Cell-based therapies in pulmonary hypertension: who, what, and when?

Susan Majka, Ellen Burnham, Kurt R. Stenmark

pulmonary hypertension (PH), diagnosed when mean pulmonary arterial pressure exceeds the upper limits of normal (i.e., >25 mmHg) at rest (2), occurs in a variety of clinical situations and is associated with a broad spectrum of histological patterns and abnormalities. PH is currently classified into five distinct World Health Organization (WHO) groups, based on common clinical parameters, potential etiological mechanisms, and responses to treatment (22). Although any form of PH can contribute to increased patient morbidity and mortality, pulmonary arterial hypertension (PAH) (WHO group 1) is a particularly severe and progressive form associated with right heart failure and premature death (1). At present, therapeutic approaches to stabilize or reverse this debilitating condition involve treatment with one or a combination of up to three specific classes of agents, including prostacyclin analogs, endothelin-1 receptor antagonists, and/or phosphodiesterase-5 inhibitors. Retrospective (meta)analyses of these therapeutic strategies have demonstrated a reduction in mortality with their use (7, 12); however, many experts believe that current PAH treatment is inadequate given the persistently high mortality rate and functional hemodynamic impairment in many patients. These observations have led to continued intensive investigation into pathogenetic mechanisms and many proposals for additional alternative new therapies (20, 24). Among the potential new therapies, increasing interest in the role of endothelial progenitor cells (EPCs) as a cell-based therapy has emerged. However, issues remain regarding what group of PH patients are most likely to benefit from treatment, at what point in the disease is treatment most likely to be successful, and what types of cells should be utilized for therapy.

The rationale for EPC treatment of PAH is based on the concept that endothelial dysfunction is at the center of pathogenesis in PH (3, 6). Clearly, the endothelium plays a critical role in modulating tone and structure of the pulmonary circulation. Furthermore, there are some reported successes in preventing or even reversing PH in animal models with EPCs, specifically in monocrotaline (MCT)-induced PH in rats (16, 19, 25). The rat model of MCT involves endothelial damage and a dramatic perivascular accumulation of inflammatory cells. It does not optimally represent all histopathological changes associated with human PAH, such as occlusive intimal lesions in the peripheral pulmonary arteries and a protracted time course of development (17, 23). Importantly, the infusion of similar cells has been unsuccessful in reversing hypoxia-induced pulmonary vascular remodeling and hypertension, a model not characterized by massive endothelial cell injury or death (13). Different models used to study experimental PH can make comparisons between studies that utilize the same cell based therapies difficult; however, they also can provide insight to how different clinical types of PH may be more or less amenable to specific types of cell-based therapy.

A major challenge of cell-based therapies for vascular dysfunction has been the lack of understanding how specific populations of infused cells might remedy the pathology associated with this disorder. For instance, to treat PAH, a paracrine role for these cells in vasodilation and immune modulation could be just as important clinically as a role in repairing or replacing abnormally remodeled vascular endothelium. Therefore, appropriate identification and classification of the cell type or types being infused is paramount to understanding how cells will behave in vivo. In studies reported here, Mirsky and colleagues (15) utilized a heterogeneous population of circulating angiogenic cells (CACs; also referred to by others as early outgrowth EPC) in their experiments; less than 1% of the total number infused bore surface markers consistent with endothelial cells [e.g., kinase domain receptor (KDR) positivity], and although the majority of CACs bound lectin and took up LDL, this feature may also be observed in nonendothelial cell types (15). It is thus not surprising that from this small starting number of CD34+/KDR+ and CD133+/KDR+ endothelial cells, none of this type was identified in the lungs of the transplanted animals; however, the possibility that the other 99% of noncharacterized infused cells might modulate pulmonary vascular dysfunction should not be discounted. In fact, lack of persistence of endothelial-like, culture-modified monocytes in the lung, despite improved pulmonary vascular hemodynamics, was observed by Ormiston (17) and colleagues when similar, early outgrowth CD14+/KDR+/CD34 cells were infused in a model of monocrotaline-induced PH. Notably, the effect of infusion of late outgrowth EPCs (CD14/KDR+/CD34+) cells, thought by some to give rise to truly functional and engraftable endothelial cells, in this same model was totally indistinguishable from that of vehicle infusion. Observations such as these point to the importance of clearly delineating cell phenotypes used in therapy to guarantee consistent results across experiments and provide a rationale to study the efficacy of specific progenitor populations in PAH therapy.

