Although the original hygiene hypothesis predicts that some infections inhibit the development of atopic diseases, there is increasing recognition that infectious agents could paradoxically promote the pathogenesis of asthma. We show here that respiratory infection with Chlamydia pneumoniae
, a common respiratory pathogen, creates a pro-inflammatory environment in the lung that can exert an adjuvant effect on allergen sensitization. We found that DCs were centrally involved in triggering allergic sensitization, since either adoptive transfer of allergen-presenting DCs from infected mice or bone marrow-derived DCs infected with CP and challenged with HSA ex vivo
could induce eosinophilic airway inflammation in recipient mice. Furthermore, MyD88 expression by adoptively transferred donor DCs was required, but MyD88-expression was not necessary in recipient animals for sensitization (see Online Repository
, Fig. E4B to D
). Therefore, intact MyD88-dependent signaling in DCs was sufficient to induce allergic airway sensitization after CP infection and exposure to allergen.
However, airway sensitization depended upon severity and timing of the infection: low-dose infection and antigen exposure within 5 days of infection induced allergic airway sensitization, while high-dose infection or antigen exposure 10 days after infection did not. The time window during which successful sensitization could be performed appears to be directly linked to an immunoregulatory pathway that involves Treg function: by depleting the CD4+
Treg subset, we were able to prolong the time window in which sensitization could occur. Recent studies suggest that Tregs normally inhibit development of allergic asthma 19
, but in studies focusing on Th1-biased immune responses, the suppressive activity of Tregs on DCs is countered by DCs themselves through TLR-mediated IL-6 release triggered by exposure to microbial ligands 20, 27
. Indeed, we found increased IL-6 levels during the time window in which sensitization could occur. Collectively, our results appear most consistent with the interpretation that allergen sensitization during bacterial lung infection is controlled by both the activation status of airway DCs in a MyD88-dependent manner and by CD4+
Treg numbers and function.
Our data indicate that mild pulmonary infections favor the development of allergic sensitization and asthma, but severe infections do not. One possible explanation for the lack of allergic antigen sensitization is that high-dose infection could accelerate DC maturation and production of Th1-skewing cytokines by DCs. This is consistent with reports that low-dose LPS exposure enhances allergic sensitization, but high-dose LPS has the opposite effect 16
. This appears to be due to the fact that high-dose LPS exposure induces predominantly IgG2a
production and Th1 skewing, which prevents allergen sensitization 16, 17
. In fact, during high-dose infection, we observed a trend towards less pronounced Th2 helper cell responses, as manifested by lower levels of HSA-specific IgG1
and IgE compared to low-dose infected mice (). Furthermore, high-dose infections were accompanied by higher levels of IFN-γ in the BAL and increased production of IL-12 but not IL-10 by DCs (Online Repository. E1B and E2A). Most importantly, our data also show that high-dose infection is associated with decreased antigen presentation (lower MHC class IIhigh
expression in lung DCs as well as lower numbers of FITC-HSA+
DCs in the lymph nodes). This finding may be related to the increased numbers of neutrophils observed in high-dose infected mice, since neutrophil degranulation inhibits DC-maturation in vitro 28
. Therefore, allergic antigen sensitization in high-dose infected mice may be impaired by the cytokine pattern elicited in DCs, which favors both Th1-responses, and an overall impairment of antigen presentation. Our data are consistent with a model that MyD88-dependent cellular activation in the presence of microbial antigens is sufficient to induce airway sensitization. Thus, it is tempting to speculate that comparable mechanisms apply to other bacteria and viruses that have been linked to the pathogenesis of asthma, such as M. pneumoniae
or RSV. Additionally, our data show that moderate, but not severe infection predisposes to the development of asthma, thus yielding a possible explanation to the fact that so far, agents causing atypical pneumonia that rarely require hospitalization have been linked to the onset and exacerbation of asthma 9-11
Previous studies addressing the impact of Chlamydia
on allergic airway inflammation in murine models have focused on exacerbation of pre-existing asthma by infecting previously sensitized mice with C. trachomatis or C. muridarum 29, 30
. These studies yielded conflicting results; C. trachomatis
inhibited airway inflammation 30
, but C. muridarum
infection was associated with an increase in inflammatory parameters 29
. In contrast to these studies we investigated the mechanisms causing allergen sensitization in previously healthy mice sensitized with allergen during a concomitant CP infection. Our findings are therefore consistent with previous studies 7,8
, which collectively emphasize that timing of infection relative to allergen exposure critically determines whether allergen sensitization is promoted or suppressed. However, in addition, our results now suggest that there is a time window during which moderate but not severe respiratory infections may act as adjuvants that promote allergen sensitization and the pathogenesis of asthma.
In summary, our results demonstrate that bacterial respiratory infection can elicit an innate immune environment that promotes allergen sensitization and eosinophilic airway inflammation in a temporally-restricted and severity-dependent manner. The mechanism requires intact MyD88-dependent signaling in DCs, and is importantly controlled by Tregs. The net outcome of the functional interactions between CP-activated DCs and Tregs dynamically alters the magnitude of antigen sensitization. Our data demonstrate that under certain circumstances, infectious agents may promote the pathogenesis of allergic disease by acting as adjuvants that favor Th2 differentiation. In a controlled experimental model of bacterial infection and allergen challenge, we demonstrate that the same infectious agent can promote the development of allergic sensitization under certain conditions, but fails to do so in other contexts. The development of allergic asthma was promoted by moderate infection, but suppressed by severe infection of the lungs. Our data provide important novel mechanistic insights into why and how this occurs, and what molecular and cellular participants impact both the likelihood that allergic sensitization could occur, and the duration of the time window during which this was possible.