Antigen uptake and immune responses after immersion vaccination.

Immersion vaccination is an effective and practical method for mass vaccination of fish and most commercial bacterins are currently administered by this method, even though the exact mechanisms of antigen uptake and protection still remain unknown. Immersion vaccination includes several delivery techniques including spray, direct immersion, hyperosmotic dip and flush exposure. Various factors have been shown to influence the uptake of antigen from a vaccine bath, such as the concentration of vaccine, the length of immersion time, size of the fish, stress, pH and salt concentration of the vaccine solution, the water temperature, anaesthetics, the use of adjuvants, and the physical state (particulate or soluble) of the antigen. Among these, the antigen concentration is the most important factor for antigen uptake and protection. Recently we found that the amount of antigen taken up is correlated with the length of immersion time in dilute vaccine solutions. Most authors have suggested the gills as the main site of antigen entry, but skin, lateral line and the gut have also been suggested. Our quantitative study has shown that the skin is the main site of antigen uptake and that there are no differences in rate of uptake between the lateral line and the remaining skin of the body surface. Not only phagocytes but also several types of epithelial cells are involved in antigen uptake. Cells involved in antigen uptake can be different depending on the physical state of the antigen and the site of antigen entry. In most trials with immersion vaccination, antibodies against pathogens are not detectable in the serum by micro-titration and, even when antibodies are found, the titre does not always correlate with protection. However, some authors have reported elevated specific antibody levels in the serum of fish vaccinated by immersion, and even that protection can be successfully conferred by transferring immune plasma. Thus, the role of humoral immunity on protection mechanisms after immersion vaccination has been controversial and potentially important roles for cell-mediated immunity or local immunity have been implicated.