Transfer of various polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) via placenta and through milk in a marmoset monkey

Dioxins as potential risk factors for autism spectrum disorder

Autism spectrum disorder (ASD) has emerged as a major public health concern due to its fast-growing prevalence in recent decades. Environmental factors are thought to contribute substantially to the variance in ASD. Interest in environmental toxins as causes of ASD has arisen due to the high sensitivity of the developing human brain to toxic chemicals, particularly to dioxin and certain dioxin-like compounds (dioxins). As a group of typical persistent organic pollutants, dioxins have been found to exert adverse effects on human brain development. In this paper, we review the evidence for association of exposure to dioxins with neurodevelopmental abnormalities related to ASD based on both human epidemiological and animal studies. It has been documented that exposure to dioxins during critical developmental periods increased risk for ASD. This notion has been demonstrated in different populations exposed to high or background level of dioxins. Furthermore, the effects and mechanisms of action of dioxins relevant to the pathophysiology and pathogenesis of ASD are summarized, describing potential underlying mechanisms linking dioxin exposure with ASD onset. Further studies focusing on effects of prenatal/perinatal exposure to individual dioxin congeners or to mixtures of dioxins on ASD-associated behavioral and neurobiological consequences in animal models, and on the mechanisms of actions of dioxins, are needed in order to better understand how dioxin exposure might contribute to increased risk for ASD.

Exposure to Environmental Contaminants and Lung Function in Adolescents-Is There a Link?

Dioxins (polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDF)), polychlorinated biphenyls (PCBs), and brominated flame retardants (BDEs) are well known toxic environmental contaminants. Their possible role in the incidence of respiratory disease is not yet well understood. Previous studies showed a negative effect on lung function in relation to prenatal and lactational dioxin exposure in pre-pubertal children. Effects of BDE exposure on the lung function have not previously been evaluated. As part of a longitudinal cohort study, the effects of perinatal dioxin (PCDD/F) exposure and serum PCDD/F, dl-PCB, and BDE levels on lung function in adolescents were assessed using spirometry, a body box, and diffusion measurements. Thirty-three children (born between 1986 and 1991) consented to the current follow-up study. Prenatal, lactational, and current dioxin, PCB, and BDE concentrations were determined using GC-MS. No relationship was seen between prenatal and lactational dioxin exposure, nor with current PCB body burden, and lung function. Indications of increasing airway obstruction were seen in relation to increasing current BDE exposure. This is a novel finding and certainly warrants further research.

Comparison of the concentrations of polychlorinated dibenzo-p-dioxins, dibenzofurans, and dioxin-like polychlorinated biphenyls in maternal and fetal blood, amniotic and allantoic fluids in cattle

To characterize the maternal-fetal transport of lipophilic endocrine disrupting chemicals, concentrations of polychlorinated (2,3,7,8-substituted) dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and dioxin-like polychlorinated biphenyls (PCBs) were measured in maternal and fetal blood, and amniotic and allantoic fluids in cattle. Total toxicity equivalent quantity (TEQ) was highest in amniotic fluid on a fat-weight basis, whereas it was highest in maternal blood on a total weight basis. TEQ was lowest in allantoic fluid on either basis; 26 of 29 congeners analyzed in this experiment were detected in one or more samples. The largest number of congeners was detected in amniotic fluid. O8CDD, 2,3,4,7,8-P5CDF and 2,3',4,4',5-P5CB were the major congeners in PCDDs, PCDFs and PCBs, respectively. The O8CDD concentration was higher in fetal blood than in maternal blood on a fat-weight basis, whereas concentrations of other congeners were lower in fetal blood than in maternal blood. Furthermore, on a fat-weight basis, the O8CDD concentration was considerably higher in allantoic fluid compared with other samples. Concentrations of major PCB congeners were higher in amniotic fluid than in maternal and fetal blood on a fat-weight basis. In conclusion, it is suggested that lipophilic endocrine-disrupting chemicals contained in maternal blood are all transferred to the fetal circulation via the placenta in cattle. Furthermore, the results of this experiment imply that O8CDD has different transportation systems from other dioxins in the circulation, and that a considerable amount of PCBs is excreted and accumulated in amniotic fluid during the fetal stage in cattle.

