Clonal deletion of self-Mls-reactive thymocytes at the early stage in H-2-compatible but Mls-disparate radiation chimeras

Clonal deletion of T cells capable of recognizing both host-type Mls and donor-type Mls occurred in the peripheral mature T-cell pool in radiation bone marrow chimeras of two H-2-compatible Mls-disparate strain combinations of AKR/J(H-2(k),Thy-1.1,Mls-1^a) and C3H/He(H-2(k),Thy-1.2-Mls-1^b). In order to determine further the stage at which the clonal deletion occurs in thymus, we examined the kinetics of thymocytes bearing Vβ6 capable of recognizing Mls-1^a in both C3H/He → AKR/J and AKR/J → C3H/He chimeras. An almost complete replacement from host-derived cells to donor-derived cells occurred by Day 21 after reconstitution in both chimeras. At this stage, CD4⁺CD8⁺ double-positive thymocytes contained an appreciable number of cells that expressed Vβ6 on their surface, albeit at low intensity, whereas CD4 or CD8 single-positive thymocytes which expressed a high density of Vβ6 were virtually abolished in both C3H → AKR and AKR → C3H chimeras on Day 21. These results suggest that clonal deletion of self-Mls-reactive T cells begins at an early stage when the thymocytes interact with the early appearing donor-derived haematopoietic cells and relatively radio-resistant host-derived cells in thymus of radiation bone marrow chimeras.