T-cell development during ageing in congenitally athymic (nude) rats.

We studied the thymus-dependent immune system in congenitally athymic (nude) rats of the WAG (RT-1u) strain, at the ages of 2, 3, 5, 9, and 17 months. This included the histology of spleen, lymph nodes, and Peyer's patches, immunohistochemistry on tissue sections, and immunocytology on cell suspensions using a panel of monoclonal antibodies to T-lymphocyte subpopulations, in vitro mitogen responsiveness, and in vivo responsiveness to ovalbumin immunization. The results were compared with those in euthymic immunocompetent littermates of the same age. The cell marker analysis, especially in nude animals, was hampered by the staining of non-T cells by some of the antibodies (staining of putative macrophages, natural killer cells, cells of B-lymphocyte lineage, and of myeloid lineage). Despite this, with age an increase in cells with these markers was observed; for instance, in spleen suspensions, cells labelled by the pan-T reagent MRC OX-19 increased from 5% at 2 months to 19% at 17 months (value in euthymic animals 34%). In the other assays too a gradual increase of T-cell reactivity with age was observed, from almost absent in 2- and 3-month-old nude rats to values in 17-month-old animals of about half the level found in euthymic rats. However, none of the nude animals responded to ovalbumin immunization. These results indicate that in nude animals processes occur which compensate for the absent intrathymic T-cell generation. This applies in particular to changes in T-cell phenotype and mitogen responsiveness, but not to antigen responsiveness.