Particulate antigen uptake during immersion immunisation of fish: The effectiveness of prolonged exposure and the roles of skin and gill

Abstract This study examined the uptake of BSA-conjugated 1  μ m fluorescent latex microspheres from bath suspensions by 1·5–3·0 g rainbow trout, Oncorhynchus mykiss (Walbaum). Uptake was quantified using a novel method based on dissolving fish tissues (excluding the gastrointestinal tract) in an alkaline solution and capturing microspheres on a filter for microscopic counting. In suspensions of 1·0×10 7 particles ml −1 , the rate of uptake was approximately linear over 48 h of continuous immersion, with a 24 h mean±SE of 98 000±12 000 microspheres fish −1 . Uptake was logarithmically proportional to particle concentration in suspensions of 1·0×10 5 , 1·0×10 6 and 1·0×10 7 particles ml −1 . Prolonged immersion in a dilute suspension resulted in greater uptake than brief immersion in concentrated suspensions. Quantification of microspheres in separate tissues indicated that both skin and gill participate in uptake, with the skin playing a major role. Particles remained in skin and gill tissues for at least 24 days following exposure, while a minority were transported to the spleen and kidney. This observation supports an important role for local immunity in the protective responses generated during immersion immunisation. Histologically, microspheres were observed primarily within epithelial cells and underlying phagocytes of both skin and gill. Collectively these findings describe a new method for quantifying antigen uptake, which was used to demonstrate the effectiveness of prolonged exposure to dilute antigen suspensions and the importance of the skin as a site of antigen uptake during immersion immunisation.