What are the stages of T cell maturation?

T cell maturation involves several key stages starting thymocyte progenitors migrating to the thymus, a specialized organ where further development occurs.

Thymocytes or immature T cells in the thymus are known as double negative (DN) cells as they do not express either the CD4 or CD8 antigen. As they mature, the double-negative (CD4-CD8-) cells develop into double-positive cells (CD4+CD8+), setting the stage for selection.  

Double-positives (DP) travel deep into the thymic cortex where they encounter self-antigens that are expressed by thymic cortical epithelial cells. These cells express two types of Major Histocompatibility Complex (MHC) molecules – MHC I and MHC II – on the cortical cell surface. Only those thymocytes that bind with MHC I or MHC II survive this stage of maturation. Those that don’t bind are eliminated by apoptosis.  

Thymocytes with weak TCR-MHC interactions undergo positive selection, resulting in CD4+ or CD8+ single-positive thymocytes. Those with strong TCR-MHC interactions undergo negative selection, eliminating potentially auto-reactive T cells.

Thymocytes that go through positive and negative selection successfully differentiate into functionally mature single-positive CD4+ or CD8+ T cells that are capable of targeted immune responses.