The Modification of H3K4me3 Enhanced the Expression of Cg TLR3 in Hemocytes to Increase Cg IL17-1 Production in the Immune Priming of Crassostrea gigas .

Increasing evidence confirms that histone modification plays a critical role in preserving long-term immunological memory. Immune priming is a novel form of immunological memory recently verified in invertebrates. Toll-like receptor (TLR) signaling and cytokines have been reported to be involved in the immune priming of the Pacific oyster Crassostrea gigas . In the present study, the expression of Toll-like receptor 3 ( Cg TLR3), myeloid differentiation factor 88-2 ( Cg Myd88-2) and interleukin 17-1 ( Cg IL17-1) was found to be elevated in the hemocytes of C. gigas at 6 h after the secondary stimulation with Vibrio splendidus , which was significantly higher than that at 6 h after the primary stimulation ( p < 0.05). A significant increase in histone H3 lysine 4 trimethylation (H3K4me3) enrichment was detected in the promoter region of the Cg TLR3 gene at 7 d after the primary stimulation with inactivated V. splendidus ( p < 0.05). After the treatment with a histone methyltransferase inhibitor (5'-methylthioadenosine, MTA), the level of H3K4me3 at the promoter of the Cg TLR3 gene decreased significantly at 7 d after the primary stimulation with inactivated V. splendidus ( p < 0.05), and the expression of Cg TLR3, Cg MyD88-2 and Cg IL17-1 was significantly repressed at 6 h after the secondary stimulation with V. splendidus ( p < 0.05). Conversely, the treatment with monomethyl fumarate (MEF, an inhibitor of histone demethylases) resulted in a significant increase in H3K4me3 enrichment levels at the Cg TLR3 promoter at 7 d after the primary stimulation ( p < 0.05), and the expression of Cg TLR3, Cg MyD88-2 and Cg IL17-1 was observed to increase significantly at 6 h after the secondary stimulation ( p < 0.05). These results suggested that H3K4me3 regulated MyD88-dependent TLR signaling in the hemocytes of C. gigas , which defined the role of histone modifications in invertebrate immune priming.