Progress towards a vaccine for Huntington's disease
Authors: Ruth Luthi-Carter
Journal: Molecular Therapy (2003)

Huntington's disease (HD) is a hereditary neurodegenerative disorder characterized by motor, psychiatric, and cognitive impairments [1Young A.B. Huntingtin in health and disease.J. Clin. Invest. 2003; 111: 299-302Crossref PubMed Scopus (92) Google Scholar]. This disease is a member of a family of similar disorders (known as polyglutamine diseases) that result from a trinucleotide repeat expansion in a single gene that causes the corresponding protein to carry an abnormally long polyglutamine tract. These mutant polyglutamine tracts seem to confer neurotoxic properties upon the proteins carrying them, resulting in neurological deficits and neuronal cell death. Sadly, there are currently no effective treatments for Huntington's disease or any of the other diseases of this class, which are ultimately fatal. The first report of successful DNA immunization against a polyglutamine disease protein appears in this issue of Molecular Therapy [2Miller T.W. et al.DNA Vaccination against mutant huntington ameliorates the HDR6/2 diabetic phenotype.Mol. Ther. 2003; (this issue)Google Scholar]. This report describes therapeutic vaccination against huntingtin, the protein that causes HD. Previous studies, including one from this same group, had shown that intracellular expression of antibody fragments specific to huntingtin protein could inhibit the aggregation [3Lecerf J.-M. et al.Human single-chain Fv intrabodies counteract in situ huningtin aggregation in cellular models of Huntington's disease.Proc. Natl. Acad. Sci. USA. 2001; 98: 4764-4769Crossref PubMed Scopus (184) Google Scholar, 4Khoshnan A. et al.Effects of intracellular expression of anti-huntingtin antibodies of various specificities on mutant huntingtin aggregation and toxicity.Proc. Natl. Acad. Sci. USA. 2002; 99: 1002-1007Crossref PubMed Scopus (132) Google Scholar] and toxicity [4Khoshnan A. et al.Effects of intracellular expression of anti-huntingtin antibodies of various specificities on mutant huntingtin aggregation and toxicity.Proc. Natl. Acad. Sci. USA. 2002; 99: 1002-1007Crossref PubMed Scopus (132) Google Scholar] of mutant huntingtin. The present study extends the previous data on this type of therapy in several important ways. First, the antigen was introduced into the system as a cDNA produced in bacteria, a process that both decreases the need for repeated immunization and increases the efficacy of the vaccine. Second, immunization against an intracellular neuronal target is a new foray into the treatment of neurodegenerative disease. Previous studies had tested this strategy for efficacy in animal models of Alzheimer's disease, stroke, and spinal cord injury, but in these cases, the target protein was either extracellular or situated on the plasma membrane. Third, the study by Miller et al. [2Miller T.W. et al.DNA Vaccination against mutant huntington ameliorates the HDR6/2 diabetic phenotype.Mol. Ther. 2003; (this issue)Google Scholar] used a peripheral marker of disease (diabetes mellitus) to follow the effect of immunotherapy in HD mice. This endpoint is relevant to human disease to the extent that HD patients exhibit cachexia and have an increased incidence of diabetes. Finally, the developmental effects of anti-huntingtin vaccination were explored, and no adverse effects of vaccination were observed in the offspring of immunoresponsive females. The issue of whether antibodies specific to huntingtin or to polyglutamine would interfere with development is important in light of previous evidence that huntingtin plays a critical role in the development of multiple organ systems [1Young A.B. Huntingtin in health and disease.J. Clin. Invest. 2003; 111: 299-302Crossref PubMed Scopus (92) Google Scholar]. Miller et al. [2Miller T.W. et al.DNA Vaccination against mutant huntington ameliorates the HDR6/2 diabetic phenotype.Mol. Ther. 2003; (this issue)Google Scholar] found that vaccination of HD R6/2 mice (which are transgenic for exon 1 of the human gene encoding huntingtin) with a plasmid encoding a fusion protein composed of the first 17 amino acids of huntingtin plus 103 glutamines plus green fluorescent protein (GFP) resulted in the selective amelioration of diabetes and pancreatic insulin immunoreactivity in mice showing an immune response, as detected by the presence of serum antibodies (Fig. 1). The presence of antibodies specific to huntingtin was not in itself sufficient for the response, however, because immunization with a 17-amino acid huntingtin peptide, which resulted in higher antibody titers, did not produce the same effect. A component of the protection afforded by plasmid immunization appears to be an unexpected benefical effect of unmethylated CpG DNA that is independent of an immune response to the disease protein. This presumably occurs by stimulation of cells of the innate immune system through binding to pattern recognition receptors [5Krieg A.M. CpG motifs in bacterial DNA and their immune effects.Annu. Rev. Immunol. 