Studies on the recirculating cells in the mouse thymus

The cells in the mouse thymus which respond to PHA and have the distribution characteristics of recirculating lymphocytes upon infusion into isologous recipients are markedly enriched 2 days after treatment of mice with proper doses of cortisone acetate. However, a similar increase is not observed in lymph nodes and spleen. During regeneration of the steroidinvoluted thymus, the proportion of recirculating thymic cells disappeared more rapidly than the relative PHA-responsiveness of the cell population. These findings indicate that all PHA-responsive thymocytes may not have recirculating capacity. The recirculating cells present in the thymus and lymph nodes have no tendency to distribute to the thymus, but tend to accumulate in lymph nodes. This fact, together with the finding that in vitro trypsin treatment of the cells temporarily abolishes their lymph node-seeking capacity, are taken as evidence that the recirculating cells in the thymus have undergone their maturation within this organ whereby they have gained membrane receptors which determine their distribution characteristics.