We previously discussed that interferon beta therapy is associated with upregulation of T regulatory cell (Treg) activity in patients with multiple sclerosis, and that this may be one of the mechanisms by which it mediates its therapeutic effect. We also discussed the point that current methods of measuring Treg activity are somewhat limited in that they do not measure the specific Treg that protect the body against the specific autoreactive T cells that are attacking the myelin, but measure activity of all the Tregs as a whole.
Here we are going to discuss a publication (Andersson et al. Impaired autoimmune T helper 17 cell responses following DNA vaccination against rat experimental autoimmune encephalomyelitis. PLoS One 2008;3(11):e3682) that seems to demonstrate, at least in the rat model, that protection from multiple sclerosis (well actually EAE) by DNA immunization with autoantigen is dependent on production of interferon beta by the immunized animal.
To induce EAE (experimental allergic encephalomyelitis) the investigators vaccinated the rats with peptide 91-108 of the myelin oligodendrocyte glycoprotein in Complete Freund’s Adjuvant. This stimulates an autoimmune reaction against the rat myelin, and rats develop a progressive disease that resembles multiple sclerosis.
When rats were administered a DNA vaccine encoding the same peptide that the animals were immunized with to induce EAE, the progress of the disease was potently inhibited. This is similar to what we discussed about in multiple sclerosis patients who were administered the DNA vaccine made by Bayhill Therapeutics against myelin basis protein. In that situation, specific inhibition of the autoimmune responses was also observed.
What is interesting about the current study is that experiments were performed to determine through what molecular mechanism the DNA vaccine was mediating protection against disease. It was found that T cells isolated from animals at the peak of disease had a much higher expression of the cytokine IL-17 when restimulated in vitro with the antigen myelin oligodendrocyte glycoprotein 91-108. Animals that were treated with the DNA vaccine had a lower amount of IL-17 production. Since IL-17 is associated with multiple sclerosis in both mice and human, it would make sense that a protective vaccine would reduce IL-17 production. As a quick aside, the anti-diabetes drug metformin, which inhibits EAE, also suppresses production of IL-17 by autoreactive T cells. Additionally, vitamin D has been demonstrated to reduce IL-17.
With the knowledge that the DNA vaccine seems to reduce ability of autoreactive T cells to produce the “bad” cytokine IL-17, the investigators next wanted to see what exactly is it that is responsible for this reduction in IL-17 producing cells. So the DNA microarray technique was used to analyze 6240 genes at once to randomly see expression of which genes goes and which goes down after DNA vaccination.
What was found was that the gene for interferon beta, as well as other genes that are associated with interferon beta were upregulated after immunization. We know that interferon beta is part of the current-day treatment of multiple sclerosis, but also that it has several systemic side effects. Therefore this study may be suggesting that DNA vaccination seems to be stimulating the body to make its own interferon beta. This would be a much more attractive approach as compared to what is currently being performed for two reasons. Firstly, the DNA vaccination encodes a specific antigen, therefore in addition to receiving interferon beta, the vaccination may offer the possibility of inducing tolerance to the autoantigen selectively. Secondly, the production of interferon beta stimulating by the DNA vaccine would hypothetically be released at smaller and more localized concentrations, thus not having to “flood” the body with interferon beta from the outside and hopefully reducing the side effects.
But here is the catch…how do we know that the interferon beta being produced as a result of the DNA vaccination is actually responsible for suppression of disease progression? In order to address this, the investigators created a DNA plasmid for vaccination that not only induced expression of the myelin oligodendrocyte glycoprotein peptide, but also short interferon RNA to target gene expression of interferon beta.
In other words, the investigators repeated their experiments using two sets of DNA plasmids, one that has previously been shown to protect from disease by inducing expression of the myelin oligodendrocyte glycoprotein peptide, and another one in which the peptide is expressed but interferon beta gene is silenced (actually they used more controls, but these are the main ones to make the point). It was found that suppression of the interferon beta gene expression resulted in loss of the therapeutic effect ! Also, recall response to peptide in terms of IL-17 secretion was augmented 20-fold if interferon beta was silenced.
What does all this mean?
Firstly, why would interferon beta be made by the simple procedure of DNA vaccination? After all, DNA vaccination is performed as an alternative delivery of antigen. Instead of injecting protein or peptides of the protein with adjuvant (which in the case of myelin oligodendrocyte glycoprotein results in disease) one injects DNA encoding the protein or peptide so that the cell generates the protein or peptide internally. So if its the same protein or peptide being made, then why would one cause disease (direct injection of protein or peptide) but the other (DNA vaccination of the same thing) cause protection from disease and interferon beta production?
The authors state that one likely explanation is that the DNA vaccine backbone (the other parts of the DNA plasmid that do not encode the protein or peptide) has CpG motifs. CpG motifs are parts of DNA that are recognized by human cells as being foreign and induce production of interferons, including interferon beta. So it may be the interact effect of the CpG motifs that is responsible for protection. In fact, the authors cite several papers, such as (Lobell et al. J Immunol 2003 Feb 15;170(4):1806-13) that demonstrate removal of CpG motifs from plasmid backbone used to “immunize” animals from autoimmunity, results in loss of effect.
In conclusion, the study discussed seems to provide a very interesting concept…that DNA vaccination can concurrently induce a cytokine that is therapeutic for multiple sclerosis, while at the same time antigen-specifically modulating an immune response. It will be very interesting to see how the trials of Bayhill Therapeutics progress using a similar antigen-specific DNA-”vaccine” based approach.