Abstract
Protein minimization is an attractive approach for designing vaccines against rapidly evolving pathogens such as human immunodeficiency virus, type 1 (HIV-1), because it can help in focusing the immune response toward conserved conformational epitopes present on complex targets. The outer domain (OD) of HIV-1 gp120 contains epitopes for a large number of neutralizing antibodies and therefore is a primary target for structure-based vaccine design. We have previously designed a bacterially expressed outer-domain immunogen (ODEC) that bound CD4-binding site (CD4bs) ligands with 3-12 μm affinity and elicited a modest neutralizing antibody response in rabbits. In this study, we have optimized ODEC using consensus sequence design, cyclic permutation, and structure-guided mutations to generate a number of variants with improved yields, biophysical properties, stabilities, and affinities (KD of 10-50 nm) for various CD4bs targeting broadly neutralizing antibodies, including the germline-reverted version of the broadly neutralizing antibody VRC01. In contrast to ODEC, the optimized immunogens elicited high anti-gp120 titers in rabbits as early as 6 weeks post-immunization, before any gp120 boost was given. Following two gp120 boosts, sera collected at week 22 showed cross-clade neutralization of tier 1 HIV-1 viruses. Using a number of different prime/boost combinations, we have identified a cyclically permuted OD fragment as the best priming immunogen, and a trimeric, cyclically permuted gp120 as the most suitable boosting molecule among the tested immunogens. This study also provides insights into some of the biophysical correlates of improved immunogenicity.
Highlights
Protein minimization is an attractive approach for designing vaccines against rapidly evolving pathogens such as human immunodeficiency virus, type 1 (HIV-1), because it can help in focusing the immune response toward conserved conformational epitopes present on complex targets
We previously reported the design of a nonglycosylated Escherichia coli– expressed outer-domain fragment immunogen (ODEC) that bound CD4 with micromolar affinity and elicited a modest neutralizing antibody response in rabbits [7]
ODEC design was focused on the CD4-binding site (CD4bs) of gp120 glycoprotein (Fig. 1, A and B; Table 1)
Summary
Protein minimization is an attractive approach for designing vaccines against rapidly evolving pathogens such as human immunodeficiency virus, type 1 (HIV-1), because it can help in focusing the immune response toward conserved conformational epitopes present on complex targets. The outer domain (OD) of HIV-1 gp120 contains epitopes for a large number of neutralizing antibodies and is a primary target for structure-based vaccine design. We have previously designed a bacterially expressed outer-domain immunogen (ODEC) that bound CD4-binding site (CD4bs) ligands with 3–12 M affinity and elicited a modest neutralizing antibody response in rabbits. We previously reported the design of a nonglycosylated Escherichia coli– expressed outer-domain fragment immunogen (ODEC) that bound CD4 with micromolar affinity and elicited a modest neutralizing antibody response in rabbits [7]. We rationally modified the initial ODEC design to generate several derivatives with improved yield, biophysical
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