Immune priming of honey bees protects against a major microsporidian pathogen

Immune priming (IP) is an emerging approach, and we provide the first evidence that the immune priming of an insect can protect against a microsporidian infection. Honey bees were immune primed with heat-killed Nosema ceranae spores as larvae grown in vitro (E1), larvae fed in vivo by nurse bees inside field colonies (E2), 1-day-old adults reared in incubated in vitro cages (E3), or as 1-day old adults kept in field colonies in vivo (E4). Immune-primed bees subsequently challenged with live spores had significantly lower infection levels and rates than control bees fed live spores without immune priming. IP efficacy at reducing infection was higher in larvae than in adults and in vitro than in vivo (E1>E2>E3>E4). However, in E4, IP bees that were challenged had higher survival and lower infection rates than challenged bees without IP. Interestingly, IP alone reduced life spans in E2, and E3. The expression of Toll pathway genes associated with Nosema infection was differentially altered by IP in some experiments, with defensin-1 expression showing the most consistent patterns (E1 and E2). These results suggest that immune priming can protect honey bees from microsporidian infection and deserves further study as a tool against insect pathogens.