Space and time: New considerations about the relationship between Toll-like receptors (TLRs) and type I interferons (IFNs)

Abstract

The Toll like receptors (TLRs) and the type I interferons have critical roles to play in innate immunity. In this review we will discuss new developments relating to the important area of TLR/IFN cross regulation.

Introduction

Within the field of innate immunity, type I interferons (IFNs) and the Toll-like receptors (TLRs) have long been recognized as two central pillars governing responses to infectious microbes. These two critical elements of the innate response do not exist in isolation, but rather, exhibit multiple levels of cross-talk that is essential for the full efficacy of each system. This review will detail and comment on several areas of TLR–IFN counter-regulation that are highly significant within contemporary research. In particular, we hope to highlight an ongoing evolution in the model that describes how members of the TLR family relate to the signaling pathways that govern induction of type I IFN and vice versa. The longstanding traditional model was largely static and based on the identity of the receptor. In this schema, individual TLRs were grouped based on their ability to directly induce IFN secretion in response to a purified ligand traditionally assayed in an innate immune cell such as a macrophage. This static understanding is being revised by new work we will discuss that shows the importance of place, i.e., where in a cell and/or on which type of cell a TLR is located, as well as time, i.e., when we are looking during a sequence of signaling events, when discussing a TLR’s role in IFN induction.

The TLRs are a family of type I transmembrane pattern recognition receptors (PRRs) that have the capacity to recognize a broad array of widely distinct microbes, ranging from bacteria to viruses, fungi, and parasites. In mammals, the TLR family presently consists of 13 members. TLRs 1–9 are conserved between humans and mice with TLR11, TLR12, and TLR13 being functionally lost in humans. Members of the TLR family share a common structural organization that is the basis of their identification as TLRs. The structural TLR prototype is composed of an N-terminal ectodomain, a single type I transmembrane domain, and an intracellular Toll/interleukin (IL)-1R Resistance (TIR) domain. The N-terminal ectodomain is typified by the presence of leucine-rich repeat sequences (LRRs) that mediate recognition by TLRs of their respective ligands. The TLR TIR region allows for interactions among TLRs and adapter proteins, and initiates signal transduction cascades [1].

The individual ligands recognized by TLRs vary in their structure and chemistries. Microbial ligands (referred to as “pathogen or microbial-associated molecular patterns (PAMPs or MAMPs) identified to date include: triacylated and diacylated lipopeptides, respectively (TLR 1/2 and 2/6 heterodimers) [2], extracellular double-stranded RNA (TLR3) [3], lipopolysaccharide and many others (TLR4) [2], bacterial flagellin (TLR5) [4], single-stranded extracellular RNA as well as select small molecules (TLR7/8) [5], [6], CpG DNA (TLR9) [7]. TLR4 is the only TLR that does not directly bind ligand, but rather, is dependent upon a non-covalently associated co-receptor called MD2 to bind ligand and initiate dimerization of the TLR4 molecules.

Ligation of TLR3 or TLR4 leads to direct induction of the IFN-β promoter. TLR3 is associated with endosomes and TLR4 becomes internalized into endosomes after LPS stimulation. TRAM and TRIF are a pair of TLR-recruited adaptor proteins [8], [9]. Ligation and dimerization of TLR4 leads to association with TRAM and subsequent recruitment of TRIF [10]. TRIF, in turn, nucleates a signaling complex comprised of the ubiquitin ligase TRAF3 and the TANK-binding kinase 1 (TBK-1) [11], [12]. Activated TBK-1 phosphorylates and activates the transcription factor IRF3. Once activated, IRF3 primarily serves to drive robust transcriptional activation of the type I IFN-β response, as well as induction of other IRF-3-dependent genes [13]. IFN-β can also be induced through the MyD88 adaptor following ligation of endosomally located TLRs 7, 8, and 9 [14]. In such cases, MyD88 recruitment to the TIR domain mediates direct binding and auto-activation of the transcription factor IRF7. Once activated, IRF7 (as with IRF3) translocates to the nucleus where it drives expression from the IFN-β promoter [15], [16].

The type I IFNs are a family of ubiquitously expressed pleiotropic cytokines that utilize a common type I IFN receptor (IFNAR). In human, the type I IFN family consists of a single IFN-β gene and 13 IFN-α genes. The large number of IFN-α genes are thought to have arisen through gene duplication and display some cell type specificity of expression. The type I IFNs are potently and rapidly induced by select TLRs and, once induced, are secreted and act on a broad array of tissue types. Downstream of the IFNAR, IFNs induce hundreds of genes through the hierarchical action of the JAK-STAT proteins. In brief, binding of IFNs to IFNAR changes the conformation of the intracellular portion of this heterodimeric receptor allowing auto- and trans-phosphorylation of kinases tyk2 and JAK1. These events permit association of the transcription factors STAT1 and STAT2 with the IFNAR that allows the phosphorylation and activation of these transcription factors [17]. Following activation, the STAT proteins enter the nucleus and drive transcription of hundreds of IFN-responsive genes. Many of the induced genes are directly antimicrobial and, in particular, anti-viral. However, many of the IFN-induced genes modulate other aspects of the innate immune response including TLR signaling and function [18]. As well as governing the innate response, IFNs serve as an essential point of contact between the innate and adaptive responses.