Department of Host Defense

Our laboratory studies pathogen recognition by the innate immune system and the mechanisms by which these recognition events lead to innate immune responses. The innate immune system senses invading microbial pathogens such as bacteria, virus and parasite and plays an essential role in inducing inflammatory responses and assisting adaptive immune responses. Pattern-recognition receptors (PRRs) expressed on innate immune cells such as macrophages and dendritic cells recognize pathogen-associated molecular patterns (PAMPs), which are conserved molecular features of microbial pathogens. By generating knockout mice, we are currently investigating the in vivo roles of PRRs and their downstream signaling molecules in innate immunity.

1) Characterization of the pathogen recognition by Toll-like receptors (TLRs) and their signaling pathways
TLR family members play essential roles in the recognition of pathogens by the innate immune system, and their signaling pathways play an important role in the gene induction involved in inflammation and immune responses. We have identified TLR family members and their signaling molecules and have characterized their functions by generating knockout mice. As a result, we have identified most of the ligands for TLR family members and their signaling pathways (Figure 1). We also found that stimulation of TLRs induces not only proinflammatory cytokine genes but also type I interferon genes. For example, the TRIF-TBK1/IKK-i-IRF-3 pathway plays an important role in TLR3- and TLR4-mediated IFN-β induction (Figure 2). Moreover, TLR7 and TLR9 are preferentially expressed in plasmacytoid dendritic cells (pDCs), which produce large amounts of IFN-α upon viral infection. We identified a specific signaling pathway in pDCs that is stimulated by TLR7 and TLR9 ligands and induces IFN-α expression (Figure 2). In summary, TLR signaling is regulated by distinct and complex mechanisms that operate in a ligand- and cell-type specific manner. We are currently expanding our understanding of the in vivo functions of TLRs and their signaling pathways.

Figure 1 : Pathogen recognition by TLRs. TLRs recognize molecular patterns associated with a broad range of pathogens, including bacteria, fungi, protozoa and viruses.

Figure 2 : TLR signaling pathways. All TLR family members apart from TLR3 share a common pathway called the MyD88-dependent pathway that induces inflammatory cytokine production. Each TLR family member also has its own specific signaling pathway. Thus, TLR3 and TLR4 operate via a TRIF-dependent pathway while TLR7 and TLR9 act in pDCs via a unique pathway to induce IFN-α expression.

2) Therapeutic applications of TLR agonists and antagonists
Appropriate agonist-induced stimulation of TLRs could stimulate an innate immune response that boosts host resistance to cancer, allergy, and infectious diseases (Figure 3). This approach could also be used to promote the development of an adaptive immune response to a co-administered vaccine. TLR antagonists may also have therapeutic potential, as they could prevent or ameliorate the inappropriate or exaggerated TLR stimulation that leads to deleterious outcomes such as autoimmune diseases, sepsis or atherosclerosis (Figure 3).

Figure 3: Therapeutic applications of TLR agonists and antagonists. The immunomodulatory effects of TLR agonists (red) and antagonists (yellow) are being harnessed for a variety of therapeutic purposes. TLR agonists (e.g. CpG DNA) may be useful as adjuvants that boost host resistance to cancer, allergy, and infectious diseases. TLR antagonists in turn may be useful for treating TLR-mediated autoimmune diseases, sepsis or atherosclerosis (blue).

Figure 4 : Signaling pathways employed by anti-viral RNA helicases. Viruses produce dsRNA during their replication in the host cell cytoplasm. RIG-I and Mda5 recognize this viral RNA and initiate antiviral signaling. In this signaling pathway, IPS-1 interacts with RIG-I and Mda5 via the CARD-like domain, and this leads to the TBK1- and IKKi-dependent phosphorylation and activation of IRF3 and IRF7. IPS-1 also activates NF-kB via FADD/RIP1-dependent pathways. In addition, synthetic dsDNA activates type I IFN promoters, although the receptor responsible for the dsDNA recognition has not yet been identified.

3) Investigation of TLR-independent pathogen recognition mechanisms.
Infection with pathogens such as viruses induces type I IFNs in both a TLR-dependent and a TLR-independent manner. Recently, the RNA helicases RIG-I and MAD5 were found to be proteins that recognize dsRNA in the cytoplasm in a TLR-independent manner. By generating RIG-I- and MDA5-deficient mice, we found that RIG-I and MDA5 distinguish between different RNA viruses and play critical roles in host antiviral responses (Figure 4). We also identified a new adaptor molecule, IPS-1, that plays an essential role in RIG-I- and MDA5-mediated antiviral responses (Figure 4). We are currently exploring these TLR-independent mechanisms further by generating knockout mice.

4) Characterization of the functional role played by the C/EBP family in host defense.
C/EBP family members are bZIP transcription factors. Analysis of knockout mice has revealed that NF-IL6 (C/EBPβ) in particular plays an essential role in eliminating intracellular bacterial infections in macrophages. We are currently exploring the host defense mechanisms that employ NF-IL6 by identifying the NF-IL6 target genes in macrophages.