Aquafeed containing raw or hydrolysed Nannochloropsis gaditana differently modulate the intestinal immune system of gilthead seabream

Aquaculture development is key for sustainable blue-based economy in the Mediterranean countries, with gilthead seabream (Sparus aurata) being one of the most important commercial species. The use of nutraceuticals is a well-known strategy to improve fish welfare and the sustainability of the sector. Microalgae are good sources of biologically active compounds, although most of their effects on fish immune responses remain poorly characterized. Resident leucocyte populations have been described in fish gut; however, their modulation by dietary nutraceuticals has been mostly limited to immune-related gene expression profile studies. Here, we evaluated the presence, abundance and distribution of leucocytes in the posterior gut by immunohistochemistry and correlated these data with gene expression profiles. Gilthead seabream juveniles were feed for 60 days with aquafeeds containing Nannocloropsis gaditana biomass at low inclusion levels (2.5 and 5%) in two formats (raw and cellulase-hydrolysed), or with a microalgae-free diet (control). Acidophilic granulocytes were recruited to the posterior gut in fish fed with hydrolysed N. gaditana diets, whereas hepcidin-positive cells increased in fish fed crude microalgae diets. Remarkably, in both cases, transcriptional profiles indicated a decrease in inflammatory responses. Accordingly, the number of Histamine-positive cells remained stable regardless of diet, as did NK-like (Nkl-positive) cells. However, the expression of adaptative immune cell markers and their correlation with other adaptive and innate immune-relevant genes revealed differences attributable to the microalga biomass format. Specially, hydrolysed N. gaditana diets induced a more coordinated response between innate and adaptative immunity.