Gilthead seabream (Sparus aurata) immune responses are modulated after feeding with purified antinutrients

Highlights

• Dietary purified saponins and phytosterols differentially stimulate the fish immune system.

• Dietary antinutrients induce changes related to the development of inflammation.

• An acquired response mediated by B and probably T lymphocytes appears to be established at a more chronic stage.

• Plant-derived ingredients reach in antinutrients should be used moderately to avoid a chronic inflammatory state.

Abstract

The present study aimed to evaluate the effects of two purified antinutrients, soy saponins and phytosterols, in an important species for Mediterranean aquaculture. For this purpose, gilthead seabream (Sparus aurata) were fed six experimental diets containing two levels of those antinutrients, alone or in combination, and a control diet, to apparent visual satiation under controlled conditions. Blood and head-kidney were collected at 7, 15 and 48 days following first feeding in order to assess immune parameters and the expression of immune-related genes. Plasma bactericidal and alternative complement pathway activities increased in fish fed antinutrients compared to fish fed the control diet during the course of the experiment, with more important changes at 7 and 48 days for bactericidal activity and at 7 and 15 days for complement values. In contrast, plasma total immunoglobulins (Ig) increased in fish fed antinutrients only at 48 days. Caspase 1 (casp1), interleukin 18 (il18), colony-stimulating factor-1 receptor (csfr) and hepcidin (hep) presented similar patterns of expression with more important changes at 7 and 48 days, while interleukin 10 (il10) and β-defensin (def) were mainly up-regulated in fish fed antinutrients at 48 days. The level of expression of IgM increased already at 7 days in fish fed the low concentration of both saponins and phytosterols while a general up-regulation was observed at 48 days compared to fish fed the control diet. Results suggest that feeding seabream a diet with purified saponins and phytosterols, alone or in combination, induces a number of changes that are related to the development of inflammation, with most important changes in fish fed the lower phytosterols concentration.

Introduction

The aquaculture industry should become sustainable from every angle and measures are nowadays being directed towards an eco-friendly and sustainable production for a protracted growth and development [1]. Replacement of fish meal and fish oil by alternative sources in aquafeeds is thus necessary, with plant feedstuffs as the most important alternative to fish meal and fish oil due to their worldwide availability and acceptable prices. Still, those plant feedstuffs must provide nutritious diets that will effectively grow aquatic species with minimal environmental impact and produce high-quality fish flesh to confer human health benefits in a cost-effective manner [2]. A recognized disadvantage in most plant-derived nutrient sources is the presence of heat stable antinutritional factors that may condition its nutritional value by altering digestion and nutrient utilization [3]. Moreover, antinutrients by themselves or through their metabolic products may also interfere with physiology and/or health of farmed fish [2].

Soybean meal (SBM) is among the most commonly used vegetable ingredients as alternative to fish meal, although it contains antinutritional factors and antigens that can compromise fish health and welfare [4], [5]. For instance, full-fat and extracted SBM induced inflammation (enteropathy) in the gastro-intestinal tract of salmonids, common carp (Cyprinus carpio) and zebrafish (Danio rerio) [6], [7], [8], [9], [10], [11], [12]. The specific agents causing such condition are not yet known, but one or more of the alcohol-soluble components of full fat soybean meal, such as saponins and phytosterols, are likely involved, as alcohol extracted soy protein concentrate does not cause pathological changes in the intestine of salmonids [13], [14].

Saponins are naturally occurring surface-active glycosides with great complexity, consisting of a sugar moiety glycosidically linked to a hydrophobic aglycone (sapogenin) which may be triterpenoid or steroid in nature [15]. These substances have been reported to have physiological, immunological and pharmacological properties. For instance, saponins have been reported to lower growth performance of Chinook salmon (Oncorhynchus tshawytscha), rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar) and common carp [16], [17], [18], while other authors suggest that saponins are involved in the stimulation of cytokine production [15] and induction of inflammation [5]. The negative effects of saponins could be triggered by the well-known effects of these surface-active components on biological membranes [2]. Phytosterols (also called plant sterols) are a group of steroid alcohols, naturally occurring in plants, which can act as a structural component in the plant cell membrane, a role played in mammalian cells by cholesterol [19]. In fish, several studies have reported reproductive and endocrine disruptive effects after exposure to large amount of phytosterols, in particular β-sitosterol [20], [21]. However, data on the effects of such compounds when incorporated in fish diets are scarce [22], [23], [24].

A comparison of immunological parameters in fish fed a regular diet and fish exposed to antinutrients represents a unique tool for the investigation of the immunological mechanisms that are involved in the protection of the gut and the organism as a whole [5]. In the last few years, efforts have focused on immune-related changes due to SBM-induced enteritis at different stages of development through both histopathological and gene expression approaches in several fish species [11], [12], [25], [26], [27], [28], [29], including gilthead seabream [30]. Among the histopathological changes observed in fish fed SBM rich diets, infiltration of inflammatory cells such as T-cells, eosinophilic granular cells, macrophages, neutrophilic granulocytes and diffuse immunoglobulin M has been observed [6], [9], [11], [12], [14], [25], [26], with visible signs of inflammation after 7 days of feeding [9], [26]. The SBM-induced enteritis appears to involve T cell mobilization and has therefore been considered to be a hypersensitivity reaction [25], [26]. In contrast, the distal intestinal mucosa of gilthead seabream juveniles fed purified antinutrients was not characterized as inflammatory, but some changes in accumulation of substances in the absorptive vacuoles and leukocyte mobilization to the intraepithelial space were observed [30]. Still, only fragmentary information on the impacts of plant-derived antinutrients on fish immune responses is currently available, and the molecular mechanisms remain unknown.

The present study intends to evaluate the effects of two purified antinutrients, soy saponins and phytosterols alone or in combination, on the immunomodulation of an important species for Mediterranean aquaculture, the gilthead seabream. Given the limited knowledge of saponin and phytosterol effects on fish, gene expression profiling in the head-kidney and several blood immune parameters were assessed to study the transcriptomic and cell-mediated immune responses of gilthead seabream exposed to dietary antinutrients. This work was part of a larger feeding trial, and fish performance and physiological data have been reported in detail elsewhere [31].