Immunotoxicologic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in rabbits.

Male white New Zealand rabbits were exposed orally to 0, 0.01, 0.1, 1 and 10 micrograms/kg/wk of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for a period of 8 weeks. After 4 and 6 weeks of first TCDD administration, the rabbits were inoculated with a mixture of tetanus toxoid and Freund's adjuvant. TCDD exposure reduced the serum antitoxin titers, skin sensitivity to tuberculin, and the number of antibody producing cells in popliteal lymph nodes. At the end of the treatment period serum IgG levels were increased at the lowest dose of TCDD treatment while a marked depression was noticed at the highest dose level. An increase in the thymidine uptake by splenic lymphocytes in culture was noted at all levels of TCDD treatment whereas the response of these cells to phytomitogens was decreased at high levels of TCDD exposure. All different immunologic effects were not altered at the lowest TCDD treatment but both humoral and cell-mediated immune responses were depressed at the highest level of TCDD exposure.

Styrene epoxide intermediate as a possible stimulant of lymphocytic function.

Mice treated with styrene showed an increased lymphocyte transformation in splenic cell cultures. The effects were observed both on spontaneous transformation and phytomitogen induced blast formation. Addition of styrene oxide directly to splenic cultures, but not of styrene, increased blast formation at low concentrations but was cytotoxic at high concentrations. In this test system styrene exhibited a generalized immunostimulation. In vitro data suggest that this effect of styrene is mediated possibly through its conversion to styrene oxide, its transient metabolic intermediate.

Immunotoxicologic studies with vinyl chloride in rabbits and mice.

Our previous studies in mice indicated that the exposure to vinyl chloride (VC) produced a state of immunostimulation. The metabolism of VC was an important factor in this phenomenon. The present paper describes the effects of VC exposure on induced immunologic responses in rabbits. No consistent effect of VC exposure was noticed on skin reactivity to tuberculin or serum anti-tetanus titers in sensitized rabbits. Vinyl chloride produced no change in the number of antibody secreting cells in the lymph nodes of immunized rabbits. An increase in the spontaneous splenic lymphocyte transformation in immunized rabbits was observed when the animals were exposed to VC. Two known metabolites of VC, namely thiodiglycolic acid and N-acetyl-S-(hydroxyethyl)-cysteine produced little or no effect when added to mouse splenic lymphocyte cultures in vitro but in vivo administration of thiodiglycolic acid produced apparent immune stimulation in mice. The study indicated that although VC may cause an apparent enhancement of immune reactivity, it does not alter the immunologic response to simultaneously administered antigens.

Non-linear pharmacokinetic parameters need to be considered in high dose/low dose extrapolation.

It is impossible to prove that any chemical, natural or man-made, cannot cause cancer in man. However, it is possible to estimate the relative degrees of risk associated with various agents. The precision of these estimations increases as experimental procedures elucidate the basic type of mechanism associated with carcinogenesis, the role of absorption, metabolism and distribution, and excretion in increasing or decreasing activity, and the dose dependency of metabolic pathways. We must constantly strive to make the most accurate risk estimations possible so that the complex issues of risk/benefit may be properly considered.

Teratogenic effects of the plant hormone indole-3-acetic acid in mice and rats.

These studies evaluated the teratogenic potential of indole-3-acetic acid (IAA), a naturally occurring plant hormone, in CF-1 mice and Sprague-Dawley rats. Mice were given 5, 50, 200, or 500 mg IAA/kg/day by gavage on days 7 through 15 of gestation. Rats were given 50, 200, or 500 mg IAA/kg/day by gavage on days 7 through 15 of gestation. IAA was teratogenic in mice and rats at 500 mg/kg/day; cleft palate was induced in both species at this dose level. In mice, other malformations including exencephaly, ablepharia, dilated cerebral ventricles, and crooked tail were also observed. Mice given 500 mg/kg of IAA gained less than control mice during gestation; no evidence of maternal toxicity was observed in rats. IAA did not cause fetal resorptions in either species and was not teratogenic at dose levels below 500 mg/kg.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on splenic lymphocyte transformation in mice after single and repeated exposures.

