Gnathostomata

Figure I. Simplified evolutionary tree showing the major divisions into the protostome and dcuierostomc invertebrates. Vertebrates are derived from the deuterostome line yet certain ncuroendocrincimmunc characteristics have been shown lobe common among ihc members of these different group?. The protostome and deuterostome characterization is based upon the formation of the mouth. Modified from a version by Dr. Arnaud Lacoste, Post-Doctoral Fellow, Psychoncu-roimmunology Program UCLA (Based upon: Giribet, G. 2002 Current advances in the phylogcnetic reconstruction of melazoan evolution. A new paradigm for the Cambrian explosion? Molecular Phyiogenetics and Evolution. 24: 343-357).

Figure I. Simplified evolutionary tree showing the major divisions into the protostome and dcuierostomc invertebrates. Vertebrates are derived from the deuterostome line yet certain ncuroendocrincimmunc characteristics have been shown lobe common among ihc members of these different group?. The protostome and deuterostome characterization is based upon the formation of the mouth. Modified from a version by Dr. Arnaud Lacoste, Post-Doctoral Fellow, Psychoncu-roimmunology Program UCLA (Based upon: Giribet, G. 2002 Current advances in the phylogcnetic reconstruction of melazoan evolution. A new paradigm for the Cambrian explosion? Molecular Phyiogenetics and Evolution. 24: 343-357).

luiar pathogens against non-specific imrmi node fence [8]. Analysing mammalian innate immune responses has revealed similarities with the invertebrate immune system. For example, insects possess an acute response that resembles human responses, implicating similar effectors, receptors and regulation of gene expression. Mussels that arc inolhisks and most other invertebrates evolved intracellular phagocytosis resembling that effected by vertebrate neutrophils, using cati-onic antibacterial peptides in phagolysosomes. Vertebrate innate immunity resembles a mosaic of invertebrate immune responses [9],

13. Bilaterians and the origins of chordates

A more precise taxonomy is essential at this point fhat shows the position of the invertebrates and vertebrates (Fig. I). There are fwo major groups, (he pre-bilaterians and the bilateria, that are sub-divided themselves into two other major groups, the protostomes and deuterostomes. Among protostomes there are fwo subgroups, the lophotrocho/.oa, which includes the annelids (e.g., segmented earthworms Eisenta fetida and leeches) and mollusks (e.g., oysters, snails, Aplysia) and the ecdysozoa to which the arthropods (e.g., flies, lobsters, shrimp, crabs) and nematodes (e.g., round worms C. elegans) belong. The deuterostomes consists of the echinoderms (e.g., starfish), hemichordates and chordates (urochordates [e.g., tufiicates], cephalochordates and vertebrates [e.g., fish, amphibians, reptiles, birds and mammals]). Although not discussed

Table I General features of natural immunity in invertebrates.

Phagocytic cells; Encapsulation (granuloma/nodule formation see ERP)

Evidence of accelerated second set transplant rejection

Protease inhibitors; Lysozyine; Lysins

Agglutinins; C-Type lectins (Ca"-dependent lectins)

S-Type lectins (Galectins); Prophenoloxidase System (proPO)

Metal-binding proteins (cysteine-rich metallothioneins)

Cytokines (functional homologues; no sequence data)

Transferrin and lactoferrin

Bacterial Permeability Increasing Protein (BPI)

in great detail here, the dominant evidence with respect to innate immunity and its relation to vertebrates (mammalian) and plants concern findings in Drosophila and C. elegans. Arthropods, especially insects, represent an important group of invertebrates because of their sheer numbers. Vertebrates, in particular mammals and including humans, are really insignificant. Drosophila as in times past with respect to genetics and development provides ample evidence that arthropods possess effective innate immune responses. Over-generalizing about the immune system of all invertebrates based upon this single species may in the long run be unwise since for example examining metazoan phylogeny based on small subunit rRNA, will yield certain interesting revelations, i.e., relationships that are perhaps closer than using other criteria. With more intense analysis as has been accomplished for fruit flies and round worms, it may be revealed that mol-lusks for example have solved problems of immunodefence in a special manner and so may have the annelids or even the urochordates the vertebrate's closest relatives [10].

Evolution of distinctive innate immunity genes concerns the urochordates where the very recent draft genome of Ciona intestinalis, a tunicate, focuses on chordate and vertebrate origins. This report finds no evidence of the pivotal genes implicated in adaptive immunity, such as immunoglobulin, T cell receptor, and major histocompatibility complex (MHC) class I and II genes. Evidence is still needed to confirm the possible existence of highly divergent orthologs of one or more of these genes. The numerous authors of this report found a large number of possible complement genes, including Clq-like and C6-like genes, three Toll-like receptor genes, and a variety of lectin genes. An interleukin-1 (IL-1) receptor and an IL-17 receptor gene were identified [11], Genetic findings confirm a wealth of functional information performed long before defining genes became a prevailing technology in comparative immunology [2]. Invertebrates are endowed with natural immunity that is effected by cells and their humoral products, (Tables I + II). Cells synthesize and secrete molecular products, a survival strategy that protects invertebrates from microorganisms that they encounter in hostile, natural environments. These molecules lyse targets, naturally occurring microorganisms or experimentally introduced eukaryotic cells, including cancers providing a common effector mechanism in all invertebrates even one-cell protozoans [12-16], Amoebapore, isolated from Entamoeba histolytica, a parasite that causes life-threatening disease in humans, is a polypeptide that forms ion channels in target cell membranes [17]. Amoebapore resembles NK-lysin, the novel lytic effector molecule from porcine cytotoxic lymphocytes [18].

1.4 Natural immunity in arthropods: Drosophila

Although evidence is mounting from the use of other invertebrate models, the most widely studied innate immune system is that of Drosophila. Aside from the fruit fly and its immune system for itself, we have learned an enormous amount about related immuno-defence systems in

Table II Invertebrate innate immune components.

Component/Characteristic

Metazoan Model

Encapsulating-Relating Factors (ERPs)

Insect (Tenebrio molitor)

Eicosanoids

Insects

Inducible inflammatory system (Toll, NF-kB)

Insects/arthropods C elegans

Acute phase proteins: Pentraxins, Ig superfamily, complement

Insects/Arthropods

a, macroblobulin family (protease inhibitor)

Nematodes, crustaceans, molluscs, echinoderms,

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