Fusion of influenza virus with the endosomal membrane is inhibited by monoclonal antibodies to defined epitopes on the hemagglutinin

A recombinant protein containing influenza viral conserved epitopes and superantigen induces broad-spectrum protection

Influenza pandemics pose public health threats annually for lacking vaccine that provides cross-protection against novel and emerging influenza viruses. Combining conserved antigens that induce cross-protective antibody responses with epitopes that activate cross-protective T cell responses might be an attractive strategy for developing a universal vaccine. In this study, we constructed a recombinant protein named NMHC that consists of influenza viral conserved epitopes and a superantigen fragment. NMHC promoted the maturation of bone marrow-derived dendritic cells and induced CD4+ T cells to differentiate into Th1, Th2, and Th17 subtypes. Mice vaccinated with NMHC produced high levels of immunoglobulins that cross-bound to HA fragments from six influenza virus subtypes with high antibody titers. Anti-NMHC serum showed potent hemagglutinin inhibition effects to highly divergent group 1 (H1 subtype) and group 2 (H3 subtype) influenza virus strains. Furthermore, purified anti-NMHC antibodies bound to multiple HAs with high affinities. NMHC vaccination effectively protected mice from infection and lung damage when exposed to two subtypes of H1N1 influenza virus. Moreover, NMHC vaccination elicited CD4+ and CD8+ T cell responses that cleared the virus from infected tissues and prevented virus spread. In conclusion, this study provides proof of concept that NMHC vaccination triggers B and T cell immune responses against multiple influenza virus infections. Therefore, NMHC might be a candidate universal broad-spectrum vaccine for the prevention and treatment of multiple influenza viruses.

Influenza hemagglutinin-specific IgA Fc-effector functionality is restricted to stalk epitopes

In this study, we utilized a panel of human immunoglobulin (Ig) IgA monoclonal antibodies isolated from the plasmablasts of eight donors after 2014/2015 influenza virus vaccination (Fluarix) to study the binding and functional specificities of this isotype. In this cohort, isolated IgA monoclonal antibodies were primarily elicited against the hemagglutinin protein of the H1N1 component of the vaccine. To compare effector functionalities, an H1-specific subset of antibodies targeting distinct epitopes were expressed as monomeric, dimeric, or secretory IgA, as well as in an IgG1 backbone. When expressed with an IgG Fc domain, all antibodies elicited Fc-effector activity in a primary polymorphonuclear cell-based assay which differs from previous observations that found only stalk-specific antibodies activate the low-affinity FcγRIIIa. However, when expressed with IgA Fc domains, only antibodies targeting the stalk domain showed Fc-effector activity in line with these previous findings. To identify the cause of this discrepancy, we then confirmed that IgG signaling through the high-affinity FcγI receptor was not restricted to stalk epitopes. Since no corresponding high-affinity Fcα receptor exists, the IgA repertoire may therefore be limited to stalk-specific epitopes in the context of Fc receptor signaling.

A Bimodal Nanosensor for Probing Influenza Fusion Protein Activity Using Magnetic Relaxation

Viral fusion is a critical step in the entry pathway of enveloped viruses and remains a viable target for antiviral exploration. The current approaches for studying fusion mechanisms include ensemble fusion assays, high-resolution cryo-TEM, and single-molecule fluorescence-based methods. While these methods have provided invaluable insights into the dynamic events underlying fusion processes, they come with their own limitations. These often include extensive data and image analysis in addition to experimental time and technical requirements. This work proposes the use of the spin-spin T2 relaxation technique as a sensitive bioanalytical method for the rapid quantification of interactions between viral fusion proteins and lipids in real time. In this study, new liposome-coated iron oxide nanosensors (LIONs), which mimic as magnetic-labeled host membranes, are reported to detect minute interactions occurring between the membrane and influenza's fusion glycoprotein, hemagglutinin (HA). The influenza fusion protein's interaction with the LION membrane is detected by measuring changes in the sensitive spin-spin T2 magnetic relaxation time using a bench-top NMR instrument. More data is gleaned from including the fluorescent dye DiI into the LION membrane. In addition, the effects of environmental factors on protein-lipid interaction that affect fusion such as pH, time of incubation, trypsin, and cholesterol were also examined. Furthermore, the efficacy and sensitivity of the spin-spin T2 relaxation assay in quantifying similar protein/lipid interactions with more native configurations of HA were demonstrated using virus-like particles (VLPs). Shorter domains derived from HA were used to start a reductionist path to identify the parts of HA responsible for the NMR changes observed. Finally, the known fusion inhibitor Arbidol was employed in our spin-spin T2 relaxation-based fusion assay to demonstrate the application of LIONs in real-time monitoring of this aspect of fusion for evaluation of potential fusion inhibitors.

