Abstract
Although several efforts have been made to describe the immunoendocrine interaction in fish, there are no studies to date focusing on the characterization of the immune response and glucocorticoid synthesis using the host–pathogen interaction on larval stage as an early developmental stage model of study. Therefore, the aim of this study was to evaluate the glucocorticoid synthesis and the modulation of stress- and innate immune-related genes in European sea bass (Dicentrarchus labrax) larvae challenged with Vibrio anguillarum. For this purpose, we challenged by bath full-sibling gnotobiotic sea bass larvae with 107 CFU mL−1 of V. anguillarum strain HI 610 on day 5 post-hatching (dph). The mortality was monitored up to the end of the experiment [120 hours post-challenge (hpc)]. While no variations were registered in non-challenged larvae maintained under gnotobiotic conditions (93.20% survival at 120 hpc), in the challenged group a constant and sustained mortality was observed from 36 hpc onward, dropping to 18.31% survival at 120 hpc. Glucocorticoid quantification and expression analysis of stress- and innate immunity-related genes were carried out in single larvae. The increase of cortisol, cortisone and 20β-dihydrocortisone was observed at 120 hpc, although did not influence upon the modulation of stress-related genes (glucocorticoid receptor 1 [gr1], gr2, and heat shock protein 70 [hsp70]). On the other hand, the expression of lysozyme, transferrin, and il-10 differentially increased at 120 hpc together with a marked upregulation of the pro-inflammatory cytokines (il-1β and il-8) and hepcidin, suggesting a late activation of defense mechanisms against V. anguillarum. Importantly, this response coincided with the lowest survival observed in challenged groups. Therefore, the increase in markers associated with glucocorticoid synthesis together with the upregulation of genes associated with the anti-inflammatory response suggests that in larvae infected with V. anguillarum a pro-inflammatory response at systemic level takes place, which then leads to the participation of other physiological mechanisms at systemic level to counteract the effect and the consequences of such response. However, this late systemic response could be related to the previous high mortality observed in sea bass larvae challenged with V. anguillarum.
Highlights
The innate immune response plays a pivotal role in the activation of the host defense mechanisms and determines the nature of the adaptive immune response [1]
Expression at basal level in all time points evaluated. These results indicate that at 120 hpc cortisol did not exert any modulatory effect on the expression of gr1, gr2, and hsp70 in sea bass larvae challenged with V. anguillarum at those time points
The expression of hepcidin was markedly augmented (1,027-fold increase) in the group challenged with V. anguillarum. These results suggest the increase in the expression of genes associated with bacterial clearance at 120 hpc in sea bass larvae challenged with V. anguillarum
Summary
The innate immune response plays a pivotal role in the activation of the host defense mechanisms and determines the nature of the adaptive immune response [1]. The components of the innate immune system are divided into physical (mucus layer, which acts as a physical and chemical barrier) [2]; cellular (phagocytic cells such as neutrophils and monocytes/macrophages); and humoral factors (based on pattern-recognition specificities or effector functions) [3] In this framework, the presence of receptors able to activate pathways responsible for the cell signaling cascade are crucial to promote a pro-inflammatory reaction, modulating the innate and adaptive immune response [4]. Pentraxins (C-reactive protein and serum amyloid protein) are a phylogenetically conserved superfamily of proteins characterized by the presence of around 200 amino acid–pentraxin domain in their carboxy-terminal region These proteins are considered lectins acting as a non-redundant component of the humoral arm of innate immunity mediating agglutination, complement activation, and opsonization [6]
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