3-Aminobenzamide enhances dexamethasone-mediated mouse thymocyte depletion in vivo: implication for a role of poly ADP-ribosylation in the negative selection of immature thymocytes

Abstract

3-Aminobenzamide (3-ABAm), an inhibitor of poly ADP-ribosylation, was here found to remarkably enhance the dexamethasone (Dex)-mediated depletion of total mouse thymocytes within 24 h post-injection, when given i.p. in combination with Dex. After treatment the total thymocytes were fractionated by Percoll gradient centrifugation into two mitogen-unresponsive (high- and medium-density) and one mitogen-reactive (low-density) subpopulations and these were analyzed for the phenotypic expression of CD4 and CD8 antigens. Treatment with Dex alone most extensively depleted the high- and medium-density thymocytes and also expressing both CD4 and CD8 double positive (DP) phenotypes in all three subpopulations. The CD4 + and CD8 + single positive (SP) and CD4 −CD8 − double negative (DN) subsets, in the low-density subpoulation in particular, were most resistant to the Dex-mediated depletion, thus giving rise to an enrichment of SP (2-fold) and particularly DN subset in the medium- and low-density populations (5-fold) recovered. 3-ABAm, which alone increased the total thymocyte number up to 2-fold, had no effect on the distribution of phenotypic subsets. However, the inhibitor, when given in combination with Dex, additionally depleted all four phenotypic subsets up to one-third of the levels with Dex alone, except for those of medium-density subpopulation. Becase the non-inhibitor, 3-aminonenzoate, had no potentiating effect, our present results, together with our previous in vitro studies, indicate a role for the DNA repair cofactor poly ADP-ribose in the intrathymic death by apoptosis and depletion of thymocytes, especially those of DP subset in the high-density, functionally immature population.