B-lymphocyte colony-forming cells in the SJL/J mouse thymus graft repopulation: independence on host thymus and graft genotype.

We have studied the development of B-lymphocyte colony-forming cells (BL-CFC) in SJL and A.SW mouse thymus grafts transplanted into lethally irradiated A.SW mice protected with SJL mouse bone marrow (A.SW-SJL bone marrow chimaeras). The chimaeric state was confirmed by Ly allotype analysis. High incidence of BL-CJC was found in both the A.SW and the SJL thymus grafts, indicating that SJL bone marrow generates BL-CFC, or cells capable of generating BL-CFC, with a high affinity for homing in the thymus irrespective of the genotype of the thymus reticulum. The incidence of BL-CFC in the SJL thymus grafts, however, was approximately 20 times higher than the incidence of BK-CFC in the A.SW thymus grafts. Thus, the extensive infiltration of ageing SJL thymus glands by BL-CFC demonstrated recently is caused not only by SJL bone marrow B cells with a high affinity for thymus homing but also by the specific property of the SJL thymus environment. The extensive infiltration of syngeneic thymus grafts by BL-CFC observed in ageing SJL mice was found to occur independent of an intact host thymus function, indicating the extrathymic origin of these cells.