Persistent antigenic variation of influenza A viruses after incomplete neutralization in ovo with heterologous immune serum

Antigenic relationships between the serotypes of Pasteurella haemolytica demonstrable by enzyme-linked immunosorbent assay (ELISA)

Mice and rabbits were immunised with sodium salicylate extracts (SSE) prepared from each of 12 serotypes of Pasteurella haemolytica, and the antisera to each were used in cross-indirect haemagglutination (IHA) tests and cross-enzyme-linked immunosorbent assays (ELISA) to study antigenic relationships between the serotypes. An indirect micro-ELISA demonstrated common antigenic relationships which were not apparent by IHA. Antisera from both species revealed considerable shared antigenicity between all the serotypes. Rabbit antisera presented clearer differences between the A biotypes on one hand and the T biotypes on the other, the T biotypes exhibiting much less cross-relatedness than that shown between the A serotypes.

[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)

Antigenic properties of some equine influenza viruses

The antigenic relationships between the haemagglutinins of five A/equine-1 viruses and between six A/equine-2 viruses were examined using post-infection ferret and immunized pony sera. Similar results were obtained with sera from both species for the A/equine-1 viruses and these confirmed minor antigenic differences between the prototype A/Prague 1/56 virus and viruses isolated in England in 1973 and 1977. Considerable antigenic differences were found between five of the A/equine-2 viruses, using ferret sera, but these differences were less evident using pony sera. The response of ponies to the inactivated viruses indicated that there was little need to change the virus strains used in the manufacture of commercial equine influenza vaccines.

Differentiation of foot-and-mouth disease virus strains using a competition enzyme-linked immunosorbent assay

Foot-and-mouth disease virus isolates were compared using solid-phase competition and indirect microenzyme-linked immunosorbent assays. Results were compared to those obtained from complement fixation tests. Similar relationships between the isolates were obtained using the indirect enzyme immunoassay and complement fixation tests. The competition assay was more discriminatory and the results did not always correlate with the other two assays.

Australian infectious bronchitis viruses: identification of nine subtypes by a neutralisation test

The antigenic relationships between 17 Australian infectious bronchitis viruses, including six vaccine viruses, were studied by a neutralisation test using a plaque reduction method in chick embryo kidney cell monolayers. The 17 viruses formed nine distinct subtypes. Antiserum to each subtype had a high titre to viruses of the same subtype and a lower titre to viruses of different subtypes. The heterologous titres of sera varied widely.

Identification of a strain of infectious bursal disease virus isolated from mosquitoes

The Becht strain of infectious bursal disease virus was compared with a virus isolated from Aedes vexans mosquitoes and designed 743 virus. The viruses were compared with respect to cell culture host range, cellular changes resulting from viral infections, growth curves, antigenic relationship, and physicochemical characteristics. The viruses are closely comparable in all these properties, and they are considered to be strains of the same virus. In cross comparisons by the enzyme-linked immunosorbent assay, 743 virus and infectious bursal disease virus were found to be antigenically identical, confirming the results of the neutralization test. The 743 virus differs from most strains of infectious bursal disease virus in that it is nonpathogenic for chickens.

In vitro identification and characterization of a virus isolated from a dog with neurological dysfunction

A virus, 78-238, isolated from the cerebrospinal fluid of a dog with neurological dysfunction, was characterized as a paramyxovirus. This conclusion was supported by viral cytopathic effects and morphological appearance of virions and nucleocapsids in infected cells. Nucleocapsids were found in the cytoplasm of all infected cells and in the nuclei of 0.001% of these cells. Growth curves revealed that a high percentage (>/=76%) of infectious progeny virus was cell released. Persistent infection of Vero cells with 78-238 showed a consistently high percentage of fluorescence-positive cells and a low proportion of hemadsorption-positive cells. Serological studies indicate that the virus was closely related to Simian virus 5 and reference canine parainfluenza virus.

