Acute respiratory distress syndrome (ARDS) in COVID-19 is associated with high mortality. Mesenchymal stem cells are known to exert immunomodulatory and anti-inflammatory effects and could yield beneficial effects in COVID-19 ARDS. The objective of this study was to determine safety and explore efficacy of umbilical cord mesenchymal stem cell (UC-MSC) infusions in subjects with COVID-19 ARDS. A double-blind, phase 1/2a, randomized, controlled trial was performed. Randomization and stratification by ARDS severity was used to foster balance among groups. All subjects were analyzed under intention to treat design. Twenty-four subjects were randomized 1:1 to either UC-MSC treatment (n = 12) or the control group (n = 12). Subjects in the UC-MSC treatment group received two intravenous infusions (at day 0 and 3) of 100 ± 20 × 106 UC-MSCs; controls received two infusions of vehicle solution. Both groups received best standard of care. Primary endpoint was safety (adverse events [AEs]) within 6 hours; cardiac arrest or death within 24 hours postinfusion). Secondary endpoints included patient survival at 31 days after the first infusion and time to recovery. No difference was observed between groups in infusion-associated AEs. No serious adverse events (SAEs) were observed related to UC-MSC infusions. UC-MSC infusions in COVID-19 ARDS were found to be safe. Inflammatory cytokines were significantly decreased in UC-MSC-treated subjects at day 6. Treatment was associated with significantly improved patient survival (91% vs 42%, P = .015), SAE-free survival (P = .008), and time to recovery (P = .03). UC-MSC infusions are safe and could be beneficial in treating subjects with COVID-19 ARDS.

Bone fractures and segmental bone defects are a significant source of patient morbidity and place a staggering economic burden on the healthcare system. The annual cost of treating bone defects in the US has been estimated to be $5 billion, while enormous costs are spent on bone grafts for bone injuries, tumors, and other pathologies associated with defective fracture healing. Autologous bone grafts represent the gold standard for the treatment of bone defects. However, they are associated with variable clinical outcomes, postsurgical morbidity, especially at the donor site, and increased surgical costs. In an effort to circumvent these limitations, tissue engineering and cell-based therapies have been proposed as alternatives to induce and promote bone repair. This review focuses on the recent advances in bone tissue engineering (BTE), specifically looking at its role in treating delayed fracture healing (non-unions) and the resulting segmental bone defects. Herein we discuss: (1) the processes of endochondral and intramembranous bone formation; (2) the role of stem cells, looking specifically at mesenchymal (MSC), embryonic (ESC), and induced pluripotent (iPSC) stem cells as viable building blocks to engineer bone implants; (3) the biomaterials used to direct tissue growth, with a focus on ceramic, biodegradable polymers, and composite materials; (4) the growth factors and molecular signals used to induce differentiation of stem cells into the osteoblastic lineage, which ultimately leads to active bone formation; and (5) the mechanical stimulation protocols used to maintain the integrity of the bone repair and their role in successful cell engraftment. Finally, a couple clinical scenarios are presented (non-unions and avascular necrosis—AVN), to illustrate how novel cell-based therapy approaches can be used. A thorough understanding of tissue engineering and cell-based therapies may allow for better incorporation of these potential therapeutic approaches in bone defects allowing for proper bone repair and regeneration.

Objective: To test the hypothesis that CD45^lowCD271+ bone marrow multipotential stromal cells (MSCs) are abundant in the trabecular bone niche and to explore their functional “fitness” in health and osteoarthritis (OA).Methods: Following enzymatic extraction, MSC release was evaluated using colony‐forming unit–fibroblast (CFU‐F) and colony‐forming unit–osteoblast assays, flow cytometry, and confocal microscopy. CD45^lowCD271+ cells isolated by fluorescence‐activated cell sorting were enumerated and expanded under standard and clonal conditions. Their proliferative and osteogenic potencies were assessed in relation to donor age and compared with those of aspirated CD45^lowCD271+ cells. In vitro and in vivo MSC “aging” was measured using quantitative polymerase chain reaction–based telomere length analysis, and standard differentiation assays were utilized to demonstrate multipotentiality.Results: Cellular isolates from trabecular bone cavities contained ∼65‐fold more CD45^lowCD271+ cells compared with aspirates (P < 0.0001) (median 1.89% [n = 39] and 0.029% [n = 46], respectively), concordant with increased CFU‐F release. Aspirated and enzymatically released CD45^lowCD271+ cells had identical MSC phenotypes (∼100% CD73+CD105+CD13+, ∼50–60% CD146+CD106+CD166+) and contained large proportions of highly clonogenic multipotential cells. In vitro osteogenic potency of freshly isolated CD45^lowCD271+ cells was comparable with, and often above, that of early‐passage MSCs (8–14%). Their frequency and in vivo telomere status in OA bone were similar to those in bone from age‐matched controls.Conclusion: Our findings show that CD45^lowCD271+ MSCs are abundant in the trabecular bone cavity and indistinguishable from aspirated CD45^lowCD271+ MSCs. In OA they display aging‐related loss of proliferation but no gross osteogenic abnormality. These findings offer new opportunities for direct study of MSCs in musculoskeletal diseases without the requirement for culture expansion. They are also relevant for direct therapeutic exploitation of prospectively isolated, minimally cultured MSCs in trauma and OA.