An amphiphilic invertible polymer as a delivery vehicle for a M2e-HA2-HA1 peptide vaccine against an Influenza A virus in pigs

Influenza A viruses (IAVs) are a group of genetically diverse and economically important zoonotic pathogens. Despite decades of research, effective and broadly protective vaccines are yet to be developed. Recent breakthroughs in epitope-based immunization for influenza viruses identify certain conserved regions of the HA2 and M2e proteins as capable of inducing broad protection against multiple influenza strains. The M2e and HA2 peptides have been evaluated in mice but not as a combination in pigs, which play an important role in the transmission and evolution of IAV. Peptides are inherently weak immunogens; and effective delivery of peptide antigens is challenging. To enhance the delivery and immunogenicity of peptide-based vaccines, the conserved M2e and HA2 and a strain-specific HA1 epitope of Influenza A (H1N1) pdm09 were expressed as a chain in a bacterial expression system and entrapped in a novel amphiphilic invertible polymer made from polyethyelene glycol (PEG, molecular weight 600 g/mol) and polytetrahydrofuran (PTHF, molecular weight 650 g/mol), PEG₆₀₀PTHF₆₅₀. Piglets vaccinated with polymeric peptide vaccine mounted significantly stronger antibody responses against the peptide construct when compared to piglets immunized with the multi-epitope peptide alone. When vaccinated pigs were challenged with Influenza A (H1N1) pdm09, viral shedding in nasal secretions and lung lesion scores were significantly reduced when compared to the unvaccinated controls and pigs vaccinated with the peptide alone at six days post-challenge. Thus, the combination of the PEG₆₀₀PTHF₆₅₀ polymer and trimeric peptide construct enhanced delivery of the peptide antigen, acted as an adjuvant in stimulating strong antibody responses, reduced the effects of viral infection in vaccinated pigs.

Practical aspects of vaccination of poultry against avian influenza virus

Although little has changed in vaccine technology for avian influenza virus (AIV) in the past 20 years, the approach to vaccination of poultry (chickens, turkeys and ducks) for avian influenza has evolved as highly pathogenic AIV has become endemic in several regions of the world. Vaccination for low pathogenicity AIV is also becoming routine in regions where there is a high level of field challenge. In contrast, some countries will not use vaccination at all and some will only use it on an emergency basis during eradication efforts (i.e. stamping-out). There are pros and cons to each approach and, since every outbreak situation is different, no one method will work equally well in all situations. Numerous practical aspects must be considered when developing an AIV control program with vaccination as a component, such as: (1) the goals of vaccination must be defined; (2) the population to be vaccinated must be clearly identified; (3) there must be a plan to obtain and administer good quality vaccine in a timely manner and to achieve adequate coverage with the available resources; (4) risk factors for vaccine failure should be mitigated as much as possible; and, most importantly, (5) biosecurity must be maintained as much as possible, if not enhanced, during the vaccination period.

In silico structural homology modelling and docking for assessment of pandemic potential of a novel H7N9 influenza virus and its ability to be neutralized by existing anti-hemagglutinin antibodies

The unpredictable nature of pandemic influenza and difficulties in early prediction of pandemic potential of new isolates present a major challenge for health planners. Vaccine manufacturers, in particular, are reluctant to commit resources to development of a new vaccine until after a pandemic is declared. We hypothesized that a structural bioinformatics approach utilising homology-based molecular modelling and docking approaches would assist prediction of pandemic potential of new influenza strains alongside more traditional laboratory and sequence-based methods. The newly emerged Chinese A/Hangzhou/1/2013 (H7N9) influenza virus provided a real-life opportunity to test this hypothesis. We used sequence data and a homology-based approach to construct a 3D-structural model of H7-Hangzhou hemagglutinin (HA) protein. This model was then used to perform docking to human and avian sialic acid receptors to assess respective binding affinities. The model was also used to perform docking simulations with known neutralizing antibodies to assess their ability to neutralize the newly emerged virus. The model predicted H7N9 could bind to human sialic acid receptors thereby indicating pandemic potential. The model also confirmed that existing antibodies against the HA head region are unable to neutralise H7N9 whereas antibodies, e.g. Cr9114, targeting the HA stalk region should bind with high affinity to H7N9. This indicates that existing stalk antibodies initially raised against H5N1 or other influenza A viruses could be therapeutically beneficial in prevention and/or treatment of H7N9 infections. The subsequent publication of the H7N9 HA crystal structure confirmed the accuracy of our in-silico structural model. Antibody docking studies performed using the H7N9 HA crystal structure supported the model's prediction that existing stalk antibodies could cross-neutralise the H7N9 virus. This study demonstrates the value of using in-silico structural modelling approaches to complement physical studies in characterization of new influenza viruses.

Antigenic analyses of highly pathogenic avian influenza a viruses

In response to the ongoing threat to animal and human health posed by HPAI endemic in poultry, Asia (H5N1) and North America (H7N3) have revived efforts to reduce pandemic risk by disease control at the source and improved pandemic vaccines. Discovery of conserved neutralization epitopes in the HA, which mediate broad protection within and across HA subtypes have changed the paradigm of "broadly reactive" or "universal" vaccine design. Development of such vaccines would benefit from comparative antigenic analysis of viruses with increasing divergence within (and between) HA subtypes. A review of recent work to define the antigenic properties of HPAI viruses revealed data generated through an array of experimental approaches. This information has supported diagnostics and vaccine development for animal and human health. Further harmonization of analytical methods is needed to determine the antigenic relationships among multiple lineages of rapidly evolving HPAI viruses.

