Studies On The Effects Of Dietary Supplementation Of Mannan Oligosaccharide On The Growth Performance And Innate Immunity-associated Gene Expression In Broiler Chickens

The gene-encoded antimicrobial peptides (AMPs, also named host defence peptides) are widely distributed throughout insects, plants, animals and human. These molecules serve as crucial components of the innate immune system and play an essential role in the host defense against infection. In addition to their direct broad-spectrum antimicrobial activities against Gram-positive and Gram-negative bacteria, fungi and some enveloped viruses, AMPs can function as potent immune regulators and act as a bridge between innate and adaptive immunity. AMPs can act as signal molecules between cells to play multiple functions in immune defense against infection. Defensins and cathelicidins are the two major families of naturally occurring host defense peptides. Mannan-oligosaccharide (MOS) can promote competitive exclusion of pathogenic microbes and selective colonization by beneficial microbes, and thus protect gut health and enchance immune function. However, it is still not clear about the underlying mechanism of action by which MOS exerts probiotic effects especially the immunomodulatory effect. Therefore, the objectives of this study were to determine the effect of dietary addition of MOS to diets free of antibiotics on growth performance and innate immune responses including cytokine content in serum and innate immunity-associated gene expressions in tissue in broiler chickens, which is expected to provide further academic basis for the use of MOS as alternatives to antibiotic growth promoters.A total of176one-day old Cobb-500male broiler chickens without significant difference in body weight (P>0.05) were randomly allotted to4groups with4replicates of11birds each. Each group was fed the antibiotic-free basal diet supplemented with0.0(control),0.3,0.6, or0.9g/kg MOS to42days of age, respectively. On d21and42, average daily feed intake (ADFI), average daily gain (ADG) and feed to gain ratio (F/G) were calculated. Two birds were randomly selected from each replicate on d21and42, blood samples were collected from the wing vein, and then killed by bleeding. Bursa of fabricius, thymus, spleen, liver, ileum and cecum were immediately collected on ice bath. The cytokines in serum, immune organ index, and innate immunity-associated gene expressions were determined. The results showed as follows:(1)The growth performance of broiler chickens was improved by dietary supplementation of MOS. Dietary supplementation of MOS at0.3-0.9g/kg significantly increased ADG and decreased F/G (P<0.05or P<0.01) with the best feed conversion being obtained when supplementation level of MOS was0.9g/kg during1-21days of age. The maximal AFDI and ADG were observed in birds fed0.6g/kg MOS-supplemented diet during22-42days of age (P<0.05or P<0.01), while the better feed conversion could be obtained when dietary supplementation level of MOS was0.3g/kg. AFDI and ADG were significantly improved in0.3-0.9g/kg MOS treatments compared with control during1-42days of age (P<0.05or P<0.01).(2) Birds fed MOS-supplemented diet showed higher relative organ weights of bursa of fabricius on d21and thymus on d42than birds fed control diet (P<0.05). No significant difference was observed in the relative organ weights of thymus and spleen on d21, and of burda of fabricius and spleen on d42among treatment groups (P>0.05). The best relative weights of immune organs could be obtained when dietary supplementation level of MOS was0.6g/kg.(3) Higher TNF-a content in serum was observed in0.6g/kg MOS treatment group compared with control group on d21(P<0.05) and in0.3-0.9g/kg MOS treatment group compared with control group on d42(P<0.01). Birds receiving0.3-0.9g/kg MOS treatment showed higher IFN-y content in serum on d42compared with control treatment (P<0.05).(4) Toll-like receptor2(TLR2) mRNA expression in liver, spleen, ileum and cecum on d21, and in spleen on d42was not significantly affected by dietary supplementation of MOS (P>0.05), while the TLR2mRNA expression in liver, ileum and cecum on d42was significantly influenced by MOS treatment (P<0.05or P<0.01). Maximal upregulation of TLR2mRNA expression in ileum was observed in0.3g/kg MOS treatment group, and in liver and cecum in0.9g/kg MOS treatment group. Dietary supplementation of MOS did not affected toll-like receptor4(TLR4) mRNA expression in spleen and cecum on d21, and in cecum on d42(P>0.05), while the TLR4mRNA expression in liver and ileum on d21, and in liver, spleen and ileum on d42was significantly influenced by MOS treatment (P<0.05or P<0.01). Maximal upregulation of TLR4mRNA expression in liver on d21and ileum on d42was observed in0.6g/kg MOS treatment group, and in ileum on d21and liver and spleen on d42in0.9g/kg MOS treatment group.(5) β-defensin9(AvBD9) mRNA expression in liver on d21, and in liver, spleen on d42was not significantly affected by dietary supplementation of MOS (P>0.05), while the AvBD9mRNA expression in spleen, ileum and cecum on d21, and in ileum and cecum on d42was significantly influenced by MOS treatment (P<0.05or P<0.01). Maximal upregulation of AvBD9mRNA expression in spleen on d21was observed in0.3g/kg MOS treatment group, and in ileum, cecum on d21and cecum on d42in0.6g/kg MOS treatment group, and in ileum on d42in0.9g/kg MOS treatment group. Dietary supplementation of MOS did not affected Cathelicidin-B1(Cath-B1) mRNA expression in liver on d21, and ileum on d42(P>0.05), while the Cath-B1mRNA expression in spleen, ileum and cecum on d21, and in liver, spleen and cecum on d42was significantly influenced by MOS treatment (P<0.05or P<0.01). Maximal upregulation of Cath-B1mRNA expression in spleen and ileum on d21was observed in0.3g/kg MOS treatment group, and in liver on d42in0.6g/kg MOS treatment group, and in cecum on d21and spleen, cecum on d42in0.9g/kg MOS treatment group.In conclusion, dietary supplementation of0.3-0.9g/kg MOS could improve the growth performance and proinflammatory cytokine profile in serum, and tends to improve relative weights of immune organs. Dietary supplementation with MOS could upregulate innate immunity-associated genes (TLR2、TLR4、AvBD9and Cath-B1) expression, which implys MOS improves innate immue defence probably through TLRs-mediated upregulation of antimicrobial peptides such as β-defensins and Cathelicidins expression. The better growth performance, proinflammatory cytokine profile in serum, relative weights of immune organs and innate immunity-associated genes could be obtained when dietary supplementation level of MOS was0.6g/kg.