Mesenchymal Stem Cells: A New Approach for Treating Bronchopulmonary Dysplasia

Abstract

In spite of improved clinical management premature babies are highly susceptible to several neonatal related-morbidities, namely respiratory, neurological, and gastrointestinal. Bronchopulmonary dysplasia (BPD) is the most common lung disorder in premature babies. Present therapeutic options are mainly supportive and do not completely cure BPD, mainly because of its multifactorial pathogenesis. Mesenchymal stem/stromal cells (MSCs) have materialized as a highly promising therapeutic option for alleviating BPD. The many facets of this novel therapy are discussed in this review.

Keywords: Prematurity; Bronchopulmonary dysplasia; Mesenchymal stem/stromal cells; Lung disease

Abbrevation: BPD: Bronchopulmonary Dysplasia; MSCs: Mesenchymal stem/stromal cells; IUGR: Intrauterine Growth Restriction; CPAP: Continuous Positive Airway Pressure; PPV: Positive Pressure Ventilation; PDA: Patent ductus Arteriosus.

Introduction

Worldwide approximately 15 million babies are born prematurely before 37 weeks gestational age, of which an estimated 1 million of these babies die due to complications directly associated with preterm delivery [1]. Premature babies are highly susceptible to several neonatal related-morbidities, namely respiratory, neurological, and gastrointestinal [2]. Several of these illnesses can persist past infancy and childhood and ultimately become a considerable financial burden to both families and healthcare systems [3]. A type of chronic lung disease, bronchopulmonary dysplasia (BPD), particularly prevalent in preterm babies that had received supplemental oxygen and supported by mechanical ventilation for alleviating respiratory failure was first communicated in 1967 [4]. At that time, BPD manifested itself as lung injury with marked inflammation and fibrosis [4]. Since then, significant progress in perinatal and neonatal medicine, together with innovative practices, and procedures have greatly improved survival of extremely premature babies born as early as 22-24 weeks gestational age [5]. At the same time, the incidence of BPD has increased; 45% of preterm babies that are delivered between 22-27 weeks gestational age will go on to develop BPD; in the United States there are up to 10,000 reported incidents of preterm babies with BPD each year [5].

Concurrent with these changes, the pathophysiology of BPD has altered and is now characterized by reduced alveolarization, impaired development of blood vessels and the microvasculature, enlarged airspaces, and poor lung function [6]. The etiology of BPD in the preterm baby is multifactorial; the extent of prematurity, several antenatal insults, and anomalies followed by postnatal influences and other co-morbidities which occur as a consequence of preterm birth all contribute to the onset and progression of BPD, summarized in Figure 1 [6-8]. Anomalies in lung function that began at birth owing to prematurity, subsequent extrauterine adaptations, and the manifestation of BPD persist throughout life [9,10]. Studies have reported that children formerly born premature with BPD were more likely to exhibit poor lung function by the time they reached school-age [10]. Long-term follow-up studies have also reported sub-optimal lung function and increased incidence of emphysema in young adults who were former BPD patients [11,12].

Figure 1: Left, The multifactorial risk factors of bronchopulmonary dysplasia. Right, Present therapeutic options and clinical strategies with the aim to reduce the severity of bronchopulmonary dysplasia

Existing Therapeutic Options for BPD

Despite considerable advances in neonatal clinical management BPD persistently presents itself as a significant illness for premature babies. As the etiology of BPD is multifactorial this presents a confounding therapeutic conundrum. In the ideal setting prevention of premature birth is the sole solution to avoiding the onset of BPD; a feat that has yet to be achieved. Current therapies are mainly supportive and directed towards minimizing lung injury (summarized in Figure 1), but neither significantly diminish the incidence of BPD nor do they alter the pathophysiological course of the disease process [13-18]. In follow-up studies, certain treatment regimens such as postnatal dexamethasone has been associated with adverse neurological outcomes [19]. Clearly, there is a pressing requirement to identify alternative therapies that are both effective and safe for alleviation of this debilitating, multifactorial lung disease.

An Untapped Therapeutic Option: Mesenchymal Stromal/Stem Cells

Bone marrow-derived mesenchymal stromal/stem cells (MSCs) were first described by Friedenstein and colleagues and their intrinsically diverse properties, attributes (e.g. self-renewal, differentiation, pro- angiogenic, anti-inflammatory, anti-fibrotic, and antioxidant), and classifications have been previously reviewed [20-22]. MSCs have been identified in almost all fetal and adult tissues and play crucial roles in promoting tissue development and the reparative responses to the injured host tissues/cells [21,23,24]. In the fetal lung resident or endogenous MSCs coordinate and foster alveolar development, tissue reparative processes, and growth of the pulmonary vasculature [23,25]. Recently Collins and colleagues reported that the repair potential of resident fetal lung MSCs isolated from an oxygen-induced rat BPD model was altered [26]. In addition, studies by Popova and colleagues showed that cultured MSCs isolated from tracheal aspirates of preterm babies that subsequently developed BPD exhibited a myofibroblast phenotype suggesting these cells exhibit a dual mode of action that is dependent upon their environment; Specifically, under normal circumstances resident lung MSCs promote lung growth, repair and development, whereas under constant injury these cells very likely switch to a pathogenic pathway [27].

