Design and Stabilization of Stem Derived Immunogens from HA of Influenza A Viruses
Najar, Tariq Ahmad
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Influenza virus belongs to the Orthomyxovirus family of viruses that causes respiratory infection in humans, leading to morbidity and mortality. The mature influenza A virion has an envelope that contains two major surface glycoproteins proteins – hemagglutinin (HA) and neuraminidase (NA). HA is a highly antigenic molecules and is responsible binding to host cell surface receptors (Sialic acid), and membrane fusion between the viral membrane and the host endosomal membrane. Most of the antibody response generated against influenza virus either by vaccination or by natural infection is directed against HA. Influenza virus has segmented negative–sense RNA genome which gives the virus the ability to evade the host immune response by incorporating mutations (antigenic drift) and/or by reassotment with other subtypes of influenza A viruses (antigenic shift). Currently licensed vaccines which include an inactivated vaccine, a live attenuated vaccine, and recombinant subunit vaccine are beneficial for providing protection against seasonal influenza viruses that are closely related to the vaccine strain but fail to provide protection against drifted strains. This limits their breadth of protection and thus requires annual revaccination with reformulated vaccines. Also, because selection of a vaccine strain for the next season is purely based on surveillance and prediction, sometimes mismatches do happen between the selected vaccine strains and circulating viruses, resulting in a drastic decrease in vaccine efficacy and thus high morbidity and mortality. Furthermore, the production of these seasonal vaccines takes 6-8 months on an average, and does not guarantee protection against infection with novel reassortant viruses which can cause pandemics. To overcome the draw-backs of seasonal influenza virus vaccines and to enhance our pandemic preparedness, there is an increasing need for game-changing influenza virus vaccines that can confer robust, long-lasting protection against a broad spectrum of influenza virus isolates. Influenza hemagglutinin (HA) is highly immunogenic and thus a major target for vaccine design. HA is synthesized as a precursor polypeptide (HA0), assembles into a trimer, matures by proteolytic cleavage along the secretory pathway and is transported to the cell surface. Mature HA has a globular head domain, primarily composed of the HA1 subunit, which mediates receptor binding, while the stem domain, predominantly comprises of the HA2 subunit, and houses the fusion peptide. At neutral pH, the HA stem is trapped in a metastable state but undergoes an extensive conformational rearrangement at low pH in the late endosome (host-cell endosome) to trigger the fusion of virus and host membranes. Clusters of ‘antigenic sites’ have been identified in the head domain of HA, indicating that it harbors an almost continuous carpet of epitopes that are targeted by antibodies. However, these immunodominant sites constantly accumulate mutations to escape immune pressure, and thereby narrow the breadth of head-directed neutralizing antibodies (nAbs). In contrast to the highly-variable head domain, the membrane-proximal HA stem subdomain has much less sequence variability and, thus, is a desirable target for influenza vaccine development. In the recent past, several broadly neutralizing antibodies (bnAbs) targeting this subdomain with neutralizing activity against diverse influenza A virus subtypes have been isolated from infected people, further proving that this subdomain of HA can be targeted as a vaccine candidate. Steering the immune response towards this conserved, subimmunodominant stem subdomain in the presence of the variable immunodominant head domain of HA has been quite challenging. Alternatively, mimicking the epitome of these stem-directed bnAbs in the native, pre-fusion conformation in a ‘headless’ stem immunogenic capable of eliciting a broadly protective immune response has been difficult because of the metastable nature of HA. Addressing the aforementioned challenges, here we describe the design, stabilization and characterization of novel stem derived immunogens from HA of influenza A viruses using a protein minimization approach. Chapter 1 gives an overview of the influenza virus life cycle, nomenclature and classification of influenza virus; outlines the structural organization and functional properties of different viral proteins. An introduction to the kind of immune responses generated during vaccination or natural infection with the virus is discussed. The conventional vaccines that are currently used and their limitations, recent progress in the field of novel vaccine developmental approaches targeting the conserved epitopes on HA, is also described in this chapter. This chapter also gives a broad overview of bnAbs that have been isolated in the recent past, which target the novel antigenic signatures on HA. The design of a stem domain construct from an H3N2 virus (A/HK/68) is described in Chapter 2. In order to ensure that HA2 folds into the neutral pH conformation, regions of HA1 interacting with it were included in the design. Additionally, two Asp mutations were introduced in the B loop of HA2 to destabilize the low pH conformation and stabilize the desired native, neutral pH conformation. Studies using small peptides (57-98 of HA2) indicated that Asp mutations at positions 63 and 73 destabilized the low pH conformation. Studies on mutants with additional pairs of introduced Cys residues showed that the designed protein H3HA6 was folded into the neutral pH