Immunomodulatory properties of infused cells, whether paracrine or mediated by cell contact, is an additional issue that must be addressed in future studies evaluating efficacy of EPC cell-based therapies for human PH (5). The well-characterized early outgrowth monocytes described by Ormiston et al. (17) were likened to immunomodulatory dendritic cells. Immunomodulation of T cell and mononuclear subpopulations has been reported for mesenchymal stem cells (MSCs) and suggested to be important for their therapeutic effects in rodent models of acute and chronic lung injuries (8, 10, 14, 18, 21). In fact, in a recent study by Liang et al. (11), MSCs expressing heme oxygenase-1 reversed established hypoxic PH, a problem not reversed by early EPC. Furthermore, many studies employing MSCs were conducted by using autograft models, meaning mouse cells transplanted into syngeneic mouse recipients. In contrast, given the lack of immune tolerance to infused CACs/early outgrowth progenitors, both Mirsky et al. (15) and Ormiston et al. employed xenograft models transplanting human cells into immunocompromised recipients (athymic nude rats). These systems alone complicate the interpretation of therapeutic cell function, since these animals are not totally devoid of immune cells nor are their immune systems reflective of a human PAH patient. Studies analyzing the effect of rodent-derived donor cells in rodent recipients are paramount to determining the efficacy of cell-based therapies in human PAH. Additionally, further characterizing the route of introduction, along with the optimal number of cells required for a therapeutic effect, remains to be determined.

An additional confounding factor that complicates the seemingly conflicting data in the literature regarding efficacy of EPC in PH includes differences in timing of cell-based therapy administration. The studies described by Mirsky et al. (15) employed a MCT model of PAH and introduced their circulating angiogenic cells on either day 14 or 21 following injury, a time in this model when PH is either established (14 days) or advanced (21 days). These studies are in contrast to those performed by Ormiston et al. (17), who introduced their early or late outgrowth EPC populations 3 days post-MCT injury. Only the early administration of the early EPCs abrogated the increase in right ventricular systolic pressure and right ventricular hypertrophy associated with MCT (17). Therefore, the timing of cell administration is an important factor to be considered. Failure of delayed administration of cells with previously demonstrated efficacy in ameliorating PH pathogenesis is disappointing, but not wholly surprising, and suggests that that the introduction of even a highly selected cell population may not be able to reverse established pulmonary vascular disease. However, introduction of the therapeutic cells at the onset of injury while acute inflammation is present and advanced remodeling is minimal could be more efficacious.

Thus, despite the imperative to develop new therapies for idiopathic PAH (iPAH), the efficacy of a cell-based therapy for this disorder remains uncertain. By the time individuals with iPAH present for evaluation of nonspecific symptoms associated with this disorder, they frequently have advanced disease with development of extensive vascular remodeling and microvascular destruction. Such pathology would be expected to influence the success of cell-based therapy, as it may well be more difficult to reverse or even to attenuate progress of such advanced disease. A potentially more optimal target population to consider might be individuals with diseases associated with PAH who have not yet developed frank PAH, such as those with scleroderma. Similar to what is observed in patients with PAH, the majority of patients with scleroderma will be diagnosed with advanced functional class PAH when therapies are less likely to be beneficial (9), although these patients are known to be at risk for its development. Group III WHO classification patients with hypoxemia-mediated PH, such as patients with chronic obstructive pulmonary disease, could be targeted as another potential population. In these patient subgroups, cell-based therapy could be explored as a means of PH prophylaxis. Consideration could also be given to treatment with cell-based therapies of individuals who have acutely developed PH in the setting of critical illness, including those with acute lung injury (ALI). PH in the setting of ALI can be diagnosed rapidly, at the bedside, with noninvasive or minimally invasive techniques. The association of pulmonary vascular dysfunction with important clinical outcomes in ALI, including survival, length of time on the ventilator, and length of stay in the intensive care unit has recently been established (4). The diverse mechanisms underlying pulmonary vascular dysfunction in the setting of ALI, including vasoconstriction, vascular compression due to fibrosis, and vascular wall remodeling, coupled with the acute nature of the disorder, might be expected to respond more favorably to cell-based therapies than more chronic forms of PAH or PH.

In summary, it is premature to consider cell-based therapies for PH (or PAH), without first considering the “who, when, and what.” Who with PH will most benefit from therapy with an endothelial progenitor-based approach: individuals with long-standing disease, or those earlier in the course of their illness, where inflammation is more active and remodeling less? Determining “when” therapy is most beneficial for PH in animal models and in preclinical investigations is critical in defining the “who”; therefore, expanding our knowledge of when PH is most amenable to therapy is necessary prior to clinical trials. Finally, what type of cells are the most appropriate and efficacious to administer for this disorder? To date, success of a cell-based approach in PH has been tied most closely to cells with paracrine capabilities, whereas less robust capabilities have been observed when cells are utilized that have a delineated structural role in endothelial repair. Answering these questions will require developing models that are clearly translatable to the bedside if we are to hope for improvement in therapy for individuals with this devastating disorder.

GRANTS

This work was funded by National Institutes of Health (NIH) SCCOR HL-084923-05 and NIH Program Project Grant HL-014985-39, HL091105-01, and American Physiological Society Giles Filley Award.

DISCLOSURES

No conflicts of interest, financial or otherwise are declared by the author(s).

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