The 2009 “swine flu” H1N1 Influenza A virus pandemic demonstrated the need for influenza vaccines providing broad coverage across multiple strains and subtypes. Current vaccine preparation methods are problematic because they function by inducing antibodies to the highly variable surface glycoproteins of the Influenza A virus, hemagglutinin (HA) and neuraminidase. Because these proteins are susceptible to mutations, i.e., genetic drift, as new strains and subtypes of the virus arise, new vaccines must be formulated that select for the current circulating strain/subtype. This issue is compounded by the long production time of several months, time needed for delivery and preparation for large scale immunization. The “swine flu” pandemic highlighted these problems as the newly emerged virus was identified in April, but it wasn’t until October that sufficient vaccine had been prepared.
An alternative approach is targeting conserved viral antigens or the generation of so-called “universal” influenza vaccines that would provide immunity in a strain and subtype independent manner. Conserved antigen vaccines do not provide the sterilizing immunity (which implies complete prevention of infection) afforded by the HA-specific, neutralizing antibody vaccines. However, universal vaccines would lead to a more mild, shorter infection and reduce morbidity and mortality by speeding up viral clearance and reducing transmission of the virus. Additionally, the mild, transient infection would boost immunity and lead to the production of neutralizing antibodies against the specific strain in circulation, thereby preventing re-infection. Ideally, these heterosubtypic, “universal” vaccines would, in the case of an unexpected pandemic, provide a stop gap measure that could limit the disease and reduce the impact of newly emergent Influenza strains until sufficient supplies of a specific vaccine has been produced.
The M2 channel, or matrix 2 protein, of influenza A is the target of the amantane-derived antiviral drugs (e.g. amantadine and rimantadine). These drugs are now ineffective because virtually all circulating strains have acquired mutations in the channel that confer drug resistance. A recent article discussed new developments made studying the structure of the M2 channel by solid-state nuclear magnetic resonance produced images. Clarification of functional and structural mechanisms underlying proton transport in the M2 channel is critical in development of new antiflu drugs.
The M2 channel is a single-pass integral membrane protein that associates into tetrameric, proton-selective ion pores on the cell membranes of infected cells and in lower levels on virion membranes. Acting as an ion channel that modulates the pH of intraviral compartments with exceptional proton-selectivity—essential for viral uncoating—the M2 protein is essential to the life cycle of the Influenza A virus and is therefore highly conserved. Additionally, it is small (97 amino acids), and the N-terminal ectodomain, M2e, is especially conserved (23 amino acids). Thus, immune-escape mutations are not as likely. The article reporting the new images of the M2 virus stated that the scientists behind the study knew all of the “handful” of possible changes to the channel that could occur and still allow it to function. All of these factors give the matrix 2 protein channel great potential as a “universal” vaccine target. How much this potential comes to fruition is yet to be determined.
There are many aspects in vaccine development: types of vaccines (e.g recombinant vector vaccines, adjuvant killed vaccines, prime-boost regimens, live-attenuated viruses, etc): methods of application (intramuscular vaccination or intranasal vaccination); and additional variables (e.g. adjuvants used, hapten-carrier conjugates, targeting multiple antigens, etc.) These issues complicate new vaccine development. And, there are other factors that will have to be assessed, such as the stability of the vaccine , vaccine doses, and the effects of different genetic backgrounds on effectiveness of immunization.
The M2 targeting vaccines have shown some significant progress and promising results. Preliminary results in phase I clinical trials with a M2e -HBc fusion particle vaccine showed 90% seroconversion rates after two doses. While these results are indicative of the efficacy of M2 vaccines in humans, it has been suggested that the number of individuals in the study is not sufficient to mitigate concerns that M2 vaccines alone may not be effective in all of the population, because of different genetic backgrounds and major histocompatibility (MHC) types. The concern regarding different genetic backgrounds is based on the differences in immune responses (including differences in antibody production, T cell responses, and protection against challenge) observed when an M2 vaccine was given to different inbred mouse strains representing different MHC types.
The small size of M2 and the limited number of epitopes it represents makes it likely that M2 would not have a productive epitope for every human MHC type. In addition to use of hapten-carrier conjugates and adjuvants, one very promising idea is to increase the proportion of the population that the vaccine protects by the addition of another influenza epitope. Additional antigens could include nucleoprotein and matrix 1 protein. Clinical trials of such combinations are underway.
Clinical trials:
Intramuscular/subcutaneous universal Influenza vaccine comparison
Recombinant M2e Influenza A vaccine