Monoclonal antibodies for prophylactic and therapeutic use against viral infections

Neutralizing antibodies play an essential part in antiviral immunity and are instrumental in preventing or modulating viral diseases. Polyclonal antibody preparations are increasingly being replaced by highly potent monoclonal antibodies (mAbs). Cocktails of mAbs and bispecific constructs can be used to simultaneously target multiple viral epitopes and to overcome issues of neutralization escape. Advances in antibody engineering have led to a large array of novel mAb formats, while deeper insight into the biology of several viruses and increasing knowledge of their neutralizing epitopes has extended the list of potential targets. In addition, progress in developing inexpensive production platforms will make antiviral mAbs more widely available and affordable.

The antigenic architecture of the hemagglutinin of influenza H5N1 viruses

Human infection with the highly pathogenic avian influenza A virus H5N1 is associated with a high mortality and morbidity. H5N1 continues to transmit from poultry to the human population, raising serious concerns about its pandemic potential. Current influenza H5N1 vaccines are based upon the elicitation of a neutralizing antibody (Ab) response against the major epitope regions of the viral surface glycoprotein, hemagglutinin (HA). However, antigenic drift mutations in immune-dominant regions on the HA structure allow the virus to escape Ab neutralization. Epitope mapping using neutralizing monoclonal antibodies (mAb) helps define mechanisms of antigenic drift, neutralizing escape and can facilitate pre-pandemic vaccine design. This review explores the current knowledge base of the antigenic sites of the H5N1 HA molecule. The relationship between the epitope architecture of the H5N1 HA, antigenic evolution of the different H5N1 lineages and the antigenic complexity of the H5N1 virus lineages that constitute potential pandemic strains are discussed in detail.

Monoclonal antibodies for prophylactic and therapeutic use against viral infections

Neutralizing antibodies play an essential part in antiviral immunity and are instrumental in preventing or modulating viral diseases. Polyclonal antibody preparations are increasingly being replaced by highly potent monoclonal antibodies (mAbs). Cocktails of mAbs and bispecific constructs can be used to simultaneously target multiple viral epitopes and to overcome issues of neutralization escape. Advances in antibody engineering have led to a large array of novel mAb formats, while deeper insight into the biology of several viruses and increasing knowledge of their neutralizing epitopes has extended the list of potential targets. In addition, progress in developing inexpensive production platforms will make antiviral mAbs more widely available and affordable.

Immunization with a hemagglutinin-derived synthetic peptide formulated with a CpG-DNA-liposome complex induced protection against lethal influenza virus infection in mice

Whole-virus vaccines, including inactivated or live-attenuated influenza vaccines, have been conventionally developed and supported as a prophylaxis. These currently available virus-based influenza vaccines are widely used in the clinic, but the vaccine production takes a long time and a huge number of embryonated chicken eggs. To overcome the imperfection of egg-based influenza vaccines, epitope-based peptide vaccines have been studied as an alternative approach. Here, we formulated an efficacious peptide vaccine without carriers using phosphodiester CpG-DNA and a special liposome complex. Potential epitope peptides predicted from the hemagglutinin (HA) protein of the H5N1 A/Viet Nam/1203/2004 strain (NCBI database, AAW80717) were used to immunize mice along with phosphodiester CpG-DNA co-encapsulated in a phosphatidyl-β-oleoyl-γ-palmitoyl ethanolamine (DOPE):cholesterol hemisuccinate (CHEMS) complex (Lipoplex(O)) without carriers. We identified a B cell epitope peptide (hH5N1 HA233 epitope, 14 amino acids) that can potently induce epitope-specific antibodies. Furthermore, immunization with a complex of the B cell epitope and Lipoplex(O) completely protects mice challenged with a lethal dose of recombinant H5N1 virus. These results suggest that our improved peptide vaccine technology can be promptly applied to vaccine development against pandemic influenza. Furthermore our results suggest that potent epitopes, which cannot be easily found using proteins or a virus as an antigen, can be screened when we use a complex of peptide epitopes and Lipoplex(O).

Evaluation of influenza virus A/H3N2 and B vaccines on the basis of cross-reactivity of postvaccination human serum antibodies against influenza viruses A/H3N2 and B isolated in MDCK cells and embryonated hen eggs

The vaccine strains against influenza virus A/H3N2 for the 2010-2011 season and influenza virus B for the 2009-2010 and 2010-2011 seasons in Japan are a high-growth reassortant A/Victoria/210/2009 (X-187) strain and an egg-adapted B/Brisbane/60/2008 (Victoria lineage) strain, respectively. Hemagglutination inhibition (HI) tests with postinfection ferret antisera indicated that the antisera raised against the X-187 and egg-adapted B/Brisbane/60/2008 vaccine production strains poorly inhibited recent epidemic isolates of MDCK-grown A/H3N2 and B/Victoria lineage viruses, respectively. The low reactivity of the ferret antisera may be attributable to changes in the hemagglutinin (HA) protein of production strains during egg adaptation. To evaluate the efficacy of A/H3N2 and B vaccines, the cross-reactivities of postvaccination human serum antibodies against A/H3N2 and B/Victoria lineage epidemic isolates were assessed by a comparison of the geometric mean titers (GMTs) of HI and neutralization (NT) tests. Serum antibodies elicited by the X-187 vaccine had low cross-reactivity to both MDCK- and egg-grown A/H3N2 isolates by HI test and narrow cross-reactivity by NT test in all age groups. On the other hand, the GMTs to B viruses detected by HI test were below the marginal level, so the cross-reactivity was assessed by NT test. The serum neutralizing antibodies elicited by the B/Brisbane/60/2008 vaccine reacted well with egg-grown B viruses but exhibited remarkably low reactivity to MDCK-grown B viruses. The results of these human serological studies suggest that the influenza A/H3N2 vaccine for the 2010-2011 season and B vaccine for the 2009-2010 and 2010-2011 seasons may possess insufficient efficacy and low efficacy, respectively.

