Simple SummaryVitamin A is an anti-inflammatory vitamin and essential for the health of humans and animals. Necrotic enteritis is an enteric inflammatory disease in poultry caused byClostridium perfringensinfection. Due to the ban on using antibiotics in feed, necrotic enteritis has caused great economic losses in poultry production. Previous observations indicated that phospholipase C secreted byC. perfringensmight interfere with the transformation of vitamin A into retinoic acid via the stimulation of prostaglandin E2production, which impaired the modulation of vitamin A on immune responses. Therefore, the present study investigated the effects of dietary supplementation with a large dose of vitamin A on the immune responses and vitamin A metabolism in broilers suffering from necrotic enteritis and explored the underlying mechanisms. The results showed that vitamin A supplementation in chicken diets improved hepatic vitamin A deposition and modulated the expression of Th2 cell-related cytokines in the jejunum and spleen. In addition, dietary supplementation with vitamin A inhibited the expression of genes involved in the Janus kinase/signal transducer and activator of the transcription pathway and genes encoding retinoic acid receptors in the spleen of broilers. The present study indicated the modulatory effects of vitamin A supplementation in diets on the immune responses and vitamin A metabolism in necrotic enteritis-challenged broilers.AbstractNecrotic enteritis (NE) is an important enteric inflammatory disease of poultry, and the effects of vitamin A (VitA) on NE birds are largely unknown. The present study was conducted to investigate the effects of VitA on the immune responses and VitA metabolism of NE broilers as well as the underlying mechanisms. Using a 2 × 2 factorial arrangement, 336 1-day-old Ross 308 broiler chicks were randomly assigned to 4 groups with 7 replicates. Broilers in the control (Ctrl) group were fed a basal diet without extra VitA supplementation. Broilers in the VitA group were fed a basal diet supplemented with 12,000 IU/kg of VitA. Birds in NE and VitA + NE groups were fed corresponding diets and, in addition, co-infected withEimeriaspp. andClostridium perfringenson days 14 to 20. Samples of the blood, jejunum, spleen and liver were obtained on day 28 for analysis, and meanwhile, lesion scores were also recorded. The results showed that NE challenge increased lesion score in the jejunum and decreased serum glucose, total glyceride, calcium, phosphorus and uric acid levels (p< 0.05). VitA supplementation reduced the levels of serum phosphorus, uric acid and alkaline phosphatase in NE-challenged birds and increased serum low-density lipoprotein content and the activity of aspartate aminotransferase and creatine kinase (p< 0.05). Compared with the Ctrl group, the VitA and NE groups had higher mRNA expression of interferon-γ in the jejunum (p< 0.05). NE challenge up-regulated mRNA expression of interleukin (IL)-13, transforming growth factor-β4, aldehyde dehydrogenase (RALDH)-2 andRALDH-3in the jejunum, while VitA supplementation increased jejunalIL-13mRNA expression and hepatic VitA content, but down-regulated splenicIL-13mRNA expression (p< 0.05). The VitA + NE group had higher serum prostaglandin E2levels and the Ctrl group had higher splenicRALDH-3mRNA expression than that of the other three groups (p< 0.05). NE challenge up-regulated jejunal retinoic acid receptor (RAR)-β and retinoid X receptor (RXR)-α as well as splenicRAR-αandRAR-βmRNA expression (p< 0.05). VitA supplementation up-regulated jejunalRAR-βexpression but down-regulated mRNA expression ofRXR-α,RXR-γ, signal transducers and activators of transcription (STAT) 5 andSTAT6in the spleen (p< 0.05). Moreover, compared with the Ctrl group, the VitA and NE groups had down-regulated mRNA expression of jejunal and splenic Janus kinase (JAK) 1 (p< 0.05). In conclusion, NE challenge induced jejunal injury and expression of Th2 and Treg cell-related cytokines and enhancedRALDHandRAR/RXRmRNA expression, mainly in the jejunum of broilers. VitA supplementation did not alleviate jejunal injury or Th2 cell-related cytokine expression; however, it improved hepatic VitA deposition and inhibited the expression ofRALDH-3,RXRand the JAK/STAT signaling pathway in the spleen of broilers. In short, the present study suggested the modulatory effects of vitamin A on the immune responses and vitamin A metabolism in broiler chickens challenged with necrotic enteritis.