Self-harm caused by an insect's innate immunity

Abstract

It has been a long-held assumption that the innate immune system of insects causes self-harm when used to combat an immune insult. We show empirically that this assumption is correct. Invertebrate innate immunity relies heavily on effector systems which, on activation, produce cytotoxins that kill pathogens. Reliance on these robust, fast-acting, generic killing mechanisms ensures a potent and rapid response to pathogen invasion, but has the potential disadvantage of causing self-damage. We show that the innate immune response against an immune insult produces measurable phenotypic and functional damage to self-tissue in the beetle Tenebrio molitor. This type of self-harm (autoreactivity) and the life-history implications that arise from it are important to understand evolutionary phenomena such as the dynamics between hosts and parasites as well as the nature of immune system costs.

Figure 1

Melanization of Malpighian tubule implants (mean+1 s.e.) measured as reduction in weighted average luminescence in the tubule between implantation and harvesting after 24 h (n=12 for each treatment). There is a highly significant effect of treatment (ANOVA, F_2,32=24.00, p<0.001) and all treatment groups differ significantly from each other (Bonferroni-corrected pairwise t-test, p<0.01).

Figure 2

Malpighian tubule function 24 h after treatment. The fluid secretion rate in control animals is significantly (T₁₄=−3.4, p=0.004) higher than in animals subjected to a single nylon implant for 24 h (mean+1 s.e.).