Dietary ß-glucan (MacroGard®) improves innate immune responses and disease resistance in Nile tilapia regardless of the administration period.

The effects of dietary ß-glucan on innate immune responses have been shown in a number of different vertebrate species. However, there is conflicting information about the period of administration (shorter vs. longer), and it is also unclear to what extent ß-glucan's effects can be observed post-treatment in fish. Thus, we fed Nile tilapia for 0 (control group; 45 days of control diet), 15 (30 days of control followed by 15 days of ß-glucan), 30 (15 days of control followed by 30 days of ß-glucan) or 45 days with a diet containing 0.1% of ß-glucan (MacroGard®). We evaluated the growth performance at the end of the ß-glucan feeding trial and the innate immune function immediately after the feeding trial and 7 and 14 days post-feeding trial. In addition, at day 10 post-feeding trial, we assessed the tilapia's resistance against a bacterial infection. No significant differences were observed in growth performance between the groups; however, fish fed with ß-glucan for 30 and 45 days had higher (approx. 8%) relative weight gain compared to the control. Regardless of the administration period, fish fed with ß-glucan had higher innate immune responses immediately after the feeding trial such as lysozyme activity in plasma, liver and intestine and respiratory burst compared to the control, and in general these differences were gradually reduced over the withdrawal period (up to 14 days). No differences were observed in the plasma hemolytic activity of the complement or myeloperoxidase activity in plasma or intestine. Moreover, fish from the control group had early mortalities (2 vs. 4-5 days post-infection, respectively) and a lower survival rate (60 vs. 80%, respectively) compared to fish fed with ß-glucan for 15 or 30 days, and, interestingly, fish fed for 45 days with ß-glucan had no mortality. This study indicates that regardless of the administration period (i.e., 15 up to 45 days), the ß-glucan improved the innate immune responses and the tilapia's resistance to disease, and this protection could be observed up to 10 days post-feeding trial, adding in vivo evidence that ß-glucan may contribute to a trained innate immunity. Additionally, we showed that a longer period of administration did not cause immunosuppression as previously hypothesized but promoted further growth and immune performance. These findings are relevant to the aquaculture industry and demonstrate that a longer ß-glucan feeding protocol may be considered to achieve better results.

ß-Glucan enhances respiratory activity of leukocytes suppressed by stress and modulates blood glucose levels in pacu (Piaractus mesopotamicus).

We evaluated the immune response of pacu fed with a ß-glucan diet (0.5%) for 10 days. After the feeding period, fish were subjected to handling and 3 h after, inoculated with Aeromonas hydrophila. Fish were sampled before handling (baseline condition), 3, 6, and 24 h and 1 week after inoculation. A higher level of blood glucose was found in fish treated with ß-glucan in baseline conditions. Handling and bacterial inoculation increased the circulating levels of cortisol and glucose and promoted the acute inflammatory response (lymphopenia and neutrophilia). ß-Glucan prevented the decrease in the respiratory activity of leukocytes observed in the control group at 3 h sampling. ß-Glucan did not affect the complement and lysozyme, which were activated 24 h after the bacterial challenge in control fish. A reduction in the number of leukocytes was found in fish treated with ß-glucan 1 week after the challenge. We suggest two plausible hypotheses for this event: (1) it could be attributed to a depletion of the immune responses or (2) it could be due to a mobilization of the leukocytes to the spleen for antigen presenting/processing. In general, ß-glucan avoided the reduction of the activity of leukocytes after stress and the bacterial challenge and increased the baseline glucose levels. Our findings confirm the immunomodulatory action of glucan and add evidence showing that glucan can have a role in stress response.

ß-glucan modulates cortisol levels in stressed pacu (Piaractus mesopotamicus) inoculated with heat-killed Aeromonas hydrophila.

