Beta-defensins are one of the most representative antimicrobial peptides throughout vertebrates and the unique class of defensins identified in fish. As in humans, beta-defensins are highly expressed in the skin, where these small cysteine-rich peptides contribute to shape microbiota and the innate immune response to danger signals. Aquaculture poses many different stressors to farmed fish. Acute stress impacts organismal immunity, requiring fast and specific responses toward reestablishing homeostasis. The skin is one of the first tissues that senses and responds to stress and, thus, requires efficient mechanisms of immune regulation in place.
Here, we investigated the effects of acute stress associated with either elevated temperature or transportation on the expression of six beta-defensin (omDB) in Rainbow trout skin. In the temperature stress experiment, we sampled fish kept at 16°C (control) and those exposed to increasing temperatures (19, 22, and 25°C). In the transportation stress experiment, we sampled before (control) and after 5h-transportation. From both experiments, we collect skin and blood plasma.
Our data show that beta-defensins are expressed in a tissue-specific manner in Rainbow trout at the steady state. For instance, omDB-1a, 1b, and 4 genes are highly constitutively expressed in the skin and swim bladder; omDB-2 and 5 in the heart, and omDB-3 in the liver and gut. In both acute stress models, beta-defensin genes were strongly modulated, which occurred at earlier time points than significant changes in cortisol and glucose levels. Specifically, temperature stress caused a down-regulation of omDB-1b, 3, and 5 genes, while up-regulated omDB-1a and 4 genes. On the other hand, transport stress resulted in a down-regulation of omDB-2.
This suggests that different types of stress impact skin immunity differently as well as likely regulated skin microbiota. Our work demonstrates that beta-defensins are associated with the early fish response to acute stress in aquaculture in a stress-specific way, which is evident in the skin, a crucial physical and functional component of the host immune defense.