Mesenchymal stem/stromal cell (MSC) exist within their in vivo niches as part of heterogeneous cell populations, exhibiting variable stemness potential and supportive functionalities. Conventional extensive 2D in vitro MSC expansion, aimed at obtaining clinically relevant therapeutic cell numbers, results in detrimental effects on both cellular characteristics (e.g., phenotypic changes and senescence) and functions (e.g., differentiation capacity and immunomodulatory effects). These deleterious effects, added to the inherent inter-donor variability, negatively affect the standardization and reproducibility of MSC therapeutic potential. The resulting manufacturing challenges that drive the qualitative variability of MSC-based products is evident in various clinical trials where MSC therapeutic efficacy is moderate or, in some cases, totally insufficient. To circumvent these limitations, various in vitro/ex vivo techniques have been applied to manufacturing protocols to induce specific features, attributes, and functions in expanding cells. Exposure to inflammatory cues (cell priming) is one of them, however, with untoward effects such as transient expression of HLA-DR preventing allogeneic therapeutic schemes. MSC functionalization can be also achieved by in vitro 3D culturing techniques, in an effort to more closely recapitulate the in vivo MSC niche. The resulting spheroid structures provide spatial cell organization with increased cell–cell interactions, stable, or even enhanced phenotypic profiles, and increased trophic and immunomodulatory functionalities. In that context, MSC 3D spheroids have shown enhanced “medicinal signaling” activities and increased homing and survival capacities upon transplantation in vivo. Importantly, MSC spheroids have been applied in various preclinical animal models including wound healing, bone and osteochondral defects, and cardiovascular diseases showing safety and efficacy in vivo. Therefore, the incorporation of 3D MSC culturing approach into cell-based therapy would significantly impact the field, as more reproducible clinical outcomes may be achieved without requiring ex vivo stimulatory regimes. In the present review, we discuss the MSC functionalization in 3D settings and how this strategy can contribute to an improved MSC-based product for safer and more effective therapeutic applications.

Objective: Controversy surrounds the identity and functionality of rare bone marrow–derived multipotential stromal cells (BM‐MSCs), including their differentiation capabilities, their relationship to pericytes and hematopoiesis‐supporting stromal cells, and the relevance of their culture‐expanded progeny in studies of skeletal biology and development of cell‐based therapies. The aim of this study was to clarify the nature of candidate BM‐MSCs by profiling transcripts that reflect different aspects of their putative functions in vivo.Methods: Rare, sorted BM‐derived CD45^−/low CD271^bright (CD271) cells were analyzed using 96‐gene expression arrays focused on transcripts relevant to mesenchymal‐lineage differentiation (toward bone, cartilage, fat, or muscle), hematopoietic and stromal support, and molecules critical to skeletal homeostasis. These cells were compared to matched CD45+ CD271− hematopoietic‐lineage cells, culture‐expanded MSCs, and skin fibroblasts. When feasible, transcription was validated using flow cytometry.Results: CD271 cells had a transcriptional profile consistent with the multiple fates of in vivo MSCs, evident from the observed simultaneous expression of osteogenic, adipogenic, pericytic, and hematopoiesis‐supporting genes (e.g., SP7 [osterix], FABP4 [fatty acid binding protein 4], ANGPT1 [angiopoietin 1], and CXCL12 [stromal cell–derived factor 1], respectively). Compared to culture‐expanded MSCs and fibroblasts, CD271 cells exhibited greater transcriptional activity, particularly with respect to Wnt‐related genes (>1,000‐fold increased expression of FRZB [secreted frizzled‐related protein 3] and WIF1 [Wnt inhibitory factor 1]). A number of transcripts were identified as novel markers of MSCs.Conclusion: The native, BM‐derived in vivo MSC population is endowed with a gene signature that is compatible with multiple functions, reflecting the topographic bone niche of these cells, and their signature is significantly different from that of culture‐expanded MSCs. This indicates that studies of the biologic functions of MSCs in musculoskeletal diseases, including osteoporosis and osteoarthritis, should focus on in vivo MSCs, rather than their culture‐adapted progeny.