Department of Host Defense

Our laboratory studies pathogen recognition by the innate immune system and the mechanisms that regulate innate immune responses. The innate immune system senses invading microbial pathogens, such as bacteria, viruses and parasites, 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. We are seeking to clarify the complex regulatory mechanisms of the innate immune system.

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. Their signaling pathways also play an important role in the gene induction involved in inflammation and immune responses. We have identified many 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 these TLR family members and their signaling pathways (Figure 1). We also found that the 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 the TLR3- and TLR4-mediated induction of IFN-β (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 . 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.

3) Investigation of pathogen recognition mechanisms by cytoplasmic receptors.
Infection with pathogens such as viruses induces type I IFNs in both a TLR-dependent and a TLR-independent manner. RIG-I and MAD5, which are RNA helicases that recognize viral RNAs, recognize different viruses and are important for host antiviral responses (Figure 3). We also identified a new adaptor molecule, IPS-1, which plays an essential role in RIG-I- and MDA5-mediated antiviral responses (Figure 3). We are currently exploring these TLR-independent mechanisms further by generating knockout mice.

Figure 3: 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.

Figure 4 : Mechanisms that regulate the inflammatory responses generated by TLR-inducible genes. TLR-inducible IkBz induces the transcription of genes such as Il6 via NFkBp50. In contrast, Zc3h12a, another TLR-inducible gene, functions as an Rnase that degrades the mRNAs for Il6 and Il12, among others, and negatively regulates inflammatory responses. TTP is also known to be involved in the degradation of TNF mRNA.

4) Investigation of the mechanisms that regulate inflammatory responses.
The inflammatory responses that are elicited by the activation of innate immunity are properly regulated by various mechanisms. Our recent studies revealed that TLR signal-inducible molecules further positively and negatively regulate inflammatory responses in response to infection. For example, the TLR-inducible nuclear factor IkBz functions as a transcriptional modulator that is responsible for inflammatory cytokine production (Figure 4). In contrast, TLR-inducible Zc3h12a, an RNase, destabilizes a set of mRNAs such as Il6 and negatively regulates inflammation. Mice that lack Zc3h12a spontaneously develop severe autoimmune inflammatory diseases. Currently, we are examining the posttranscriptional regulation of inflammatory responses.