Effect of lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin on the susceptibility to Listeria infection

We studied the effect of lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the susceptibility to Listeria infection of offspring in C57BL /6NCji mice. The offspring were nursed by TCDD-treated dams and exposed to TCDD from birth to weaning via milk. The exposure had little effect on the weights of immune organs and the spleen or the thymus cell population in the dams and offspring, but it enhanced the production of tumor necrosis factor alpha (TNF-alpha) and interferon gamma (IFN-gamma) in the serum after Listeria infection. The clearance of Listeria monocytogenes from the spleen was impaired in the off-spring. These results suggest that the exposure to TCDD of the offspring via milk disrupted the host resistance of the offspring, even though the main immune parameters were unchanged.

Reproductive toxicity and tissue concentrations of low doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin in male offspring rats exposed throughout pregnancy and lactation

The effects of low doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the reproductive system of male offspring rats were examined. The dams were treated subcutaneously 2 weeks prior to mating and throughout mating, pregnancy, and lactation. They received an initial loading dose of 25, 60, or 300 ng TCDD/kg body wt, followed by a weekly maintenance dose of 5, 12, or 60 ng TCCD/kg body wt (TCDD 25/5, TCDD 60/12, and TCDD 300/60). Three dams per group were killed on Gestation Day 21 and the fetuses were removed. The concentration of TCDD in the maternal liver and fat was measured. After birth, developmental landmarks in male rats were monitored. At weaning, the concentration of TCDD in the offspring liver and testis was determined. Effects on male reproduction were studied on Postnatal Days (PND) 70 and 170. At weaning, the concentration of TCDD in the offspring liver was 0.24, 0.39, and 1.78 ng/g in the TCDD 25/5, TCDD 60/12, and TCDD 300/60 groups, respectively. In the testes, the concentration of TCDD was 0.25 ng/g in the TCDD 25/5 and TCDD 60/12 groups and 0.28 ng/g in the TCDD 300/60 group. The number of sperm per cauda epididymis was reduced in TCDD groups at puberty and at adulthood. Daily sperm production was permanently decreased as was the sperm transit rate in the TCDD-exposed male rats, thus increasing the time required by the sperm to pass through the cauda epididymis. Moreover, the male rats of the TCDD groups showed an increased number of abnormal sperm when investigated at adulthood. Similarly, mounting and intromission latencies were significantly increased in the TCDD 25/5 and TCDD 300/60 groups. In the highest dose group, serum testosterone concentration was decreased at adulthood. Likewise, in this dose group permanent changes including pyknotic nuclei and the occurrence of cell debris in the lumen were revealed. The lowest adverse effect level and the no observed effect level can be estimated to be substantially lower than the estimated daily dose of the lowest dose which is 0.8 ng/kg body wt/day. Sperm parameters were more susceptible than the other end points investigated. However, the question as to whether such doses exposed throughout gestation and lactation induce subtle changes in humans remains to be determined.

Hexachlorobenzene accumulated by dams during pregnancy is transferred to suckling rats during early lactation

The distribution of ingested stable, lipophilic environmental pollutants in dams and their transfer to fetuses and sucklings were investigated in rats fed a diet containing a small amount (35.1 nmol/100 g diet) of hexachlorobenzene (HCB). In the first experiment, we examined the distribution of HCB in pregnant and nursing rats fed the HCB diet during pregnancy and lactation. Its transfer to their sucklings was also studied. On d 16 after parturition, HCB concentrations in the blood, and subcutaneous and perirenal fat of nursing rats fed the HCB diet during pregnancy and lactation were approximately 1/3.5, 1/15 and 1/2.8, respectively, those of pregnant rats fed the HCB diet only during pregnancy. On the other hand, the HCB concentrations in the blood, and subcutaneous and perirenal fat of sucklings were approximately 6, 29 and 15 times higher than those of their dams. Therefore, a large amount of HCB apparently was transferred from dams to suckling pups through the milk. In the second experiment, we fed dams the HCB diet only during pregnancy and determined the distribution of HCB in the pregnant rats and fetuses as well as in the nursing rats and suckling pups. The estimated amount of HCB transferred from a dam to her fetuses corresponded to about 0.39% of her total intake during pregnancy. The amount of HCB detected in nursing rats on d 16 after parturition was much smaller than that in the pregnant rats, suggesting that a large proportion of the HCB that accumulated during pregnancy disappeared from the organs and fat tissues during lactation. The HCB concentration in the stomach contents of suckling pups fed by the dams who had consumed HCB before parturition was highest on d 2 after birth and decreased gradually during the 16 d after birth. In the blood, liver and fat tissues of suckling rats, the HCB concentrations increased until 7 d after birth and then decreased gradually. We conclude that the HCB that accumulated in dams during pregnancy was transferred to their suckling pups through milk in the early days after birth.

