DISSOCIATION OF HEMAGGLUTINATING AND ANTIBODY-MEASURING CAPACITIES OF INFLUENZA VIRUS

Structural restrictions for influenza neuraminidase activity promote adaptation and diversification

Influenza neuraminidase (NA) is a sialidase that contributes to viral mobility by removing the extracellular receptors for the haemagglutinin (HA) glycoprotein. However, it remains unclear why influenza NAs evolved to function as Ca²⁺-dependent tetramers that display variable stability. Here, we show that the Ca²⁺ ion located at the centre of the NA tetramer is a major stability determinant, as this Ca²⁺ ion is required for catalysis and its binding affinity varies between NAs. By examining NAs from 2009 pandemic-like H1N1 viruses, we traced the affinity variation to local substitutions that cause residues in the central Ca²⁺-binding pocket to reposition. A temporal analysis revealed that these local substitutions predictably alter the stability of the 2009 pandemic-like NAs and contribute to the tendency for the stability to vary up and down over time. In addition to the changes in stability, the structural plasticity of NA was also shown to support the formation of heterotetramers, which creates a mechanism for NA to obtain hybrid properties and propagate suboptimal mutants. Together, these results demonstrate how the structural restrictions for activity provide influenza NA with several mechanisms for adaptation and diversification.

Assessing the Protective Potential of H1N1 Influenza Virus Hemagglutinin Head and Stalk Antibodies in Humans

Seasonal influenza viruses are a major cause of human disease worldwide. Most neutralizing antibodies (Abs) elicited by influenza viruses target the head domain of the hemagglutinin (HA) protein. Anti-HA head Abs can be highly potent, but they have limited breadth since the HA head is variable. There is great interest in developing new universal immunization strategies that elicit broadly neutralizing Abs against conserved regions of HA, such as the stalk domain. Although HA stalk Abs can provide protection in animal models, it is unknown if they are present at sufficient levels in humans to provide protection against naturally acquired influenza virus infections. Here, we quantified H1N1 HA head- and stalk-specific Abs in 179 adults hospitalized during the 2015-2016 influenza virus season. We found that HA head Abs, as measured by hemagglutinin inhibition (HAI) assays, were associated with protection against naturally acquired H1N1 infection. HA stalk-specific serum total IgG titers were also associated with protection, but this association was attenuated and not statistically significant after adjustment for HA head-specific Ab titers. We found slightly higher titers of HA stalk-specific IgG1 and IgA Abs in sera from uninfected participants than in sera from infected participants; however, we found no difference in serum in vitro antibody-dependent cellular cytotoxicity activity. In passive transfer experiments, sera from participants with high HAI activity efficiently protected mice, while sera with low HAI activity protected mice to a lower extent. Our data suggest that HA head Abs are more efficient at protecting against H1N1 infection than HA stalk Abs.IMPORTANCE Abs targeting the HA head of influenza viruses are often associated with protection from influenza virus infections. These Abs typically have limited breadth, since mutations frequently arise in HA head epitopes. New vaccines targeting the more conserved HA stalk domain are being developed. Abs that target the HA stalk are protective in animal models, but it is unknown if these Abs exist at protective levels in humans. Here, we completed experiments to determine if Abs against the HA head and stalk were associated with protection from naturally acquired human influenza virus infections during the 2015-2016 influenza season.

Inability of Kaolin Treatment to Remove Nonspecific Inhibitors from Equine Serum for the Hemagglutination Inhibition Test Against Equine H7N7 Influenza Virus

The hemagglutination inhibition test is used by many diagnostic and surveillance laboratories for detection of antibodies to influenza viruses. It is well known that the hemagglutination inhibition test is affected by nonspecific inhibitors present in equine serum. Several serum treatments are in use to remove these inhibitors, including treatment with kaolin. Discrepant results were observed in the authors' laboratories when using kaolin treatment before testing equine sera for antibodies against equine influenza virus (EIV) subtype-1 (H7N7). It is demonstrated here that kaolin treatment leads to false positive results when testing for antibodies against EIV subtype-1, as compared to other standard serum treatments (trypsin-periodate, receptor-destroying enzyme). Against EIV subtype-2 (H3N8), however, false positive results were not evident. Trypsin-periodate and receptor-destroying enzyme (RDE) treatments appear to be superior to kaolin for removal of nonspecific inhibitors from equine serum and should be used for serological diagnosis and surveillance of equine influenza virus.