The role of season in the epidemiology of influenza

Four types of observations have been used to illustrate the seasonal characteristics of epidemic influenza: (1) The experience of a small population during 28 consecutive years, 1946-74, (2) world influenza outbreaks 1964-75 reported to the World Health Organization, (3) the experience of two widely separated localities at about the same latitude, 1969-74, and (4) the experience of two places at latitudes 30 degrees + on opposite sides of the Equator, 1968-74. The following tendencies are shown. (1) Outbreaks of influenza even in the small community came at approximately the same season almost every year. (2) Outbreaks are globally ubiquitous and epidemic loci move smoothly to and fro across the surface of the earth almost every year in a sinuous curve that runs parallel with the 'midsummer' curve of vertical solar radiation, but lags about six months behind it. Such findings exclude the mediation of seasonal control by any agencies of local distribution, and suggest a direct effect of variations in some component of solar radiation on virus or human host. (3) Antigenic variations in influenza A virus tended to have the same seasonal characteristics as epidemicity. This suggests that epidemicity and virus variation are two facets of one seasonally controlled process. None of these seasonal characteristics can be explained by the current concept of influenzal epidemiology. A new hypothesis recently proposed and recapitulated in the Appendix offers a possible explanation. The primary agency mediating seasonal control remains unidentified.

Australian infectious bronchitis viruses: identification of nine subtypes by a neutralisation test

The antigenic relationships between 17 Australian infectious bronchitis viruses, including six vaccine viruses, were studied by a neutralisation test using a plaque reduction method in chick embryo kidney cell monolayers. The 17 viruses formed nine distinct subtypes. Antiserum to each subtype had a high titre to viruses of the same subtype and a lower titre to viruses of different subtypes. The heterologous titres of sera varied widely.

The mechanism of antigenic drift in influenza viruses: analysis of Hong Kong (H3N2) variants with monoclonal antibodies to the hemagglutinin molecule

Monoclonal antibodies to the hemagglutinin molecule of the Hong Kong variant, Mem/1/71 (H3N2), influenza virus were used to study antigenic drift in the H3N2 subtype of influenza viruses. Antigenic variants of Mem/1/71 (H3N2) were selected after a single passage of the virus in chick embryos in the presence of monoclonal antibody. The variants showed a marked reduction in the ability to react with the monoclonal antibody used in selection. The monoclonal antibodies could be divided into three groups based on their reactions with the variants, providing evidence for at least three nonoverlapping antigenic areas on the hemagglutinin molecule. Amino acid analysis of tryptic peptides of the hemagglutinin from these variants showed that a single amino acid substitution in the heavy polypeptide chain (HA1) of the hemagglutinin molecule accounted for the reduced antibody interactions, and that variants from each group exhibited sequence changes in different areas of the molecule. Sequence changes were also detected in the HA1 polypeptides of naturally occurring H3N2 variants, but in most cases the changes in the monoclonal antibody selected variants were different from the field strains. Antigenic analysis showed that most of the variants selected with monoclonal antibody could not be distinguished from parental viruses with heterogeneous sera, suggesting that they are probably epidemiologically irrelevant. One variant, however, could be distinguished from parental virus with heterogeneous sera. This variant showed a change in sequence at residue 144 of the HA1 polypeptide from glycine in the parent to aspartic acid in the variant. Similar substitutions have been found in naturally occurring variants at this position. These studies suggest that some amino acid substitutions are more important than other for producing viruses with epidemiological potential. Antigenic analysis of naturally occurring H3N2 strains with monoclonal antibodies established that two variants co-circulated in 1968; Hong Kong/1/68 being distinguishable from Aichi/2/68 in at least two antigenic areas. It would appear that there may have been two separate lineages of H3N2 viruses, Hong Kong/1/68 giving rise to variants in England and Aichi/2/68 to variants in USA and Australia.

Fourteen serotypes of ureaplasma urealyticum (T-mycoplasmas) demonstrated by the complement-dependent mycoplasmacidal test

Previously, we found that strains of Ureaplasma urealyticum could be divided into 11 serotypes using the mycoplasmacidal (MC) method for serotyping. Subsequent study has shown that two pairs of these strains were sufficiently closely related to cause us to revise the typing scheme into nine distinct serotypes. Other investigators, using different methods to detect serotypic diversity, have found eight serotypes. We have now compared our nine strains with the eight prototype strains developed by Ford and Black, and we have used reciprocal MC titers of antisera for the comparison. Three pairs of strains from our group and the group prepared by Ford and Black were found to be serologically closely related or identical, leaving six serotypes in our series and five in the other series that were serologically unique. Thus 14 serotypes of U. urealyticum have been identified in these studies, and a 15th strain that is apparently serologically distinct from the others, has recently been described in Vancouver.