Inducibility of cytochromes P-450 by dioxin in liver and extrahepatic tissues of the marmoset monkey (Callithrix jacchus)

Cytochrome P-450 induction was investigated in the marmoset monkey, a non-human primate, using dioxins as inducing agents. Animals received a single subcutaneous dose of 1.6 nmol tetrachlorodibenzo-p-dioxin or tetrabromodibenzo-p-dioxin/kg body weight. Microsomal fractions were prepared from liver, lung and kidney, and homogenates were prepared from gut and adrenal glands. Anti-peptide antibodies which bind to CYP1A1, CYP1A2, CYP2B6 and CYP3A4 in human were used to identify related forms in the marmoset. The results indicate that CYP1A2 is constitutively expressed in liver, but not in lung, kidney, gut or adrenal gland and that CYP1A1 is not expressed in any of these tissues in untreated animals. Treatment with dioxin induced both CYP1A1 and CYP1A2 in liver, but only CYP1A1 in lung. No induction of CYP1A1 or CYP1A2 was found in kidney, small intestine or adrenal glands. Methoxy-, ethoxy-, pentoxy- and benzoyloxyresorufin O-dealkylases and high affinity phenacetin O-deethylase activities were induced in the liver, whereas ethoxycoumarin O-deethylase and aryl hydrocarbon hydroxylase activities were not affected by dioxin treatment. High-affinity phenacetin O-deethylase and CYP1A2 apoprotein were detected only in liver, consistent with this activity being specifically catalysed by CYP1A2. Furafylline was found to be a competitive inhibitor of methoxyresorufin O-demethylase activity with a Ki of 10 microM. In the lung the induction of CYP1A1 was accompanied by 15- and 23-fold increases in ethoxyresorufin O-deethylase and methoxyresorufin O-demethylase activities, respectively, suggesting that both activities are catalysed by CYP1A1. In contrast, there was no induction of aryl hydrocarbon hydroxylase activity in lung or liver showing that, unlike in many other species, marmoset CYP1A1 does not catalyse this reaction efficiently. The expression, distribution, induction and substrate specificities of marmoset monkey P-450 enzymes differ from the situation found in rodents and other species, demonstrating that caution has to be exercised when making cross-species extrapolations.

Intake, fecal excretion, and body burden of polychlorinated dibenzo-p-dioxins and dibenzofurans in breast-fed and formula-fed infants

To assess toxicokinetics of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), oral intake and fecal excretion were measured in two breast-fed infants and one formula-fed infant during the 1st y of life. The intake of these compounds was up to 50 times higher in the breast-fed infants. In these children, fecal excretion of the main tetra- to hexachlorinated congeners was less than 9% of the intake at age of 1 and 5 mo, indicating almost complete intestinal absorption during breast-feeding. In contrast, distinctly higher fecal excretion rates were observed for the hepta- and octachlorinated compounds. Despite much lower PCDD/PCDF intake after weaning, concentrations in stool fat did not decrease substantially. We conclude that concentrations in fecal fat more or less reflect those in body fat. Additionally, PCDD/PCDF concentrations were measured in blood fat of all infants (and in a second formula-fed baby) at the age of 11 mo. International toxicity equivalent (I-TEq) concentrations in the formula-fed infants were less than 25% of maternal values and about 10 times lower than in the infants breast-fed for 6-7 mo. In the latter, a distinct accumulation was found for the tetra- to hexachlorinated congeners compared with maternal concentrations. We conclude that accumulation of PCDDs and PCDFs in infants is as high as expected on the basis of intake data and assuming complete absorption and negligible elimination during the 1st y of life.

