FURIN regulates cytotoxic T-lymphocyte effector function and memory cell transition in mice.

The proprotein convertase subtilisin/kexins (PCSKs) regulate biological actions by cleaving immature substrate proteins. The archetype PCSK, FURIN, promotes the pathogenicity of viruses by proteolytically processing viral proteins. FURIN has also important regulatory functions in both innate and adaptive immune responses but its role in the CD8+ CTLs remains enigmatic. We used a T-cell-specific FURIN deletion in vivo to demonstrate that FURIN promotes host response against the CTL-dependent lymphocytic choriomeningitis virus by virtue of restricting viral burden and augmenting interferon gamma (IFNG) production. We also characterized Furin KO CD8+ T cells ex vivo, including after their activation with FURIN regulating cytokines IL12 or TGFB1. Furin KO CD8+ T cells show an inherently activated phenotype characterized by the upregulation of effector genes and increased frequencies of CD44+ , TNF+ , and IFNG+ cells. In the activated CTLs, FURIN regulates the productions of IL2, TNF, and GZMB and the genes associated with the TGFBR-signaling pathway. FURIN also controls the expression of Eomes, Foxo1, and Bcl6 and the levels of ITGAE and CD62L, which implies a role in the development of CTL memory. Collectively, our data suggest that the T-cell expressed FURIN is important for host responses in viral infections, CTL homeostasis/activation, and memory development.

Proteomics Analysis of R-Ras Deficiency in Oxygen Induced Retinopathy.

Small GTPase R-Ras regulates vascular permeability in angiogenesis. In the eye, abnormal angiogenesis and hyperpermeability are the leading causes of vision loss in several ischemic retinal diseases such as proliferative diabetic retinopathy (PDR), retinal vein occlusion (RVO), and retinopathy of prematurity (ROP). Oxygen-induced retinopathy (OIR) is the most widely used experimental model for these ischemic retinopathies. To shed more light on how the R-Ras regulates vascular permeability in pathological angiogenesis, we performed a comprehensive (>2900 proteins) characterization of OIR in R-Ras knockout (KO) and wild-type (WT) mice by sequential window acquisition of all theoretical mass spectra (SWATH-MS) proteomics. OIR and age-matched normoxic control retinas were collected at P13, P17, and P42 from R-Ras KO and WT mice and were subjected to SWATH-MS and data analysis. The most significant difference between the R-Ras KO and WT retinas was an accumulation of plasma proteins. The pathological vascular hyperpermeability during OIR in the R-Ras KO retina took place very early, P13. This led to simultaneous hypoxic cell injury/death (ferroptosis), glycolytic metabolism as well compensatory mechanisms to counter the pathological leakage from angiogenic blood vessels in the OIR retina of R-Ras deficient mice.

Pathological Angiogenesis Requires Syndecan-4 for Efficient VEGFA-Induced VE-Cadherin Internalization.

[Figure: see text].

Exposed CendR Domain in Homing Peptide Yields Skin-Targeted Therapeutic in Epidermolysis Bullosa.

Systemic skin-selective therapeutics would be a major advancement in the treatment of diseases affecting the entire skin, such as recessive dystrophic epidermolysis bullosa (RDEB), which is caused by mutations in the COL7A1 gene and manifests in transforming growth factor-ß (TGF-ß)-driven fibrosis and malignant transformation. Homing peptides containing a C-terminal R/KXXR/K motif (C-end rule [CendR] sequence) activate an extravasation and tissue penetration pathway for tumor-specific drug delivery. We have previously described a homing peptide CRKDKC (CRK) that contains a cryptic CendR motif and homes to angiogenic blood vessels in wounds and tumors, but it cannot penetrate cells or tissues. In this study, we demonstrate that removal of the cysteine from CRK to expose the CendR sequence confers the peptide novel ability to home to normal skin. Fusion of the truncated CRK (tCRK) peptide to the C terminus of an extracellular matrix protein decorin (DCN), a natural TGF-ß inhibitor, resulted in a skin-homing therapeutic molecule (DCN-tCRK). Systemic DCN-tCRK administration in RDEB mice led to inhibition of TGF-ß signaling in the skin and significant improvement in the survival of RDEB mice. These results suggest that DCN-tCRK has the potential to be utilized as a novel therapeutic compound for the treatment of dermatological diseases such as RDEB.

