Recent research published in Nature Communications has regenerated a functional thymus in mice, making several other discoveries along the way .
A new sub-type of thymus cell emerges
Although it is imperative to our immune health, the thymus is an under-studied organ that slowly disappears as we age. Its main function is to provide a maturation facility for T cells, which are part of the adaptive immune system.
The main cells of the thymus are generally considered to be thymic epithelial cells (TECs), which differ in whether they come from the thymic medulla (mTECs) or cortex (cTECs), along with thymic interstitial cells (TICs). Researchers from The Francis Crick Institute have recently characterized human thymus cells using RNA sequencing, which is a relatively new technique that allows for much more in-depth observation of cell behaviors. Surprisingly, the researchers identified a sub-population of both TECs and TICs that express characteristics in common with mesenchymal stromal cells (MSCs), progenitor cells that are very popular in regenerative medicine applications. However, the researchers weren’t yet finished. After their discovery, they turned to improving the biomaterial on which they planned to grow these cells.
Improving thymic decellularization
The thymus is uniquely difficult to decellularize because of its blood supply. Unlike most organs, which each have a main artery and vein, the thymus is supplied by many, smaller vessels. The researchers developed a (now patented) microsurgery technique to overcome the difficulty of perfusing the thymus with decellularization detergents. They accomplished this in rat tissue by sewing shut the various smaller vessels of the thymus. Additionally, they intentionally did not dissect some extra, non-thymic tissue, including the carotid artery, so it could serve as the entry point for perfusion. They showed success with this technique, as perfusion had reached every part of the thymus and successfully cleared it of cells.
Successful mouse thymus regeneration
Finally, the researchers brought these two findings together by perfusing the decellularized rat thymi with the human cell types that they had characterized earlier. After five to six days in culture, the thymi with a combination of TECs and TICs showed the best regeneration, which was similar to that of an embryonic thymus. Each regenerated thymus was also able to mature T cells in vitro. Finally, they transplanted these rat thymi into immunocompromised mice, which had also undergone irradiation and bone marrow transplants to give them more human-like immune systems. After transplantation, the mice were followed for slightly over five months. Most transplanted organs were functionally producing T cells by 11 weeks, and various thymic markers increased throughout the post-transplantation period.
In conclusion, we have used a multidisciplinary approach to generate an in vivo, long-lasting thymus where both the haematopoietic and stroma compartments are of human origin. Such a system opens the possibility of addressing very many immunological questions including the development and functional maturation of conventional and unconventional human T cells (e.g. Treg, NK, γδ); positive and negative selection of MHC-I and II-restricted human T cells; and the roles of additional factors in the establishment and maintenance of tolerance. Future work and further optimisation will be needed to estimate the full potential and limitations of our system in relation to each of these crucial questions. Notwithstanding, the successful completion of these steps can allow medically relevant applications such as thymus transplantation in primary immune deficiencies as athymic DiGeorge syndrome and Foxn1null (nude) babies; the control of tolerance in congenital conditions as autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) patients, and in immunosuppression-free organ transplantations.
This was a particularly impressive study, both in terms of the quantity and quality of discoveries reported. The characterization of human thymic cells and the novel decellularization technique both serve to broadly benefit future thymus research beyond the scope of this study. Additionally, their results in vivo showed what might be the most advanced thymus regeneration published to date.
However, there are still several hurdles that must be overcome before a similar strategy can be implemented in humans. The thymic cells used to populate the decellularized tissue were isolated and expanded from children’s thymi that had to be removed during chest surgery, which is a limited, non-autologous source. Furthermore, rat thymi are much smaller than those of humans. While diffusion of nutrients did not appear to be a limiting factor in this study, their thymi are only 1 cubic centimeter in volume. Vascularization of the tissue will be needed in order to regenerate a thymus large enough to fully function in a human being. Finally, the thymus all but disappears with age, making the oldest among us the most in need of this technology. While there are also plenty of unmet needs for thymic regeneration among younger people, this technique relies heavily on regeneration taking place in vivo, an ability that is significantly impaired as we age.
 Campinoti, S., Gjinovci, A., Ragazzini, R., Zanieri, L., Ariza-McNaughton, L., … & Bonfanti, P. (2020). Reconstitution of a functional human thymus by postnatal stromal progenitor cells and natural whole-organ scaffolds. Nature Communications, 11, 6372. https://doi.org/10.1038/s41467-020-20082-7