Peptides: An Emerging Force in Host Responses to Parasitism

Abstract

This chapter provides an overview of the impact of parasites on the endocrine system of their host. The review is concerned primarily with helminth parasites of vertebrates and specifically parasites that inhabit the gastrointestinal tract, because that is the area in which the overwhelming bulk of the experimental work has been carried out. In preparing the review, however, it soon became apparent that to discuss endocrine responses in isolation would be a very limiting exercise. In addition to causing changes in the hormonal status of the host, parasites elicit immune responses from the host, induce pathological changes in host tissues, and alter the normal functioning of the gut by disrupting neuroendocrine control mechanisms, epithelial cell functioning, and differentiation of mucosal cells, for example. Moreover, it is becoming increasingly apparent that the various facets of the host response are interlinked with each other and—of particular significance to this review—are involved with peptides, because peptidergic molecules are common to the endocrine, nervous, and immune systems. Consequently, the chapter will follow four main themes. Initially, it will briefly review pathophysiological changes in the gut resulting from parasitism. It will then examine the roles of gut neuroendocrine and immune systems in these responses, highlighting the disruption of peptidergic mechanisms in both the gut endocrine and nervous systems. The review will then consider interactions between the neuroendocrine and immune systems, first establishing general principles, then discussing data relating specifically to peptides and cytokines. Finally, an attempt will be made to draw together the different themes of the review and explore what progress has been made in unravelling connections among the main tissues discussed (namely, endocrine system, nervous system, immune system, and epithelium plus muscle). A number of schemes have been proposed to integrate such interactions into the overall picture of the host response. The models will be examined to determine how comprehensive they are and any weaknesses exposed will serve to highlight potential lines of future research.