Optimization of an enzyme-linked lectin assay suitable for rapid antigenic characterization of the neuraminidase of human influenza A(H3N2) viruses

Antibodies to neuraminidase (NA), the second most abundant surface protein of the influenza virus, contribute to protection against influenza virus infection. Although traditional and miniaturized thiobarbituric acid (TBA) neuraminidase inhibition (NI) assays have been successfully used to characterize the antigenic properties of NA, these methods are cumbersome and not easily amendable to rapid screening. An additional difficulty of the NI assay is the interference by hemagglutinin (HA)-specific antibodies. To prevent interference of HA-specific antibodies, most NI assays are performed with recombinant viruses containing a mismatched HA. However, generation of these viruses is time consuming and unsuitable for large-scale surveillance. The feasibility of using the recently developed enzyme-linked lectin assay (ELLA) to evaluate the antigenic relatedness of NA of wild type A(H3N2) viruses was assessed. Rather than using recombinant viruses, wild type A(H3N2) viruses were used as antigen with ferret sera elicited against recombinant viruses with a mismatched HA. In this study, details of the critical steps that are needed to modify and optimize the NI ELLA in a format that is reproducible, highly sensitive, and useful for influenza virus surveillance to monitor antigenic drift of NA are provided.

Influenza vaccines: from whole virus preparations to recombinant protein technology

Vaccination against influenza represents our most effective form of prevention. Historical approaches toward vaccine creation and production have yielded highly effective vaccines that are safe and immunogenic. Despite their effectiveness, these historical approaches do not allow for the incorporation of changes into the vaccine in a timely manner. In 2013, a recombinant protein-based vaccine that induces immunity toward the influenza virus hemagglutinin was approved for use in the USA. This vaccine represents the first approved vaccine formulation that does not require an influenza virus intermediate for production. This review presents a brief history of influenza vaccines, with insight into the potential future application of vaccines generated using recombinant technology.

Contribution of murine innate serum inhibitors toward interference within influenza virus immune assays

Please cite this paper as: Cwach et al. (2011) Contribution of murine innate serum inhibitors toward interference within influenza virus immune assays. Influenza and Other Respiratory Viruses DOI: 10.1111/j.1750‐2659.2011.00283.x.

Background  Prior to detection of an antibody response toward influenza viruses using the hemagglutination inhibition assay (HAI), sera are routinely treated to inactivate innate inhibitors using both heat inactivation (56°C) and recombinant neuraminidase [receptor‐destroying enzyme (RDE)].

Objectives  We revisited the contributions of innate serum inhibitors toward interference with influenza viruses in immune assays, using murine sera, with emphasis on the interactions with influenza A viruses of the H3N2 subtype.

Methods  We used individual serum treatments: 56°C alone, RDE alone, or RDE + 56°C, to treat sera prior to evaluation within HAI, microneutralization, and macrophage uptake assays.

Results  Our data demonstrate that inhibitors present within untreated murine sera interfere with the HAI assay in a manner that is different from that seen for the microneutralization assay. Specifically, the γ class inhibitor α₂‐Macroglobulin (A2‐M) can inhibit H3N2 viruses within the HAI assay, but not in the microneutralization assay. Based on these findings, we used a macrophage uptake assay to demonstrate that these inhibitors can increase uptake by macrophages when the influenza viruses express an HA from a 1968 H3N2 virus isolate, but not a 1997 H3N2 isolate.