Targeting B cell responses in universal influenza vaccine design

Since its first administration in the 1940s, the influenza vaccine has provided tremendous relief against influenza infections. However, time has revealed the ultimate limit of the vaccine and the call for its reinvention has now come, just as we are beginning to appreciate the antibody immune responses vital in preventing infections. New strategies to design the influenza vaccine rely on selectively inducing broadly neutralizing antibodies that are specific for highly conserved viral epitopes. Such approaches take us away from the limited range of protection provided by current seasonal influenza vaccines and towards a future with a pan-influenza vaccine capable of providing universal strain coverage.

Antibodies in infectious diseases: polyclonals, monoclonals and niche biotechnology

Antibody preparations have a long history of providing protection from infectious diseases. Although antibodies remain the only natural host-derived defense mechanism capable of completely preventing infection, as products, they compete against inexpensive therapeutics such as antibiotics, small molecule inhibitors and active vaccines. The continued discovery in the monoclonal antibody (mAb) field of leads with broadened cross neutralization of viruses and demonstrable synergy of antibody with antibiotics for bacterial diseases, clearly show that innovation remains. The commercial success of mAbs in chronic disease has not been paralleled in infectious diseases for several reasons. Infectious disease immunotherapeutics are limited in scope as endemic diseases necessitate active vaccine development. Also, the complexity of these small markets draws the interest of niche companies rather than big pharmaceutical corporations. Lastly, the cost of goods for mAb therapeutics is inherently high for infectious agents due to the need for antibody cocktails, which better mimic polyclonal immunoglobulin preparations and prevent antigenic escape. In cases where vaccine or convalescent populations are available, current polyclonal hyperimmune immunoglobulin preparations (pIgG), with modern and highly efficient purification technology and standardized assays for potency, can make economic sense. Recent innovations to broaden the potency of mAb therapies, while reducing cost of production, are discussed herein. On the basis of centuries of effective use of Ab treatments, and with growing immunocompromised populations, the question is not whether antibodies have a bright future for infectious agents, but rather what formats are cost effective and generate safe and efficacious treatments to satisfy regulatory approval.

Conserved epitopes of influenza A virus inducing protective immunity and their prospects for universal vaccine development

Influenza A viruses belong to the best studied viruses, however no effective prevention against influenza infection has been developed. The emerging of still new escape variants of influenza A viruses causing epidemics and periodic worldwide pandemics represents a threat for human population. Therefore, current, hot task of influenza virus research is to look for a way how to get us closer to a universal vaccine. Combination of chosen conserved antigens inducing cross-protective antibody response with epitopes activating also cross-protective cytotoxic T-cells would offer an attractive strategy for improving protection against drift variants of seasonal influenza viruses and reduces the impact of future pandemic strains. Antigenically conserved fusion-active subunit of hemagglutinin (HA2 gp) and ectodomain of matrix protein 2 (eM2) are promising candidates for preparation of broadly protective HA2- or eM2-based vaccine that may aid in pandemic preparedness. Overall protective effect could be achieved by contribution of epitopes recognized by cytotoxic T-lymphocytes (CTL) that have been studied extensively to reach much broader control of influenza infection. In this review we present the state-of-art in this field. We describe known adaptive immune mechanisms mediated by influenza specific B- and T-cells involved in the anti-influenza immune defense together with the contribution of innate immunity. We discuss the mechanisms of neutralization of influenza infection mediated by antibodies, the role of CTL in viral elimination and new approaches to develop epitope based vaccine inducing cross-protective influenza virus-specific immune response.

Ebolavirus is internalized into host cells via macropinocytosis in a viral glycoprotein-dependent manner

Ebolavirus (EBOV) is an enveloped, single-stranded, negative-sense RNA virus that causes severe hemorrhagic fever with mortality rates of up to 90% in humans and nonhuman primates. Previous studies suggest roles for clathrin- or caveolae-mediated endocytosis in EBOV entry; however, ebolavirus virions are long, filamentous particles that are larger than the plasma membrane invaginations that characterize clathrin- or caveolae-mediated endocytosis. The mechanism of EBOV entry remains, therefore, poorly understood. To better understand Ebolavirus entry, we carried out internalization studies with fluorescently labeled, biologically contained Ebolavirus and Ebolavirus-like particles (Ebola VLPs), both of which resemble authentic Ebolavirus in their morphology. We examined the mechanism of Ebolavirus internalization by real-time analysis of these fluorescently labeled Ebolavirus particles and found that their internalization was independent of clathrin- or caveolae-mediated endocytosis, but that they co-localized with sorting nexin (SNX) 5, a marker of macropinocytosis-specific endosomes (macropinosomes). Moreover, the internalization of Ebolavirus virions accelerated the uptake of a macropinocytosis-specific cargo, was associated with plasma membrane ruffling, and was dependent on cellular GTPases and kinases involved in macropinocytosis. A pseudotyped vesicular stomatitis virus possessing the Ebolavirus glycoprotein (GP) also co-localized with SNX5 and its internalization and infectivity were affected by macropinocytosis inhibitors. Taken together, our data suggest that Ebolavirus is internalized into cells by stimulating macropinocytosis in a GP-dependent manner. These findings provide new insights into the lifecycle of Ebolavirus and may aid in the development of therapeutics for Ebolavirus infection.

