Abstract

There is a resurgence of interest in the duality of the immune system: innate, natural, non-specific, non-anticipatory, non-clonal responses of invertebrates; adaptive, induced, specific, anticipatory, clonal responses of vertebrates. Natural immunity is considered the most primitive and found in both groups, although invertebrates do not possess the adaptive system. Using monoclonal antibodies to human adhesion molecules, earthworm, leech and sipunculan leukocytes are positive for many CD markers common to vertebrate leukocytes, especially macrophages and natural killer cells. In earthworms, only those leukocytes active as killers in cytotoxic responses are positive, whereas a larger primarily phagocytic leukocyte is negative. Growing functional evidence focuses on invertebrate cytokines but prevailing controversy concerns molecular structure and presumed homology. Most credible information has been derived from the tunicatcs especially with the appearance of the draft genome of Ciona intestinalis. The evolutionary history of complement can be traced from sea urchins to the teleosts and tetrapods, exhibiting at each level a corresponding increase in the numbers of complement components and duplications in complement pathways. Invertebrates and vertebrates seem to possess common signalling molecules e.g. neuropeptides. These signalling molecules act as immunomodulators in circulating blood. In vertebrates, release occurs during stress that triggers the hypothalamo-hypophyseal-adrenal (HPA) axis. Neuropeptides also act as conserved messengers that initiate and stimulate innate immune responses in invertebrates and in humans. This system probably evolved in "simple" animals. Cross talk between nervous and immune systems has an ancient evolutionary origin essential to homeostasis. Comparative analyses using simpler animal models have enriched these views.

1. INTRODUCTION: INNATE VS ADAPTIVE IMMUNITY 1.1 Early history and scope

The world of immunology was split when Metchnikoff discovered invertebrate phagocytosis. It was then that the monolithic field underwent a major change [1], Before Metchnikoff, immune systems were believed to be wholly humoral with little or no emphasis relegated to the role of cellular effectors (leukocytes) and the molecular products that they synthesize and secrete (mediators). Metchnikoff's discovery, however, initiated an enduring schism that we accept as fact; i.e., we added cellular immunity to the prevailing humoral immunity. Now almost 100 years later, this cellular component, and the animal models from which it was derived is, once again, at the centre of immunology. As a result, immunobiologists are more willing now to accept data and emerging concepts derived from intense analyses using invertebrate models than from work done in the fifty previous years when modern immunology was establishing itself. Knowing his strength of conviction, Metchnikoff would have savoured this intellectual turn of events! Because of his prescient influence and the discovery of phagocytosis we are now considerably further along toward a more complete understanding of the natural immune response and some of the mechanisms at the molecular and cellular levels. Otherwise we might not have emerged from what could have been a 19lh century one-way "dark alley".

The scope of this chapter alerts us to the utility of models that offer more clues to the nature of animal immune systems than is available from the utilization of vertebrate models alone. This chapter, will focus on two major themes: a) natural immunity emphasizing certain characteristics that seem to be common for invertebrates and vertebrates (e.g. CD markers, cytokines, and complement); b) neuroendocrine components; c) linkages between the immune and neuroendocrine systems. These proposed overlaps of the three regulatory systems can be seen in the simplified figure.

1.2. Organization of the immune system

We are witnessing a resurgence of interest in the dual functional and morphological division of the immune system: innate, natural, non-specific, non-anticipatory, non-clonal responses in contrast to the adaptive, induced, specific, anticipatory, clonal responses typical of vertebrates. This duality is realized at every level: molecules, cells, tissues, organs, and whole organisms. Metchnikoff's discovery of phagocytosis an invertebrate cellular immune mechanism in an echinoderm (sea stars) became applicable to vertebrates. Vertebrate adaptive immunity utilizes the macrophage, T/B lymphocyte system that depends upon rearranging genes (RAG), whereas invertebrate innate immunity depends upon activities of complex leukocytes and molecules (non-clonally derived) that they synthesize and secrete [2], Vertebrates have innate immunity and adaptive immunity and the innate capacity may act independently but overlap as a continuum with adaptive immunity [3-5]. Fearon and Locksley proposed that vertebrate natural or innate immunity effects a rapid, non-specific, incomplete antimicrobial host defence, until the slower, more specific acquired immune response develops [6]. Innate immunity may effect a supplementary role in determining which antigens the adaptive immune system responds to and the nature of that response. Innate immunity has also been considered vis a vis its cell products, those ubiquitous antimicrobial peptides [7]. Yet another view classifies vertebrate immunity as a specialized inflammatory response against infections. Adaptive immunity of vertebrates is considered to have evolved in response to counter measures successfully used by intracel-

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