Conclusions  The practice of treating sera to inactivate innate inhibitors of influenza viruses prior to evaluation within immune assays has allowed us to effectively detect influenza virus‐specific antibodies for decades. However, this practice has yielded an under‐appreciation for the contribution of innate serum inhibitors toward host immune responses against these viruses, including contributions toward neutralization and macrophage uptake.

The multiplication of influenza viruses in the fertile egg: A Report to the Medical Research Council

Hoyle & Fairbrother (1937) showed that tissues infected with influenza virus contained two distinct particles, the infective virus elementary body, and a smaller particle, the soluble antigen, which could be demonstrated by complement-fixation tests.

INHIBITION BY CERTAIN POLYSACCHARIDES OF HEMAGGLUTINATION AND OF MULTIPLICATION OF INFLUENZA VIRUS

The complex carbohydrates apple pectin, citrus pectin, flaxseed mucilage, blood group A substance, gum acacia, and gum myrrh as well as an extract of RBC, when examined in a pattern test, were shown to inhibit the agglutination of chicken RBC by influenza A virus. A number of other simple and complex carbohydrates showed no inhibitory effect. The hemagglutination-inhibiting action of apple pectin was examined in some detail and evidence was adduced to show that it affected both virus and red cell. Apple pectin was also found to inhibit the multiplication of influenza A virus in embryonated eggs.

THE NATURE OF NON-SPECIFIC INHIBITION OF VIRUS HEMAGGLUTINATION

A study of the component in serum and tissue extracts responsible for non-specific inhibition of hemagglutination with mumps virus and the PR8 and Lee strains of influenza virus has yielded the following results: 1. The inhibitory factor was found in high titer in human serum and in saline extracts of various organs procured at autopsy (lung, liver,kidney, spleen). The inhibition titers of extracts of these organs were usually higher than the serum titers, whereas the titers of muscle extracts were invariably lower. 2. Similar results were obtained with serum and tissue extracts from normal rabbits and guinea pigs. 3. The serum inhibition titers were not affected by heating to 75°C. for 30 minutes, whereas the titers of the tissue extracts were usually reduced by heating at 65°C. or 75°C. for 30 minutes. 4. Saline extracts of human and chicken red blood cells also contained an inhibitory substance in high titer, and these cells showed marked agglutination with influenza and mumps viruses. Rabbit red cells, on the other band, underwent little or no agglutination with these viruses and extracts of these cells failed to cause inhibition. Sheep red cells varied in their capacity to agglutinate and also in their yield of the inhibitory substance. 5. When the virus receptor substance was removed from chicken red cells by adsorption and elution with influenza virus, extracts of the cells no longer yielded the inhibitory factor. 6. The inhibitory substance did not neutralize influenza virus in mice and it failed to fix complement when mixed with influenza or mumps viruses. 7. Evidence was obtained that some virus was released from the inhibitory substance after incubation for 6 hours at 22°C. or 37°C. The implications of these findings are discussed.

The nature of the virus receptors of red cells; the effect of partial heat inactivation of influenza virus on the destruction of red cell receptors and the use of inactivated virus in the measurement of serum inhibitor

Evidence has been offered that influenza virus which has been heated at 56°C. for 30 or more minutes loses some of its capacity to agglutinate red cells and may completely lose its power to elute from cells on which it has been adsorbed. Such heat-inactivated virus does not possess the capacity to destroy the virus inhibitor in normal rabbit serum and this appears to be the explanation of the higher agglutinin inhibitory levels obtained with serum and heated virus as compared with serum and untreated virus. The heat-inactivated virus can be used to measure the inhibitor substance in normal rabbit serum. By two different methods it has been demonstrated that the inhibitor is destroyed in the presence of unheated influenza virus, as measured by inhibition titrations with virus inactivated at 56°C. The destruction of inhibitor by virus of either type A or B can be measured by virus of either type with similar results.

