Definition of the thymic generative lineage by selective expression of high molecular weight isoforms of CD45 (T200)

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Abstract
Selective expression of CD45 isoforms distinguishes naive and memory T cells in peripheral blood. Paradoxically, although the most recent thymic emigrants are CD45R+ CD45 p180−, the majority of thymocytes are CD45 p180+. Speculating that the small subset of thymocytes selectively expressing only the high molecular weight isoforms of CD45 constitute the thymic generative lineage giving rise to peripheral T cells, we characterized the phenotypic and functional properties of CD45 p180− thymocytes. All cells bearing CD45 p180 were removed by rigorous depletion or all CD45R+ thymocytes were removed in a parallel depletion. CD45R− thymocytes were essentially the same in phenotype and CD4/CD8 subset distribution as unfractionated thymus, and dissimilar to naive peripheral blood lymphocyte (PBL) T cells. In contrast, CD45 p180− thymocytes, mainly CD45R+, were CD1− CD38− pgp 1+, corresponding closely to the phenotype of naive CD45R+ PBL T cells. This subset is enriched in CD4+ or CD8+ single positives, includes a high proportion of CD4−8− thymocytes which are predominantly CD3−, and appears to have a medullary location. Approximately 40%–50% of CD45 p180− thymocytes expressed a high density of CDw29 (4B4), which in the periphery is expressed at high density only on CD45 p180+ memory T cells and at low density on CD45R+ naive T cells. However, the expression of high density CDw29 in the absence of CD45 p180 indicates a close resemblance to fetal lymphocytes and suggests an essential role for CDw29 in both the least and the most mature of T cells. If CD45 p180− thymocytes constitute the generative lineage and CD45 p180+ cells are commited to intrathymic death, then the CD45 p180− subset should have enhanced proliferative potential. By combining depletion methods with a limiting dilution assay for clonogenic potential, we found that 100% of the clonogenic precursors present in unfractionated thymus were CD45R+ CD45 p180− cells. This indicates that the CD45 p180+ majority of thymocytes has a very limited capability for proliferation consistent with a commitment to intrathymic death. The clonogenic potential of CD45 p180− thymocytes indicates a greater functional resemblance to PBL T cells than to CD45 p180+ thymocytes. In so far as clonogenic potential in vitro reflects generative potential in vivo, expression of high molecular weight CD45 isoforms appears to define the generative thymic lineage. Our working hypothesis proposes that expression of CD45 p180 implements the mechanism for eliminating thymocytes with self-reactive receptor specificities.