Abstract
Thymectomized (Tx), irradiated hooded rats, reconstituted with 200 × 10 6 syngeneic normal bone marrow cells, showed variable responsiveness to challenge with sheep red blood cells (SRBC) at four and eight weeks after irradiation. If Tx recipients were restored instead with only 10 × 10 6 marrow cells from lymphocyte-depleted (i.e. thymectomized, thoracic duct-drained) donors, these rats were unresponsive to SRBC challenge at four weeks. Injection of 200 × 10 6 syngeneic thymus cells into such rats restored the hemolysin response to SRBC and the antibody was made by marrow-derived cells. Thoracic duct lymphocytes obtained from Tx irradiated rats reconstituted with lymphocyte-depleted marrow were able to transfer responsiveness to this antigen to irradiated secondary recipients if thymocytes were injected one week or more before cannulation. The restoration of competence in the thoracic duct population required the presence of thymus-derived cells but all antibody made in secondary recipients was synthesized by marrow-derived cells. It is argued that the competence of normal thoracic duct lymphocytes is due to the presence of a mixture of thymus-derived and marrow-derived cells.
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