Immature T lineage lymphocytes in athymic mice. Presence of TL, lifespan and homeostatic regulation.

Lymphocytes belonging to the T lineage were described in athymic nude mice. We showed previously that they were distinguished from usual peripheral lymphocytes by their low density of theta antigen, their slow electrophoretic mobility and their absence of recirculation through the thoracic duct. We now report that they also express the TL antigen, have a life span of 1 to 2 days, are produced in the bone marrow and are under the homeostatic influence of the thymus. Indeed they appear rapidly in surgically T-deprived mice and their production is blocked in both surgically or congenitally athymic mice receiving a thymus graft. This homeostatic control may be mediated via a humoral factor. Cells with similar characteristics are present in the thymus at early stages of embryogenesis. The characteristics of these T lineage lymphocytes strongly suggest that they may represent "pre-thymocytes", i.e. cells already committed to the T pathway independently of thymic influence but needing the thymus microenvironment to differentiate further.