Mechanisms of immunosuppression by organotins : apoptosis vs. proliferative arrest

Abstract

Mechanisms of immunosuppression by organotins-apoptosis vs. proliferative arrest. The organotin compounds di-n-butyltin dichloride (DBTC) and trin-butyltin chloride (TBTC), used as stabilizers and biocides respectively, induce thymus atrophy inhibiting immature thymocyte proliferation. The aim of the study was to examine whether apoptosis has a role in this atrophy and whether DBTC, like TBTC, induces apoptosis in vitro. Thymi from rats treated with a dose (15 mg/kg) of organotin known to reduce thymocyte proliferation, did not show DNA fragmentation. This indicates that apoptosis is not evident in organotin-induced thymus atrophy at low doses. In vitro data showed that 3-5 µM of DBTC or TBTC significantly increased the percentage of apoptotic nuclei in rat thymocytes. Further mechanistic studies indicated a relation between the cytotoxic effects of the compounds and their capacity to induce apoptosis. At lower concentrations than required to induce apoptosis, both organotins inhibited protein and DNA synthesis and increased RNA and heat shock proteins synthesis. We demenstrated that the increase of RNA synthesis occurred in small thymocytes, which comprised the same subset of cells sensitive to apoptosis by organotins. Co-exposure to RNA or protein synthesis inhibitors protected cells from apoptosis by DBTC or TBTC, indicating that macromolecular synthesis is required for the initiation of the prcess. Moreover, two genes (glutathione S-transferase and nur77) were found to be activated under influence of apoptotic concentrations of DBTC. Besides effects on macromolecular synthesis, organotins disrupt energy metabolism and affect mitochondria. Previously, TBTC has been shown to increase intracellular calcium level, to produce reactive oxygen species (ROS) and to release pro-apoptotic factors. We showed similar changes in case of DBTC, i.e. increase of calcium, ROS production, release of cytochrome c and activation of caspase 3. Thus, induction of apoptosis is a relevant mechanism at relatively high concentrations/doses of organotin compounds, while lower concentrations/doses cause a proliferative arrest without signs of apoptosis. Finally, it has been investigated whether DBTC, in vivo was thymus-selective or had a direct effect on peripheral T cell responsiveness as well. We found that DBTC can rapidly and directly inhibit a hapten-specific immune respons, reducing the number of peripheral cells isolated from lymph nodes of sensitized rats. This finding may not have important implications for studying the mechanisms of organotin-induced immunosuppression