Histopathology and immunohistochemical analysis of 5-fluorouracil and triamcinolone treated keloids in double-blinded randomized controlled trial.

Keloids are a major complication related to surgical wound healing and very challenging condition to treat. Many treatment options are available, but the efficacy of the treatment is poor in most of cases and some keloids do not respond to the treatment at all. We compared the efficacy of intralesional 5-fluorouracil (5-FU) and triamcinolone (TAC) injections in a double-blind randomized controlled trial (RCT). Forty-three patients with 50 keloid scars were treated with either intralesional TAC or 5-FU-injections over 6 months. We wanted to find out whether biological features (cell density, cell proliferation rate, vascular density, myofibroblast numbers, steroid hormone receptor expression) in keloids could be used to predict the response to therapy and define the biological changes that take place in patients receiving a response. As there was no statistically significant difference in the remission rate between TAC and 5-FU treatments, all patients were combined and analyzed as responders and nonresponders. Although responders have slightly more myofibroblasts than the nonresponders in their keloids in the pretreatment biopsy samples, we could not identify a single predictive factor that could identify those patients that respond to drug injections. The good clinical response to therapy is associated with the simultaneous reduction of myofibroblasts in the keloid. This study demonstrates that myofibroblasts are reduced in number in those keloids that were responsive to therapy, and that both 5-FU and TAC injections are useful for keloid treatment.

Carbonic Anhydrase VI in Skin Wound Healing Study on Car6 Knockout Mice.

Carbonic anhydrases (CAs) contribute to tumor cell migration by generating an acidic environment through the conversion of carbon dioxide to bicarbonate and a proton. CA VI is secreted to milk and saliva, and it could contribute to wound closure, as a potential trophic factor, in animals that typically lick their wounds. Our aim was to investigate whether human CA VI improves skin-wound healing in full-thickness skin-wound models. The effect was studied in Car6 -/- knockout mice and wild type littermates. Half of both mice strains were given topically administered, milk-derived CA VI after wounding and eight hours later. The amount of topically given CA VI exceeded the predicted amount of natural saliva-delivered CA VI. The healing was followed for seven days and studied from photographs and histological sections. Our results showed no significant differences between the treatment groups in wound closure, re-epithelization, or granulation tissue formation, nor did the Car6 genotype affect the healing. Our results demonstrate that CA VI does not play a major role in skin-wound healing and also suggest that saliva-derived CA VI is not responsible for the licking-associated improved wound healing in animals.

R-Ras regulates vascular permeability, but not overall healing in skin wounds.

Wounds close by keratinocytes migrating from the edge of the wound and re-epithelializing the epidermis. It has been proposed that the major stimuli for wound closure are blood-derived growth factors, chemokines and cytokines. The small GTPase R-Ras, a known integrin activator, also regulates vascular permeability during angiogenesis, and blood vessels lacking R-Ras leak plasma proteins constantly. We explored whether the access to blood-derived proteins influences skin wound healing in R-Ras knockout (KO) mice. In skin wounds, R-Ras expression was mostly restricted to the vasculature in the granulation tissue. Angiogenic blood vessels in the R-Ras KO mice were significantly more permeable than in wild-type (WT) controls. Although the distances between epidermal tongues, and the panniculus carnosus muscles, were significantly longer in R-Ras KO than WT controls before the granulation tissue formation took place, there were no differences in the wound closure or re-epithelialization rates or granulation tissue formation. These findings were also corroborated in a special splint excision wound model. Our study shows that although R-Ras does not influence the skin wound healing itself, the blood vessels lacking R-Ras are leaky and thus could facilitate the access of blood-derived proteins to the wound.

