Efficient compartmentalization in insect bacteriomes protects symbiotic bacteria from host immune system

Background: Many insects house symbiotic intracellular bacteria (endosymbionts) that provide them with essential nutrients, thus promoting the usage of nutrient-poor habitats. Endosymbiont seclusion within host specialized cells, called bacteriocytes, often organized in a dedicated organ, the bacteriome, is crucial in protecting them from host immune defenses while avoiding chronic host immune activation. Previous evidence obtained in the cereal weevil Sitophilus oryzae has shown that bacteriome immunity is activated against invading pathogens, suggesting endosymbionts might be targeted and impacted by immune effectors during an immune challenge. To pinpoint any molecular determinants associated with such challenges, we conducted a dual transcriptomic analysis of S. oryzae's bacteriome subjected to immunogenic peptidoglycan fragments. Results: We show that upon immune challenge, the bacteriome actively participates in the innate immune response via induction of antimicrobial peptides (AMPs). Surprisingly, endosymbionts do not undergo any transcriptomic changes, indicating that this potential threat goes unnoticed. Immunohistochemistry showed that TCT-induced AMPs are located outside the bacteriome, excluding direct contact with the endosymbionts. Conclusions: This work demonstrates that endosymbiont protection during an immune challenge is mainly achieved by efficient confinement within bacteriomes, which provides physical separation between host systemic response and endosymbionts.

Automated summary

This study reveals the active involvement of the bacteriome in the innate immune response of insects, showing the induction of antimicrobial peptides upon immune challenge. Surprisingly, the endosymbionts do not undergo any changes in their gene expression, indicating that they are protected from immune effectors mainly through confinement within the bacteriome. The physical separation between the host immune response and the endosymbionts ensures their survival during immune challenges.