Several other adjuvants are widely used experimentally in animals but are not approved for use in humans. Many of these are sterile constituents of bacteria, particularly of their cell walls. Freund’s complete adjuvant is an oil-in-water emulsion containing killed mycobacteria. The peptidoglycan muramyl dipeptide and the glycolipid trehalose dimycolate (TDM) found in mycobacterial cell walls contain much of the adjuvant activity of the whole killed organism. Other bacterial adjuvants include killed B. pertussis, bacterial polysaccharides, bacterial heat-shock proteins, and bacterial DNA. Many of these adjuvants cause quite marked inflammation and so are not suitable for use in vaccines for humans.
Many adjuvants seem to work by triggering the innate viral and bacterial sensor pathways in APCs, via TLRs and proteins of the NOD-like receptor family such as NLRP3 (see Chapter 3), and thereby activating them to initiate an adaptive immune response. The TLR-4 agonist lipopolysaccharide (LPS) has adjuvant effects, but its use is limited by its toxicity. Small amounts of injected LPS can induce a state of shock and systemic inflammation that mimics Gram-negative sepsis, raising the question whether its adjuvant effect can be separated from its toxic effects. Monophosphoryl lipid A, an LPS derivative and TLR-4 ligand, partly achieves this, retaining adjuvant effects but being associated with much lower toxicity than LPS. Both unmethylated CpG DNA, which activates TLR-9, and imiquimod, a small-molecule drug that acts as a TLR-7 agonist, can provide adjuvant activity experimentally, but neither is approved as an adjuvant in human vaccines.
16-30 Protective immunity can be induced by DNA-based vaccination.
Surprisingly, when bacterial plasmids were used to express proteins in vivo for gene therapy, some were found to stimulate an immune response. Later, it was found that DNA encoding a viral immunogen, when injected intramuscularly in mice, induced antibody responses and cytotoxic T cells that could protect against subsequent infection from the live virus. This response does not seem to damage the muscle tissue, is safe and effective, and, because it uses only a single microbial gene or a stretch of DNA encoding sets of antigenic peptides, does not carry the risk of active infection. This procedure is termed DNA vaccination, and can be carried out in various ways. In one, DNA coated onto minute metal particles can be administered by a gene gun, so that particles penetrate the skin and potentially some underlying muscle, but other approaches, such as electroporation, are also possible. Because of DNA’s stability, DNA vaccination is suitable for mass immunization. One problem with DNA-based vaccines, however, is that they are comparatively weak. Mixing in plasmids that encode cytokines such as IL-12, IL-23, or GM-CSF makes immunization with genes encoding protective antigens much more effective. In DNA vaccination, the antigen is produced by cells that are directly transfected, such as skin or muscle, but CD8 T-cell activation requires cross-presentation of the antigen by dendritic cells. Current approaches are identifying how best to transfect DNA into these dendritic cell populations. DNA vaccines are being tested in human trials for malaria, influenza, HIV infection, and breast cancer.
16-31 Vaccination and checkpoint blockade may be useful in controlling existing chronic infections.
There are many chronic diseases in which infection persists because of a failure of the immune system to eliminate disease. Such infections can be divided into two groups: those in which there is an obvious immune response that fails to eliminate the organism, and those that seem to be invisible to the immune system and evoke a barely detectable immune response.