Arthroscopic partial meniscectomy versus placebo surgery for a degenerative meniscus tear: a 2-year follow-up of the randomised controlled trial.

OBJECTIVE: To assess if arthroscopic partial meniscectomy (APM) is superior to placebo surgery in the treatment of patients with degenerative tear of the medial meniscus. METHODS: In this multicentre, randomised, participant-blinded and outcome assessor-blinded, placebo-surgery controlled trial, 146 adults, aged 35-65 years, with knee symptoms consistent with degenerative medial meniscus tear and no knee osteoarthritis were randomised to APM or placebo surgery. The primary outcome was the between-group difference in the change from baseline in the Western Ontario Meniscal Evaluation Tool (WOMET) and Lysholm knee scores and knee pain after exercise at 24 months after surgery. Secondary outcomes included the frequency of unblinding of the treatment-group allocation, participants' satisfaction, impression of change, return to normal activities, the incidence of serious adverse events and the presence of meniscal symptoms in clinical examination. Two subgroup analyses, assessing the outcome on those with mechanical symptoms and those with unstable meniscus tears, were also carried out. RESULTS: In the intention-to-treat analysis, there were no significant between-group differences in the mean changes from baseline to 24 months in WOMET score: 27.3 in the APM group as compared with 31.6 in the placebo-surgery group (between-group difference, -4.3; 95% CI, -11.3 to 2.6); Lysholm knee score: 23.1 and 26.3, respectively (-3.2; -8.9 to 2.4) or knee pain after exercise, 3.5 and 3.9, respectively (-0.4; -1.3 to 0.5). There were no statistically significant differences between the two groups in any of the secondary outcomes or within the analysed subgroups. CONCLUSIONS: In this 2-year follow-up of patients without knee osteoarthritis but with symptoms of a degenerative medial meniscus tear, the outcomes after APM were no better than those after placebo surgery. No evidence could be found to support the prevailing ideas that patients with presence of mechanical symptoms or certain meniscus tear characteristics or those who have failed initial conservative treatment are more likely to benefit from APM.

Furin deficiency in myeloid cells leads to attenuated revascularization in a mouse-model of oxygen-induced retinopathy.

Ischemic retinopathy is a vision-threatening disease associated with chronic retinal inflammation and hypoxia leading to abnormal angiogenesis. Furin, a member of the proprotein convertase family of proteins, has been implicated in the regulation of angiogenesis due to its essential role in the activation of several angiogenic growth factors, including vascular endothelial growth factor-C (VEGF-C), VEGF-D and transforming growth factor - ß (TGF- ß). In the present study, we evaluated expression of furin in the retina and its role in retinal angiogenesis. As both inflammation and hypoxia contribute to angiogenesis, the role of furin was evaluated using myeloid-cell specific furin knockout (KO) mice (designated LysMCre-fur(fl/fl)) both in developmental retinal angiogenesis as well as in hypoxia-driven angiogenesis using the oxygen-induced retinopathy (OIR) model. In the retina, furin expression was detected in endothelial cells, macrophages and, to some extent, in neurons. The rate of angiogenesis was not different in LysMCre-fur(fl/fl) mice when compared to their wild-type littermates during development. In the OIR model, the revascularization of retina was significantly delayed in LysMCre-fur(fl/fl) mice compared to their wild-type littermates, while there was no compensatory increase in the preretinal neovascularization in LysMCre-fur(fl/fl) mice. These results demonstrate that furin expression in myeloid cells plays a significant role in hypoxia-induced angiogenesis in retina.

A novel vascular homing peptide strategy to selectively enhance pulmonary drug efficacy in pulmonary arterial hypertension.

