Abstract
The third conference on immunopotentiators in modern vaccines was held in Montego Bay, Jamaica, and attracted delegates from all around the world. This meeting is devoted to the lively field of vaccine immunopotentiator systems, aimed at facilitation of better responses to vaccine antigens. The half-moon resort was a very congenial venue and proved ideal for interactive scientific discussions throughout the meeting. Selected highlights of the meeting will be discussed.
Novel vaccine antigen-delivery systems & immunostimulants
Vaccines have had a tremendous impact on global health and are expected to provide novel prophylactic, as well as therapeutic, treatments for infectious and non-microbial syndromes in the future. There is an obvious role for novel adjuvants in these products; aluminium-based adjuvants are the only widely used adjuvants in human vaccines, although they are not a suitable or efficacious option for many of the vaccine candidates in the pipeline. Many novel vaccines require adjuvants with broader efficacy, as well as the capacity to induce T-cell immunity. In addition, there is a general lack of understanding of many existing vaccine adjuvants, such as aluminium, as explained by Jim Brewer (University of Strathclyde, Glasgow, UK). Rational vaccine design-based concepts were generally advocated by most speakers (see later), including those involved in vaccine regulation, such as Baijesh Patel (Medicines and Healthcare products Regulatory Agency, UK), Danuta Herzyk (Merck, USA) and Charmaine Gittleson (CSL, Australia). Virgil Schijns (Intervet Schering-Plough, The Netherlands) gave an introductory overview of mechanistic concepts based on the two-signal model (as published in several reviews) and reviewed the trends in the vaccine immunopotentiator field, including the searches for combinations of signal 2 ligands (predominantly Toll-like receptor [TLR] agonists), combinations of signal 2 with signal 1 formulations and blocking of natural immune response inhibitors. Indeed, numerous examples were presented in detail during the meeting but they cannot all be covered here.
Mode of action of vaccine adjuvants
When compared with the first and second Immunopotentiators in Modern Vaccines (IMV) meetings, IMV 2008 had a greater focus on the mechanistic activity of vaccine immunopotentiators. Interestingly, several new ideas on most upstream adjuvant-induced events were presented.
Geert van den Bossche (Bill and Melinda Gates Foundation, WA, USA) proposed that most upstream events of adjuvant activity could be investigated through increased use of biophysical characterization tools. He advocated that lipid membranes and lipid rafts are the gatekeepers of adjuvant sensing and signaling. Furthermore, membrane perturbation may represent one of the first interactions between the vaccine (adjuvant) formulation and the host, and is likely to occur independently of specific receptor interactions. He proposed that changes in membrane integrity and lipid raft dynamics should be investigated in more detail and may provide clues for common features of adjuvant activity.
The importance of biophysical aspects of vaccine adjuvants was emphasized further by Latoya Braun (University of Colorado, CO, USA), who illustrated that model proteins show reduced thermal stability and altered structure in the presence of adjuvants. These interactions may explain why antigens that interact strongly with alum can be less immunogenic.
Martin Bachmann (Cytos, Switzerland) showed that Qb virus-like particles (VLPs) contain naturally entrapped RNA, which activates the immune system through TLR7, resulting in IgG2a isotype production. This response is B-cell intrinsic, as demonstrated in mice selectively deficient for TLR7 on B cells. Using the M2e influenza antigen, it was shown that vaccine-induced protection against PR-8 challenge requires IgG2a production and the presence of ssRNA in the VLP. Clinical trial data showed that the Qb-VLP–angiotensin II peptide vaccine reduced blood pressure, with an effective serum antibody half-life of 4 months. These data suggest that the vaccine response is reversible and controllable, probably requiring a booster shot every 3–6 months. Such a vaccine would obviate problems with drug compliance and provide more consistent treatment of hypertension.
Martijn Gebbink (Cross Beta Biosciences, The Netherlands) discussed the inherent immunogenicity of misfolded proteins, a feature exploited by incorporating β-structures of misfolded proteins into antigens. This approach provides intrinsic adjuvant activity, circumventing the need for additional immunopotentiators, with a good safety profile.
