Thymocytes that express high levels of homing receptors for peripheral lymph nodes can be detected with the monoclonal antibody MEL-14. We have shown that in adult mice these rare MEL-14hi thymocytes a) are cortical in location and typically constitute 1 to 3% of the total thymocyte population, b) may be a major source of thymus emigrants, and c) contain a high frequency of precursors of alloreactive cytotoxic T lymphocytes. In this study we have analyzed the phenotype of the MEL-14hi thymocyte subset. Most normal adult MEL-14hi thymocytes are midsize and express the mature phenotype typical of thymus emigrants, medullary thymocytes, and peripheral T cells: they are predominantly PNAlo, H-2K+, Thy-1+, Ly-1hi, and either Lyt-2-/L3T4+ or Lyt-2+/L3T4-. These findings argue strongly for the presence of rare MEL-14hi immunocompetent cortical thymocytes that, aside from their homing receptor expression, are phenotypically indistinguishable from medullary thymocytes. However, a minority (20 to 30%) of MEL-14hi thymocytes are large and phenotypically nonmature: they express intermediate to high levels of PNA binding sites, and are H-2K- to H-2Klo, Thy-1hi, Ly-1+, and either Lyt-2+/L3T4+ or Lyt-2-/L3T4-. Through a technique that selectively labels outer cortical cells, phenotypically nonmature MEL-14hi thymocytes have been shown to be concentrated in the subcapsular blast region of the outer cortex. Although we have no direct evidence of a precursor-product relationship, we consider it likely that the phenotypically nonmature outer cortical MEL-14hi lymphoblasts give rise to phenotypically mature MEL-14hi cells located deeper in the cortex. These results are consistent with our previous proposal that MEL-14hi thymocytes are a major source of thymus emigrants, and indicate that expression of high levels of MEL-14-defined homing receptors may be closely linked to the intrathymic selection process.

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