Recirculating lymphocytes in the mouse thymus are part of the relatively cortisone resistant cell population.

A quantitative analysis of the distribution patterns of intravenously infused Cr-labelled thymus and lymph node cells was carried out. It was observed that only a small fraction of the thymic cells distributed to the lymph nodes of the recipients. These lymph node seeking thymic cells could be enriched by treating the donor mice with cortisone acetate whereas this treatment did not change the distribution pattern of lymph node cells. Thymectomy followed by lethal irradiation and bone marrow reconstitution of the recipients did not change the distribution patterns of normal or cortisone resistant thymic cells. The cells in lymph nodes which home to lymph nodes upon intravenous infusion were found to be thymus-dependent, since they were almost absent in the lymph node cells obtained after thymectomy irradiation and bone marrow protection of the donors. The results indicate that only a small proportion of the cells in the thymus, namely the relatively cortisone resistant cells known to exhibit the cell-bound type of immunological reactivity, behave like recirculating lymphocytes upon infusion into isologous recipients.