We investigated the role of DCs in instructing lung responses against CS and C. pneumoniae
infection. We demonstrated that exposure to CSE modulates the functional activity of CD11c+
DCs in the lung, with and without concomitant C. pneumoniae
infection. The BMDCs exposed to both CSE and C. pneumoniae in vitro
resulted in a strong Th2 skew, the release of Th2 cytokines (IL-4 and IL-5), and a decrease in the Th1 cytokine IL-12. Although DCs are not generally considered to produce the Th2 cytokine, several studies documented their ability to secrete Th2 cytokine, and our data support those studies (21
). In addition, BMDCs alone exposed to C. pneumoniae
or LPS were shown to drive Th2 sensitization and subsequent allergic airway inflammation (6
). The transfer of CSE-pulsed as well as C. pneumoniae–
pulsed BMDCs into the lungs increased the influx of CD11c+
DCs (mDCs). Although BMDCs exposed to both CSE and C. pneumoniae
resulted in a strong Th2 response in vitro
, this response was not evident in vivo
. Instead, when we adoptively transferred DCs pulsed ex vivo
with both CSE and C. pneumoniae
into the lungs of naive mice, we observed a decrease in the number of mDCs compared with either treatment alone, and more importantly, an influx of pDCs. The depletion of these pDCs resulted in a greatly increased Th2 response and subsequent inflammation, similar to the findings in our in vitro
data. We also observed an increased expression of both IFN-α and IDO by BMDCs pulsed with both CSE and C. pneumoniae,
both of which were reduced during pDC depletion, suggesting their potential involvement in the mechanism of altered polarization.
Exposure to CS can lead to increased hyperresponsiveness to infections, mainly because of the production of oxidants that can facilitate the nuclear translocation of NF-κB and activator protein-1 (9
). The activation of these transcription factors can mediate the production of proinflammatory cytokines and chemokines, influencing the influx of immune cells to the lung and the bacteria-mediated inflammatory process (9
). In this study, the adoptive transfer of BMDCs pulsed with CSE or C. pneumoniae
alone increased the presence of macrophages and DCs in recipient mice, both of which can be activated by TLR signaling. In vitro
studies revealed that CSE-induced cytokine release by BMDCs occurred in a TLR2-dependent and TLR4-dependent manner, a finding that is consistent with recent reports that TLR2-dependent and TLR4-dependent signaling is activated by oxidants and CSE in models of peritonitis and lung inflammation (9
). Collectively, our data are consistent with the interpretation that CS initiates inflammatory responses via TLR2/TLR4-dependent pathways, and this activation may modify host responses to a pathogen such as C. pneumoniae
by altering the activity of DCs and thus the adaptive immunity polarization.
Our experimental model aimed to analyze the activity of CSE-pulsed and/or C. pneumoniae–
pulsed DCs in the lung. Although this model is limited because the mice are not directly infected and do not receive first-hand smoke, it does allow us to investigate the specific role of mDCs in directing and instructing the immune response to these stimuli by the adaptive transfer of ex vivo
pulsed cells intratracheally. The activation and transfer of these cells to the lung could induce the production of cytokines and chemokines capable of recruiting other immune cells to the lung. The CSE also increased the number of innate immune cells in the lungs of mice after an adoptive transfer of BMDCs pulsed with C. pneumoniae
. Interestingly, beyond the recruitment of macrophages, neutrophils, and myeloid DCs, a large influx of pDCs into the lungs occurred after the transfer of BMDCs pulsed with both CSE and C. pneumoniae.
D'Hulst and colleagues reported an increased infiltration of DCs in the airways and lung parenchyma of mice after chronic (24-wk) exposure to CS (3
). Our report describes the relevance of pDCs in lung inflammation induced by CSE and/or C. pneumoniae
. The adoptive transfer of BMDCs pulsed with both CSE and C. pneumoniae
led to higher numbers of pDCs in the lungs of mice compared with each stimulation alone, and pDCs attenuated CSE-induced and/or C. pneumoniae
–induced lung inflammation by suppressing the Th2-like immune response of mDCs. The expression of IFN-α and IDO correlated with the suppression of Th2 inflammatory cells and cytokines associated with pDCs.
Previous studies showed that exposure to CS can exacerbate asthma. The increased incidence of asthma and airway hyperresponsiveness in smokers suggests that smoking alters the handling of inhaled antigens by the lungs, favoring the development of Th2-biased airway inflammation. In an ovalbumin-induced model of asthma, the exposure of mice to CS increased numbers of lung eosinophils, goblet cells, dendritic cells, and CD4-positive T cells (25
). Moreover, immature human blood-derived DCs preincubated with CSE for 7 days showed a decreased release of typical Th1 cytokines such as IFN-γ and IL-12, and the concomitant augmented production of Th2 cytokines such as IL-4 and IL-10 (7
), a pattern similar to that observed in our experimental model. Furthermore, IL-5 and IL-13, cytokines capable of inducing airway hyperresponsiveness, were increased in the lymph-node cell cultures of ovalbumin-exposed and CSE-exposed mice (25
In this study, we demonstrate that the adoptive transfer of BMDCs pulsed ex vivo
with both C. pneumoniae
and CSE into naive mice increased numbers of pDCs in the lung, and that these pDCs appeared to suppress the ability of mDCs to skew the lung toward Th2-type inflammation. The depletion of pDCs resulted in acute inflammation of the lung and increased Th2 cytokines, in both in vivo
and ex vivo
experiments. The BMDCs stimulated in vitro
with CSE and C. pneumoniae
naturally skewed toward Th2 cytokine expression, but in vivo
, the BMDCs directed a pDC response that could reverse the Th2 skew and prevent inflammation. One potential mechanism by which pDCs reduced the cellular influx to the lung would occur via the up-regulation of the immunosuppressive enzyme IDO. The absence of pDCs correlated with decreased levels of IDO, and the levels of apoptosis in the lung were also decreased. Beyond its metabolic activity, IDO can either promote the apoptosis of T cells or induce regulatory T cells (18
). The activation of TLR9 signaling in pDCs promotes the expression of IFN-α, which in turn can promote the expression of IDO. Concentrations of IDO are also influenced by oxidant concentrations, which are necessary for IDO activity (21
). The presence of both CSE and C. pneumonia
e significantly increased the expression of IDO and the number of TUNEL-positive cells, suggesting that increased apoptosis may have been mediated by the higher expression of IDO in pDCs. It is interesting to speculate that the CSE-induced suppression of host DC function may also be relevant to the pathology of cancer, where the imbalance in IL-12/IL-10 may lead to a tolerogenic state (21
). This possibility calls for further study. Indeed, the Th2 bias of mDCs and the increased presence of pDCs, which correlated with a higher expression of IDO, could lead to an immunosuppressive condition in a chronic inflammation model. That possibility would also be interesting to investigate in future studies.
In conclusion, our study demonstrates the ability of CSE to modulate the ability of DCs to direct the appropriate immune response to infection with C. pneumoniae. The increase in pDCs resulted in reduced numbers of mDCs, an increase in the expression of IDO, increased positive TUNEL staining in the lung, and the prevention of acute inflammation. On the other hand, the depletion of pDCs reversed this phenotype, and allowed an acute Th2-like inflammation to occur.