Introduction
In the past decades, the improvement of production efficiency has resulted in the tremendous success in the poultry industry. Feed Conversion Ratio has decreased from 2.8 in the 1950s to less than 1.6 currently. Over the past 25 years, the number of eggs laid by a single laying hen increased by at least one egg per year. As the production efficiency has improved, the poultry industry is also changing according to customer demands. The trend of antibiotic-free chickens and cage-free egg production is requiring alternative approaches to maintain animal performance, primarily one that focuses on a balanced intestinal microflora. Two of the most detrimental enteric diseases in poultry are coccidiosis and necrotic enteritis (NE). It has been reported that the annual economic losses due to coccidiosis and NE are approximately 3 billion (Kadykalo et al., 2018) and 6 billion dollars (Wade and Keyburn, 2015), respectively.
The main function of amino acids (AAs) in poultry nutrition is for protein synthesis. Traditionally, the requirement of AAs has been determined based on the response of growth or egg production. In addition to protein deposition, many AAs support other functions. Some AAs are reported to play an important role in maintaining intestinal barrier or immune functions. For instance, threonine and serine account for a great proportion in intestinal mucus which is a physical barrier to prevent pathogens from contact with enterocytes. Additionally, AAs are important as they are involved in multiple metabolic pathways in immune responses. AAs are also used to synthesize critical proteins such as acute phase protein, cytokines and immunoglobulins. In poultry, there have been several research trials investigating the function of AAs on immune responses and gut function. This manuscript briefly describes the functions of threonine and methionine on the immune system and their role on mediating intestinal challenges.
Threonine
Threonine is the third limiting amino acid in poultry. It is an important component in mucin, which is secreted by intestinal goblet cell as a functional protein in mucus. Mucus acts as a physical barrier between enterocytes and lumen in the intestine. In the mucus layer, mucin can trap hazardous agents including pathogens and limit their mobility to prevent them from approaching enterocytes (Khan, 2008). Mucin is a glycoprotein that is composed by very unique polypeptides. The core protein of mucin consists of a considerable amount of proline-threonine-serine sequence. The hydroxyl groups on the side chain of threonine and serine are used to attach the sugar to form the glycoprotein (Strous, 1979). This could be the reason why the percentages of threonine and serine are high in mucin. Threonine has been reported to account for 30% of total AAs in certain mucins (Faure et al., 2002). The relationship between dietary threonine and mucus secretion was investigated by Chee and coworkers (Chee et al., 2010). They found a thinner mucus when dietary threonine was deficient in chicken. In the same study, when manno-oligosaccharide was added in the diet, a thicker mucus was observed as broilers (1 to 35 day of age) were fed excessive threonine (120% of NRC 1994 recommendation) in comparison to broilers fed recommended threonine level diets (NRC 1994 recommendation). This may suggest that excessive dietary threonine could support the intestine mediating challenges by increasing mucus secretion. Pathogenic infections can stimulate the secretion of mucin by goblet cells (Kim and Khan, 2013). Because the mucin in the lumen cannot be reabsorbed by animals (Rémond et al., 2009), sufficient threonine supply is required to support mucin synthesis, especially under pathogenic infections. Besides the involvement in mucin synthesis, threonine may also play an important role for antibody synthesis (Van Der Boog et al., 2005). Antibody is a Y shape protein with two light polypeptide chains bound to two heavy polypeptide chains. Threonine and serine are rich in the hinge region of the antibody in poultry (Bateman et al., 2017). It can be postulated that poultry may need more threonine to support a higher antibody production due to the great presence of threonine in antibodies.
Research has shown that the requirement of threonine could vary under different intestinal conditions. Two independent experiments estimated the digestible THR/LYS ratios of broilers were from 0.67 to 0.69 under Clostridium perfringens challenge from 9 to 20 day of age (Star et al., 2012). This ratio is equal to or slightly above the recommendation of the primary breeder company (digestible THR/LYS: 0.67). In this study, 0.63 digestible THR/LYS was able to meet the requirement of non-challenged birds. This suggested an increased digestible THR/LYS ratio by 0.04 to 0.06 under Clostridium perfringens challenges. In a following experiment, the researchers were able to validate this findings. Using the same challenge model, significant increases on ADG and FI were observed when birds fed 0.67 digestible THR/LYS in comparison to 0.63 digestible THR/LYS. In a Eimeria maxima challenge study, the weight gain of broilers (1 to 16 day of age) fed high threonine (0.84% vs. 0.49%) were close to broilers under uninfected condition, while weight gain of challenged broilers fed deficient threonine diets was inferior to uninfected non-challenged birds (Wils-Plotz et al., 2013). In a Salmonella Enteritidis infection model, excessive dietary threonine (0.956%) was reported to improve the negative effects on weight gain in chicks (2 to 10 day of age) (De Barros Moreira Filho et al., 2015). Bhargava and coworkers (Bhargava et al., 1971) reported a threonine requirement was 0.7% under a Newcastle disease infection model in chicks from 1 to 18 day of age. This requirement was very close to the NRC at that time. The requirement for antibody titers could be much higher since the antibody titer still increased as dietary threonine concentration increased from 0.7 to 1.1%. In another study, the threonine requirement has been reported to be higher in broilers (21 to 42 day of age) reared on build-up litter than new litter (Corzo et al., 2007). Based on growth and carcass yield, total threonine requirement was 0.02 to 0.04% higher of birds reared on built-up litter than those on new litter. This equals an increase of 0.013 to 0.034 on total THR/LYS ratio. According to those researchers, the shift of intestinal microflora could be one of reason altering the threonine requirement. On the other hand, results from a study by Kidd and coworkers (Kidd et al., 2003) reported that there were no interactions between mild Eimeria acervulina challenge and dietary threonine concentrations (0.6, 0.7 and 0.8%) in broilers from 4 to 15 day of age. In this report, threonine requirement was not estimated since the three threonine levels all fall on the linear curve and no plateau was observed on growth performance.
