Transcriptome features of innate immune memory in Drosophila

Immune memory is the ability of organisms to elicit potentiated immune responses at secondary infection. Current studies have revealed that similar to adaptive immunity, innate immunity exhibits memory characteristics (called innate immune memory). Although epigenetic reprogramming plays an important role in innate immune memory, the underlying mechanisms have not been elucidated, especially at the individual level. Here, we established experimental systems for detecting innate immune memory in Drosophila melanogaster. Training infection with low-pathogenic bacteria enhanced the survival rate of the flies at subsequent challenge infection with high-pathogenic bacteria. Among low-pathogenic bacteria, Micrococcus luteus (Ml) and Salmonella typhimurium (St) exerted apparent training effects in the fly but exhibited different mechanisms of action. Ml exerted training effects even after its clearance from flies, while live St persisted in the flies for a prolonged duration. RNA sequencing (RNA-Seq) analysis revealed that Ml training enhanced the expression of the immune-related genes under the challenge condition but not under the non-challenge condition. In contrast, St training upregulated the expression of the immune-related genes independent of challenge. These results suggest that training effects with Ml and St are due to memory and persistence of immune responses, respectively. Furthermore, we searched for the gene involved in immune memory, and identified a candidate gene, Ada2b, which encodes a component of the histone modification complex. The Ada2b mutant suppressed Ml training effects on survival and disrupted the expression of some genes under the training + challenge condition. These results suggest that the gene expression regulated by Ada2b may contribute to innate immune memory in Drosophila. Author summary Innate immune memory is a memory-like character of the innate immune system and is one of the current hot topics, because of a great possibility for clinical applications. It is known that Bacillus Calmette Guerin (BCG) vaccine promotes the protection against not only tuberculosis but also a wide variety of pathogens in heterologous manners. Recent studies have demonstrated that innate immune memory is responsible for the vaccine effects of BCG. It has also been revealed that epigenetic reprogramming plays important roles in innate immune memory. However, the mechanisms underlying it are not fully understood yet. It is also unclear how much the mechanisms of innate immune memory are evolutionarily conserved among organisms. Here we established an experimental system for detecting innate immune memory in Drosophila. We found that training infection of low-pathogenic bacteria enhanced the survival rate of flies after challenge infection of high-pathogenic bacteria. Especially, Micrococcus luteus (Ml) and Salmonella typhimurium (St) showed apparent training effects in fly. Our RNA-Seq analysis revealed that training effects of Ml and St could be attributed to memory and persistence of immune reactions, respectively. We also identified Ada2b, a component of the histone modification complex, as a factor involved in innate immune memory.

Automated summary

Immune memory is the ability of organisms to activate enhanced immune responses upon secondary infection. This study established an experimental system in Drosophila to detect innate immune memory. Training infection with low-pathogenic bacteria increased the survival rate of flies upon challenge infection with high-pathogenic bacteria. RNA sequencing analysis revealed that different bacteria have different mechanisms of action and that the gene Ada2b is involved in innate immune memory.