Vitamin D serves as a modulator of immune organs in grass carp (Ctenopharyngodon idella) infected with Aeromonas hydrophila

Vitamin D regulates biological processes via vitamin D receptors (VDRs) and exerts a pleiotropic effect in the prevention of pathogen infections. In various fish, the existence of two unique VDR isoforms (VDRa and VDRb) has been reported; however, the effects of vitamin D and VDR isoforms on the function of immune-related organs (e.g., the head kidney and spleen) remain poorly understood. In the present study, diets containing different vitamin D concentrations (0, 364.3, 782.5, 1167.9, 1573.8, and 1980.1 IU/kg) were provided to on-growing grass carp for 70 d. Subsequently, each group was subjected to a challenge trial by infection with Aeromonas hydrophila for 14 d. Dietary vitamin D decreased the oxidative biomarker content and upregulated the transcriptional responses of antioxidant enzymes potentially involved in Nrf2 signaling; alleviated cell apoptosis by down-regulating caspase expression, potentially in relation to the inhibition of p38MAPK molecules; enhanced immune component activity; downregulated the transcriptional levels of pro-inflammatory cytokines; and upregulated those of anti-inflammatory cytokines. Furthermore, the optimal vitamin D dose decreased the expression of nuclear factor kappa B p65, c-Rel, IKKα, IKKβ, and IKKγ and increased the transcriptional levels of iκBα, VDRb (but not VDRa), macrophage stimulating 1 (MST1), and signal transducers and activators of transcription (STAT) 3b1 and 3b2. Moreover, the protein levels of VDR, p-STAT3Tyr705, and p-MST1Thr108 were elevated both in the head kidney and spleen after treatment. In summary, vitamin D protected the immune organs from oxidative stress, apoptosis, and immunological injury. Based on lysozyme activity in the head kidney and spleen, the vitamin D requirements of on-growing grass carp (256.89–1129.67 g) were estimated to be 1150.71 and 1355.37 IU/kg, respectively. In conclusion, our findings confirmed that vitamin D can regulate molecular pathways related to immune organ function and pathogen defense in an aquaculture setting, supporting the feasibility of vitamin D supplementation to support immunological functions under infection conditions.