A major limitation in the pharmacological treatment of pulmonary arterial hypertension (PAH) is the lack of pulmonary vascular selectivity. Recent studies have identified a tissue-penetrating homing peptide, CARSKNKDC (CAR), which specifically homes to hypertensive pulmonary arteries but not to normal pulmonary vessels or other tissues. Some tissue-penetrating vascular homing peptides have a unique ability to facilitate transport of co-administered drugs into the targeted cells/tissues without requiring physical conjugation of the drug to the peptide (bystander effect). We tested the hypothesis that co-administered CAR would selectively enhance the pulmonary vascular effects of i.v. vasodilators in Sugen5416/hypoxia/normoxia-exposed PAH rats. Systemically administered CAR was predominantly detected in cells of remodeled pulmonary arteries. Intravenously co-administered CAR enhanced pulmonary, but not systemic, effects of the vasodilators, fasudil and imatinib, in PAH rats. CAR increased lung tissue imatinib concentration in isolated PAH lungs without increasing pulmonary vascular permeability. Sublingual CAR was also effective in selectively enhancing the pulmonary vasodilation by imatinib and sildenafil. Our results suggest a new paradigm in the treatment of PAH, using an i.v./sublingual tissue-penetrating homing peptide to selectively augment pulmonary vascular effects of nonselective drugs without the potentially problematic conjugation process. CAR may be particularly useful as an add-on therapy to selectively enhance the pulmonary vascular efficacy of any ongoing drug treatment in patients with PAH.

Systemically Administered, Target Organ-Specific Therapies for Regenerative Medicine.

Growth factors and other agents that could potentially enhance tissue regeneration have been identified, but their therapeutic value in clinical medicine has been limited for reasons such as difficulty to maintain bioactivity of locally applied therapeutics in the protease-rich environment of regenerating tissues. Although human diseases are treated with systemically administered drugs in general, all current efforts aimed at enhancing tissue repair with biological drugs have been based on their local application. The systemic administration of growth factors has been ruled out due to concerns about their safety. These concerns are warranted. In addition, only a small proportion of systemically administered drugs reach their intended target. Selective delivery of the drug to the target tissue and use of functional protein domains capable of penetrating cells and tissues could alleviate these problems in certain circumstances. We will present in this review a novel approach utilizing unique molecular fingerprints ("Zip/postal codes") in the vasculature of regenerating tissues that allows target organ-specific delivery of systemically administered therapeutic molecules by affinity-based physical targeting (using peptides or antibodies as an "address tag") to injured tissues undergoing repair. The desired outcome of targeted therapies is increased local accumulation and lower systemic concentration of the therapeutic payload. We believe that the physical targeting of systemically administered therapeutic molecules could be rapidly adapted in the field of regenerative medicine.

Myofibroblasts reside in the middle dermis of the keloids but do not predict the response to injection therapies: a double-blinded, randomized, controlled trial.

Introduction: Keloids form as a pathological response to skin wound healing, and their etiopathology is poorly understood. Myofibroblasts, which are cells transformed from normal fibroblasts, are believed to contribute to pathological scar formation in wounds. Methods: We carried out a double-blinded randomized controlled trial (RCT) comparing the efficacy of intralesional 5-fluorouracil (5-FU) and triamcinolone (TAC) injections in treating keloids. A total of 43 patients with 50 keloids were treated with either intralesional TAC or 5-FU injections, and their clinical response was evaluated. Biopsies were collected before, during, and after injection therapy from the active border of a keloid. To understand the role of myofibroblasts in keloids, we conducted an immunohistochemical analysis to identify myofibroblasts [α-smooth muscle actin (αSMA)] from the biopsies. We first defined the three histologically distinct regions-superficial, middle, and deep dermis-in each keloid. Results: We then demonstrated that myofibroblasts almost exclusively exist in the middle dermis of the keloids as 80% of the cells in the middle dermis were αSMA positive. However, both the percentage of myofibroblasts as well as the area covered by them was substantially lower in the superficial and deep dermis than in the middle dermis of the keloids. Myofibroblasts do not predict the clinical response to intralesional injection therapies. There is no difference in the myofibroblast numbers in keloids or in the induced change in myofibroblasts between the responders and non-responders after treatment. Discussion: This study demonstrates that myofibroblasts reside almost exclusively in the middle dermis layer of the keloids, but their numbers do not predict the clinical response to intralesional injection therapies in the RCT.