Reactions between influenza virus and a component of allantoic fluid

Evidence is presented which shows that there is a component present in normal allantoic fluid, probably mucoprotein in nature, capable of combining with influenza A virus (PR8), and that following combination between this component and the virus only partial dissociation of the complex occurs. Evidence is also presented which strongly suggests that the component is present in virus-infected allantoic fluid in which it is in part combined with the virus and in part free although altered by viral action. The probability that the component is present as well in highly purified preparations of influenza virus, and its effect upon various reactions obtained with this agent are discussed.

Purification of an influenza virus substrate, and demonstration of its competitive antagonism to apple pectin

A substance was demonstrated in erythrocytes which antagonized the inhibiting effect of apple pectin on influenza virus hemagglutination. This substance was purified and found to be a water-soluble material, rather labile, and with some properties which suggested that it contained a polysaccharide. It was destroyed in vitro by highly purified preparations of the virus. It occurred in greatest amount in human erythrocytes and to a lesser extent in the red cells of species not susceptible to the virus. It was also found in normal rabbit serum. Calcium ions were found to be essential to the action of apple pectin in causing inhibition of virus hemagglutination. A second substance was purified from an alkaline extract of erythrocytes, and shown likewise to have an effect antagonistic to that of pectin. However, this latter material was not destroyed by the virus, and seemed to owe its effect to the binding of calcium ions.

A labile component of normal serum which combines with various viruses; neutralization of infectivity and inhibition of hemagglutination by the component

A labile component present in the serum of human beings, guinea pigs, and rabbits neutralizes the infectivity of mumps, Newcastle disease, influenza A and B viruses. The labile component of these sera and of mouse serum also inhibits hemagglutination of chicken RBC by these viruses. The component is inactivated by heating at 56 degrees C. for 30 minutes and upon storage at 4 degrees C. for periods longer than 2 weeks. The virus-neutralizing and hemagglutination-inhibiting properties result from serum component-virus combination in the presence of calcium. The combination is stable, and does not undergo spontaneous dissociation. Partial separation of virus can be brought about by heating mixtures held for 24 hours or by removal of calcium ions with sodium citrate. The labile serum component appears to be distinct from hemolytic complement.

Inactivation of haemagglutinin and infectivity of influenza and Newcastle disease viruses by heat and by formalin

Progressive heat inactivation of N.D.V. at 54–58° C. for 15 min. is accompanied by parallel destruction of its haemagglutinating activity. Progressive heat inactivation of Lee virus at 50–54° C. for 15 min. is accompanied by parallel loss of eluting power and ‘enzymic’ action on ‘Francis inhibitor’. Formalin in low concentrations rapidly inactivates both N.D.V. and Lee virus without appreciable impairment of their haemagglutinating activity or ‘enzymic’ function. Higher formalin concentrations acting over several days may, however, produce a slow deterioration of the haemagglutinin of both viruses. It is suggested that heat destroys the infectivity of N.D.V. and influenza virus primarily by abolishing their haemagglutinating ‘enzyme’. The possibility of the existence of independent virus components, all of which are essential for full infectivity of these viruses, is discussed.

Agglutination of red blood cells of different animal species by influenza and Newcastle disease viruses

The agglutination of red cells of seventeen animal species by influenza and Newcastle disease viruses has been quantitatively investigated. The peculiar behaviour of one group of red cells, illustrated by ox, toward influenza virus heated for 30 min. at 56° C. and toward purified N.D.V. has been studied in detail. It is concluded that heated influenza virus acquires an apparently increased titre against ox cells because it has lost its eluting power. Similarly, purified N.D.V. is less readily eluted from red cells, due to the elimination of an ‘elution-hastening factor’ present in infected fluid. The present study illustrates the importance of careful adjustment of all possible variable conditions for individual viruses and individual red cell systems.

