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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 hone marrow chimaeras). The chimaeric state was confirmed by Ly allotype analysis. High incidence of BL‐CFC 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 properly 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.