DNA vaccines have the ability to induce humoral and cellular immune responses but their efficiency needs further improvement. One strategy is to target the antigen to surface receptors on antigen-presenting cells (APCs), another is to use structure-based antigens that display a conformation relevant for induction of neutralizing antibodies (Abs). Recently, glycoprotein 120 (gp120) from human immunodeficiency virus-1 (HIV-1) was developed into a resurfaced stabilized core 3 (RSC3) molecule, in which the CD4 binding site is exposed and immunogenic residues are deleted or substituted with non-immunogenic residues. In this study, our aim was to investigate the RSC3 molecule for its ability to induce Ab responses, when inserted into a bivalent vaccine format that can target antigen to APCs. The vaccine format is denoted vaccibody. In this study, we also included gp120 as an antigen in the vaccibody format, as well as a vaccine construct encoding monomeric free gp120.
We used molecular techniques to insert the DNA encoding the antigens into the vaccibody format. The targeting units employed were a single chain fragment variable (scFv) specific for MHC class II and the chemokine MIP-1α/CCL3 which is specific for the chemokine receptors CCR1, 3, and 5. The vaccine constructs were tested in vitro by transfection into HEK293E cells and by analyses of the cell supernatants utilizing ELISA, western, and flow cytometry. The assays revealed that the vaccine constructs were secreted, formed dimers, and that the targeting units were functional. Inserting antigens into a vaccine format might disrupt the conformation they possess as free antigens. However, gp120 as well as RSC3 were recognized by a number of various monoclonal Abs that bind to discontinuous epitopes suggesting that the antigens retained their native conformation. When delivered in BALB/c mice, all vaccine constructs elicited anti-gp120 Abs, but there was no apparent effect of the targeting. Sera form immunized mice could also inhibit infection by HIV-1 in an in vitro neutralization assay. Taken together, this study suggests that RSC3 could be used as an HIV-1 vaccine immunogen when inserted into a vaccibody format and is worthy of further investigation.