Cellular and molecular effects of di-n-octyltin dichloride on the rat thymus

Triphenyltin acetate-induced cytotoxicity and CD4(+) and CD8(+) depletion in mouse thymocyte primary cultures

Organotin compounds (OTs) find application worldwide as catalysts, stabilizers and biocides. Triphenyltin derivatives (TPs), including the fungicide triphenyltin acetate (TPTA), are OTs mostly used in our country. Some OTs were proved to be immunotoxic and in this paper the cytotoxicity, the possible selective activity upon definite lymphocyte subsets as well as the antiproliferative effect of TPTA was investigated in vitro by using primary cultures of mouse thymocytes. TPTA (5, 10 and 25 microM) was cytotoxic to these cells, as demonstrated by the significant (P<0.05) reduction of the cell viability percentage (trypan blue dye exclusion test), the neutral red uptake and the reduction of tetrazolium salts to formazan products (MTT assay). These overt effects were already noticed after 4 h of exposure to TPTA. The fungicide otherwise significantly reduced, after 24 h of incubation, the percentage of mature single positive thymocytes, particularly the CD4(+)/CD8(-) one. Finally, a significative dose-dependent inhibition of the T-cell mitogen-induced cell proliferation was observed in thymocytes exposed to 1 and 8 microM TPTA. These results are indicative of the TPTA immunotoxic properties, according to previous published reports concerning the in vitro and in vivo toxicity of some di- and triorganotin compounds.

In vitro effects of triphenyltin acetate (TPTA) on mouse lymphocyte proliferation

Organotin compounds (OTs) are used worldwide in industry and in agricultural practice. In Italy, the OT mostly used is the fungicide triphenyltin acetate (TPTA), although in past years it has been submitted to revision for occupational and environmental grounds. In the present study, the possible effect of the fungicide on cell proliferation has been investigated, using [(3)H]thymidine incorporation radiometric assay. Mouse thymocytes in primary culture were stimulated for 24 h with T- and B-cell mitogens (concanavalin A, CON A and phytohemagglutinin, PHA or pokeweed mitogen, PWM, respectively). Cultures were then exposed 24 h to 0, 1 and 8 microM TPTA. At the end of incubation, cells were pulsed with methyl-[(3)H]thymidine and harvested for total radioactivity counts after a further 24 h of incubation. An overall dose-dependent significant (P<0.05) reduction of proliferative response was observed with all mitogens tested. Interestingly, CON A proved to be more sensitive (P<0.05) to the TPTA toxic effect compared with PWM and PHA. TPTA is cytotoxic to mouse thymocytes in primary culture, particularly towards the mature lymphocytes (CD4(+) and CD8(+)). The present results support the hypothesis of a TPTA-induced decrease of lymphoproliferative response to T- and B-cell mitogens, previously observed with other OTs.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

Immunotoxic organotins as possible model compounds in studying apoptosis and thymocyte differentiation

In the mid-seventies it appeared that some organotin compounds selectively caused thymus atrophy. From that time onward efforts were made to reveal molecular and cellular mechanisms involved. In this review recent studies into organotin-sensitive stages and processes of thymocyte maturation are discussed. Together these studies resulted in the recognition of organotin compounds as possible model compounds in studying immature thymocyte differentiation and protein synthesis-independent apoptotic cell death of thymocytes.

The evaluation of data-derived safety factors for bis(tri-n-butyltin)oxide

In this report the evaluation of a safety factor is assessed, according to the decision tree suggested by Renwick (1991a,b), to determine a Tolerable Daily Intake (TDI) value for the environmental contaminant bis(tri-n-butyltin)oxide (TBTO). Based on available literature the nature of the most sensitive parameter of TBTO-toxicity was perceived to be on lymphoid organs and lymphoid function. Subsequently, safety factors were derived in relation to published data on inter-species and inter-individual differences in both kinetics and dynamics of TBTO. Lack of information on human data concerning the nature of toxicity as well as kinetics and dynamics of TBTO finally resulted in a safety factor of 100. A TDI of 5 or 0.25 mg/kg bw per day was assessed based on reductions of lymphoid organ weights (thymus) or lymphoid function (resistance to T. spiralis), respectively. In addition, based on available data in rodents on kinetics and dynamics of TBTO, it is suggested that a combined TDI value for both tri- and dibutyltin compounds might have to be considered.

