YCWs are proven immune response modifiers holding an important position among multifarious paraprobiotics and, in fish, have been implicated in the maintenance of (intestinal ) mucosal health and integrity, disease resilience and animal performance. T he major structural components of YCWs are β-glucans and mannanoligosaccharides (MOS) which act as microbe-associated molecular patterns (MAMPs) that can be recognized by the pattern recognition receptors of the host immune system. YCW can vary in their biochemical composition and molecular structure due to the yeast species, strain, fermentation, and down-processing conditions . Such diversity of MAMPs structure and profile could elicit distinct host immune responses and to date has been under evaluated. The current investigation aimed to characterise the biochemical and molecular properties of four proprietary S. cerevisiae YCW fractions of discrete origin. Our hypothesis was that the biophysical characteristics of given S. cerevisiae will elicit distinct immune responses, which has not been previously addressed using a purposely designed comparative study.
Materials and Methods
A 5-week trial (Plymouth, UK) tested 5 dietary groups in triplicate using wild-type adult zebrafish (BWi = 0.80 ± 0.02 g; 25 fish/tank) and a basal diet formulated to NRC requirements for cyprinids (36% protein, 8% lipid). Treatments consisted of 1] non-supplemented basal diet (Control), and basal diet supplemented with 2] YCW1, 3] YCW2, 4] YCW3, 5] YCW4 . A ll YCW fractions were supplemented at 2.0 kg/T of feed and consisted of contrasted proprietary YCW strains provided by Lallemand SAS (France). YCW fractions were assessed by sulfuric acid method. D iets were fed at fixed rate of 4 % biomass daily over 3 daily rations . At the end of the trial, intestinal and skin histomorphometry (n = 9 fish/ treatment) as well as the intestinal gene expression analysis (n = 6 fish/ treatment) was assessed . For gene expression analysis d ifferences between control and experimental groups were assessed by non-parametric permutation test with significance accepted at p < 0.05. All other data was analysed by O ne-way ANOVA and significance accepted at p < 0.05.
The biochemical analysis revealed significant differences in percentage composition of β-glucan and alpha - mannan polysaccharides in each YCW fraction (Fig 1 ; p < 0.05). Histological appraisal revealed significantly elevated GCD, IELs and acidomucins in YCW1, 2 and 3 groups containing higher levels of mannan content compared to YCW4 (Fig 2; p < 0.05). Gene expression analysis revealed differential modulation in markers for innate immune pattern recognition receptors (PRRs) and signal transduction factors, transcription and innate cytokines that suggest polarisation of responses to Th1, Th17, Tr1 and Foxp3+-Tregs according to each YCW group (Fig 3 ).
This study identifies marked intra-species variability in the molecular properties of S. cerevisiae yeast cell wall fraction. T he α-mannan content was associated with GCD and IEL hyperplasia suggestive of fortifying intestinal barrier integrity and immune competence. The gene expression analysis revealed marked modification in the expression pattern of PRRs when fed contrasted YCW fractions . Furthermore, the gene expression patterns for transcription factors and cytokine responses suggest the preferential mobilisation of distinct T-helper cell subsets for Th1, Th17, TR1 and Foxp3+ -Tregs, indicating a particular potential for each YCW fraction to confer protection against infectious agents and, or non-infectious pathologies . Accordingly, this study highlights the potential of considering the biophysical characteristics of the YCW to apprehend their specific immune properties and elicit targeted immune functionalities towards precision functional nutrition in Aquaculture.