A mucoprotein derived from human urine which reacts with influenza, mumps, and Newcastle disease viruses

A mucoprotein, present in normal human urine, has been isolated and obtained in a state of a high degree of purity. A number of the biological, chemical, and physicochemical properties of the substance have been studied. From the results obtained in the present investigation and those reported in succeeding papers (34, 35) it appears that the mucoprotein has a high molecular weight, i.e., of the order of 7.0 x 10⁶, consists of thread-like molecules which have axial ratios of approximately 100, and is specifically antigenic. This substance, which appears to be free of contaminating material, possesses in extraordinary degree the capacity to react with influenza, mumps, and Newcastle disease viruses. At equilibrium, with influenza virus, the minimal amount of the substance capable of giving a demonstrable reaction with one hemagglutinating unit of virus appears to be about 0.0003 µg. The mucoprotein is altered by preparations of influenza viruses and its capacity to react with these agents or others is lost. The kinetics of the inactivation process brought about by influenza viruses is in accord with those of well known enzyme-substrate systems. With the exception of the capacity to react with viruses, altered mucoprotein did not differ from the native substance relative to any of the properties examined in the present study. That certain physicochemical properties of the altered mucoprotein are different from those of the native substance is demonstrated in succeeding papers (34, 35).

Studies on host-virus interactions in the chick embryo-influenza virus system. IV. The role of inhibitors of hemagglutination in the evaluation of viral multiplication

The role of inhibitors of hemagglutination in the evaluation of host-virus interactions in the chick embryo-influenza virus system has been analyzed. Comparisons were made between materials (allantoic fluids and membrane suspensions) derived from in vivo (growth curve) experiments at hourly intervals after inoculation, and from in vitro tests in which normal allantoic fluids and membrane suspensions were incubated with virus at 37°C. for various periods of time. In both instances large amounts of virus were added to the systems, resulting in comparable concentrations of the agent. The seeds employed were either fully active or irradiated by ultraviolet light to the extent that the virus lost its capacity to increase but kept its interfering and hemagglutinating properties. The various materials were assayed for (a) the hemagglutinating titers of the virus present in the systems before and after heating to 56°C.; (b) the concentration of inhibitor in the materials at various stages of incubation after heating to 70°C. for 30 minutes as measured by the hemagglutination-inhibition reaction with native or heated test virus (30 minutes 56°C.); and (c) the degree of adsorption of the hemagglutinins present in the materials onto chicken red cells at 0°C. and their subsequent elution at 37°C. The effects of receptor-destroying enzyme (RDE), treatment with sodium periodate, or high speed centrifugation on the inhibitory activities were studied in some of the tests. The essential results which indicate certain sources of error in the evaluation of host-virus interactions as well as means for studying virus activity at the early stages of the infectious process, were as follows: 1. Though some inhibitory effects on hemagglutination were noticeable in the allantoic fluid during the 1st hour after inoculation they were, as a rule, no longer apparent after this interval, and treatment with RDE did not increase the hemagglutinin titers. Thus, the interpretation of growth curve data concerning allantoic fluids hardly seems to be affected by inhibitor. On the other hand, striking effects were noted with the membrane suspensions of growth curve experiments in that RDE shortened the latent period to 2 hours and the titers in the first few positive samples (4 to 5 hours) increased) whereas in later harvests no such effect was noted. Under these conditions complement-fixation antigens and hemagglutinins made their appearance in the tissues simultaneously and not as previously reported the former prior to the latter. However, the infectivity showed increments only several hours after these two activities had become measurable. Thus the hypothesis of the stagewise development of influenza virus is still supported by these data. 2. Using the inhibition of hemagglutination technic it was found that the inhibitor in allantoic fluid rapidly decreased as a result of the action of active and irradiated virus, but destruction was never complete. In the membranes of the in vivo series only active seed led to loss of inhibitor, again without complete destruction, beginning at the time complement-fixing antigen and hemagglutinins became measurable. Irradiated seed was without effect in vivo whereas, in the in vitro tests it equalled the activity of the active virus. The implications of this difference in the effectiveness of active and irradiated seed in vivo with regard to the understanding of the mode of viral multiplication are discussed. 3. Although many factors may influence the shape of adsorption-elution curves it is felt that at 0°C. the extent of adsorption is directly related to the amount of inhibitor present in the systems. In the early hours after inoculation the degree of adsorption was relatively small but it increased gradually with the time of incubation. The inhibitor of adsorption was destroyed by RDE and NaIO₄ and was only partially sedimentable by high speed centrifugation. In every respect studied its properties corresponded with the findings obtained with inhibitors in the hemagglutination-inhibition technic. Although the difference in the rapidity of inhibitor destruction as measured by the various technics might suggest a multiplicity of inhibitors it is felt that it rather denotes a greater sensitivity of the adsorption technic as compared to the others.

