAM-null fibroblasts, TGF-ß induces the expression of A20, compared to wild-type fibroblasts. An anti-fibrotic small molecule targeting cellular adiponectin receptors stimulates A20 expression in vitro in wild-type but not A20-deficient fibroblasts and in bleomycin-treated mice. Thus, A20 has a novel cell-intrinsic function in restraining fibroblast activation, and together with DREAM, constitutes a critical regulatory network governing the fibrotic process in SSc. A20 and DREAM represent novel druggable targets for fibrosis therapy.

Pathological pulmonary vascular remodeling is induced by type V collagen in a model of scleroderma.

OBJECTIVE: The pulmonary vascular remodeling in systemic sclerosis (SSc) is poorly understood and animal models are lacking. Type V collagen (COLV) is elevated in SSc and is implicated in the pathogenesis, and immunization with human COLV induces SSc-like skin and lung changes in rabbits and mice. Here we tested the hypothesis that COLV immunization will induce pathological and functional changes that phenocopy SSc-associated pulmonary vascular disease. METHODS: Pulmonary vascular changes in rabbits immunized with human COLV were extensively characterized by a combination of histology, electron microscopy and immunohistochemistry. Physiologic changes induced by COLV in explanted pulmonary artery rings were evaluated. The pattern of histopathologic alterations and gene expression induced in immunized rabbits were compared to those in SSc patients. RESULTS: COLV immunization was accompanied by striking pulmonary vascular abnormalities, characterized by reduced capillary density, perivascular inflammation, endothelial cell injury and collagen accumulation, that closely phenocopy changes seen in SSc patients. Moreover, pulmonary arteries from immunized rabbits showed impaired ex vivo vascular relaxation. Expression of COL5A2 was significantly increased in the lungs from immunized rabbits (p = 0.02), as well as in patients with SSc (P = 0.02). CONCLUSION: COLV immunity in rabbits is associated with marked vascular remodeling in the lung that phenocopies early-stage human SSc-associated pulmonary vascular disease. COLV immunization therefore represents a novel approach to model SSc pulmonary vascular pathology. Moreover, our findings suggest that COLV might represent a novel pathogenic autoantigen in SSc and future studies with the present model should be developed for possible association with PAH.

Targeting CD38-dependent NAD+ metabolism to mitigate multiple organ fibrosis.

The processes underlying synchronous multiple organ fibrosis in systemic sclerosis (SSc) remain poorly understood. Age-related pathologies are associated with organismal decline in nicotinamide adenine dinucleotide (NAD+) that is due to dysregulation of NAD+ homeostasis and involves the NADase CD38. We now show that CD38 is upregulated in patients with diffuse cutaneous SSc, and CD38 levels in the skin associate with molecular fibrosis signatures, as well as clinical fibrosis scores, while expression of key NAD+-synthesizing enzymes is unaltered. Boosting NAD+ via genetic or pharmacological CD38 targeting or NAD+ precursor supplementation protected mice from skin, lung, and peritoneal fibrosis. In mechanistic experiments, CD38 was found to reduce NAD+ levels and sirtuin activity to augment cellular fibrotic responses, while inhibiting CD38 had the opposite effect. Thus, we identify CD38 upregulation and resulting disrupted NAD+ homeostasis as a fundamental mechanism driving fibrosis in SSc, suggesting that CD38 might represent a novel therapeutic target.

The JAK/STAT pathway is activated in systemic sclerosis and is effectively targeted by tofacitinib.

Rationale: Fibrosis leads to failure of the skin, lungs, and other organs in systemic sclerosis; accounts for substantial morbidity and mortality; and lacks effective therapy. Myofibroblast activation underlies organ fibrosis, but the key extracellular cues driving persistence of the process remain incompletely characterized. Objectives: The objectives were to evaluate activation of the IL6/JAK/STAT axis associated with fibrosis in skin and lung biopsies from systemic sclerosis patients and effects of the Food and Drug Administration-approved JAK/STAT inhibitor, tofacitinib, on skin and lung fibrosis in animal models. Methods: Bioinformatic analysis showed that IL6/JAK/STAT3 and tofacitinib gene signatures were aberrant in biopsies from systemic sclerosis patients in four independent cohorts. The results were confirmed by JAK and STAT3 phosphorylation in both skin and lung biopsies from patients with systemic sclerosis. Furthermore, treatment of mice with the selective JAK inhibitor tofacitinib not only prevented bleomycin-induced skin and lung fibrosis but also reduced skin fibrosis in TSK1/+ mice. Conclusion: These findings implicate the JAK/STAT pathway in systemic sclerosis skin and lung fibrosis and identify tofacitinib as a potential antifibrotic agent for the treatment of systemic sclerosis and other fibrotic diseases.

