January - March 2009: 
Volume 22, Issue 1

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Repair of respiratory system lesions using stem cells
A few years ago, the possibility of regeneration of tissues and organs was the stuff of science fiction. Today it has become a practicality and many scientists are focusing their interest on the possible applications of embryonic and adult stem cells. Autologous transplantation is a novel effective treatment for many diseases and studies are now providing evidence of the possible repair of lesions of the lung.
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Stem cells are defined by two minimum criteria: the capacity for long term self-renewal without senescence, and the ability to differentiate into one or more specialized somatic cell types, given the appropriate stimuli. Stem cells are divided into embryonic and adult stem cells1. Embryonic stem cells are derived from the blastocyst of a developing embryo and are able to produce progeny of all cell lineages; however their use has generated moral and political objections2. Furthermore, embryonic cells have a greater tumorigenic potential than adult stem cells3.

Adult stem cells are located in many tissues throughout the body and play an essential role in their growth, maintenance and repair1. Bone marrow derived stem cells consist of haematopoietic stem cells, which produce not only progenitors of all types of mature blood cells, but also mesenchymal stem cells, which, despite previous beliefs to the contrary, retain a developmental plasticity that allows them to differentiate, adopting the phenotypes of other tissues4,5. They appear to have the ability to be specifically recruited to areas of inflammation and injured tissues and to be involved in their repair2. A potential advantage of using adult stem cells, expanded in culture and reintroduced into the same patient, is the avoidance of immunologic rejection, as this process constitutes an autologous transplantation.

In 2001 an early study was published in which a single haematopoietic stem cell was transplanted from an adult male mouse into a female mouse that had previously been irradiated5. This cell repopulated all bone marrow cell lineages but also cells that engrafted in several organs, such as the lungs in which up to 20% of the parenchyma cells were subsequently found to contain the donor's Y chromosome. Later, similar animal studies showed varying levels of engraftment of bone marrow stem cells as alveolar and airway epithelial tissue. In humans, similar findings have been recorded following bone marrow transplantation. Female recipients of male bone marrow were shown to have male donor cells engrafted as epithelial and endothelial cells4. This finding was also replicated in male recipients of a female lung allograft, and in this case the greater levels of engraftment occurred at sites of greatest injury following rejection or infection. However, more recent studies showed that the previously used methods of detecting engraftment were not sufficiently rigorous and there was an overestimation of the amount of engraftment2, and other studies do not confirm the engraftment of stem cells to the lungs and to the alveolar epithelium.

There is also evidence for engraftment into the pulmonary vasculature by other bone marrow cells, namely the endothelial progenitor cells. In animals endothelial progenitor cells were engrafted into areas of vascular injury in models of pulmonary hypertension. In human studies an increase in circulating endothelial progenitor cells was associated with improved outcome from acute lung injury and bacterial pneumonia4. All of the above examples lead to the theory that blood marrow transplantation could contribute to the repair of a variety of tissues, as stem cells appear to be recruited to areas of injury, where chemokines are produced4. Studies have demonstrated that when the process of stem cell migration was augmented, lung injury could be reduced. Intraperitoneal injection of granulocyte colony stimulating factor increased bone marrow stem cell engraftment in the lungs of mice suffering from emphysema induced by intranasal elastase, and reduced the damage compared with the control group. Mesenchymal stem cell administration immediately after exposure to bleomycin in mice was associated with a significant reduction in bleomycin induced inflammation and collagen deposition in the lung tissue.

Concerning the possible use of stem cells for the treatment of pulmonary hypertension, a study showed that the intravenous implantation of endothelial Nitric Oxid Synthase (eNOS) gene-transduced mesenchymal stem cells improved the progression of RV impairment caused by pulmonary hypertension induced by monocrotaline (n.b., the monocrotaline induced pulmonary hypertension model is known to represent similar pathology to that of primary pulmonary hypertension)6. Another study showed that intratracheal mesenchymal stem cell administration attenuated monocrotaline induced pulmonary hypertension and endothelial dysfunction7. It was suggested that the decrease in the pulmonary vascular resistance and improvement in the response to achetylcholine results from a paracrine effect of the transplanted mesenchymal stem cells in the lung parenchyma, which improves vascular endothelial function. As a result of these findings, a phase I safety trial has commenced in Canada, whereby patients with refractory pulmonary hypertension are receiving autologous epithelial progenitor cells transfected with endothelial nitric oxide synthase via a pulmonary artery catheter4.

Many studies concerning mesenchymal stem cells have been reported. Mesenchymal stem cells were transduced to express interleukin-12, with the rationale of improving anti-cancer immune surveillance by activating cytotoxic lymphocytes and producing IFN- γ. The mesenchymal stem cells expressing IL-12 were introduced before tumour inoculation and prevented the development of lung cancer.