Ebolavirus (EBOV) is an enveloped, single-stranded, negative-sense RNA virus that causes severe hemorrhagic fever with high mortality rates in humans and nonhuman primates. Previous studies suggest roles for clathrin- or caveolae-mediated endocytosis in EBOV entry; however, questions remain regarding the mechanism of EBOV entry. Here, we demonstrate that internalization of EBOV particles is independent of clathrin- or caveolae-mediated endocytosis. Specifically, we show that internalized EBOV particles co-localize with macropinocytosis-specific endosomes (macropinosomes) and that their entry is negatively affected by treatment with macropinocytosis inhibitors. Moreover, the internalization of Ebola virions accelerated the uptake of a macropinocytosis-specific cargo, was associated with plasma membrane ruffling, and was dependent on cellular GTPases and kinases involved in macropinocytosis. We further demonstrate that a pseudotyped vesicular stomatitis virus possessing the EBOV glycoprotein (GP) also co-localizes with macropinosomes and its internalization is similarly affected by macropinocytosis inhibitors. Our results indicate that EBOV uptake into cells involves the macropinocytic pathway and is GP-dependent. These findings provide new insights into the lifecycle of EBOV and may aid in the development of therapeutics for EBOV infection.

Respiratory syncytial virus-neutralizing monoclonal antibodies motavizumab and palivizumab inhibit fusion

Respiratory syncytial virus (RSV) is a major cause of virus-induced respiratory disease and hospitalization in infants. Palivizumab, an RSV-neutralizing monoclonal antibody, is used clinically to prevent serious RSV-related respiratory disease in high-risk infants. Motavizumab, an affinity-optimized version of palivizumab, was developed to improve protection against RSV. These antibodies bind RSV F protein, which plays a role in virus attachment and mediates fusion. Determining how these antibodies neutralize RSV is important to help guide development of new antibody drugs against RSV and, potentially, other viruses. This study aims to uncover the mechanism(s) by which palivizumab and motavizumab neutralize RSV. Assays were developed to test the effects of these antibodies at distinct steps during RSV replication. Pretreatment of virus with palivizumab or motavizumab did not inhibit virus attachment or the ability of F protein to interact with the target cell membrane. However, pretreatment of virus with either of these antibodies resulted in the absence of detectable viral transcription. These results show that palivizumab and motavizumab act at a point after F protein initiates interaction with the cell membrane and before virus transcription. Palivizumab and motavizumab also inhibited F protein-mediated cell-to-cell fusion. Therefore, these results strongly suggest that these antibodies block both cell-to-cell and virus-to-cell fusion, since these processes are likely similar. Finally, palivizumab and motavizumab did not reduce viral budding. Based on models developed from numerous studies of viral fusion proteins, our results indicate that these antibodies may prevent conformational changes in F protein required for the fusion process.

Evaluation of diagnostic applications of monoclonal antibodies against avian influenza H7 viruses

A panel of monoclonal antibodies (MAbs) was generated from mice immunized with binary ethylenimine (BEI)-inactivated H7N1 (A/TK/ON/18-2/00) virus. Using a dot blot assay, six of seven MAbs reacted with viruses of the H7 subtype, but not with any of the other 15 hemagglutinin (HA) subtypes tested. Four of the seven MAbs reacted with 14 different H7 isolates, indicating that the MAbs binding epitopes are conserved among viruses of the H7 subtype. The binding epitopes of all seven MAbs were conformational and reacted with the HA1 fraction of the HA protein in Western blots under nonreducing conditions. Applications of these MAbs in the development of rapid tests for H7 subtype viruses were evaluated. The MAbs demonstrated reactivity with AI virus H7 antigen in immunofluorescence and immunohistochemistry assays. Monoclonal antibody 3 showed a very strong immunostaining in the formalin-fixed and paraffin-embedded tissue from the H7N3 virus-infected chicken. A double-antibody sandwich (DAS) enzyme-linked immunosorbent assay (ELISA) was developed using two of the MAbs. The DAS ELISA specifically detected all H7 strains tested in this study. A competitive ELISA (cELISA) for the detection of H7-specific antibodies was evaluated using one MAb and BEI-inactivated H7N1 virus as the antigen. All infected birds showed positive antibody responses at 7 days postinfection. The sensitivity of this cELISA was comparable with that of an influenza A nucleoprotein-based cELISA. This panel of MAbs is valuable in the development of various immunoassays.

