Abstract
A role for vitamin D in the immune system is emerging from human research but data in the bovine is limited. In the current study, 48 Holstein–Friesian calves were randomly assigned to one of 4 groups designed to expose calves to divergent vitamin D levels for a 7 month period and to determine its effects on circulating immunity in young calves. Concentrations of circulating 25-hydroxyvitamin D (25OHD) was measured in serum using a commercial ELISA with validated bovine standards. Results showed that mean circulating concentrations of 25OHD at birth was 7.64 ± 3.21 ng/ml indicating vitamin D deficiency. Neither the injection of Vit D3 at birth nor the elevated levels in milk replacer yield discernible changes to pre-weaning circulating concentration of 25OHD. No calf reached the recommended level of vitamin D immune sufficiencyof 30 ng/ml of 25OHD until at least 3 months of age (T4). Increasing dietary Vit D3 via ration in the post-weaning period significantly elevated 25OHD concentrations in serum in VitD-In calves. Maximal levels of circulating 25OHD were achieved in VitD-Out calves, reaching 60.86 ± 7.32 ng/ml at 5 months of age (T7). Greatest divergence in haematology profile was observed between Ctl-In vs VitD-In groups with Ctl-In calves showing an elevated count of neutrophils, eosinophils, and basophils associated with reduced 25OHD concentrations. Neither IL-8 expression nor ROS production in serum were significantly different between calves with high and low 25OHD, indicating that other vitamin D-dependent mechanisms may contribute to the divergent circulating cellular profiles observed. This novel data on the vitamin D status of neonatal calves identifies a significant window of vitamin D insufficiency which is associated with significant differences in circulating immune cell profiles. Vitamin D insufficiency may therefore exacerbate pre-weaning disease susceptibility, and further work in now warranted.
Highlights
Vitamin D (Vit D) is the collective term used to describe a group of closely related fat-soluble steroids
Vitamin D deficiency in calves at birth extends until 3 months of age and is not addressed by supplemented Vit D3 in milk replacer or ration pre‐weaning
Despite a sub-dermal injection of Vit D 3 (50,000 IU) at birth and inclusion of supplementary Vit D3 in milk replacer (6000 IU/kg in Ctl groups and 10,000 IU/kg in Vit D groups), a change in serum 25OHD concentration at 15 days post-Vit D3 injection (T2) was not detected (p > 0.05) (Fig. 1)
Summary
Vitamin D (Vit D) is the collective term used to describe a group of closely related fat-soluble steroids. Vit D 2 (ergocalciferol) is obtained from the roughage used for cattle feed (e.g., hay and silage) and Vit D 3 (cholecalciferol) is synthesized in the skin during exposure to sunlight[1], and it is supplied as a synthetic supplement in feed[2] Both molecules, are metabolized in the same way, Vit D2 is less physiologically effective and less efficient at ensuring a sufficient blood levels of Vit D in cattle[3]. The immune system in calves develops gradually from conception to maturity at approximately 6 months after birth[11] During this time, calves are vulnerable to infection with respiratory and enteric bacteria and viruses (respiratory syncytial virus, BVD, herpesvirus, E. coli, rotavirus, Salmonella). In this study we aimed to develop a model to drive divergent Vit D status under the current European supplementation guidelines and to investigate how variation in the circulating concentration of 25OHD affected the immune cell profiles in dairy calves
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