An efficacious recombinant subunit vaccine against the salmonid rickettsial pathogen Piscirickettsia salmonis

Piscirickettsia salmonis is the aetiological agent of salmonid rickettsial septicaemia, an economically devastating rickettsial disease of farmed salm onids. Infected salmonids respond poorly to antibiotic treatment and no eff ective vaccine is available for the control of P. salmonis. Bacterin prepar ations of P. salmonis were found to elicit a dose-dependent response in coh o salmon (Oncorhynchus kisutch). which varied from inadequate protection to exacerbation of the disease. However, an outer surface lipoprotein of P. s almonis, OspA, recombinantly produced in Escherichia coli elicited a high l evel of protection in vaccinated coho salmon with a relative percent surviv al as high as 59%, for this single antigen. In an effort to further improve the efficacy of the OspA recombinant vaccine. T cell epitopes (TCE's) from tetanus toxin and measles virus fusion protein. that are universally immun ogenic in mammalian immune systems, were incorporated tandemly into an OspA fusion protein. Addition of these TCE's dramatically enhanced the efficacy of the OspA vaccine, reflected by a three-fold increase in vaccine efficac y. These results represent a highly effective monovalent recombinant subuni t vaccine for a rickettsia-like pathogen, P. salmonis, and for the first ti me demonstrate the immunostimulatory effect of mammalian TCE's in the salmo nid immune model. These results may also be particularly pertinent to salmo nid aquaculture in which the various subspecies are outbred and of heterolo gous haplotypes. (C) 2001 Elsevier Science Ltd. All rights reserved.