Researchers at the Francis Crick Institute and at University College London (UCL) have rebuilt a human thymus using human stem cells and a bioengineered scaffold. The thymus is an essential organ in the immune system, and the achievement represents an important step towards being able to construct an artificial thymus for transplant therapy. Sara Campinoti, PhD, a researcher at the Crick Institute’s Epithelial Stem Cell Biology and Regenerative Medicine Laboratory, said, “Showing it is possible to build a working thymus from human cells is a crucial step towards being able to grow thymi which could one day be used as transplants.”

Reporting in Nature Communications (“Reconstruction of a functional human thymus by postnatal stromal progenitor cells and natural whole-organ scaffolds,”) first author Campinoti and colleagues, concluded, “This is an important proof-of-principle that a long-lived phenocopy of a human thymus utilizing only postnatal, cultivated cells is achievable.”

Located in the chest, the thymus is a primary lymphoid organ, and is “essential for T cell maturation and selection,” the authors explained. When the thymus doesn’t function properly, or if the organ does not form during fetal development, the result may include severe immunodeficiency, when the body cannot fight infectious diseases or cancerous cells, or autoimmunity, whereby the immune system mistakenly attacks the patient’s own healthy tissue. “Since the thymus controls the development of both immune competence and tolerance, its functional dissection and subsequent reconstruction with desired cell populations might provide powerful tools applicable to many medical conditions, including primary or acquired immune deficiencies,” the researchers continued. “Indeed, such is the clinical unmet need that cultivated thymus slices are currently used for transplantation into athymic patients.”

However, to date, attempts to build a fully functional thymus have resulted in “limited success,” the authors noted, possibly because the organ is so complex. But also, they pointed out, there has been some “uncertainty” about whether this could be achieved using postnatal epithelial stem/progenitor and mesenchymal cells, and if so, whether such cells could even be expanded in sufficient numbers, ex vivo. In their newly reported proof-of-concept study, the scientists describe how they have now successfully rebuilt thymi using stem cells taken from patients who had to have the organ removed during surgery. When transplanted into mice, the bioengineered thymi were able to support the development of mature and functional human T lymphocytes. While researchers have previously rebuilt other organs or sections of organs, the team suggests this is the first time that scientists have successfully rebuilt a whole working human thymus.

To create the complete organ, the researchers started by collecting thymi from patients who underwent open chest cardiac surgery, and in the laboratory, grew thymic epithelial cells (TEC) and thymic interstitial cells (TIC) from the donated tissue into many colonies of billions of cells. The next step was to obtain a structural scaffold of thymi, which they could repopulate with the thymic cells they had cultured. For this, co-author Asllan Gjinovci, PhD, a senior laboratory research scientist at the Crick, developed a new approach to remove all the cells from rat thymi, so only the structural scaffolds remained. The researchers had to use a new microvascular surgical approach for this, as conventional methods are not effective for the thymus. Gjinovci commented, “This new approach is important because it enables us to obtain scaffolds from larger organs like the human thymus, something essential to bringing this beautiful work to the clinic.”

The researchers then injected the organ scaffolds with up to six million of the thymic epithelial cells and thymic interstitial cells from the colonies they had grown in the lab. The cells grew onto the scaffolds and after only five days, the organs had developed to a similar stage as those seen in a nine-week-old fetus. “… it was possible to cultivate organ-size, 3D structures that supported the survival and appropriate spatial organization of thymic stromal cells, that in turn facilitated thymocyte differentiation,” the authors commented.

When the team then implanted the lab-grown thymi into “humanized” mice, they found that in over 75% of transplants, the thymi were able to support the development of human lymphocytes. Co-author Roberta Ragazzini, PhD, postdoctoral research scientist at the Crick, further noted, “The fact that we can extensively expand thymic stem cells taken from human donors into large colonies is really exciting. It makes it possible to scale up the process with a view to build ‘human size’ thymi.”

The team suggests the work represents an important step, not only for further research into and treatment of severe immune deficiencies, but also more broadly for developing new techniques to grow artificial organs. As the authors concluded, “The data described in this study demonstrate that postnatal human thymus harbors epithelial (TEC) and interstitial cells (TIC) that can expand to clinically relevant numbers in vitro, suitable for contributing to the reconstruction of a human functional thymus in vivo … by overcoming obstacles to constructing a functional thymus with only expanded, postnatal donor stromal cells, our findings offer practical prospects for treating immune disorders including congenital athymia for which current therapies are limited.”

The scientists are continuing to refine and scale up the process for rebuilding thymi. “As well as providing a new source of transplants for people without a working thymus, our work has other potential future applications,” stated Paola Bonfanti, PhD, senior author and group leader at the Crick and professor in the division of infection and immunity at UCL. “For example, as the thymus helps the immune system to recognize self from non-self, it poses a problem for organ transplants as it can cause the immune system to attack the transplant. It is possible that we could overcome this by also transplanting a thymus regrown from cells taken from the thymus of the organ donor. We are confident that this may prevent the body from attacking the transplant. The research behind this is still in the early days, but it is an exciting concept which could remove the need for patients to take immune suppressors for the rest of their life.”