LPS induces complement gene expression in absence of acute-phase proteins in the liver of Cururu toads (Rhinella diptycha)

Amphibians face threats from emerging bacterial, viral, and fungal pathogens, and their innate immune response plays a key role in early defense. The liver is a central immune organ that produces acute-phase proteins (APPs) and complement system proteins, which are essential for early pathogen neutralization and a positive disease outcome. Here, we investigated the temporal dynamics of hepatic gene expression for APPs [C-reactive protein (CRP), serum amyloid A (SAA), fibrinogen-like protein 1 (FGL1)] and complement components (C1s, C3, C4, C5, C9) in Rhinella diptycha toads injected with lipopolysaccharide (LPS) or saline. Using RT-qPCR at four time points post-injection (1h, 3h, 6h, and 18h), we detected a significant effect of LPS on C1s expression and a treatment x time interaction for C4, C5, and C9, indicating time-dependent activation of different complement proteins. In contrast, we found no evidence for LPS-induced changes in APPs expression. However, several APPs were positively correlated with complement components, suggesting coordinated regulation even in the absence of differential expression. Our findings provide a temporal analysis of hepatic acute-phase responses to LPS in Rhinella diptycha toads, unraveling a complex network of complement system activation. By showing that complement components, rather than APPs, dominate the early hepatic response to a bacterial antigen in a widespread Neotropical toad species, our study provides a temporal and mechanistic baseline for future investigations of amphibian innate immunity and underscores the need for future work to determine how these early pathways respond to different pathogens and contribute to variation in disease outcomes among amphibians.