form. Immunization studies using mice showed that the protein was highly immunogenic and provided complete protection against a lethal dose of a homologous virus. Two constructs H3HA6a and H3HA6b, designed from the stem region of drifted H3N2 viruses (A/Phil/2/82 and A/Bris/10/07) were tested for protection against HK/68 to determine the extent of cross-strain protection provided by HA6. While HA6a (from A/Phil/2/82) provided near complete protection against HK/68, HA6b could protect against challenge only partially, possibly because of lower titers of antibodies elicited by this antigen. Studies using FcRγ chain knockout mice indicated that majority of the protection mediated by anti-HA6 antibodies was because of antibody mediated effectors functions, although neutralization as a mechanism of protection was also likely to contribute. In all the 18 subtypes of HA, the B loop contains residues that form the hydrophobic core of the extended coiled coil of the low pH form. As in the case of H3HA6, we suggest that these residues could be mutated to Asp to destabilize the low pH conformation. Two circularly permuted stem domain constructs from an H1N1 virus (A/PR/8/34) and an H5N1 virus (A/Viet/1203/04) were made. The design and characterization of these proteins is described in Chapter 3. H1HA6, H1HA0HA6 and H5HA6 were purified from inclusion bodies and refolded. The proteins H1HA6 and H1HA0HA6 were highly immunogenic and provided protection against a lethal challenge with homologous PR/8/34 virus. Anti-H1HA6 sera had higher titres of antibodies against heterogonous HAs as compared to convalescent sera. Stem derived immunogens from drifted H1N1 viruses (A/NC/20/99 and A/Cal/7/09) have been made and tested for cross-protection with PR/8/34 challenge. While H5HA6 also elicited high titers of antibodies, it could only protect partially against PR/8/34 challenge probably because high enough titers of cross-reactive protective antibodies were not elicited by this protein. These stem immunogens conferred robust subtype specific and modest heterosubtypic protection in vivo against lethal virus challenge. However, the immunogens, especially H1HA6, a stem immunogen from group 1 (PR8) virus is aggregation prone when expressed in E.coli. The strategy used to improve the biophysical and biochemical properties and thus the immunogenicity of these stem derived immunogens is discussed in Chapter 4. A random mutagenesis library of H1HA6 was constructed by error prone PCR using modified nucleotide analogues. The library was displayed on the yeast cell surface to isolate mutants showing better surface expression and improvement in binding to the broadly neutralizing antibody CR6261 compared to the wild-type protein. We isolated few clones, of which one mutant (H1HA6P2) dominated the enriched population. The other mutants differed slightly from H1HA6P2. This mutant differs from the wild-type by two mutations K314E and M317T (H1 numbering) which are close to the CR6261 binding site but outside the antibody foot-print (epitope). This mutant showed improved binding to CR6261 and exhibited significant improvement in surface expression. Improvement was also observed in binding of this mutant to F16v3-ScFv (another broadly neutralizing antibody). Two cysteine mutations were also introduced to further stabilize the trimeric form of the protein. Chapter 5 describes the biophysical and biochemical characterization of the high affinity isolated mutant at the protein level. We expressed this affinity matured mutant gene in E.coli and purified the protein from inclusion bodies. The stabilized mutant protein showed remarkable improvement in biophysical and biochemical properties and was recognized by stem directed conformation sensitive broadly neutralizing antibodies CR6261, F10 and F16v3 with affinity comparable to the full-length HA ectodomain. These results clearly suggest that this mutant protein is properly folded in its native pre-fusion conformation and thus can be an excellent candidate for eliciting stem directed broadly neutralizing antibodies. All these stabilized versions of stem derived immunogens will be tested for immunogenicity and cross-protection with different viral challenges. Chapter 6 describes the development of a method for mapping antibody epitopes (especially conformational epitopes) down to the residue level. Using a panel of single cysteine mutants, displayed on the yeast cell surface, this bypasses the need for laborious and time consuming protein purifications steps used in conventional methods for epitope mapping. We made a panel of single cysteine mutants, covering the entire surface of the antigen (CcdB, a bacterial toxin protein), displayed each mutant individually as well as in a pool, representing all mutants together on the yeast cell surface, and covalently labeled the cysteine with biotin-PEG2-maleimide to mask the area. The effect on antibody binding was monitored to identify the residues and relative positions important for antibody interactions with the displayed antigen by flow cytometry. By using this method we were able to map the conformational as well as linear epitopes of a panel of monoclonal antibodies down to the residue level with ease, and also identify the regions on the antigen which contribute to the antigen city during immunization in different animals. Since, this method is quite easy, rapid and gives in-depth information about antigenic epitopes, it can be useful in rational design of epitomes specific vaccines and other antibody therapeutics. It can easily be extended to other display systems and is a general approach to probe macromolecular interfaces.
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