Mechanism of action.
The immunostimulatory effect of bacterial DNA is due to the presence of unmethylated CpG dinucleotides which are both rare
and methylated in vertebrate DNA.53–55 CpG's effect is mediated by endocytic TLR9 receptors56 expressed on B cells and plasmacytoid dendritic cells in humans, triggering the release of inflammatory cytokines57 and biasing responses towards Th1 immunity and induction of CTL.58 CPG 7909, developed by Coley Pharmaceuticals, has been tested in conjunction with an alum-adjuvanted Hepatitis B vaccine.
This vaccine resulted in faster achievement of protective antibody levels and higher overall titer. There was an indication
of enhanced CD8 CTL responses, but only in higher CpG dose groups.59 In Phase 2 cancer trials using a CpG adjuvanted Melan-A vaccine in melanoma patients, there was evidence of induction of
CD8 CTL's specific for Melan-A expressed by tumor cells, but little effect on outcome.60
Limitations of CpG.
In human trials of CpG adjuvants, adverse events included injection site reactions, flu-like symptoms, and headache, and
were all more frequent in CpG versus alum adjuvanted groups.59 This is due to TLR9 activating NK-kB, a major inducer of inflammatory cytokines such as TNF-alpha, which are largely responsible
for reactogenicity of adjuvants using TLR pathways.61–63 Overall, reactogenicity, toxicity, and safety remain a barrier to acceptance of CpG adjuvants for human prophylactic vaccines.
Additionally, TLR9 signalling has shown to play a critical role in experimental allergic encephalitis (EAE), a model of human
multiple sclerosis,64 and can even break tolerance and trigger EAE in otherwise healthy animals,65 raising concern that CpG adjuvants could induce or exacerbate multiple sclerosis or other autoimmune diseases in susceptible
individuals. Activation by CpG-DNA also has been shown to trigger and exacerbate systemic lupus erythematosus (SLE), with
TLR9 activation in genetically prone mice triggering lupus nephritis.66 CpG-DNA triggers lupus nephritis due to its potent immunostimulatory effects at multiple levels, including B-cell IL12p40
production, B-cell proliferation, double-stranded DNA autoantibody secretion, and dendritic cell IFN-alpha production.67
Mechanism of action.
QS21 is derived from Quil A, itself a collection of triterpenoid glycosides (saponins) derived from the bark of the South
American soap bark tree, Quillaja saponaria. QS21 induces Th1 cytokines and antibodies of the IgG2a isotype in mice, consistent with a Th1 bias.68–70 Saponins integrate into cell membranes through interaction with cholesterol, resulting in pores71 through which antigens enter. Subsequently, peptides from these antigens may be processed and presented via MHC class I,
stimulating a CD8 CTL response. Numerous clinical trials have been conducted using QS21 in cancer vaccines and infectious
disease, including HIV-1, influenza, herpes, malaria, and hepatitis B.72 The saponins have also been used in adjuvant formulations such as immunostimulatory complexes (ISCOMs) which will be discussed
Limitations of QS21.
Severe injection site pain is a major limiting factor in QS21 use. In addition to pain on injection and granulomas, toxicity
of QS21 includes severe hemolysis,3,6,69,73,74 making such adjuvants unsuitable for human prophylactic uses. This was highlighted in a recent trial of a QS21 adjuvanted
influenza vaccine in healthy young adults where vaccination site pain and postvaccination myalgias were far greater in the
QS21 group, and the QS21-containing vaccine had no advantage in terms of antibody response compared with the unadjuvanted
vaccine.75 In a trial of QS21 in a cancer vaccine, virtually all of the patients experienced inflammation and/or pruritis at the site
of injection attributed to the QS21 adjuvant.76 Other common side effects were fever (71%), fatigue (44%), flu-like symptoms (58%), chills (29%), myalgias (48%), and headache
(66%). These toxicities were thought by the investigators to be all due to QS21, given there was no correlation between vaccine
dose and toxicity.77