Local immune response induced by intra-fin antigen injection in Japanese medaka (Oryzias latipes) is a useful model for immunological studies

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Teleost fishes play a pivotal role in advancing our understanding of immune system evolution because they retain the ancient characteristics of vertebrate immunity, encompassing both innate and adaptive immune systems. Among these, innate immunity plays a critical role in fish as the first line of defense, coordinating rapid responses to pathogen infections. However, the lack of fish-specific immunological methodologies has limited progress in elucidating fish immune mechanisms. To better understand how the innate immune response develops and resolves in fish, detailed observation and integrative analysis of leukocytes at multiple time points is necessary. In the present study, an intra-fin injection method for observing local immune responses in Japanese medaka ( Oryzias latipes ) was tested and optimized to analyze the progression of zymosan-induced innate immune responses. Zymosan-injected medaka showed a rapid immune response characterized by leukocyte recruitment and phagocytosis. Using TG(F mpx P:mCherry) transgenic medaka with mCherry fluorescence driven by myeloperoxidase ( mpx ) promoter, granulocyte chemotaxis towards the site of zymosan entry was successfully visualized. The rapid increase in tumor necrosis factor α ( tnfa ), interleukin-1β ( il1b ), interleukin-6 ( il6 ) , and CXC motif chemokine ligand 8 ( cxcl8 ) expressions in zymosan-injected anal fins provided a molecular basis for the visualized tissue-specific cellular response. Our study underscores the dynamic orchestration of immune components during the innate immune response in Japanese medaka and highlights their potential as a promising model for immunological research. • An ‘inter-ray injection’ method was optimized to micro-scale for medaka. •Local immune response was measured both qualitatively and quantitatively in medaka. •The progression of zymosan-induced immune response was visualized in medaka. •Use of TG(F mpx P:mCherry) transgenic medaka enables imaging of immune response. •Multiple timepoint analyses covered initiation to resolution phases.