Description
After positive selection in the thymus, the newly generated single positive (SP) thymocytes are phenotypically and functionally immature and undergo apoptosis upon antigen stimulation. In the thymic medullary microenvironment, SP cells progressively acquire immunocompetence. Negative selection to remove autoreactive T cells also occur at this stage. We have defined four subsets of CD4 SP, namely, SP1, SP2, SP3, and SP4 that follow a functional maturation program and a sequential emergence during mouse ontogeny.