AhR activation decreases survival from infection with a non-lethal dose of influenza A virus [15
]. Others have reported similar effects on host resistance using lethal viral challenges [17
]. Initially, it was postulated that decreased survival was due to an inability to clear virus; however, TCDD-treated mice clear virus from their lungs with kinetics similar to control-treated mice, and there is no change in the overall pulmonary viral burden [17
]. In fact, some studies have shown that influenza A virus replication may actually be inhibited by neutrophil recruitment [43
], however whether or not this inhibition is affected by AhR activation remains unknown. These findings suggest that other mechanisms underlie the decreased survival from influenza virus infection. One likely mechanism is the increased recruitment of neutrophils to the lung. Compared to infected controls, about twice as many neutrophils are found in the lungs of infected mice treated with TCDD (). Excess neutrophils are observed in airways and lung interstitium of TCDD-treated wild-type mice, but are not observed in lungs of TCDD-treated AhR−/−
Figure 2 AhR activation modulates innate and adaptive immune responses in the lung and mediastinal lymph node (MLN). Mice were treated (p.o.) with 10 μg/kg TCDD (closed circles) or peanut oil vehicle control (open circles) and intranasally-infected with (more ...)
This increase in neutrophils is of great interest for several reasons. During uncomplicated respiratory viral infection, the recruitment of neutrophils to the lung is commonly observed; however, excessive accumulation of neutrophils during infection has been linked to immune-mediated pathology and death [45
]. For example, recent reports suggest that during the 1918 Spanish influenza pandemic, uncontrolled neutrophilia was associated with mortality from influenza-related complications [46
]. Neutrophil recruitment has also been implicated in the acute pathology of other viral infections such as respiratory syncytial virus (RSV), herpes simplex virus-1 (HSV-1) and cytomegalovirus (CMV) [45
]. What factors trigger heightened pulmonary neutrophilia during viral infection remain uncertain; but recent findings suggest that controlling neutrophil recruitment may provide an important but overlooked means to improve clinical outcomes during viral infection [36
Not only has the relationship between excessive neutrophil recruitment to the lung and enhanced immune-mediated pathology been demonstrated in other systems, it has been established that AhR-mediated excessive neutrophil recruitment contributes to decreased survival and more severe bronchopneumonia in TCDD-treated, infected mice [33
]. Moreover, this effect of TCDD on neutrophil recruitment may not be unique to the response to influenza virus, as it has been reported in model systems using non-infectious antigens, in which excessive neutrophilia is observed at the site of antigen challenge [51
]. However, the type of antigen likely influences the nature of AhR-mediated changes in neutrophil recruitment, as TCDD-treated mice infected with Streptococcus pneumoniae
have markedly reduced neutrophil recruitment compared to vehicle-treated mice [54
]. Differences in the consequences of AhR activation during infection with influenza virus and S. pneumonaie
may be explained by the action of pattern recognition receptors (PRRs), which recognize different types of pathogens and activate distinct pro-inflammatory signaling pathways [55
]. It is possible that AhR activation impacts neutrophil recruitment by modulating signaling pathways important for PRR function or expression, but this concept has yet to be experimentally tested. Nevertheless, it is becoming clear that AhR activation impacts pathways that control neutrophil migration, and as we decipher the underlying mechanism new targets for regulating neutrophil trafficking may be revealed.
An important new concept to emerge from these studies is that AhR specifically targets regulatory pathways that are involved in the host’s response to infection, such that they are primed to respond inappropriately. The basis for this idea stems from the observation that, in the absence of infection, AhR activation does not alter the number of neutrophils at the site of antigen challenge [16
]. This suggests that AhR activation synergizes with or potentiates infection-associated signals important for neutrophil recruitment. Within the context of infection with influenza virus, we have systematically examined mechanisms by which AhR could mediate an increase in the number of neutrophils in the lungs of infected mice. While infection increases the expression of soluble chemoattractants in the lung, TCDD-treatment does not further enhance the levels of these molecules [33
]. Furthermore, treatment with TCDD does not increase expression of adhesion molecules on neutrophils, endothelial or epithelial cells in the lung [33
]. Also unaffected by AhR activation are the number of circulating neutrophils, their function, or the level of neutrophil death in the lung [33
]. Thus, it is likely that AhR activation during viral infection targets a novel pathway for neutrophil recruitment, which has yet to be identified.
While AhR activation does not affect the levels of known neutrophil chemoattractants, it does increase levels of the important anti-viral cytokine interferon (IFN)-γ in the lungs of mice infected with influenza virus (). However, the number of neutrophils in the lung is the same in infected wild-type and IFN-γ-deficient mice treated with TCDD. This indicates that elevated IFN-γ levels do not underlie the increased recruitment of neutrophils [56
]. Instead, we have recently discovered that it is the neutrophils themselves that produce much of the excess IFN-γ in the infected lung [58
], which suggests that AhR-mediated increases in IFN-γ levels may be due to the elevated number of IFN-γ producing cells rather than a direct impact on IFN-γ gene expression.
Interestingly, in the same timeframe in which pulmonary neutrophilia and IFN-γ levels are increased, there is enhanced expression of inducible nitric oxide synthase (iNOS) in the lung. AhR-mediated increases in iNOS are observed in alveolar macrophages as well as lung epithelial cells, suggesting AhR targets immune and non-immune cells of the lung. Increased iNOS expression has been associated with influenza-mediated pathology, and its product, nitric oxide (NO), has been implicated as a mediator of both beneficial and detrimental effects during viral infections [58
]. Similar to increases in neutrophil recruitment and IFN-γ levels in the lung, AhR activation alone is not sufficient to induce iNOS. A second signal, in this case viral infection, appears to be required, which again suggests AhR influences the nature of the response to viral infection. One explanation for increased iNOS is that it is downstream of elevated IFN-γ, which is a known inducer of iNOS expression [63
]. However, recent studies using iNOS-deficient mice revealed that AhR-mediated excesses in IFN-γ production during influenza virus infection requires iNOS expression [58
]. Therefore, it appears that AhR-activation stimulates a novel, iNOS-dependent pathway for IFN-γ production in the lung by neutrophils. These findings also highlight iNOS as a potentially new AhR target gene.