Hemagglutinin stalk antibodies elicited by the 2009 pandemic influenza virus as a mechanism for the extinction of seasonal H1N1 viruses.

After the emergence of pandemic influenza viruses in 1957, 1968, and 2009, existing seasonal viruses were observed to be replaced in the human population by the novel pandemic strains. We have previously hypothesized that the replacement of seasonal strains was mediated, in part, by a population-scale boost in antibodies specific for conserved regions of the hemagglutinin stalk and the viral neuraminidase. Numerous recent studies have shown the role of stalk-specific antibodies in neutralization of influenza viruses; the finding that stalk antibodies can effectively neutralize virus alters the existing dogma that influenza virus neutralization is mediated solely by antibodies that react with the globular head of the viral hemagglutinin. The present study explores the possibility that stalk-specific antibodies were boosted by infection with the 2009 H1N1 pandemic virus and that those antibodies could have contributed to the disappearance of existing seasonal H1N1 influenza virus strains. To study stalk-specific antibodies, we have developed chimeric hemagglutinin constructs that enable the measurement of antibodies that bind the hemagglutinin protein and neutralize virus but do not have hemagglutination inhibition activity. Using these chimeric hemagglutinin reagents, we show that infection with the 2009 pandemic H1N1 virus elicited a boost in titer of virus-neutralizing antibodies directed against the hemagglutinin stalk. In addition, we describe assays that can be used to measure influenza virus-neutralizing antibodies that are not detected in the traditional hemagglutination inhibition assay.

Antibody recognition of the pandemic H1N1 Influenza virus hemagglutinin receptor binding site.

Influenza virus is a global health concern due to its unpredictable pandemic potential. This potential threat was realized in 2009 when an H1N1 virus emerged that resembled the 1918 virus in antigenicity but fortunately was not nearly as deadly. 5J8 is a human antibody that potently neutralizes a broad spectrum of H1N1 viruses, including the 1918 and 2009 pandemic viruses. Here, we present the crystal structure of 5J8 Fab in complex with a bacterially expressed and refolded globular head domain from the hemagglutinin (HA) of the A/California/07/2009 (H1N1) pandemic virus. 5J8 recognizes a conserved epitope in and around the receptor binding site (RBS), and its HCDR3 closely mimics interactions of the sialic acid receptor. Electron microscopy (EM) reconstructions of 5J8 Fab in complex with an HA trimer from a 1986 H1 strain and with an engineered stabilized HA trimer from the 2009 H1 pandemic virus showed a similar mode of binding. As for other characterized RBS-targeted antibodies, 5J8 uses avidity to extend its breadth and affinity against divergent H1 strains. 5J8 selectively interacts with HA insertion residue 133a, which is conserved in pandemic H1 strains and has precluded binding of other RBS-targeted antibodies. Thus, the RBS of divergent HAs is targeted by 5J8 and adds to the growing arsenal of common recognition motifs for design of therapeutics and vaccines. Moreover, consistent with previous studies, the bacterially expressed H1 HA properly refolds, retaining its antigenic structure, and presents a low-cost and rapid alternative for engineering and manufacturing candidate flu vaccines.

Recombinant IgA is sufficient to prevent influenza virus transmission in guinea pigs.

A serum hemagglutination inhibition (HAI) titer of 40 or greater is thought to be associated with reduced influenza virus pathogenesis in humans and is often used as a correlate of protection in influenza vaccine studies. We have previously demonstrated that intramuscular vaccination of guinea pigs with inactivated influenza virus generates HAI titers greater than 300 but does not protect vaccinated animals from becoming infected with influenza virus by transmission from an infected cage mate. Only guinea pigs intranasally inoculated with a live influenza virus or a live attenuated virus vaccine, prior to challenge, were protected from transmission (A. C. Lowen et al., J. Virol. 83:2803-2818, 2009.). Because the serum HAI titer is mostly determined by IgG content, these results led us to speculate that prevention of viral transmission may require IgA antibodies or cellular immune responses. To evaluate this hypothesis, guinea pigs and ferrets were administered a potent, neutralizing mouse IgG monoclonal antibody, 30D1 (Ms 30D1 IgG), against the A/California/04/2009 (H1N1) virus hemagglutinin and exposed to respiratory droplets from animals infected with this virus. Even though HAI titers were greater than 160 1 day postadministration, Ms 30D1 IgG did not prevent airborne transmission to passively immunized recipient animals. In contrast, intramuscular administration of recombinant 30D1 IgA (Ms 30D1 IgA) prevented transmission to 88% of recipient guinea pigs, and Ms 30D1 IgA was detected in animal nasal washes. Ms 30D1 IgG administered intranasally also prevented transmission, suggesting the importance of mucosal immunity in preventing influenza virus transmission. Collectively, our data indicate that IgG antibodies may prevent pathogenesis associated with influenza virus infection but do not protect from virus infection by airborne transmission, while IgA antibodies are more important for preventing transmission of influenza viruses.

Influenza human monoclonal antibody 1F1 interacts with three major antigenic sites and residues mediating human receptor specificity in H1N1 viruses.

Most monoclonal antibodies (mAbs) to the influenza A virus hemagglutinin (HA) head domain exhibit very limited breadth of inhibitory activity due to antigenic drift in field strains. However, mAb 1F1, isolated from a 1918 influenza pandemic survivor, inhibits select human H1 viruses (1918, 1943, 1947, and 1977 isolates). The crystal structure of 1F1 in complex with the 1918 HA shows that 1F1 contacts residues that are classically defined as belonging to three distinct antigenic sites, Sa, Sb and Ca(2). The 1F1 heavy chain also reaches into the receptor binding site (RBS) and interacts with residues that contact sialoglycan receptors and determine HA receptor specificity. The 1F1 epitope is remarkably similar to the previously described murine HC63 H3 epitope, despite significant sequence differences between H1 and H3 HAs. Both antibodies potently inhibit receptor binding, but only HC63 can block the pH-induced conformational changes in HA that drive membrane fusion. Contacts within the RBS suggested that 1F1 may be sensitive to changes that alter HA receptor binding activity. Affinity assays confirmed that sequence changes that switch the HA to avian receptor specificity affect binding of 1F1 and a mAb possessing a closely related heavy chain, 1I20. To characterize 1F1 cross-reactivity, additional escape mutant selection and site-directed mutagenesis were performed. Residues 190 and 227 in the 1F1 epitope were found to be critical for 1F1 reactivity towards 1918, 1943 and 1977 HAs, as well as for 1I20 reactivity towards the 1918 HA. Therefore, 1F1 heavy-chain interactions with conserved RBS residues likely contribute to its ability to inhibit divergent HAs.

Human monoclonal antibodies to pandemic 1957 H2N2 and pandemic 1968 H3N2 influenza viruses.

Investigation of the human antibody response to the 1957 pandemic H2N2 influenza A virus has been largely limited to serologic studies. We generated five influenza virus hemagglutinin (HA)-reactive human monoclonal antibodies (MAbs) by hybridoma technology from the peripheral blood of healthy donors who were born between 1950 and 1968. Two MAbs reacted with the pandemic H2N2 virus, two recognized the pandemic H3N2 virus, and remarkably, one reacted with both the pandemic H2N2 and H3N2 viruses. Each of these five naturally occurring MAbs displayed hemagglutination inhibition activity, suggesting specificity for the globular head domain of influenza virus HA. When incubated with virus, MAbs 8F8, 8M2, and 2G1 each elicited H2N2 escape mutations immediately adjacent to the receptor-binding domain on the HA globular head in embryonated chicken eggs. All H2N2-specific MAbs were able to inhibit a 2006 swine H2N3 influenza virus. MAbs 8M2 and 2G1 shared the V(H)1-69 germ line gene, but these antibodies were otherwise not genetically related. Each antibody was able to protect mice in a lethal H2N2 virus challenge. Thus, even 43 years after circulation of H2N2 viruses, these subjects possessed peripheral blood B cells encoding potent inhibiting antibodies specific for a conserved region on the globular head of the pandemic H2 HA.

Epitope-specific human influenza antibody repertoires diversify by B cell intraclonal sequence divergence and interclonal convergence.

We generated from a single blood sample five independent human mAbs that recognized the Sa antigenic site on the head of influenza hemagglutinin and exhibited inhibitory activity against a broad panel of H1N1 strains. All five Abs used the V(H)3-7 and J(H)6 gene segments, but at least four independent clones were identified by junctional analysis. High-throughput sequence analysis of circulating B cells revealed that each of the independent clones were members of complex phylogenetic lineages that had diversified widely using a pattern of progressive diversification through somatic mutation. Unexpectedly, B cells encoding multiple diverging lineages of these clones, including many containing very few mutations in the Ab genes, persisted in the circulation. Conversely, we noted frequent instances of amino acid sequence convergence in the Ag combining sites exhibited by members of independent clones, suggesting a strong selection for optimal binding sites. We suggest that maintenance in circulation of a wide diversity of somatic variants of dominant clones may facilitate recognition of drift variant virus epitopes that occur in rapidly mutating virus Ags, such as influenza hemagglutinin. In fact, these Ab clones recognize an epitope that acquired three glycosylation sites mediating escape from previously isolated human Abs.

A broadly neutralizing human monoclonal antibody that recognizes a conserved, novel epitope on the globular head of the influenza H1N1 virus hemagglutinin.

The conserved influenza virus hemagglutinin (HA) stem domain elicits cross-reactive antibodies, but epitopes in the globular head typically elicit strain-specific responses because of the hypervariability of this region. We isolated human monoclonal antibody 5J8, which neutralized a broad spectrum of 20th century H1N1 viruses and the 2009 pandemic H1N1 virus. Fine mapping of the interaction unexpectedly revealed a novel epitope between the receptor-binding pocket and the Ca2 antigenic site on HA. This antibody exposes a new mechanism underlying broad immunity against H1N1 influenza viruses and identifies a conserved epitope that might be incorporated into engineered H1 virus vaccines.

Naturally occurring human monoclonal antibodies neutralize both 1918 and 2009 pandemic influenza A (H1N1) viruses.

The 2009 pandemic influenza A (H1N1) virus exhibits hemagglutinin protein sequence homology with the 1918 pandemic influenza virus. We found that human monoclonal antibodies recognized the Sa antigenic site on the head domains of both 1918 and 2009 hemagglutinins, a site that is hypervariable due to immune selection. These antibodies exhibited high potency against the 2009 virus in vitro, and one exerted a marked therapeutic effect in vivo.

Very late-onset group B Streptococcus meningitis, sepsis, and systemic shigellosis due to interleukin-1 receptor-associated kinase-4 deficiency.

We describe a child with very late-onset group B Streptococcus sepsis and meningitis, systemic shigellosis, and chronic osteomyelitis. Peripheral blood cells obtained from the patient and her brother did not respond to stimulation with either interleukin-1beta or lipopolysaccharide. Sequencing of the interleukin-1 receptor-associated kinase-4 gene revealed 2 novel mutations.

Metabolic abnormalities in human immunodeficiency virus-infected children: two-year follow-up.

BACKGROUND: HIV-associated lipodystrophy (LD) manifests with fat maldistribution, dyslipidemia, and insulin resistance in some HIV-infected children on antiretroviral therapy. AIM: To assess whether lipid abnormalities in patients with HIV are stable over time. PATIENTS: The perinatally HIV-infected cohort at a medium-sized urban US teaching hospital. METHODS: This prospective, observational study consisted of five visits (at entry and 3, 6, 24, and 30 months after entry) during which fasting venous blood samples were drawn for HIV-1 RNA, CD4 lymphocytes, lipid profile, free fatty acids (FFA), glucose, insulin, and adiponectin. IGF-I/IGFBP-3 levels were measured at the first and fifth visits. RESULTS: Of 36 study participants, seven were lipodystrophic, and 30 patients completed all five study visits. LDL-cholesterol, total cholesterol (TC), triglycerides (TG), and FFA levels were significantly higher in patients taking protease inhibitors (PIs). Patients with LD had higher TC and TG levels (both p < 0.05), and higher FFA (p = 0.0532). Adiponectin levels did not differ between PI/non-PI and LD/non-LD groups. HDL-cholesterol seemed to decrease, and FFA to increase over time. All IGF-I and all but one IGFBP-3 level were within normal range for age and Tanner stage. CONCLUSION: Dyslipidemia remained relatively constant over our study period. Adiponectin was not useful as a marker of LD in our population.

HIV--associated lipodystrophy in children.

Lipodystrophy (LD) with varying degrees of lipohypertrophy, lipoatrophy, hyperlipidemia, and insulin resistance is one of the complications of highly active antiretroviral therapy (HAART) and occurs in one to 33 % of HAART-treated, HIV infected children. We summarize the data on the role of leptin, adiponectin, the growth hormone axis, glucocorticoids, sterol response element binding protein 1c (SREBP-1c), the tumor necrosis factor alpha axis (TNF-alpha), interleukin-6 (IL-6), interleukin- 18 (IL-18), interferon-alpha (IFN-alpha), tissue plasminogen activator (tPA), and plasminogen activator inhibitor (PAI-1) in the pathophysiology of LD. Adiponectin levels are generally decreased in LD, whereas leptin levels are increased. Systemic cortisol levels are not elevated in LD, even though glucocorticoids seem to play an important role in LD and the phenotype can be reminiscent of Cushing syndrome. GH resistance in LD needs to be better characterized. While some cytokines show promise as markers for LD, it is difficult to tell whether their derangement is a cause of or the effect of LD.

Committing the Oldest Sins in the Newest Kind of Ways-Antibodies Targeting the Influenza Virus Type A Hemagglutinin Globular Head.

The globular head of the trimeric influenza hemagglutinin (HA) contains the receptor-binding domain (RBD) and is the target of potently neutralizing human monoclonal antibodies (mAbs) with high in vivo activity. In general, these mAbs are induced easily by vaccination, but only infrequently display cross-neutralizing activity against antigenic drift variants or even against HA molecules from viruses of heterologous subtypes. Recently, the atomic resolution structures of several such antibodies in complex with HA have been determined by X-ray crystallography. Not surprisingly, cross-reactive globular head antibodies target, at least partially, the conserved RBD. The cross-reactive potential of such mAbs is limited by contacts of hypervariable HA residues outside the conserved RBD. The RBD of H2 HA seems especially immunogenic. Increasing the immunogenicity of the RBD of other HA subtypes may be a step toward a universal influenza vaccine. The germ line-encoded Phe54 residue of the CDR-H2 of the VH1-69 germ line sequence appears to be ideally suited not only to reach into a conserved, hydrophobic pocket on the HA stem, but also to reach into the conserved, hydrophobic pocket that is the RBD. We have cloned antibodies from different individuals that are encoded by the VH1-69 germ line gene segment that contact the universally conserved Trp153 on the bottom of the RBD. These antibodies serve as further evidence of antibody genetic sequence convergence across individuals.

A recurring motif for antibody recognition of the receptor-binding site of influenza hemagglutinin.

Influenza virus hemagglutinin (HA) mediates receptor binding and viral entry during influenza infection. The development of receptor analogs as viral-entry blockers has not been successful, which suggests that sialic acid may not be an ideal scaffold to obtain broad, potent HA inhibitors. Here, we report crystal structures of Fab fragments from three human antibodies that neutralize the 1957 pandemic H2N2 influenza virus in complex with H2 HA. All three antibodies use an aromatic residue to plug a conserved cavity in the HA receptor-binding site. Each antibody interacts with the absolutely conserved HA1 Trp153 at the cavity base through π-π stacking with the signature Phe54 of two VH1-69-encoded antibodies or a tyrosine from HCDR3 in the other antibody. This highly conserved interaction can be used as a starting point to design inhibitors targeting this conserved hydrophobic pocket in influenza viruses.

Human antibodies that neutralize respiratory droplet transmissible H5N1 influenza viruses.

Recent studies described the experimental adaptation of influenza H5 HAs that confers respiratory droplet transmission (rdt) to influenza virus in ferrets. Acquisition of the ability to transmit via aerosol may lead to the development of a highly pathogenic pandemic H5 virus. Vaccines are predicted to play an important role in H5N1 control should the virus become readily transmissible between humans. We obtained PBMCs from patients who received an A/Vietnam/1203/2004 H5N1 subunit vaccine. Human hybridomas were then generated and characterized. We identified antibodies that bound the HA head domain and recognized both WT and rdt H5 HAs. We used a combination of structural techniques to define a mechanism of antibody recognition of an H5 HA receptor-binding site that neutralized H5N1 influenza viruses and pseudoviruses carrying the HA rdt variants that have mutations near the receptor-binding site. Incorporation or retention of this critical antigenic site should be considered in the design of novel H5 HA immunogens to protect against mammalian-adapted H5N1 mutants.

A carboxy-terminal trimerization domain stabilizes conformational epitopes on the stalk domain of soluble recombinant hemagglutinin substrates.

Recently, a new class of broadly neutralizing anti-influenza virus antibodies that target the stalk domain of the viral hemagglutinin was discovered. As such, induction, isolation, characterization, and quantification of these novel antibodies has become an area of intense research and great interest. Since most of these antibodies bind to conformational epitopes, the structural integrity of hemagglutinin substrates for the detection and quantification of these antibodies is of high importance. Here we evaluate the binding of these antibodies to soluble, secreted hemagglutinins with or without a carboxy-terminal trimerization domain based on the natural trimerization domain of T4 phage fibritin. The lack of such a domain completely abolishes binding to group 1 hemagglutinins and also affects binding to group 2 hemagglutinins. Additionally, the presence of a trimerization domain positively influences soluble hemagglutinin stability during expression and purification. Our findings suggest that a carboxy-terminal trimerization domain is a necessary requirement for the structural integrity of stalk epitopes on recombinant soluble influenza virus hemagglutinin.

Structure of the human metapneumovirus fusion protein with neutralizing antibody identifies a pneumovirus antigenic site.

Human metapneumovirus and respiratory syncytial virus cause lower respiratory tract infections. The virus fusion (F) glycoprotein promotes membrane fusion by refolding from a metastable pre-fusion to a stable post-fusion conformation. F is also a major target of the neutralizing antibody response. Here we show that a potently neutralizing anti-human metapneumovirus antibody (DS7) binds a structurally invariant domain of F, revealing a new epitope that could be targeted in vaccine development.

An insertion mutation that distorts antibody binding site architecture enhances function of a human antibody.

The structural and functional significance of somatic insertions and deletions in antibody chains is unclear. Here, we demonstrate that a naturally occurring three-amino-acid insertion within the influenza virus-specific human monoclonal antibody 2D1 heavy-chain variable region reconfigures the antibody-combining site and contributes to its high potency against the 1918 and 2009 pandemic H1N1 influenza viruses. The insertion arose through a series of events, including a somatic point mutation in a predicted hot-spot motif, introduction of a new hot-spot motif, a molecular duplication due to polymerase slippage, a deletion due to misalignment, and additional somatic point mutations. Atomic resolution structures of the wild-type antibody and a variant in which the insertion was removed revealed that the three-amino-acid insertion near the base of heavy-chain complementarity-determining region (CDR) H2 resulted in a bulge in that loop. This enlarged CDR H2 loop impinges on adjacent regions, causing distortion of the CDR H1 architecture and its displacement away from the antigen-combining site. Removal of the insertion restores the canonical structure of CDR H1 and CDR H2, but binding, neutralization activity, and in vivo activity were reduced markedly because of steric conflict of CDR H1 with the hemagglutinin antigen.

Severe pandemic 2009 H1N1 influenza disease due to pathogenic immune complexes.

Pandemic influenza viruses often cause severe disease in middle-aged adults without preexisting comorbidities. The mechanism of illness associated with severe disease in this age group is not well understood. Here we find preexisting serum antibodies that cross-react with, but do not protect against, 2009 H1N1 influenza virus in middle-aged adults. Nonprotective antibody is associated with immune complex-mediated disease after infection. We detected high titers of serum antibody of low avidity for H1-2009 antigen, and low-avidity pulmonary immune complexes against the same protein, in severely ill individuals. Moreover, C4d deposition--a marker of complement activation mediated by immune complexes--was present in lung sections of fatal cases. Archived lung sections from middle-aged adults with confirmed fatal influenza 1957 H2N2 infection revealed a similar mechanism of illness. These observations provide a previously unknown biological mechanism for the unusual age distribution of severe cases during influenza pandemics.

Specificity for human hemoglobin enhances Staphylococcus aureus infection.

Iron is required for bacterial proliferation, and Staphylococcus aureus steals this metal from host hemoglobin during invasive infections. This process involves hemoglobin binding to the cell wall of S. aureus, heme extraction, passage through the cell envelope, and degradation to release free iron. Herein, we demonstrate an enhanced ability of S. aureus to bind hemoglobin derived from humans as compared to other mammals. Increased specificity for human hemoglobin (hHb) translates into an improved ability to acquire iron and is entirely dependent on the staphylococcal hemoglobin receptor IsdB. This feature affects host-pathogen interaction as demonstrated by the increased susceptibility of hHb-expressing mice to systemic staphylococcal infection. Interestingly, enhanced utilization of human hemoglobin is not a uniform property of all bacterial pathogens. These results suggest a step in the evolution of S. aureus to better colonize the human host and establish hHb-expressing mice as a model of S. aureus pathogenesis.

Structural basis of preexisting immunity to the 2009 H1N1 pandemic influenza virus.

The 2009 H1N1 swine flu is the first influenza pandemic in decades. The crystal structure of the hemagglutinin from the A/California/04/2009 H1N1 virus shows that its antigenic structure, particularly within the Sa antigenic site, is extremely similar to those of human H1N1 viruses circulating early in the 20th century. The cocrystal structure of the 1918 hemagglutinin with 2D1, an antibody from a survivor of the 1918 Spanish flu that neutralizes both 1918 and 2009 H1N1 viruses, reveals an epitope that is conserved in both pandemic viruses. Thus, antigenic similarity between the 2009 and 1918-like viruses provides an explanation for the age-related immunity to the current influenza pandemic.