2,3,7,8-Tetrachlorodibenzo-p-dioxin impairs stable establishment of oral tolerance in mice.

The toxic environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunomodulatory chemical. TCDD activates the aryl hydrocarbon receptor (AhR) and suppresses peripheral humoral and cellular adaptive immune responses. Though the major route of uptake is via food, little is known until now on the immunotoxic effects of TCDD on the gut-associated lymphoid tissue. We show here that AhR is strongly expressed along the small intestine, especially in intestinal epithelial cells (IEC). The AhR marker gene cyp1a1 is induced in IEC by oral TCDD exposure. We asked how TCDD affects oral tolerance, a unique function of mucosal immunity. C57BL/6 mice were injected with 10 μg/kg body weight TCDD and fed with ovalbumin (OVA) in a high-dose tolerization protocol. Mice were immunized and boosted with OVA on days 12, 23, and 55 after tolerization. Five of 14, 6 of 15, and 13 of 14 TCDD-treated mice generated OVA-specific immunoglobulin (Ig)G1 antibodies after the first, second, and third immunization with OVA, respectively. Only one mouse harbored anti-OVA IgG1 antibodies in the control group even after the third immunization with OVA. OVA-specific IgA in fecal samples of tolerized and TCDD-exposed mice could be detected at the levels of nontolerized mice, whereas completely absent in tolerant control mice. Correlated to this, we found in TCDD-treated mice an increase in interleukin-6 producing CD103+ dendritic cells (DC) present in the gut-draining mesenteric lymph nodes (MLN) and a small increase in the frequency of Th17 cells. Neither the frequencies nor the absolute numbers of immune cells in the lamina propria (LP) or in intraepithelial lymphocytes were changed by TCDD treatment. Our data not only have implications for food allergies in settings of environmental exposure but also raise concerns regarding the harmlessness of overdosing potential AhR agonist in food, which needs to be studied further.