Cross-Neutralization Activity of Single-Chain Variable Fragment (scFv) Derived from Anti-V3 Monoclonal Antibodies Mediated by Post-Attachment Binding

Production of an Antibody Fragment (scFv) Targeting PcrV Protein of Pseudomonas aeruginosa in Fed-Batch Cultivation Mode

Background:

Pseudomonas aeruginosa is one of the opportunistic pathogens causing frequent hospital-acquired life-threatening infections in mechanically ventilated patients. The most significant virulence factor of P. aeruginosa is T3SS. PcrV is an important structural protein of the T3SS.

Methods:

In the current investigation, a recombinant scFv mAb against the PcrV protein was expressed in EnBase® (fed-batch) cultivation mode. The pETiteTM N-His SUMO Kan vector, including anti-PcrV scFv gene, was transformed into Escherichia coli (BL21) cells. The expression and solubility of anti-PcrV scFv protein were investigated at two different temperatures (25 °C and 30 °C) and at different induction times (4, 6, 8, 12, and 24 hours).

Results:

Increased efficiency was achieved by EnBase® compared to LB broth; owing to the slow release of glucose, the maximum level of solubility and total protein expression was observed in EnBase® cultivation system at 30 °C and 24 h post induction. Furthermore, IC₅₀ for anti-PcrV scFv protein was determined to be approximately 7 μg/mL.

Conclusion:

Anti-PcrV scFv produced in this study showed promising in vitro results, protecting RBC from lysis by P. aeruginosa (exoU+).

Synergistic inhibition of cell-to-cell HIV-1 infection by combinations of single chain variable fragments and fusion inhibitors

Cell-to-cell spread of HIV permits ongoing viral replication in the presence of antiretroviral therapy and is suggested to be a major contributor to sexual transmission by mucosal routes. Fusion inhibitors that prevent viral entry have been developed, but their clinical applications have been limited by weak antiviral activity, short half-life, and the low genetic barrier to development of resistance. We examined the inhibitory activities of a series of single-chain variable fragments (scFvs) targeting the V3 and CD4i epitopes against both cell-free and cell-to-cell HIV infection. We found that all anti-V3 scFvs, including two newly constructed scFvs, showed broad neutralization activity against a panel of subtype B viruses compared with the corresponding IgGs. All scFvs neutralized cell-free infection by HIV-1_{JR-FL WT} and fusion inhibitor-resistant mutants. In addition, all anti-V3 scFvs and some CD4i scFvs significantly inhibited cell fusion, while their IgG counterparts did not. Furthermore, scFvs-fusion inhibitors combinations, such as C34 and SC34, showed synergistic inhibition of cell fusion by both HIV-1_{JR-FL WT} and fusion inhibitor-resistant mutants. The most prominent combinational effect was observed for 916B2 CD4i scFv with SC34. The delayed fusion kinetics of fusion inhibitor-resistant mutants partly explain their synergistic inhibition by such combinations. Our data demonstrate the advantages of using scFvs over their parent IgGs for inhibiting both cell-free and cell-to-cell infection. High synergistic inhibition of cell fusion by using scFvs-fusion inhibitors combinations suggests the possibility of intensification therapy adding this combination to current anti-HIV treatment regimens.

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Newly constructed anti-V3 scFvs showed broader HIV-1 neutralization activity.

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HIV-1 cell fusion was inhibited by scFvs better than the corresponding IgGs.

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Combinations of scFvs with fusion inhibitors synergistically inhibit cell fusion.

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Combination therapy with scFvs and fusion inhibitors may be effective.

Exploration of broadly neutralizing antibody fragments produced in bacteria for the control of HIV

While broadly neutralizing antibodies (bnAbs) are a promising preventative and therapeutic tool for HIV infection, production is difficult and expensive. Production of antibody-like fragments in bacterial cytoplasm provides a cheaper alternative. This work explored the transplantation of the complementarity determining regions of the anti-HIV bnAbs PGT121 and 10E8 onto a single-chain variable fragment (scFv) scaffold, previously discovered through a novel screening platform. The scaffolded 10E8 scFv, but not the scaffolded PGT121 scFv, was soluble in bacterial cytoplasm, enabling efficient production in bacteria. Three additional multimeric constructs employing the scaffolded 10E8 scFv were also generated and soluble versions produced in bacteria. However, the constructs were found to have substantially lost anti-HIV binding function and had completely abrogated neutralizing activity. Overall, while this study provides a proof-of-concept for anti-HIV bnAb construct production in bacterial cytoplasm, future refinement of these technologies will be required to realize the goal of producing inexpensive and effective bnAb-like tools for the control of HIV.

Unique binding modes for the broad neutralizing activity of single-chain variable fragments (scFv) targeting CD4-induced epitopes

Background

The CD4-induced (CD4i) epitopes in gp120 includes the co-receptor binding site, which are formed and exposed after interaction with CD4. Monoclonal antibodies (mAbs) to the CD4i epitopes exhibit limited neutralizing activity because of restricted access to their epitopes. However, small fragment counterparts such as single-chain variable fragments (scFvs) have been reported to neutralize a broad range of viruses compared with the full-size IgG molecule. To identify the CD4i epitope site responsible for this broad neutralization we constructed three scFvs of anti-CD4i mAbs from a human immunodeficiency virus type 1 (HIV-1)-infected elite controller, and investigated the neutralization coverage and precise binding site in the CD4i epitopes.

Results

We constructed scFvs from the anti-CD4i mAbs, 916B2, 4E9C, and 25C4b and tested their neutralization activity against a panel of 66 viruses of multi-subtype. Coverage of neutralization by the scFvs against this panel of pseudoviruses was 89% (59/66) for 4E9C, 95% (63/66) for 25C4b and 100% (66/66) for 916B2. Analysis using a series of envelope glycoprotein mutants revealed that individual anti-CD4i mAbs showed various dependencies on the hairpin 1 (H1) and V3 base. The binding profiles of 25C4b were similar to those of 17b, and 25C4b bound the region spanning multiple domains of H1 and hairpin 2 (H2) of the bridging sheet and V3 base. For 4E9C, the V3-base dependent binding was apparent from no binding to mutants containing the ΔV3 truncation. In contrast, binding of 916B2 was dependent on the H1 region, which is composed of β2 and β3 strands, because mutants containing the H1 truncation did not show any reactivity to 916B2. Although the H1 region structure is affected by CD4 engagement, the results indicate the unique nature of the 916B2 epitope, which may be structurally conserved before and after conformational changes of gp120.

Conclusions

Identification of a unique structure of the H1 region that can be targeted by 916B2 may have an important implication in the development of small molecules to inhibit infection by a broad range of HIV-1 for the purpose of HIV treatment and prevention.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-017-0369-y) contains supplementary material, which is available to authorized users.