Human monoclonal antibodies to SARS-coronavirus inhibit infection by different mechanisms

SARS-CoV causes an acute infection making targeted passive immunotherapy an attractive treatment strategy. We previously generated human mAbs specific to the S1 region of SARS-CoV S protein. These mAbs bind epitopes within the receptor binding domain (RBD) or upstream of the RBD. We show that mAbs recognizing epitopes within the RBD inhibit infection by preventing viral attachment to the cellular receptor. One mAb binds upstream of the RBD and prevents viral entry by inhibiting a post-binding event. Evaluation of several mAbs demonstrated varying ability of the mAbs to select escape mutants when used individually. However, a mixture of antibodies could effectively neutralize a range of mutant viruses. These data strongly suggest that a mixture containing antibodies recognizing distinct regions and targeting more than one step in viral entry is likely to be more effective in neutralizing the virus and suppressing the generation of escape mutants, and thus potentially constitute a highly effective passive immunotherapy.

Monoclonal antibodies against the fusion peptide of hemagglutinin protect mice from lethal influenza A virus H5N1 infection

The HA2 glycopolypeptide (gp) is highly conserved in all influenza A virus strains, and it is known to play a major role in the fusion of the virus with the endosomal membrane in host cells during the course of viral infection. Vaccines and therapeutics targeting this HA2 gp could induce efficient broad-spectrum immunity against influenza A virus infections. So far, there have been no studies on the possible therapeutic effects of monoclonal antibodies (MAbs), specifically against the fusion peptide of hemagglutinin (HA), upon lethal infections with highly pathogenic avian influenza (HPAI) H5N1 virus. We have identified MAb 1C9, which binds to GLFGAIAGF, a part of the fusion peptide of the HA2 gp. We evaluated the efficacy of MAb 1C9 as a therapy for influenza A virus infections. This MAb, which inhibited cell fusion in vitro when administered passively, protected 100% of mice from challenge with five 50% mouse lethal doses of HPAI H5N1 influenza A viruses from two different clades. Furthermore, it caused earlier clearance of the virus from the lung. The influenza virus load was assessed in lung samples from mice challenged after pretreatment with MAb 1C9 (24 h prior to challenge) and from mice receiving early treatment (24 h after challenge). The study shows that MAb 1C9, which is specific to the antigenically conserved fusion peptide of HA2, can contribute to the cross-clade protection of mice infected with H5N1 virus and mediate more effective recovery from infection.

Genetic Analyses of an H3N8 Influenza Virus Isolate, Causative Strain of the Outbreak of Equine Influenza at the Kanazawa Racecourse in Japan in 2007

In August 2007, an outbreak of equine influenza occurred among vaccinated racehorses with Japanese commercial equine influenza vaccine at Kanazawa Racecourse in Ishikawa prefecture in Japan. Apparent symptoms were pyrexia (38.2-41.0 degrees C) and nasal discharge with or without coughing, although approximately half of the infected horses were subclinical. All horses had been shot with a vaccine that contained two inactivated H3N8 influenza virus strains [A/equine/La Plata/93 (La Plata/93) of American lineage and A/equine/Avesta/93 (Avesta/93) of European lineage] and an H7N7 strain (A/equine/Newmarket/1/77). Influenza virus, A/equine/Kanazawa/1/2007 (H3N8) (Kanazawa/07), was isolated from one of the nasal swab samples of diseased horses. Phylogenetic analysis indicated that Kanazawa/07 was classified into the American sublineage Florida. In addition, four amino acid substitutions were found in the antigenic sites B and E in the HA1 subunit protein of Kanazawa/07 in comparison with that of La Plata/93. Hemagglutination-inhibition (HI) test using 16 serum samples from recovering horses revealed that 1.4- to 8-fold difference in titers between Kanazawa/07 and either of the vaccine strains. The present findings suggest that Japanese commercial inactivated vaccine contributed to reducing the morbidity rate and manifestation of the clinical signs of horses infected with Kanazawa/07 that may be antigenically different from the vaccine strains.

Sindbis virus conformational changes induced by a neutralizing anti-E1 monoclonal antibody

A rare Sindbis virus anti-E1 neutralizing monoclonal antibody, Sin-33, was investigated to determine the mechanism of in vitro neutralization. A cryoelectron microscopic reconstruction of Sindbis virus (SVHR) neutralized with FAb from Sin-33 (FAb-33) revealed conformational changes on the surface of the virion at a resolution of 24 A. FAb-33 was found to bind E1 in less than 1:1 molar ratios, as shown by the absence of FAb density in the reconstruction and stoichiometric measurements using radiolabeled FAb-33, which determined that about 60 molecules of FAb-33 bound to the 240 possible sites in a single virus particle. FAb-33-neutralized virus particles became sensitive to digestion by endoproteinase Glu-C, providing further evidence of antibody-induced structural changes within the virus particle. The treatment of FAb-33-neutralized or Sin-33-neutralized SVHR with low pH did not induce the conformational rearrangements required for virus membrane-cell membrane fusion. Exposure to low pH, however, increased the amount of Sin-33 or FAb-33 that bound to the virus particles, indicating the exposure of additional epitopes. The neutralization of SVHR infection by FAb-33 or Sin-33 did not prevent the association of virus with host cells. These data are in agreement with the results of previous studies that demonstrated that specific antibodies can inactivate the infectious state of a metastable virus in vitro by the induction of conformational changes to produce an inactive structure. A model is proposed which postulates that the induction of conformational changes in the infectious state of a metastable enveloped virus may be a general mechanism of antibody inactivation of virus infectivity.

