Immunosuppression can cause concurrent or increased incidence rates of, secondary infections and reduce the protective effect of vaccines on poultry, leading to decreased chicken production capacity and even death [31]. In this study, chickens injected with Cy exhibited decreased excitability, appetite, and weight loss. Furthermore, poor growth performance, low ADG and ADFI, and high FCR were observed in Cy-treated chickens. In addition, immune organ indices, globulin concentrations, and antioxidant enzyme activity in the serum decreased, relative to those in the control group. The above results indicate that the immune function of black-bone silky chickens was dramatically affected by Cy, and demonstrate the successful establishment of an immunosuppression model.
We obtained two acidic heteropolysaccharides (DPR and DPL), with different structural features, from dandelion roots and leaves, using water extraction, ethanol precipitation and gel chromatography. To improve their purity, Sevag reagent was used to remove proteins during preparation of the polysaccharides. The effects of DPR and DPL on the immune and antioxidant systems of black-bone silky chickens were investigated, and the results showed that DPR and DPL had positive effects on growth performance, particularly at 900 mg/kg, compared with the model group. Within the range of 300 to 900 mg/kg, supplementation with DPR and DPL improved ADG and ADFI in immunosuppressed chickens, and decreased FCR during the overall experimental period. The thymus, spleen, and bursa of Fabricius are the main immune organs in chickens, and are typically used to evaluate immunological status in these birds. Lymphocyte proliferation is the key event in activation process of both cellular and humoral immune responses. Lymphocyte proliferation induced by ConA or LPS can be used to evaluate T or B lymphocyte activity. In this work, both DPR and DPL enhanced T and B-cell proliferation. However, DPL exhibited better proliferation effects of T and B cells than DPR did, indicating that it is a possible potential immunopotentiator for use in livestock and poultry production.
Serum globulin concentration is also a vital indicator of the humoral immunological status of an organism [15]. Our results showed that DPR and DPL improved immune organ indices and serum concentrations of IL-2, IL-6 and INF-γ, IgM, IgA, and IgG, compared with those of the model group, indicating that DPR and DPL can antagonize Cy-induced immunosuppression reactions. Serum immunoglobulin levels are important markers of humoral immune function [32]. Immunoglobulins, including IgG, IgA, IgM, IgD, and IgE, are related to immune responses and regulation, and several diseases are closely associated with changes in the concentrations of IgA, IgG, and IgM in serum [33]. In this study, the immune organ index, and the serum concentrations of IL-2, IL-6, INF-γ, IgM, IgA, and IgG in the model group decreased compared with those of the control group. Moreover, dandelion polysaccharides antagonized Cy-induced immunosuppression reactions. DPL at 600 and 900 mg/kg significantly increased serum IgA and IgG levels in black-bone silky chickens. However, the enhancing effects of DPR and DPL on immune function in immunosuppressed chickens differ. At the same dose, both polysaccharides improved the above indicators, showing a trend of DPL > DPR, indicating that DPR has stronger effects in resisting Cy-induced immune suppression.
Polysaccharides are biomacromolecules with complex structures, and their biological activities may be influenced by several factors, including monosaccharide composition, Mw, category of glycosidic bond and chain conformation [34]. Based on our results, DPL which had lower Mw, higher GalA content than DPL, showed lower immunoenhancement effects. Interestingly, polysaccharides are rich in hydroxyl groups, which can form numerous intermolecular and intramolecular hydrogen bonds, leading to the formation of different conformations. Triplex conformation is an important characteristic of immunoactive polysaccharides [35], and our conformation analysis showed that DPR and DPL possessed triplex chain conformations, but with different morphological structures. Therefore, we speculate that the difference in immunoenhancement between the dandelion polysaccharides may result from the synergistic effects of multiple factors.
Cyclophosphamide weakens the immune system and causes endocrine dyscrasia in poultry, leading to metabolic disorders and increasing the concentration of free radicals [36]. Excessive free radicals cause oxidative stress and reduce the growth performance of animals. Based on our results, compared to the control group, serum MDA in the Cy-treated group increased, and the activities of SOD, GSH-Px, and T-AOC in serum decreased significantly, suggesting that Cy administration weakened the antioxidant capacity of black-bone silky chickens. In addition, the levels of serum ALT and AST in the model group were significantly higher than those in the control group, which may have been due to the consumption of microsomal cytochrome P450 by Cy activation in the liver, leading to an increase in ROS and lipid peroxidation, ultimately leading to liver cell damage. Recently, the antioxidant activities of plant polysaccharides have received increasing attention. Studies have shown that plant polysaccharides can restore Cy-induced oxidative damage [14]. In our results, compared with the model group, supplementation with DPR or DPL markedly reduced MDA levels, improved the activity of SOD, CAT, and GSH-Px, and improved T-AOC, demonstrating the protective effect of dandelion polysaccharides on the antioxidant system of immunosuppressed black-bone silky chickens. Interestingly, the antioxidant ability in immunosuppressed chickens was better in the DPL groups than in the DPR groups, indicating that DPL plays a stronger role in resisting oxidative injury induced by Cy. Zhang et al also noted that dandelion leaf polysaccharides, which are rich in GalA, showed stronger antioxidant activity than polysaccharides from dandelion roots [11, 37]. Meanwhile, significant decreases in ALT and AST levels and increases in TP and ALB levels were observed in serum from DPL-treated black-bone silky chickens, indicating that DPL supplementation can improve physiological protein synthesis capacity.
Most plant polysaccharides are considered potential prebiotics because of their complex structures. They can be easily utilized by gut microbiota to produce short-chain fatty acids (SCFAs) and other beneficial metabolites, thereby affecting animal health [38]. Various polysaccharides can improve the structure and abundance of gut microbiota in chickens, including Bifidobacterium and Lactobacillus, which enhance the activity of digestive enzymes and improve protein absorption and utilization. Furthermore, such polysaccharides can increase intestinal acidity and promote chemical barrier formation by promoting SCFA production and reducing the physiological generation of harmful nitrogen [39]. A similar phenomenon was also observed in this study, where dandelion polysaccharides improved serum biochemical indicators related to metabolism, and DPL exhibited superior effects to DPR; however, further research is needed to determine whether this effect is related to intestinal microbiota regulation.
Based on the above analysis, we conclude that DPL has stronger effects on immune enhancement and antioxidant stress in immunosuppressed black-bone silky chickens, possibly due to differences in its structural features relative to DPR.