Lymphocyte Development and Function

T-Cell Development and Differentiation

The primitive thymic rudiment is formed from the ectoderm of the 3rd branchial cleft and endoderm of the 3rd branchial pouch at 4 wk gestation. Beginning at 7-8 wk, the right and left rudiments fuse in the midline. Bloodborne T-cell precursors from the fetal liver then begin to colonize the perithymic mesenchyme at 8 wk gestation and move into the thymus at 8.0-8.5 wk. The earliest cells to enter the thymus are found in the subcapsular region and do not express CD3, CD4, CD8, or either type of T-cell receptor (TCR). These lymphoid cell precursors are triggered to proliferate and become thymocytes through interactions with the thymic stroma. The cells are arrested at this stage until they productively rearrange the β-chain locus of the TCR. The β chain then pairs with the surrogate pre-T α chain. This tests the function of the β chain, and if signaling occurs, β-chain rearrangement ceases. CD4 and CD8 are then expressed simultaneously (i.e., they are double-positive thymocytes). Fetal cortical thymocytes are among the most rapidly dividing cells in the body and increase in number by 100,000-fold within 2 wk after stem cells enter the thymus. As these cells proliferate and mature, they migrate deeper into the thymic cortex. The double-positive thymocytes begin efficient rearrangement of the α-chain locus. TCR gene rearrangement occurs by a process in which large, noncontiguous blocks of DNA are spliced together. V (variable) , D (diversity) , and J (joining) blocks exist in families of minimally different segments. Random combinations of the segments account for much of the enormous diversity of TCRs that enables humans to recognize millions of different antigens. TCR gene rearrangement requires the presence of recombinase-activating genes , RAG1 and RAG2, as well as other recombinase components.

As immature cortical thymocytes begin to express TCRs, the processes of positive and negative selection take place. Positive selection occurs in immature thymocytes, recognizing major histocompatibility complex (MHC) antigens present on cortical thymic epithelial cells. Some cells are selected to mature into CD4 or CD8 single-positive cells. Negative selection occurs next in the thymic medulla on medullary thymic epithelial cells. Autoreactive T cells undergo apoptosis and die. T cells begin to emigrate from the thymus to the spleen, lymph nodes, and appendix at 11-12 wk of embryonic life and to the tonsils by 14-15 wk. They leave the thymus via the bloodstream and are distributed throughout the body, with the heaviest concentrations in the paracortical areas of lymph nodes, the periarteriolar areas of the spleen, and the thoracic lymph duct. Recent thymic emigrants co-express the CD45RA isoforms and CD62L ( l -selectin).

Rearrangement of the TCR locus during intrathymic T-cell development results in the excision of DNA and the excised elements form circular episomes as a by-product. These TCR recombination excision circles can be detected in T cells that are recent thymic emigrants. TCR recombination excision circles detected in dried-blood spots collected from infants shortly after birth is the test used for newborn screening for severe combined immunodeficiency (SCID). By 12 wk gestation, T cells can proliferate in response to plant lectins, such as phytohemagglutinin and concanavalin A. Antigen-specific T cells have been found by 20 wk gestation. Hassall corpuscles (bodies), which are swirls of terminally differentiated medullary epithelial cells, are first seen in the thymic medulla at 16-18 wk of embryonic life.