Traumatic injuries demand immediate hemorrhage control and resuscitation, as well as definitive repair and infection management to prevent death from hemorrhagic shock.1-3 Recent advances in hemorrhage control and blood-based, hemostatic resuscitation can reduce the risk of early death due to bleeding, but a significant burden of morbidity and mortality thought to be caused by inflammation leading to multiorgan failure (MOF) persists and, currently, adequate treatment modalities are lacking. Stem cells, which include embryonic stem cells (ES cells), induced pluripotent stem cells (iPS cells), and mesenchymal stem cells (MSCs) are unspecialized, multipotent progenitor cells with the capability of self-renewal followed by differentiation.4-15 They respond to inflammatory mediators and home to the site of injury and can release paracrine factors for growth signaling and immunomodulation.16, 17 By utilizing these mechanisms, stem cells may reduce mortality due to MOF and promote wound healing. Stem cells have been shown to hold great promise in treating other pathophysiologic states in which inflammation plays a major role such as acute graft-versus-host disease, acute respiratory distress syndrome, myocardial infarction, and stroke.18-23 Thus, cellular therapy with stem cells for traumatic injuries also holds great promise. However, a major limitation to the use of either ES or iPS cells has been the formation of teratomas.24-28 MSCs are adult stem cells generally harvested from marrow, but also found in the stromal vascular fraction of other tissues.29-32 Upon appropriate signaling, MSCs differentiate into bone, cartilage, or fat cells.33-35 Additionally, they appear to have a paracrine function and have also been termed mesenchymal signaling cells or mesenchymal stromal cells.36-42 While MSCs have been shown to home to tumors, unlike ES or iPS cells, MSCs do not form teratomas.43-45 The therapeutic potential of MSCs rests upon their regenerative capabilities, medicinal anti-inflammatory properties, and improved safety profile over other stem cells.46, 47 In considering the potential use of MSCs in the early management of civilian or military trauma, an understanding of the pathophysiologic context of their use is required. For both civilians and military personnel, hemorrhage is the leading potentially reversible cause of traumatic death.48 In the last two conflicts in Southwest Asia, 26% of 4090 “killed in action” deaths were deemed potentially survivable. Fully 92% of these, or 984 deaths, were due to hemorrhage.49 Deaths due to hemorrhage are due to anatomic disruption and coagulopathy. In Kandahar, Afghanistan, 1.5% of the admissions were multiamputees; they required initiation of massive transfusion protocols with a mean of 54 units of blood products. Overall, in the last two conflicts of Operation Iraqi Freedom and Operation Enduring Freedom 13.6% of patients received transfusions. Coagulopathy, as evidenced by a mean international normalized ratio of more than 1.5 in patients receiving a transfusion, correlated with a fivefold increase in mortality.50, 51 In severe trauma complicated by coagulopathy, there is massive fibrinogen consumption, with altered clotting kinetics and fibrinolysis, as well as platelet (PLT) consumption and dysfunction. Crystalloid fluid administration causes hemodilution and acidosis, and frequently hypothermia, adding iatrogenic injury that worsens bleeding. Decreased tissue perfusion with blood loss leads to tissue anoxia and acidosis. This completes the lethal triad of acidosis, hypothermia and coagulopathy.52-55 The breach of the endothelium with tissue disruption leads to exposure of tissue factor (TF) and activation of the coagulation pathway as well as suppression of endothelial cell function. Nitric oxide metabolism is disrupted with decreased expression of endothelial nitric oxide synthase and thus decreased synthesis of nitric oxide, increased PLT, and neutrophil adhesion results. With lengthy hemorrhagic shock, tissues begin to express inducible nitric oxide synthase leading to a profound hypotension.56 The endothelial disruption also leads to loss of local anti-inflammatory properties and induction of fibrinolysis. How will MSCs be used in the early management of trauma? As immunomodulatory cells, it has been proposed that systemically administered MSCs may reduce the risk of multiorgan failure. The question is whether acute use of MSCs would alleviate or potentially exacerbate acute traumatic coagulopathy and how they will affect the risk of postresuscitation thrombosis, a leading cause of trauma-related morbidity and mortality. How this envisaged use of MSCs will influence wound healing, which is thought to require MSC homing to damaged tissues where they play an innate immune as well as trophic role during tissue remodeling remains to be determined. Systemic administration of MSCs would require placing multifunctional cells into a highly perturbed physiologic setting undergoing complex clinical interventions. MSCs have been isolated from multiple organs in addition to marrow, including heart, liver, kidney, umbilical cord blood, and adipose tissue.29-31 They are not isolated as a pure cell type but are a minute population selected from the total cell population. Within