Immunological consequences of dioctyltin dichloride (DOTC)-induced thymic injury

Acute toxic effects of dioctyltin on immune system of rats

In the present study, dioctyltin chloride (DOTC: 100mg/kg, BW) was orally administered to immature (30-day-old) male rats, and the acute toxic effects were studied. Di- and monooctyltin (its metabolite) accumulations were mainly detected in the liver, and peaked 48h later. A similar pattern was also found in the kidney, but the levels were low or trace amounts. Significantly low thymus and spleen weights were detected in DOTC-treated animals. Increased apoptotic cell numbers in the thymus and spleen were observed in DOTC-treated animals also. Although the expression of 97 genes involved in apoptosis was studied in the thymus, at least 24h after treatment, we could not detect clearly different expressions between DOTC- and vehicle-treated animals. The present results suggest that DOTC was selectively immunotoxic. One of the mechanisms for its immunotoxicity would be via its stimulation of immune cell apoptosis.

Species differences in the structure and function of the immune system

With the recent publication of regulatory guidelines from both the FDA and the CPMP addressing the investigation of immunotoxicity of new chemical entities has come the requisite increased application of immunotoxicology protocols. Importantly, the fulfillment of these protocols may require the use of different species, and while in many cases information concerning the structure and function of the immune system can be readily translated across species, there are numerous and significant species differences that need to be considered. In some cases, the generation of meaningful immunotoxicology data can be adversely affected by the choice of a species that does not adequately share the immune function of concern with man. Likewise immunotoxicology testing in one species may produce negative data in one species but positive data in another. Knowing the mechanistic basis through an understanding of species differences in the structure and function of the immune system is pivotal to success. This becomes especially true as pharmaceutical companies design and develop highly specific immunomodulatory molecules that demonstrate species-specific pharmacology. This review is an exploration of various species differences in the structure and function of the immune system and an attempt to identify those differences that may be important in the conduct of immunotoxicity tests.

Developmental toxicity of an octyltin stabilizer in NMRI mice

The octyltin stabilizer ZK 30.434 is a mixture of 80% dioctyltin diisooctylthioglycolate (DOTTG) and 20% of monooctyltin triisooctylthioglycolate (MOTTG) and is used as stabilizer for rigid polyvinylchloride (PVC) materials. One of the applications of such stabilized films is the packaging of foodstuffs. Exposure to humans occurs via migration of DOTTG/MOTTG from PVC materials. In the present study the developmental toxicity of DOTTG/MOTTG in NMRI mice was investigated. Dams were treated orally with doses of 20, 30, 45, 67, or 100 mg/kg/day DOTTG/MOTTG from gestation day 6 through 17 (plug = day 1). Resorption rates were significantly increased and fetal weights significantly reduced in the study group at the 2 highest doses. External anomalies, such as bent forelimbs, cleft palate, and exencephaly were reported in the group treated with 100 mg/kg/day DOTTG/MOTTG, with the 67-mg/kd dose also exhibiting a significant increase in cleft palate. Moreover, an increase in skeletal anomalies was reported in fetuses exposed to 100 mg/kg/day. The doses of 20, 30, and 45 mg/kg/day elicited a significant increase in supernumerary lumbar ribs. It can be concluded that DOTTG/MOTTG is embryo-fetotoxic and induces developmental effects. The study revealed the need for the establishment of different No-Observed Adverse Effect Levels (NOAEL) for the endpoints investigated.

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.

Immune alterations in rats following subacute exposure to tributyltin oxide

Adult male Fischer 344 rats were dosed by oral gavage with bis(tri-n-butyltin)oxide (TBTO) in peanut oil for 10 consecutive days, at dosages ranging from 1.25 to 15 mg/kg/day. Other groups of rats were dosed daily for 10 days by oral gavage with cyclophosphamide (CY) at dosages ranging from 0.75 to 6 mg/kg/day. These rats served as positive controls for the immune assays employed. The immune function parameters examined included the following: delayed-type hypersensitivity (DTH) and antibody responses to bovine serum albumin (BSA), primary antibody responses to sheep red blood cells (SRBC) and trinitrophenyl lipopolysaccharide (TNP-LPS) and enumeration of splenic lymphocyte populations. The DTH and antibody responses to BSA were not affected by TBTO exposure; however these responses were suppressed in rats dosed with CY at 6 mg/kg/day. The plaque forming cell (PFC) response to the T cell-dependent antigen SRBC was enhanced in rats dosed with TBTO at from 5 to 15 mg/kg/day. On the other hand, the PFC response to the T cell-independent antigen TNP-LPS was unaffected by TBTO exposure. Rats dosed with CY had suppressed PFC responses to SRBC and TNP-LPS at dosages of 3 and 6 mg/kg/day, respectively. Enumeration of splenic lymphocyte populations from TBTO-exposed rats revealed a reduction in OX8- but not W3/25- or IgG-positive cells. These results, as well as results from an earlier study from this lab, suggest that T lymphocytes are a primary target for TBTO-induced immune alterations and that the enhancement of the PFC response to SRBC in TBTO-exposed rats may be mediated by alterations in the suppressor (OX8-positive) T lymphocyte population.

