Fewer T lymphocytes and decreased pulmonary influenza virus burden in mice exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The immune system is a sensitive target for the toxicity of the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin ( TCDD). In mice, immunotoxicity associated with exposure to TCDD includes suppression of humoral and cell-mediated immunity and impaired host resistance to infectious agents. However, the underlying mechanisms for these effects of TCDD are unknown. We previously reported that treatment of C57Bl/6 mice with TCDD and influenza A virus increases mortality, impairs cytokine production, and suppresses T cell expansion and the generation of cytotoxic T lymphocytes (CTL) in the draining lymph node. However, the overall virus-specific cytolytic activity in the lung is unaffected. This enigmatic finding left several questions unanswered, including whether decreased CD8(+) lymphocytes in the lung are the consequence of either delayed or suppressed recruitment of cells to the lung; whether exposure to TCDD affects the recruitment of CD4(+) cells to the lung; and what effect TCDD treatment has on pulmonary virus burden. To compare the kinetics of the response in vehicle- and TCDD-treated mice, we examined the number of bronchoalveolar lavage (BAL) cells and CTL activity in the lung through d 10 postinfection. We found that the peak day for cellular influx and cytolytic activity in the lung is 9 d after infection. Immunophenotypic analysis of BAL cells shows that, when compared with BAL cells from infected controls, exposure to TCDD caused a 50% decrease in the percentage and number of both CD4(+) and CD8(+) cells. When the pulmonary virus burden was examined over time, we found that on d 1-5 postinfection, lungs from mice exposed to TCDD generally had lower virus titers than lung homogenates from vehicle- treated controls. By d 9 postinfection, no influenza virus was detected in lung homogenates from either vehicle- or TCDD-treat ed mice. These findings are likely not related to the observed decrease in CD4(+) and CD8(+) BAL cells; moreover, the diminished CD4(+) population in the lung indicates that CD4(+) cells are probably not compensating for the decreased CTL generation in TCDD-treated mice. Our observation that mice exposed to TCDD and infected with influenza virus do not have an increased pulmonary virus burden suggests either that TCDD treatment alters the host response to infection, creating a cellular environment that is less supportive for viral growth, or that exposure to TCDD directly affects influenza virus, leading to impaired virus replication within lung epithelial cells.