Currently, inflammatory bowel disease (IBD) is a serious public health problem on the rise worldwide. In this work, we utilized the zebrafish to introduce a new model of intestinal inflammation triggered by food intake. Taking advantage of the translucency of the larvae and the availability of transgenic zebrafish lines with fluorescently labeled macrophages, neutrophils, or lymphocytes, we studied the behavior of these cell types in vivo during the course of inflammation. We established two feeding strategies, the first using fish that were not previously exposed to food (naïve strategy) and the second in which fish were initially exposed to normal food (developed strategy). In both strategies, we analyzed the effect of subsequent intake of a control or a soybean meal diet. Our results showed increased numbers of innate immune cells in the gut in both the naïve or developed protocols. Likewise, macrophages underwent drastic morphological changes after feeding, switching from a small and rounded contour to a larger and dendritic shape. Lymphocytes colonized the intestine as early as 5 days post fertilization and increased in numbers during the inflammatory process. Gene expression analysis indicated that lymphocytes present in the intestine correspond to T helper cells. Interestingly, control diet only induced a regulatory T cell profile in the developed model. On the contrary, soybean meal diet induced a Th17 response both in naïve and developed model. In addition, when feeding was performed in rag1 -deficient fish, intestinal inflammation was not induced indicating that inflammation induced by soybean meal is T cell-dependent.

Intestinal inflammation is a harmful condition in fish that can be triggered by the ingestion of soybean meal. Due to the positive costs-benefits ratio of including soybean meal in farmed fish diets, identifying additives with intestinal anti-inflammatory effects could contribute to solving the issues caused by this plant protein. This study evaluated the effect of incorporating lactoferrin (LF) into a soybean meal-based diet on intestinal inflammation in zebrafish. Larvae were fed with diets containing 50% soybean meal (50SBM) or 50SBM supplemented with LF to 0.5, 1, 1.5 g/kg (50SBM+LF0.5; 50SBM+LF1.0; 50SBM+LF1.5). The 50SBM+LF1.5 diet was the most efficient and larvae had a reduced number of neutrophils in the intestine compared with 50SBM larvae and an indistinguishable number compared with control larvae. Likewise, the transcription of genes involved in neutrophil migration and intestinal mucosal barrier functions (mmp9, muc2.2, and β-def-1) were increased in 50SBM larvae but were normally expressed in 50SBM+LF1.5 larvae. To determine the influence of intestinal inflammation on the general immune response, larvae were challenged with Edwardsiella tarda. Larvae with intestinal inflammation had increased mortality rate compared to control larvae. Importantly, 50SBM+LF1.5 larvae had a mortality rate lower than control larvae. These results demonstrate that LF displays a dual effect in zebrafish, acting as an intestinal anti-inflammatory agent and improving performance against bacterial infection.

Soybean meal has been used in many commercial diets for farm fish; despite this component inducing intestinal inflammation. On the other hand, microalgae have increasingly been used as dietary supplements in fish feed. Nevertheless, the vast quantity of microalgae species means that many remain under- or unstudied, thus limiting wide scale commercial application. In this work, we evaluated the effects to zebrafish (Danio rerio) of including Tetraselmis sp (Ts); Phaeodactylum tricornutum (Pt); Chlorella sp (Ch); Nannochloropsis oculata (No); or Nannochloropsis gaditana (Ng) as additives in a soybean meal-based diet on intestinal inflammation and survival after Edwardsiella tarda infection. In larvae fed a soybean meal diet supplemented with Ts, Pt, Ch, or Ng, the quantity of neutrophils present in the intestine drastically decreased as compared to larvae fed only the soybean meal diet. Likewise, Ts or Ch supplements in soybean meal or fishmeal increased zebrafish survival by more than 20% after being challenged. In the case of Ts, the observed effect correlated with an increased number of neutrophils present at the infection site. These results suggest that the inclusion of Ts or Ch in fish diets could allow the use of SBM and at the same time improve performance against pathogen.