Comparisons of some bluetongue virus isolates by plaque neutralization and relatedness tests

Seven North American bluetongue virus isolates, cloned by plaquing, and sera to five of them were reacted in a plaque neutralization test. Using a paired controls system, each virus-serum reaction was studied in terms of the regression of percent neutralization on log serum dilution. Antigen-antibody interaction terms in the analysis of effective dose estimates were used to assess the relatedness of the virus isolates. The degree of cross reactivity formed a spectrum from virtually no evidence of unrelatedness to clear antigenic differences.

Taxonomic studies on strains of avian infectious bronchitis virus using neutralisation tests in tracheal organ cultures

The antigenic relationships of 24 strains of avian infectious bronchitis virus (IBV) were investigated by serum neutralisation tests performed in chick embryo tracheal organ cultures. The serum dilution that neutralised 100 median ciliostatic doses (CD50) of virus was estimated from the linear relationship between varying concentrations of each virus strain and the neutralisation titre of homologous antiserum; this dilution defined 1 antibody unit. Antisera diluted to contain 20 antibody units were then tested by neutralisation against 1.5--2.5 log10 CD50 of each strain. Clusters of both strains and antisera in turn were established by methods of numerical taxonomy using as measures of resemblance Euclidean distance and correlation coefficient, and by analysis by principal components. These analyses identified a group of 8 similar strains; neutralisation of the remaining 16 strains was slight. Similar results were obtained by classifying antisera, except that a further group of 3 antisera was demonstrated, each having a neutralising capacity for most strains. Implications for vaccine formulation are discussed.

Epidemic mechanisms of type A influenza

The antigenic varieties of influenza A virus isolated from 1968 to 1976 in a surveillance of a small, rather remote population were similar to those from England and Wales as a whole, despite frequent antigenic changes during the period. Household studies in the first two H3N2 influenza A epidemics found low attack rates within households, a high proportion (70%) of affected households with only one case of influenza, similar distributions of affected households in the two epidemics by the number of cases of influenza and similar distributions of the influenza cases by the day of their onset in the household outbreak. No serial interval could be demonstrated by cumulating household outbreaks. More than one minor variant was causing influenza contemporaneously in the same villages in several seasons, and different variants were on one occasion found on successive days in bedfellows. The regular occurrence of epidemics in winter was often accompanied by the disappearance of the epidemic variants and their replacement, after a virus-free interval, by new variants. These epidemiological findings seem best interpreted on the following tentative hypothesis. Influenza A sufferers do not transmit the virus during their illness; instead it rapidly becomes latent in their tissues so that they become symptomless carrier-hosts and develop specific immunity. Next season an extraneous seasonally mediated stimulus reactivates the latent virus residues so that the carrier-host becomes briefly infectious, though symptomless. Antigenic drift occurs because particles reconstituted to be identical with the progenitor virus cannot escape the specific immunity it has provoked in the carrier host. He can shed only mutants also determined by the progenitor virus. From the assortment of mutants shed by the carrier-host, his non-immune companions select that (those) which is best fitted to survive, and it rapidly causes influenzal illness. Epidemics consist largely or entirely of such persons sick with influenza caused by reactivated virus caught from symptomless carrier-hosts.

Antigenic drift in type A influenza virus: peptide mapping and antigenic analysis of A/PR/8/34 (HON1) variants selected with monoclonal antibodies

Variants of A/PR/8/34 (HON1) influenza virus, having hemagglutinin molecules with probably a single altered antigenic determinant, were isolated by growing the virus in the presence of the monoclonal hybridoma antibody PEG-1. The variants were analyzed by peptide mapping and characterized antigenically by using PEG-1 and four other monoclonal hybridoma antibodies to PR8 hemagglutinin. Peptide maps of the large hemagglutinin polypeptide, HA1, from 8 out of 10 variants showed a single changed peptide. This peptide from two of the variants was analyzed, and in each case a serine residue in the wild-type hemagglutinin was replaced by leucine in the variant. Although these eight variants showed identical peptide maps, one could be discriminated antigenically from the others with one of the hybridomas. (The peptide maps represented about one-third of the HA1 molecule.) Of the other two variants, one gave the same HA1 map as the wild type, but could be distinguished antigenically from wild-type virus by two of the hybridomas. The other was unique, and could be distinguished, both antigenically and by peptide mapping, from the other variants. Since a large number of the variants selected with PEG-1 showed the same peptide change, it is likely that this alteration in amino acid sequence (serine to leucine) was responsible for the inability of the variants to bind PEG-1 monoclonal antibody. We do not know, however, whether the changed amino acids were located within the antigenic sites or whether the change occurred somewhere else in the hemagglutinin molecule and altered the determinants through conformational changes.

