Invertebrate CD96 inhibits metabolic epigenetic regulatory axis upon sensing microbes to suppress inflammation

Mammalian CD96 is a novel immune checkpoint expressed on NK or activated T cell surface, playing important roles in immune surveillance. In the present study, CD96 homolog is identified from oysters and exhibits binding activities towards various polysaccharides and microbes. CgCD96 upon binding V. splendidus recruits CgSHIP2 to gill cell membrane, which then transduces inhibitor signals to reduce CgERK phosphorylation, ultimately leading to the decrease in LDH activities as well as Lactate, ACSS2 and KAT2A contents, while increase in pyruvate and α-KG contents. After LDH activity is inhibited, the generation of lactate decreases and pyruvate accumulates, leading to a decline in H3K4me2/3 and H3K18la levels, which eventually suppress the inflammatory responses. These results indicate that invertebrate CD96 is conserved with some unique features, which function as an immune checkpoint to sense microbes and regulate the inflammatory responses through special mechanism by inhibiting ERK-α-KG-H3K4me2/3 and ERK-Lactate-H3K18la axis. The findings have profound implications for comprehensive understanding of the immunosuppressive role of CD96 as well as the novel inflammatory regulatory mechanism. Mechanistic characterization of oyster CD96 sheds light on the role of this invertebrate immune checkpoint and its downstream ERK-metabolic-epigenetic regulatory axis in the context of microbial-induced inflammatory regulation.