ANTIBODY RESPONSE OF HUMAN BEINGS FOLLOWING VACCINATION WITH INFLUENZA VIRUSES

Role of Damage-Associated Molecular Pattern/Cell Death Pathways in Vaccine-Induced Immunity

Immune responses induced by natural infection and vaccination are known to be initiated by the recognition of microbial patterns by cognate receptors, since microbes and most vaccine components contain pathogen-associated molecular patterns. Recent discoveries on the roles of damage-associated molecular patterns (DAMPs) and cell death in immunogenicity have improved our understanding of the mechanism underlying vaccine-induced immunity. DAMPs are usually immunologically inert, but can transform into alarming signals to activate the resting immune system in response to pathogenic infection, cellular stress and death, or tissue damage. The activation of DAMPs and cell death pathways can trigger local inflammation, occasionally mediating adaptive immunity, including antibody- and cell-mediated immune responses. Emerging evidence indicates that the components of vaccines and adjuvants induce immunogenicity via the stimulation of DAMP/cell death pathways. Furthermore, strategies for targeting this pathway to enhance immunogenicity are being investigated actively. In this review, we describe various DAMPs and focus on the roles of DAMP/cell death pathways in the context of vaccines for infectious diseases and cancer.

Influenza vaccines: Past, present, and future

Globally, infection by seasonal influenza viruses causes 3-5 million cases of severe illness and 290,000-650,000 respiratory deaths each year. Various influenza vaccines, including inactivated split- and subunit-type, recombinant and live attenuated vaccines, have been developed since the 1930s when it was discovered that influenza viruses could be cultivated in embryonated eggs. However, the protection rate offered by these vaccines is rather low, especially in very young children and the elderly. In this review, we describe the history of influenza vaccine development, the immune responses induced by the vaccines and the adjuvants applied. Further, we suggest future directions for improving the effectiveness of influenza vaccines in all age groups. This includes the development of an influenza vaccine that induces a balanced T helper cell type 1 and type 2 immune responses based on the understanding of the immune system, and the development of a broad-spectrum influenza vaccine that can increase effectiveness despite antigen shifts and drifts, which are characteristics of the influenza virus. A brighter future can be envisaged if the development of an adjuvant that is safe and effective is realized.

Challenges of Making Effective Influenza Vaccines

Seasonal influenza vaccines prevent influenza-related illnesses, hospitalizations, and deaths. However, these vaccines are not as effective as other viral vaccines, and there is clearly room for improvement. Here, we review the history of seasonal influenza vaccines, describe challenges associated with producing influenza vaccine antigens, and discuss the inherent difficulties of updating influenza vaccine strains each influenza season. We argue that seasonal influenza vaccines can be dramatically improved by modernizing antigen production processes and developing models that are better at predicting viral evolution. Resources should be specifically dedicated to improving seasonal influenza vaccines while developing entirely new vaccine platforms.

Standardisation of inactivated influenza vaccines-Learning from history

The single radial immunodiffusion assay has been the accepted method for determining the potency of inactivated influenza vaccines since 1978. The worldwide adoption of this assay for vaccine standardisation was facilitated through collaborative studies that demonstrated a high level of reproducibility and its applicability to the different types of influenza vaccine being produced at that time. Clinical evidence indicated the relevance of SRID as a potency assay. Unique features of the SRID assay are likely responsible for its longevity even as newer technologies for vaccine characterisation have been developed and refined. Nevertheless, there are significant limitations to the SRID assay that indicate the need for improvement, and there has been a substantial amount of work undertaken in recent years to develop and evaluate alternative potency assays, including collaborative studies involving research laboratories, regulatory agencies and vaccine manufacturers. Here, we provide an overview of the history of inactivated influenza vaccine potency testing, the current state of alternative assay development and the some of the major challenges to be overcome before implementation of new assays for potency determination.

Seasonal influenza vaccination and technologies

Seasonal influenza is a serious respiratory illness that causes annual worldwide epidemics resulting in significant morbidity and mortality. Influenza pandemics occur about every 40 yrs, and may carry a greater burden of illness and death than seasonal influenza. Both seasonal influenza and pandemic influenza have profound economic consequences. The combination of current vaccine efficacy and viral antigenic drifts and shifts necessitates annual vaccination. New manufacturing technologies in influenza vaccine development employ cell culture and recombinant techniques. Both allow more rapid vaccine creation and production. In the past 5 years, brisk, highly creative activity in influenza vaccine research and development has begun. New vaccine technologies and vaccination strategies are addressing the need for viable alternatives to egg production methods and improved efficacy. At present, stubborn problems of sub-optimal efficacy and the need for annual immunization persist. There is an obvious need for more efficacious vaccines and improved vaccination strategies to make immunization easier for providers and patients. Mitigating this serious annual health threat remains an important public health priority.

