Mesenchymal Stem Cells in the Treatment of Knee Osteoarthritis

Osteoarthritis (OA) is the most common progressive musculoskeletal disorder, affecting 303 million people worldwide [1]. Although OA can affect any joint in the body, it most commonly affects weight-bearing hip and knee joints [2]. The prevalence of knee OA among adults 60 years-of-age or older is approximately 10% in men and 13% in women and is constantly increasing due to the growing number of obese population, longer life expectancy and aging population [3].

The role of cytokines, chemokines, miRNA, gene expression and imbalance between anabolic and catabolic pathways is crucial in understanding the pathogenesis of OA [4]. Knee OA results in structural modifications in articular cartilage and the subchondral bone, but also Hoffa's pad, synovia, ligaments and muscles [4]. Therefore, OA can be observed and interpreted as a whole joint disease.

Patients suffering from OA present with pain, stiffness, swelling and reduced range of motion in the affected joint, all of which result in reduced quality of life and mental health impairment due to chronic pain [5]. While joint replacement surgery is the gold standard for knee OA treatment, new treatment options have drawn clinicians’ attention as a potentially preferable alternative method for OA treatment. These methods can be divided into pharmacological and non-pharmacological therapies. An unavoidable part of successful OA treatment is physical therapy and bodyweight reduction, i.e. reducing the load in the joints that present major non-pharmacological methods. Optimal pharmacologic treatment for pain and disability caused by OA should be tailored in accordance with individual patient needs and their comorbidities. The emerging field of pharmacogenomics provides the clinician with new options to guide precision treatment with a maximal therapeutic effect and minimal risk to the patient [6,7]. A new class of drugs called S/DMOADs (symptomatic or disease-modifying osteoarthritic drugs), including chondroitin sulfate and glucosamine, are commonly used in pharmacological therapy for OA, while injections of autologous mesenchymal stem cells, Hyaluronic Acid (HA), Platelet-Rich Plasma (PRP) are becoming increasingly used methods [8]. In this short commentary review, we will focus on current findings on mesenchymal stem cells in the treatment of knee OA.

Stem cells emerged as a promising line of treatment for OA due to their characteristics, such as self-renewal, proliferation and differentiation into several cell types [9]. Mesenchymal Stem Cells (MSCs) are adult stem cells present in various tissues throughout the body. For instance, they can be found in bone marrow, adipose tissue, peripheral blood, skeletal muscle, heart and umbilical cord [10]. Their ability to differentiate towards osteoblasts, chondrocytes and adipocytes, together with generating immunomodulatory and paracrine mechanisms around damaged tissue have put them in the spotlight in the field of regenerative medicine [11]. The mechanisms by which MSCs repair damaged cartilage are inhibition of cell apoptosis, reduction of inflammation by suppressing activation, proliferation and infiltration of macrophages, T and B- lymphocytes; secretion of trophic, chondrogenic, angiogenic, anti-fibrotic and anti-catabolic factors [12]. Many studies have been conducted with the aim of selecting the best source of MSCs. Factors such as the amount of harvest volume, cell isolation procedure, the regenerative capacity of certain cells and the risks of harvesting procedure should be assessed [13]. A systematic review conducted in 2018 included 28 scientific articles and confirmed the safety of MSCs in the treatment of various musculoskeletal pathologies [14].

Bone-marrow mesenchymal stem cells

Bone-Marrow Mesenchymal Stem Cells (BM-MSCs) are usually obtained by aspiration from the posterior or anterior iliac crest. Since it is estimated that only about 0.001% of nucleated cells from BM aspirate are MSCs, density-gradient centrifugation of the aspirate is needed to produce a Bone Marrow Aspirate Concentrate (BMAC) [15]. This process increases not only the number of MSCs but also hematopoietic stem cells and platelets containing growth factors, important for stem cell migration and chondrogenesis [16]. The observed clinical results of BM-MSCs therapy are generally positive. A recent meta-analysis indicated improvement in pain level measured by the Visual Analogue Scale (VAS), International Knee Documentation Committee (IKDC) function score, Tegner Activity Scale and Lysholm Knee score when compared to respective results before treatment with BM-MSCs [17]. A recent literature review of clinical data published between 2014 and 2019 regarding intraarticular autologous BM-MSCs injections also showed predominantly positive results, pointing out that a moderate-high number of cells (40 × 10⁶) achieves optimal responses in individuals with grade ≥ 2 knee OA on the Kellgren-Lawrence scale, while lower (24 × 10⁶) and higher (100 × 10⁶) cell numbers, despite showing significant improvement, are associated with a larger number of adverse effects, such as persistent knee pain and swelling [18]. Although the intraarticular injection is the most commonly used method of BM-MSCs application, a recently published clinical trial determined that the BM-MSCs injection in the subchondral bone of an osteoarthritic knee is more effective to postpone total knee arthroplasty comparing to intraarticular injection [19].

