Long‐term β‐adrenergic receptor blockade increases levels of the most mature thymocyte subsets in aged rats

Age‐related increase in the density of thymic noradrenergic fibres and noradrenaline (NA) concentration is proposed to be associated with thymic involution and altered thymopoiesis. To test this hypothesis thymocyte differentiation/maturation and thymic structure were studied in 18‐month‐old male Wistar rats subjected to 14‐day‐long propranolol (P) blockade of β‐adrenoceptors (β‐ARs). The treatment primarily resulted in changes in the T‐cell receptor (TCR)‐dependent stages of thymopoiesis, which led to an increase in both the relative and absolute numbers of the most mature single positive (SP) CD4+CD8− (including cells with the CD4+CD25+ regulatory phenotype) and CD4−CD8+ TCRαβhigh thymocytes. Accordingly, in the thymi of these rats an increase in both numerical density and absolute number of medullary thymocytes encompassing mainly the most mature SP cells was found. These findings, together with an increase in the thymocyte surface expression of the regulatory molecule Thy‐1 (CD90) (implicated in negative regulation of TCRαβ‐dependent thymocyte selection thresholds) in the same rats, may suggest increased positive/reduced negative thymocyte selection. Collectively, the results indicate that a decline in thymic efficiency in generating both conventional and regulatory T cells, and consequently in immune function, in aged rats may be, at least partly, attenuated by long‐term blockade of β‐ARs with P.