Growth factors, stem cells and bronchopulmonary dysplasia

Molecular physiopathogenetic mechanisms and development of new potential therapeutic strategies in persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN) is a cyanogenic plurifactorial disorder characterized by failed postnatal drop of pulmonary vascular resistance and maintenance of right-to-left shunt across ductus arteriosus and foramen ovale typical of intrauterine life. The pathogenesis of PPHN is very complex and can result from functional (vasoconstriction) or structural (arteriolar remodeling, reduced pulmonary vessels density) anomalies of pulmonary circulation. Etiopathogenetic factors heterogeneity can strongly condition therapeutical results and prognosis of PPHN that is particularly severe in organic forms that are usually refractory to selective pulmonary vasodilator therapy with inhaled nitric oxide. This paper reports the more recent acquisitions on molecular physiopathogenetic mechanisms underlying functional and structural forms of PPHN and illustrates the bases for adoption of new potential treatment strategies for organic PPHN. These strategies aim to reverse pulmonary vascular remodeling in PPHN with arteriolar smooth muscle hypertrophy and stimulate pulmonary vascular and alveolar growth in PPHN associated with lung hypoplasia.In order to restore lung growth in this severe form of PPHN, attention is focused on the results of studies of mesenchymal stem cells and their therapeutical paracrine effects on bronchopulmonry dysplasia, a chronic neonatal lung disease characterized by arrested vascular and alveolar growth and development of pulmonary hypertension.

Bronchopulmonary dysplasia: structural challenges and stem cell treatment potential

Bronchopulmonary dysplasia (BPD) continues to be a significant cause of morbidity and mortality for premature infants. Currently, most treatment strategies are mainly palliative and do not address the underlying structural changes of the lungs leading to the symptoms. New research and ongoing experiments with mesenchymal stem cells are showing capabilities to mitigate structural damage and promote vascular growth that leads to normal lung architecture in animal models. Looking at the pathophysiology that contributes to BPD and assessing current treatment options available, there still appears to be a gap in treatment that addresses the structural issues within the lungs. This article reviews the findings of several mesenchymal stem cell experiments and the potential for future treatment to help repair the lungs in infants with BPD.

Human amniotic fluid stem cells protect rat lungs exposed to moderate hyperoxia

BACKGROUND

Treatment of bronchopulmonary dysplasia (BPD) remains as yet an unmet clinical need and recently stem cells have been proposed as a therapeutic tool in animal models. We investigated the role of amniotic fluid stem cells (AFS) in an adult rat model of hyperoxia lung injury.

METHODS

Fifty Sprague-Dawley rats were, at birth, randomly exposed to moderate hyperoxia or room air for 14 days and a single dose of human amniotic fluid stem (hAFS) or human Fibroblasts (hF), cells was delivered intratracheally (P21). At P42 animals were euthanized and lung tissue examined using histology, immunohistochemistry, PCR, and ELISA. hAFS cells characterization and homing were studied by immunofluorescence.

RESULTS

In rats treated with hAFS and hF cells 16S human rRNA fragment was detected. Despite a low level of pulmonary hAFS cell retention (1.43 ± 0.2% anti-human-mitochondria-positive cells), the lungs of the treated animals revealed higher secondary crest numbers and lower mean linear intercept and alveolar size, than those exposed to hyperoxia, those left untreated or treated with hF cells. Except for those treated with hAFS cells, moderate hyperoxia induced an increase in protein content of IL-6, IL-1β, as well as IF-γ and TGF-1β in lung tissues. High VEGF expression and arrangement of capillary architecture in hAFS cell group were also detected.

CONCLUSIONS

Treatment with hAFS cells has a reparative potential through active involvement of cells in alveolarization and angiogenesis. A downstream paracrine action was also taken into account, in order to understand the immunodulatory response.

Ex vivo expanded human cord blood-derived hematopoietic progenitor cells induce lung growth and alveolarization in injured newborn lungs

BACKGROUND

We investigated the capacity of expanded cord blood-derived CD34+ hematopoietic progenitor cells to undergo respiratory epithelial differentiation ex vivo, and to engraft and attenuate alveolar disruption in injured newborn murine lungs in vivo.

