Age-related changes in the capacity of bone marrow cells to differentiate in thymic organ cultures.

The capacity of the bone marrow to give rise to T cells in advanced age was studied in vitro by reconstituting fetal thymic lobes from 14-day C57BL/Ka (Thy-1.1) mice with bone marrow cells from old (24-month) or young (3-month) C57BL/6 (Thy-1.2) mice. The use of these congenic strains enabled distinguishing between donor and host contribution to the developing T cells. We found that bone marrow cells from aged mice maintained their capacity to reconstitute fetal thymic explants and to differentiate into various T-cell subsets as assessed by distinct T-cell-specific surface markers (Thy-1, Lyt-1, Lyt-2, and L3T4) and functions (concanavalin A-induced proliferative and cytotoxic responses). However, when mixtures of old and young bone marrow cells reconstituted fetal thymic explants, the cells of old mice were less efficient than those of young in their capacity to give rise to T cells. These results indicate that bone marrow cells from aged mice can reconstitute the thymus and differentiate into T cells; however, their reconstituting capacity is inferior to that of bone marrow cells from young mice.