The size of the thymus is age-related. During puberty, the parenchyma is continuously replaced by non-functional fatty and connective tissue. Accordingly, the thymus output also decreases steadily. This output can be measured via the surface marker CD31 (PECAM-1) on naive T lymphocytes. These cells are called recent thymic emigrants and have only recently left the thymus. Naïve T lymphocytes circulating in the blood then lose the CD31 and migrate to peripheral lymphatic organs and tissues. The proportion of CD31+ cells is therefore a measure of the remaining thymus function.
In the rare disease di George syndrome (CATCH 22 syndrome), the thymus may be completely absent. Patients therefore do not have a cellular immune response. The resulting extreme susceptibility to a variety of infectious diseases shows how important the thymus is.
A reduced thymus reserve is generally found in autoimmune diseases such as multiple sclerosis, as well as in viral infections with the HI virus or the hepatitis C virus or in patients with T-cell abnormalities such as common variable immunodeficiency (CVID) syndrome. Therapy with thymus peptides to strengthen cellular immune function, the inhibition of pro-inflammatory mediators such as TNF or a stem cell transplant, on the other hand, lead to an increased number of thymus emigrants.
Diagnostically, the measurement of thymus emigrants is particularly important for immunosuppressive treatments such as chemotherapy or radiotherapy. Here, the analysis is used to estimate the T-cell regeneration capacity. However, measuring the thymus reserve can also provide additional assistance in the general clarification of unclear persistent lymphocytopenia.