Traditionally, immune memory was thought to be restricted to vertebrates; such adaptive immunity is the reason why early vaccination protects us from later infection. However, a large body of recent work suggests that many insects also show a form of immune memory, whereby exposure to a low dose of a pathogen improves an individual’s response to a subsequent infection with the same pathogen. Although we do not understand the physiological and molecular mechanisms responsible for such immune “priming”, the response may be costly and is often specific to the pathogen (even the strain), suggesting that priming may be an adaptive trait. If so, the strength of the priming response should vary depending on, e.g., selection imposed by local pathogens and the specific costs of mounting the priming response. However, no one had quantified variation in the priming response. We tackled this problem in our latest paper in Ecology and Evolution.
Imroze and Arun did an enormous study to measure the immune priming response of 10 different wild-collected populations of flour beetles as a function of sex and life stage of priming. As predicted, they found substantial variation in the priming response across populations. Interestingly, they also found that ontogenic priming (when larvae are primed, and then infected as adults) conferred the largest survival benefits compared to within-stage or trans-generation priming. These are exciting results because they show that priming is not a ubiquitous response, and set up clear predictions about the relative benefits and costs of priming as a function of stage and sex. The results are also consistent with (though they do not directly support) the hypothesis that priming should evolve under local, specific and strong pathogen pressure. We are now moving ahead with efforts to determine factors responsible for the observed variability in the priming response. Watch this space for more!