PITUITARY EPITHELIAL-CELL IMPLANTS REVERSE THE ACCUMULATION OF CD4-CD8- LYMPHOCYTES IN THYMUS GLANDS OF AGED RATS

Although implantation of GH3 pituitary epithelial cells has been shown to reverse thymic aging in rats, the differentiation pattern of T-lymphocytes within the reconstituted thymus glands has not been investigated. Syngeneic GH3 cells were implanted into 22-month-old female (old) Wistar-Furth rats. Eight weeks later, thymus glands and thymocyte subpopulations were compared to those in aged (24-month-old) and young (3-month-old) female Wistar Furth rats. We confirmed that implantation of GH3 cells increased (P < 0.05) not only thymus size but also the number of thymocytes isolated from aged rats. Flow cytometric analysis using dual color direct immunofluorescence with fluorescein isothiocyanate-labeled anti-CD4 (W3/25) and phycoerythrin-labeled anti-CD8 (OX 8) monoclonal antibodies revealed that thymus glands from young rats had approximately 20% CD4-CD8- and 30% CD4+CD8+ cells. Thymus glands from old rats contained greater than 50% more CD4-CD8- cells and a reduced percentage of CD4+CD8+ lymphocytes compared to those of young rats (P < 0.05). Moreover, both of these age-associated changes were reversed (P > 0.05) by implanting GH3 cells. GH3-treated aged rats also had a significantly (P < 0.05) greater proportion of CD4+CD8- thymocytes compared to aged control rats. There were no differences among the three groups of rats in the percentage of CD4-CD8+ thymocytes or in the percentage or intensity of cells expressing the T-cell markers CD3, T-cell receptor, or OX19. These results show that in aged rats, intrathymic maturation is inhibited at the key transitional stage where double negative cells differentiate into double positive cells, which may limit the diversity of the T-cell repertoire. The data also extend earlier results by demonstrating that GH3 epithelial cells promote not only growth, but also the differentiation of T-lymphocytes in the aging rat thymus.