Induction of humoral immunity with plasmid DNA encoding an immunogenic fragment of Pasteurella haemolytica A1 leukotoxin

Pneumonic pasteurellosis is an acute respiratory disorder that results from infection of the bovine lung by Pasteurella haemolytica A1. The primary virulence determinant is a secreted, ruminant-specific leukotoxin (LktA). The leukotoxin is a pore forming toxin that promotes inflammation and bacterial proliferation in the airways of infected animals. Studies continue to show that leukotoxin-neutralizing antibodies are required for significant levels of protective immunity. This laboratory has focused on characterizing this toxin in an attempt to define important immunogenic epitopes which could be incorporated into an effective vaccine. These studies were undertaken to explore the ability of a novel vaccination approach to elicit protective leukotoxin-specific antibodies in immunized mice. A plasmid encoding the immunogenic LktA fragment corresponding to amino acids 715-953 was used in a DNA-based vaccine and was examined for its efficacy in generating a leukotoxin-specific immune response in mice. In addition, the potential immunostimulatory peptides, human IL-1β 163-171 and human acidic isoferritin 172-185, were incorporated into DNA-based vaccines and into GSTfusion- protein vaccines, and monitored for the ability to augment the developing antibody response. Results from these studies indicated 1) the leukotoxin-encoding plasmid used in DNA-based vaccination is capable of generating protein-specific antibodies, however, these antibodies are not neutralizing; 2) the adjuvant peptides are capable of augmenting the immune response when incorporated in a DNA-based vaccine, although they do not contribute to the development of neutralizing antibodies; and 3) the adjuvant peptides do not display immunostimulatory effects when incorporated between the GST moiety and the LktA fragment in recombinant protein vaccines.