Antigenic drift in influenza A viruses. I. Selection and characterization of antigenic variants of A/PR/8/34 (HON1) influenza virus with monoclonal antibodies

Antigenic variants of A/PR/8/34 [HON1] influenza virus were selected after a single passage of the parent virus in embryonated chicken eggs in the presence of monoclonal antibodies to this virus. The monoclonal antibodies were produced by a hybridoma and were specific for an antigenic determinant on the HA molecule of the parent virus. Seven antigenic variants were analyzed with 95 monoclonal anti-HA antibodies prepared in vitro in the splenic fragment culture system. Three subgroups of antigenic variants were distinguished. The antigenic changes were primarily recognized by monoclonal antibodies to the strain- specific determinants of the parental hemagglutinin (HA) molecule. Monoclonal antibodies to HA determinants shared (in an identical or cross-reactive form) by parental virus and more than three heterologous viruses of the HON1 and H1N1 subtypes were unable to recognize the antigenic change on the variants. Similarly, heterogeneous antibody preparations could not differentiate between parental and variant viruses. The results are compatible with the idea that the HA of PR8 has available a large repertoire of antigenic modifications that may result from single amino acid substitutions, and that antigenic changes can occur in the strain- specific determinants on the HA molecule in the absence of concomitant changes in the cross-reactive HA determinants. The findings suggest that antigenic drift, in order to be epidemiologically significant, probably requires a series of amino acid substitutions in, or close to, the antigenic area on the HA molecule.

Elimination of inter-species reactive anti-IgG antibodies by affinity chromatography

Inter-species reactive anti-IgG antibodies are disturbing in blocking and interference experiments with the sensitive mixed hemadsorption (MH) technique. By affinity chromatography this disturbance could be efficiently eliminated. Anti-IgG sera were passed through columns containing heterologous IgG. The absorption technique is rapid and simple. After one absorption the inter-species reactivity in anti-IgG sera had been decreased thousand-fold, while any loss of species-specific reactivity could not be detected. Such absorbed anti-IgG sera reacted only with homologous antibodies. The effect is exemplified with e.g. blocking of human HLA antibodies by anti-beta2-microglobulin from rabbits.

Description of a technique for the analysis of antineuraminidase antibodies oriented to H2N2 and H3N2 influenza virus strains

A method for the analysis of mixtures of antineuraminidase (AN) antibodies oriented to antigenically different neuraminidases of H2N2 and H3N2 influenza virus strains is described. The method is based on the finding that the different classes of AN antibodies give different titer ratios when reacting with the recombinants A/Bel/42 (H0)-A/Sing/1/57 (N2) and A/equine/Prague/1/56 (Heg1)-A/Hong Kong/1/68 (N2) and a mixture of both recombinants. These titer ratios are determined under the experimental conditions of the photometric ACU (antibody concentration unit) method for a series of reference sera and test sera. By mathematical treatment of these data an exact analysis of AN antibodies of test sera is achieved. This paper gives a detailed description of the conduct of the test and summarizes the results of experiments to designed to assess the accuracy of analysis.

Antigenic variation of poliovirus caused by antibody components with different specificities

The possible role of antibody as a selective pressure on antigenic mutants of poliovirus in nature was investigated in vitro. A mutant resistant to a monospecific antibody with a definite specificity was readily obtained by several cycles of neutralization of Mahoney strain with a monospecific antibody and multiplication in monkey kidney (MS) cells. Mutants resistant to more than two different monospecific antibodies were also readily obtained in a similar manner. Studies on the antigenicity of these mutants by kinetic neutralization tests revealed that the Mahoney strain underwent a progressive serological variation as it became successively resistant to one to five different monospecific antibodies isolated from anti-Mahoney serum.

