The Very Defensive Caterpillar

Caterpillars are bleeding defensive! Insects are known to lack an antibody-mediated immune response, and research in caterpillars has recently shown that, instead, they produce protective proteins in response to bacterial infection. The pattern recognition receptors (PRR) and antibacterial effectors produced at a first infection still function to protect against a repeated challenge. These results raise important issues in insect research which will be reported by Dr. Ioannis Eleftherianos at the Society of Experimental Biology Annual Meeting at the Universitat Autonoma de Barcelona on Wednesday 13th of July.

After being fed on a diet of antibiotics, hawkmoth (Manduca sexta) caterpillars were infected by non-pathogenic bacteria (E. coli), followed by exposure to a second, but lethal insect pathogen (Photorhabdus). Investigation of their blood then showed that antibacterial peptides were being produced by the so-called ‘fat-body’, an organ specialised in protein production. These proteins appear to be able to persist from the initial benign E. coli infection and then confer resistance against the second, usually lethal, infection by the pathogen. Using RNAi techniques workers at the University of Bath have shown that several different proteins can confer this protective effect against subsequent infections.

Micro-organisms such as bacteria, fungi and nematodes are often used as biocontrol agents against insects. It had always been assumed that insects in the field would be naïve to such control agents but these results raise the possibility that control with one pathogen may confer resistance to another. Most experiments on insect immunity are conducted in the laboratory on insects often fed on antibiotic containing diet so these results suggest that the immune response of insect constantly exposed to pathogens in the field may be very different from all the work described in the laboratory. This raises new challenges for the field and places into question the relevance of laboratory based studies on immunity.