The human IgG3 hinge mediates the formation of antigen dimers that enhance humoral immune responses to DNA immunisation.

A series of plasmid DNA constructs containing the 45W antigen gene from Taenia ovis were used to investigate the impact of antigen dimerisation on the humoral immune response to genetic immunisation. Genes encoding dimeric 45W were generated via fusion to the hinge region of human IgG3 (hIg). This region was selected because it is compact and contains 11 inter-chain disulphide-bridges. The DNA encoding the IgG3 hinge contains four exons, with the last three exons being repeats and possibly superfluous. Plasmids containing the 45W gene linked to exons 1-2, 1-3 or 1-4 of the hIgG3 hinge, were compared to a control plasmid containing a form of the 45W gene which encodes secreted, monomeric 45W protein. Western blot analysis was used to investigate the formation of the fusion-proteins in transfected Cos-7 cells. The full-length fusion construct expressed predominantly dimeric forms of the fusion-protein, while truncation of the hinge region decreased the abundance of dimeric fusion-protein and increased the proportion monomeric fusion antigen. In immunised BALB/c mice, 45W-specific antibody titres were increased 3 to 4-fold via fusion to the full-length hinge region, whereas the truncated constructs were similar to the control. IgG subclass analysis indicated that all mice generated predominantly IgG1, IgG2a and IgG2b antibodies. Therefore, these results suggest that the efficient formation of dimeric antigen, via fusion to the full-length hinge of human IgG3, can increase the immunogenicity of expressed antigens without altering the form of the immune response elicited by DNA immunisation.