Mesenchymal Stromal Cells in Acute Respiratory Distress Syndrome: More Questions Than Answers

Abstract

An effective treatment for acute respiratory distress syndrome (ARDS) remains elusive. Preclinical studies have demonstrated therapeutic potential of multiple mesenchymal stromal cell (MSC) and MSC-related therapies in the context of ARDS, including autologous, allogenic, xenogenic, and induced pluripotent stem cell–derived MSCs; multipotent adult progenitor cells; and MSC-derived extracellular vesicles and exosomes (1). These cells, or cell-free products, have been sourced from various tissues, including bone marrow, umbilical cord, perinatal tissue, and adipose. However, few preclinical studies, and no clinical trials, have compared these head to head. Considerable debate persists as to what is the most efficacious cell product. The situation is further complicated when factors such as scalability, storage, delivery, dose, safety, and regulatory compliance are considered. In this issue of the Journal, Batchinsky and colleagues (pp. 1283–1292) use a porcine model of smoke inhalation injury to compare the therapeutic efficacy of autologous bone marrow aspirate concentrate (BMAC) (n = 10), allogenic bone marrow–derived MSCs (n = 10), and sham injection (n = 12) (2). Their well-established model combined wood smoke inhalation, targeting a carboxyhemoglobin concentration >70%, with a full-thickness burn affecting 40% of the total body surface area, inducing moderate ARDS in the sham group by 24 hours. Animals were supported with protocolized intensive care until death or 72 hours. Autologous BMAC was harvested after injury, at 24 and 48 hours, and concentrated at the bedside, using a point-of-care, automated, closed-loop apheresis device. Allogenic MSCs were procured from bone marrow aspirate before the experiments and were expanded by serial passage. Each animal group received a total of three treatments: immediately after injury, at 24 hours, and at 48 hours using a pulmonary artery catheter. The study showed that BMAC, and to a lesser extent allogenic MSCs, effectively delayed the decline in PaO2 :Fi O2 ratio associated with injury, postponing the onset of ARDS. Notably, 4 of the 10 animals treated with BMAC did not meet ARDS criteria by 72 hours. The BMAC-treated group demonstrated longer mean survival time and a reduced mortality rate compared with the sham and allogeneic MSC groups. Furthermore, the BMAC-treated group had a lower incidence of acute kidney injury; lower systemic IL-6, IL-8, and HMGB1 (high mobility group box 1) concentrations; and larger amounts of normally aerated lung tissue on computed tomography imaging. This was a complex and resource-intensive study with key strengths. The challenges of successfully executing a large-animal model that requires clinical-level intensive care and, at the same time, deeply characterizing the effect of the interventions should not be underestimated. The study was clinically relevant by testing MSCs and BMAC after injury occurred (albeit before the onset of ARDS). Although the authors conclude that autologous MSCs (from BMAC) appear more potent than allogenic MSCs, the situation is more nuanced. Allogenic cells in this study had some protective effects, but the dose used was much lower than those used in previous large-animal studies or clinical trials of ARDS (usually 1 million to 10 million cells/kg compared with a total dose of approximately 0.4 million cells/kg in this study). The relatively modest therapeutic effect in the allogenic MSC–treated group is therefore unsurprising. It is also an oversimplification to describe autologous BMAC as “autologous MSCs.” BMAC is an enriched mixture of mononucleated cells, platelets, cytokines, and growth factors, many of which have immunomodulatory effects in ARDS (3, 4). MSCs account for only ∼0.001–0.01% of the total mononuclear cell population of bone marrow (5). The dose of MSCs in the BMAC in this study was not known, but rather a total white-cell count was available. Notably, this was less than the dose of allogeneic MSCs in the comparator group. It is plausible that some beneficial effects demonstrated by BMAC in this study relate to the non-MSC components (Table 1). Table 1. Comparison of Allogenic Mesenchymal Stromal Cell Products and Bone Marrow Aspirate Concentrate   Allogenic MSC Preparations Autologous BMAC MSCs Pure population: fixed number per dose Variable percentage of total cell count Platelets — Present (variable amount) Leukocytes — Neutrophils, erythroblasts, lymphocytes, eosinophils, monocytes, and basophils (variable amount) Cytokines — IL-1Ra, IL-1β, IL-6, IL-8, GM-CSF, TNF-α variably reported Growth factors — TGF-β, VEGF, PDGF, FGF-2/18, IGF-1 Other proteins — BMPs, osteoprotegerin reported in some but not all BMAC preparations Potentially Immunogenic* Yes No Persistence of cellular components Rapidly cleared Not yet known, but not foreign Expansion/passage in vitro † Yes No Freeze–thaw cycle ‡ Usually (for clinical grade product) No Excipients Usually DMSO None Exogenous animal product used in manufacture Fetal bovine serum None Release criteria for clinical grade product Known Unknown Potency assay No standard assay Unknown Is suitability age dependent? No Likely Definition of abbreviations: BMAC = bone marrow aspirate concentrate; BMP = bone morphogenic protein; FGF = fibroblast growth factor; GM-CSF = granulocyte–macrophage colony-stimulating factor; IGF-1 = insulin-like growth factor 1; MSC = mesenchymal stromal cell; PDGF = platelet-derived growth factor; TGF-β = transforming growth factor-β; VEGF = vascular endothelial growth factor. * Clinical studies have not shown clear anti-HLA antibody development, but long-term studies are needed. † Serial passage linked to reduction in therapeutic efficacy. ‡ Freeze–thaw cycle may affect viability. Although the present study supports further exploration of its therapeutic potential, BMAC presents substantial challenges. The extent of MSC enrichment after concentration in BMAC production varies according to site of aspiration, donor sex, and the device used to perform concentration (6, 7). The concentration process may induce phenotypic alterations within MSCs, modifying their in vivo actions (8), and in swine, the onset of lung injury has been shown to render the MSCs retrieved from autologous bone marrow less effective in terms of regenerative and immunomodulatory properties compared with uninjured control animals (9). The issues of batch-to-batch variation and lack of definitive potency assays for efficacy that have plagued the field of MSC therapy are even more relevant for BMAC. Although BMAC offers the potential advantage of retrieving MSCs that will not lose efficacy by undergoing repeated passage in vitro and avoids the use of animal products such as fetal bovine serum or excipients such as DMSO, it is difficult to understand how we might standardize BMAC composition at the bedside for clinical trials and therapeutic use. Furthermore, with increasing age, the cell yield from BMAC falls rapidly. A recent study showed a greater than 67% reduction in mononuclear cell yield per milliliter of marrow by 55–60 years of age compared with 19–20 years (6). In the LUNG SAFE (Large Observational Study to Understand the Global Impact of Severe Acute Respiratory Failure) study, the mean age of patients with ARDS was 61 years (95% confidence interval, 60–62 years) (10). Even if therapeutically effective in other preclinical models of ARDS, clinical trials may not be feasible if the substrate for autologous BMAC is insufficient in most patients. The heterogeneity of BMAC reflects the heterogeneity evident in the entirety of ARDS cell therapy. Phenotypically different cell populations, obtained from diverse sources using diverse protocols, and applied to a heterogeneous syndrome cannot be expected to produce consistent outcomes, a fact evident in the results of recent clinical trials in both coronavirus disease (COVID-19)–related and non–COVID-19–related ARDS (11–15). So how can researchers address the heterogeneity, and can preclinical models contribute? First, there should be a concerted effort to better characterize “MSCs” and their related products used in experimentation, ideally beyond the minimal criteria required to identify MSCs and including assays of function and relevant biological activity. An example is tissue factor/CD142 (cluster of differentiation 142) expression, which varies widely among MSCs but is implicated in the risk of thromboembolic complications (16). Second, we need to develop an approach that allows comparison of different cell products in a common model or a consistent product in alternative models. Given the cost and complexity of high-fidelity models, such as the one described in the present study, no single research group is likely to achieve this. The solution may be to adopt protocols and collaborations that permit multicenter platform studies, as is an evolving norm among clinical trials. The recent endeavor of the Stroke Preclinical Assessment Network, which tested six treatment candidates across multiple models in 2,615 mice, serves as an exemplar (17).