Time-of-Day-Dependent Host Defence Against Edwardsiella Infection in Medaka (Oryzias latipes)

Circadian rhythms regulate immune function in vertebrates; however, their contribution to infection outcomes in teleost fish remains unclear. Here, we examined whether infection timing influences host defence against Edwardsiella piscicida in Japanese medaka (Oryzias latipes). Fish were challenged by immersion with E. piscicida at ZT2 or ZT14 under a 12 h:12 h light-dark cycle, and survival was monitored for 16 days. To evaluate early immune responses, the expression of Toll-like receptors (TLRs) and pro-inflammatory cytokines in head kidney was analysed 6 h after infection by quantitative real-time PCR. Mortality began on day 3 post-infection, and survival differed significantly based on infection time: fish infected at ZT2 showed 75% survival, whereas those infected at ZT14 exhibited 25% survival. Expression levels of tlr1, tlr5m, tlr21, il1b and tnfa were significantly higher in fish infected at ZT2 than in those infected at ZT14, while tlr9 showed a similar, but non-significant trend. These results indicate that early induction of pathogen recognition receptors and inflammatory cytokines in medaka differs according to the time of infection and is associated with different outcomes in host survival. Our findings suggest that time-of-day-dependent factors are important determinants of E. piscicida infection outcomes in teleost fish.