Role of the thymus in the proliferation and migration of donor marrow cells in irradiated allogeneic host mice

SUMMARY CBA/H-T6T6 bone marrow cells (1 × 107) were injected into adult C57BL/6 mice that had received various amounts of radiation. Hosts were either thymectomized or sham thymectomized 10 days before irradiation. Donor marrow cells did not proliferate in the host organs after 400 R of irradiation regardless of thymectomy of the hosts. Donor marrow cells proliferated and comprised more than 65% on day 5 in the spleens and bone marrows of hosts irradiated with 600 R, but were quickly rejected by hosts. Thymectomy of the hosts did not influence the rejection. Donor marrow cells fully repopulated the bone marrows and spleens of hosts irradiated with 800 R by day 10 but not the thymuses or lymph nodes. Donor marrow cells started to appear in the latter organs after day 15, and comprised 100% of the dividing cell population after day 20. The percentages of the dividing cells increased gradually in the lymph nodes of thymectomized and 800 R irradiated hosts. Donor-type cells comprised only 50% of the dividing cell population in the lymph nodes on day 30. These data indicate that rejection of donor marrow cells in host marrows or spleens was a thymus-independent process, but that thymectomy clearly influenced the pattern of donor cell proliferation and migration in irradiated allogeneic hosts. Since allogeneic thymus cells directly settled and divided in the thymuses and lymph nodes of 800 R irradiated hosts, donor marrow cells may have to be modified in their membrane characteristics so that they are accepted in the peripheral lymph nodes. In the absence of the thymus, donor marrow cells that once settled and differentiated in the host marrows later migrated to the lymph nodes.