Thymus transplantation is a form of

organ transplantation Organ transplantation is a medical procedure in which an organ is removed from one body and placed in the body of a recipient, to replace a damaged or missing organ. The donor and recipient may be at the same location, or organs may be transpor ...

where the

thymus The thymus is a specialized primary lymphoid organ of the immune system. Within the thymus, thymus cell lymphocytes or ''T cells'' mature. T cells are critical to the adaptive immune system, where the body adapts to specific foreign invaders. ...

is moved from one body to another. It is used in certain immunodeficiencies.

Indications

Thymus transplantation is used to treat infants with

DiGeorge syndrome DiGeorge syndrome, also known as 22q11.2 deletion syndrome, is a syndrome caused by a microdeletion on the long arm of chromosome 22. While the symptoms can vary, they often include congenital heart problems, specific facial features, frequent i ...

, which results in an absent or hypoplastic thymus, in turn causing problems with the

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...

T-cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...

mediated response. It is used in people with complete DiGeorge anomaly, which are entirely athymic. This subgroup represents less than 1% of DiGeorge syndrome patients. Nezelof syndrome is another thymus-related disease where it can be used.

Co-transplantation with other organs

In the 2000s, promising animal experiments into transplanting thymic tissue and another organ at the same time were carried out, in order to improve the recipient's tolerance of the transplanted organ, and to reduce the need for immunosuppressing drugs like

tacrolimus Tacrolimus, sold under the brand name Prograf among others, is an immunosuppressive drug. After allogeneic organ transplant, the risk of organ rejection is moderate. To lower the risk of organ rejection, tacrolimus is given. The drug can also ...

. Such trials have been performed with kidney and heart transplants, drastically extending the time the animals were surviving without immunosuppressing drugs. The first human heart-and-thymus co-transplantation was performed on Easton Sinnamon in 2022, a newborn who suffered from both a lack of T cells, and a serious heart defect. Depending on the development, it is planned to wean him off immunosuppressant drugs, but it remains to be seen whether the same technique is viable in adults, as they lack a thymus, with the bone marrow taking over T cell production.

Effects and prognosis

A study of 54 DiGeorge syndrome infants resulted in all tested subjects having developed polyclonal T-cell repertoires and proliferative responses to mitogens. The procedure was well tolerated and resulted in stable immunoreconstitution in these infants. It had a survival rate of 75%, having a follow-up as long as 13 years. Complications include an increased susceptibility to infections while the T cells have not yet developed, rashes and erythema.

Graft-versus-host disease

Theoretically, thymus transplantation could cause two types of graft-versus-host disease (GVHD): First, it could cause a donor T cell-related GVHD, because of T cells from the donor that are present in the transplanted thymus that recognizes the recipient as foreign. Donor T cells can be detected in the recipient after transplantation, but there is no evidence of any donor T cell-related graft-versus-host disease. Second, a thymus transplantation can cause a non-donor T cell-related GVHD because the recipients

thymocyte A Thymocyte is an immune cell present in the thymus, before it undergoes transformation into a T cell. Thymocytes are produced as stem cells in the bone marrow and reach the thymus via the blood. Thymopoiesis describes the process which turns thymo ...

s would use the donor thymus cells as models when going through the negative selection to recognize self-antigens, and could therefore still mistake own structures in the rest of the body for being non-self. This is a rather indirect GVHD because it is not directly cells in the graft itself that causes it, but cells in the graft that make the recipient's T cells act like donor T cells. It would also be of relatively late-onset because it requires the formation of new T cells. It can be seen as a multiple-organ autoimmunity in

xenotransplantation Xenotransplantation (''xenos-'' from the Greek meaning "foreign" or strange), or heterologous transplant, is the transplantation of living cells, tissues or organs from one species to another. Such cells, tissues or organs are called xenograft ...

experiments of the thymus between different species. Autoimmune disease is a frequent complication after human allogeneic thymus transplantation, found in 42% of subjects over 1 year post transplantation.Thymus Transplantation Book Thymus Gland Pathology Publisher Springer Milan DOI 10.1007/978-88-470-0828-1 Copyright 2008 (Print) 978-88-470-0828-1 (Online) DOI 10.1007/978-88-470-0828-1_30 Pages 255-267 However, this is partially explained by that the indication itself, that is, complete DiGeorge syndrome, increases the risk of autoimmune disease.

References

{{Lymphatic organ procedures Organ transplantation Immunology Lymphatic organ surgery