Suppression of humoral immune responses by 2,3,7,8-tetrachlorodibenzo-p-dioxin intercalated in smectite clay

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic environmental contaminant found in soils and sediments. Because of its exceptionally low water solubility, this compound exists predominantly in the sorbed state in natural environments. Clay minerals, especially expandable smectite clays, are one of the major component geosorbents in soils and sediments that can function as an effective adsorbent for environmental dioxins, including TCDD. In this study, TCDD was intercalated in the smectite clay saponite by an incipient wetness method. The primary goal of this study was to intercalate TCDD in natural K-saponite clay and evaluate its immunotoxic effects in vivo. The relative bioavailability of TCDD was evaluated by comparing the metabolic activity of TCDD administered in the adsorbed state as an intercalate in saponite and freely dissolved in corn oil. This comparison revealed nearly identical TCDD-induced suppression of humoral immunity, a well-established and sensitive sequela, in a mammalian (mouse) model. This result suggests that TCDD adsorbed by clays is likely to be available for biouptake and biodistribution in mammals, consistent with previous observations of TCDD in livestock exposed to dioxin-contaminated ball clays that were used as feed additives. Adsorption of TCDD by clay minerals does not appear to mitigate risk associated with TCDD exposure substantially.

Fig. 1

X-ray powder diffraction pattern of natural K-saponite clay treated with dimethylsulfoxide and dioxin heated at 60°C (A) and after isothermal heat treatment at 200°C (B).

Fig. 2

Thermogravimetric analysis (TGA) of tetrachlorodibenzo-p-dioxin–dimethylsulfoxide (DMSO)–K-saponite mixture heated at 60°C or heated at 1,000°C (A) and undergoing isothermal heat treatment at 200°C (B). The solid curve indicates DMSO mass loss; the dashed line curve is the TGA curves of the first derivative.

Fig. 3

Comparison of Fourier transform infrared (FTIR) and Raman spectra of tetrachlorodibenzo-p-dioxin (TCDD). A: Theoretical Raman spectra. B: Measured Raman crystal TCDD spectra. C: Theoretical FTIR TCDD spectra. D: Measured micro-FTIR crystal TCDD spectra. E: Measured FTIR spectra of TCDD adsorbed to K-saponite clay.

Fig. 4

X-ray powder diffraction pattern of synthetic saponite. a.u.= Arbitrary units.

Fig. 5

Transmission electron microscopy image of synthetic saponite formed at 90°C.

Fig. 6

Tetrachlorodibenzo-p-dioxin (TCDD)-induced cyp1a1 gene expression in liver. Mice were treated with TCDD in corn oil (CO) or TCDD-adsorbed synthetic or natural K-saponite (0–10 µg/kg/d) for 4 d by oral gavage. 0 in each group indicates CO, natural or synthetic K-saponite without TCDD. On day 3, mice were sensitized with sRBC. Expression of cyp1a1 gene was evaluated 4 d after sRBC by real-time PCR. ^ap < 0.05 as compared with vehicle (0); ^bp < 0.05 as compared with TCDD-adsorbed synthetic saponite; ^cp < 0.05 as compared with TCDD-adsorbed natural K-saponite.

Fig. 7

Tetrachlorodibenzo-p-dioxin (TCDD)-induced suppression of humoral immunity. Mice were treated as described in Fig. 6. The number of IgM-producing cells (AFC) was quantified 4 d after sRBC by plaque assay. ^a–cp < 0.05 as compared with vehicles (0) for TCDD in CO, TCDD-adsorbed natural K-saponite and TCDD, and TCDD-adsorbed synthetic saponite, respectively.