Comparative serological studies with mammalian adenoviruses

Nine serotypes of bovine adenovirus, five serotypes of ovine adenovirus, and four serotypes of porcine adenovirus were compared in reciprocal cross-neutralisation tests, to determine if viruses isolated from different species were indeed distinct serotypes. In addition, the above serotypes were tested for possible antigenic relationships with 28 of 32 human adenoviruses, by one-way cross neutralisation tests with human adenovirus antisera. The results indicated that all viruses tested were distinct serotypes. Ovine adenovirus types 4 and 5, until now not compared by neutralisation tests, were confirmed as separate serotypes.

Attempts to serologically classify feline caliciviruses on a national and an international basis

A serological classification of five feline calicivirus (FCV) strains of Swiss origin and 13 isolates of Austrian origin was attempted. The antisera used had been prepared in rabbits against the five Swiss strains and in goats against six American strains. Homologous and heterologous neutralization tests were made in tube cultures using sera at dilutions that contained 20 antibody units, in unabsorbed sera at low dilution, and in sera after subjection to three consecutive absorptions with lyophilized feline liver powder. None of these procedures resulted in the delineation of serotypes. A study using 20 antibody units yeilded FCV strains with broad-spectrum antigenicity, which seem promising for vaccination trials and for covering global needs. In addition, readily neutralized strains suitable for epidemiological work on cat sera, and possibly also for measuring humoral response to vaccines, could be recognized. The extent of serum titer variance, after 10-fold variance of virus input, was established as being only two- to threefold. Five Swiss FCV strains have been shown to remain antigenically stable over 10 years of laboratory passage. FCV strains and the procedure using 20 antibody units offered a suitable approach for international comparative work. FCV strains with little cross-neutralization should be subjected to higher antibody concentrations of existing antisera before attempting to create serotypes. Liver powder absorptions, which reduced homologous and heterologous neutralization titers to similar extents, subsequently proved to be unsuitable for use in FCV classification.

Serological relatedness of herpes simplex viruses. Type-specificity of antibody response

The serological relatedness of forty-seven strains of type 1 and type 2 herpes simplex virus was investigated by reciprocal and non-reciprocal neutralization kinetics. Early rabbit antisera divided the virus strains into two distinct groups where confident indentification of virus type was possible. Hyperimmune mouse and rabbit antisera did not divide the two virus types into two distinct non-over-lapping groups. The extent of overlap varied with the particular attribute of the virus being studied. The virus types were best discriminated by their neutralizability by type 1 antisera and least well by their neutralizability by type 2 antisera. The results of reciprocal kinetic neutralization test with hyperimmune mouse antisera were analysed by multi-dimensional cluster analysis. Hyperimmune mouse or rabbit antisera could not be discriminated with respect to their immunogenic type by their absolute neutralization rate constants against either type 1 or type 2 virus, but could be distinguished on a group basis by their relative neutralizability against both virus types (antiserum specificity attribute); however, using this latter criterion, the type of immunogen could only be predicted in seven of the forty antisera under test. 'Early' mouse antisera could also be distinguished as groups by their absolute k-values against type 1 herpes virus. Thus, immunogenic identification, on other than a group basis, was unreliable. The specificity of a given serum was inversely related to its titre. There was a positive correlation between the specificity of a given virus strain and of its corresponding antiserum.

Antigenic variants of influenza A virus obtained in vitro

The aim of this study was to investigate antigenic "drift" in the haemagglutinin and neuraminidase antigens of influenza A virus in vitro under immunological pressure. Variants of the "Asian" influenza strains A/England/12/64 (H2N2) and A/Tokyo/3/67 (H2N2) were isolated in the allantois-on-shell system in the presence of homologous postinfection ferret sera. For each of these two viruses three generations of variants were isolated and characterized. It was found that the successive antigenic variants of A/Eng/12/64 did not resemble A/Tokyo/3/67. Thus it is probable that the pathway of antigenic drift in vitro was not the same as that which occurred in nature during the evolution of A/Tokyo/3/67 from A/Eng/12/64. In addition, A/Tokyo/3/67, which was the last strain to be prevalent before the A/Hong Kong subtype appeared, underwent significant antigenic drift from "junior" to "senior" variants. This finding did not support the concept that, when antigenic drift occurs, resulting in the appearence of viruses with new haemagglutinin antigen subtypes, the previously prevalent strain has no capacity for further antigenic drift. The study did not result in the production of strains that were identifiable as "bridging" viruses between the H2 and H3 haemagglutinin subtypes. The present paper includes the first report of antigenic variation in the neuraminidase antigens of influenza A viruses occurring in vitro under immunological pressure.

