A homing receptor-bearing cortical thymocyte subset: implications for thymus cell migration and the nature of cortisone-resistant thymocytes.

The thymus exports a selected subset of virgin T lymphocytes to the peripheral lymphoid organs. The mature phenotype of these thymus emigrants is similar to that of medullary thymocytes and has been cited as supporting a medullary rather than cortical exit site. Using the monoclonal antibody MEL-14, we identify a 1%-3% subpopulation of thymocytes that expresses high levels of a receptor molecule involved in lymphocyte homing to peripheral lymph nodes. We present evidence that these rare MEL-14hi thymocytes are predominantly of mature phenotype and represent the major source of thymus emigrants. Surprisingly, MEL-14hi thymocytes are exclusively cortical in location, although their mature phenotype may allow them to masquerade as medullary cells in conventional studies. We also demonstrate that unlike medullary thymocytes, many cortisone-resistant thymocytes (CRT) are MEL-14hi. Thus, in contrast to current dogma, CRT do not represent a sample of medullary thymocytes as they are found in situ and their level of immunocompetence does not necessarily reflect that of the medullary population. Our findings refute the hypothesis that phenotypically and functionally mature cells are restricted to the medulla, and support our proposition that most thymus emigrants are derived from the MEL-14hi cortical subset.