bis(tri-n-butyltin)oxide and Trichinellosis

Bis(tri-n-butyltin)oxide (TBTO) has been shown to be immunotoxic in rodents, resulting in decreased resistance to infections. The no-effect level assessed by estimating effects on host resistance in rats has been found to lie between 0.5 and 5.0 mg TBTO/kg food (0.025 and 0.25 mg/kg body weight). For risk assessment such animal data need to be extrapolated to the human situation. In risk assessment procedures uncertainty factors are used to account for interspecies variation (extrapolation from animal to man) and for variation within the human species. For both factors a value of 10 is often used, based on international guidelines. Hence, exposures below 0.00025 mg/kg body weight should not pose a risk for the human population. In the present study we have taken an alternative approach. We have produced dose-response curves for the effect of TBTO exposure on resistance to Trichinella spiralis. To extrapolate this curve to the human situation, we produced additional dose response data concerning in vitro effects of TBTO exposure on the mitogen responsiveness of both rat lymphoid cells and human blood cells. Using regression analyses of these dose-response data, we calculated a factor that accounts for interspecies variation (IEV) and a factor that accounts for intraspecies variation (IAV) within the human samples. Using these factors, we estimated the dose that decreases resistance in man to an infection. We choose 10% increase of the infectious load as a reference point which in our view is of biological significance. Based on these considerations, we estimated the dose that may affect resistance in adult humans at 0.04 mg/kg body weight. Pre- and postnatal exposure will probably result in effects at lower concentrations, due to the vulnerability of the developing immune system.

Topics: Adult; Animals; Female; Humans; Immune System Diseases; Immunosuppressive Agents; Lymphocyte Activation; Male; Rats; Risk Assessment; Trialkyltin Compounds; Trichinella spiralis; Trichinellosis

To investigate whether immune function suppression observed in an earlier study after short-term bis(tri-n-butyltin)oxide (TBTO) exposure also occurred after long-term treatment, function studies for specific and nonspecific resistance were performed after exposure of weaned male rats to diets containing 0, 0.5, 5, or 50 mg TBTO/kg for 4-6 and 15-17 months. Treatment for 4.5 months had no effect on body weight but reduced thymus weight at 50 mg/kg. Regarding the thymus-dependent immunity, delayed-type hypersensitivity reactions to ovalbumin and tuberculin were not depressed, in contrast to the results of the short-term study. The resistance to the nematode Trichinella spiralis was dose-relatedly suppressed at the 5 and 50 mg/kg levels, in both experiments (5.5 and 16.5 months exposure), as shown by increased counts of muscle larvae and depressed serum IgE titers. Also the inflammatory reaction around cysts in parasitized musculature was reduced. No significant reduction was found in IgM and IgG titers to T. spiralis, ovalbumin, and sheep red blood cells as determined by enzyme-linked immunosorbent assay. TBTO exposure at 50 mg/kg for 4.5 months significantly reduced thymus weight, but the response of thymocytes to T-cell mitogens was unaltered. TBTO treatment for 4.5 or 16 months did not influence the response of spleen cells to T-and B-cell mitogens and neither influenced spleen weight. A dose-related shift was observed in T- and B- cell numbers in mesenteric lymph nodes as shown by flow cytometry using monoclonal antibodies: treatment for 6 and 18 months reduced the relative count of T-lymphocytes and consequently increased the percentage of B-lymphocytes. As a result, the T:B ratio was reduced in the 5 and 50 mg/kg groups. Concerning the nonspecific resistance, TBTO exposure for 5 and 17 months reduced macrophage function at 50 mg/kg as shown by impaired splenic clearance of Listeria monocytogenes bacteria. Natural cell-mediated cytotoxicity of spleen and peritoneal cells was investigated in a 51Cr-release assay with YAC-lymphoma target cells. TBTO treatment significantly suppressed natural killer (NK) activity in spleen cells. Significant suppression was noted in all treatment groups following 16 months TBTO exposure; in contrast to treatment for 4.5 months. No significant alterations were observed in the spontaneous cytotoxicity of nonadherent and adherent peritoneal cells following 4.5 months treatment. Treatment of aged (i.e., 1-year-old) male rats

Topics: Aging; Animals; Antibody Formation; Cytotoxicity, Immunologic; Hypersensitivity, Delayed; Immunity; Immunity, Innate; Immunosuppressive Agents; Listeria monocytogenes; Lymphocytes; Male; Random Allocation; Rats; Rats, Inbred Strains; Spleen; Thymus Gland; Time Factors; Trialkyltin Compounds; Trichinellosis

To evaluate the functional significance of bis(tri-n-butyltin)oxide (TBTO)-induced thymus atrophy, lymphocyte depletion in spleen and lymph nodes, lymphopenia, and increased serum IgM and decreased IgG concentrations, in vivo and in vitro function studies were performed for specific and nonspecific resistance. Weaned male rats were fed diets containing 0, 20, or 80 mg TBTO/kg for at least 6 weeks. Regarding the thymus-dependent immunity, delayed-type hypersensitivity reactions to ovalbumin as well as tuberculin were significantly depressed at both dietary concentrations. Resistance to the nematode Trichinella spiralis was significantly suppressed as shown by a retarded expulsion of adult worms from the small intestine, increased counts of muscle larvae, reduced inflammatory reaction in parasitized musculature, and suppressed serum IgE titers. Also the secondary mercaptoethanol-resistant (presumably IgG) hemagglutinating antibody titer to sheep red blood cells was significantly reduced, while no significant alterations were found in IgM and IgG titers to T. spiralis, ovalbumin, and tetanus toxoid. TBTO exposure reduced the response of thymocytes in both treatment groups and of spleen cells in the 80-mg/kg group upon stimulation with T-cell mitogens and increased the response of spleen cells to B-cell mitogens. When calculated per whole spleen, the response to T-cell mitogens was strongly impaired but unaltered by B-cell mitogens. This difference can be explained by a relative increase of splenic B cells as a result of reduced numbers of T cells, as shown by cell surface marker analysis using monoclonal antibodies. Reduced splenic T-cell numbers appeared equally due to a decreased number of T helper and to T suppressor cells. From these data and from results of a time-sequence study in which effects of TBTO on cell count and cell viability of thymus, spleen, and bone marrow were investigated, it is concluded that TBTO-induced immunodeficiency was primarily due to its direct toxic action on thymocytes. When cultured in vitro in the presence of TBTO, viability of thymus and bone marrow cells was equally reduced, while after in vivo treatment viability of bone marrow cells was unaffected. Thus, the in vitro situation does not mimic the in vivo one. Concerning the nonspecific resistance, TBTO reduced macrophage function as shown by impaired splenic clearance of Listeria monocytogenes bacteria. From in vitro studies it is concluded that impaired in vivo splenic

Topics: Animals; Antibody Formation; Bone Marrow; Cell Count; Cell Survival; Cytotoxicity, Immunologic; Hypersensitivity, Delayed; Immunity, Innate; Immunoglobulin G; Immunoglobulin M; Immunosuppressive Agents; In Vitro Techniques; Lymphocyte Activation; Lymphocytes; Mitogens; Phagocytosis; Rats; Rats, Inbred Strains; Thymus Gland; Trialkyltin Compounds; Trichinellosis