Induced Pluripotent Stem Cells Make Lungs

Since my father died of disseminated lung cancer (squamous cell carcinoma), this report has particular meaning to me.

When a person dies, their lungs can be harvested and stripped of their cells. This leaves a so-called “lung scaffold” that can then be used to build new lungs by means of tissue engineering techniques. Lung scaffolds consist of a protein called collagen, and sugar-rich proteins called “proteoglycans” (say that fast five times) and a rubber band-like protein called elastin. Depending on how the lung scaffolds are made more or less of these components can remain in the lung scaffold (see TH Peterson, and others, Cells Tissues Organs. Feb 2012; 195(3): 222–231). The important thing is that the cells are gone and this greatly reduces the tendency for the lung scaffold to be rejected by someone else’s immune system.

Once a lung scaffold is generated from a whole lung, cells can be used to reconstitute the lung. The key is to use the right cell type or mix of cell types and to induce them to form mature lung tissue.

The laboratory of Harald Ott at Harvard University Medical School used a technique called “perfusion decellularization” to make lung scaffolds from the lungs of cadavers. Then he and his co-workers used lung progenitor cells that were derived from induced pluripotent stem cells (iPSCs). This study was published in The Annals of Thoracic Surgery, and it examined the ability of iPSCs to regenerate a functional pulmonary organ

Whole lungs from rat and human cadavers were stripped of their living material by means of constant-pressure perfusion with a strong detergent called sodium dodecyl sulfate (SDS; 0.1% if anyone is interested). Ott and his crew then sectioned some of the resulting lung scaffolds and left others intact, and then applied human iPSCs that had been differentiated into developing lung tissue.

Lung tissue develops from the front part of the developing gut. This tissue is called “endoderm,” since it is in the very innermost layer of the embryo.

Lung Development

Therefore, the iPSCs were differentiated into endoderm with a cocktail of growth factors (FGF, Wnt, Retinoic acid), and then further differentiated in the anterior endoderm (foregut; treated cells with Activin-A, followed by transforming growth factor-β inhibition), and then even further differentiated into anterior, ventral endoderm, which is the precise tissue from which lungs form. In order to be sure that this tissue is lung tissue, they must express a gene called NK2 homeobox 1 (Nkx2.1). If these cells express this gene, then they are certainly lung cells.

Ott and his group showed that their differentiate iPSCs strongly expressed Nkx2.1, and then seeded them on slices and whole lung scaffolds. Then Otts’s group maintained these tissues in a culture system that was meant to mimic physiological conditions.

Those cells cultured on decellularized lung slices divided robustly and committed to the lung lineage after 5 days. Within whole-lung scaffolds and under the physiological mimicking culture, cells upgraded their expression of Nkx2.1. When the culture-grown rat lungs were transplanted into rats, they were perfused and ventilated by host vasculature and airways.

Thus these decellularized lung scaffolds supports the culture and lineage commitment of human iPSC-derived lung progenitor cells. Furthermore, whole-organ scaffolds and a culture system that mimics physiological conditions, allows scientists to enable seeding a combination of iPSC-derived endothelial and epithelial progenitors and enhance early lung fate. Transplantation of these laboratory-grown lungs seem to further maturation of these grafted lung tissues.

Induced Pluripotent Stem Cells Used to Make New Bone In Monkeys

Cynthia Dunbar, MD and her colleagues at the National Heart, Lung, and Blood Institute, which is a division of the National Institutes of Health (NIH) in Bethesda, Maryland have shown for the first time that it is possible to make new bone from induced pluripotent stem cells that are derived from a patient’s own skin cells.

This study, which was done in monkeys, shows that there is some risk that induced pluripotent stem cells (iPSCs) can form tumors, but that the risk of tumor formation is less than what was shown in immuno-compromised mice.

iPSCs are made from adult cells by means of a process called “reprogramming.” To reprogram adult cells, genetic engineering techniques are used to introduce specific genes into adult cells. These introduced genes drive the adult cells to de-differentiate into a less mature state, until they eventually become pluripotent, much like embryonic stem cells.

Originally, discovered by Nobel-prize winner Shinya Yamanaka, reprogramming was initially done with genetically engineered viruses that insert genes into the genome of cells. Even though these viruses do a passable job of reprogramming cells, they also introduce insertion mutations. Yamanaka and others originally used four transcription factors (Oct4, Sox2, Klf4, c-Myc) to reprogram adult cells. Several of these genes are overexpressed in a variety of tumors, and therefore, the use of these genes does create a risk of forming cells that overgrown and become tumorous. Secondly, The reprogramming process does put cells under the types of stresses that increase the mutation rate, and these mutations can also increase the risk of forming tumor cells. However, it is clear that not all reprogramming protocols cause the same rate of mutations, and that the mutation rate of iPSCs was originally overestimated. What is required is a good way to screen iPSC lines for mutations and for safety, especially since not all iPSC lines are equal when it comes to their safety.

The advantage of using iPSCs over embryonic stem cells is that the immune system of the patient should not reject tissues and cells made from iPSCs. This would eliminate the need for immune suppression drugs, which can be rather toxic.

Cynthis Dunbar from the National Heart, Lung, and Blood Institute said of her experiments, “We have been able to design an animal model for testing of pluripotent stem cell therapies using the rhesus macaque, a small monkey that is readily available and has been validated as being closely related physiologically to humans.

Dr. Dunbar continued: “We have used this model to demonstrate that tumor formation of a type called a ‘teratoma’ from undifferentiated autologous iPSCs does occur; however, tumor formation is very slow and requires large numbers of iPSCs given under very hospitable conditions. We have also shown that new bone can be produced from autologous iPSCs as a model for their possible clinical application.”

Dunbar and her team used a excisable polycistronic lentiviral vector called STEMCCA (Sommer et al., 2010) that expressed four genes: human OCT4, SOX2, MYC, and KLF4 to make iPSCs from skin cells. After they had derived culturable iPSCs from rhesus monkeys (made under feeder-free conditions), Dunbar and her group seeded them on ceramic scaffolds that are used by reconstructive surgeons to fill in or rebuild bone. Interestingly, these cells regrew bone in the monkeys.

The differentiated iPSCs formed no teratomas, but monkeys that had received implantations of undifferentiated iPSCs formed teratomas in a dose-specific manner.

Dunbar and her colleagues note that this approach might be beneficial for people with large congenital bone defects or other types of traumatic injuries. Having said that, it is doubtful that bone replacement therapies will be the first human iPSC-based treatment, since bone defects are not life-threatening, even though they can seriously compromise the quality of a patient’s life.

“A large animal preclinical model for the development of pluripotent or other high-risk/high-reward generative cell therapies is absolutely issues of tissue integration of homing, risk of tumor formation, and immunogenicity,” said Dunbar. “The testing of human-derived cells in vitro or in profoundly immunodeficient mice simply cannot model these crucial preclinical safety and efficiency issues.”

This NIH team is now collaborating with other labs to differentiate macaque iPSCs into liver, heart, and white blood cells for to test them for eventual pre-clinical trials in hepatitis C, heart failure, and chronic granulomatous disease, respectively.