Recent thymic emigrants as a cellular basis for the formation of immune homeostasis

Morphological basis of the homeostatic immune system is made up of lymphoid and hematopoietic organs, as well as numerous clusters of lymphoid cells scattered throughout various organs and tissues of the body. According to their morpho-functional significance, they are divided into central and peripheral organs. The thymic gland is the only place where T lymphocytes are produced. In the thymus, thymocytes undergo differentiation and proliferation, eventually leading to the formation of two T cell populations. The main issue is that there is no differentiation of T cells in the thymus into T cells of effector subpopulations. This is the prerogative of the periphery. However, before becoming effector cells at the periphery, T cells migrate from the thymus and remain in circulation for a certain time without settling in secondary lymphoid organs. These cells are no longer thymocytes, but they are not yet naive T cells on the periphery, being recent thymic emigrants (NTE). Thus, they represent a separate population of T cells, one of three dominant populations of T cells. Moreover, the cells of all these three macropopulations differ from each other in a number of morphofunctional characteristics. NTE cells become the object of evaluating their quantitative and qualitative characteristics. It turned out that in many diseases with immunopathogenetic component (and, possibly, in all of them), the number of NETs decreases depending on the type of disease and its stage of development. In some cases, there is evidence of changes in percentage of Treg cells and other T cells among NETs, as well as changing ratios of CD4⁺ and CD8⁺T cells. These changes in the relative contents of different subpopulations among NETs are associated with pathogenesis of underlying disease. Thus, it seems to be a strong necessity to develop comprehensive methods of quantitative and qualitative assessment of the population of NTE cells as targets for both diagnosis and therapy of immunocompromised diseases. One may assume that such an assessment will form the diagnostic basis before clinical detection of the disease and/or aggravation of its course.

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