the marrow, MSCs differ in phenotype depending on their niche. Endosteal MSCs supply hematopoietic stem cell (HSC) quiescence factors, while perivascular MSCs, at some distance from the endosteum, supply factors stimulating HSC activation and differentiation.57 Consider also that endothelial cells usually separate MSCs from blood, which begs the question of how compatible MSCs are with blood. MSCs are known to express TF.58, 59 Phosphatidylserine (PS) is exposed during trypsin release from tissue culture plastic or during the recovery from frozen storage. Primary MSC products contain contaminating cells and other biomaterial including microvesicles. TF, PS surface expression, and microvesicles are all procoagulant.60, 61 What are the effects of MSC expansion on these variables and what are the potential interactions with the recipient, particularly in the setting of severe trauma? It is possible that MSCs will behave as beneficial hemostatic agents, or they could produce an excessively procoagulant environment, depending on dose and timing, and require concomitant anticoagulant administration to prevent venous thromboembolism or disseminated coagulopathy. Stephenne and colleagues62 found that MSCs accelerated clotting due to increased TF expression. When using human adult liver progenitor cells, a type of MSC isolated from liver, they were able to limit thrombotic events by coinfusion of bivalirudin and unfractionated heparin. Such an approach would not be feasible in a still-coagulopathic trauma patient that may require further operative interventions. TF expression has been found to be up regulated by multiple agents including IL-1α, TGF-β. CD40, VEGF, LPS, hypoxia, and shear stress.61 NF-κB has been found to play a critical role in regulation of TF expression.63 MSCs infused after trauma will encounter an environment containing elevated levels of these factors. While MSC anti-inflammatory effects may reduce the expression of these factors, it is also possible that the MSC phenotype will be rendered more procoagulant by the host. Thus, in addition to determining how collection, expansion, and processing of MSCs may contribute to potential infusional toxicities, it will be important to study cell–host interactions to understand the safety and efficacy variables of this cell therapy product. As might be expected, there are myriad complexities in MSC–blood interactions. In addition to expressing the procoagulant compounds TF and PS on their surfaces, MSCs also can express the anticoagulant TF pathway inhibitor and fibrinolytics such as tissue plasminogen activator and urokinase plasminogen activator and its receptor.58 MSCs also express the antifibrinolytic, plasminogen activator inhibitor.65, 66 MSCs express long pentraxin 3 (PTX3), an acute-phase multimeric glycoprotein. PTX3 is involved in inflammation, innate immunity, extracellular matrix deposition, and wound healing through potentiation of fibrinolysis.67, 68 In an acidic environment, PTX3 accelerates fibrinolysis by enhancing plasminogen and fibrin binding. However, persistently high levels of PTX3 during sepsis are associated with worse outcomes. It is unknown how the expression of all these critical factors in coagulation and fibrinolysis varies across the different MSC sources, it is unknown how culture conditions affect them, and it is unknown what effects dosage or route of administration may have. Moll and colleagues64 found fibronectin 1, collagen type 1, and TF surface expression on MSCs significantly increased at Passage 6 versus Passage 3. Although they observed profound donor variability, in an in vitro measure of instant blood-mediated coagulation and inflammatory response, at 15,000 MSCs/mL, free PLTs were reduced approximately 50% at 30 minutes for all donors. This PLT activation was MSC dose and time dependent. The coagulation cascade was initiated—both visible clots were seen and the levels of thrombin–anti-thrombin complexes increased, again in a dose- and time-dependent manner. Tellingly, PLT activation and elevated thrombin–anti-thrombin complexes complexes were dependent on cell passage; later-passage MSCs were more procoagulant. The consequences of these MSC phenotypic changes for the trauma patient are anything but clear: addition of MSCs could exacerbate or resolve coagulopathy and PLT dysfunction; it could result in enhanced fibrinolysis and, thus, enhanced wound healing, but possibly cause late rebleeding. The question of MSC compatibility with stored blood products and hemostatic adjuncts is also relevant for the trauma patient. Should timing and route of administration be different for MSCs and other resuscitation products? Consider the increased use of tranexamic acid (TXA) to control bleeding in trauma since the publication of the CRASH-2 trial.69 Widely used TXA protocols call for delivery to the patient within 3 hours. Within this time frame, TXA demonstrated a reduced odds ratio (OR) for mortality. However, given after 3 hours from injury, TXA was associated with an increased OR for mortality. Timing of administration is critical for this antifibrinolytic.70 Will similar considerations hold for MSCs in trauma? Route of delivery of MSCs represents another important consideration in optimizing this therapy. Wounds are