Significance Early-life exposure to dogs is protective against allergic disease development, and dog ownership is associated with a distinct milieu of house dust microbial exposures. Here, we show that mice exposed to dog-associated house dust are protected against airway allergen challenge. These animals exhibit reduced Th2 cytokine production, fewer activated T cells, and a distinct gut microbiome composition, highly enriched for Lactobacillus johnsonii , which itself can confer airway protection when orally supplemented as a single species. This study supports the possibility that host–environment interactions that govern allergic or infectious airway disease may be mediated, at least in part, by the impact of environmental exposures on the gastrointestinal microbiome composition and, by extension, its impact on the host immune response.

Nucleotide-binding oligomerization domain proteins (NODs) are modular cytoplasmic proteins implicated in the recognition of peptidoglycan-derived molecules. NOD2 has recently been shown to be important for host cell cytokine responses to Mycobacterium tuberculosis, to synergize with Toll-like receptor 2 (TLR2) in mediating these responses, and thus to serve as a nonredundant recognition receptor for M. tuberculosis. Here, we demonstrate that macrophages and dendritic cells from NOD2-deficient mice were impaired in the production of proinflammatory cytokines and nitric oxide following infection with live, virulent M. tuberculosis. Mycolylarabinogalactan peptidoglycan (PGN), the cell wall core of M. tuberculosis, stimulated macrophages to release tumor necrosis factor (TNF) and interleukin-12p40 in a partially NOD2-dependent manner, and M. tuberculosis PGN required NOD2 for the optimal induction of TNF. However, NOD2-deficient mice were no more susceptible to infection with virulent M. tuberculosis than wild-type mice: they controlled the replication of M. tuberculosis in lung, spleen, and liver as well as wild-type mice, and both genotypes displayed similar lung pathologies. In addition, mice doubly deficient for NOD2 and TLR2 were similarly able to control an M. tuberculosis infection. Thus, NOD2 appears to participate in the recognition of M. tuberculosis by antigenpresenting cells in vitro yet is dispensable for the control of the pathogen during in vivo infection.

The initial TLR-mediated interaction between Mycobacterium tuberculosis and dendritic cells is critical, since the cytokine production that ensues can greatly influence the class of adaptive immunity that is generated to the pathogen. In this study, we therefore determined the dependency on TLR2 and TLR4 for M. tuberculosis-induced cytokine production by murine dendritic cells. A key new finding of this study is that production of IL-6 and IL-10 from dendritic cells in response to M. tuberculosis is principally dependent on TLR2. The study also indicates that M. tuberculosis can induce IL-12 production in the absence of either TLR2 or TLR4, suggesting redundancy or possibly involvement of other receptors in IL-12 production. In addition, the data also reveal that lack of TLR2 or TLR4 does not impact on dendritic cell maturation or on their ability to influence the polarity of differentiating naive T cells. Collectively, data presented here provide a mechanistic insight for the contribution of TLR2 and TLR4 to tuberculosis disease progression and offer strategies for regulating IL-6 and IL-10 production in dendritic cell-based vaccine strategies.

The control of IL-12 production from dendritic cells (DCs) and macrophages in response to Mycobacterium tuberculosis (Mtb) is not well understood. The objective of this study was to pursue the mechanism underlying our previous report that in response to Mtb infection, DCs release abundant IL-12, whereas secretion is limited in macrophages. An initial comparison of IL-12p35 and IL-12p40 gene induction showed that p35 transcription is similar in murine bone marrow-derived DCs and macrophages, but a rapid and enhanced IL-12p40 transcription occurs only in DCs. Consistent with the p40 gene transcription profile, Mtb-induced remodeling at nucleosome 1 of the p40 promoter also occurs rapidly and extensively in DCs in comparison to macrophages. Removal of IL-10 or addition of IFNγ enhances macrophage IL-12 release to Mtb, but without affecting the kinetics of remodeling at the macrophage p40 promoter. Furthermore, we show that Mtb-induced remodeling at the p40 promoter and IL-12 release in DCs is TLR9 dependent, and in contrast, TLR2 dependent, in macrophages. Data are also presented to demonstrate that a TLR9 agonist induces quantitatively more extensive remodeling at the IL-12p40 promoter and larger IL-12 release in comparison to a TLR2 agonist. Collectively, these findings suggest that DCs and macrophages handle Mtb differently resulting in only DCs being able to engage the more efficient TLR9 pathway for IL-12 gene induction. Our results also imply that TLR2 signaling is not a good inducer of IL-12, supporting the increasingly strong paradigm that TLR2 favors Th2 responses.

Recent work has demonstrated the importance of macroautophagy in dendritic cell (DC) maturation and innate cytokine production upon viral infection through delivery of cytoplasmic viral components to intracellular toll-like receptors. To study the functional consequences of impaired autophagosome formation during a Respiratory Syncytial Virus (RSV) infection, mice harboring significant autophagy defects due to Beclin-1 haploinsufficiency (Beclin-1+/−) were used. Upon RSV infection in vivo, lungs of Beclin-1+/− mice showed increased Th2 cytokine production, mucus secretion, and lung infiltration of eosinophils and inflammatory DCs. While isolated airway epithelial cells from Beclin-1+/− mice demonstrated little change compared to wildtype, Beclin-1+/− pulmonary and bone marrow-derived DCs (BMDCs) showed decreased expression of MHC-II and innate cytokine production upon RSV infection. Further examination indicated that Beclin-1+/− DC stimulated less IFNγ and IL-17 production by co-cultured CD4+ T cells and increased Th2 cytokine production in comparison to wild-type controls. Finally, adoptive transfer of RSV-infected Beclin-1+/− DCs into the airways of wild-type mice produced severe lung pathology and increased Th2 cytokine production upon subsequent RSV challenge compared to wild-type DC transfer controls. These results indicate a critical role of autophagy in dendritic cells during pulmonary viral infection, facilitating appropriate antiviral adaptive immune responses.