High-throughput quantitative histology in systemic sclerosis skin disease using computer vision.

BACKGROUND: Skin fibrosis is the clinical hallmark of systemic sclerosis (SSc), where collagen deposition and remodeling of the dermis occur over time. The most widely used outcome measure in SSc clinical trials is the modified Rodnan skin score (mRSS), which is a semi-quantitative assessment of skin stiffness at seventeen body sites. However, the mRSS is confounded by obesity, edema, and high inter-rater variability. In order to develop a new histopathological outcome measure for SSc, we applied a computer vision technology called a deep neural network (DNN) to stained sections of SSc skin. We tested the hypotheses that DNN analysis could reliably assess mRSS and discriminate SSc from normal skin. METHODS: We analyzed biopsies from two independent (primary and secondary) cohorts. One investigator performed mRSS assessments and forearm biopsies, and trichrome-stained biopsy sections were photomicrographed. We used the AlexNet DNN to generate a numerical signature of 4096 quantitative image features (QIFs) for 100 randomly selected dermal image patches/biopsy. In the primary cohort, we used principal components analysis (PCA) to summarize the QIFs into a Biopsy Score for comparison with mRSS. In the secondary cohort, using QIF signatures as the input, we fit a logistic regression model to discriminate between SSc vs. control biopsy, and a linear regression model to estimate mRSS, yielding Diagnostic Scores and Fibrosis Scores, respectively. We determined the correlation between Fibrosis Scores and the published Scleroderma Skin Severity Score (4S) and between Fibrosis Scores and longitudinal changes in mRSS on a per patient basis. RESULTS: In the primary cohort (n = 6, 26 SSc biopsies), Biopsy Scores significantly correlated with mRSS (R = 0.55, p = 0.01). In the secondary cohort (n = 60 SSc and 16 controls, 164 biopsies; divided into 70% training and 30% test sets), the Diagnostic Score was significantly associated with SSc-status (misclassification rate = 1.9% [training], 6.6% [test]), and the Fibrosis Score significantly correlated with mRSS (R = 0.70 [training], 0.55 [test]). The DNN-derived Fibrosis Score significantly correlated with 4S (R = 0.69, p = 3 × 10- 17). CONCLUSIONS: DNN analysis of SSc biopsies is an unbiased, quantitative, and reproducible outcome that is associated with validated SSc outcomes.

Tumor Necrosis Factor Induces Obliterative Pulmonary Vascular Disease in a Novel Model of Connective Tissue Disease-Associated Pulmonary Arterial Hypertension.

OBJECTIVE: Connective tissue disease (CTD)-associated pulmonary arterial hypertension (PAH) is the second most common etiology of PAH and carries a poor prognosis. Recently, it has been shown that female human tumor necrosis factor (TNF)-transgenic (Tg) mice die of cardiopulmonary disease by 6 months of age. This study was undertaken to characterize this pathophysiology and assess its potential as a novel model of CTD-PAH. METHODS: Histologic analysis was performed on TNF-Tg and wild-type (WT) mice to characterize pulmonary vascular and right ventricular (RV) pathology (n = 40 [4-5 mice per group per time point]). Mice underwent right-sided heart catheterization (n = 29) and micro-computed tomographic angiography (n = 8) to assess vascular disease. Bone marrow chimeric mice (n = 12), and anti-TNF-treated mice versus placebo-treated mice (n = 12), were assessed. RNA sequencing was performed on mouse lung tissue (n = 6). RESULTS: TNF-Tg mice displayed a pulmonary vasculopathy marked by collagen deposition (P < 0.001) and vascular occlusion (P < 0.001) with associated RV hypertrophy (P < 0.001) and severely increased RV systolic pressure (mean ± SD 75.1 ± 19.3 mm Hg versus 26.7 ± 1.7 mm Hg in WT animals; P < 0.0001). TNF-Tg mice had increased α-smooth muscle actin (α-SMA) staining, which corresponded to proliferation and loss of von Willebrand factor (vWF)-positive endothelial cells (P < 0.01). There was an increase in α-SMA-positive, vWF-positive cells (P < 0.01), implicating endothelial-mesenchymal transition. Bone marrow chimera experiments revealed that mesenchymal but not bone marrow-derived cells are necessary to drive this process. Treatment with anti-TNF therapy halted the progression of disease. This pathology closely mimics human CTD-PAH, in which patient lungs demonstrate increased TNF signaling and significant similarities in genomic pathway dysregulation. CONCLUSION: The TNF-Tg mouse represents a novel model of CTD-PAH, recapitulates key disease features, and can serve as a valuable tool for discovery and assessment of therapeutics.