Target-seeking antifibrotic compound enhances wound healing and suppresses scar formation in mice.

Permanent scars form upon healing of tissue injuries such as those caused by ischemia (myocardial infarction, stroke), trauma, surgery, and inflammation. Current options in reducing scar formation are limited to local intervention. We have designed a systemically administered, target-seeking biotherapeutic for scar prevention. It consists of a vascular targeting peptide that specifically recognizes angiogenic blood vessels and extravasates into sites of injury, fused with a therapeutic molecule, decorin. Decorin prevents tissue fibrosis and promotes tissue regeneration by inhibiting TGF-ß activity and by other regulatory activities. The decorin-targeting peptide fusion protein had substantially increased neutralizing activity against TGF-ß1 in vitro compared with untargeted decorin. In vivo, the fusion protein selectively accumulated in wounds, and promoted wound healing and suppressed scar formation at doses where nontargeted decorin was inactive. These results show that selective targeting yields a tissue-healing and scar-reducing compound with enhanced specificity and potency. This approach may help make reducing scar formation by systemic drug delivery a feasible option for surgery and for the treatment of pathological processes in which scar formation is a problem.

Low IL-13Rα1 expression on mast cells tunes them unresponsive to IL-13.

Cytokine-mediated mast cell regulation enables precise optimization of their own proinflammatory cytokine production. During allergic inflammation, interleukin (IL)-4 regulates mast cell functions, tissue homing, and proliferation, but the direct role of closely related IL-13 for mast cell activation remains unclear. Previous work has shown that mast cells are potent IL-13 producers, but here we show that mouse mast cells do not directly respond to IL-13 by Stat6 activation, as they do not express measurable amount of IL-13 receptor α1 (IL-4Rα1) messenger RNA. Consequently, IL-4 responses are mediated via type I IL-4R (IL-4/IL4Rα/γC), and IL-4-induced Stat6 activation is abolished in γC-deficient mast cells. Type II IL-4R deficiency (IL-13Rα1 knockout) has no effect on IL-4-induced Stat6 activation. In basophils, both IL-4 and IL-13 induce Stat6 activation in wild-type and γC-deficient cells, while in type II IL-4R-deficient basophils, IL-4 signaling is impaired at low ligand concentration. Thus, mast cell and basophil sensitivity to IL-4/IL-13 is different, and in mast cells, lack of IL-13Rα1 expression likely explains their unresponsiveness to IL-13.

Traumatic muscle injury.

Traumatic muscle injury represents a collection of skeletal muscle pathologies caused by trauma to the muscle tissue and is defined as damage to the muscle tissue that can result in a functional deficit. Traumatic muscle injury can affect people across the lifespan and can result from high stresses and strains to skeletal muscle tissue, often due to muscle activation while the muscle is lengthening, resulting in indirect and non-contact muscle injuries (strains or ruptures), or from external impact, resulting in direct muscle injuries (contusion or laceration). At a microscopic level, muscle fibres can repair focal damage but must be completely regenerated after full myofibre necrosis. The diagnosis of muscle injury is based on patient history and physical examination. Imaging may be indicated to eliminate differential diagnoses. The management of muscle injury has changed within the past 5 years from initial rest, immobilization and (over)protection to early activation and progressive loading using an active approach. One challenge of muscle injury management is that numerous medical treatment options, such as medications and injections, are often used or proposed to try to accelerate muscle recovery despite very limited efficacy evidence. Another challenge is the prevention of muscle injury owing to the multifactorial and complex nature of this injury.

Systemically administered wound-homing peptide accelerates wound healing by modulating syndecan-4 function.