Adipose-derived mesenchymal stem cells

Adipose-Derived Mesenchymal Stem Cells (AD-MSCs) are usually obtained from abdominal subcutaneous adipose tissue by lipoaspiration [20]. Those procedures are less invasive compared to BM-MSCs extraction procedures. Moreover, adipose tissue contains 500 times more MSCs compared to the same volume of bone marrow [13,21]. Adipose tissue provides a significant, easily accessible source of cells contained in stromal vascular fraction (SVF) for prompt administration and provides a compelling amount of cells from which multipotent AD-MSCs can be isolated [22]. At present, AD-MSCs in the form of Stromal Vascular Fraction from Lipoaspirate (SVF-LA) or Stromal Vascular Fraction from Microfragmented Lipoaspirate (SVF-MLA) are used in most clinical trials and treatment protocols. A study that analyzed cell phenotypes within CD45 - fraction in these two samples (SVF-LA and SVF-MLA) identified the following cell types: Endothelial Progenitor Cells (EPC), endothelial mature cells, pericytes, transitional pericytes, and Supra Adventitial-Adipose Stromal Cells (SA-ASC), with a surprising and intriguing result of increased EPC number, and reduction of leukocytes and SA-ASC in SVF-MLA compared with SVF-LA [23].

When applied intraarticularly, AD-MSCs therapy showed significant clinical improvement in numerous studies. Improvements in clinical scores such as The Knee Injury and Osteoarthritis Outcome Score (KOOS), The Western Ontario and McMaster Universities Arthritis Index (WOMAC), Timed Up and Go test (TUG) and VAS (both in resting and in movement), as well as an increase in range of motion (ROM), were observed [24-26]. Also, structural analysis, measured by MRI Osteoarthritis Knee Scores (MOAKS), showed a significant rate of cartilage loss regression and less osteophyte formation comparing the group treated with AD-MCSs with the group treated conservatively [27,28]. Furthermore, studies have shown that intraarticular application of AD-MSCs has an impact on proteoglycan synthesis in the cartilage of patients with knee OA. Measured by the dGEMRIC index (delayed gadolinium-enhanced magnetic resonance imaging of cartilage), hyaline cartilage Glycosaminoglycan (GAG) content significantly increased six and twelve months after the treatment [29]. The observed effects of AD-MSCs were visible in a two-year follow-up period after the initial intraarticular intervention [30]. A recent study showed a systemic effect on circulating immune cells after intraarticular application of AD-MSCs in patients suffering from knee OA. The increased percentage of regulatory T cells, as well as transitional B cells, persisted for at least 3 months after treatment, whilst the monocyte level decreased and remained low at 3 months after AD-MSC injection. The studies, therefore, indicate that AD-MSCs treatment provides a long-lasting clinical and systemic immunomodulatory effect [25,31]. Current concepts that include co-administration of AD-MSCs alongside HA or PRP could offer interesting results. It is stated that HA provides an environment in which AD-MSCs can more easily adhere to the target area around the lesion and differentiate into cells needed to build damaged bone and cartilage components whereas PRP contains highly concentrated platelets and a wide range of growth factors providing AD-MSCs proliferation [32,33] (Table 1).