Seasonal influenza vaccinations: specialized products for different target groups

Traditionally, inactivated influenza vaccines have all been treated as virtually identical, at least in terms of recommendations for use. This has mostly been the case since their development over 60 years ago. The concept, still often quoted, that they are 70-90% protective against laboratory-confirmed clinical influenza comes from multiple studies carried out with different preparations in the US military; studies which ended in 1969 [1]. During this period, there were only gradual advances in improved potency and purity of the vaccines, so that it was appropriate to consider them as being comparable. However, we are currently witnessing a change, which started slowly, but is now accelerating, in which very different types of vaccine are becoming available. This has already begun in some parts of the world, but will soon be universal. The process is being accelerated by questions concerning the actual effectiveness of the current vaccines in specific risk groups. In this paper, we will take a look at the developments in the formulation of the vaccine to address the needs that have been identified. We will also consider different strategies for vaccine use which might be applied to traditional or future vaccines to improve population protection.

AN EVALUATION OF METHODS FOR THE CONCENTRATION AND PURIFICATION OF INFLUENZA VIRUS

The concentration and purification of influenza virus by means of differential centrifugation in a vacuum type centrifuge, by adsorption on and elution from adult chicken red cells, by elution of the precipitate formed on freezing and thawing of allantoic fluid, by adsorption on and elution from embryonic chick red cells, and by combinations of the first method with each of the three succeeding methods, have been studied. Over-all yields of virus of about 50 to 70 per cent were obtained by these methods and combinations of methods except for somewhat lower yields when adsorption on and elution from adult chicken red cells was employed. However, the purified products obtained by methods involving only the use of red cells or the freezing and thawing technique were found to contain about 80 per cent of non-virus protein. The purified products obtained when differential centrifugation was used either alone or in combination with any one of the other methods were found to be indistinguishable and to consist of a fairly homogeneous component having a sedimentation constant of about 600 S. Such preparations possessed about 22,000 chicken red cell agglutinating units per mg. of protein nitrogen and solutions containing only about 10^–14 gm. of the materials gave 50 per cent infectivity end points in chick embryos. The Sharples centrifuge was found to be almost as efficient as the vacuum type centrifuge for the concentration and purification of influenza virus and, because of its larger capacity, is recommended for the preparation of purified virus on a large scale.

STUDIES IN HUMAN IMMUNIZATION AGAINST INFLUENZA : DURATION OF IMMUNITY INDUCED BY INACTIVE VIRUS

The administration to human beings of formalin-killed influenza virus, concentrated from allantoic fluid, resulted in a high order of antibody response within 2 weeks after injection. Even after 1 year the great majority of individuals vaccinated had antibody levels considerably above their prevaccination titer for the PR8, Lee, and a current 1943 strain. An investigation of the occurrence of epidemic influenza A in seven widely separated populations, 1 year after vaccination of part of these groups, showed that the attack rate among vaccinated persons was consistently lower than that of control individuals. The average reduction in attack rate was of the order of 35 per cent.

