Addressing challenges in the design of vaccines for the future

Little did Edward Jenner know, upon performing his „vaccination“ experiments in 18th century England, that his findings would change the course of medicine and lead to the first worldwide eradication of an infectious agent, the smallpox (Variola virus). A practice that was already known to the Chinese around the year 1000 AC, Jenner improved upon it by inoculating his gardener´s 8-year-old son in 1796 with a related virus that caused a milder form of the disease in cows and humans called „cowpox“. Jenner was able to achieve immunity against the smallpox virus in his research subjects and the term „vaccination“ was coined (from the Latin vaccinus, or cow-related).1

Airborn infection

The next milestone in the history of vaccines was the development of the rabies vaccine by Louis Pasteur in 1885 based on an attenuated („weakened“) virus obtained by drying out the nerve tissues of rabbits that had been used to propagate the virus.2 The first anthrax and cholera vaccines, likewise based on the principle of attenuation, were also developed by Pasteur. By the time World War I was over, devastating diseases such as typhoid fever, tuberculosis, and the plague had been tackled with either live or killed vaccines. Between 1930 and 1960, and thanks to major technological improvements such as the ability to culture viruses in embryonated chick eggs or mammalian cells, and continuous-flow-centrifugation for virus purification, new vaccines appeared on the radar, namely for yellow fever, Japanese B encephalitis, polio, and influenza. Bacterial vaccines were first developed after 1950, based on sub-unit capsular polysaccharide preparations for pneumococcus and meningococcus, followed by live attenuated paediatric vaccines for diseases such as measles, mumps, rubella, and varicella. 3, 4

Vaccines in the Genomic Era

Modern vaccines are much safer than their predecessors, as they undergo strict scrutiny by the authorities and are based on highly-purified microorganisms or antigenic proteins selected using techniques such as functional genomics and reverse genetics.5, 6 Designing vaccines that meet the high standards of the required safety and effectiveness continues to be a significant challenge to researchers.

One of the fronts where major progress has occurred in recent years is in the field of adjuvants.7 An adjuvant (from the Latin adiuvare, or „to help“) is any substance that potentiates the immune response to the antigen included in the vaccine. Aluminium salts were discovered to be effective adjuvants about 80 years ago, and have been included in numerous vaccines, due to their excellent safety record. „Alum“ is used in vaccines such as the combination diphteria-tetanus-pertusis (Dtap), Hepatitis A and B vaccines, and against TBE (tick-born encephalitis, or FSME, as it is known in Austria). A plethora of new adjuvants are being intensively researched and the immunological mechanism of action has been elucidated for some of these. MF59 (a squalene-based-oil-in-water emulsion developed by Novartis) has already being lisenced for seasonal (Fluad®) and pandemic Influenza vaccines (Aflunov® for H5N1 plus Focetria® and Celtura® for H1N1). Similarly AS03, another squalene emulsion developed by GSK is lisenced for Pandemrix® (H1N1). Its successor, AS04 is a mixture of Alum and monophosphoryl lipid A (MPL). AS04 is successfully being used as an adjuvant for Cervarix® (against Human Papilloma Virus infection, which can cause cervical cancer) and Hepatitis B (Fendrix®). While „Alum“ potentiates a Th2-biased immune response (generation of neutralizing antibodies), these new adjuvants are able to activate both a Th2 and a Th1 response (the latter causing activation of immune cells for destruction of intracellular pathogens). Virosomes, a lipid bilayer carrying active antigenic proteins (e.g. hemagglutinin and neuraminidase from the influenza virus) have been developed by Crucell and are also licensed as adjuvants in Inflexal® (influenza) and Epaxal® (Hepatitis A). Another example are ISCOMs (produced by Isconova) which are a mixture of cholesterol, phospholipids, and saponins, also capable of generating strong antibody and cell-based immune reponses.8 Recently, a Phase I clinical trial with an H5N1 vaccine showed that the inclusion of this potent adjuvant allows significant antigen-sparing (dose-sparing), which could be of critical importance during pandemic.9

Modern Vaccine Delivery Systems

            Recently, new avenues for vaccine delivery have been explored. Mucosal-associated infections such as HIV, gastric ulcer, diahrrea, and sexually-transmited diseases could be tackled in the future by applying the vaccine directly on the entry route of the pathogen (e.g. orally, intranasally, vaginally)10,11. To date, oral vaccines are available only for polio and cholera and a nasal vaccine (FluMist® by Medimmune) is available for influenza. In addition to providing needle-free immunization, which might increase market-acceptance, these vaccines offer a first-line-of-defence against invading pathogens by stimulating generation of secretory antibody (IgA). A diverse array of molecules that function by activating „toll-like receptors“ (TLRs), and therefore innate immunity, are being studied intensively as a source of potent new adjuvants for traditionally-injected as well as for mucosal vaccines.12 MPL is the only TLR4 ligand approved to date and it is included in Cervarix®. Unmethylated CpG-oligonucleotides activate TLR9 and have given promising results in human clinical trials against hepatitis B, malaria, influenza, and anthrax.13 Flagellin activates TLR5 and has been tested in animal models as a fusion protein with the conserved M2e influenza protein with the aim of developing a future universal vaccination independent of seasonal-strain variations.12

To substitute the traditional, but unpopular „shot“ in the arm approach, new creative approaches have been employed to develop needle-free devices. Pharmajet has developed an intradermal-delivery needle-free „gun“ that has completed two successful clinical trials (rabies and influenza) and a massive immunization campaign in children against measles.14, 17 Vaginal delivery of HIV antigens is currently being explored by use of gels or intravaginal rings as vehicles.15, 16 Skin patches that can deliver the antigen of interest (e.g. influenza, rotavirus, measles, Herpex simple virus 2) directly to antigen-presenting cells in the skin by painless microabrasion are also being developed.17 This technology would seem ideally suited to treat skin-related disorders, such as psoriasis or acne. Furthermore, they would not require refrigeration and would therefore bypass the cold transport chain to countries in the southern hemisphere leading to a significant lowering of cost of transport and distribution. Vaccination through consumption of edible plants (for example by expressing the Hepatitis surface B antigen in maize) could be an ideal delivery system, although this technology is still in the very early stages.18

New Approaches and Applications

Increasingly vaccines are used not only to prevent acute diseases, but also to cure chronic infectious diseases and non-infectious diseases such as cancer. Moreover, many diseases which were traditionally thought to be caused by genetic and environmental factors, have been recently associated with underlying bacterial or viral infections that either initiate or exacerbate the conditions. Examples of such diseases are Chagas disease (caused by Trypanosoma cruzi), cervical cancer (human papilloma virus), and peptic ulcer (Helicobacter pylori). More and more evidence for the role of bacteria in other complex human diseases is gradually coming to surface. Likewise, acne vulgaris, a skin inflammatory condition that was traditionally attributed mostly to genetics and hormonal influences, has been recently re-defined as a complex chronic disease associated with bacterial infection.19-21

Just like Jenner, 200 years later, we are still faced with the challenge of developing safe, effective, and efficient ways of vaccinating the population. The road ahead is still long and winding, but never before has the field of vaccinology had so many tools at hand to tackle infectious diseases, as well as cancer, allergies, and age-related diseases.




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