Acute Inflammatory Response to Sheep Red Blood Cell Challenge in Mice Treated with 2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD): Phenotypic and Functional Analysis of Peritoneal Exudate Cells

Abstract

TCDD is a widespread environmental contaminant of concern to human health because of its well-recognized immunotoxicity in laboratory animals. Suppression of the murine antibody response to xenogeneic erythrocytes has been shown to be one of the most sensitive assays for TCDD immunotoxicity. However, the cellular mechanisms underlying the suppressed immune function have not been fully elucidated. In the present studies, peritoneal macrophage recruitment, activation, and antigen-presenting function in response to sheep red blood cell (SRBC) injection were compared in C57B1/6 mice treated with a single oral dose of 0 or 5 μg TCDD/kg. In vehicle-treated mice, SRBC injection induced a typical inflammatory response in the peritoneal cavity. Within 6 hr, the number of neutrophils increased and remained elevated until 40 hr. Macrophage numbers increased at 24 hr and remained elevated through 72 hr. In TCDD-treated mice, a hyperinflammatory response to SRBC was observed. The total number of peritoneal exudate cells was significantly greater at 16, 24, and 40 hr after SRBC challenge when compared to that of vehicle-treated mice. The increased number of peritoneal cells reflected significant increases in both neutrophils and macrophages. Mac-1 + peritoneal cells were examined by two-color flow cytometric analysis on Days 0-3 after SRBC injection for expression of the activation markers F4/80 and I-A. The intensity of F4/80 fluorescence significantly decreased 24-72 hr following SRBC challenge, while fluorescence associated with I-A significantly increased at 72 hr. These changes are consistent with macrophage activation. TCDD did not significantly alter F4180 expression on Mac-1 + cells, whereas I-A expression was increased earlier on cells from TCDD-treated mice. However, TCDD treatment did not alter the antigen presentation function of peritoneal cells, assessed by their ability to induce the proliferation of SRBC-primed T cells in vitro. The antigen-presenting function of adherent spleen cells was also not altered by TCDD exposure. To test the hypothesis that an excess number of phagocytes in TCDD-treated mice were clearing the antigen more efficiently, leading to a smaller (e.g., suppressed) antibody response, we attempted to overcome TCDD suppression by increasing the amount of SRBC antigen used for challenge. However, the magnitude of the anti-SRBC response in TCDD-treated mice was not significantly altered by increasing the antigen challenge dose, suggesting that enhanced clearance of antigen by macrophage is not a mechanism for TCDD-induced suppression of the anti-SRBC response. The mechanism(s) for the enhanced inflammatory response and its relationship, if any, to TCDD immunotoxicity remain to be elucidated.