QUALITATIVE DIFFERENCES IN THE ANTIGENIC COMPOSITION OF INFLUENZA A VIRUS STRAINS

Next-generation influenza vaccines: opportunities and challenges

Seasonal influenza vaccines lack efficacy against drifted or pandemic influenza strains. Developing improved vaccines that elicit broader immunity remains a public health priority. Immune responses to current vaccines focus on the haemagglutinin head domain, whereas next-generation vaccines target less variable virus structures, including the haemagglutinin stem. Strategies employed to improve vaccine efficacy involve using structure-based design and nanoparticle display to optimize the antigenicity and immunogenicity of target antigens; increasing the antigen dose; using novel adjuvants; stimulating cellular immunity; and targeting other viral proteins, including neuraminidase, matrix protein 2 or nucleoprotein. Improved understanding of influenza antigen structure and immunobiology is advancing novel vaccine candidates into human trials.

ADJUVANTS IN IMMUNIZATION WITH INFLUENZA VIRUS VACCINES

Subcutaneous inoculation, of PR8 allantoic fluid, or watery suspensions of the virus obtained from allantoic fluid by high-speed centrifugation or by elution after adsorption on red cells induced serum antibodies in experimental animals, which reached the highest levels within 2 weeks after inoculation and were gradually lost thereafter. The addition of killed acid-fast bacteria (Myco. tuberculosis or butyricum), paraffin oil, and a proprietary adsorption base (Falba) to form a stable water-in-oil emulsion of influenza virus suspensions greatly enhanced and maintained immunity and antibody response to the virus. These adjuvants provided a much more effective method of increasing antibody production to the virus than the use of concentrated preparations of virus alone. Paraffin oil and Falba without the acid-fast bacilli were less effective as adjuvants, although the antibody levels induced were higher than those produced by watery suspensions of the virus and were maintained at a constant level for at least 6 months. Myco. butyricum appeared to be more effective in producing antibodies against the virus than the tubercle bacilli in the emulsions of paraffin oil and Falba. Immunization with these adjuvants and suspensions of influenza virus obtained from allantoic fluid induced antibodies not only against the virus but against antigenic material contained in normal allantoic fluid, although the latter titers were considerably lower. A suspension of influenza virus (sedimented by high-speed centrifugation) and Myco. butyricum in sesame oil induced about four times as much antibody as when the virus was suspended in saline, in sesame oil alone, or in combination with typhoid bacilli.

One-step growth curves of various strains of influenza A and B viruses and their inhibition by inactivated virus of the homologous type

One-step growth curves of five strains of influenza A, one strain of swine influenza, and three strains of influenza B virus have been analyzed. The influenza A and swine influenza strains showed constant periods of 5 to 6 hours before newly formed virus was liberated from the infected cells, whereas 8 to 10 hours elapsed in the case of the influenza B strains. The yield of virus in the allantoic fluids, i.e. the number of ID₅₀ released for every ID₅₀ of seed virus adsorbed, was consistently higher in the case of the influenza A and swine influenza strains than in that of the influenza B viruses. Interruption of the cycle by injection of inactivated virus subsequent to infection can be achieved by any of the strains of the homologous type. However, cross-tests between influenza A and swine influenza virus led only to partial inhibition of virus growth.

Lack of identity in neutralizing and hemagglutination-inhibiting antibodies against influenza viruses

There is an exponential linear relationship between the quantity of influenza virus neutralized and the quantity of immune serum employed in in ovo neutralization. The slope of the neutralization line is extremely steep. The concentration of neutralizing antibody can be measured with considerable precision in ovo if the constant virus-varying serum technique is utilized. The amounts of hemagglutination-inhibiting and neutralizing antibodies which are absorbed by a given quantity of influenza virus (PR8) were found to be predictable and the degree of reactivity of these two antibodies was shown to be directly related to the extent of immunization. It was demonstrated that there are marked discrepancies in correlation between antibody titers obtained by in vitro hemagglutination-inhibition and in vivo neutralization techniques and that neutralizing antibody is preferentially absorbed by a given quantity of virus. Inasmuch as the results were found not to be attributable to peculiarities of the techniques employed, it appears that the antibodies measured by hemagglutination-inhibition in vitro and by neutralization in vivo are not identical.

