Hypochlorous acid and taurine chloramine co-mediate the innate arm of anti-Vibrio immunity in oyster hemocytes

Balanced and temporally coordinated generation of reactive oxygen species (ROS) and antioxidants is crucial to productive clearance of pathogens in marine molluscan invertebrates, such as the oyster. Despite the presence of an oyster homolog of the myeloperoxidase (MPO) gene in its genome and a hyperabundant pool of intracellular taurine, it remains unclear how precisely ROS-based antibacterial immunity and pro-survival antioxidant production are orchestrated during microbial infections. Herein, we leveraged HKOCl-3, an established probe selective for hypochlorous acid (HOCl), to visualize the dynamics of bactericidal HOCl production in oyster hemocytes challenged with Vibrio parahaemolyticus, a clinically significant Gram-negative pathogen. We hypothesized that taurine (2-aminoethanesulphonic acid), an organosulfur osmolyte that scavenges HOCl efficiently, forms taurine chloramine (TauCl) in vivo to modulate hemocytic immune response, while mitigating host injury. In molecular imaging and flow cytometry, rapid production of HOCl was observed following V. parahaemolyticus challenge. TMB (3,3′,5,5′-tetramethylbenzidine) assay confirmed the concomitant formation of TauCl in oyster hemocytes under V. parahaemolyticus stimulation. This TauCl biosynthesis was executed by the cysteine dioxygenase and cysteine sulfinic acid decarboxylase (CDO/CSAD) pathway, as verified by qRT-PCR. The potency of TauCl bactericidal activity was broadly comparable to that of HOCl in bacterial clearance assay. As a pro-survival agent, TauCl mitigated apoptosis in V. parahaemolyticus-challenged hemocytes, while this effect could be nullified by the chloramine scavenger bilirubin. In terms of transcriptomic changes, TauCl stimulated the phagosomes, endocytosis, FoxO signaling pathways, and induced upregulation of various antioxidant enzymes, thereby favoring redox homeostasis in hemocytes. Activation of pro-survival transcription was achieved by TauCl direct binding to Keap1. Overall, our work demonstrated for the first time how phagocyte-derived HOCl and TauCl act as immune effectors within an intimately overlapping window of host defense, with theoretical implications for understanding ROS metabolism and immunomodulation in broader disease contexts.