The cortical epithelium of the rat thymus after in vivo exposure to bis(tri-n-butyltin)oxide (TBTO). An (immuno)histological and ultrastructural study

Bis(tri-n-butyltin)oxide (TBTO) induces cortical atrophy in the rat thymus. We studied the potential involvement of the cortical epithelium in TBTO-induced thymotoxicity by (immuno) histology and electron microscopy. Juvenile male Wistar rats were orally intubated once with either 30 or 90 mg/kg TBTO and sacrificed 4 or 10 days later. A dose-dependent thymic atrophy occurred. Anti-keratin labelling showed epithelial cell aggregation in some animals at 10 days after exposure to 90 mg/kg TBTO, when recovery of the thymus was apparent. At the ultrastructural level, a relative shift was observed from "pale" to darker epithelial cell subtypes at the 30 mg/kg dose level, both at day 4 and day 10 after intubation. This phenomenon was not observed after exposure to 90 mg/kg TBTO. Both the altered keratin distribution and the increased electron density of the epithelium probably represent non-specific phenomena. The present morphological observations support the concept that TBTO affects the thymus via its action on lymphoid cells rather than on the epithelial compartment.

Immunotoxicity of bis(tri-n-butyltin) oxide in the rat

Previous studies in the rat have shown that bis(tri-n-butyltin) oxide (TBTO), used as a biocide, was immunotoxic at dose levels that did not affect other organs. In order to determine a no-effect level, weanling rats were treated for at least 28 consecutive days with TBTO at 0, 0.5, 2, 5, or 50 mg/kg of diet. Studies on clinical chemistry, hematology, pathology, and immune function, that is, plaque-forming cell (PFC) assay, delayed-type hypersensitivity (DTH) response, and the splenic clearance of Listeria monocytogenes, were performed at the end of treatment. No treatment-related effects were noted on clinical chemistry and hematology parameters and on PFC and DTH response, whereas thymic atrophy and impaired clearance of L. monocytogenes were noted only at a dietary concentration of 50 mg/kg. These results confirm the thymus as a target organ of TBTO immunotoxicity. Under the conditions of these experiments the dietary concentration of 5 mg/kg, equivalent to a dose of 0.5 mg/kg body weight, represents a no observed effect level (NOEL) for immunotoxicity in the Sprague-Dawley rat.

Reversibility of thymic atrophy induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO)

We studied the reversibility of thymic atrophy induced by intubation of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 10 days after a single dose of 50 micrograms/kg, or bis(tri-n-butyltin)oxide (TBTO), 4 days after a single dose of 75 mg/kg. This was done by an experimental design in which the atrophic thymus was placed in an in vivo situation in which the toxic chemical was no longer present, e.g. by transplantation of atrophic thymic lobes in untreated normal rats with connection to the vasculature of the recipient. At 20 days after the transplantation, the atrophic thymus showed the morphology and architecture of a normal uninvoluted thymus: lymphocyte counts and phenotypic expression of markers on lymphocytes, epithelium, and macrophages in the transplanted lobe did not differ from those in untreated donor rats or those in the normal uninvoluted thymus. Considering the mechanism of action of the toxic chemical, TBTO has been claimed to affect preferentially (passenger) lymphocytes in the thymus: the recovery after transplantation therefore is explained on the mere influx of newly-recruited precursor cells from the bone marrow. For TCDD a toxic action on the stationary epithelial component of the thymus has been claimed. We conclude that this epithelial damage is reversible within the 3-week period of the present experiment, with respect to both the morphology and immunologic phenotype of epithelium and other cell populations, as well as the recruitment of lymphocytes.