Hepatitis B core
(HBc) virus-like particles (VLPs) and flagellin
are highly immunogenic and widely explored vaccine delivery platforms.
Yet, HBc VLPs mainly allow the insertion of relatively short antigenic
epitopes into the immunodominant c/e1 loop without affecting VLP assembly,
and flagellin-based vaccines carry the risk of inducing systemic adverse
reactions. This study explored a hybrid flagellin/HBc VLP (FH VLP)
platform to present heterologous antigens by replacing the surface-exposed
D3 domain of flagellin. FH VLPs were prepared by the insertion of
flagellin gene into the c/e1 loop of HBc, followed by E. coli expression, purification, and self-assembly
into VLPs. Using the ectodomain of influenza matrix protein 2 (M2e)
and ovalbumin (OVA) as models, we found that the D3 domain of flagellin
could be replaced with four tandem copies of M2e or the cytotoxic
T lymphocyte (CTL) epitope of OVA without interfering with the FH
VLP assembly, while the insertion of four tandem copies of M2e into
the c/e1 loop of HBc disrupted the VLP assembly. FH VLP-based M2e
vaccine elicited potent anti-M2e antibody responses and conferred
significant protection against multiple influenza A viral strains,
while FljB- or HBc-based M2e vaccine failed to elicit significant
protection. FH VLP-based OVA peptide vaccine elicited more potent
CTL responses and protection against OVA-expressing lymphoma or melanoma
challenges than FljB- or HBc-based OVA peptide vaccine. FH VLP-based
vaccines showed a good systemic safety, while flagellin-based vaccines
significantly increased serum interleukin 6 and tumor necrosis factor
α levels and also rectal temperature at increased doses. We
further found that the incorporation of a clinical CpG 1018 adjuvant
could enhance the efficacy of FH VLP-based vaccines. Our data support
FH VLPs to be a highly immunogenic, safe, and versatile platform for
vaccine development to elicit potent humoral and cellular immune responses.