Distinct inflammatory macrophage populations sequentially infiltrate bone‐to‐tendon interface tissue after ACL reconstruction surgery in mice

Remodeling Process of the Tendon Graft After Anterior Cruciate Ligament Reconstruction: Comprehensive Analysis With RNA Sequencing in a Murine Model

The tendon graft is known to undergo a remodeling process after anterior cruciate ligament (ACL) reconstruction. However, little is known about the transcriptional profile of this process. The aim of the present study is to identify differentially expressed genes inside the remodeling ACL graft in the early phase after ACL reconstruction in our murine model using RNA sequencing (RNAseq). Fifty four male C57BL/6 mice were used in this study. The mice were euthanized at 1, 2, and 4 weeks after surgery and used for histological evaluations and RNAseq of the tendon graft. Histologically, there was a progressive decrease in the tendon-bone interface gap space and increased tissue continuity between the grafted tendon and the bone tunnel over time. At 1 and 2 weeks after surgery, cell increase and loss of collagen fiber organization inside the tendon graft were observed. RNAseq showed that genes related to inflammation, matrix metalloproteinases, bone metabolism, chemokines and signaling pathways were upregulated at 1 and 2 weeks after surgery compared to the control group (p < 0.0001). Our transcriptional profiling data suggests that expression of inflammatory mediators and bone remodeling genes may play an important role in the early events in graft-to-bone healing. Further validation at the protein level is necessary to draw firm conclusions about the role of these mediators in graft remodeling and healing. Understanding the remodeling process of the grafted tendons may lead to the identification of new approaches to improve clinical outcomes after ACL reconstruction.

Sustained release of ubiquitin-like protein ISG-15 enhances tendon-to-bone healing following anterior cruciate ligament reconstruction in a mouse model

The process of tendon-to-bone healing is regulated by several proteins and cytokines that play critical roles in shaping biomechanical properties and functional recovery. Among these, the ubiquitin-like protein ISG-15 has been reported to have a beneficial effect on tissue repair. However, its specific function in tendon-to-bone interface regeneration has not been well characterized. This study investigated the function of ISG15 in vitro and addressed its in vivo effects on tendon and bone healing. In this study, wild-type C57/BL6 mice underwent anterior cruciate ligament (ACL) reconstruction surgery, with a sustained-release hydrogel containing ISG15 protein injected into the bone tunnels in the treatment group. To assess its therapeutic potential, bone-tendon interface growth was evaluated through histological staining, while micro-computed tomography (Micro-CT) was employed to quantify newly formed bone and bone density within the bone tunnels. Additionally, biomechanical testing was performed to measure the mechanical strength of the grafted tendons, and immunohistochemistry was conducted to detect the expression of Runx2 and osteocalcin (OCN) at the bone-tendon interface. In vitro results showed that an appropriate concentration of ISG-15 has the ability to promote osteogenic differentiation of bone marrow mesenchymal stem cells. Also, In the in vivo experiments, the local application of ISG15 protein significantly reduced inflammatory tissue growth during the early stages of healing and minimized bone resorption in the later stages. Furthermore, Micro-CT analysis showed an increased volume of newly formed bone in the treatment group, while biomechanical testing demonstrated enhanced mechanical strength of the grafted tendons. In summary, this study suggests that the localized sustained release of ISG15 protein during ACL reconstruction facilitates tendon-to-bone interface repair by promoting bone ingrowth, ultimately leading to improved biomechanical properties and functional recovery.