Luke O’Neill (Trinity College, Dublin, Ireland) discussed the role of innate immune receptors in signaling responses following adjuvant challenge. He highlighted the situation with DNA vaccines where, although TLR9 appears to be the obvious target, the adjuvant activity of the plasmid DNA appears to be dependent on the NALP3 inflammasome. The importance and influence of the cell surface versus endosomal location of TLR4 in controlling TIRAP-MyD88-induced TNF production versus TRIF/TRAM production of IFN-β, respectively, was also discussed.
Jim Brewer presented data on interactions between aluminium adjuvants and antigen-presenting cells using a variety of imaging approaches. These studies highlighted the limitations of studying dynamic biological systems as ‘snapshots’, and cellular movies from studies employing multiphoton laser scanning microscopy in vivo demonstrated dynamic differences in cell behavior during antigen challenge in the presence or absence of adjuvants.
Microbe-derived immunostimulants: prominent TLR9 agonists
A substantial number of presentations addressed TLR agonists as promising adjuvants for a variety of vaccine antigens.
Gary Ott (Dynavax, USA) showed that conjugates of allergens linked to an immunostimulatory sequence (a TLR9 agonist) reduced hay fever symptoms and lowered the use of antihistamine medication.
Karin Riedl (Intercell, Austria) presented IC31®, a two-component synthetic adjuvant that signals through TLR9 and augments humoral and cellular immune responses to seasonal influenza vaccines. Experiments performed in young adult mice showed increased hemagglutination inhibition titers and higher levels of IgG2a antibodies that were accompanied by the induction of IFN-γ-producing CD4+ T cells after a single vaccination with reduced doses of vaccine antigens; even 200 days after the single immunization. Importantly, similar effects were seen in aged mice, although these were most pronounced upon booster immunization.
Alain Vicari (Pfizer, USA) explained the positive effects of CpG 7909 when applied to an anthrax vaccine in humans. He also reported increases in seroprotective titers against hepatitis B in HIV patients. In addition, he presented data of a Melan-A (MART-1) vaccine that, when administered in montanide plus CpG 7909, evoked antigen-specific CD8+ T cells in all subjects, which was not observed in the group of control subjects vaccinated without the CpG component. Also, NY-ESO-1 evoked measurable CD4 and CD8 responses, even without culturing the cells for restimulation with antigen.
Ruth Ellis (NIH/NIAID, USA) described results from a malaria vaccine adjuvant (alum plus CpG 7909) combined with a new malaria antigen, apical membrane antigen (AMA)-1, and showed safety and in vitro growth inhibition of falciparum parasites.
Particles
Alex Shneider (Curelab, USA) described self-aggregating nanoparticles generated by adding segments of polyglutamine repeats (□36; originally discovered from PolyQ tails in Huntington disease) to antigens. Green fluorescent protein fused to long PolyQ domains given in a prime–boost regimen induced higher antibody responses and cytotoxic T-lymphocyte reactions.
Stephane Ascarateil (SEPPIC, France) explained basic concepts of different emulsion adjuvants. He emphasized the importance of a stable, reproducible particle size for vaccine efficacy and showed that T-connecter devices, for use in the clinic as a bedside formulation, need at least 100 cycles to reach the required particle size. The water-in-oil emulsions used in the clinic were able to evoke tumor antigen-specific T-cell responses.
Nikolai Petrovsky (Flinders University, Australia) demonstrated a polysaccharide particle adjuvant (VX 69 and VX 70), ‘Advax’, which evoked no inflammatory response (i.e., no cytokine nor neutrophil influx), when tested in humans. Pathway analysis studies have been performed but were not presented.
Asa Schiott (Nordic Vaccine A/S, Denmark) presented Posintro®, a small positively charged immunostimulating complex (ISCOM). Efficacy studies for comparison of activity and safety relative to normal ISCOMs have not been performed.
