3-AMINOBENZAMIDE ENHANCES DEXAMETHASONE-MEDIATED MOUSE THYMOCYTE DEPLETION IN-VIVO - IMPLICATION FOR A ROLE OF POLY ADP-RIBOSYLATION IN THENEGATIVE SELECTION OF IMMATURE THYMOCYTES

3-Aminobenzamide (3-ABAm), an inhibitor of poly ADP-ribosylation, was here found fo remarkably enhance the dexamethasone (Dex)-mediated depl etion of total mouse thymocytes within 24 h post-injection, when given i.p. in combination with Dex. After treatment the total thymocytes we re fractionated by Percoll gradient centrifugation into two mitogen-un responsive (high- and medium-density) and one mitogen-reactive (low-de nsity) subpopulations and these were analyzed for the phenotypic expre ssion of CD4 and CD8 antigens. Treatment with Dex alone most extensive ly depleted the high- and medium-density thymocytes and also those exp ressing 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 subpopulation in particular, were most resistant to the Dex-mediated depletion, thus giving rise to an enrichment of SP (2-fold) and particularly DN subse t in the medium- and low-density populations (5-fold) recovered. 3-ABA m, which alone increased the total thymocyte number up to 2-fold, had no effect on the distribution of phenotypic subsets. However, the inhi bitor, when given in combination with Dex, additionally depleted all f our phenotypic subsets up to one-third of. the levels with Dex alone, except for those of medium-density subpopulation. Because the non-inhi bitor, 3-aminobenzoate, had no potentiating effect, our present result s, together with our previous in vitro studies, indicate a role for th e DNA repair cofactor poly ADP-ribose in the intrathymic death by apop tosis and depletion of thymocytes, especially those of DP subset in th e high-density, functionally immature population.