Synovial macrophage diversity and activation of M-CSF signaling in post-traumatic osteoarthritis

Synovium is home to immune and stromal cell types that orchestrate inflammation following a joint injury; in particular, macrophages are central protagonists in this process. We sought to define the cellular and temporal dynamics of the synovial immune niche in a mouse model of post-traumatic osteoarthritis (PTOA), and to identify stromal-immune crosstalk mechanisms that coordinate macrophage function and phenotype. We induced PTOA in mice using a non-invasive tibial compression model of anterior cruciate ligament rupture (ACLR). Single-cell RNA-sequencing and flow cytometry were used to assess immune cell populations in healthy (Sham) and injured (7 and 28 days post-ACLR) synovium. Characterization of synovial macrophage polarization states was performed, alongside computational modeling of macrophage differentiation, as well as implicated transcriptional regulators and stromal-immune communication axes. Immune cell types are broadly represented in healthy synovium, but experience drastic expansion and speciation in PTOA, most notably in the macrophage portion. We identified several polarization states of macrophages in synovium following joint injury, underpinned by distinct transcriptomic signatures, and regulated in part by stromal-derived macrophage colony-stimulating factor signaling. The transcription factors Pu.1, Cebpα, Cebpβ, and Jun were predicted to control differentiation of systemically derived monocytes into pro-inflammatory synovial macrophages. In summary, we defined different synovial macrophage subpopulations present in healthy and injured mouse synovium. Nuanced characterization of the distinct functions, origins, and disease kinetics of macrophage subtypes in PTOA will be critical for targeting these highly versatile cells for therapeutic purposes.

Allograft and autograft anterior cruciate ligament reconstructions exhibit a similar biological response to cyclic loading

OBJECTIVE

Anterior cruciate ligament (ACL) reconstruction is one of the most commonly performed orthopaedic procedures. While outcomes are similar in the general patient population, the rerupture rate of non-irradiated allografts are 3-4 times higher than autografts in young active individuals. Previous studies suggest that the difference in clinical performance between graft types is due to impaired remodeling in allografts in response to loading. The objective of this study was to compare the remodeling response of autografts and allografts to cyclic loading. Furthermore, given that allografts are a foreign object and that immune cell signaling affects fibroblast mechanobiology, we compared markers of the immune cell composition between graft types.

METHODS

ACL reconstructions were performed on New Zealand white rabbits, harvested 8 weeks post-surgery, and cyclically loaded to 2 MPa in a tensile bioreactor. Expression of markers for anabolic and catabolic tissue remodeling, as well as inflammatory cytokines and immune cells, were quantified using quantitative reverse transcription polymerase chain reaction.

RESULTS

We found that the expression of markers for tissue remodeling were not different between allografts and autografts. Similarly, we found that the expression of markers for immune cells were not different between allografts and autografts.

CONCLUSIONS

These data suggest that the poor clinical outcomes and impaired remodeling of allograft reconstructions compared to autografts is not due to a difference in graft mechanobiology.

Biologic Impact of Anterior Cruciate Ligament Injury and Reconstruction

Surgical intervention after anterior cruciate ligament (ACL) tears is typically required because of the limited healing capacity of the ACL. However, mechanical factors and the inflammatory response triggered by the injury and surgery can impact patient outcomes. This review explores key aspects of ACL injury and reconstruction biology, including the inflammatory response, limited spontaneous healing, secondary inflammation after reconstruction, and graft healing processes. Understanding these biologic mechanisms is crucial for developing new treatment strategies and enhancing patient well-being. By shedding light on these aspects, clinicians and researchers can work toward improving quality of life for individuals affected by ACL tears.

The role of macrophage polarization in tendon healing and therapeutic strategies: Insights from animal models

Tendon injuries, a common musculoskeletal issue, usually result in adhesions to the surrounding tissue, that will impact functional recovery. Macrophages, particularly through their M1 and M2 polarizations, play a pivotal role in the inflammatory and healing phases of tendon repair. In this review, we explore the role of macrophage polarization in tendon healing, focusing on insights from animal models. The review delves into the complex interplay of macrophages in tendon pathology, detailing how various macrophage phenotypes contribute to both healing and adhesion formation. It also explores the potential of modulating macrophage activity to enhance tendon repair and minimize adhesions. With advancements in understanding macrophage behavior and the development of innovative biomaterials, this review highlights promising therapeutic strategies for tendon injuries.

