Developmental interactions of CD4 T cells and thymocytes: age-related differential effects

The study was designed to determine whether the developmental potential of immature thymocytes in the thymus is altered in aging, and whether concomitantly present mature T cells have any feedback effect. The strategy was to seed sorted double negative, CD4^-CD8^-(DN) thymocytes on their own, or in the presence of mature T cells, onto lymphoid depleted fetal thymus (FT) explants, and to examine the resulting T cell subsets. Thymocyte donors were young (2-3 months) and old (24 months) C57BL/6J, Thy1.2 mice and splenocytes were from C57BL/Ka, Thy1.1 mice. The DN cells of the old gave rise to lower values of double positive CD4⁺CD8⁺ (DP) cells than those of the young. Cocultures containing a mixture of DN thymocytes and CD4⁺CD8^- splenocytes showed higher CD4⁺CD8^- and DN, and lower DP and CD4^-CD8⁺ levels in the old-donor derived cells, as compared with the young ones. Similar results were obtained with CD4⁺CD8^- thymocytes. In contrast, the presence of CD4^-CD8⁺ splenocytes had no effect on the pattern of DN cell development. Our data indicate that differentiation of CD4/CD8 thymocyte phenotypes is affected by CD4⁺ cells, in an age-associated differential manner.