Introduction:
Vaccination is known as the most efficient and adequate method to induce immune activation and thus protecting fish against pathogens since fishes live in an antigen-enriched environment (Tafalla et al., 2013). Beside the importance of the systemic immune system to protect fish against pathogens in the environment, mucosal-associated lymphoid tissues (MALTs) are also implicated in protection and play a crucial role in maintaining homeostasis (Parra et al., 2015; Khansari et al., 2018). There is also an extensive communication between the neuroendocrine and immune system where activation of neuroendocrine system elicits immune activation, thus either suppressing or enhancing the immune response. Most of our knowledge on the effect of vaccination in rainbow trout comes from studies in freshwater, but transfer of the fish to seawater after vaccination can be hypothesized to be a challenge for the fish that influences the immune response. In the present study, we aimed to compare how the mucosal immune system after long-term vaccination is affected by stress in both fresh- and seawater acclimated fish.
Material and Method:
Juvenile rainbow trout weighing on average 70 g was obtained from a local hatchery (Vänneåns fish farm AB, Knäred, Sweden). The fish were held in recirculating tanks at ca 10°C and fed daily with ≤2% of body weight. At the start of experiment, fish were immunized with Alpha Ject 3000 or injected with saline as controls. Starting 12 days post injection (dpi), salinity was gradually increased to 31 ppt over 10 days for half of the vaccinated and control fish, while the other half (vaccinated and non-vaccinated) remained in fresh water. To stress fish, they were chased for 5 min and after 48 hours of recovery, all fish were euthanized, length and weight measured, and skin mucus, blood and tissues skin, gills and gut were collected for further analysis.
Results and discussion:
Our results show an overall significant interaction between salinity groups (FW and SW) and treatments. The cortisol level in skin mucus were significantly higher in the stress and in vaccine+stress fish in both fresh water (FW) and sea water fish (SW) groups (Figure 1). In addition, cortisol was augmented in the vaccine+stress group in SW versus FW (Figure 1). This reflects a general stress response of animals to external stimuli that results in activation of the hypothalamic-pituitary-interrenal axis (HPI) axis to secretes cortisol by head kidney interrenal cells (Schreck and Tort, 2016). It is not yet clear if cortisol secretion at mucosal levels is delivered by the circulation or is secreted by local cells since cortisol secretion by immune cells has also been demonstrated. However, previous results showed that abiotic and biotic stressors are capable of inducing cortisol release in both systemic and mucosal system (Khansari et al., 2018; Ordóñez-Grande et al., 2020) and therefore cortisol has proven to be a reliable stress indicator.
Genes transcriptions were also analysed, and they were classified in three groups including stress related genes (gr1, gr2, mr, β-ar, hsp70), innate immune related genes (il1β, il6, tnfα, il10, tgfβ1, nadph oxidase, c3, enolase and lysozyme) and adaptive immune related genes (igm, igt and cd8). In the gills of FW fish, stress and vaccine+stress enhanced the transcription level of il1β. The il1β transcript expression in stress and vaccine+stress was significantly lower in SW versus FW. In the skin, FW (stress and vaccine+stress) and SW (vaccine) fish, the mRNA level of il1β was decreased, whereas a significant increase was observed for the vaccine+stress group in SW. There was a significantly higher il1β expression in the skin of the stress and vaccine+stress groups in SW compared with FW. In the intestine, vaccine and vaccine+stress was found to induce il1β gene expression in FW while no alteration was registered in SW. The same expression pattern as gills was observed in intestine; a suppression of il1β transcript expression in the vaccine and vaccine+stress groups in SW (Figure 1). The results support that an inflammatory response was induced by the vaccine and stress in the peripheral tissues that is characterized by a first wave of expression of proinflammatory cytokines such as il1β, mainly produced by activated macrophages, that plays a crucial role in host defence (Stolte et al. 2008).
Overall, our results show that mucosal immune response at cellular level is triggered by intraperitoneal vaccine injection. Furthermore, the cellular immune response induced by vaccine/stress is significantly suppressed by salinity in rainbow trout. Thus, more studies should be devoted to investigating the effect of salinity on immune response elicited by vaccine in order how to mitigate the negative effect of hypersalinity within smoltification stage.
References:
- Khansari, A. R., Balasch, J. C., Vallejos-Vidal, E., Parra, D., Reyes-Lopez, F. E., and Tort, L. (2018). Comparative immune- and stress-related transcript response induced by air exposure and Vibrio anguillarum bacterin in rainbow trout (Oncorhynchus mykiss) and Gilthead seabream (Sparus aurata) mucosal surfaces. Front. Immunol. 9, 856. doi:10.3389/fimmu.2018.00856
- Ordóñez-Grande, B., Guerreiro, P. M., Sanahuja, I., Fernandez-Alacid, L., and Ibarz, A. (2020). Evaluation of an Acute Osmotic Stress in European Sea Bass via Skin Mucus Biomarkers. Animals, 10, 1546, 2-19.
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