Fight Aging!

FOXN1 is the master regulator of thymic growth and activity; the thymus is an unusually straightforward organ in this respect. One can make it grow and perform to a greater degree just by dialing up expression of this one gene in thymic tissue. Thymic regrowth is a desirable goal for the elderly, given that thymic atrophy occurs in everyone, and limits the production of T cells. It is a major contribution to immune aging.

One of the approaches that has been taken to achieve this goal of thymic regrowth is the delivery of a FOXN1 recombinant protein attached to the TAT domain derived from the HIV-1 virus, allowing it to enter the cell. Researchers published a study some years back in which intrathymic injection was used, an approach that is probably too risky to serve as a basis for human therapies. Without direct injection, one can't get enough of the protein into the thymus to be worth the effort.

Today's open access paper discusses an advance on that earlier work by the same team. The researchers further attach an additional binding domain to the FOXN1-TAT fusion protein to optimize uptake in thymic tissue, and inject the protein intravenously instead of directly into the thymus. This necessarily requires higher doses (and recombinant proteins remain expensive), but the team reports good results. This is an approach that could in principle be developed for human use, setting aside the more conservative concerns one might encounter regarding the use of TAT versus the adoption of other options for cell entry.

Recombinant FOXN1 fusion protein increases T cell generation in aged mice

The thymus is a specified immune organ that provides an inductive environment for the generation of T cells that play a critical role in the adaptive immune system. Although the thymus continues to export T cells throughout life, it undergoes a profound atrophy with age, a process termed thymic involution, resulting in decreased numbers and functional capacity of T cells in the older adult, which has direct etiological linkages with many diseases. Furthermore, T cell immune deficiency in the older adult is exacerbated when the immune system is insulted by chemotherapy, radiotherapy, infections (e.g. HIV), and preparative regimens for foreign tissue or organ transplants. Therefore, restoring thymus function in the older adult has important implications.

We have previously reported that intrathymic injection (i.t.) of a recombinant (r) protein containing FOXN1 and a protein transduction domain embedded in the HIV transactivator of transcription (TAT) protein increases the number of thymic epithelial cells (TECs) in mice that have undergone hematopoietic stem cell transplantation. Consequently, these mice had enhanced thymopoiesis, an improved thymic output and an increased number of naïve T cells in the periphery. However, i.t. injection may not be an ideal choice for clinical applications. 

It has been reported that chemokine CCL25 is highly expressed in thymic tissue, especially thymic stroma. CCR9 is the receptor for CCL25. Unlike other CC chemokine receptors, CCR9 shows a strict specificity for its ligand CCL25. It has been shown intramural injection of a fusion protein containing the N-terminal of CCR9 and IL-7 increased the content of IL-7 in the thymus as compared to injection of IL-7 alone.

In this study, we develop a rFOXN1 fusion protein that contains the N-terminal of CCR9, FOXN1, and TAT. We show here that, when injected intravenously (i.v.) into aged mice, the rFOXN1 fusion protein can migrate into the thymus and enhance T cell generation in the thymus, resulting in increased number of peripheral T cells. Our results suggest that the rFOXN1 fusion protein has the potential to be used in preventing and treating T cell immunodeficiency in the older adult.