Aspects of mucosal immunity in rainbow trout, Oncorhynchus mykiss (Walbaum. 1792)
The immunological characteristics of the mucosae of rainbow trout, Oncorhynchus mykiss, were investigated. A technique for the isolation of viable cells from the intestinal and cutaneous mucosae was developed which enabled differential cell counts to be performed on suspensions of cells liberated from these sites. This was achieved using light and electron microscopical, enzyme histochemical and flow cytometric techniques. In cell suspensions derived from the intestinal mucosa, lymphocytes, macrophages and eosinophilic granulocytes (EGC) were present, lymphocytes being by far the most numerous. Goblet cells and epithelial cells were also present but neutrophils were not apparent. In cell suspensions derived from the cutaneous mucosa, lymphocytes, macrophages, neutrophils and several types of cell with hitherto undescribed morphologies were identified. In addition, goblet cells and epithelial cells were present. A small proportion of the cells from the cutaneous mucosa were leucocytes compared with cells from the intestinal mucosa. A small number (about 4%) of lymphocytes from the intestinal mucosa reacted with a monoclonal anti-trout immunoglobulin antibody, suggesting that these cells may be B cells. In the case of the intestinal mucosa it was found to be possible to enrich for certain cell types using Percoll gradients. Functional studies were performed using cells from the intestinal mucosa. These cells were capable of phagocytosing latex microspheres and releasing reactive oxygen species, although only in small quantities compared with cells from the head kidney. Intestinally-derived cells secreted a factor(s) with the ability to induce head kidney cells to migrate and this could be enhanced by treatment of the gut cells with calcium ionophore. Intestinal cells were unable to migrate themselves under the conditions investigated. In addition, intestinal cells were able to proliferate in response to stimulation by the T-cell mitogen phytohaemagglutinin, as determined by incorporation of triated thymidine. Cells isolated from the intestinal mucosa were also able to secrete a macrophage-activating factor (MAF) with the ability to upregulate the production of reactive oxygen species in normal head kidney macrophages.