Male CD-1 mice were orally treated with 0.01, 0.1, 1 and 10 micrograms TCDD/kg body wt./week for up to 8 weeks. Randomly selected animals were sacrificed at 2, 4, and 8 weeks of exposure. An additional group was given orally 10 micrograms TCDD/kg and the animals similarly sacrificed. Splenic lymphocytes from these animals were cultured in vitro with or without the presence of phytomitogens, phytohemagglutinin, and pokeweed mitogen. The incorporation of 3H-thymidine was measured as an indication of relative blast formation. Exposure of animals to TCDD, even at the lowest level (0.01 microgram/kg/wk for 2 weeks) caused a marked increase in the thymidine uptake by cultured lymphocytes. The blastogenic response of mitogens was reduced at high levels of TCDD exposure, indicating an immunosuppressive effect. Following a single treatment with 10 micrograms TCDD/kg, the increase in the blast formation was noted at 2 weeks, the effect reduced at 4 weeks, and no difference noticed in treated vs. control cultures 8 weeks after the treatment. Small doses of TCDD stimulate the splenic lymphocyte transformation and this effect, although somewhat dose-related, can be reversed in a relatively short period of time.

Immunologic effects of vinyl chloride in mice.

Male CD-1 mice were exposed to 10, 100 or 1000 ppm vinyl chloride (VC) for 2--8 weeks at 6 hr/day, 5 days/week. A slight increase in the spleen weight of mice was noted at the highest exposure level. Spleens were obtained from these animals (4 mice/group after 2, 4, and 8 weeks of exposure) and their lymphocytes cultured in vitro with or without the presence of phytomitogens, phytohemagglutinin (PHA) and pokeweed mitogen (PWM). Relative blast formation and the DNA synthesis was measured by the incorporation of 3H-thymidine in the cultured cells. The response of splenic lymphocytes to the phytomitogens was increased several-fold by VC exposure. The effects were apparent at 1000 ppm VC after 2 weeks of exposure and at all levels of VC exposure after 4--8 weeks. The effects were generally more pronounced at 100 ppm VC exposure than those at 1000 ppm. In vitro culture of splenic lymphocytes from control or VC-exposed mice in the VC atmosphere did not show an enhancement of blast formation. Alteration of VC metabolism during the VC exposure in vivo yielded results that indicated that metabolites of VC may be responsible for the stimulation of lymphocyte transformation observed in splenic cultures.

The dose-dependent fate of 1,4-dioxane in rats.

A pharmacokinetic study was conducted to determine the fate of dioxane in rats at doses equivalent to those given in toxicological studies conducted previously. The results show that the fate of dioxane in rats is markedly dose-dependent due to a limited capacity to metabolize dioxane to beta-hydroxyethoxyacetic acid (HEAA). The pharmacokinetic data collected in support of these conclusions include plasma concentration-time curves for dioxane given to rats intravenously at dose levels from 3 to 1000 mg/kg and an inhalation study of 50 ppm dioxane vapors for 6 hr. The plasma curves at low doses by each route were linear, with half life values of about 1 hr. As the dose was increased above 10 mg/kg the plasma clearance rate decreased, the fraction of the dose excreted as HEAA decreased, and the fraction of the dose excreted as dioxane per se in the urine and expired in the breath increased. These data could be described by a one-compartment open system model with parallel first order (urinary and pulmonary excretion) and Michaelis-Menten (metabolism) type elimination kinetics. At saturation, the maximum velocity of the metabolism of dioxane ato HEAA was about 18 mg/kg/hr. Multiple daily oral doses of 1000 mg/kg, but not 10 mg/kg, were excreted more rapidly than equivalent single doses, indicating that at high daily doses dioxane induced its own metabolism. The correlation of the dose-dependent fate of dioxane with the results of toxicological studies in rats supports the conclusion that there is an apparent threshold for the toxic effects of dioxane which coincides with saturation of the metabolic pathway for its detoxification.