The pathogenesis of infection with a virulent (CG 179) and an avirulent (B) strain of Newcastle disease virus in the chicken. II. Development of antibody

Circulating antibody appeared in the convalescing NDV-infected chicken concomitantly with the disappearance of virus from the tissues. The antigenic response to the CG 179 and B strains was demonstrated to be approximately equal. The neutralization test in the embryo and the hemagglutination inhibition technique yielded parallel results in the measurement of antibody early in convalescence, but late in convalescence the hemagglutination inhibition titers were relatively lower. This disparity indicates the possible duality of the antibodies. There was a wide ratio between the neutralizing antibody titers found in the brain and in the serum after an asymptomatic infection with NDV. The antibody level in the brain appeared to be related to the extent of virus growth and damage in the central nervous system. It appeared likely that a major factor in determining the virulence of the CG 179 strain was the more rapid attainment in the central nervous system of high virus concentration which outstripped the defense mechanisms of the host.

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

Antigenic variants of influenza A virus strains emerge on serial passage in ovo in the presence of immune serum against different but related strains. An old laboratory strain (PR8) which had been through hundreds of animal passages was as readily modified by this procedure as recently recovered strains. Such variants apparently can be obtained at will and show antigenic patterns which are reproducible and appear to be predictable in terms of the immune serum used for their selection. Variant strains retain their new antigenic patterns on serial passage in ovo in the absence of immune serum. Limited serial passage in ovo of strains in the absence of immune serum did not result in the emergence of antigenic variants. Similarly, serial passages of strains in ovo in the presence of immune serum against widely different strains, which failed to show significant cross-neutralization, did not lead to the appearance of antigenic variants.

Antigenic analysis of certain group B arthropodborne viruses by antibody absorption

A method for carrying out antibody absorption studies for antigenic analysis of group B arthropod-borne (arbor) viruses is described and examples of homologous and heterologous absorption curves are presented. Evidence that antigenic structure can be a stable property was obtained with three strains of West Nile virus isolated from different hosts in different countries over a period of years. Comparative studies with viruses of the Japanese B-St. Louis-West Nile subgroup indicate that each virus contains a completely specific antigen as well as one or more cross-reactive components. Strains of yellow fever virus isolated in America were shown to lack an antigen present in strains of African origin although no differences were found between isolates from the same geographical area. The attenuated 17 D vaccine strain of yellow fever was found to have acquired an additional antigen not present in the unadapted parent or in other strains tested. However, alteration in pathogenicity for man was not found to be necessarily attended by any antigenic modification, as shown by the antigenic identity of the French neurotropic vaccine strain with its pantropic parent.

Serological behaviour of influenza viruses. II. Patterns of antigenic relationship

By antibody absorption it was found that strains of influenza virus exhibiting P-Q differences were related according to certain patterns.In the course of this investigation it was also revealed that some viruses possessed masked antigens capable of stimulating antibody production but incapable of combining efficiently with antibody.

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.

The influenza virus: its morphology, immunology, and kinetics of multiplication

The morphology of influenza virus is described, and the properties of its two components, the elementary body and the soluble substance, differentiated. The relationship between the filamentous and spherical structures observed in the infected allantoic fluid is still obscure, but the filaments are thought to represent either a form of influenza virus or a stage in its multiplication.The antigenic constitution of A and B viruses is discussed and variations in the pattern of individual strains over the past two decades described. The author outlines his speculations on the origin and mechanism of these variations. Other differences in serological behaviour, such as non-specific inhibition and the avidity effect, are discussed.The results of experimental investigation, with various animal hosts, of the kinetics of influenza virus multiplication are reviewed, and the effects of such factors as incubation temperature and concentration of the seed virus considered. The bearing of these studies of enzymatic virus/host-cell interactions on the chemotherapeutic control of influenza is emphasized.

Strain-specific elements in influenza antigens

Rabbit antisera were prepared against ten antigenically different influenza A strains. These sera were absorbed with one or more heterologous strains and in each case all the heterologous or crossing antibody was removed; the anti-bodies remaining after this treatment were specific for the immunizing strain or group of strains. On the basis of reactions with absorbed sera, the strains fell into seven groups. Absorbed specific antisera of these groups were used to test the HI titer against a large number of influenza A viruses. Most of the strains were inhibited by a single serum, a few were inhibited by none of the sera, and only one strain was inhibited by two antisera. The grouping of strains by this method was less equivocal than classifications based on previous tests. When more fully developed, this technique promises to be of interest and assistance in the study of influenza, especially from the epidemiological and prophylactic standpoints.