Memory and Specificity in the Insect Immune System: Current Perspectives and Future Challenges

Abstract

The immune response of a host to a pathogen is typically described as either innate or adaptive. The innate form of the immune response is conserved across all organisms, including insects. Previous and recent research has focused on the nature of the insect immune system and the results imply that the innate immune response of insects is more robust and specific than previously thought. Priming of the insect innate immune system involves the exposure of insects to dead or a sublethal dose of microbes in order to elicit an initial response. Comparing subsequent infections in primed insects to non-primed individuals indicates that the insect innate immune response may possess some of the qualities of an adaptive immune system. Although some studies demonstrate that the protective effects of priming are due to a "loitering" innate immune response, others have presented more convincing elements of adaptivity. While an immune mechanism capable of producing the same degree of recognition specificity as seen in vertebrates has yet to be discovered in insects, a few interesting cases have been identified and discussed.

Keywords: adaptive immunity; immune memory; immune priming; innate immunity; insects.

Figure 1

Evidence of immune priming in insects has emerged in many different forms and to varying extents. (A) Infection of Manduca sexta larvae with non-pathogenic Escherichia coli leads to the upregulation of microbial pattern-recognition receptors and antimicrobial peptides such that the insect survives better against a secondary infection with a pathogenic microbe, such as Photorhabdus luminescens. (B) Queen Apis mellifera honeybees injected with heat-killed Paenibacillus larvae give rise to progeny, which contain as much as three times as many differentiated hemocytes and which survive better against P. larvae infection than honeybees whose parents had not been injected. (C) The Bombus terrestris immune response exhibits a great deal of memory and specificity after being primed with one of three pathogens (Pseudomonas fluorescens, Paenibacillus alvei, or P. larvae). Survival against a homologous secondary infection is increased; however, no change in survival is seen against heterologous secondary infections.

Figure 2

Anopheles gambiae Down syndrome cell adhesion molecule (AgDscam), a member of the Ig superfamily, generates semi-specific splice variants in response to various immune elicitors. In A. gambiae, immune elicitors such as Escherichia coli (yellow) and P. veronii (green) have been shown to lead to the generation of pathogen-specific splice variants (purple) of the germ line-encoded AgDscam. AgDscam (blue bar) contains four exons (black squares), which exhibit alternative splicing, capable of producing 31,920 different isoforms (represented in rows). When mosquitoes are exposed to various bacteria, the repertoire of AgDscam splice variants not only differ but also contain a majority of variants capable of binding to the bacteria (inside red square) to which the insect is exposed.