Breast-feeding exposure of infants to environmental contaminants--a public health risk assessment viewpoint: chlorinated dibenzodioxins and chlorinated dibenzofurans

Exposure of children to chlorinated dibenzodioxins and chlorinated dibenzofurans via breast-feeding has been well-documented in industrialized countries. Recent studies indicate a possible link between development of subtle health effects in children and their exposure to dioxin-like chemicals from maternal milk. Some examples of the effects are lower vitamin K levels, increased thyroxine levels, and mild changes in liver enzymes. The projected daily intakes of chlorinated dibenzodioxins and chlorinated dibenzofurans are compared with minimal risk levels for intermediate duration oral exposure (15-365 days) derived for these chemicals. Public health recommendations for future actions related to infant intake of chlorinated dibenzodioxin- and chlorinated dibenzofuran-contaminated breast milk are also addressed.

Peri- and postnatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin: effects on physiological development, reflexes, locomotor activity and learning behaviour in Wistar rats

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the development of rat offspring were studied after administration of a loading dose of 300 or 1000 ng TCDD/kg body wt on day 19 of pregnancy, followed by weekly maintenance doses of 120 or 400 ng TCDD/kg body wt. The dose regimens led to a fluctuation of average TCDD concentrations in the liver of the offspring of 4.9-14.9 ng/g (TCDD1000/400 group) or 1.4-6.3 ng/g (TCDD300/120 group) during the course of the experiment. In both TCDD-exposed groups the body weight of the offspring was significantly lower on postnatal day 7 (PND 7); in the high dose group from PND 7 to PND 31. Some landmarks of postnatal development were retarded in the exposed groups; in particular, the vaginal opening was delayed for several days in both TCDD-exposed groups. The TCDD-exposed animals revealed a reduced ability to remain on a rotating rod. During reflex testing, the rate of successfully responding animals was higher in the exposed groups. No statistically significant differences in the locomotor activity between controls and TCDD-exposed offspring were detectable under our experimental conditions. In a discrimination learning test no effects on the learning ability were found. However, TCDD-exposed offspring showed an increase in unanswered trials during critical phases of the task. They also exhibited increased locomotor activity in a novel environment; prior to an amphetamine challenge dose of 1 mg/kg body weight. Amphetamine-induced activity was decreased in a dose-dependent manner.

Intake and fecal excretion of PCDDs, PCDFs, HCB and PCBs (138, 153, 180) in a breast-fed and a formula-fed infant

Intake and fecal excretion of PCDDs, PCDFs, HCB and PCBs (IUPAC Nos. 138, 153, 180) were measured in a breast-fed and a formula-fed infant at the age of 1 and 5 months. As expected, the intake of these compounds was clearly higher in the breast-fed infant. In this baby an almost complete absorption was observed for lower chlorinated PCDDs and PCDFs and also for HCB and PCBs, whereas for hepta- and octachlorinated PCDDs and PCDFs fecal excretion was considerably higher (from 20% up to nearly 100% of the intake). Due to low concentrations in diet and feces of the formula-fed infant an evaluation was possible only for a few compounds at the age of 5 months. These values were in the same range when compared with those of the breast-fed infant. For collection of feces new cotton diapers were used which were pre-extracted in order to reduce the levels of polychlorinated compounds. Unexpectedly, after washing the tissue a much higher contamination was observed which made a calculation of fecal excretion rates in the formula-fed infant at the age of 1 month impossible.

The toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and their relevance for toxicity

This article reviews the present state of the art regarding the toxicokinetics and metabolism of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs). The absorption, body distribution, and metabolism can vary greatly between species and also may depend on the congener and dose. In biota, the 2,3,7,8-substituted PCDDs and PCDFs are almost exclusively retained in all tissue types, preferably liver and fat. This selective tissue retention and bioaccumulation are caused by a reduced rate of biotransformation and subsequent elimination of congeners with chlorine substitution at the 2,3,7, and 8 positions. 2,3,7,8-Substituted PCDDs and PCDFs also have the greatest toxic and biological activity and affinity for the cytosolic arylhydrocarbon (Ah)-receptor protein. The parent compound is the causal agent for Ah-receptor-mediated toxic and biological effects, with metabolism and subsequent elimination of 2,3,7,8- substituted congeners representing a detoxification process. Congener-specific affinity of PCDDs and PCDFs for the Ah-receptor, the genetic events following receptor binding, and toxicokinetics are factors that contribute to the relative in vivo potency of an individual PCDD or PCDF in a given species. Limited human data indicate that marked species differences exist in the toxicokinetics of these compounds. Thus, human risk assessment for PCDDs and PCDFs needs to consider species-, congener-, and dose-specific toxicokinetic data. In addition, exposure to complex mixtures, including PCBs, has the potential to alter the toxicokinetics of individual compounds. These alterations in toxicokinetics may be involved in some of the nonadditive toxic or biological effects that are observed after exposure to mixtures of PCDDs or PCDFs with PCBs.

Reproductive toxicity and toxicokinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin. 3. Effects of single doses on the testis of male rats

The effects of a single dose of TCDD on the testis were studied in rats. The animals were treated (subcutaneously) once with TCDD doses of 0, 0.5, 1.0, 3.0, 5.0 micrograms/kg body weight. Doses of 3.0 or 5.0 micrograms TCDD/kg reduced the number of spermatids/testis significantly (60% of the controls). Electron microscopic inspection revealed that both doses led to a dissolution on the germinal epithelium. Altered germ cells at all developmental stages occurred in all testes evaluated. Doses of 0.5 or 1.0 micrograms TCDD/kg did not induce any effects in the testis; therefore, under these experimental conditions of single exposure to rats the dose of 1.0 micrograms TCDD/kg can be considered as NOAEL.

Reproductive toxicity and toxicokinetics of 2,3,7,8-tetrachlorodibenzo-p-dioxin. 2. Problem of paternally-mediated abnormalities in the progeny of rat

Possible effects on the next generation after long-term exposure (subcutaneous administration) of male rats to very high doses of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied. Two dose regimes were applied: TCDD-25 (initial dose: 25 micrograms/kg body wt; maintenance dose: 5 micrograms/kg body wt, once weekly) and TCDD-75 (initial dose: 75 micrograms/kg body wt; maintenance dose: 15 micrograms/kg body wt). Male rats were treated for 10 weeks before mating and then throughout the entire 12 week mating period. They were mated to unexposed virgin females. One group of pregnant females was used for teratological evaluations, and another group was allowed to deliver. No significant differences were observed in the number of implantations or fetuses per litter, and resorption rate, and fetal weight between the controls and TCDD-treated groups. No gross-structural anomalies occurred in any of the fetuses sired by TCDD-treated males. In the TCDD-25 group an increased frequency of two types of variations was observed which also occur in controls: incompletely ossified fingers (TCDD-25 = 5.1%, controls = 2.6%), and incompletely ossified ossa zygomatica (TCDD-25 = 1.8%, controls = 0.5%). In the TCDD-25 group a slight but statistically significant increase was observed in the rate of stillbirths (TCDD-25 = 1.3%, controls = 0.1%), apparently due to an unusually low frequency occurring in the controls (overall historical controls = 0.6%). There was no difference in postnatal mortality (TCDD-25 = 1.3%, controls = 1.3%). Taken together, despite the very high doses of TCDD used, the data do not provide evidence for biologically significant paternally-mediated developmental toxicity in the fetuses and newborn.

