chapter 1 - Bentham Science

Immune Response of Insects and Crustaceans HPFP: Recent Advances in Insects and Other Arthropods Vol. 1 63
excellent model organism to decipher the basic principles of innate immune responses [5]. Following the
discovery of AMPs, which are infection-dependent inducible effector molecules, the ability to monitor
AMP expression was rapidly established [6]. Genetic screening of these AMP reporter systems has led to
the identification of two key nuclear factors (NF-B), DIF (dorsal-related immunity factor) and Relish,
which control two distinct innate immune signaling pathways, the Toll and immune deficiency (imd)
pathways, respectively [7, 8]. The Toll pathway is activated by fungal and Gram-positive bacteria and the
imd pathway is predominantly activated by Gram-negative bacteria, which are mechanistically conserved in
the mammalian Toll-like-receptor/Interleukin-1 receptor and tumor necrosis factor (TNF) receptor
signaling pathways, respectively [5]. Subsequent genetic and RNAi screens led to the functional
characterization of PRRs and proteases involved in activating the Toll pathway or melanization. The Janus
kinase-signal transducers and activators of transcription (JAK-STAT) pathway, which were originally
found to be involved in developmental processes, are also involved in the antiviral response, tissue damage
recovery, and hemocyte proliferation and differentiation [9, 10]. A recent proteomic analysis also identified
crucial factors involved in hemolymph coagulation [11].
The horseshoe crab, belonging to the class of Merostomata, is commonly found in East Asia and North
America. It is called a living fossil and there are at least four known species (Limulus polyphemus,
Tachypleus tridentatus, Tachypleus gigas, and Carcinoscorpius rotundicauda). The horseshoe crab is
highly useful for biochemical analysis and several hundred milliliters of hemolymph can be obtained from
one horseshoe crab without threatening its life, and the largest single population of hemocytes in the
hemolymph, granular hemocytes (~99%), is extremely sensitive to bacterial endotoxin LPS [12]. In the
1990s, using T. tridentatus whose habitats are coastal areas of the Kyushu Islands in Japan, Professor
Iwanaga’s group at Kyushu University successively identified novel hemolymph proteins and clarified the
detailed molecular mechanisms of the hemolymph coagulation cascades [13-15].
In this review, we provide an overview of current advances in our understanding of invertebrate innate
immunity, based on studies performed mainly in Drosophila and horseshoe crab. Further elucidation of the
detailed molecular mechanisms on both cellular and humoral innate immune reactions will enhance our
understanding of the complexities of mammalian innate immune reactions.
RECOGNITION OF PATHOGENS AND DANGERS
The ingenious predictions of Professor Charles Janeway (1943-2003) led to the pattern recognition theory
(e.g., host cells can discriminate ‘non-infectious self’ from ‘infectious non-self’) and it is now established
that PAMP-PRR interactions are key initial processes in activating innate immune responses [1-4].
As summarized in Fig. 1, the innate immune response comprises multiple steps. Although this picture
represents a combined overview from Drosophila and horseshoe crab studies, the basic principles can be
applied to almost all metazoans, including mammals. Once host cells sense either a pathogen (e.g., PAMPs)
or danger signal (damage-associated molecular pattern; DAMP), two pillars of the innate immune system,
cellular and humoral responses are initiated to activate numerous defense reactions such as phagocytosis,
encapusulation, AMPs and effector gene expression, and clot and melanization formation (Fig. 1). Using
appropriate combinations of these defense reactions, hosts can efficiently eliminate invading pathogens and
maintain homeostasis.
Among various types of PRRs, one of the hallmark signaling PRRs in Drosophila is the peptidoglycan
recognition receptor (PGRP) family, which detects peptidoglycans (PGN), an essential cell wall component
in nearly all bacteria [16, 17]. The name PGRP was first introduced by Professor Ashida’s group [18] who
purified a 19-kDa protein from silkworm (Bombyx mori) hemolymph based on its high binding affinity to
bacterial PGNs and demonstrated its involvement in activating prophenoloxidase (proPO) cascades [18].
Subsequent cDNA cloning studies revealed that this PGRP family is evolutionarily conserved from insects
to mammals. Several recently completed genome projects have also demonstrated at least 13 PGRPs with
19 spliced variants in D. melanogaster, 7 PGRPs with 9 spliced protein variants in Anopheles gambiae, and
4 PGRPs in both humans and mice [19-21]. All PGRPs have a conserved PGRP-domain (~165 amino acid