Dioxin-enhanced expression of interleukin-1 beta in human epidermal keratinocytes: potential role in the modulation of immune and inflammatory responses

Trajectory Shifts in Interdisciplinary Research of the Aryl Hydrocarbon Receptor-A Personal Perspective on Thymus and Skin

Identifying historical trajectories is a useful exercise in research, as it helps clarify important, perhaps even “paradigmatic”, shifts in thinking and moving forward in science. In this review, the development of research regarding the role of the transcription factor “aryl hydrocarbon receptor” (AHR) as a mediator of the toxicity of environmental pollution towards a link between the environment and a healthy adaptive response of the immune system and the skin is discussed. From this fascinating development, the opportunities for targeting the AHR in the therapy of many diseases become clear.

Mechanistic exploration of AhR-mediated host protection against Streptococcus pneumoniae infection

Streptococcus pneumoniae is a primary cause of invasive bacterial infection and pneumonia and is one of the leading causes of death worldwide. In prior studies we showed that pre-treating mice with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent agonist of the aryl hydrocarbon receptor (AhR), protects against S. pneumoniae-induced mortality and reduces pulmonary bacterial burden. The current studies were conducted to help elucidate the mechanism for this protective effect, and to characterize the response in the lung during the first 10h following infection. C57Bl/6 mice were treated with TCDD one day prior to intranasal infection with serotype 3 S. pneumoniae. Monitoring of bacteria in the lung airways revealed that bacterial growth was inhibited in the TCDD-treated animals within 10h of infection. To address the mechanism of this rapid protective response, macrophages, neutrophils, and invariant Natural Killer T (iNKT) cells were quantified, and levels of natural antibodies produced by B-1 B cells were evaluated. Functional assays addressed whether AhR activation reduced the capacity of lung epithelial cells to bind bacteria, and whether TCDD treatment enhanced production of antimicrobial agents in the lung or blood. None of the hypothesized mechanisms was able to explain the protective effect. Finally, the exposure paradigm was manipulated to test whether administration of TCDD after instillation of the bacteria was also protective. Results showed that TCDD must be administered in advance of exposure to bacteria, suggesting that the lung environment is rendered inhospitable to the pathogens.

The aryl hydrocarbon receptor (AhR) pathway as a regulatory pathway for cell adhesion and matrix metabolism

The aryl hydrocarbon receptor (AhR) is an orphan receptor in the basic helix-loop-helix PAS family of transcriptional regulators. Although the endogenous regulator of this pathway has not been identified, the AhR is known to bind and be activated by a variety of compounds ranging from environmental contaminants to flavanoids. The function of this receptor is still unclear; however, animal models indicate that the AhR is important for normal development. One hypothesis is that the AhR senses cellular stress and initiates the cellular response by altering gene expression and inhibiting cell cycle progression and that activation of the AhR by exogenous environmental chemicals results in the dysregulation of this normal function. In this review we will examine the role of the AhR in the regulation of genes and proteins involved in cell adhesion and matrix remodeling, and discuss the implications of these changes in development and disease. In addition, we will discuss evidence suggesting that the AhR pathway is responsive to changes in matrix composition as well as cell-cell and cell-matrix interactions.

Protection against lethal challenge with Streptococcus pneumoniae is conferred by aryl hydrocarbon receptor activation but is not associated with an enhanced inflammatory response

Streptococcus pneumoniae is a common respiratory pathogen and a major cause of morbidity and mortality in humans, particularly in the elderly and young children. The pulmonary immune response to S. pneumoniae is initiated very rapidly, and, ideally, innate immune responses are able to contain bacterial colonization. In the studies presented here, we sought to determine whether activation of the aryl hydrocarbon receptor (AhR) would protect mice from an otherwise lethal infection with S. pneumoniae. The rationale for this hypothesis is that, although most AhR agonists are potent immunosuppressants, AhR activation enhances the inflammatory response to pathogenic and nonpathogenic stimuli. Specifically, neutrophil numbers and levels of inflammatory cytokines are often increased in mice treated with the potent AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To test the hypothesis, vehicle control- or TCDD-treated mice were intranasally infected with S. pneumoniae. Mortality, pulmonary bacterial burden, cytokine/chemokine levels, and influx of immune cells to the lung were analyzed at various times postinfection. As predicted, survival was substantially improved in the mice treated with TCDD, and the pulmonary bacterial burden was decreased. Surprisingly, however, there was no evidence suggesting that protection resulted from an enhanced inflammatory response. In fact, neutrophil numbers and inflammatory chemokines and cytokines were all decreased in the TCDD-treated mice relative to vehicle control-treated mice. This suggests that the protective effect of AhR activation is not the result of altered immune function but instead may reflect a direct effect on the response of lung cells to infection.