Natural and synthetic sialic acid-containing inhibitors of influenza virus receptor binding

Influenza viruses attach to susceptible cells via multivalent interactions of their haemagglutinins with sialyloligosaccharide moieties of cellular glycoconjugates. Soluble macromolecules containing sialic acid from animal sera and mucosal fluids can act as decoy receptors and competitively inhibit virus-mediated haemagglutination and infection. Although a role for these natural inhibitors in the innate anti-influenza immunity is still not clear, studies are in progress on the design of synthetic sialic acid-containing inhibitors of receptor binding which could be used as anti-influenza drugs.

A study in vitro of components in the transmission cycle of swine influenza virus

Swine lungworm extracts and suspensions of swine lungworms contain receptor-like substances capable of adsorbing influenza virus, a result consonant with the hypothesis (5–8) that the lungworm may be involved in the swine influenza cycle. Yet no evidence for multiplication of virus or even persistence of infectious virus in lungworms at undimished titer was found. Clearly much more information is needed, and it is hoped that the present demonstration of the practicality of studying the components of the transmission cycle proposed by Shope, will provide important tools requisite for further investigation of this problem. Studies on the role of the earthworm in the transmission of swine influenza suggest that, at best, that role would be a passive one.

Surfactant protein A, but not surfactant protein D, is an opsonin for influenza A virus phagocytosis by rat alveolar macrophages

Surfactant protein A (SP-A) and surfactant protein D (SP-D) are collectins, and both proteins were shown to interact with influenza A virus and alveolar macrophages. However, it is not known whether SP-A and SP-D can serve as opsonins for the phagocytosis of influenza A virus by alveolar macrophages. In the present study, we investigated the opsonic activities of SP-A and SP-D for phagocytosis of fluorescein isothiocyanate (FITC)-labeled influenza A (H3N2) virus by rat alveolar macrophages using flow cytometry. SP-A enhanced the association of the virus with macrophages in a dose-dependent manner, reaching a maximum at an SP-A concentration of 60 microg/ml. An approximate threefold increase in association of influenza A virus with alveolar macrophages in the presence of SP-A over control incubations which contained no SP-A was observed. Half of the total cell-associated fluorescence could be quenched as demonstrated using the extracellular quenching dye trypan blue. These results indicate that SP-A mediates internalization of FITC-labeled influenza A (H3N2) virus by alveolar macrophages. Removal of the carbohydrate moiety of SP-A by N-glycosidase F treatment or cleavage of its sialic acid residues by neuraminidase abolished the enhancement of the phagocytosis of FITC-labeled influenza A virus by alveolar macrophages. Mannan, a mannose homopolysaccharide known to bind to the carbohydrate-binding domain of SP-A, did not affect the SP-A-mediated phagocytosis of FITC-labeled influenza by alveolar macrophages. In contrast, SP-D neither enhanced the association of FITC-labeled influenza A virus with alveolar macrophages nor affected the opsonic activity of SP-A for FITC-labeled influenza A (H3N2) virus at the SP-D concentrations tested. It is concluded that SP-A acts via its sialic acid residues as an opsonin in the phagocytosis of influenza A virus by alveolar macrophages.

