The Development of a DNA Vaccine that Provides Protective Immunity against H5N1 Influenza Virus Infections This is an interactive module based upon the following paper: Kodihalli, S., H. Goto, D. L. Kobasa, S. Krauss, Y. Kawaoka, and R. G. Webster. 1999. DNA vaccine encoding hemagglutinin provides protective immunity against H5N1 influenza virus infection in mice. Journal of Virology. 73(3):2094-2098. This module was created by Tammi Waters for Biology 426: Virology at California State University, Fullerton In Hong Kong in 1997, a lethal Type A H5N1 avian influenza virus was transmitted directly from chickens to humans causing 18 humans to be infected. One-third of the infected individuals died. Prior to this outbreak, avian influenza viruses were considered unable to transmit directly to humans. The ability of this virus to transmit directly from birds to humans suggests the possibility of future pandemics of lethal viruses. A strategy needs to be developed to combat these novel viruses. Antiviral agents and inactivated vaccines are the primary defenses against influenza virus infections today. These methods, however, are not sufficient enough to prevent a catastrophic pandemic from occuring. Immunization with purified viral DNA has been shown to induce an immune response against many infectious diseases, including influenza, malaria and tuberculosis. DNA vaccines offer a unique advantage over other strategies in that the candidate vaccine can be recovered from infected tissue and rapidly cloned into an expression vector, thus eliminating the time otherwise required to culture the virus. The primary focus of this study was to develop a DNA vaccine that would provide protective immunity against H5N1 influenza viruses. Questions to Ponder Why are avian influenza viruses unable to infect human beings directly? Why was this H5N1 influenza virus so lethal to humans? What if the virus was capable of transmission between human hosts? What are some drawbacks to using antiviral drugs? How would you go about developing a DNA vaccine against H5N1 influenza viruses? Answers Experimental Methods and Results Experimental infection of mice with HK97 and CkHK97 Since the H5N1 influenza viruses in Hong Kong caused severe infection in humans, the properties of these viruses in mammalian systems needed to be established. Mice are often used as models for mammalian systems. Influenza virus genes and expression vectors The HA gene needs to be cloned into an expression vector. Hemadsorption The hemagglutinin on the surface of the influenza virus adsorbs to red blood cells (RBC) and can cause clumping of RBC's. This phenomenon can confirm the presence of an infuenza virus. Immunization with pHKHA The extent of protection provided by the DNA encoding HA of HK97 against challenge with homologous H5 virus (HK97) or CkHK97 needs to be determined. Immunization with pTyIrHA Is it possible to provide protection against HK97 using an HA-DNA vaccine of an antigenic variant (TyIr83)? Antibody responses After immunization, antibody production is usually the major mechanism of protection against influenza infection. Antibodies to the HA molecule are necessary for influenza viurs neutralization and infection prevention. Do HA-DNA vaccinated mice have antibodies to the HA? What does all this mean? Need more information? Answers to Questions Why are avian influenza viruses unable to infect human beings directly? The answer lies in the cellular receptors of the host and the receptor specificity of the virus. The specificity of avian influenza viruses differs from that of human influenza viruses. The avian influenza virus binds an SA2,3 Gal linkage on the cell surface sialyloligosaccharides cell surface proteins containing sialic acid groups that assist in viral entry into cell whereas human strains bind SA2,6 Gal linkages. In addition to the receptor specificity, the optimal temperature for replication of avian viruses is 41C, whereas human viruses have an optimal temperature of 37C. All previous avian influenza viruses that have infected humans have first undergone reassortment the virus enters an intermediate host, such as a pig, which is capable of being infected by both avian and human viruses. The genome of the various viruses 'reassort' within the host to form new strains of virus. with human viruses, thus allowing mutations to occur that allow the newly reassorted virus to infect humans. Why was this H5N1 influenza virus so lethal to humans? This was the first H5 influenza virus to infect humans. All subtypes of influenza A have been observed in birds (hemagglutinins H1 to H15 and neuraminidases N1 to N9), whereas only H1 to H3 subtypes have been observed in humans. Humans have protective immunity against strains with H1, H2 and H3 due to previous infection when influenza virus infects a host, the host produces antibodies to the hemagglutinin (antigen). The response to the H1 antigen would be an antibody to H1. The memory response developed against one antigen affect subsequent immune responses to antigens that are antigenically and structurally related to the primary antigen . When a virus with a novel HA or NA is introduced, a sudden, major change in virus antigenicity occurs (antigenic shift). The host has no immunity to the new virus due to the lack of antibodies specific for that subtype. What if the virus was capable of transmission between human hosts? Two things must be true for an influenza pandemic to occur: the virus must be of a novel subtype of HA and/or NA, AND the virus must be transmissible between human hosts. This particular H5N1 virus satisfied the first requirement, but luckily, was incapable of transmission between humans. Because it was unable to jump from human host to human host, the virus was easily contained In order to stop the spread of this H5N1 virus in Hong Kong in 1997, all hosts (fowl) of the virus were killed. This mass murder of fowl in the area cost millions of dollars to both the people of Hong Kong and the government. . If the virus had been capable of transmission between human hosts, with a death rate of 33%, the results would have been catastrophic. What are some drawbacks to using antiviral drugs? Antiviral drugs, such as amantadine, are very useful in treating influenza A. However, the feasibility for their widespread use as part of a response to an influenza A pandemic depends on a number of issues such as limited supply, their side effects, antiviral spectrum and potency, ease of administration in outpatient settings and the emmergence of drug-resistant variants. All of these factors, and many more, need to be considered when responding to a pandemic. How would you go about developing a DNA vaccine against H5N1 influenza viruses? A likely target for a vaccine would be the hemagglutinin. This protein is used in attachment and entry of the virus into the host cell. If the entry into the host is inhibited or interfered with, this would greatly decrease the virulence of the virus. Thus, with this H5N1 virus, a DNA vaccine encoding for H5 would be ideal. Back to top
The Development of a DNA Vaccine that Provides Protective Immunity against H5N1 Influenza Virus Infections
This is an interactive module based upon the following paper: Kodihalli, S., H. Goto, D. L. Kobasa, S. Krauss, Y. Kawaoka, and R. G. Webster. 1999. DNA vaccine encoding hemagglutinin provides protective immunity against H5N1 influenza virus infection in mice. Journal of Virology. 73(3):2094-2098.
This module was created by Tammi Waters for Biology 426: Virology at California State University, Fullerton
In Hong Kong in 1997, a lethal Type A H5N1 avian influenza virus was transmitted directly from chickens to humans causing 18 humans to be infected. One-third of the infected individuals died. Prior to this outbreak, avian influenza viruses were considered unable to transmit directly to humans. The ability of this virus to transmit directly from birds to humans suggests the possibility of future pandemics of lethal viruses. A strategy needs to be developed to combat these novel viruses. Antiviral agents and inactivated vaccines are the primary defenses against influenza virus infections today. These methods, however, are not sufficient enough to prevent a catastrophic pandemic from occuring. Immunization with purified viral DNA has been shown to induce an immune response against many infectious diseases, including influenza, malaria and tuberculosis. DNA vaccines offer a unique advantage over other strategies in that the candidate vaccine can be recovered from infected tissue and rapidly cloned into an expression vector, thus eliminating the time otherwise required to culture the virus.
The primary focus of this study was to develop a DNA vaccine that would provide protective immunity against H5N1 influenza viruses.
Questions to Ponder
Experimental Methods and Results
Answers to Questions
Back to top