Comparison of epitope structures of H3HAs through protein modeling of influenza A virus hemagglutinin: mechanism for selection of antigenic variants in the presence of a monoclonal antibody

Starting with nine plaques of influenza A/Kamata/14/91(H3N2) virus, we selected mutants in the presence of monoclonal antibody 203 (mAb203). In total, amino acid substitutions were found at nine positions (77, 80, 131, 135, 141, 142, 143, 144 and 146), which localized in the antigenic site A of the hemagglutinin (HA). The escape mutants differed in the extent to which they had lost binding to mAb203. HA protein with substitutions of some amino acid residues created by site-directed mutagenesis in the escape mutants retained the ability to bind to mAb203. Changes in the amino acid character affecting charge or hydrophobicity accounted for the binding capacity to the antibody of the HA with most of the substitutions in the escape mutants and binding-positive mutants. However, the effect of some amino acid substitutions remained unexplained. A three-dimensional model of the 1991 HA was constructed and used to analyze substituted amino acids in these mutants for the accessible surface hydrophobic and hydrophilic characters. One amino acid substitution in an escape mutant and another amino acid substitution in a binding-positive mutant seemed to be explained by the changes noted on this model.

The growth and potential of human antiviral monoclonal antibody therapeutics

Monoclonal antibodies (mAbs) have long provided powerful research tools for virologists to understand the mechanisms of virus entry into host cells and of antiviral immunity. Even so, commercial development of human (or humanized) mAbs for the prophylaxis, preemptive and acute treatment of viral infections has been slow. This is surprising, as new antibody discovery tools have increased the speed and precision with which potent neutralizing human antiviral mAbs can be identified. As longstanding barriers to antiviral mAb development, such as antigenic variability of circulating viral strains and the ability of viruses to undergo neutralization escape, are being overcome, deeper insight into the mechanisms of mAb action and engineering of effector functions are also improving the efficacy of antiviral mAbs. These successes, in both industrial and academic laboratories, coupled with ongoing changes in the biomedical and regulatory environments, herald an era when the commercial development of human antiviral mAb therapies will likely surge.

A vaccine prepared from a non-pathogenic H7N7 virus isolated from natural reservoir conferred protective immunity against the challenge with lethal dose of highly pathogenic avian influenza virus in chickens

During 2001-2004, 41 H7 influenza viruses (2 H7N1 and 39 H7N7 strains) were isolated from fecal samples of migratory ducks that flew from Siberia in the autumn of each year to Japan and Mongolia. A phylogenetic analysis of the hemagglutinin (HA) genes of the nine representative isolates revealed that they belonged to the Eurasian lineage and the deduced amino acid sequence at the cleavage site of the HAs represented apathogenic profiles. One of the H7 isolates A/duck/Mongolia/736/02 (H7N7) was chosen from these H7 isolates for the preparation of the test vaccine. To improve the growth potential of A/duck/Mongolia/736/02 (H7N7) in chicken embryos, A/duck/Hokkaido/Vac-2/04 (H7N7) was generated by genetic reassortment between A/duck/Mongolia/736/02 (H7N7) as the donor of the PB2, PB1, PA, HA, NA, and NS genes and A/duck/Hokkaido/49/98 (H9N2) as that of NP and M genes. The test vaccine was prepared as follows; A/duck/Hokkaido/Vac-2/04 (H7N7) was propagated in chicken embryos and the virus in the allantoic fluid was inactivated and adjuvanted to form an oil-in-water emulsion. The test vaccine conferred immunity to chickens, completely protecting the manifestation of clinical signs against the challenge with lethal dose of H7 highly pathogenic avian influenza virus. These results indicate that influenza viruses isolated from natural reservoirs are useful for vaccine strains.

Enhancement of neutralizing activity of influenza virus-specific antibodies by serum components

The role of serum components in enhancing virus neutralizing (VN) activity of influenza virus A/PR/8/34 hemagglutinin (HA)-specific MAbs in vitro was investigated. The degree of enhancement depended on the MAb's fine specificity and heavy chain isotype and on type of serum. Greatest enhancement (>100-fold) was seen with sera from immunodeficient mice that lacked serum immunoglobulin. At least two serum components were involved: C1q and a heat-resistant factor. C1q was mandatory for enhancement, and other components of the complement system were not required. C1q appeared to operate by improving MAb-mediated inhibition of virus attachment to host cells and was most effective with MAbs that inhibited virus attachment poorly on their own. The heat-resistant factor enhanced VN activity only in the presence of C1q and appeared to operate by enhancing VN activity at a post-attachment stage.

Neutralizing and hemagglutination-inhibiting activities of antibodies elicited by the 2004-2005 influenza vaccine against drifted viruses

Evaluation of the antibody responses induced by the 2004-2005 influenza vaccine strain against the homologous variant, the 2004-2005 field isolates, and a previous circulating strain showed that a correlation between neutralizing and hemagglutination-inhibiting activities exists only when the antigen is very close to the vaccine strain.

Antigenic alteration of influenza B virus associated with loss of a glycosylation site due to host-cell adaptation

Effects of host-cell adaptation of the hemagglutinin (HA) protein were evaluated by the analyses of four pairs of recent influenza B field isolates, each pair consisting of an Madin Darby canine kidney (MDCK)- and an embryonated chicken egg-derived isolates from the same clinical specimen. Among the isolates examined, all of the MDCK-derived isolates retained glycosylation site at amino acid 197 on the HA1 molecule, whereas three egg-derived isolates lost it. Antigenic difference in the HA molecule between an MDCK- and an egg-derived isolates of three of these pairs was demonstrated to be associated with the glycosylation 197. Replication of the MDCK-derived isolates was suppressed in eggs, suggesting that the presence of the glycosylation 197 was disadvantageous to replication in eggs. Virus-binding affinity assay revealed that the loss of carbohydrate chain did not significantly alter the preferential recognition of sialic acid linkage. Immunogenicity and vaccine efficacy of an MDCK- and an egg-derived clones of B/Akita/27/2001, the former retained the glycosylation 197 and the latter lost it, were compared in a hamster model. When formalin-inactivated whole virion vaccines prepared from the paired isolates were administered into hamsters, no significant difference between them was observed in protective ability against challenges by the homologous and heterologous clones. Implication of the egg adaptation of influenza virus to antigenic surveillance of the field isolates as well as the selection of vaccine strains, and possibility of the involvement of the viral protein(s) other than the HA in the egg adaptation were discussed.

CD4 binding site antibodies inhibit human immunodeficiency virus gp120 envelope glycoprotein interaction with CCR5

The human immunodeficiency virus type 1 (HIV-1) gp120 exterior glycoprotein is conformationally flexible. Upon binding the host cell receptor, CD4, gp120 assumes a conformation that is able to bind the chemokine receptors CCR5 or CXCR4, which act as coreceptors for the virus. CD4-binding-site (CD4BS) antibodies are neutralizing antibodies elicited during natural infection that are directed against gp120 epitopes that overlap the binding site for CD4. Recent studies (S. H. Xiang et al., J. Virol. 76:9888-9899, 2002) suggest that CD4BS antibodies recognize conformations of gp120 distinct from the CD4-bound conformation. This predicts that the binding of CD4BS antibodies will inhibit chemokine receptor binding. Here, we show that Fab fragments and complete immunoglobulin molecules of CD4BS antibodies inhibit CD4-independent gp120 binding to CCR5 and cell-cell fusion mediated by CD4-independent HIV-1 envelope glycoproteins. These results are consistent with a model in which the binding of CD4BS antibodies limits the ability of gp120 to assume a conformation required for coreceptor binding.

Inhibition of rotavirus replication by a non-neutralizing, rotavirus VP6-specific IgA mAb

Rotaviruses are the leading cause of severe diarrheal disease in young children. Intestinal mucosal IgA responses play a critical role in protective immunity against rotavirus reinfection. Rotaviruses consist of three concentric capsid layers surrounding a genome of 11 segments of double-stranded RNA. The outer layer proteins, VP4 and VP7, which are responsible for viral attachment and entry, are targets for protective neutralizing antibodies. However, IgA mAb's directed against the intermediate capsid protein VP6, which do not neutralize the virus, have also been shown to protect mice from rotavirus infection and clear chronic infection in SCID mice. We investigated whether the anti-VP6 IgA (7D9) mAb could inhibit rotavirus replication inside epithelial cells and found that 7D9 acted at an early stage of infection to neutralize rotavirus following antibody lipofection. Using electron cryomicroscopy, we determined the three-dimensional structure of the virus-antibody complex. The attachment of 7D9 IgA to VP6 introduces a conformational change in the VP6 trimer, rendering the particle transcriptionally incompetent and preventing the elongation of initiated transcripts. Based on these observations, we suggest that anti-VP6 IgA antibodies confers protection in vivo by inhibiting viral transcription at the start of the intracellular phase of the viral replication cycle.

An epitope of the Semliki Forest virus fusion protein exposed during virus-membrane fusion

Semliki Forest virus (SFV) is an enveloped alphavirus that infects cells via a membrane fusion reaction triggered by acidic pH in the endocytic pathway. Fusion is mediated by the spike protein E1 subunit, an integral membrane protein that contains the viral fusion peptide and forms a stable homotrimer during fusion. We have characterized four monoclonal antibodies (MAbs) specific for the acid conformation of E1. These MAbs did not inhibit fusion, suggesting that they bind to an E1 region different from the fusion peptide. Competition analyses demonstrated that all four MAbs bound to spatially related sites on acid-treated virions or isolated spike proteins. To map the binding site, we selected for virus mutants resistant to one of the MAbs, E1a-1. One virus isolate, SFV 4-2, showed reduced binding of three acid-specific MAbs including E1a-1, while its binding of one acid-specific MAb as well as non-acid-specific MAbs to E1 and E2 was unchanged. The SFV 4-2 mutant was fully infectious, formed the E1 homotrimer, and had the wild-type pH dependence of infection. Sequence analysis demonstrated that the relevant mutation in SFV 4-2 was a change of E1 glycine 157 to arginine (G157R). Decreased binding of MAb E1a-1 was observed under a wide range of assay conditions, strongly suggesting that the E1 G157R mutation directly affects the MAb binding site. These data thus localize an E1 region that is normally hidden in the neutral pH structure and becomes exposed as part of the reorganization of the spike protein to its fusion-active conformation.