The entry of the prothymocyte into the thymus after lethal irradiation and bone marrow transplantation. II. Time of entry.
The time of entry of prothymocytes into the thymus after lethal irradiation and bone marrow transplantation (BMT) was determined by exposing the thymus only or the whole body with the thymus shielded to a second irradiation after different intervals. The repopulation of the thymus by donor type cells was determined by a thymus repopulation assay using donor specific markers. Reirradiation of the thymus kills the prothymocytes that have entered the thymus during the interval. It was found that reirradiation of the thymus from 48 hours after BMT onwards increasingly delayed thymus regeneration. This shows that donor prothymocytes do not enter the thymus until about 2 days after BMT and that they continue to do so during at least three subsequent days. In the second reirradiation protocol thymus regeneration occurred earlier in the shielded thymus than in thymuses of whole body irradiated mice. Earlier thymus regeneration was not seen in mice that were reirradiated at 24 hours after BMT, but occurred only when irradiation took place at 48 hours and later. These data are consistent with those obtained in the first protocol. The results are in contradiction with results of direct homing experiments, which showed entrance of donor cells within 3 hours after BMT. Our functional assay demonstrated that the early appearing cells cannot be prothymocytes. In retransplantation experiments it was shown that the bone marrow (BM) may indeed be the initial homing site of prothymocytes.
Mulder
A H
AH
Radiobiological Institute TNO, Rijswijk, The Netherlands.
Visser
J W
JW
Zoetelief
J
J
van Bekkum
D W
DW
D000818:Animals; D016026:Bone Marrow Transplantation; D051379:Mice; D008815:Mice, Inbred Strains; D011835:Radiation Protection; D013234:Stem Cells / Q000166:cytology*; D013950:Thymus Gland / Q000166:cytology* / Q000528:radiation effects; D013997:Time Factors; D014916:Whole-Body Irradiation