In this study, we show that ß-glucan can modulate cortisol release in fish. We simulated a common situation in aquaculture: the transport of fish followed by contact with an opportunistic pathogen and observed what effect glucan had on the immune and stress response in these conditions. Pacu (Piaractus mesopotamicus) were fed with a diet containing ß-glucan (0.1%) for 15 days prior to transport followed by an injection with heat-killed Aeromonas hydrophila. We sampled fish before transport, at arrival and at 3 and 24 h after bacterial injection. ß-Glucans are used in aquaculture and have a known immunostimulatory effect, which was observed in this study. The results showed that ß-glucan modulated the plasma cortisol levels differently by increasing these levels up to 24 h after transport and preventing the increase caused by bacterial inoculum injection. In addition, ß-glucan enhanced the activity of the complement system at 24 h and reduced the monocytes and lymphocytes number in peripheral blood at 3 and 24 h after bacterial inoculation. Our results suggest that ß-glucan modulated a bidirectional interaction between the stress and the immune responses. The modulation of cortisol levels and the immunostimulation by ß-glucan at different moments in our study suggest the compound has a protective effect by avoiding higher levels of the hormone and improving resistance against bacterial infection in pacu. These results add evidence to support the use of ß-glucan as an immunomodulator in the aquaculture industry.

Immunomodulation of Juvenile Pacu, Piaractus mesopotamicus, by Different ß(1-3)(1-6)-D glucan Products

Abstract Stress in intensive fish farming hamper immune function of fish and cause losses by disease outbreaks, a situation that can be minimized, but cannot be completely circumvented, by the use of immunomodulators. Addition of immunomodulators to aquafeeds has thus become a common practice. β-glucan (BG) is one of most studied and effective immunomodulators, aquaculture purposes included. Extracted from cell walls of bacteria, fungi and selected cereals, BG activity depends on the source and extraction methods. This study evaluated effects of two BG products (BG1 and BG2), extracted from Saccharomyces cerevisiae under varying extraction methods and with different immune activity, on the feeding of pacu Piaractus mesopotamicus juveniles. BG1 provided higher leukocytes respiratory activity when fed at 0.5% inclusion for 10 days and 0.1% inclusion for 15 days. Both products seems to cause negative effect on lysozyme concentration and monocytes profile when fed to pacu for 15 days at 0.5% inclusion. Although the results for BG2 did not differ from control (diet devoid of BG), the proximity with the BG1 behavior is a indicative that a commercial product with smaller BG concentration can be effective when more refined technology is used in extraction process.

Distinct ß-glucan molecules modulates differently the circulating cortisol levels and innate immune responses in matrinxã (Brycon amazonicus).

This study investigated the effects of two ß-glucan molecules with different purities and isolated by different biotechnological processes on the immune response of matrinxã (Brycon amazonicus) prior and after challenge with Aeromonas hydrophila. In this sense, we evaluated serum cortisol and plasma glucose levels, the number of leukocytes (lymphocytes, neutrophils and monocytes), as well as the respiratory activity of leukocytes prior to, 6 and 24 h post infection (hpi). During 15 days, fish were fed with diets containing 0.1% of two ß-glucans (ß-G 1 and ß-G 2, with 71 and 62% of purity, respectively) and then submitted to challenge. Results were compared with a positive control group fed with a ß-glucan-free diet. A negative control group, also fed with ß-glucan-free diet but inoculated with PBS, was established to evaluate the effect of handling during injection. Our results showed that different ß-glucans affected differently the biological responses of matrinxã. The ßG 2 modulated the cortisol profile prior to and after the acute infection with A. hydrophila, and increased the mobilization and activity of leukocytes. The infection promoted lymphopenia at 6 hpi and both ß-glucans increased the circulating lymphocyte population 24 hpi. Moreover, the ß-G 2 prevented the infection-induced neutrophilia at 6 and 24 hpi. Finally, the ß-G 2 caused a marked increase in the circulating monocytes prior to infection, and a reduction at 6 hpi that was reversed at 24 hpi. In summary, our study demonstrates that ß-G 2 was more efficient on the induction of the cell-mediate immunity in matrinxã.

Different ß-glucans improve the growth performance and bacterial resistance in Nile tilapia.

The role of glucan as a biologically active immunomodulator has been well documented for more than 40 years. However, the wide diversity of ß-glucan forms and the extraction process has implications for the benefits of these compounds. Biorigin developed two samples of ß-glucans using different biotechnological processes. Thus, in the present study, we fed Nile tilapia (Oreochromis niloticus) diets containing these two ß-glucan molecules (BG01 and BG02) for 30 days prior to bacterial infection with Streptococcus agalactiae. The results showed that the different ß-glucan samples exhibited biologically differently behaviors, but both increased the resistance against bacterial infection. Specifically, BG01 increased immunostimulation, while BG02 improved growth performance. In summary, these findings confirm the benefits of ß-glucans in aquaculture and also provide further evidence of the growth promotion of these compounds.