ADSORPTION OF INFLUENZA HEMAGGLUTININS AND VIRUS BY RED BLOOD CELLS

A number of experiments were performed on the adsorption of influenza hemagglutinins on chicken red blood cells, from which the following conclusions were drawn:— 1. When chicken red blood cells and preparations of influenza viruses were mixed together, the influenza hemagglutinins present were rapidly adsorbed onto the cells. After varying lengths of time, dependent on the conditions of the experiment, the adsorbed hemagglutinins began to elute from the cells. With the Lee strain at 23°C. and the PR8 strain at 37°C. almost all of the adsorbed agglutinin was released in 4 to 6 hours. 2. When the number of red cells used for adsorption was increased, the speed and degree of adsorption of the hemagglutinins increased. The time of maximum adsorption of hemagglutinins was the same, regardiess of red cell concentration, and with the larger amounts of red cells the speed and degree of elution was decreased. 3. When adsorption of PR8 virus agglutinins was carried out at 4°C. the adsorption was rapid and nearly complete. When the reaction was carried out at higher temperatures (27° and 37°C.), the adsorption was equally rapid but was progressively less complete with rise in temperature. At 4°C. the maximum adsorption was not reached for 5 hours; at 27°C. it was reached in 25 minutes; and at 37°C. the greatest degree of adsorption was attained between 3 and 5 minutes. The amount of elution observed at 4°C. at 18 hours was negligible, but the degree of elution increased with temperature so that at 37°C. almost all of the adsorbed agglutinin was released in 6 hours' time. 4. Red cells which had adsorbed and then fully eluted the agglutinin were not capable of adsorbing a detectable amount of fresh agglutinin. In addition, such cells would no longer agglutinate even though exposed to fresh virus suspensions. 5. The hemagglutinin of influenza B virus was capable of being adsorbed on and eluted from several successive lots of chicken red cells without appreciable loss of agglutinating activity. 6. The hemagglutinins of the PR8 and Lee strains were rapidly inactivated at 60°C. The presence of active virus was not necessary for the occurrence of the adsorption-elution reaction on chicken red cells. 7. The activity of the portion of the red cells responsible for the adsorption of the hemagglutinins persisted, though in reduced amount, even after heating for 5 minutes at 100°C. Hemagglutinins were adsorbed and eluted from red cell stroma. 8. The infective agent in influenza virus suspensions was adsorbed by chicken red cells simultaneously with the adsorption of hemagglutinins. 95 per cent of the infective agent was removed from suspension by the red cells after contact for 15 minutes. From then on the infective agent was gradually released from the red cells. After 4 hours the 50 per cent mortality titer of the supernatant fluid was as high as at the beginning of the experiment.

THE PREPARATION AND PROPERTIES OF INFLUENZA VIRUS VACCINES CONCENTRATED AND PURIFIED BY DIFFERENTIAL CENTRIFUGATION

Influenza virus vaccines containing from 1 to 10 mg. of virus materials per cc. concentrated and purified from infectious allantoic fluids by means of one or two cycles of differential centrifugation and inactivated by different treatments have been prepared and subjected to laboratory tests. Suitable inactivation of the virus preparations with retention of full red cell agglutinating activity and immunizing potency in mice was achieved by treatment with minimal amounts of formaldehyde or ultraviolet light. Treatment with phenol or chloroform failed to cause adequate loss of virus activity. Excessive amounts of formaldehyde or of ultraviolet light were found to cause a loss in red cell agglutinating activity and in immunizing potency. Freezing resulted in the immediate loss of red cell agglutinating activity of the formalinized vaccine. Storage of the vaccines in the frozen state was accompanied by a gradual decrease in red cell agglutinating activity. Drying of the vaccines from the frozen state resulted in a loss of red cell agglutinating activity and, in the case of the formalinized vaccine, in a loss in immunizing potency. There appeared to be at least a rough correlation between red cell agglutinating activity and immunizing potency. The immunizing potency and red cell agglutinating activity of a purified formalinized vaccine containing 2 mg. of virus material per cc. were unchanged following 2 months' storage at 4 degrees but were measurably decreased following storage for 2 months at 18 to 25 degrees and at 37 degrees . At equivalent dosages of virus material the immunizing potency of formalinized centrifugally purified virus, of formalinized virus purified by the red cell elution method, and of infectious allantoic fluid was not measurably different. The immunizing potency of a formalinized polyvalent vaccine containing centrifugally purified Lee, PR8, and Weiss influenza virus materials at concentrations of 5, 2.5, and 2.5 mg. per cc., respectively, was found to be essentially the same as that of a similar vaccine prepared commercially. In both cases the protection afforded against the Weiss strain appeared to be better than that against the Lee and PR8 strains. The commercially prepared vaccine is being subjected to clinical tests in man at dosage levels ranging from 0.01 mg. to 10 mg. The latter corresponds to a level approximately 100 times that of infectious allantoic fluid. It was found that the bacterial contamination that frequently accompanies operation on a large scale can be controlled by the addition of one part per 10,000 of formalin plus one part per 100,000 of phenyl mercuric nitrate to the allantoic fluid immediately following harvesting, without affecting the quality of the vaccine. This procedure and the use of virus materials purified and concentrated by a single cycle of differential centrifugation by means of the Sharples centrifuge were found to be suitable for the production of influenza virus vaccines on a large scale. By means of this method influenza vaccines possessing 20 or more times the immunizing potency of infectious allantoic fluid and 10 or more times the immunizing potency of the usual commercial vaccine prepared by the red cell elution method can be manufactured rapidly on a very large scale with considerable ease and efficiency.