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

Experiments have been performed to characterise early changes in the proliferative capacity and phenotypic makeup of thymocytes obtained from DOTC-treated PVG rats. The analysis of density gradient-separated thymocytes demonstrated that within 72 h of oral dosing, spontaneous in vitro thymocyte proliferation was markedly suppressed. A concomitant depletion of an MRC OX18 positive thymocyte population was also observed. These events occurred prior to both overt thymic weight loss and characteristic, treatment-induced histopathological changes. Since recent evidences suggest that the growth factor interleukin-2 (IL-2) may play an important role in thymocyte proliferation, additional molecular biological studies were performed to evaluate whether DOTC may exert its anti-proliferative effects by compromising IL-2 production. Using a mRNA cytoplasmic dot blot technique, the examination of thymocyte lysates from treated and control animals revealed that DOTC markedly down regulated and at high doses abolished, the normal expression of the IL-2 gene. The high turnover gene alpha-actin was, however, unaffected by its action, thus demonstrating the selective effects of DOTC.

Immunologic effects of perinatal exposure of rats to dioctyltin dichloride

Studies were conducted to determine the period of immune system development that was most sensitive to perturbation by the known immunotoxicant di-n-octyltin dichloride (DOTC). Fischer 344 rats were exposed prenatally, both pre- and postnatally, or postnatally to DOTC by oral gavage of pregnant and/or lactating females. At various ages, ranging from 3 to 16 wk of age, offspring were examined for a number of immune functions. These included body and lymphoid organ weights; lymphoproliferative responses to B- and T-cell mitogens; natural killer cell activity; and primary antibody response to sheep erythrocytes. Prenatal (10-20 of gestation), pre- and postnatal (d 11-20 of gestation and 2-11 d of age), or postnatal (2-13 d of age) oral dosing of dams with 20-50 mg/kg DOTC resulted in no consistent alteration in immune function in offspring. However, direct oral dosing of rat pups to 5-15 mg/kg DOTC, beginning at 3 d of age and then 3 times per week up to 24 d of age for a total of 10 doses, resulted in significant suppression of the lymphoproliferative response of splenocytes to a T-cell mitogen in 10-wk-old rats (i.e., 7 wk after the last exposure to DOTC). Lymphoproliferative responses returned to control levels by 12 wk of age. In comparison young adult (8 wk old) rats dosed with 10 or 20 mg/kg DOTC under an identical dosing schedule (i.e., 3 times per week for a total of 10 doses) showed no suppression in the mitogen response of splenocytes 4 wk after the last exposure to DOTC. These results suggest that direct dosing of pups during early postnatal life may be the most effective means of inducing immunosuppression with DOTC during immune system development. The results also provide evidence for the greater sensitivity of the developing immune system compared with the fully developed immune system for a known immunotoxicant.

Differences in immunological susceptibility to cadmium toxicity between two rat strains as demonstrated with cell biological methods. Effect of cadmium on DNA synthesis of thymus lymphocytes

When 2 inbred rat strains, the Brown-Norway rat and the Lewis rat were exposed to the same amount of CdCl2 for 15 days, a completely different immunological reaction pattern could be demonstrated. Despite the same amount of intrathymic cadmium in both strains, the Brown-Norway rat showed a significant decrease in thymocytes in the S-phase and a significant increase of thymocytes in the G2 phase and mitosis, in contrast with findings in the Lewis rats. A new method for estimating subtle forms of thymus atrophy showed a slight decrease in the number of the smallest thymocytes in the Brown-Norway rat after exposure to cadmium, in contrast with that in the Lewis rat. Evidence is presented that the approximately 1.7 times larger number of thymocytes/mg thymus in the Lewis rat, compared to the Brown-Norway rat, as well as the approximately 2.5 times lower proliferation rate of the thymocytes, and an approximately 1.5 times higher metallothionein content of the thymus medulla epithelial cells in the Lewis rat, might be responsible for the observed difference in toxicity. The zinc content of the thymus was not significantly decreased by exposure to CdCl2, and did not differ significantly between both strains.

Suppression of thymocyte proliferation in vitro by a dioctyltin dichloride-induced serum factor

Oral administration of dioctyltin dichloride (DOTC) results in the production or release of factors which appear to prevent thymocyte maturation at an early stage. Sera obtained from rats gavaged with 75 mg DOTC/kg on alternate days, significantly inhibited PHA-induced blastogenesis of normal thymocytes. In vitro uptake of tritiated thymidine by thymocytes obtained from DOTC-gavaged rats was markedly reduced. These events occurred 24 to 72 h after commencement of DOTC treatment and preceded overt thymic weight loss. The effects of DOTC may therefore be a consequence of factor(s) capable of blocking the intrathymic differentiation of T cell precursors.