Variation among strains of type A foot-and-mouth disease virus in the Eastern Mediterranean region 1964-1972

Variants of type A FMD virus from the Eastern Mediterranean region over the years 1964-72 have been shown to belong to a group distinct from the Western European strains as represented by A5 Westerwald. This group appears to derive from the A22 strain first recognized in 1964 and indicates the possibility of new strains supplanting old in the field.

Serological relationships among feline caliciviruses

A total of 46 strains of feline calicivirus isolates from the United Kingdom, United States, Australia, and New Zealand were used in an investigation of their serological relationships based on the serum neutralization test. Although demonstrable antigenic variation exists between these isolates, it is shown that significant in vitro cross-activity exists between all these isolates to greater or lesser extent. All isolates tested may be regarded as serological variants of a single serotype of feline calicivirus. It is postulated that this relationship would provide for considerable cross-protection during successive exposures of cats to various feline caliciviruses.

Antigenic relationships among Newcastle disease virus mutants obtained from laboratory strains and from recent California isolates

The antigenic relationship between pairs of plaque mutants of Newcastle disease virus (NDV) derived from laboratory strains and from isolates from the 1971-72 California epizootic were examined by kinetic neutralization test. Comparing four sets of mutants from laboratory strains by both kinetic neutralization and hemagglutination inhibition tests, a similarity was found in the antigenic relationship expressed as an r value with both tests. However, kinetic neutralization was the more precise as well as sensitive assay. Antigenic diversity was greatest between pairs of mutants from different strains, but distinctions could also be made between mutants from the same strains such as Herts-L and Herts-S with an r value of 36%. Examination of mutants from the California epizootic isolated from separate locations and at different times showed antigenic divergence which was greatest between two red-plaque mutants with an r value of 39%. Antigenic distinctions were found between a red- and clear-plaque mutant obtained from isolates taken from brain and tracheal swabs of one infected chicken. In addition to antigenic divergence found between pairs of some mutants, two of the clear-plaque mutants reacted more avidly with antibody than did the corresponding red-plaque partner. Thus, both differences in antigenicity and avidity can be found among these NDV mutants. The antigenic variation found among these mutants is similar to that found within a serotype. This would imply that at the present NDV is a single serological type.

Antigenic analysis of prototype influenza A (H3N2) strains by the antiserum absorption method

Prototype strains of the influenza A (H3N2) virus can be arranged on a gradient showing the degree of the antigenic drift which the haemagglutinins of the strains have undergone. The demonstration of fine antigenic differences is based on an antiserum absorption test which allows a detailed antigenic analysis of strains. The gradient provides information on variation in strains occurring in different geographical areas and its use may be helpful in differentiating between introduced strains and locally developing variants.

Interferon sensitivity of Venezuelan equine encephalomyelitis virus

Two strains of Venezuelan equine encephalomyelitis virus, which differ in virulence for mice, have been studied for their production of and sensitivity to chick and mouse interferon. Little interferon was produced by chick cells in response to the virulent Trinidad strain or the attenuated TC-83 strain without either aging or priming the cultures. Consistent differences in the production of chick interferon were not found between the two strains. Plaque variants of the Trinidad strain produced higher titers of mouse interferon than the TC-83 strain in both primed and control L-cell cultures. The TC-83 strain was found to be more sensitive than the Trinidad strain to the inhibitory effects of interferon. The greater sensitivity of the TC-83 strain was observed at both high and low multiplicities and for both chick and mouse interferons. These results are consistent with the hypothesis that interferon sensitivity may have a role as a determinant of virulence in some virus-host systems.

Comparative complement-fixation studies with subtype strains of foot-and-mouth disease virus

A comparison was made between a macrotechnique in tubes and a microtechnique in plastic plates for complement-fixation tests, using strains of three subtypes of the Asia 1 type of foot-and-mouth disease (FMD) virus. The results obtained with these techniques were found to be comparable and delineated the antigenic relationships of the three strains employed. The microtechnique was considered to be both economical with reagents and capable of similar accuracy and reproducibility to that of the standard method in tubes. It was concluded that the antigenic analysis of subtype strains of FMD virus can be conveniently carried out by the use of the microtechnique as described.