Identification of anterior cruciate ligament fibroblasts and their contribution to knee osteoarthritis progression using single-cell analyses

The mechanical properties of the anterior cruciate ligament (ACL) in the knee have been highlighted, but its role in the regulation of the joint microenvironment remains unclear, especially in the progression of Knee Osteoarthritis (KOA). Here, single-cell RNA sequencing (scRNA-seq) and single-cell assay for transposase-accessible chromatin sequencing (scATAC-seq) data were integrated to reveal the transcriptional and epigenomic landscape of ACL in normal and OA states. We identified a novel subpopulation of fibroblasts in ACL, which provides new insights into the role of the ACL in knee homeostasis and disease. Degeneration of the ACL during OA mechanically alters the knee joint homeostasis and influences the microenvironment by regulating inflammatory- and osteogenic-related factors, thereby contributing to the progression of KOA. Additionally, the specific mechanism by which these Inflammation-associated Fibroblasts (IAFs) regulate KOA progression was uncovered, providing new foundation for the development of targeted treatments for KOA.

Synovial macrophage diversity and activation of M-CSF signaling in post-traumatic osteoarthritis

Objective

Synovium is home to immune and stromal cell types that orchestrate inflammation following a joint injury; in particular, macrophages are central protagonists in this process. We sought to define the cellular and temporal dynamics of the synovial immune niche in a mouse model of post-traumatic osteoarthritis (PTOA), and to identify stromal-immune crosstalk mechanisms that coordinate macrophage function and phenotype.

Design

We induced PTOA in mice using a non-invasive tibial compression model of anterior cruciate ligament rupture (ACLR). Single cell RNA-seq and flow cytometry were used to assess immune cell populations in healthy (Sham) and injured (7d and 28d post-ACLR) synovium. Characterization of synovial macrophage polarization states was performed, alongside computational modeling of macrophage differentiation, as well as implicated transcriptional regulators and stromal-immune communication axes.

Results

Immune cell types are broadly represented in healthy synovium, but experience drastic expansion and speciation in PTOA, most notably in the macrophage portion. We identified several polarization states of macrophages in synovium following joint injury, underpinned by distinct transcriptomic signatures, and regulated in part by stromal-derived macrophage colony-stimulating factor signaling. The transcription factors Pu.1, Cebpα, Cebpβ, and Jun were predicted to control differentiation of systemically derived monocytes into pro-inflammatory synovial macrophages.

Conclusions

We defined different synovial macrophage subpopulations present in healthy and injured mouse synovium. Nuanced characterization of the distinct functions, origins, and disease kinetics of macrophage subtypes in PTOA will be critical for targeting these highly versatile cells for therapeutic purposes.

Preclinical tendon and ligament models: Beyond the 3Rs (replacement, reduction, and refinement) to 5W1H (why, who, what, where, when, how)

Several tendon and ligament animal models were presented at the 2022 Orthopaedic Research Society Tendon Section Conference held at the University of Pennsylvania, May 5 to 7, 2022. A key objective of the breakout sessions at this meeting was to develop guidelines for the field, including for preclinical tendon and ligament animal models. This review summarizes the perspectives of experts for eight surgical small and large animal models of rotator cuff tear, flexor tendon transection, anterior cruciate ligament tear, and Achilles tendon injury using the framework: "Why, Who, What, Where, When, and How" (5W1H). A notable conclusion is that the perfect tendon model does not exist; there is no single gold standard animal model that represents the totality of tendon and ligament disease. Each model has advantages and disadvantages and should be carefully considered in light of the specific research question. There are also circumstances when an animal model is not the best approach. The wide variety of tendon and ligament pathologies necessitates choices between small and large animal models, different anatomic sites, and a range of factors associated with each model during the planning phase. Attendees agreed on some guiding principles including: providing clear justification for the model selected, providing animal model details at publication, encouraging sharing of protocols and expertise, improving training of research personnel, and considering greater collaboration with veterinarians. A clear path for translating from animal models to clinical practice was also considered as a critical next step for accelerating progress in the tendon and ligament field.

