Response to ConA and PHA of host and donor thymic cells in radiation bone marrow chimeras

The response of thymic cells to ConA and PHA was followed during 49 days in 9 Gy-irradiated C3H mice reconstituted with (C3H X AKR)F1 BM. Thymic suspension were fractionated firstly according to their capacity to bind PNA, and secondly by their Thy-1 surface antigen: Thy-1.1 antiserum was used either to lyse doner-derived cells or to separate them from host cells by panning. From days 14 to 25, when the donor-derived elements expand rapidly, the PNA- fraction remains stable and equal to 6%, suggesting that PNA+ and PNA- cell populations develop independently of each other. On the contrary, the PNA- fraction which initially represents 6% of the surviving cells rises up to 12% after day 20 and remains at this level in the long-lived host population until the end of the experiment (49 days). The response to ConA returns to normal as early as 15 days after X-rays in the PNA- fraction, whereas the response to PHA is still impaired at the end of the experiment. In both cases host cells express a higher level of responsiveness to mitogens than donor cells.