A key hallmark of the vertebrate adaptive immune system is the generation of antigen-specific antibodies from B cells. Fish are the most primitive gnathostomes (jawed vertebrates) possessing an adaptive immune system. Vaccination of rainbow trout against enteric redmouth disease (ERM) by immersion in Yersinia ruckeri bacterin confers a high degree of protection to the fish. The immune mechanisms responsible for protection may comprise both cellular and humoral elements but the role of specific immunoglobulins in this system has been questioned and not previously described. The present study demonstrate significant increase in plasma antibody titers following immersion vaccination and significantly reduced mortality during Y. ruckeri challenge.
Rainbow trout were immersion-vaccinated, using either a commercial ERM vaccine (AquaVacTM ERM vet) or an experimental Y. ruckeri bacterin. Half of the trout vaccinated with AquaVacTM ERM vet received an oral booster (AquaVacTM ERM Oral vet). Sub-groups of the fish from each group were subsequently exposed to 1x109 CFU Y. ruckeri/ml either eight or twenty-six weeks post vaccination (wpv). All vaccinated groups showed 0% mortality when challenged, which was highly significant compared to the non-vaccinated controls (40 and 28 % mortality eight and twenty-six weeks post vaccination (wpv), respectively) (P<0.0001). Plasma samples from all groups of vaccinated fish were taken 0, 4, 8, 12, 16 and 26 wpv. and Y. ruckeri specific IgM antibody levels were measured with ELISA. A significant increase in titers was recorded in vaccinated fish, which also showed a reduced bacteremia during challenge. In vitro plasma studies showed a significantly increased bactericidal effect of fresh plasma from vaccinated fish indicating that plasma proteins may play a role in protection of vaccinated rainbow trout.
In order to test whether Y. ruckeri specific antibodies can confer protection against the disease, have we injected plasma containing specific antibodies against Y. ruckeri in naive rainbow trout and challenged them with Y. ruckeri. Passive transfer of specific antibodies significantly increased the survival. The present work has shown that immersion vaccination with Y. ruckeri bacterin induced significant protection, and that the increase of specific antibodies is part of the protective mechanism behind the successful immersion vaccination against ERM.