Soft tissue necrosis in patients treated with transoral robotic surgery and postoperative radiotherapy: preliminary results.

OBJECTIVE: Postoperative radiotherapy (PORT) is indicated in almost two-thirds of patients treated with transoral robotic surgery (TORS) for head and neck tumors. The aim of this study was to quantify the toxicity profile of patients treated with PORT after TORS in oropharyngeal and supraglottic laryngeal cancer focusing on soft tissue necrosis (STN). METHODS: We retrospectively reviewed 28 patients. Acute and late toxicity were examined. Incidence and severity of STN were recorded. RESULTS: No patient experienced acute grade 3 skin or mucosal toxicity; 1 patient had grade 3 dysphagia. At 12 months, no evaluated patient required enteral nutrition and 2 patients had tracheostomy. STN occurred in 4 (14%) patients: 3 out of 4 (75%) patients with STN had diabetes, whereas 6 out of 13 (25%) patients without STN had diabetes (p = 0.05). CONCLUSION: We found an acceptable toxicity profile of PORT performed after a TORS procedure. Diabetes mellitus might be a risk factor for STN.

Adipocyte-specific Repression of PPAR-gamma by NCoR Contributes to Scleroderma Skin Fibrosis.

BACKGROUND: A pivotal role for adipose tissue homeostasis in systemic sclerosis (SSc) skin fibrosis is increasingly recognized. The nuclear receptor PPAR-γ is the master regulator of adipogenesis. Peroxisome proliferator activated receptor-γ (PPAR-γ) has antifibrotic effects by blocking transforming growth factor-ß (TGF-ß) and is dysregulated in SSc. To unravel the impact of dysregulated PPAR-γ in SSc, we focused on nuclear corepressor (NCoR), which negatively regulates PPAR-γ activity and suppresses adipogenesis. METHODS: An NCoR-regulated gene signature was measured in the SSc skin transcriptome. Experimental skin fibrosis was examined in mice with adipocyte-specific NCoR ablation. RESULTS: SSc skin biopsies demonstrated deregulated NCoR signaling. A 43-gene NCoR gene signature showed strong positive correlation with PPAR-γ signaling (R = 0.919, p < 0.0001), whereas negative correlations with TGF-ß signaling (R = - 0.796, p < 0.0001) and the modified Rodnan skin score (R = - 0.49, p = 0.004) were found. Mice with adipocyte-specific NCoR ablation demonstrated significant protection from experimental skin fibrosis and inflammation. The protective effects were mediated primarily through endogenous PPAR-γ. CONCLUSIONS: Our results implicate, for the first time, to our knowledge, deregulated NCoR/PPAR-γ pathways in SSc, and they support a role of adipocyte modulation of skin fibrosis. Pharmacologic restoration of NCoR/PPAR-γ signaling may represent a novel strategy to control skin fibrosis in SSc.

The non-neuronal cyclin-dependent kinase 5 is a fibrotic mediator potentially implicated in systemic sclerosis and a novel therapeutic target.

The mechanisms underlying persistent fibroblast activation and myofibroblast phenoconversion in underlying multi-organ fibrosis in systemic sclerosis (SSc) remain incompletely understood, hindering effective therapies to slow or reverse the process. Cyclin-dependent kinase 5 (CDK5) is a pleiotropic member of the CDK family originally identified in neuronal cells. In contrast to other CDKs, CDK5 activity depends on its CDK5R1 subunit p35. Here we demonstrate that expression of p35 and CDK5 activity are induced by TGF-ß in fibroblasts and adipocytic cell types. Levels of p35 are markedly elevated in both SSc skin biopsies and explanted SSc fibroblasts, as well as in fibrotic skin in mice. Ectopic p35 and CDK5 suppressed adipogenic markers while stimulating collagen production and myofibroblast markers, whereas RNAi-mediated CDK5 knockdown abrogated TGF-ß fibrotic responses in a Smad-independent manner. Pharmacological inhibitors of CDK5 likewise prevented and reversed TGF-ß responses in fibroblast monolayers and in ex vivo human skin organ cultures, ameliorated collagen overproduction in SSc fibroblasts, and prevented and reversed skin fibrosis in two distinct mouse models of SSc. Together, these results reveal a previously unrecognized key function for p35/CDK5 as a mediator of mesenchymal cell fibrotic responses. The results suggest a potential pathogenic role for elevated p35 expression and CDK5 activity in SSc, and raise the possibility that their selective pharmacological targeting might represent a novel treatment approach in fibrosis.