LYMPHOCYTE AGING IN BONE-MARROW CHIMERAS

Chimeric mice provide a unique approach to the analysis of genetic fac tors associated with aging since cells with two genetically distinct b ackgrounds can be analyzed in the same animal. In this study, bone mar row chimeras were produced by reconstituting lethally irradiated femal e B6AF1 [(C57BL/6 female x A male)F1] mice with varying mixtures of T cell-depleted bone marrow cells from A (short-lived) and C57BL/6 (long -lived) mice. The phenotypic composition of the peripheral blood lymph ocytes was analyzed using either a cytotoxicity assay or flow cytometr y with indirect immunofluorescence. The percentage of A-derived lympho cytes in the peripheral blood following reconstitution was generally h igher than the percentage of A bone marrow cells with which the irradi ated mice were inoculated, suggesting that the cells from the A donor bone marrow were more efficient at marrow reconstitution than the cell s from the C67BL/6 donor bone marrow. In order to determine whether th e percentage of A- versus C57BL/6-derived cells changed with age in ea ch animal, the chimeric mice were bled for phenotype analysis of perip heral blood lymphocytes between 26 months following reconstitution and at 2-3 month intervals until death. For most animals [93/127 (73%)], there was no consistent pattern of increase or decrease (>20%) with re gard to the percentage of A lymphocytes in the peripheral blood over t ime. However, in 34/127 (27%) of the chimeras, a change greater than 2 0% in the phenotypic composition of the peripheral blood lymphocytes w as observed and these animals were considered unstable. Among these 34 unstable animals, 6 (18%) showed an overall increase in A-derived lym phocytes, 24 (71%) showed an overall decrease in A-derived lymphocytes , and 4 (12%) showed fluctuating increases and decreases over their li fespan. While the lifespans of the chimeric animals in these studies w ere considerably shorter than those reported for untreated mice of the same strain and gender, in these animals increased proportions of A c ells were associated with significantly longer lifespans. In addition, the lifespan of the B6AF1 chimeric mice was a function of the proport ion of A lymphocytes present in the peripheral blood over the course o f the animal's life.