Table 1: Clinical outcomes from different sources of MSCs used in clinical studies reviewed in the article. BM-MSCs - Bone Marrow-Mesenchymal Stem Cells; VAS - Visual Analog Scale; MSCs - Mesenchymal Stem Cells; AD-MSCs - Adipose-Derived Mesenchymal Stem Cells; SVF - Stromal Vascular Fraction; WOMAC -The Western Ontario and McMaster Universities Arthritis Index; ROM - Range Of Motion; TUG - Timed Up and Go; KOOS - The Knee Injury and Osteoarthritis Outcome Score; MFAT - Microfragmented Adipose Tissue; NPRS - Numeric Pain Rating Scale; QOL - Quality of Life; ADL - Activities Of Daily Living; GAG -Glycosaminoglycans; dGEMRIC - delayed Gadolinium (Gd)-Enhanced Magnetic Resonance Imaging of Cartilage; AMUC - Amniotic Membrane/Umbilical Cord Particulate; PLMSCs - Placental Derived Mesenchymal Stem Cells; S/R - Sport and Recreation; ASA - Amniotic Suspension Allograft; HA – Hyaluronic Acid; BMAC - Bone Marrow Aspirate Concentrate; hUCB - MSCs- Human Umbilical Cord Blood-Derived Mesenchymal Stem Cells; IKDC - International Knee Documentation Committee.

Placental tissue

Placental tissue also represents a source of stem cells. Neonatal MSCs can be isolated from amniotic fluid, amnion, chorion, umbilical cord tissue, and blood. Containing not only MSCs, but also a collagen-rich structural matrix, epithelial cells, fibroblasts and several biologically active factors, placental tissue has been successfully used as a treatment for burns and wounds, and recent data suggest potential benefits in orthopedic [34,38]. Although the number of trials is still small, the described outcomes are generally positive. The available literature demonstrated that allogenic placental mesenchymal stem cells are safe and efficient in terms of clinical improvements regarding knee ROM, quality of life, the activity of daily living, sport/recreational activity, pain reduction and even chondral thickness in a 24-week follow-up period [35]. Amniotic tissue, a distinct placental tissue, contains stromal cells which have the chondrogenic and osteogenic capacity, and also provides a rich source of HA and proteoglycans, essential for cartilage structure [39]. A randomized controlled single-blind study including 200 patients with moderate knee OA evaluated the efficacy of symptom modulation with Amniotic Suspension Allograft (ASA) injection compared with saline and HA. Patients receiving ASA presented with both statistically significant and clinically meaningful improvements compared with the other two groups [36]. Another study compared the effects of BMAC and allogeneic human umbilical cord blood-derived MSC implantation in the osteoarthritic knee, and results showed that there is no significant difference in clinical outcome between the two methods [37]. Even though, more studies are required to investigate potential risks of allogeneic cell implantations before such methods find a place in everyday clinical practice.

In recent years, due to the increasing use of MSCs in clinical practice around the world, scientific research about MSCs has become increasingly extensive and relevant. Although the results so far are promising, pointing out the advantages of MSCs treatment such as reduction of patient’s knee pain, regression of affected joint damage, less invasiveness and shorter hospital stay, additional studies that would address the optimal procedure steps, timing and number of injections, as well as patient selection criteria are still needed to establish these methods in everyday clinical practice. Novel methods, including a combination of currently available therapeutic approaches and different administration methods (intraarticular, subchondral) could offer better treatment outcomes in the future.

Comparing the two most commonly used sources of MSCs for the knee OA treatment: AD-MSCs showed to be more easily harvested than BM-MSCs, with less invasive and less painful harvesting procedures. Furthermore, adipose tissue provided a higher count of MSCs when compared to the same harvested volume of bone marrow. In vitro studies show different regenerative capacities of AD-MSCs and BM-MSCs, however, the in vivo effect is still measured in a subjective, symptom-specific outcome rather than a thorough biochemical, histological and pathophysiologic effect on OA [40]. More studies are required to address and compare the regenerative capacity of different sources of MSCs before definitive conclusions can be drawn. Also, the benefits of placental tissue should be further investigated, together with potential side effects from allogeneic transplants.

The main limitations of the conducted studies include lack of standardization associated with patient selection and assessment parameters, as well as undefined and insufficiently long post-treatment follow-up period. Furthermore, there is still no sufficiently effective, inexpensive and reliable method, by which the number of applied MSCs could be determined. Such a method could serve as the foundation of advances in personalized medicine and could provide even greater advances in OA therapy. 

While joint replacement surgery still represents the gold standard in the treatment of OA, MSCs therapy provides a possibly great alternative and is assumed that it will take a major role in future OA treatment.

VP, TK and VMo reviewed the literature and drafted the manuscript, VMa, F? and DP critically revised the manuscript. All of the authors approved the final version of the manuscript.

This research received no external funding.

No funding was received to support the research or data analysis in this article.

The authors declare no conflict of interest.