FIG 15.1  Subcellular localization of the TLR pattern-recognition receptor family in dendritic cells.

TLR2, 4, 5, and 11 are cell surface receptors that recognize membrane components of invading microorganisms. TLR3, 7, 8, and 9 are present in the endosomal compartment, where they recognize different forms of nucleic acids taken up by the dendritic cell.6,17 TLR1 and TLR6, involved in the recognition of diacyl/triacyl lipopeptides and lipoteichoic acid, are not shown.

and, importantly, function independently of immunological memory. They recognize highly conserved components that are released by invading microbes, collectively named the pathogen-associated molecular patterns (PAMS).4 Among the PRRs are the Toll-like receptors that recognize essential components of microorganisms, such as membrane proteins, lipids, and nucleic acids5 (Figure 15.1 and Table 15.1). This chapter deals with the signalling pathway downstream of the Toll 4 (TLR4) receptor.

The toll receptor in Drosophila

Flies possess two strategies of defence against bacterial and fungal invasion:

(1)a cellular pathway involving plasmatocytes (the major blood cell type) and

(2)a humoral pathway that involves the production of bacteriolytic peptides (such as cecropins, diptericin, drosocin, and defensin), and antifungal peptides (drosomycin and metchnikowin).7

The events leading to the expression of the drosomycin gene were revealed by comparing signal transduction pathways in mammals and flies that have similar components but are instrumental in completely different processes.8,9 Thus, there are striking parallels between the pathway operated by the morphogen ‘dorsal’ in embryonic flies, controlled by the Toll receptor and the mediation of the inflammatory response in mammals by IL-1.10–12 Also, the promoter sequence of the diptericin gene has DNA motifs related to NF- B binding sites.

Toll. Lack-of-function mutants produce dorsalized Drosophila embryos, dominant gain- of-function alleles result in ventralized embryos. The German word toll means ‘amazing’, ‘fantastic’. According to Christiane Nüsslein-Volhard, ‘When Eric Wieschaus and I

first saw the mutant phenotype, we were amazed, because it was so novel and unexpected. I must have yelled “toll”, and the name stuck’.

Beyond its role in development, Toll plays a key role in the host defence mechanisms of adult flies. While

Drosophila Toll recognizes the cytokine spätzle, the mammalian TLRs (TLR1– 12) recognize specific microbial components, ranging from singlestranded DNA to lipopolysaccharides6 (see

Figure 15.4).

Table 15.2  Toll and IL-1 pathways in Drosophila and mammals

through the dorsal pathway. Ablation of the Toll receptors results in flies that are susceptible to microbial infection, particularly by fungi. The Toll–dorsal pathway is a costimulant for the expression of antimicrobial peptides.13

Spätzle and IL-1 are, respectively, the ligands of   Toll and IL-1 receptors.14 Although these ligands have no sequence homology, spätzle being closer to the mammalian nerve growth factors (NGF), the processing of their inactive precursors is controlled, in part, by immune challenge. During development, spätzle is processed by three extracellular serine proteases (‘easter’, ‘gastrulation defective’, and ‘snake’). An alternative proteolytic cascade, activated by an immune challenge, may constitute the activation mechanism in host defence.15 IL-1 is processed by an intracellular protease (interleukin- 1 -converting enzyme, ICE, also called caspase-1). In monocytes the activity of this enzyme is controlled by, among other factors, lipopolysaccharides.16

Although the ligands are dissimilar, the receptors, Toll and IL-1R share a common intracellular protein–protein interacting TIR domain, through which they bind the adaptors TIRAP, TRIF, TRAM, and MyD88 (in mammalian cells) and MyD88 (in Drosophila)19,20 (see Table 15.2 and Figure 15.2). The extracellular domains are completely different. The adaptors Tube and MyD88 both contain a death domain (DD) and are involved in the activation of the serine/threonine kinases pelle and IRAK-4 respectively. The protein interaction domain DD is also present in the C-terminal region of the TNF receptor (TNFR-1) and involved in TNF-mediated signalling for apoptosis (see page 489). The DD mediates selfassociation of these receptors, so directing the signal to downstream events.

The downstream targets of these two pathways are Dorsal and NF- B, both of which contain a REL-homology domain (RHD) that binds DNA at the B sequence motif (Figure 15.3) (first discerned in the kappa light chain enhancer region of immunoglobulin24). Classical NF- B is a transcription

The TIR domain is the target of an escape strategy of vaccinia virus. It encodes two TIRdomain proteins which block receptor signalling by sequestering MyD88 and TRIF.21

Proteins carrying a death domain are not necessarily involved in the process of death signalling (apoptosis). Examples of death domain proteins are IRAK, pelle, tube, ankyrin (cytoskeleton), MyD88, RIP (apoptosis), and TRADD.