We investigated the role of NKT cells in immunity to Chlamydia pneumoniae and Chlamydia muridarum infections using a combination of knockout mice and specific cellular activation approaches. The NKT-deficient mice showed exacerbated susceptibility to C. pneumoniae infection, but more resistance to C. muridarum infection. Activation of NKT reduced C. pneumoniae in vivo growth, but enhanced C. muridarum infection. Cellular analysis of invariant NKT cells revealed distinct cytokine patterns following C. pneumoniae and C. muridarum infections, i.e., predominant IFN-γ in the former, while predominant IL-4 in the latter. The cytokine patterns of CD4+ and CD8+ T cells matched those of NKT cells. Our data provide in vivo evidence for a functionally diverse role of NKT cells in immune response to two intracellular bacterial pathogens. These results suggest that distinct NKT subsets are induced by even biologically closely related pathogens, thus leading to differential adaptive immune response and infection outcomes.

We used two approaches to examine the role of NK T cells (NKT) in an intracellular bacterial (Chlamydia trachomatis mouse pneumonitis (C. muridarum)) infection. One is to use CD1 gene knockout (KO) mice, which lack NKT, and the other is to activate NKT using α-galactosylceramide (α-GalCer), a natural ligand of these cells. The data showed a promoting effect of NKT activation on Chlamydia lung infection. Specifically, CD1 KO mice exhibited significantly lower levels of body weight loss, less severe pathological change and lower chlamydial in vivo growth than wild-type mice. Immunological analysis showed that CD1 KO mice exhibited significantly lower C. muridarum-specific IL-4 and serum IgE Ab responses as well as more pronounced delayed-type hypersensitivity response compared with wild-type controls. In line with the finding in KO mice, the in vivo stimulation of NKT using α-GalCer enhanced chlamydial growth in vivo, which were correlated with reduced delayed-type hypersensitivity response and increased C. muridarum-driven IL-4/IgE production. Moreover, neutralization of IL-4 activity in the α-GalCer-treated BALB/c mice significantly reduced the promoting effect of α-GalCer treatment on chlamydial growth in vivo. These data provide in vivo evidence for the involvement of NKT in a bacterial pathogenesis and its role in promoting Th2 responses during infection.

Although NKT cells have been found to be capable of modulating immune responses in several model systems, the role of NKT cells in allergy remains unclear. Using CD1 gene knockout (KO) mice, which lack NKT cells, we examined the function of NKT cells in the development of allergic inflammation induced by a common airborne human allergen, ragweed. The data showed that airway eosinophilia and mucus overproduction induced by ragweed were significantly reduced in CD1 KO mice, which was correlated with significantly lower allergendriven IL-4 production and lower eotaxin responses in the airways of CD1 KO mice. Moreover, both ragweed-specific and total serum IgE levels in CD1 KO mice were significantly lower than those in control BALB/c mice. The reduced allergic reaction in CD1 KO mice is not due to intrinsic deficiency because they showed normal levels of immune cells and function. In addition, in vivo stimulation of NKT cells using their natural ligand, § -galactosylceramide, enhanced ragweed-induced airway eosinophilia, IL-4, and eotaxin production in control, but not CD1 KO mice. These data provide in vivo evidence for the involvement of NKT cells in the allergic mechanisms responsible for allergen-driven cytokine and chemokine production and airway inflammation.

Our previous study has shown that Chlamydia lung infection can inhibit local eosinophilic inflammation induced by allergen sensitization and challenge, which is correlated with altered cytokine production. In the present study, we examined the role played by dendritic cells (DC) in chlamydial infection-mediated modulation of allergic responses. The results showed that DC freshly isolated from Chlamydia-infected mice (iIDC), unlike those from naive control mice (iNDC), could efficiently modulate immune responses to ovalbumin in vitro and in vivo. Co-culture of freshly isolated DC with naive CD4 cells from T cell receptor transgenic mice (DO11.10) showed that iIDC directed Th1-dominant, while iNDC directed Th2-dominant, allergen-specific CD4 T cell responses. Moreover, adoptive transfer of iIDC, but not iNDC, could inhibit systemic and local eosinophilia induced by allergen exposure. The reduction of eosinophilia was associated with a decrease in IL-5 receptor expression on bone marrow cells and the production of IL-5 and IL-13 by T lymphocytes. Analysis of the DC showed that iIDC expressed significantly higher levels of mRNA for Toll-like receptor 9 and produced more IL-12 compared to iNDC. The data demonstrate a critical role played by DC in infection-mediated inhibition of allergic responses.

SUMMARYOur previous studies, as well as those of others, have demonstrated that local or systemic Mycobacterium bovis bacille Calmette±Gue Ârin (BCG) infection can inhibit de novo allergeninduced asthma-like reactions, but the effect of this infection on established allergic responses is unknown. The aim of this study was therefore to examine the effect of mycobacterial infection on established allergy in a murine model of asthma-like reaction. Mice were sensitized with ovalbumin (OVA) in alum followed by infection with BCG and subsequent intranasal challenge with the same allergen. In some experiments, mice were sensitized with OVA followed by intranasal challenge with OVA and then given BCG infection with subsequent rechallenge with OVA. Mice without BCG infection but treated with OVA in the same manner, were used as a control. The mice were examined for immunoglobulin E (IgE) response and eosinophilic in¯ammation, mucus production, cytokine/chemokine patterns and adhesion molecule expression in the lung. The results showed that postallergen BCG infection suppressed the established airway eosinophilia and mucus overproduction, but not IgE responses. The inhibition of asthma-like reactions by BCG infection was correlated with a shift of allergen-driven cytokine production pattern and, more interestingly, with a dramatic decrease of vascular cell adhesion molecule-1 (VCAM-1) expression in the lung. These ®ndings suggest that intracellular bacterial infection can inhibit established allergic responses via alteration of local cytokine production and the expression of adhesion molecules.