METHODS

Respiratory epithelial differentiation was studied in CD34+ cells expanded in the presence of growth factors and cytokines ("basic" medium), in one group supplemented with dexamethasone ("DEX"). Expanded or freshly isolated CD34+ cells were inoculated intranasally in newborn mice with apoptosis-induced lung injury. Pulmonary engraftment, lung growth and alveolarization were studied at 8 weeks post-inoculation.

RESULTS

SP-C mRNA expression was seen in 2/7 CD34+ cell isolates expanded in basic media and in 6/7 isolates expanded in DEX, associated with cytoplasmic SP-C immunoreactivity and ultrastructural features suggestive of type II cell-like differentiation. Administration of expanding CD34+ cells was associated with increased lung growth and, in animals treated with DEX-exposed cells, enhanced alveolar septation. Freshly isolated CD34+ cells had no effect of lung growth or remodeling. Lungs of animals treated with expanded CD34+ cells contained intraalveolar aggregates of replicating alu-FISH-positive mononuclear cells, whereas epithelial engraftment was extremely rare.

CONCLUSION

Expanded cord blood CD34+ cells can induce lung growth and alveolarization in injured newborn lungs. These growth-promoting effects may be linked to paracrine or immunomodulatory effects of persistent cord blood-derived mononuclear cells, as expanded cells showed limited respiratory epithelial transdifferentiation.

Endogenous lung regeneration: potential and limitations

The exploration of the endogenous regenerative potential of the diseased adult human lung represents an innovative and exciting task. In this pulmonary perspective, we discuss three major components essential for endogenous lung repair and regeneration: epithelial progenitor populations, developmental signaling pathways that regulate their reparative and regenerative potential, and the surrounding extracellular matrix in the human diseased lung. Over the past years, several distinct epithelial progenitor populations have been discovered within the lung, all of which most likely respond to different injuries by varying degrees. It has become evident that several progenitor populations are mutually involved in maintenance and repair, which is highly regulated by developmental pathways, such as Wnt or Notch signaling. Third, endogenous progenitor cells and developmental signaling pathways act in close spatiotemporal synergy with the extracellular matrix. These three components define and refine the highly dynamic microenvironment of the lung, which is altered in a disease-specific fashion in several chronic lung diseases. The search for the right mixture to induce efficient and controlled repair and regeneration of the diseased lung is ongoing and will open completely novel avenues for the treatment of patients with chronic lung disease.

Amnion epithelial cells as a candidate therapy for acute and chronic lung injury

Acute and chronic lung injury represents a major and growing global burden of disease. For many of these lung diseases, the damage is irreparable, exhausting the host's ability to regenerate new lung, and current therapies are simply supportive rather than restorative. Cell-based therapies offer the promise of tissue regeneration for many organs. In this paper, we examine the potential application of amnion epithelial cells, derived from the term placenta, to lung regeneration. We discuss their unique properties of plasticity and immunomodulation, reviewing the experimental evidence that amnion epithelial cells can prevent and repair lung injury, offering the potential to be applied to both neonatal, childhood, and adult lung disease. It is amazing to suggest that the placenta may offer renewed life after birth as well as securing new life before.

Bronchopulmonary dysplasia: then and now

When bronchopulmonary dysplasia (BPD) was first described in 1967, the use of assisted ventilation in neonates was in its infancy. High concentrations of oxygen were implicated, and BPD was equated with 'pulmonary oxygen toxicity'. The etiologic role of not only oxygen but also peak inspiratory pressures and the duration of exposure to both was emphasized in the 1970s, but BPD remained a dreaded complication of managing respiratory distress syndrome in the 1980s. It was only after exogenous surfactant became commercially available for endotracheal administration that 'classical' BPD began to disappear and was replaced by the 'new' BPD. 'Classical' BPD was seen in more mature preterm infants (>28 weeks' gestational age) and in its severe form was characterized radiographically by micro- and macrocysts of the lung, lung hyperinflation and flattening of the diaphragms. In contrast, 'new' BPD is seen in less mature infants (<28 weeks' gestational age), has comparatively mild radiographic abnormalities and has been defined as continued oxygen requirement at 36 weeks' postmenstrual age. Pathologically, 'classical' BPD frequently revealed obstructive bronchiolitis and fibrosis of lung parenchyma, whereas 'new' BPD demonstrates minimal fibrosis but uniform arrest of development. Herein, factors which may contribute to the etiology of BPD are described, as well as possible preventative and therapeutic strategies.