Proteomic and Functional Analysis Reveals Temperature-Driven Immune Evasion Strategies of Streptococcus iniae in Yellowfin Seabream (Acanthopagrus latus)

Simple SummaryStreptococcus iniae(S. iniae) is a major aquaculture pathogen causing significant economic losses. While infections show seasonal patterns tied to water temperature, the mechanisms behind its thermal adaptation remain unclear. This study explored the proteomic changes inS. iniaeat different temperatures and their effects on immune responses in yellowfin seabream (Acanthopagrus latus,A. latus). Results demonstrated that elevated temperatures markedly upregulated bacterial virulence factor expression, resulting in significantly higher host mortality rates. Furthermore, thermal stress enhances the destruction of key immune functions of the host byS. iniae, including immune cell survival, ROS production, and phagocytic activity. These findings provide novel insights into the heat-regulated immune evasion strategies ofS. iniae, offering valuable theoretical foundations for developing temperature-aware disease management protocols in aquaculture systems.Streptococcus iniae(S. iniae) is a globally significant aquatic pathogen responsible for severe economic losses in aquaculture. While theS. iniaeinfection often exhibits distinct seasonal patterns strongly correlated with water temperature, there is limited knowledge regarding the temperature-dependent immune evasion strategies ofS. iniae. Our results demonstrated a striking temperature-dependent virulence phenotype, with significantly higherA. latusmortality rates observed at high temperature (HT, 33 °C) compared to low temperature (LT, 23 °C). Proteomic analysis revealed temperature-dependent upregulation of key virulence factors, including streptolysin S-related proteins (SagG, SagH), antioxidant-related proteins (SodA), and multiple capsular polysaccharide (cps) synthesis proteins (cpsD, cpsH, cpsL, cpsY). Flow cytometry analysis showed that HT infection significantly reduced the percentage of lymphocyte and myeloid cell populations in the head kidney leukocytes ofA. latus, which was associated with elevatedcaspase-3/7expression and increased apoptosis. In addition, HT infection significantly inhibited the release of reactive oxygen species (ROS) but not nitric oxide (NO) production. UsingS. iniaecps-deficient mutant, Δcps, we demonstrated that the cps is essential for temperature-dependent phagocytosis resistance inS. iniae, as phagocytic activity against Δcps remained unchanged across temperatures, while NS-1 showed significantly reduced uptake at HT. These findings provide new insights into the immune evasion ofS. iniaeunder thermal regulation, deepening our understanding of the thermal adaptation of aquatic bacterial pathogens.