often poorly perfused and hypoxic. Hypoxia increases the MSC microvesicle concentrations of UPAR, VEGF, and IL-6 and other cytokines and trophic factors. In situations such as traumatic brain injury where the injury core is surrounded by a less-damaged perilesional area, which is more receptive to treatment, direct injection of MSCs into the hypoxic wound may be a more effective route than IV delivery. Studies are required to optimize the possible combination of local versus systemic administration and frequency of dosing. Feasibility of such treatments and costs will also need to be evaluated.71, 72 The immunomodulatory properties of MSCs and their potential capacity to reduce the risk of MOF should be studied in relation to the observed capacity of stored blood products to cause immunosuppression or transfusion-related immunomodulation.73, 74 If MSCs are given during a massive transfusion protocol with multiple blood products, would there be additive effects? It would be helpful to have biomarkers to differentiate “good” immunomodulation from “bad” transfusion-related immunomodulation. If MSCs are shown to be beneficial for the trauma patient, what would be the transfusion trigger? Could vital signs, such as heart rate, systolic blood pressure, shock index, or lactate measurement guide the decision?74 What endpoint is targeted? How will the need for redosing be determined? Should MSCs be integrated into a point-of-injury protocol for tactical damage control resuscitation or given in the ICU or much later during reconstruction and rehabilitation?75 Clearly, it would be helpful to have biomarkers for MSC safety and efficacy to rationally design treatment algorithms. While responding to the challenges outlined above, the field of cellular therapy is also undergoing an organizational evolution. One model for the field's development is patterned on the pharmaceutical industry, in which highly characterized, perhaps uniform or genetically modified MSCs will be produced for off-the-shelf use. Another model is the “blood banking” model in which allogeneic or autologous MSCs from many donors are expanded and stored with minimal modification or characterization across many organizations. Finally, there is the “clinical” model, in which autologous mixed MSC and mononuclear cell populations are reapplied to the patient locally or systemically with minimal manipulation. Each of these models presents certain advantages and disadvantages. Clearly, the “pharma” model offers the highest degree of controls on product characteristics and comparability across clinical trials, but is likely to result in constrained supplies and high costs. The blood bank model offers broader availability and cost advantages at the price of product variability. The clinical model reduces risks associated with allogeneic products, but may be very limited in its practicability and perhaps efficacy due the complexities of ensuring timely and adequate collections to allow proper dosing and timing in critically ill patients. Which development model is most appropriate? Can we identify potency markers for MSCs that allow us to define an envelope of acceptable performance that facilitates simultaneous product development along all three pathways? Current investigational metrics examine tropism, endothelial permeability, cytokine release, and synthesis of the extracellular matrix. Are these reliable and appropriate biomarkers of MSC efficacy in trauma or do we need additional functional correlates such as alterations in T-cell activation or macrophage polarization? Ideally, positive outcomes should correlate with known biomarkers. This will require further preclinical testing of MSCs, but the payoff will be more rational clinical trial design and improved comparability between clinical trials. Adopting broadly accepted standards for MSC evaluation will greatly facilitate growth in this branch of cellular therapy. In summary, application of MSCs in the setting of trauma and coagulopathy presents unique opportunities and challenges that have received comparatively little attention to date. The field of MSC therapy lacks adequate potency and safety biomarkers generally, but certainly none have been rigorously validated in complex, critically ill patients receiving significant quantities of stored blood products, hemostatic adjuncts, and other interventions that alter the coagulation and inflammation host environment so dramatically. MSCs have not even been adequately studied in relevant animal models to guide clinical development. Questions that require urgent attention include source of cells (marrow, other?), mechanism of action, selection of patients, dose, route of delivery (IV, IO, other?), timing, preclinical efficacy testing, clinical biomarkers of potency, endpoint selection, and integration with other blood products and hemostatic adjuncts. These challenges are not insurmountable and while one does not want “perfect” to be the enemy of “good,” a robust investment in defining acceptable performance metrics is the catalyst that will spur the development of the field. The authors have disclosed no conflicts of interest.
No takes yet. Share an insight, caveat, or question.
Maryanne C. Herzig (2016) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: