The molecular biology of influenza virus pathogenicity

Influenza A virus hemagglutinin is a B cell-superstimulatory lectin

Influenza A viruses display T cell-independent polyclonal B cell-activating properties which are mediated by the B cell-superstimulatory envelope glycoprotein hemagglutinin (HA). In this report, the receptor-binding requirements for B cell activation by influenza viruses were expected. Neuraminidase treatment of resting mature B cells from BALB/c mice abrogated late (proliferation/immunoglobulin synthesis), early (up-regulation of cell surface markers, including CD25, B220, and B7-1) and very-early events (homotypic adhesion) in virus-responding B lymphocytes. Similarly, pretreatment of murine responder cells with different inhibitors of N-glycosylation (tunicamycin, deoxymannojirimycin) significantly suppressed subsequent B lymphocyte activation by HA, but not control responses to lipopolysaccharide or anti-mu. Assays with chimeric HA transfectants, expressing the loop region of epitope B (amino acids 155-160) of the globular head of H2 (high B cell-stimulatory subtype) or H3 (medium-stimulatory subtype) on the protein backbone of a low-stimulatory subtype (H1) failed to alter the B cell-stimulatory activity of the virus, suggesting that the hypervariable loop region is not crucial in determining the B cell-activating properties of the protein. Collectively, our results imply that the B cell-superstimulatory function of influenza virus HA is not mediated by a direct protein/protein interaction, but via binding of HA to terminal sialic acid residues on cell surface receptor glycoproteins. These findings identify the influenza virus HA glycoprotein as the first viral lectin with lymphocyte-activating properties.

Immunology taught by viruses

The survival of viruses depends on the survival of susceptible hosts. The vertebrate immune system and viruses have therefore coevolved complementary facets. Evidence from various balanced virus-host relationships illustrates that immunological specificity and memory may best be defined biologically and that the mature immune system does not discriminate between "self" and "nonself." Rather, B cells distinguish antigen patterns, whereas T cell responses depend on localization, transport, and kinetics of antigen within lymphatic organs.

Superstimulatory influenza virus and highly organized BCR-ligands act synergistically on B cell activation

The influenza virus glycoprotein hemagglutinin (HA) behaves as a superstimulatory protein for B lymphocytes from various species. Polyclonal B cell stimulation mediated by HA can be blocked by soluble anti-Ig antibodies. We here report that, if presented in a highly organized form, i.e., as anti-Ig mAb coupled to dextran (anti-Ig-Dex), conventional BCR-ligands and influenza viruses act synergistically on murine B cell activation. Proliferative responses of both spellen-derived and peritoneal B cells mediated by suboptimal amounts of HA were significantly augmented by costimulation with anti-Ig-Dex, and vice versa. Similarly, anti-Ig-Dex, which on its own cannot induce Ig production in the absence of added cytokines, significantly enhanced Ig synthesis in response to superstimulatory HA. By contrast, poorly organized BCR-ligands (i.e. the same anti-Ig mAb in a soluble form) had either no, or a strong inhibitory effect on virus-triggered lymphocyte activation. Assays with various second messenger-antagonists, however, revealed clear differences in the signaling pathway employed by anti-Ig-Dex and HA, suggesting that the functional synergy between the two multimeric agents is mediated by engagement of distinct transducing elements. Taken together, these results indicate that the superstimulatory function of influenza virus HA represents a molecular strategy to mimick B cell activation by conventional, highly organized particulate-antigens.

Cellular proteases involved in the pathogenicity of enveloped animal viruses, human immunodeficiency virus, influenza virus A and Sendai virus

In enveloped viruses, post-translational proteolytic activation is a critical step for the fusion activity and thus for the infectivity of the virus. In addition to the membrane receptors for the viruses, proteolytic activation is indispensable for effective virus spread in the infected host and it is a prime determinant for pathogenicity. Here we described the host cellular processing proteases, tryptase Clara and tryptase TL2, which proteolytically activate the infectivity of influenza A and Sendai viruses in the respiratory tract and HIV-1 in human CD4+ T cells, respectively. A novel trypsin-like protease, designated tryptase Clara, was purified from rat lung. The enzyme is localized in Clara cells of the bronchial epithelium and is secreted into the airway lumen. The enzyme specifically recognizes the consensus cleavage motif Gln(Glu)-X-Arg of influenza A and Sendai viruses and proteolytically activates the envelope fusion glycoproteins of the progeny viruses extracellularly in the airway lumen. Human mucus protease inhibitor and pulmonary surfactant in airway fluid inhibited the proteolytic activation of these viruses and also suppressed multiple cycles of viral replication in vitro. These results suggest that an imbalance between the amount of tryptase Clara and that of endogenous inhibitors in airway fluid is a prime determinant for pneumopathogenicity of the viruses. Therefore supplementing an endogenous inhibitor at therapeutic doses may protect against virus infection. In HIV-1 infection, binding of the gp120 envelope glycoprotein to the CD4 receptor is not sufficient in itself to allow virus entry, and an additional component(s) in the membrane is required for cell infection as a cofactor. We isolated a serine protease named tryptase TL2, in the membrane of CD4+ lymphocytes, which specifically binds to the V3 loop of HIV-1 gp120 as a cofactor. After binding, tryptase TL2 proteolytically processed gp120 into two protein species of 70 and 50 kDa and the cleavage was suppressed by a neutralizing antibody against the V3 loop. The amino acids that constitute the cleavage sites in the V3 loop of almost all HIV isolates are variable, but they are restricted to those which are susceptible to chymotryptic and/or tryptic enzyme. The multi-substrate specificity of tryptase TL2, which has tryptic and chymotryptic specificities, may correspond tot he variability of the V3 loop. The selective cleavage of the V3 loop by tryptase TL2 may lead to a conformational change of gp120, resulting in the dissociation of gp120 from gp41, exposing the fusogenic domain of the transmembrane protein gp41 following virus-host cell fusion.

Influenza viruses, cell enzymes, and pathogenicity

Proteolytic cleavage of the influenza virus hemagglutinin glycoprotein (HA) by cellular proteases is a prerequisite for virus infectivity, spread of the virus in the infected organism, tissue tropism, and viral pathogenicity. Production of infectious virus depends upon the structure at the HA cleavage site as well as the substrate specificity and the distribution of appropriate enzymes. Differences exist in the specificities of the endoproteases that recognize the different sequence motifs at the cleavage site. With avian influenza viruses that cause lethal systemic infections, the cleavage site consists of multibasic amino acids. Furin, which activates this type of HA, is a member of the subtilisin family and represents the prototype of ubiquitously occurring membrane-bound proteases. On the other hand, serine proteases secreted from a restricted number of cell types and some bacterial enzymes recognize a monobasic cleavage signal at HA of the mammalian and the apathogenic avian influenza viruses. The limited occurrence of these proteases results in only localized infection. Implementation of these defined conditions for virus activation may represent a novel type of disease control.

Molecular evolution of influenza viruses

There are two different mechanisms by which influenza viruses might evolve: (1) Because the RNA genome of influenza viruses is segmented, new strains can suddenly be produced by reassortment, as happens, for example, during antigenic shift, creating new pandemic strains. (2) New viruses evolve relatively slowly by stepwise mutation and selection, for example, during antigenic or genetic drift. Influenza A viruses were found in various vertebrate species, where they form reservoirs that do not easily mix. While human influenza A viruses do not spread in birds and vice versa, the species barrier to pigs is relatively low, so that pigs might function as "mixing vessels" for the creation of new pandemic reassortants in Southeast Asia, where the probability is greatest for double infection of pigs by human and avian influenza viruses. Phylogenetic studies revealed that about 100 years ago, an avian influenza A virus had crossed the species barrier, presumably first to pigs, and from there to humans, forming the new stable human and classical swine lineages. In 1979, again, an avian virus showed up in the North European swine population, forming another stable swine lineage. The North European swine isolates from 1979 until about 1985 were genetically extremely unstable. A hypothesis is put forward stating that a mutator mutation is necessary to enable influenza virus to cross the species barrier by providing the new host with sufficient variants from which it can select the best fitting ones. As long as the mutator mutation is still present, such a virus should be able to cross the species barrier a second time, as happened about 100 years ago. Although the most recent swine isolates from northern Germany are again genetically stable, we nevertheless should be on the lookout to see if a North European swine virus shows up in the human population in the near future.

Proteolytic processing is required for viral superantigen activity

The mouse mammary tumor virus-7 superantigen (vSAG7) is proteolytically processed in B cells at as many as three positions. Proteolytic processing appears to be important for superantigen activity because a processed form of vSAG7 was predominant among those forms that were found to bind to major histocompatibility complex class II molecules. To determine the functional significance of proteolytic processing, a mutation was introduced in vSAG7 at one of the sites where proteolytic cleavage is thought to take place in B cells. Elimination of the putative processing site at position 171 abrogated detectable vSAG7 surface expression in B cells, indicating that proteolytic processing is required for vSAG7 function. Coexpression in insect cells of vSAG7 and furin, a proprotein-processing enzyme, also demonstrated that furin could process vSAG7 at position 171.

Pathogenicity of influenza A/Seal/Mass/1/80 virus mutants for mammalian species

Increases in infectiousness, neurotropism and virulence were found in a laboratory variant of influenza A/Seal/Massachussets/1/80 (H7N7) virus having a highly cleavable hemagglutinin. Sequential passage from host to host further increased pathogenicity of the H7N7 virus in mice, ferrets and rats.

Proteolytic processing of human cytomegalovirus glycoprotein B (gpUL55) is mediated by the human endoprotease furin

Inhibition of endoproteolytic cleavage of glycoprotein B (gB; gpUL55) of human cytomegalovirus was achieved by treatment of infected fibroblasts with decanoyl peptidyl chloromethyl ketone (decRVKR-CMK), which inhibits the action of cellular subtilisin-like endoproteases with the amino acid recognition motif R x K/R R. Uncleaved gB precursor molecules of 160 kDa that were accumulated were endoglycosidase H resistant, suggesting that correct cellular transport occurred in the presence of the drug. The inhibitor also prevented endoproteolytic gB processing in CV-1 cells infected with a recombinant vaccinia virus-gB construct (VVgB). Evidence for direct involvement of the ubiquitous subtilisin-like endoprotease furin in gB cleavage was obtained from the observation that coinfection of CV-1 cells with WgB and a recombinant vaccinia-human furin construct reestablished endoproteolytic activity which was normally absent late after infection with WgB alone.

Five myths about AIDS that have misdirected research and treatment

A number of widely repeated and factually incorrect myths have pervaded the AIDS research literature, misdirecting research and treatment. Five of the most outstanding are: 1) that all risk groups develop AIDS at the same rate following HIV infection; 2) that there are no true seroreversions following HIV infection; 3) that antibody is protective against HIV infection; 4) that the only way to treat AIDS effectively is through retroviral therapies; and 5) that since HIV is so highly correlated with AIDS incidence, it must be the sole necessary and sufficient cause of AIDS. A huge body of research, reviewed in this paper, demonstrates the falsity of these myths. 1) The average number of years between HIV infection and AIDS is greater than 20 years for mild hemophiliacs, 14 years for young severe hemophiliacs, 10 years for old severe hemophiliacs, 10 years for homosexual men, 6 years for transfusion patients of all ages, 2 years for transplant patients, and 6 months for perinatally infected infants. These differences can only be explained in terms of risk-group associated cofactors. 2) Seroreversions are common. Between 10 and 20 percent of HIV-seronegative people in high risk groups have T-cell immunity to HIV, and may have had one or more verified positive HIV antibody tests in the past. 3) Antibody, far from being protective against HIV, appears to be highly diagnostic of loss of immune regulation of HIV, and some evidence of antibody-enhancement of infection exists. 4) Non-retroviral treatments of HIV infection, including safer sex practices, elimination of drug use, high nutrient diets, and limited reexposure to HIV and its cofactors have proven to be effective means of preventing or delaying onset of AIDS. 5) Many immunosuppressive factors, including drug use, multiple concurrent infections, and exposure to alloantigens, are as highly correlated with AIDS risk groups as HIV. These data are more consistent with AIDS being a multifactorial or synergistic disease than a monofactorial one.

Thermolysin activation mutants with changes in the fusogenic region of an influenza virus hemagglutinin

Influenza virus A/seal/Mass/1/80 (H7N7) mutants were obtained; the hemagglutinins (HAs) of the mutants were not activated by trypsin, as in the wild-type virus, but by thermolysin. The mutants grew efficiently under multiple replication cycle conditions and formed plaques in chicken embryo cells only when thermolysin was added to the culture medium. They exhibited hemolytic activity and induced protective immunity in chickens after an asymptomatic course of infection. Nucleotide sequencing of the HA gene and direct amino acid sequencing showed that insertion of a single leucine into the fusion peptide of the HA2 chain close to the cleavage site and a shift of the cleavage site toward the C terminus by one amino acid were responsible for the changes in the biological properties of the thermolysin activation mutants. Revertants could be obtained when trypsin or trypsin-like endoproteases were present in the virus-producing system.

Consistent occurrence of hemagglutinin variants among avian influenza virus isolates of the H7 subtype

Several field isolates of avian influenza virus of the H7 subtype were analyzed for the presence of hemagglutinin variants by labeling proteins in cells infected with virus clones, and reacting with monoclonal antibodies. Each strain was shown to contain two distinct electrophoretic variants of the uncleaved hemagglutinin. In the A/Tk/Ore/71 (H7N3) isolate, two variants remained in the population through 35 laboratory passages, indicating both are stable and may be important to expression of the viral phenotype. Nucleotide sequence analysis of the HA gene of these two variants demonstrated differences at several amino acid positions in the HA1 subunit including one glycosylation site. Three additional recent North American isolates were also each found to contain two electrophoretic variants occurring within populations as few as one embryo passage away from the original clinical specimen. Pulse-chase assays indicated none of the variant HA molecules were cleavable in chick embryo fibroblasts. In the highly pathogenic Australian isolate; A/Ck/Victoria/75, both HA variants are cleavable in fibroblasts, without added trypsin, and the differences are localized within the HA1 region. With all the strains tested, the slower migrating HA variant was associated with a consistently higher hemagglutinin titer in embryos. Finally, recent H7 isolates from imported birds (A/Soft Bill/Ill/92) also exhibit similar variants, indicating their occurrence is not limited to domestic poultry. This consistent presence of two distinct electrophoretic variants in several avian H7 isolates suggests multiple allelic forms of the H7 hemagglutinin.

Synthesis of transition-state analogues as potential inhibitors of sialidase from Influenza virus

Sodium 5-acetamido-2,6-anhydro-3,4,5-trideoxy-D-manno-non-2-enonate (2) has been synthesized from N-acetyl-4-deoxy-neuraminic acid methyl ester (4). Sodium 2,6-anhydro-3-deoxy-L-arabino-hept-2-enonate (3), 4-acetamido-2,6-anhydro-3,4-dideoxy-L-arabino-hept-2-enonic acid (18e), and 4-acetamido-2,6-anhydro-3,4-dideoxy-L-ribo-hept-2-enonic acid (18a) have been prepared from L-arabinose. The above compounds were investigated as inhibitors of sialidase from Influenza virus. Only compound 2 showed a significant inhibitory activity (Ki 8 x 10(-2) mM) against the enzyme.

Proprotein-processing endoproteases PC6 and furin both activate hemagglutinin of virulent avian influenza viruses

Among the proprotein-processing subtilisin-related endoproteases, furin has been a leading candidate for the enzyme that activates the hemagglutinin (HA) of virulent avian influenza viruses. In the present study, we examined the cleavage activity of two other recently isolated ubiquitous subtilisin-related proteases, PACE4 and PC6, using wild-type HA of A/turkey/Ireland/1378/83 (H5N8) and a series of its mutant HAs. Vaccinia virus-expressed wild-type HA was not cleaved in human colon adenocarcinoma LoVo cells, which lack active furin. This processing defect was corrected by the expression of furin and PC6 but not of PACE4 and a control wild-type vaccinia virus. PC6 showed a sequence specificity similar to that with the endogenous proteases in cultured cells. When LoVo cells were infected with a virulent avian virus, A/turkey/Ontario/7732/66 (H5N9), only noninfectious virions were produced because of the lack of HA cleavage. However, when the cells were coinfected with vaccinia virus that expressed either furin or PC6, the avian virus underwent multiple cycles of replication, indicating that both furin and PC6 specifically cleave the virulent virus HA at the authentic site. These data suggest that PC6, as well as furin, can activate virulent avian influenza viruses in vivo, implying the presence of multiple HA cleavage enzymes in animals.

Reverse genetics provides direct evidence for a correlation of hemagglutinin cleavability and virulence of an avian influenza A virus

To obtain direct evidence for a relationship between hemagglutinin (HA) cleavability and the virulence of avian influenza A viruses, we generated a series of HA cleavage mutants from a virulent virus, A/turkey/Ontario/7732/66 (H5N9), by reverse genetics. A transfectant virus containing the wild-type HA with R-R-R-K-K-R at the cleavage site, which was readily cleaved by endogenous proteases in chicken embryo fibroblasts (CEF), was highly virulent in intramuscularly or intranasally/orally inoculated chickens. By contrast, a mutant containing the HA with an avirulent-like sequence (R-E-T-R) at the cleavage site, which was not cleaved by the proteases in CEF, was avirulent in chickens, indicating that a genetic alteration confined to the HA cleavage site can affect cleavability and virulence. Mutant viruses with HA cleavage site sequences of T-R-R-K-K-R or T-T-R-K-K-R were as virulent as viruses with the wild-type HA, whereas a mutant with a two-amino-acid deletion but retention of four consecutive basic residues (R-K-K-R) was as avirulent as a virus with the avirulent-type HA. Interestingly, although a mutant containing an HA with R-R-R-K-T-R, which has reduced cleavability in CEF, was as virulent as viruses with high HA cleavability when given intramuscularly, it was less virulent when given intranasally/orally. We conclude that the degree of HA cleavability in CEF predicts the virulence of avian influenza viruses.

Rescue of vector-expressed fowl plague virus hemagglutinin in biologically active form by acidotropic agents and coexpressed M2 protein

The hemagglutinin of the Rostock strain of fowl plague virus was expressed in CV-1 cells by a simian virus 40 vector, and its stability in the exocytotic transport process was examined by a fusion assay. A 50-fold increase in the fusion activity of the hemagglutinin was observed when expression occurred in the presence of ammonium chloride, Tris-HCl, or high doses of amantadine. When chloroquine, another acidotropic agent, was used, the hemagglutinin exposed at the cell surface had to be activated by trypsin, because intracellular cleavage was inhibited by this compound. Hemagglutinin mutants resistant to intracellular cleavage did not require acidotropic agents for full expression of fusion activity, when treated with trypsin after arrival at the cell surface. These results indicate that fowl plague virus hemagglutinin expressed by a simian virus 40 vector is denatured in the acidic milieu of the exocytotic pathway and that cleavage is a major factor responsible for the pH instability. Coexpression with the M2 protein also markedly enhanced the fusion activity of the hemagglutinin, and this effect was inhibited by low doses of amantadine. These results support the concept that M2, known to have ion channel function, protects the hemagglutinin from denaturation by raising the pH in the exocytotic transport system. The data also stress the importance of acidotropic agents or coexpressed M2 for the structural and functional integrity of vector-expressed hemagglutinin.

Sequence specificity of furin, a proprotein-processing endoprotease, for the hemagglutinin of a virulent avian influenza virus

The virulence of avian influenza viruses correlates with the sensitivity of their hemagglutinin (HA) to cellular proteases. Furin, a proprotein-processing subtilisin-related endoprotease, is a leading candidate for the enzyme that cleaves the HA of virulent avian viruses. We therefore compared the specificity of furin with those of proteases in a variety of cultured cells and in a rat Golgi fraction, using the HA cleavage mutants of a virulent avian influenza virus, A/Turkey/Ireland/1378/85 (H5N8). The results indicated similar sequence specificities among the endoproteases when purified furin was used. In experiments with the vaccinia virus expression system, overexpressed furin cleaved mutant HAs that were not recognized by the endogenous proteases, resulting in an apparent broader specificity of furin. These findings authenticate the proposed role of furin as an HA-activating protease in vivo and caution against the use of expression vectors to study protease sequence specificity.

Host cell proteases controlling virus pathogenicity

The majority of viral glycoproteins that undergo post-translational proteolysis are cleaved by ubiquitous intracellular proteases; however, a minority are cleaved by secreted proteases available only in a few host systems. The interplay of viral glycoproteins and cellular proteases may have a pivotal role in the spread of infection, host range and pathogenicity.

Processing of viral glycoproteins by the subtilisin-like endoprotease furin and its inhibition by specific peptidylchloroalkylketones

The spike glycoproteins of many enveloped viruses are proteolytically cleaved at the carboxytermini of sequences containing the basic motif R-X-K/R-R. Cleavage is often necessary for the fusion capacity of the glycoproteins and, thus, for virus infectivity. Among these viruses are pathogenic avian influenza viruses, human parainfluenza virus, human cytomegalovirus, and human immunodeficiency virus; it has been demonstrated that these viruses can be activated by furin. Indigenous furin has been identified in T-lymphocytes, which are host cells for HIV. Furin has been localized in the TGN and on the surface of cells after vectorial expression. Peptidylchloroalkylketones have been designed that inhibit with high specificity cleavage and fusion activity of viral glycoproteins, as well as virus replication.

Timing of palmitoylation of influenza virus hemagglutinin

The timing of the attachment of fatty acids to the hemagglutinin (HA) of influenza A virus was studied. Treatment of virus infected cells with brefeldin A (BFA), a drug which blocks intracellular transport along the exocytic pathway at a pre-Golgi site, does not prevent palmitoylation of HA. The relationship of HA-palmitoylation to the oligomerisation and to the proteolytical cleavage of the protein revealed that the uncleaved trimer of HA is the substrate for the acylating enzyme in virus infected cells. The results are discussed with regard to the intracellular site of palmitoylation.

The synthesis of inhibitors for processing proteinases and their action on the Kex2 proteinase of yeast

Peptidyl chloromethane and sulphonium salts containing multiple Arg and Lys residues were synthesized as potential inhibitors of prohormone and pro-protein processing proteinases. The potencies of these compounds were assayed by measuring the kinetics of inactivation of the yeast Kex2 proteinase, the prototype of a growing family of eukaryotic precursor processing proteinases. The most potent inhibitor, Pro-Nvl-Tyr-Lys-Arg-chloromethane, was based on cleavage sites in the natural Kex2 substrate pro-alpha-factor. This inhibitor exhibited a Ki of 3.7 nM and a second-order inactivation rate constant (k2/Ki) of 1.3 x 10(7) M-1.s-1 comparable with the value of kcat./Km obtained with Kex2 for the corresponding peptidyl methylcoumarinylamide substrate. The enzyme exhibited sensitivity to the other peptidyl chloromethanes over a range of concentrations, depending on peptide sequence and alpha-amino decanoylation, but was completely resistant to peptidyl sulphonium salts. Kinetics of inactivation by these new inhibitors of a set of 'control' proteinases, including members of both the trypsin and subtilisin families, underscored the apparent specificity of the compounds most active against Kex2 proteinase.

Rational design of potent sialidase-based inhibitors of influenza virus replication

Two potent inhibitors based on the crystal structure of influenza virus sialidase have been designed. These compounds are effective inhibitors not only of the enzyme, but also of the virus in cell culture and in animal models. The results provide an example of the power of rational, computer-assisted drug design, as well as indicating significant progress in the development of a new therapeutic or prophylactic treatment for influenza infection.

Role of conserved glycosylation sites in maturation and transport of influenza A virus hemagglutinin

The role of three N-linked glycans which are conserved among various hemagglutinin (HA) subtypes of influenza A viruses was investigated by eliminating the conserved glycosylation (cg) sites at asparagine residues 12 (cg1), 28 (cg2), and 478 (cg3) by site-directed mutagenesis. An additional mutant was constructed by eliminating the cg3 site and introducing a novel site 4 amino acids away, at position 482. Expression of the altered HA proteins in eukaryotic cells by a panel of recombinant vaccinia viruses revealed that rates and efficiency of intracellular transport of HA are dependent upon both the number of conserved N-linked oligosaccharides and their respective positions on the polypeptide backbone. Glycosylation at two of the three sites was sufficient for maintenance of transport of the HA protein. Conserved glycosylation at either the cg1 or cg2 site alone also promoted efficient transport of HA. However, the rates of transport of these mutants were significantly reduced compared with the wild-type protein or single-site mutants of HA. The transport of HA proteins lacking all three conserved sites or both amino-terminally located sites was temperature sensitive, implying that a polypeptide folding step had been affected. Analysis of trimer assembly by these mutants indicated that the presence of a single oligosaccharide in the stem domain of the HA molecule plays an important role in preventing aggregation of molecules in the endoplasmic reticulum, possibly by maintaining the hydrophilic properties of this domain. The conformational change observed after loss of all three conserved oligosaccharides also resulted in exposure of a normally mannose-rich oligosaccharide at the tip of the large stem helix that allowed its conversion to a complex type of structure. Evidence was also obtained suggesting that carbohydrate-carbohydrate interactions between neighboring oligosaccharides at positions 12 and 28 influence the accessibility of the cg2 oligosaccharide for processing enzymes. We also showed that terminal glycosylation of the cg3 oligosaccharide is site specific, since shifting of this site 4 amino acids away, to position 482, yielded an oligosaccharide that was arrested in the mannose-rich form. In conclusion, carbohydrates at conserved positions not only act synergistically by promoting and stabilizing a conformation compatible with transport, they also enhance trimerization and/or folding rates of the HA protein.

Pulmonary surfactant is a potential endogenous inhibitor of proteolytic activation of Sendai virus and influenza A virus

The pathogenicities of influenza viruses and paramyxoviruses have been proposed to be primarily determined by a host cell protease(s) that activates viral infectivity by proteolytic cleavage of the envelope glycoproteins. We recently isolated a trypsin-type endoprotease, named tryptase Clara, from rat bronchial and bronchiolar epithelial Clara cells, which is secreted into the airway lumen and activates Sendai virus and influenza A virus proteolytically. We report here that surfactant in the bronchial fluid inhibited tryptase Clara specifically, having a Ki value of 0.13 microM, and inhibited the proteolytic activations by tryptase Clara in vitro and in organ cultures of rat lung. Intranasal infection of rats with Sendai virus was shown to stimulate secretion of tryptase Clara without changing the amount of surfactant in the bronchial lumen, resulting in a preferable condition for proteolytic viral activation and multiplication.

Deduced amino acid sequences at the haemagglutinin cleavage site of avian influenza A viruses of H5 and H7 subtypes

The amino acid sequences at the haemagglutinin cleavage sites of 9 avian influenza A viruses of H5 subtype (5 high and 4 low pathogenicity for chickens) and 21 of H7 subtype (13 high and 8 low pathogenicity for chickens) were determined by direct RNA sequencing, PCR amplification sequencing or both. None of the viruses of low pathogenicity had multiple basic amino acids at the cleavage site. All highly pathogenic viruses had an insert of basic amino acids at the cleavage site, except A/chicken/Scotland/59 (H5N1) for which the multiple basic amino acids appeared as substitutions and not insertions. All highly pathogenic viruses examined conformed to the amino acid motif of R-X-R/K-R at the cleavage site which is considered to be essential for high pathogenicity in chickens, with the notable exception of highly pathogenic virus A/turkey/England/50-92/91 (H5N1) which had the sequence R-K-R-K-T-R adjacent to the cleavage site.

Inhibition of sialidases from viral, bacterial and mammalian sources by analogues of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid modified at the C-4 position

The inhibition of sialidase activity from influenza viruses A and B, parainfluenza 2 virus, Vibrio cholerae, Arthrobacter ureafaciens, Clostridium perfringens, and sheep liver by a range of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid analogues modified at the C-4 position has been studied. All substitutions tested resulted in a decrease in the degree of inhibition of the bacterial and mammalian sialidases. For sialidases from influenza viruses A and B, on the other hand, most of the substitutions tested either had no significant effect on binding or, in the case of the basic amino and guanidino substituents, resulted in significantly stronger inhibition. The results for parainfluenza 2 virus sialidase were mostly intermediate, in that inhibition was neither significantly increased nor decreased by most of the modifications. We conclude that only the influenza A and B sialidase active sites possess acid groups correctly positioned to participate in charge-charge interactions in the region of C-4 of bound substrate, and that the C-4 binding pockets of the bacterial and mammalian sialidases examined are considerably smaller than is observed for either the influenza virus or parainfluenza virus sialidases.

A H1 hemagglutinin of a human influenza A virus with a carbohydrate-modulated receptor binding site and an unusual cleavage site

Two receptor binding variants of the influenza virus A/Tübingen/12/85 (H1N1) were separated by their different plaque formation in MDCK cells. Hemagglutination of variant I was restricted to red blood cells of guinea pigs, whereas variant II also hemagglutinated chicken cells. The variants differed also in their ability to bind to alpha 2,6-linked sialic acid. Evidence is presented that this difference is determined by a complex carbohydrate side chain at asparagine131 near the receptor binding site which is absent in variant II. With both variants, the arginine found at the cleavage site of all other human isolates analyzed so far was replaced by lysine.

Tryptase Clara, an activating protease for Sendai virus in rat lungs, is involved in pneumopathogenicity

Tryptase Clara is an arginine-specific serine protease localized exclusively in and secreted from Clara cells of the bronchial epithelium of rats (H. Kido, Y. Yokogoshi, K. Sakai, M. Tashiro, Y. Kishino, A. Fukutomi, and N. Katunuma, J. Biol. Chem. 267:13573-13579, 1992). The purified protease was shown in vitro to behave similarly to trypsin, cleaving the precursor glycoprotein F of Sendai virus at residue Arg-116 and activating viral infectivity in a dose-dependent manner. Anti-tryptase Clara antibody inhibited viral activation by the protease in vitro in lung block cultures and in vivo in infected rats. When the enzyme-specific antibody was administered intranasally to rats that were also infected intranasally with Sendai virus, activation of progeny virus in the lungs was significantly inhibited. Thus, multiple cycles of viral replication were suppressed, resulting in a reduction in lung lesions and in the mortality rate. These findings indicate that tryptase Clara is an activating protease for Sendai virus in rat lungs and is therefore involved in pulmonary pathogenicity of the virus in rats.

Mammalian subtilisin-related proteinases in cleavage activation of the paramyxovirus fusion glycoprotein: superiority of furin/PACE to PC2 or PC1/PC3

The fusion glycoprotein precursor of Newcastle disease virus is ubiquitously cleaved in the constitutive secretory pathway if it possesses an oligobasic cleavage motif (RRQR/KR), whereas the precursor is refractory to cleavage if the motif is monobasic (GR/KQGR). We examined the cleavage activity of the mammalian subtilisin-related proteinases furin/PACE, PC2, and PC1/PC3, which are thought to be responsible for proprotein processing in either the constitutive (furin/PACE) or the regulated (PC2 and PC1/PC3) secretory pathway, for the viral precursors with different cleavage motifs. Only furin/PACE was fully capable of cleaving the precursors with the oligobasic motif. PC2 and PC1/PC3 were incapable or only partially capable of cleaving at this motif. None of the proteinases cleaved the monobasic motif. These results suggest involvement of furin/PACE in viral protein processing in the constitutive secretory pathway.

The pathogenic determinant of influenza virus

Influenza viruses, like other viruses, must exhibit a genome constellation, which permits optimal virus reproduction in a given host. Besides this prerequisite the influenza virus haemagglutinin glycoprotein (HA) has been shown to be an essential determinant for pathogenicity. HA, which is synthesized as a precursor molecule, is activated by posttranslational cleavage by host proteases to obtain its full biological properties. Proteolytic activation is therefore indispensable for effective virus spread in the infected host and thus for pathogenicity. HA of the highly pathogenic avian influenza viruses inducing a systemic infection in birds is cleaved in a broad range of different host cells. On the other hand, HA of all mammalian viruses and the nonpathogenic avian strains, which cause local infection, exhibit a restricted cleavability. The prime determinant for these differences has been found to be the structure of the cleavage site. This concept was corroborated on virus mutants adapted in vitro to a new host.

Budding site of Sendai virus in polarized epithelial cells is one of the determinants for tropism and pathogenicity in mice

Wild-type Sendai virus fusion (F) glycoprotein requires trypsin or a trypsin-like protease for cleavage-activation in vitro and in vivo, respectively. The virus is pneumotropic in mice and buds at the apical domain of bronchial epithelial cells. On the other hand, the F protein of the protease-activation host range mutant, F1-R, is cleaved by ubiquitous proteases present in different cell lines and in various organs of mice. F1-R causes a systemic infection in mice and the mutant buds bipolarly at the apical and basolateral domains of infected epithelial cells. The enhanced cleavability of the F protein of F1-R has been shown to be a primary determinant for pantropism. Additionally, it has been postulated that bipolar budding of F1-R is required for the systemic spread of the virus and it has been attributed to mutations in the matrix (M) protein of F1-R (Tashiro et al., Virology 184, 227-234, 1991). In this study protease-activation mutants (KD series) were isolated from wild-type virus. They were revealed to bud at the apical domain, and the F protein was cleaved by ubiquitous proteases in mouse organs. The KD mutants were exclusively pneumotropic in mice following intranasal infection, whereas they caused a generalized infection when inoculated directly into the circulatory system. Comparative nucleotide sequence analysis of the F gene of the KD mutants revealed that the deduced amino acid substitutions responsible for enhanced cleavability of the F protein occurred removed from the cleavage site. Mutations were not at all found in the M gene of the KD mutants analyzed, in support of the role of the M protein of F1-R and of a revertant T-9 derived from the latter in bipolar budding. These results suggest that bipolar budding is necessary for the systemic spread of F1-R from the lungs and that apical budding by wild-type virus and the KD mutants leads to respiratory infections. Differential budding at the primary target of infection, in addition to the cleavage-activation of the F protein in mouse organs, is therefore also a determinant for tropism and pathogenicity of Sendai virus in mice.

Structure and assembly of hemagglutinin mutants of fowl plague virus with impaired surface transport

Five temperature-sensitive mutants of influenza virus A/FPV/Rostock/34 (H7N1), ts206, ts293, ts478, ts482, and ts651, displaying correct hemagglutinin (HA) insertion into the apical plasma membrane of MDCK cells at the permissive temperature but defective transport to the cell surface at the restrictive temperature, have been investigated. Nucleotide sequence analysis of the HA gene of the mutants and their revertants demonstrated that with each mutant a single amino acid change is responsible for the transport block. The amino acid substitutions were compared with those of mutants ts1 and ts227, which have been analyzed previously (W. Schuy, C. Will, K. Kuroda, C. Scholtissek, W. Garten, and H.-D. Klenk, EMBO J. 5:2831-2836, 1986). With the exception of ts206, the changed amino acids of all mutants and revertants accumulate in three distinct areas of the three-dimensional HA model: (i) at the tip of the 80-A (8-nm)-long alpha helix, (ii) at the connection between the globular region and stem, and (iii) in the basal domain of the stem. The concept that these areas are critical for HA assembly and hence for transport is supported by the finding that the mutants that are unable to leave the endoplasmic reticulum at the nonpermissive temperature do not correctly trimerize. Upon analysis by density gradient centrifugation, cross-linking, and digestion with trypsin and endoglucosaminidase H, two groups can be discriminated among these mutants: with ts1, ts227, and ts478, the HA forms large irreversible aggregates, whereas with ts206 and ts293, it is retained in the monomeric form in the endoplasmic reticulum. With a third group, comprising mutants ts482 and ts651 that enter the Golgi apparatus, trimerization was not impaired.

New aspects of influenza viruses

Influenza virus infections continue to cause substantial morbidity and mortality with a worldwide social and economic impact. The past five years have seen dramatic advances in our understanding of viral replication, evolution, and antigenic variation. Genetic analyses have clarified relationships between human and animal influenza virus strains, demonstrating the potential for the appearance of new pandemic reassortants as hemagglutinin and neuraminidase genes are exchanged in an intermediate host. Clinical trials of candidate live attenuated influenza virus vaccines have shown the cold-adapted reassortants to be a promising alternative to the currently available inactivated virus preparations. Modern molecular techniques have allowed serious consideration of new approaches to the development of antiviral agents and vaccines as the functions of the viral genes and proteins are further elucidated. The development of techniques whereby the genes of influenza viruses can be specifically altered to investigate those functions will undoubtedly accelerate the pace at which our knowledge expands.

Human influenza virus hemagglutinin with high sensitivity to proteolytic activation

To examine the prerequisites for cleavage activation of the hemagglutinin of human influenza viruses, a cDNA clone obtained from strain A/Port Chalmers/1/73 (serotype H3) was subjected to site-directed mutagenesis and expressed in CV-1 cells by using a simian virus 40 vector. The number of basic residues at the cleavage site, which consists of a single arginine with wild-type hemagglutinin, was increased by inserting two, three, or four additional arginines. Like wild-type hemagglutinin, mutants with up to three additional arginines were not cleaved in CV-1 cells, but insertion of four arginines resulted in activation. When the oligosaccharide at asparagine 22 of the HA1 subunit of the hemagglutinin was removed by site-directed mutagenesis of the respective glycosylation site, only three inserted arginines were required to obtain cleavage. Mutants containing a series of four basic residues were also generated by substituting arginine for uncharged amino acids immediately preceding the cleavage site. The observation that these mutants were not cleaved, even when the carbohydrate at asparagine 22 of HA1 was absent, underscores the fact that the basic peptide had to be generated by insertion to obtain cleavage. The data show that the hemagglutinin of a human influenza virus can acquire high cleavability, a property known to be an important determinant for the pathogenicity of avian influenza viruses. Factors important for cleavability are the number of basic residues at the cleavage site, the oligosaccharide at asparagine 22, and the length of the carboxy terminus of HA1.

Comparison of complete amino acid sequences and receptor-binding properties among 13 serotypes of hemagglutinins of influenza A viruses

We determined the sequences of 7 serotypes (H4, H6, H8, H9, H11, H12, and H13) of hemagglutinin (HA) genes, which have not been reported so far. The coding regions consisted of 1692 nucleotides in H4, 1698 in H6, 1695 in H8, 1680 in H9, 1695 in H11, 1692 in H12, and 1698 in H13, and specified 564, 566, 565, 560, 565, 564, and 566 amino acids, respectively. By comparison of amino acid sequences, 13 HA serotypes could be divided into two families, i.e., an H1 group (H1, H2, H5, H6, H8, H9, H11, H12, and H13) and an H3 group (H3, H4, H7, and H10). The relationship was essentially similar to that reported by Air from the comparison of 80 amino-terminal amino acid sequence of 12 HA serotypes (G.M. Air, 1981, Proc. Natl. Acad. Sci. USA 78, 7639-7643). Though a considerable amino acid sequence difference exists between certain HA serotypes, several amino acid residues in fusion peptides (HA2(1-11)) and receptor-binding sites (HA1(98), -134, -138, -153, -183, and -195) were shown to be conserved among the 13 HA serotypes. Human H1 and avian H3, H4, H8, and H10 viruses preferentially bound NeuAc alpha 2,3Gal sequences, whereas human H2 and H3 and avian H6 and H9 viruses bound NeuAc alpha 2,6Gal sequences, although the amino acid residues at position 226 of human H2 and avian H6 and H9 serotype HAs are glutamine. These results show that the amino acid residue at position 226 is not necessarily a determinant of receptor specificity for all serotypes.

Site-specific mutagenesis identifies three cysteine residues in the cytoplasmic tail as acylation sites of influenza virus hemagglutinin

The hemagglutinin (HA) of influenza virus is a type I transmembrane glycoprotein which is acylated with long-chain fatty acids. In this study we have used oligonucleotide-directed mutagenesis of cloned cDNA and a simian virus 40 expression system to determine the fatty acid binding site in HA and to examine possible functions of covalently linked fatty acids. The results show that the HA is acylated through thioester linkages at three highly conserved cysteine residues located in the cytoplasmic domain and at the carboxy-terminal end of the transmembrane region, whereas a cysteine located in the middle of the membrane-spanning domain is not acylated. Mutants lacking fatty acids at individual or all three attachment sites acquire endoglycosidase H-resistant oligosaccharide side chains, are cleaved into HA1 and HA2 subunits, and are transported to the plasma membrane at rates similar to that of wild-type HA. All mutants are membrane bound and not secreted into the medium. These results exclude transport signal and membrane-anchoring functions of covalently linked fatty acids for this integral membrane glycoprotein. Furthermore, lack of acylation has no obvious influence on the biological activities of HA: cells expressing fatty acid-free HA bind to and, after brief exposure to mildly acidic pH, fuse with erythrocytes; the HA-induced polykaryon formation is not impaired, either. Other possible functions of covalently linked fatty acids in integral membrane glycoproteins which cannot be examined in conventional cDNA expression systems are discussed.

Differences in sensitivity in haemagglutinin inhibition assays between A/equine/H3N8 viruses isolated in eggs and MDCK cells are linked to cleavage of the haemagglutinin molecule

Two primary isolates of A/equine/H3N8 viruses were obtained in embryonated hens' eggs and in Madin-Darby canine kidney (MDCK) cells. Viruses isolated in MDCK cells were significantly more sensitive as antigens in haemagglutination inhibition (HI) tests. This sensitivity appeared to be primarily linked to the extent of cleavage of the haemagglutinin molecule.

Retarded processing of influenza virus hemagglutinin in insect cells

When expressed in Spodoptera frugiperda cells by a baculovirus vector, the hemagglutinin of fowl plague virus has been found to contain palmitic acid in covalent hydroxylamine-sensitive linkage, indicating that these cells have the capacity to acylate foreign proteins at cysteine residues. Centrifugation on sucrose density gradients and immune precipitation with conformation-specific antibodies were used to compare trimerization of the hemagglutinin in insect cells and in fowl plague virus-infected MDCK cells. Trimerization of the hemagglutinin was incomplete in insect cells, and the kinetics of this reaction were about three times slower than in vertebrate cells. Similarly, post-translational proteolytic cleavage occurred in insect cells with a half-time of 90 min, and a substantial fraction of the hemagglutinin persisted in uncleaved form. In contrast, hemagglutinin was almost completely cleaved in MDCK cells, and the half-time of cleavage was only 30 min. The data indicate that in insect cells trimerization and, as a result, the subsequent processing steps of the hemagglutinin, are retarded and less efficient. The possible roles of aberrant glycosylation, acidic milieu, and lack of other influenza virus proteins in hemagglutinin trimerization are discussed.

Linkage and independence of AIDS and Kaposi disease: the interaction of human immunodeficiency virus and some coagents

Through epidemiological considerations we conclude that full-blown AIDS may occur only if the index patient is infected by the human immunodeficiency virus (HIV) and, in addition, by some other infectious coagent. Since the dynamical behavior of the spread of AIDS cases with manifestation of Kaposi's sarcoma differs fundamentally from that of the non-Kaposi cases, we conjecture that two independent coagents (together with HIV) are responsible for the outbreak of full-blown AIDS with or without manifestation of Kaposi's sarcoma, respectively. Our formal epidemiological considerations appear to be supported by recent microbiological findings.

The critical cut-off temperature of avian influenza viruses

We have measured the pathogenicity for 6-week-old chicks of infection by H7 avian influenza viruses. One virus, strain S3 from A/FPV/Rostock/34(H7N1) showed a temperature sensitive phenotype at 41.5 degrees C and reduced pathogenicity. By analysis of reassortants made between virus S3 and A/FPV/Dobson/27(H7N7), a fully pathogenic virus, two conclusions arise. (1) The critical cut-off temperature for avian influenza virus in 6-week-old chicks is 41.5 degrees. (2) RNA segment 1 of virus S3 is responsible for the lack of pathogenicity in reassortant viruses. Nucleotide sequencing of RNA segment 1 from S3 and its parent, A/FPV/Rostock/34 has revealed a single mutation at nucleotide 1561. This results in a substitution of isoleucine for leucine at amino acid position 512 in the cap-binding protein, PB2.