The use of adjuvants in studies on influenza immunization. I. Measurements in monkeys of the dimensions of antigenicity of virus-mineral oil emulsions

Untoward reactions at the site of inoculation were not observed in monkeys vaccinated with influenza virus incorporated in a water-in-oil emulsion without acid-fast bacilli. Studies were then made to measure some of the dimensions of antigenicity of these emulsions to evaluate the extent of the immunologic adjuvant effect. This included measurements of height and persistence of the antibody response to inoculation and measurements of the extent to which the vaccine could be diluted and still induce antibody formation; i.e., antigenic extinction. In addition, comparisons were made of the rates of development of hemagglutination-inhibiting, virus-neutralizing, and complement-fixing antibody activities to determine the relationship among these three properties of the serum of immunized animals. It was found that levels of antibody many fold higher were induced by the virus-adjuvant mixtures as compared with virus in an aqueous menstruum, and that the level of antibody induced was related to the quantity of antigen incorporated in the emulsion. The stock vaccine when emulsified could be diluted 100,000-fold and was still active in antibody formation whereas a 100-fold dilution of the antigen without emulsification was essentially ineffective. Equivalent quantities of virus in 0.1 ml. or 1.0 ml. of emulsion induced antibody responses that were indistinguishable with respect to level or persistence. In comparing the course of antibody development it was found that hemagglutination-inhibiting, virus-neutralizing, and complement-fixing antibodies develop at different rates; careful analysis of the data derived from the present study together with other observations warrant the conclusion that these antibody activities are not present in constant proportion and are independent of one another. The implications of this observation and of the others mentioned above are discussed.

An immune response profile model for immunogenicity quantitation

We propose an analytical model, which can simultaneously depict many fundamental characteristics of the immunogenicity of various vaccines. This model, the Immune Response (IR) profile, conveniently expresses the mathematical relation between pre- and post-vaccination titers. A vaccine's IR profile is antigen-specific, dose-dependent and post-vaccination interval-dependent. The maximal capability for serological response to a vaccine can be determined using this model irrespective of the dose administered, the post-vaccination assay interval, or the live or killed state of the vaccine. The IR profile obtained from analysis of booster vaccine responses in a limited number of seropositive study subjects can be used to predict maximal antibody titers which are expected after vaccination and can predict the geometric mean post-vaccination antibody titer of a cohort of subjects undergoing primary immunization. Using this model, it is anticipated that the immunoregulation implied by the IR profile may also prove to be correlated with cellular subpopulations and idiotypic antibody functions. Although derived from influenza vaccines analyses, the model successfully describes immune response characteristics following natural infection with malaria and following diphtheria and rubella vaccine administration.

The search for the ideal influenza vaccine

The history of the development of influenza virus vaccine is traced from its origin with experimental studies of influenza virus in ferrets and mice and the first trials in man. Knowledge of the basis of immunity to the viruses in experimental animals and in man has grown steadily over the years and has been essential to successful immunization. Virus variation affecting the surface antigens of the virus is seen as the principal obstacle to the application of vaccines in man. So significant are the changes occurring during antigenic drift that former concepts of a polyvalent vaccine cannot provide a solution of the problem of the composition of vaccines. Disrupted virus vaccines appear to provide the answer to the prevention of vaccine reactions.

Influenza immunization: clinical studies with ether-split subunit vaccines

Clinical studies of ether-split influenza antigen vaccines have been in progress for almost a decade. One series of such studies, completed before the Hong Kong virus appeared, compared identically constituted conventional and antigen vaccines for serological effectiveness in 1700 vaccinees from the staff of a metropolitan hospital. A series of 6 annual trials included both "old" subjects (vaccinated the previous year) and "new" subjects (no vaccination the previous year). The serological response to the type A2 component of the antigen vaccines was 3-4 times better than that to intact virus in both the old and new populations. The response to either vaccine by new subjects significantly exceeded the response by the old subjects. The type B component of both vaccines induced an equivalent response in both populations. Monovalent Hong Kong vaccines, both conventional and antigen, given just prior to the Hong Kong epidemic induced an anamnestic response in a geriatric group. No influenza-like disease was seen in this high-risk group during the epidemic.

Vaccination against influenza

This paper reviews studies which have been carried out during the past twenty years in the United States of America to investigate the suitability of various vaccines and vaccination methods for immunizing man against the different influenza virus strains. A number of investigations in closed communities, such as children's institutions, army and navy units, and medical schools, are described. The author discusses the comparative value of the techniques employed in preparing vaccines, and the use of adjuvants in improving the response.