[SUMMARY]The thymus is the only organ that produces T cells where immature thymocytes are properly educated and make the correct decision to become appropriate lineage T cells such as CD4+ or CD8+ T cells. This process is referred to as‘ lineage choice’. Failure in T cell development and/or lineage choice is a direct cause of severe immunodeficiency and/or severe autoimmune disorders. Thus, it is extremely important to understand the molecular mechanisms of thymocyte development and lineage choice and many immunologists have studied this issue intensively. A diverse TCR repertoire（ nearly 1018） is generated by TCR gene rearrangement in immature thymocytes. It contains autoreactive T cells as well as useless T cells that cannot recognize self-MHC molecules. These useless T cells are negatively selected（ i.e. clonal deletion）, and only cells that moderately recognize self-MHC molecules are positively selected. These processes are termed negative and positive selection. Positively selected cells further differentiate into either CD4T cells or CD8T cells, depending on their specificity to MHC-II or MHC-I respectively. How can the lineage choice of T cells, which have a huge TCR repertoire, be properly determined during their development? In this review, I will update the recent understanding of the molecular mechanisms underlying the CD4/CD8 lineage choices in the thymus and explain the mechanisms of error-free MHC-I-dependent CD8 T cell development.

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