Isolation and characterization of conglutinin as an influenza A virus inhibitor

Normal horse and guinea pig sera contain alpha 2-macroglobulin which inhibits the infectivity and hemagglutinating activity of influenza A viruses of the H2 and H3 subtypes. On the other hand, normal bovine serum contains a component termed beta inhibitor that inhibits the infectivity and hemagglutinating activity of influenza A viruses of the H1 and H3 subtypes. To investigate the nature of the beta inhibitor of influenza A virus, we purified the conglutinin and examined its characteristics. First, we found a high correlation between the hemagglutination inhibition(HI) titer and conglutinin titer in several bovine sera (r = 0.906, p less than 0.005). The HI of bovine serum was mainly dependent on conglutinin because the HI activity was abrogated by N-acetylglucosamine but not by D-mannose. The conglutinin, purified from bovine serum, had neutralizing-activity as well as HI activity on influenza A viruses of the H1 and H3 subtypes. The HI activity of conglutinin was heat stable (56 degrees C, 30 min), Ca(++)-dependent, and resistant to both neuraminidase and periodate treatments. The HI activity of purified conglutinin was blocked by N-acetylglucosamine but not by D-mannose. The conglutinin was bound to hemagglutinin which had high mannose and complex sugar chains and its binding was inhibited by N-acetylglucosamine and dependent on divalent cations. These data indicate that the beta-like inhibitor activity of bovine serum is mainly dependent on conglutinin which inhibits hemagglutination and neutralizes the virus infectivity by its binding to a carbohydrate site at the HA.

Inactivated influenza virus vaccines in man: a comparative study of subunit and split vaccines using two methods for assessment of antibody responses

The serum antibody responses and reactions of volunteers to a trivalent subunit influenza virus vaccine prepared using cetyl trimethyl ammonium bromide (CTAB) or trivalent split vaccine prepared by ether-extraction, were essentially similar, although the antibody levels to the A/Brazil/78 (H1N1) components of the vaccine were greater in volunteers receiving the subunit preparation. Antibody responses to the vaccines were assessed using both the haemagglutination-inhibition (HI) and single radial haemolysis (SRH) tests. Although good correlation was found between the tests with respect to both antibody titres in individual sera and antibody responses in serum pairs to both A(H3N2 and H1N1) and B influenza viruses, the greater reliability of SRH, indicates this test should supplant the HI test for the routine measurement of antibody responses to influenza viruses.

[Influenza]

Influenza is the last great uncontrolled plague of mankind. Pandemics and epidemics occur at regular time intervals. The influenza viruses are divided into the types A, B and C and show unique variability of their surface antigens (hemagglutinin and neuraminidase). Influenza viruses of type A show the largest degree of antigenic variation which, in turn, resulted in the definition of a number of subtypes, each comprising many strains. By comparison, influenza viruses of types B and C exhibit much less variation of their surface antigens. As a consequence, no subtypes but many different strains have been recognized. The degree of antigenic variation correlates with the epidemiologic significance of the virus types, type A being the most and type C the least important. Two different kinds of antigenic variation have been recognized: In the case of minor variation of one or both surface antigens, the term "antigenic drift" is employed. Antigenic drift occurs with all three types of virus, it is caused by point mutations which increase the chance of survival of mutants in the diseased host. In addition, influenza A viruses show sudden and complete changes of their surface antigens in regular time intervals, resulting in the appearance of new subtypes. This event is called "antigenic shift". The mechanisms responsible for antigenic shift are poorly understood, only. In addition to the recycling of preceding subtypes, reassortment resulting from double infection of cells with strains of human and animal origin are considered possible explanations. By use of modern DNA recombinant technology, the base sequences of a series of virus genes and, as a consequence, the amino acid sequence of the corresponding antigens have been determined. By means of monoclonal antibodies, the antigenic structure of many influenza antigens has been further elucidated. It can be expected that further research on the molecular basis of antigenic variation could finally result in an understanding of the causal mechanisms. It is an outstanding feature of the epidemiology of influenza A viruses that a family of related strains prevails for a certain period of time and disappears abruptly as a new subtype emerges.(ABSTRACT TRUNCATED AT 400 WORDS)

The postnatal acquisition of factors which affect the influenza haemagglutination-inhibition test

Levels of maternally transferred antibodies against the surface antigen of the A/Texas/1/77 strain of influenza virus showed the expected decline during infancy when measured by complement fixation (CF). However, this decline was not observed when these antibodies were measured by haemagglutination-inhibition (HI). It has been postulated that this discrepancy is due to the acquisition, in the early days of life, of non-specific serum factors which increase the HI activity of sera. The levels of these factors were determined indirectly by calculating HI:CF ratios and it was shown that the factors are rapidly acquired by children between the fifth and twentieth week of life.

