Aeromonas hydrophila vaccine development using immunoproteomics

Abstract

Aeromonas hydrophila is an opportunistic pathogen that causes a wide range of symptoms and diseases in fish. Development of a commercial vaccine has been problematic due to the heterogenicity between isolates of A. hydrophila. A new approach using immunoproteomics was used in this study to try to develop a vaccine that would protect against a wide range of A. hydrophila strains.

The virulence of 14 isolates of A. hydrophila from different geographical regions was determined in common carp (Cyprinus carpio) indicating that 6 isolates were virulent, while 8 isolates were avirulent. Expression of cellular and extracellular products (ECP) of six of these isolates (4 virulent and 2 avirulent isolates) were examined following culture of the bacterium in vitro, in tryptic soy broth, and in vivo, in dialysis tubing placed within the peritoneal cavity of carp. Two types of molecular weight cut off tubes (25 and 100 kDa) were used for the implants. Whole cell (WC), outer membrane protein (OMP) and ECPs of the bacteria grown in vitro and in vivo were analysed by 1 dimensional sodium dodecyl sulphate-polyacrylamide gel electrophoresis (1D SDS-PAGE). Additionally, 2D SDS-PAGE was used to analyse WC preparations of A. hydrophila grown in vitro and in vivo. The production of unique proteins and up and down-regulation of protein expression were observed in all the preparations of bacteria grown in vitro and in vivo. Unique bands were seen in the 1D SDS-PAGE at 58 and 55 kDa for WC and OMP preparations, respectively, for all the isolates cultured in vivo. Bands of increased intensity were observed at 70, 55, 50 and 25 kDa with WC preparations for the virulent isolates cultured in vivo. Analysis of WC preparations by 2D SDS-PAGE indicated differences in the expression of spots between bacteria cultured in vitro and in vivo. A number of unique spots, mostly between 30 and 80 kDa with pI values ranging from 5.0-6.0 were observed in the bacteria grown in vivo.

The protein profiles of different preparations (WC, OMP, ECP) of bacteria cultured in vitro and in vivo were screened by 1D Western blot using antibodies from carp artificially infected with different isolates of A. hydrophila to identify potential vaccine candidates. The WC preparations of A. hydrophila (T4 isolate) grown in vitro were also analysed by 2D Western blot. A 50 kDa protein of A. hydrophila was found to be the most immunogenic molecule in both WC and OMP of bacteria grown both in vitro and in vivo. The protection efficacy of this protein was determined in goldfish by vaccinating fish with electro-eluted 50 kDa protein then challenging the fish with A. hydrophila. Fish were also passively immunised with fish sera raised to the 50 kDa protein and then challenged. The relative percentage survival (RPS) was 67 % in the vaccination trial, while the results were inconclusive for the passive immunisation trial.

The 50 kDa protein was confirmed to be the S-layer protein of A. hydrophila following identification using matrix-assisted laser desorption ionisation time-of-flight mass spectrometry (MALDI-TOF MS). Recombinant S-layer protein was then produced and the cross-protection efficacy of this protein against six virulent isolates of A. hydrophila was confirmed in a large scale vaccination trial using carp. The RPS value for the 6 isolates of A. hydrophila ranged from between 56 and 87 %. The results of this project suggest that the immunogenic S-layer protein of A. hydrophila could be used as a common antigen to protect fish against infection by different isolates of this pathogenic bacterium.