The specifically enhanced immune response of oyster Crassostrea gigas against the secondary encountered same pathogen

The innate immune cells in invertebrates can mount an enhanced immune response upon re-exposure to the same pathogen, which is referred to 'immune priming', similar to the 'trained immunity' in vertebrates. However, whether this enhanced immune effects during immune priming is pathogen-specific remains contradictory. In the present study, the specificity of immune response was investigated in the Pacific oyster (Crassostrea gigas) which received pre-stimulation with either Vibrio splendidus or Staphylococcus aureus followed by a secondary challenge with V. splendidus. After the secondary stimulation with V. splendidus, phagocytic activity of haemocytes against V. splendidus, rather than against S. aureus, was significantly higher in the V. splendidus-primed group compared to that of S. aureus-primed group, while there was no significant difference in the phagocytic activity of haemocytes against S. aureus or fluorescent microspheres. The mRNA expression levels of CgβGBP2, CgC1qDC-8, CgRab24, CgBigdefensin1 and CgBigdefensin2 and nitric oxide (NO) level in haemocytes all significantly up-regulated in the oyster primed with V. splendidus compared with those of S. aureus-primed group. The lysozyme levels in the hepatopancreas and cell-free haemolymph in the V. splendidus-primed group were also increased, concomitant with stronger inhibitory effect of cell-free haemolymph on the growth of V. splendidus than that of S. aureus-primed group. Moreover, both of the first V. splendidus and S. aureus stimulation significantly increased histone modification levels including H3K4me1, H3K4me3, H3K9ac and H3K27ac. These results indicated that pre-stimulation with a microbe could specifically induce enhanced cellular and humoral immunity in oyster C. gigas upon re-exposure, which provided a fresh idea for understanding the innate immune memory in invertebrates.