Persistence of various polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) in hepatic and adipose tissue of marmoset monkeys

A defined mixture of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs and PCDFs) was subcutaneously administered to marmoset monkeys (Callithrix jacchus). Tissue concentrations in hepatic and adipose tissue were measured at different times after treatment (1-28 weeks). One week after application high concentrations could be detected for the 2,3,7,8-substituted congeners only. The percent of the administered dose in whole liver differed for the various 2,3,7,8-substituted congeners, ranging from 24.5 +/- 4.5% for 2,3,7,8-TCDD to 74.1 +/- 4.9% for 2,3,4,6,7,8-H6CDF. Therefore, the concentration ratio (liver/adipose tissue) was also very different, ranging from about 1 (2,3,7,8-T4CDD or 2,3,7,8-T4CDF) to greater than 10 in the case of some higher chlorinated PCDDs and PCDFs. Half-lives of PCDDs and PCDFs were very different for the various 2,3,7,8-substituted congeners. For the most toxic compound (2,3,7,8-T4CDD) a t/2 of about 8 weeks in hepatic tissue and about 11 weeks in adipose tissue was found when calculated from data obtained later than 6 weeks after injection. For 2,3,7,8-T4CDD and 1,2,3,7,8-P5CDD the decreases in hepatic concentrations were much faster during the first 6 weeks after administration (t/2 of 4 weeks). This was apparently due to redistribution phenomena. Half-life increased with increasing degrees of chlorination. In some cases (e.g. OCDD, OCDF) no significant decrease in tissue concentrations could be observed after 28 weeks. The shortest t/2 was determined for 2,3,7,8-T4CDF: shorter than 6 days in hepatic tissue and about 10 days in adipose tissue. Calculation of the body burden of the non-2,3,7,8-substituted PCDDs/PCDFs 1 week after injection revealed that all groups of isomers were present at less than 5%. Consequences of these findings for the use of TCDD-toxic-equivalency factors are discussed and a change in strategy is suggested.

Polyhalogenated dibenzo-p-dioxins and dibenzofurans and the immune system. 1. Effects on peripheral lymphocyte subpopulations of a non-human primate (Callithrix jacchus) after treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Monoclonal antibodies were used to analyse the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on peripheral lymphocytes from marmosets (Callithrix jacchus) following injections of single low doses of TCDD. A reduction of the percentage and the total number of lymphocytes among the white blood cells was seen 3 weeks after a single administration of 300 ng TCDD/kg body wt, but not following 167 ng TCDD/kg or less. After treatment of marmosets with the single subcutaneous dose of 10 ng TCDD/kg body wt. we observed an average decrease of about 20% in the percentage of CD4+ cells in the venous blood of treated animals. This change was not seen at the time of maximum absorption (2 weeks after the injection), but was demonstrable after 4 weeks and reached its maximum 15 weeks after the injection. The time course of the changes suggests an indirect effect (possibly via the thymus). Due to the considerable inter- and intra-individual variability in the number of peripheral lymphocytes the effect was less convincing on the basis of the total CD4+ cells per microliter blood. There was a significant concomitant increase in the percentage of cells with the CD8+ marker. A closer analysis of the lymphocyte subpopulation involved revealed a predominant effect on the cells with the CD4CDw29 ("leu 3a+4B4+") surface marker ("helper-inducer cells"). Subsequent to a dose of 10 ng TCDD/kg body wt the percentage of these cells was reduced by 50% or more when compared with the (24) controls. On an absolute basis (number of cell/microliter blood) these CD4+CDw29+ cells were clearly reduced in only two out of four marmosets. There was no significant effect on the percentage of the CD4+CD45R+ ("leu3a+2H4+") subpopulation ("suppressor-inducer cells"). The ratios: CD4+CDw29+/CD4+CD45R+, or CD4+CDw29+/CD8+ appear to be convenient measures for monitoring the effect described. Since the difference between the percentage of CD2+ cells minus the sum of CD4+ plus CD8+ cells was found to be increased following rather high doses (167 ng TCDD/kg body wt or more), this suggests that immature cells (CD2+ CD4-CD8-) are released into the periphery as a result of the exposure to TCDD. Furthermore, a decrease in the percentage of CD20+ (B1+) cells (about 50% when compared with controls) was observed in the treated animals following a single dose of 10 ng TCDD/kg body wt or higher. Subsequent to a dose of 10 ng TCDD/kg body wt the percentage of CD56+ (NKH1+) cells seemed to be slightly increased. The dose-response for the effects observed was rather poor.(ABSTRACT TRUNCATED AT 400 WORDS)