TLR6: A novel member of an expanding Toll-like receptor family

Abstract

Drosophila Toll protein is shown to activate the innate immune system in adult and regulate the dorsoventral patterning in the developing embryo. Recently, five human homologs of Drosophila Toll, designated as Toll-like receptors (TLRs), have been identified and shown to play a role in the innate immune response. We report here the molecular cloning and characterization of a new member of Toll-like receptor family, Toll-like receptor 6 (TLR6). Human and murine TLR6 are type-I transmembrane receptors that contain both an extracellular leucine-rich repeat (LRR) domain and a cytoplasmic Toll/IL-1 receptor (IL-1R)-like region. The amino acid sequence of human TLR6 (hTLR6) is most similar to that of hTLR1 with 69% identity. RT-PCR analysis revealed that murine TLR6 is expressed predominantly in spleen, thymus, ovary and lung. Like other TLR family members, constitutively active TLR6 activates both NF-κB and c-Jun N-terminal kinase (JNK). The TLR6 gene, as well as the TLR1 gene, mapped to the proximal region of murine chromosome 5 within 1.7 cM of each other. These results suggest that TLR6 is a novel member of an expanding TLR family.

Introduction

In the early phase of an infection, the host defense initiates by the production of proinflammatory cytokines by antigen-presenting cells and the induction of costimulatory molecules on their cell surfaces. These molecules activate naive T cells and transduce signals to adaptive immune system (Medzhitov et al., 1997b). In Drosophila, in which there is no adaptive immune system, the host defense is executed by the rapid synthesis of antimicrobial and/or antifungal peptide. The induction of the gene encoding antibacterial and/or antifungal peptide relies on the Toll signaling pathway, which is involved in the control of dorsoventral patterning in the embryo (Meister et al., 1997). Drosophila Toll (dToll) is a type-I transmembrane receptor that is characterized by the leucine-rich repeat (LRR) of the extracellular domain as well as the cytoplasmic domain that resembles the cytoplasmic domain of mammalian interleukin-1 receptor (IL-1R). The signaling pathway of dToll is substantially similar to that of mammalian IL-1R signaling pathway. Drosophila Toll induce the activation of the Rel type transcription factor, Dorsal via the adaptor protein Tube and the serine–threonine protein kinase Pelle. Drosophila Tube/Pelle/Dorsal are the mammalian homologs of MyD88/IL-1R associated kinase (IRAK)/NF-κB, respectively (Muzio et al., 1997, O'Neill and Greene, 1998).

Five human homologs of Drosophila Toll, designated as Toll-like receptor (TLR) 1–5, have been reported (Rock et al., 1998). Constitutively active mutant of TLR4 (also known as hToll) activates transcription factor NF-κB and induces the expression of B7.1, IL-6, and IL-8, implying the importance of TLR4 in switching from the innate immunity to acquired immunity (Medzhitov et al., 1997a). TLR4 is shown to signal through an adaptor protein MyD88, which recruits IRAK-1 and IRAK-2 to the receptor complex. Subsequently, IRAK-1 and IRAK-2 interact with tumor necrosis factor receptor-activated factor (TRAF) 6. TRAF6 relays the signal via NF-κB inducing kinase (NIK) to IκB kinases (IKKs) complex that directly phosphorylates IκBα and mediates degradation of IκBα (Medzhitov et al., 1998, Muzio et al., 1998). Several other TLR family members, TLR2 (also known as TIL4) and TLR5 (or TIL3) are implicated to activate NF-κB (Chaudhary et al., 1998).

In this study, we describe the cloning and characterization of TLR6, a novel member of an expanding Toll-like receptor family.