Immune responsiveness in vector insects.

Abstract

Insects have been particularly successful in evolution, and current estimates are that they represent three-quarters of all extant animal species. With the marked exception of the seas, insects occupy nearly all ecological niches on earth and hence are confronted to innumerable potential pathogenic bacteria, viruses, fungi, and protozoan and helminth parasites. Not surprisingly therefore, insects have developed efficient host defense mechanisms. The current view is that the insect host defense is multifaceted and involves cellular reactions, namely phagocytosis and capsule formation by blood cells and a systemic response evidenced by the rapid and transient synthesis of a battery of potent, small cationic antimicrobial peptides. After septic injury, these molecules predominantly are produced in the fat body, an equivalent of the mammalian liver, and in some blood cells. They accumulate in the blood of infected insects where they oppose the development of invading microorganisms. Additional defense reactions in insects are blood coagulation and melanization, which occur at the sites of injury as a result of almost immediate activation of proteolytic cascades. It is speculated that some of the products of these cascades can activate the synthesis of antimicrobial peptides in the fat body and blood cells. Remarkably, the signaling cascades that lead to antimicrobial peptide gene expression in these responsive tissues show significant structural and functional similarities with those involved in the cytokine-induced expression of mammalian acute phase proteins (see refs. 1–4 for review).