Imprints of the embryonic thermal environment on nasal mucosal immunity and disease resistance to Yersinia ruckeri in Atlantic salmon (Salmo salar) parr

This study investigated immune responses to Yersinia ruckeri in the olfactory organ and brain of Atlantic salmon parr and the influence of embryonic temperature history on immunity and disease resistance. Atlantic salmon were reared at 4 °C or 8 °C from fertilisation to the eyed stage of embryogenesis and maintained under standard rearing conditions until the parr stage (∼20 g). Fish were bath-exposed to Y. ruckeri, and immune responses were assessed at 1, 3, and 14 days post-infection (dpi). At 14 dpi, cumulative mortality in the 8 °C group (∼35 %) was significantly higher than in the 4 °C group (∼22 %). Low bacteria levels were detected in the olfactory organ and brain. Y. ruckeri antigens were present in the lamina propria of the olfactory lamellae and the optic tectum, including the meninges and nucleus of the vagus nerve. Mild histological changes, such as congestion and leukocyte infiltration were observed in the olfactory organ, although mucosal morphometric parameters were unaffected by infection or embryonic temperature history. Y. ruckeri modulated the expression of cytokines, antibacterial defence genes, and immunoglobulins in the olfactory organ. The most significant changes occurred at 3 and 14 dpi. Embryonic temperature history did not broadly alter immune gene expression, but interleukin-1β (il1b) and tumour necrosis factor-α (tnfa) had higher expression in the 8 °C group. In the brain, microglial markers including aif1 and cd45 were significantly upregulated at 14 dpi, particularly in the 8 °C group. This study provides insights into the nasal immune responses of Atlantic salmon to Y. ruckeri. Our findings suggest that lower embryonic temperature history influenced disease resistance against Y. ruckeri in Atlantic salmon parr, but only minimally shaped immune responses in the olfactory organ and brain following infection.