Methionine
Methionine is the first limiting amino acid in poultry. Methionine and cysteine are called sulfur amino acids (SAA) since they both contain sulfur in their molecular formulas. The functions of methionine in the immune responses are manifested by participating in related metabolic pathways or converting to important bioactive molecules such as cysteine, polyamines and glutathione. Polyamines are involved at intracellular level for rapid cell proliferation, including transcription, mRNA stabilization, and translation. Polyamines are also required for the function of receptor binding (Minois et al., 2011). In addition, induction and regulation of inflammation also requires the involvement of polyamines (Puntambekar et al., 2011). Glutathione is an antioxidant converted from cysteine. It can balance the redox status within innate immune cells by regulating the free radicals produced to kill pathogens (Iseri and Klasing, 2013; Martínez et al., 2017). Glutathione is also required for the activation and proliferation of immune cells as well as controlling intracellular infections (Droge and Breitkreutz, 2000). In addition, methionine is a methyl-donor which is vital for stem cell differentiation into progenitor cells and further into immune cells. DNA methylation can control the gene expression by methylation of the promoter (Suarez-Alvarez et al., 2012). As SAA make multiple contributions to the immune system, it can be postulated that deficiency in SAA supply may interfere with the efficiency of the immune system in many different aspects.
Research on the effects of various dietary methionine or total SAA concentrations on the immune response is limited. In 21 day old chicks, the SAA levels for optimal growth was lower than the SAA levels needed to optimize the immune functions by measuring total anybody titers and T cell proliferation responses (Tsiagbe et al., 1987). A similar result was reported by Rama Rao and coworkers (Rama Rao et al., 2003). They found that 0.39% dietary methionine in broilers was able to maximize growth by increasing methionine levels resulting in increased antibody titers as well as improved innate system reactions from 1 to 49 day of age. An experiment was conducted to investigate the SAA requirement of broilers from 11 to 21 days of age under Eimeria spp challenge (not published, Evonik internal data). The experiment was a 4×2 factorial arrangement with 4 standardized ileal digestibility (SID) SAA levels (0.6%, 0.8%, 0.9% and 1.0%) and coccidiosis challenge or not. The performance of non-challenged birds reached a plateau at 0.8% SID SAA which was consistent with the primary breeder recommendations; however, no dose-response was observed by various dietary SAA levels on challenged birds. It could be due to unknown limiting nutrients induced by the challenge that hide the SAA concentration effect. Dietary SAA levels didn’t affect the oocyst count in excreta (P>0.05). Coccidiosis challenge significantly increased cytokine secretions (IL- 10, IFN-γ) in the intestinal lumen (P<0.01) but no interaction was observed between dietary SAA level and the challenge. Coccidiosis challenge also increased the anti-Eimeria IgA titers in duodenum, jejunum, ileum, and cecum (P<0.5). There was an interaction trend in the jejunum between SAA levels and challenge indicating SAA levels increased anti-Eimeria IgA titers only in challenged broilers (P=0.07). Overall, this experiment was not able to estimate the SAA requirement under coccidiosis; nevertheless, increased dietary SAA concentrations could enhance the adaptive immune responses in the enterocytes.
Summary
Threonine is an important component in mucin and antibodies synthesis. Increased dietary threonine provides sufficient supply to potentially increase mucus synthesis under-challenged intestinal conditions. It was suggested the THR/LYS ratio could increase by 0.02 to 0.04 in broilers under pathogenic infections to maximize growth performance. The requirement to maximize mucus secretion or antibody titers could be even higher. The suggested ratio could also provide a reference for laying hens, turkeys or ducks but more research are required in the future to investigate the beneficial effects of threonine under pathogenic infections. Methionine, as a precursor of cysteine, polyamines, and glutathione, has been reported to affect the immune responses by different modes of action. Although methionine could enhance both innate and adaptive immune responses in broilers, there is no sufficient evidence to support the hypothesis that additional dietary methionine could alleviate the detrimental effects of intestinal infections on growth performance.