Mesenchymal stem cell-derived exosomes as a promising cell-free therapy for knee osteoarthritis

This review addresses the growing need for disease-modifying therapies for knee osteoarthritis (OA), a prevalent degenerative joint disorder characterized by cartilage degradation, subchondral bone remodeling, osteophyte formation, and synovial inflammation. Conventional pharmacologic treatments provide symptomatic relief but do not halt progression, and surgical options are costly and invasive. Mesenchymal stem cells (MSCs) and their exosomes (MSC-Exos) have emerged as promising regenerative and immunomodulatory approaches. The purpose of the review is to summarize current evidence on how MSC-Exos influence chondrocyte biology, inflammation, and extracellular matrix (ECM) homeostasis; to compare exosomes from different MSC sources; and to outline engineering strategies to enhance exosome therapeutic efficacy and delivery for knee OA. The importance lies in establishing MSC-Exos as a feasible, cell-free alternative to MSC therapy with potentially lower immunogenicity, improved stability, and fewer ethical concerns.

The review synthesizes preclinical findings across diverse MSC sources (bone marrow, adipose tissue, umbilical cord, synovium, embryonic stem cell-derived MSCs, infrapatellar fat pad, urine-derived stem cells). Prior studies show MSC-Exos promote chondrocyte proliferation and migration, inhibit apoptosis, modulate inflammatory pathways, and rebalance ECM synthesis/degradation by affecting markers such as COL2, SOX9, aggrecan, and reducing MMP-13/ADAMTS5. Multiple exosomal miRNAs (e.g., miR-92a-3p, miR-100-5p, miR-125a-5p, miR-129-5p, miR-140-5p, miR-155-5p, miR-212-5p, miR-320c, miR-3960) and lncRNAs/circRNAs (e.g., KLF3-AS1, LYRM4-AS1, NEAT1, circHIPK3, circRNA3503) mediate these effects through pathways including WNT, AKT/ERK, NF-κB, PTEN, mTOR, and others. Anti-inflammatory actions include promoting macrophage polarization to M2 and reducing IL-1β, TNF-α, and IL-6 while increasing IL-10. Studies also show improved outcomes with engineered exosomes via cargo loading, surface modification, culture environment modulation (3D culture, hypoxia, mechanical stimuli), and biomaterial-assisted delivery (hydrogels, ECM scaffolds). A completed clinical pilot using BM-MSC-derived ExoFlo reported pain reduction and functional improvement at 6 months, indicating early clinical promise.

This is a narrative review summarizing evidence from prior in vitro and in vivo preclinical studies and limited clinical data on MSC-derived exosomes for knee OA. The article compiles mechanistic findings (effects on chondrocyte proliferation/apoptosis, inflammation, and ECM balance), compares exosome sources, and surveys engineering strategies (cargo loading, surface targeting modifications, altering production conditions, and biomaterial carriers). No systematic search strategy, inclusion/exclusion criteria, or meta-analytic methods are described; instead, the review integrates representative studies and recent advances to provide a comprehensive overview.

