Digestive enzymes, haemolysins and symbionts in the search for vaccines against blood-sucking insects.

Control of blood-sucking insects by vaccination of the host is an appealing possibility. In this overview the potential of digestive enzymes, haemolysins and symbionts as targets for vaccines are discussed. Blood-sucking insects are largely dependent on proteases for digestion. This is advantageous in the search for a vaccine as there is a considerable fund of biochemical knowledge for this group of molecules. Also there are a range of inhibitors available which can be used to mimic the effects of a vaccine providing a useful guide to the potential usefulness of such a vaccine before the considerable investment needed in producing one. Weighing against this, proteolytic enzymes by their nature will attack antibodies approaching them making it questionable if antibodies ingested by the insect could succeed before they themselves are destroyed by their target. Proteolytic enzymes are also poor immunogens, probably because they attack the antibody making machinery approaching them and because the complexes formed between the abundant vertebrate antiproteolytic molecules and proteases provide a different antigenic target to the native enzymes. Despite their relative paucity, evidence suggests that lipid digesting enzymes deserve attention as potential vaccine targets. Because about 80% of the nutrients in blood are locked up in its formed elements haemolysins are a strong potential target of a vaccine. The obligate haematophages rely on symbionts to provide essential nutrients and in consequence these are also potential vaccine targets in these insects.