CAR (CARSKNKDC) is a wound-homing peptide that recognises angiogenic neovessels. Here we discover that systemically administered CAR peptide has inherent ability to promote wound healing: wounds close and re-epithelialise faster in CAR-treated male mice. CAR promotes keratinocyte migration in vitro. The heparan sulfate proteoglycan syndecan-4 regulates cell migration and is crucial for wound healing. We report that syndecan-4 expression is restricted to epidermis and blood vessels in mice skin wounds. Syndecan-4 regulates binding and internalisation of CAR peptide and CAR-mediated cytoskeletal remodelling. CAR induces syndecan-4-dependent activation of the small GTPase ARF6, via the guanine nucleotide exchange factor cytohesin-2, and promotes syndecan-4-, ARF6- and Cytohesin-2-mediated keratinocyte migration. Finally, we show that genetic ablation of syndecan-4 in male mice eliminates CAR-induced wound re-epithelialisation following systemic administration. We propose that CAR peptide activates syndecan-4 functions to selectively promote re-epithelialisation. Thus, CAR peptide provides a therapeutic approach to enhance wound healing in mice; systemic, yet target organ- and cell-specific.

Peptide-directed highly selective targeting of pulmonary arterial hypertension.

Pulmonary arterial hypertension (PAH) is a disorder of the pulmonary vasculature associated with elevated pulmonary vascular resistance. Despite recent advances in the treatment of PAH, with eight approved clinical therapies and additional therapies undergoing clinical trials, PAH remains a serious life-threatening condition. The lack of pulmonary vascular selectivity and associated systemic adverse effects of these therapies remain the main obstacles to successful treatment. Peptide-mediated drug delivery that specifically targets the vasculature of PAH lungs may offer a solution to the lack of drug selectivity. Herein, we show highly selective targeting of rat PAH lesions by a novel cyclic peptide, CARSKNKDC (CAR). Intravenous administration of CAR peptide resulted in intense accumulation of the peptide in monocrotaline-induced and SU5416/hypoxia-induced hypertensive lungs but not in healthy lungs or other organs of PAH rats. CAR homed to all layers of remodeled pulmonary arteries, ie, endothelium, neointima, medial smooth muscle, and adventitia, in the hypertensive lungs. CAR also homed to capillary vessels and accumulated in the interstitial space of the PAH lungs, manifesting its extravasation activity. These results demonstrated the remarkable ability of CAR to selectively target PAH lung vasculature and effectively penetrate and spread throughout the diseased lung tissue. These results suggest the clinical utility of CAR in the targeted delivery of therapeutic compounds and imaging probes to PAH lungs.

Dual drug delivery collagen vehicles for modulation of skin fibrosisin vitro.

Single molecule drug delivery systems have failed to yield functional therapeutic outcomes, triggering investigations into multi-molecular drug delivery vehicles. In the context of skin fibrosis, although multi-drug systems have been assessed, no system has assessed molecular combinations that directly and specifically reduce cell proliferation, collagen synthesis and transforming growth factorß1 (TGFß1) expression. Herein, a core-shell collagen type I hydrogel system was developed for the dual delivery of a TGFßtrap, a soluble recombinant protein that inhibits TGFßsignalling, and Trichostatin A (TSA), a small molecule inhibitor of histone deacetylases. The antifibrotic potential of the dual delivery system was assessed in anin vitroskin fibrosis model induced by macromolecular crowding (MMC) and TGFß1. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and high performance liquid chromatography analyses revealed that ∼50% of the TGFßtrap and ∼30% of the TSA were released from the core and shell compartments, respectively, of the hydrogel system after 10 d (longest time point assessed) in culture. As a direct consequence of this slow release, the core (TGFßtrap)/shell (TSA) hydrogel system induced significantly (p< 0.05) lower than the control group (MMC and TGFß1) collagen type I deposition (assessed via SDS-PAGE and immunocytochemistry),αsmooth muscle actin (αSMA) expression (assessed via immunocytochemistry) and cellular proliferation (assessed via DNA quantification) and viability (assessed via calcein AM and ethidium homodimer-I staining) after 10 d in culture. On the other hand, direct TSA-TGFßsupplementation induced the lowest (p< 0.05) collagen type I deposition,αSMA expression and cellular proliferation and viability after 10 d in culture. Our results illustrate the potential of core-shell collagen hydrogel systems for sustained delivery of antifibrotic molecules.