Efforts are made for the treatment of cystic fibrosis (CF) using bone marrow stem cells. One study used genetic transduction of bone marrow stem cells from patients with CF to enable them to express the normal CF transmembrane conductance regulator (CFTR). These cells were then mixed in a human airway-epithelial culture and they differentiated into airway epithelial cells and partially corrected the defective CFTR-dependent chloride current4.

Autologous peripheral blood stem cell transplantation is being successfully applied to patients with treatment-resistant autoimmune diseases. In 2005 a study analyzed the effects of autologous peripheral blood stem cell transplantation in four patients with treatment resistant systemic vasculitis8. One of the patients had Wegener's granulomatosis and another had Churg Strauss syndrome. The results were impressive, as the patient with Wegener's granulomatosis achieved complete remission and the other patients are in good partial remission and responding to maintenance treatment. Autologous peripheral blood stem cell transplantation is probably not curative, but it induces remission and can turn a life-threatening disease in a milder form, more amenable to treatment.

The contribution of bone marrow-derived stem cells to the pathogenesis of fibrosis is a confused issue, as these cells contribute to the fibroblast and myoblast community in the lungs. Although it has been shown that the suppression of bone marrow with busulphan led to a worsening of pulmonary fibrosis in mice, and systemic mesenchymal stem cells appear to be able to alleviate bleomycin induced lung fibrosis, another study showed that a reduction in the recruitment of these bone marrow cells to the areas of fibrosis produced a reduction in the amount of fibrosis2. Furthermore, in a case of bleomycin lung fibrosis in mice transplanted with bone marrow, 80% of type I fibroblasts at sites of fibrosis were shown to be of bone marrow donor origin.

Concerning the possibility of using adult cells for the treatment of various diseases, there are several impressive pathways other than stem cells. Studies have shown that certain human adult cells, such as fibroblasts, can be reprogrammed into cells similar to embryonic stem cells, but at present this process requires extensive genetic modification, which is not yet acceptable for clinical use 9.

In spite of the impressive research findings concerning their therapeutic possibilities, the use of stem cells could be potentially dangerous. The addition of mesenchymal stem cells to human breast carcinoma cells and to colonic tumour cells led to an increased rate of metastasis. Mesenchymal stem cells have also been shown to have immunosuppressive effects which may favour tumour growth in vivo. In addition, as well as affecting the behavior of cancer cells, stem cells may themselves have malignant potential as they have the ability for self renewal and unlimited proliferation. In vitro, it has been demonstrated that bone marrow stem cells can develop karyotypic abnormalities2. Evidence in humans of bone marrow contribution to tumour cells comes from sex-mismatched bone marrow transplants. In a patient who developed lung cancer four years after bone marrow transplantation, up to 20% of the neoplastic cells were of bone marrow origin2. A contribution of bone marrow derived stem cells to the angiogenesis of tumours has also been demonstrated .

Controversially, evidence has also been produced for the exact opposite: mesenchymal stem cells appear to have intrinsic antineoplastic properties, as it has been shown in a Kaposi's sarcoma model.

In addition to the incorporation of bone marrow-derived cells after whole bone marrow transplantation, mesenchymal stem cells alone have also been shown to have an ability to specifically target tumour tissue where chemokines are produced. The ability of these cells to home to the lungs in pathological conditions such as fibrosis or cancer makes them the perfect vectors for delivering therapeutic agents. Mesenchymal stem cells have several properties that render them suitable for the role of a vector. Firstly, they can relatively easily transduced and expanded in culture for many passages, while retaining their growth and multi-lineage potential. They also appear to be relatively immunoprivileged due to their expression of major histocompatibility complex 1 (MHC1) but lack of complex 2 (MHC2) and the molecules CD80, CD86 and CD40, properties that may allow an allogenic transplantation without prior immunomodulation10.

Little data has been published on the use of human embryonic stem cells, because of legal and ethical constraints to research.

In the first report of the detection of the lung epithelial phenotype in 2002 1 type two pneumocytes were identified in differentiated murine embryonic stem cell cultures, and later reports showed similar results. In 2007 a report studied the ability of embryonic stem cells to home to the areas of lung injury and to contribute to their repair. Their potential use requires attention as there are concerns about the potential for malignant transformation and for immune rejection in the hosts 3.

In conclusion, much work remains to be accomplished in the research on stem cells as their contribution to lung tissue repair is not yet completely understood. Nevertheless, exciting pathways are opening up in the areas of pharmacological, genetic and cellular therapy for many lung diseases such as cystic fibrosis, lung cancer, pulmonary hypertension, autoimmune diseases and lung fibrosis.


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