A pattern of influenza virus variation

INDICATIONS OF HEREDITARY, SPATIAL REARRANGEMENT OF ANTIGEN COMPLEXES, IN THE INFLUENZA VIRUS

During passage in mice which had been vaccinated with the homologous, and with closely related strains of influenza virus, the passage strain developed a lessened susceptibility to the deleterious effects of the "immune" environment, concommittant with which was a developed capacity to evoke antibodies which reacted with earlier strains of virus—a capacity which was inapparent in the parent strain. However, the parent strain exhibited a relatively broad range of surface reactivity which was not apparent in the derived strain. The data are interpreted to mean that the hereditary change resulted from spatial rearrangement and quantitative redistribution of antigens in the virus particle (in which the surface is viewed as being distinct from the inner bulk), and are viewed as enhancing the idea that influenza virus variation (i.e., "mutation") may result from a rearrangement of existing hereditary elements.

Antigenic variants of influenza A virus, PR8 strain. I. Their development during serial passage in the lungs of partially immune mice

Antigenically different strains of mouse-adapted PR8 influenza A virus have been produced by 17 serial passages of the virus in the lungs of mice immunized with the homologous agent. Comparative serological tests show that the variant strains share antigenic components with the parent strain but the dominant antigen is different. By means of antibody absorption it was shown that the "new" antigenic component of the variant was already present in minor amounts up to the eighth passage and thereafter gained prominence with continued passage in vaccinated mice. Groups of mice vaccinated with either the PR8-S or T(21) virus and having comparable antibody titers showed no growth of virus in the lungs following aid-borne challenge with homologous strains. On the other hand, following heterologous air-borne challenge no deaths occurred, but virus grew in the lungs of both groups of vaccinated mice. Almost unrestricted virus multiplication took place in the lungs of mice vaccinated with the parent strain and challenged with the PR8-T(21) virus which resulted in extensive consolidation. Less virus grew in the lungs of the mice vaccinated with the variant strains and challenged with the PR8-S virus. In these animals only microscopic evidence of changes due to virus growth in the lungs was observed. The successful serial passage of PR8 influenza A virus in immunized animals was dependent on the initial selection of mice with uniformly low H.I. antibody titers as determined on tail blood, and the intranasal instillation of sufficient virus to favor the survival of those virus particles least related to the antibodies present. The epidemiological implications of these observations are discussed briefly.

Non-specific inhibitors of influenza viruses in normal sera

The presence of non-specific inhibitors in immune influenza sera may falsify the antibody pattern as shown by the haemagglutination-inhibition test, and it is consequently often necessary to pre-treat sera in order to inactivate these inhibitors. A number of different methods are in use for this purpose. It was therefore thought useful to compare the efficacy of the various methods with different representative influenza strains and normal sera from eight animal species and acute-phase human sera. Tests were carried out with simple heating at 56 degrees C, the use of receptor-destroying enzyme, the use of potassium periodate, and two methods involving the use of trypsin. No one technique was found to be universally applicable for all types of serum against all strains, and further testing with larger numbers of sera is advocated. It is felt that a combination of potassium periodate and trypsin with a change in concentration and times of action may be found advantageous.

Propagation of influenza virus in immune environments

INFLUENZA VIRUS CAN SURVIVE, AND CAN BE PROPAGATED IN IMMUNOLOGICAL ENVIRONMENTS INDUCED IN MICE BY VACCINATION WITH THE HOMOLOGOUS STRAIN OF VIRUS: survival was associated with the emergence of variants which differed from the parent strain in antigenic characteristics. The data concerning hemagglutinating activity of the variants, on the one hand, and of the antigenicity, on the other, are compatible with the concept that the structure of the influenza virus includes a surface arrangement which is distinct from the inner virus bulk. The points (a) that propagation was accomplished with difficulty whenever the immunological environment was altered, and (b) that once established, passage was continued without difficulty, are interpreted to indicate that the mechanism of variation may involve a rearrangement of the basic hereditary mechanism.

[The viruses of the 1952-3 influenza epidemic]

The results of antigenic studies of the viruses received at the World Influenza Centre from the 1952-3 influenza epidemic are described. As in 1950-1, two main antigenic groups of influenza A viruses could be distinguished. One group, called Liverpool, contained strains closely related to 1950-1 Liverpool strains. The other, called Scandinavian, contained strains related to but not identical with 1950-1 Scandinavian viruses. Altogether, 175 Scandinavian, 33 Liverpool, and 4 influenza B virus strains were examined.A peculiar relationship between 1950-1 and 1952-3 Scandinavian viruses was investigated by antibody absorption experiments. It was concluded that A/Sweden/3/50 and A/Missouri/303/52 viruses had a rather similar antigenic composition, but that the dominant antigen in the former strain was contained within the virus particle whereas it was a surface constituent of the latter strain. The possible significance of this finding in the antigenic evolution of influenza viruses is discussed.The findings suggest that survival of influenza viruses without much antigenic alteration from one epidemic to the next, two years later, is a demonstrable but uncommon finding. Evidence is presented that this survival was followed by separate spread of each antigenic variety of virus in different countries.

The antigenic composition of influenza virus measured by antibody-absorption

The application of methods herein described for the absorption of antibody from sera by influenza virus adsorbed to erythrocytes has greatly facilitated absorption studies with this virus and has resulted in a more comprehensive demonstration of strain differences and relationships. It is now evident that there are at least 18 different antigenic components which can be measured in Type A strains of influenza virus. Utilization of quantitative absorption technics allows the simultaneous detection of variable quantities of several different antigens so that the antigenic composite of the strain is more clearly defined than was previously possible. The wide sharing of antigenic components leads to the conclusion that no completely new antigens have formed of late nor have the antigenic components of strains isolated several years ago disappeared. Some mechanisms for antigenic variation among strains are suggested, and the significance of the variation in antigenic components is discussed in relation to specifications for an effective vaccine.