- MSC-Exos modulate key OA processes: They promote chondrocyte proliferation and migration, inhibit apoptosis (e.g., via increased Bcl-2/Survivin, decreased cleaved caspase-3; suppression of mitochondrial-induced apoptosis through Akt phosphorylation and ERK/p38 inhibition), reduce inflammatory cytokines (TNF-α, IL-1β, IL-6) while increasing anti-inflammatory cytokines (IL-10, IL-4, TGF-β), and restore ECM homeostasis (upregulating COL2, aggrecan, SOX9, GAGs, TIMPs; downregulating MMP-13, ADAMTS5). - Exosomal cargo mediates therapeutic effects: miRNAs (e.g., miR-92a-3p targeting WNT5A; miR-100-5p targeting NOX4 and inhibiting mTOR; miR-125a-5p targeting E2F2; miR-129-5p targeting HMGB1; miR-140-5p; miR-212-5p targeting ELF3; miR-320c increasing SOX9 and decreasing MMP-13; miR-3960 inactivating SDC1/Wnt/β-catenin; miR-9-5p inhibiting SDC1) and lncRNAs/circRNAs (e.g., KLF3-AS1, LYRM4-AS1/GRPR/miR-6515-5p, NEAT1, circHIPK3, circRNA3503) are key effectors. - Immunomodulation: MSC-Exos promote macrophage polarization towards M2 phenotypes and can prevent macrophage ferroptosis (e.g., via GOT1/CCR2/Nrf2/HO-1), improving the intra-articular microenvironment. - Source and culture conditions matter: Exosomes from different MSC sources show varying efficacy; 3D culture substantially increases yield (reported ~7.5-fold higher vs. 2D) and can enhance therapeutic potency; hypoxic preconditioning boosts chondroprotective effects; mechanical stimulation (e.g., rotary culture, low-intensity pulsed ultrasound) can increase yield and functional activity (e.g., upregulating LncRNA H19). - Engineering strategies enhance efficacy and targeting: Pre-loading and post-loading enable enrichment with miRNAs, small molecules (e.g., curcumin), and proteins (e.g., TGF-β3, BMP-6, WNT3a). Surface modifications (e.g., chondrocyte-affinity peptides like CAP; Lamp2b fusions; nanoparticle hybrids) improve targeting and enable gene editing cargo delivery (e.g., CRISPR/Cas9 sgMMP-13) to deep cartilage zones. Biomaterial carriers (GelMA, mussel-inspired adhesive hydrogels, acellular cartilage ECM scaffolds, 3D-printed ECM/GelMA scaffolds) prolong intra-articular retention and enhance regeneration. - Preliminary clinical signal: A completed pilot trial (ExoFlo; BM-MSC-Exos) showed significant pain reduction and functional improvement at 6 months, with acceptable safety, supporting translational potential.

The compiled evidence supports MSC-Exos as a feasible cell-free therapeutic candidate for knee OA, addressing key pathogenic mechanisms by enhancing chondrocyte anabolism and survival, tempering inflammation through macrophage reprogramming, and restoring ECM balance. Compared with MSC therapy, exosomes offer lower immunogenicity, improved storage stability, and fewer ethical constraints, while retaining key paracrine functions. Engineering approaches further improve potency, specificity, and intra-articular persistence, directly addressing challenges in natural exosome yield, targeting, and functional cargo abundance. Nonetheless, translation requires overcoming variability in isolation, characterization, and dosing, and validating efficacy and safety in large-animal models and controlled clinical trials. The findings highlight a path toward disease-modifying, minimally invasive interventions for knee OA that could complement or delay surgical procedures.

MSC-derived exosomes are a promising cell-free therapy for knee osteoarthritis, capable of promoting chondrocyte regeneration, modulating immune responses, and reestablishing ECM homeostasis. Engineering strategies—cargo loading, surface modification, optimized culture environments, and biomaterial-assisted delivery—can further enhance therapeutic efficacy and targeting. To advance clinical translation, future work should prioritize standardized, scalable manufacturing and rigorous characterization, address donor selection and pathogen safety, optimize storage and delivery regimens, and conduct large-animal studies followed by well-designed clinical trials. With continued development, MSC-Exos may offer an effective alternative or adjunct to current OA treatments, potentially delaying or reducing the need for joint replacement.

- Evidence base is primarily preclinical and often limited to small-animal models; large-animal validation is needed before clinical trials. - Significant variability in MSC-Exos preparation due to source differences, culture conditions, and isolation methods; lack of standardized protocols complicates reproducibility and comparability. - Donor-related constraints (e.g., low BM-MSC yield; invasive harvesting; screening for infections/genetic diseases for UC-MSCs) and challenges in pathogen removal while preserving exosome function. - Many exosomes may carry low quantities of functional miRNAs, necessitating engineering to increase cargo load. - Current isolation methods are time-consuming, low-yield, and may not produce homogeneous, high-purity preparations; scale-up remains challenging despite advances (bioreactors, microfluidics). - Potential immunogenicity of engineered exosomes due to functional proteins/immune molecules; rapid clearance is possible. - Storage conditions, administration route, dosing, and timing significantly affect bioactivity and outcomes, and require optimization.