Sensing infection in Drosophila: Toll and beyond

Abstract

Drosophila has evolved a potent immune system that is somewhat adapted to the nature of infections through the selective activation of either one of two NF-κB-like signalling pathways, the Toll and IMD (Immune deficiency) pathways. In contrast to the mammalian system, the Toll receptor does not act as a pattern recognition receptor (PRR) but as a cytokine receptor. The sensing of microbial infections is achieved by at least four PRRs that belong to two distinct families: the peptidoglycan recognition proteins (PGRPs) and the Gram-negative binding proteins (GNBPs)/β-glucan recognition proteins (βGRPs).

Introduction

Insects constitute a real success story of metazoan evolution. They originated in the Devonian and have evolved for over 400 million years. The number of insect species could be as high as 10 millions and they are found in all terrestrial ecological niches. Such an expansion would not have been possible in the absence of a sturdy immune system. Because of its small size, its remarkable fecundity, its short life cycle, Drosophila is an ideal model animal for the study of development and immunity. Its genome has been sequenced [1]. However, its major advantages are the existence of a knowledge database and the availability of genetic tools that have now been developed for almost a century. Large collections of mutant lines are publicly accessible and, although strenuous and labour-intensive, saturation mutagenesis of the fly genome by random chemical mutagenesis can be achieved. Even though directed knock-outs of selected genes is feasible, it has not been widely used to this day and RNA interference technology is often used to obtain a first approximation of the mutant phenotype.

Insect immunity relies on three major mechanisms. A wound triggers immediately several proteolytic cascades, one of which leads to the activation of the phenoloxidase pathway. Phenoloxidase enzymatic activity is required at multiple points to synthesize melanin that is deposited at the injury site. This process is thought to release cytotoxic reactive oxygen species that may affect invading microorganisms [2]. The molecules involved in detecting the septic injury and triggering this cascade are at present poorly known in Drosophila. The cellular arm of the immune response disposes of microorganisms through phagocytosis, and signals the presence of infection to other tissues [3], [4], [5], [6]. In Drosophila, phagocytosis is performed by the major blood cell type, the plasmatocyte [7]. In contrast, larger objects such as wasp eggs, are segregated from the internal cavity by the formation of a melanized capsule formed by another type of blood cells, the lamellocyte [8]. Thanks to biochemical and genetic analyses, the best known arm of the fly immune system is the humoral response. Its effectors are small, cationic peptides that display potent antimicrobial activities [9]. They can be grouped in seven families, each of which presenting a definite spectrum of antimicrobial activities. For instance, Drosomycin and Metchnikowin are active against filamentous fungi whereas Diptericin, Drosocin, Cecropins and Attacins are antibacterial compounds that target more effectively Gram-negative bacteria. Defensin is the only antimicrobial peptide that displays activity against Gram-positive bacteria. Their synthesis is inducible and regulated at the transcriptional level. Genetic analysis has revealed that the transcription of antimicrobial peptides is achieved by two distinct, albeit similar, NF-κB-like signal transduction pathways [10], [11], [12], [13] (Fig. 1). Drosomycin is essentially controlled by the Toll pathway with long-term kinetics whereas Diptericin and other antibacterial peptide genes are mostly regulated by the Immune deficiency (IMD) pathway (Fig. 1) [14]. Toll pathway mutants are susceptible to fungal and Gram-positive bacterial infections and are unable to synthesize Drosomycin during these infections whereas IMD pathway mutants are sensitive only to Gram-negative bacterial infections and fail to synthesize antibacterial peptides in response to these elicitors. The response to viral infection appears to rely largely on Toll- and IMD-independent mechanisms [15].

The Drosophila immune system is able to discriminate, at least to some degree, between several broad categories of microorganisms [16], [17]. Indeed, the IMD pathway is better induced by Gram-negative bacteria while the Toll-dependent response is mostly elicited by fungi and Gram-positive bacteria. These observations suggest that distinct pattern recognition receptors (PRRs) exist in the fly where they may sense microbial infections by binding to microbial elicitors and then trigger the appropriate immune response. In mammals, some ten members of the Toll-like receptor family may function as PRRs [18], [19]. In Drosophila, the Toll family of receptors comprises nine members [20]. In the following section, we review our current understanding of the functions of these Toll family genes in Drosophila. Next, we shall focus on two families of PRRs with established roles in the Drosophila immune response, the peptidoglycan recognition proteins (PGRPs) and the Gram-negative binding proteins (GNBPs)/β-glucan recognition proteins (βGRPs).