Owing to the diverse properties and attributes of MSCs coupled with experimental evidence describing their dysfunction in BPD, sound judgement prevails that exogenous MSCs will be a suitable alternative treatment option for BPD. To further support the utility of MSCs as a therapeutic contender for BPD, several logistical aspects have to be considered:

a) MSCs can be easily isolated from adipose and bone marrow tissue in sufficient amounts [28]. MSCs can also be isolated devoid of ethical constraints from placental and umbilical cord tissue (Wharton’s jelly) and blood, and amniotic fluid which provides an autologous source of cells [29,30]. Regardless of the source, MSCs can be easily cultured and rapidly expand in carefully controlled environments, essentially following good manufacturing practices;

b) MSCs do not express human leukocyte antigen class II which affords allogenic treatment [31]; and

c) MSCs can move towards injured tissues and selectively adapt their reparative actions [32].

At first, it was postulated that MSCs exerted their beneficial effects by migrating to the site of injury, followed by engraftment, and then differentiating in to the compromised/damaged cells [34]. Subsequent studies in rodent models of BPD reported poor rates of engraftment of exogenous MSCs coupled with the finding that MSCs only lasted for a few days in the lung [34-36]. The accepted means by which MSCs exert their beneficial effects is via paracrine mechanisms. This includes secretion of bioactive substances with anti- apoptotic, anti-inflammatory, and pro-angiogenic properties (collectively referred to as the secretome or conditioned media) within the microenvironment and exosomes, containing proteins, microRNAs, and mRNA fragments [37,38]. Exosomes are then taken up by the damaged cells by means of vesicle fusion [40]. Also, MSCs can transfer mitochondria to target cells through nanotubes or microvesicles [37]. This is particularly beneficial to minimize apoptosis in target cells [37].

Therapeutic Effects of MSCs: Proving a Concept

The conviction that exogenous MSCs and or the secretome will be beneficial in ameliorating BPD-like lung damage in experimental animal models was validated by Augustine and colleagues, who completed a systematic review and meta-analysis study [41]. In this study the authors categorized 25 independent research reports which utilized newborn rodents (mice and rats) who were exposed to hyperoxia and displayed BPD-like lung injury. Animals received either human umbilical cord or cord blood or rat bone marrow derived MSCs which were administered either via intraperitoneal, intravenous, or intratracheal routes [41]. Variables in these studies included source and route of administration of MSCs, the number of administered cells, and long-term outcomes. In the MSCs-treated rodents, significant improvements in lung function and alveolarization, and vascular growth coupled with decreased inflammation and oxidative stress was observed [7,41-43].

Clinical Trials: Taking the Concept to the Bedside

The outcomes of the initial, groundbreaking phase I clinical trial was recently reported [44]. In this study, nine preterm babies (23-29 weeks gestational age) requiring mechanical ventilation between 5-14 days following birth were treated once with either 107 or 2X107 umbilical cord blood derived MSCs (intratracheal delivery). It is extremely important to note that these babies did not exhibit any undesirable consequences thus strongly supporting the safety and practicality of this novel therapeutic modality [44]. The same investigators went on to report that there were no indications of neurological, respiratory, or growth deficits in these babies after 2 years [45]. These pioneering studies have set the precedence for several other independent investigators to initiate their own phase I-II trials, summarized in Table 1.

Table 1:Clinical Trials of Mesenchymal Stromal Cells Therapy for Bronchopulmonary Dysplasia.

Conclusion

Identifying the most effective treatment for BPD has indeed proven an onerous task. However, owing to advancements in MSCbiology has now afforded us with a potentially very promising therapeutic option. Preclinical studies using MSCs for the treatment of BPD have essentially paved the way for the initiation of several clinical trials. Of note, given the multifactorial nature of BPD, one has to consider that MSCs combined with other drugs may yet prove to be an additional useful therapeutic option. Concurrent with the clinical trials, we must still continue to investigate the long-term efficacy and safety of this particular therapy. Also, of importance is to 1) further understand the mechanistic pathways of MSCs mode of action using both in vivo and in vitro models; and 2) investigate the effects of the microenvironment on the biological properties of lung tissue resident/endogenous MSCs. It is unknown as to whether endogenous fetal lung-MSCs recover and regain their biological properties after cessation or lessening of BPD or how for long they remain in a quiescent/altered state. In a similar context, although recent studies have clearly demonstrated the therapeutic efficacy of exogenously applied MSC or their secretome via a paracrinemediated effect in animal models of neonatal lung disease, the interaction between exogenous MSCs and compromised resident fetal lung-MSCs is unclear

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