Dietary vitamin D supplementation attenuates immune responses of pigs challenged with rotavirus potentially through the retinoic acid-inducible gene I signalling pathway

In the present study, twenty-four Duroc x Landrance x Yorkshire (initial body weight (BW) of 21.82 (SEM 2.06) kg) cross-bred pigs were used to determine whether dietary vitamin D supplementation could confer protection against viral infections through the retinoic acid-inducible gene I (RIG-I) signalling pathway in pigs. Experimental treatments were arranged in a 2 x 2 factorial manner with the main effects of immune challenge (control v. porcine rotavirus (PRV) challenge) and dietary concentrations of vitamin D (200 and 5000 IU; where 1 IU of vitamin D is defined as the biological activity of 0.025 mg of cholecalciferol). The pigs were fed a diet containing 200 or 5000 IU vitamin D in the first week of the study period. On day 8, the pigs were orally dosed with 4ml of Dulbecco's modified Eagle's medium/Ham's F-12 medium containing PRV or essential medium (control). Serum samples were collected on day 8 (pre-challenge), and 6 d after the PRV challenge, the pigs were killed to evaluate intestinal morphology and tissue gene expression following the last blood collection. Pigs challenged with PRV had decreased BW gain (P<0.01), feed intake (P<0.01), villus height (P<0.01), faecal consistency (P<0.05), and serum 1,25-dihydroxyvitamin D concentration (P<0.01) and increased (P<0.01) serum IL-2, IL-6 and interferon (IFN)-beta concentrations. Vitamin D supplementation mitigated these effects. The mRNA expression of RIG-I (P<0.01), IFN-beta promoter stimulator 1 (P<0.01), IFN-beta (P<0.01) and interferon-stimulated gene 15 (ISG(15)) (P<0.01) was up-regulated by the PRV challenge and vitamin D supplementation in the intestine. In conclusion, vitamin D supplementation could activate the RIG-I signalling pathway and thus alleviate the negative effects caused by PRV challenge.