Derek O’Hagan (Novartis Vaccines, USA) described Fluad®, a Novartis influenza vaccine containing MF59 adjuvant, which showed broader anti-influenza responsiveness against heterologous influenza strains relative to nonadjuvanted, antigen-only vaccines. It also evoked IFN-γ/IL-2 T-cell responses and is well-tolerated in young children (6–36 months of age), apart from the usual transient swelling at the injection site.
Anja Seubert (Novartis, Siena, Italy) showed that the oil-in-water emulsion MF59 is a widely used, safe and effective adjuvant, yet its mechanism of action is poorly understood. She assessed the effects of MF59 on human immune cells and found that both induce secretion of chemokines, such as CCL2 (MCP-1), CCL3 (MIP-1α), CCL4 (MIP-1β) and CXCL8 (IL-8), all of which are involved in cell recruitment from blood into peripheral tissue. MF59 appears to act predominantly on macrophages and monocytes and, additionally, targets granulocytes.
So-called ‘stealth microparticles’ (multivalent, no cold-chain requirements and self-administrable) were presented by Laszlo Stipkovits (GalenBio Inc., CA, USA). Proof-of-concept data were presented for a Mycoplasma gallisepticum model in chickens, using biocompatible polysaccharide particles (1–5 µm), coated with a number (theoretically up to 20) of selected nonescape antigens (generated by a proprietary technology based on gene sequences) and pattern-recognition receptor ligands. The coupled ligands (these may also include antibodies) are stable at room temperature. Partial protective activity observed in the chicken model was interpreted to result from cell-mediated immunity.
Charmaine Gittleson (CSL Ltd, Australia) explained the ISCOMATRIX adjuvant system. Approximately 1400 subjects have been immunized at least once with ISCOMATRIX. Currently, 14 trials have been initiated (seven completed), with some oncology patients receiving up to 12 doses. ISCOMATRIX is not supposed to work through TLR recognition. No serious adverse events have been noted, except for the expected local swelling, pain and induration.
Safety
Luke O’Neill referred to adverse complications, diagnosed as Wegener’s granulomatosis, in one subject. As a result, the clinical trial stopped testing a vaccine against hepatitis (HEPLISAV™) containing a TLR9 agonist.
Another warning came from Nikolai Petrovsky who emphasized that we need safer adjuvants, instead of stronger ones or more options. Separation of reactogenicity from immune response stimulating activity is a holy grail for adjuvant research. He presented new polysaccharide vaccine adjuvants, which showed promise in that regard.
Adjuvant platform developments
The progress in developments of a variety of distinct immunopotentiator platforms were presented as studies in preclinical and clinical stage. Performance of vaccines was based on immunopotentiator platforms, such as the heat-labile enterotoxin from Escherichia coli patch (Gregory Glenn; IOMAI, USA), microimplants (Sarah Hook, University of Otago, New Zealand), an immunostimulant protein derived from the helmith, Onchocerca volvulus (Angus MacDonald, New York Blood Center, NY, USA), a C3d-antigen fusion (Franklin Toapanta, Pittsburgh University, USA), mycobacterial cell wall extracts, mycobacterial cell wall–DNA complexes – which act as multicomponent immunostimulants, developed for the treatment of bladder cancer – (Nigel Phillips, Bioniche, Canada) and TLR5 ligand–antigen fusions (Diane Stark; VaxInnate Corporation, USA). Mark Tomai (3M Drug Delivery Systems, USA) provided an update on the well-known resiquimod and imiquimod, small-molecule TLR7/8 agonists, as vaccine adjuvants. Ali Harandi (University of Gothenburg, Gothenburg, Sweden) showed that α-Gal-Cer was an effective adjuvant for the HSV-gD protein, when administered via the mucosal route; similar to CpG motifs but without local inflammatory responses. All of these novel platforms were initially discovered in mouse systems and, while in some cases they have gone on to demonstrate efficacy in other species, some murine studies have proven nonpredictive for humans or veterinary species.
Cutting-edge progress of these promising technologies and updates on the mature immunopotentiators will be the subject of the fourth IMV meeting, to be held in 2 years time in Portugal.
For future information please contact John Herriot at Meetings Management; [email protected].
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.