IL-13Rα1 Suppresses Tumor Progression in Two-Stage Skin Carcinogenesis Model by Regulating Regulatory T Cells.

Type 2 inflammation‒related cytokine IL-13 plays a protective role in experimental papilloma induction in mice. To understand the mechanisms by which IL-13 contributes to papilloma formation, we utilized Il13rα1-knockout (KO) mice in a widely used 7,12-dimethylbenz[a]anthracene/12-O-tetradecanoyl phorbol-13-acetate two-stage skin carcinogenesis protocol that mimics the development of squamous cell carcinoma. KO mice developed more papillomas and significantly faster than wild-type mice. Papilloma development reduced regulatory T cells in wild-type mice but substantially less in KO mice. In line with this, IL-2 and IL-10 levels decreased in wild-type mice but not in KO mice. Furthermore, systemic IL-5 and TSLP levels were elevated, whereas IL-22 was decreased during papilloma formation in the skin of KO mice. Polymorphonuclear myeloid‒derived suppressor cells were decreased in the KO mice at the early phase of papilloma induction. We show that IL-13Rα1 protects from papilloma development in chemically induced skin carcinogenesis, and our results provide further insights into the protective role of functional IL-4 and IL-13 signaling through type II IL-4 receptor in tumor development.

Return to Play Prediction Accuracy of the MLG-R Classification System for Hamstring Injuries in Football Players: A Machine Learning Approach.

BACKGROUND AND OBJECTIVE: Muscle injuries are one of the main daily problems in sports medicine, football in particular. However, we do not have a reliable means to predict the outcome, i.e. return to play from severe injury. The aim of the present study was to evaluate the capability of the MLG-R classification system to grade hamstring muscle injuries by severity, offer a prognosis for the return to play, and identify injuries with a higher risk of re-injury. Furthermore, we aimed to assess the consistency of our proposed system by investigating its intra-observer and inter-observer reliability. METHODS: All male professional football players from FC Barcelona, senior A and B and the two U-19 teams, with injuries that occurred between February 2010 and February 2020 were reviewed. Only players with a clinical presentation of a hamstring muscle injury, with complete clinic information and magnetic resonance images, were included. Three different statistical and machine learning approaches (linear regression, random forest, and eXtreme Gradient Boosting) were used to assess the importance of each factor of the MLG-R classification system in determining the return to play, as well as to offer a prediction of the expected return to play. We used the Cohen's kappa and the intra-class correlation coefficient to assess the intra-observer and inter-observer reliability. RESULTS: Between 2010 and 2020, 76 hamstring injuries corresponding to 42 different players were identified, of which 50 (65.8%) were grade 3r, 54 (71.1%) affected the biceps femoris long head, and 33 of the 76 (43.4%) were located at the proximal myotendinous junction. The mean return to play for grades 2, 3, and 3r injuries were 14.3, 12.4, and 37 days, respectively. Injuries affecting the proximal myotendinous junction had a mean return to play of 31.7 days while those affecting the distal part of the myotendinous junction had a mean return to play of 23.9 days. The analysis of the grade 3r biceps femoris long head injuries located at the free tendon showed a median return to play time of 56 days while the injuries located at the central tendon had a shorter return to play of 24 days (p = 0.038). The statistical analysis showed an excellent predictive power of the MLG-R classification system with a mean absolute error of 9.8 days and an R-squared of 0.48. The most important factors to determine the return to play were if the injury was at the free tendon of the biceps femoris long head or if it was a grade 3r injury. For all the items of the MLG-R classification, the intra-observer and inter-observer reliability was excellent (k > 0.93) except for fibres blurring (κ = 0.68). CONCLUSIONS: The main determinant for a long return to play after a hamstring injury is the injury affecting the connective tissue structures of the hamstring. We developed a reliable hamstring muscle injury classification system based on magnetic resonance imaging that showed excellent results in terms of reliability, prognosis capability and objectivity. It is easy to use in clinical daily practice, and can be further adapted to future knowledge. The adoption of this system by the medical community would allow a uniform diagnosis leading to better injury management.