Responding to ACL Injury and its Treatments: Comparative Gene Expression between Articular Cartilage and Synovium

The relationship between cartilage and synovium is a rapidly growing area of osteoarthritis research. However, to the best of our knowledge, the relationships in gene expression between these two tissues have not been explored in mid-stage disease development. The current study compared the transcriptomes of these two tissues in a large animal model one year following posttraumatic osteoarthritis induction and multiple surgical treatment modalities. Thirty-six Yucatan minipigs underwent transection of the anterior cruciate ligament. Subjects were randomized to no further intervention, ligament reconstruction, or ligament repair augmented with an extracellular matrix (ECM) scaffold, followed by RNA sequencing of the articular cartilage and synovium at 52 weeks after harvest. Twelve intact contralateral knees served as controls. Across all treatment modalities, the primary difference in the transcriptomes was that the articular cartilage had greater upregulation of genes related to immune activation compared to the synovium-once baseline differences between cartilage and synovium were adjusted for. Oppositely, synovium featured greater upregulation of genes related to Wnt signaling compared to articular cartilage. After adjusting for expression differences between cartilage and synovium seen following ligament reconstruction, ligament repair with an ECM scaffold upregulated pathways related to ion homeostasis, tissue remodeling, and collagen catabolism in cartilage relative to synovium. These findings implicate inflammatory pathways within cartilage in the mid-stage development of posttraumatic osteoarthritis, independent of surgical treatment. Moreover, use of an ECM scaffold may exert a chondroprotective effect over gold-standard reconstruction through preferentially activating ion homeostatic and tissue remodeling pathways within cartilage.

Effect of CCR2 Knockout on Tendon Biomechanical Properties in a Mouse Model of Delayed Rotator Cuff Repair

BACKGROUND

The high incidence of incomplete or failed healing after rotator cuff repair (RCR) has led to an increased focus on the biologic factors that affect tendon-to-bone healing. Inflammation plays a critical role in the initial tendon-healing response. C-C chemokine receptor type 2 (CCR2) is a chemokine receptor linked to the recruitment of monocytes in early inflammatory stages and is associated with an increase in pro-inflammatory macrophages. The purpose of this study was to evaluate the role of CCR2 in tendon healing following RCR in C57BL/6J wildtype (WT) and CCR2-/- knockout (CCR2KO) mice in a delayed RCR model.

METHODS

Fifty-two 12-week-old, male mice were allocated to 2 groups (WT and CCR2KO). All mice underwent unilateral supraspinatus tendon (SST) detachment at the initial surgical procedure, followed by a delayed repair 2 weeks later. The primary outcome measure was biomechanical testing. Secondary measures included histology, gene expression analysis, flow cytometry, and gait analysis.

RESULTS

The mean load-to-failure was 1.64 ± 0.41 N in the WT group and 2.50 ± 0.42 N in the CCR2KO group (p = 0.030). The mean stiffness was 1.43 ± 0.66 N/mm in the WT group and 3.00 ± 0.95 N/mm in the CCR2KO group (p = 0.008). Transcriptional profiling demonstrated 7 differentially expressed genes (DEGs) when comparing the CCR2KO and WT groups (p < 0.05) and significant differences in Type-I and Type-II interferon pathway scores (p < 0.01). Flow cytometry demonstrated significant differences between groups for the percentage of macrophages present (8.1% for the WT group compared with 5.8% for the CCR2KO group; p = 0.035). Gait analysis demonstrated no significant differences between groups.

CONCLUSIONS

CCR2KO may potentially improve tendon biomechanical properties by decreasing macrophage infiltration and/or by suppressing inflammatory mediator pathways in the setting of delayed RCR.

CLINICAL RELEVANCE

CCR2 may be a promising target for novel therapeutics that aim to decrease failure rates following RCR.