A flagellin-poxvirus antigen vaccine: Strengths and limitations
Bacterial flagellin is a potent adjuvant that enhances adaptive immune responses to a variety of antigens. The vaccinia virus antigens L1R and B5R are highly immunogenic in the context of the parent virus, but recombinant forms of the proteins are only weakly immunogenic. Therefore, we evaluated the response to these antigens when flagellin was used as an adjuvant. Although flagellin promoted a robust antigen-specific humoral response to poxvirus antigens delivered intranasally (i.n.) or intramuscularly (i.m.), intramuscular immunization resulted in significantly high titers of anti-L1R and B5R IgG. Flagellin/poxvirus antigen fusion proteins were more potent than flagellin and L1R and B5R as separate proteins as inducers of a humoral response against the poxvirus antigens. At least three immunizations with flagellin/poxvirus fusion proteins were required to confer protection in mice against challenge with vaccinia virus. Although mice were protected and exhibited only limited signs of disease, they still exhibited significant, but reversible weight loss. When immune mice were depleted of complement using cobra venom factor, 50% of the mice succumbed to vaccinia virus infection. These results demonstrate that flagellin-poxvirus antigen fusion proteins are effective in eliciting protective immunity against vaccinia virus that is dependent, in part, on complement.
We evaluated the efficacy of additional flagellin-poxvirus antigen constructs to promote protective immunity and found that antigens can lose their immunogenicity when inserted into certain regions of flagellin. The loss of immunogenicity is dependent on the individual antigen, and was not the same for all antigens tested. When administering more than one immunogenic fusion protein antigen-specific titers decrease slightly, and the addition of excess flagellin will further decrease titers suggesting that there is a limit to the number of fusion proteins that can be administered in a single vaccine.