The effect of age on the regeneration of the B cell population was studied by cell transfer methods, using the allotype-congenic mouse strains BALB/c (Igha) and C.B-17 (Ighb) as donors of old and young bone marrow (BM) and spleen cells, and C.AL-20 (Igho) as recipients. This design allowed us to identify the origin of the sIgD+ B cells present in the recipients. It was found that in a simple cell transfer, BM cells or spleen cells of aged donors could reconstitute the peripheral B cell population of irradiated, thymectomized recipients essentially as effectively as could BM or spleen cells from young donors. However, when BM cells from aged donors and from young donors were mixed and were used to reconstitute a single recipient, the cells from the aged donor were less efficient than were the cells from the young donor. We found that sIgD+ B cells of young donor origin predominated in the peripheral B cell population of the recipient at 3 to 6 wk after cell transfer. In the BM of the recipients, however, there was no difference in the incidence of sIgD+ B cells derived from the young and the old donors. When recipients were reconstituted with a mixture of spleen cells from old and young mice, the sIgD+ cells of young donor allotype showed a tendency to predominate in the peripheral B cell population, although this predominance was not statistically significant. Under such competitive conditions, the spleen cells of aged donors were less efficient than the BM of aged donors in reconstituting the sIgD+ B cell population of the recipient's BM, but were more efficient in reconstituting the splenic sIgD+ cells. Thus, a subtle defect in the B cell precursor population of the BM and the spleen of aged mice has been demonstrated. The role of T cells in the generation of sIgD+ cells was also analyzed.

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