Thymocyte

Thymocytes are hematopoietic progenitor cells present in the thymus.Thymopoiesis is the process in the thymus by which thymocytes differentiate into mature T lymphocytes. The primary function of thymocytes is the generation of T lymphocytes (T cells). The thymus provides an inductive environment, which allows for the development and selection of physiologically useful T cells. The processes of beta-selection, positive selection, and negative selection shape the population of thymocytes into a peripheral pool of T cells that are able to respond to foreign pathogens and are immunologically tolerant towards self antigens.

Thymocytes are classified into a number of distinct maturational stages based on the expression of cell surface markers. The earliest thymocyte stage is the double negative stage (negative for both CD4 and CD8), which more recently has been better described as Lineage-negative, and which can be divided into four substages. The next major stage is the double positive stage (positive for both CD4 and CD8). The final stage in maturation is the single positive stage (positive for either CD4 or CD8).

In human, circulating CD34+ hematopoietic stem cells (HSC) reside in bone marrow. They produce precursors of T lymphocytes, which seed the thymus (thus becoming thymocytes) and differentiate under influence of the Notch and its ligands.

Early, double negative thymocytes express (and can be identified by) CD2, CD5 and CD7. Still during the double negative stage, CD34 expression stops and CD1 is expressed. Expression of both CD4 and CD8 makes them double positive, and matures into either CD4+ or CD8+ cells.

In order to pass the β-selection checkpoint, the β chain of the T cell receptor rearranged by the thymocyte must retain the structural properties allowing it to be presented on the surface of the thymocyte with pre-TCRα. This eliminates thymocytes with gross defects introduced into the T cell receptor by gene rearrangement.

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