Age-associated plasticity of α1-adrenoceptor-mediated tuning of T-cell development.

Alpha(1)-adrenoceptors (α(1)-ARs) are involved in neuro-thymic and thymic intercellular communications, and consequently modulation of T-cell development. Ageing is associated with a number of changes in noradrenergic neuro-effector transmission, and possibly intercellular noradrenaline (NA)-mediated communication resulting in altered responses of target cells to NA. Thus, in old animals an altered NA modulation of thymopoiesis via α(1)-ARs may be expected. To test this hypothesis, in old and young adult Wistar rats we examined: 1) thymic NA levels, density of noradrenergic innervation and NA synthesizing cells, as well as α(1)-AR expression, and 2) then the effects of 14-day-long treatment with the α(1)-AR blocker, urapidil, on thymocyte development. Overall, the first part of study suggested augmented NA signalling to thymic cells via α(1)-ARs due to increased NA availability and α(1)-AR thymocyte surface density in old rats. The second part of study supported this assumption. Namely, although in rats of both ages urapidil affected the same thymocyte developmental steps ultimately leading to changes in the relative number of the most mature single positive TCRαβ(high) thymocytes, its effects were generally more prominent in old animals. Following urapidil treatment, the percentages of CD4+CD8- cells, including those showing a regulatory CD4+CD25+RT6.1- phenotype, were increased, while CD4-CD8+ cells decreased. In old rats, an augmented thymic escape of immature CD4+CD8+ cells was also registered. In rats of both ages the thymic changes were accompanied by alterations in the proportions of major cell populations in the T-lymphocyte compartment of both peripheral blood and spleen, leading to an increase in the CD4+/CD8+ T-cell ratio. These alterations were also more pronounced in old rats. Moreover, in old rats following urapidil treatment the proportion of TCRαβ+cells in the periphery was slightly greater reflecting, most likely, partly enhanced thymic production of regulatory CD161+TCRαβ+cells. Thus, the study indirectly suggests an age-associated increase in the basal α(1)-AR-mediated inhibitory influence of NA on thymopoiesis.