Epidemiologic and immunologic significance of age distribution of antibody to antigenic variants of influenza virus

The effects on the antibody content of the population which result from repeated exposure to antigenic variants of influenza viruses have been studied by measuring, with many strains, the antibody content of lots of gamma globulin prepared in different years and the patterns of antibody found in sera collected in 1 year from various age groups. In all samples of gamma globulin collected from 1943 through 1951, high levels of antibody were found with strains of Type A and Type B influenza viruses isolated prior to 1941. The highest levels were found in the more recent collections of gamma globulin. Antibodies to A-prime, and to B strains of 1945 and 1952, were present at low levels in gamma globulin collected prior to the isolation of these viruses. A moderate increase in antibody was observed in the gamma globulin of recent years. The pattern of distribution of antibody by age found with most A-prime strains in serum pools exhibited high levels in infancy and childhood, but after the age of 20, little or no antibody was detected. With Type A strains antibody was usually not observed until the 11th year of age. Thereafter, high levels were present until age 20, when the amount of antibody declines to a moderate and relatively constant level which persists throughout life. Antibody against swine influenza virus did not become detectable until the 29th year. The intermediate antigenic character of a few A-prime isolates was reflected in the antibody pattern obtained with them. Antibody was not found until age 13 with the Lee (1940) strain of Type B influenza virus, but thereafter the level was high. With the type B isolates of 1945 and 1952, antibody became measurable at earlier ages. The present data clearly demonstrate that in the early years of life the range of the antibody spectrum is narrow, and that it becomes progressively broader in later life. A striking correlation was found between what is known of the periods of prevalence of certain strains of influenza viruses and the age of the people in whom strain-specific antibodies are currently found. It has been observed that the age at which antibodies to certain strains are first detectable has progressively advanced with the passage of time. From these data the following immunologic thesis is formulated. The antibody which is acquired during the initial infections of childhood is of limited scope and reflects the dominant antigens of the prevailing strains. The immunity conferred by the initial experiences with influenza is also limited. Successive experiences later in life with viruses of related but differing antigenic make-up result in a composite of antibody which is oriented toward a larger number of the common antigens which comprise influenza virus. These experiences confer a broader immunity which limits infection with, and antibody response to, the more recently encountered strains. The antibody-forming mechanisms appear to be oriented by the initial infections of childhood so that exposures later in life to antigenically related strains result in a progressive reinforcement of the primary antibody. The highest cumulative antibody levels detectable in a particular age group tend, therefore, to reflect the dominant antigens of the virus responsible for the childhood infections of that group. Hence the pattern of antibody distribution determined currently in different age groups provides a serologic recapitulation of past infection with antigenic variants of influenza viruses.

Epidemiology of influenza

The epidemiological pattern of influenza over the past century is outlined; its vagaries are ascribed to the plasticity of the influenza virus. In recent years it has been possible to carry out antigenic studies revealing variations in the strains of virus A, the agent in most major outbreaks. New antigenic variants may have a greater chance to emerge after periods of latency, since the exposed population will have less specific resistance to them. Work on the causation and spread of epidemics is reviewed; the conclusion these studies suggest is that two factors are involved-the existence of a "basic virus", as yet undetectable until activated, and responsible for multicentric outbreaks; and the element of "spread", which under favourable conditions carries the infection to susceptible groups, and results in extensive epidemics.

Influenza antibodies in the population of the USA; an epidemiological investigation

Sera taken from persons of various ages in 1951 and collected from adults yearly from 1943 to 1951, inclusive, were tested by the haemagglutination-inhibition method with influenza viruses selected to represent the subgroups of each antigenic type. These were influenza A-WS (1933) and PR8 (1934); influenza A-prime-FM1 (1947) and FW-1-50 (1950); influenza B-Lee (1940) and IB1 (1950); influenza C-1233 (1947). The sera tested with influenza A and B viruses were treated with cholera filtrate to remove non-specific inhibitor. Since influenza C virus was not affected by the non-specific substance, the sera tested against this agent were not so treated.Children's sera showed high antibody level, attained at an early age, for FM1 and FW-1-50 viruses, and essentially no antibody for WS or PR8. By contrast, adult sera revealed high antibody content for PR8 and moderate titres for WS and the A-prime viruses. In adult sera, antibody against the PR8 virus increased significantly in 1944, after the 1943-4 epidemic, and remained at a relatively constant level for the eight succeeding years. The antibody pattern for WS was similar to that for PR8, but the values for its titres were only half as great. Antibody against the A-prime strains rose steadily from inconsequential levels in 1943 to high mean titres in 1951. These findings were consistent with virus isolation studies which suggested that the A-prime viruses, such as FM1, were introduced about 1946 and have been continuously prevalent since that time, while the WS and PR8 agents have been recovered only occasionally in recent years. Sera tested with Lee and IB1 viruses showed essentially identical titres. Antibody to these strains was low in the sera of children, indicating that there had been little previous exposure to the B agents. Adult sera showed marked elevation in titre between 1944 and 1946, and the titres have remained at a high level since that time. The increase in 1946 followed the influenza B outbreak of 1945-6; the increase in the previous year occurred in spite of the fact that there was no epidemic. Children developed significant antibody to influenza C in early life, and high antibody levels were observed in adult sera collected over the nine-year period. These observations indicate that the virus was widespread in the population and was prevalent before 1943.