Pasteurella multocida is the causative agent of a range of animal, and occasionally human, diseases. Problems with antimicrobial treatment of P. multocida highlight the need to find other possible ways, such as prophylaxis, to manage infections. Current vaccines against P. multocida
include inactivated bacteria, live attenuated and nonpathogenic
bacteria; they have disadvantages such as lack of immunogenicity,
reactogenicity, or reversion to virulence. Using bioinformatics
approaches, potentially immunogenic and protective epitopes were
identified and merged to design the most optimally immunogenic triple
epitope PlpE fusion protein of P. multocida as a vaccine candidate. This triple epitope (PlpE1 + 2 + 3)
was cloned into the pBAD/gIII A plasmid (pBR322-derived expression
vectors designed for regulated, secreted recombinant protein expression
and purification in Escherichia coli), expressed in Top 10 E. coli
and purified in denatured form using Ni-NTA chromatography and 8 M
urea. The immunogenicity of the purified proteins in BALB/c mice was
assayed by measuring immunoglobulin G (IgG) responses. The protection
potential was evaluated by challenging with 10 LD50 of serotype A:1,
X-73 strain of P. multocida and compared with commercially
available inactivated fowl cholera vaccine and PlpE protein. IgG levels
elicited by the polytope fusion protein of P. multocida PlpE were
higher than both commercially available inactivated fowl cholera
vaccine and PlpE protein. Surprisingly, protection was independent of
IgG level; commercially available inactivated fowl cholera vaccine (100%
protection) was more protective than the polytope fusion protein (69%
protection) and PlpE protein (69% protection). These results also
confirm that IgG level is not a reliable indicator of protection.
Further studies to evaluate the other antibody classes, such as
immunoglobulin A or M, are required. The role of cell-mediated immunity
should also be considered as a potential protection pathway.