Thermal inactivation of Newcastle disease virus. I. Coupled inactivation rates of hemagglutinating and neuraminidase activities

The thermal stability of Newcastle disease virus has been characterized in terms of the rate constants for inactivation of hemagglutinating activity (HA), neuraminidase activity (NA), and infectivity. Inactivation of HA results in the concomitant loss of NA. Infectivity, however, is much more thermolabile. Disintegration of the virus particle is not responsible for the identical rate constants for inactivation of HA and NA, nor is their parallel inactivation uncoupled in envelope fragments produced by pretreating the virus with phospholipase-C. The data indicate that a common envelope factor(s) can influence the thermal stability of both activities.

Virus-erythrocyte interactions

This chapter summarizes the experimental evidence bearing on the nature of virus-erythrocyte reactions characteristic of several taxonomic groups.. Such evidence is culled from (1) the study of conditions necessary for hemagglutination; (2) the examination of specific factors affecting either the cell or the virion to enhance, alter, or abolish the reaction; and (3) the direct physicochemical analysis of cells, viruses, and “receptor analogs.” The hemadsorption phenomenon also provides evidence for virus-erythrocyte interactions, which is based on the attachment of erythrocytes to infected cells in culture having hemagglutinin at their surfaces. This phenomenon reflects the interaction between erythrocytes and viral envelope components. The major virus groups that react with erythrocytes include myxoviruses, paramyxoviruses, pseudomyxoviruses, adenoviruses, arboviruses, reoviruses, enteroviruses, and miscellaneous hemagglutinating viruse (rubella virus, coronaviruses, rhabdoviruses, and oncogenic viruses). The agglutination of erythrocytes by the direct action of viral particles was first described in connection with myxoviruses. This led directly to the discovery of viral neuraminidase—a property unique to myxoviruses and paramyxoviruses. A number of viruses unrelated to myxoviruses have since been shown to agglutinate erythrocytes of various species. The visible result of viral hemagglutination is the “pattern” formed at the bottom of a test tube or well plate by lattices of red cells lightly conjoined by viral hemagglutinin. Hemagglutination serves as a useful direct means of titering intact viral particles or hemagglutinating subunits.

Properties of the Hong Kong influenza virus. I. General characteristics of the Hong Kong virus

Biological properties of the Hong Kong influenza variants are described in relation to the isolation and identification of strains, sensitivity to nonspecific inhibitors, and serological diagnosis. Hong Kong virus strains were readily isolated in both eggs and monkey-kidney tissue cultures and were identified by haemagglutination-inhibition (HI) with chicken erythrocytes. The morphology, soluble antigen, and neuraminidase of Hong Kong isolates were similar to those of earlier A2 Asian influenza strains. Hong Kong haemagglutinins were related in varying degrees to previous human A2 and animal influenza viruses.The sensitivity of Hong Kong isolates to nonspecific haemagglutination inhibitors in serum varied widely. From least to most inhibitory, the ranking of sera of 8 animal species tested was: monkey, goat, chicken, human, rabbit, ferret, guinea-pig and horse. The same sera were treated with heat, trypsin, periodate, receptor-destroying enzyme and kaolin to determine the most effective way of removing nonspecific inhibitors. The results varied with the animal species involved.The A2 Asian antigen was nearly as effective as Hong Kong antigen in detecting antibody rises by HI tests. HI tests with either antigen were more efficient than complement-fixation tests for serodiagnosis of Hong Kong influenza, but for maximum efficiency both tests were required.