2002; 20: 709-760Crossref PubMed Scopus (2239) Google Scholar] The attempt to develop a vaccine for Huntington's disease is part of a larger effort to explore vaccine treatments for a variety of neurological conditions [6Weiner H.L. Selkoe D.J. Inflammation and therapeutic vaccination in CNS diseases.Nature. 2002; 420: 879-884Crossref PubMed Scopus (221) Google Scholar]. In the cases of HD and Alzheimer's disease, a vaccine might be expected to prevent or reverse the long-term accumulation of a toxic protein species that is responsible for disease. In the latter case, several mechanisms have been proposed to account for pro-tection through vaccination against the toxic peptide amyloid-p (Ap) [6Weiner H.L. Selkoe D.J. Inflammation and therapeutic vaccination in CNS diseases.Nature. 2002; 420: 879-884Crossref PubMed Scopus (221) Google Scholar, 7Shenk D. Amyloid-β immunotherapy for Alzheimer's disease: the end of the beginning.Nat. Rev. Neurosci. 2002; 3: 825-828Crossref Google Scholar]. One proposed mechanism involves an antibody-mediated change in the compartmentalization of the toxic species, resulting in a lower concentration in the brain (known as the "peripheral sink" hypothesis). Another way that antibodies may provide neuroprotection against Ap is by inhibiting the toxic activity of the peptide, either in its soluble or aggregated form. A third potential mechanism of anti-Ap-mediated neuronal rescue is amyloid clearance by "Fc-mediated phagocytosis", whereby antibody binding to extracellular protein deposits recruits monocytes and/or microglia to engulf and digest them. The potential mechanisms for immune-mediated clearance of an intracellular protein such as huntingtin from living neurons are much less clear. A previously described route for antibody-mediated clearance of undesired neuronal proteins from intracellular stores appears to require at least a limited presence of the target antigen on the neuronal cell surface [8Ubol S. et al.Roles of immunoglobulin valency and the heavy-chain constant domain in antibody-mediated downregulation of Sindbis virus replication in persistently infected neurons.J. Virol. 1995; 69: 1990-1993PubMed Google Scholar]. Here, Miller et al. [2Miller T.W. et al.DNA Vaccination against mutant huntington ameliorates the HDR6/2 diabetic phenotype.Mol. Ther. 2003; (this issue)Google Scholar] propose a THl-biased immune response as the mediator of their antigen-specific effect in HD mice on the basis of previous studies of plasmid vaccination and the absence of a similarly protective response to huntingtin peptide immunization in their study. This type of response would not necessarily yield direct clearance of the disease protein, but might instead work through the secretion of beneficial factors at sites of huntingtin accumulation. Vaccination against Ap peptide for the treatment of Alzheimer's disease has shown considerable promise in preclinical studies. Both active and passive immunization strategies have shown benefit in mouse models of Alzheimer's, in terms of both reversing Ap accumulation and improving clinical measures [6Weiner H.L. Selkoe D.J. Inflammation and therapeutic vaccination in CNS diseases.Nature. 2002; 420: 879-884Crossref PubMed Scopus (221) Google Scholar, 7Shenk D. Amyloid-β immunotherapy for Alzheimer's disease: the end of the beginning.Nat. Rev. Neurosci. 2002; 3: 825-828Crossref Google Scholar]. Efficacy data from the first human clinical trial of active Ap immunization are still being analyzed, so it is not yet known whether this strategy will be efficacious in humans. Although Miller and colleagues observed improvement in pancreatic function with their preclinical HD vaccine, they did not see an overt change in the neurological status of their immunoresponsive mice. Therefore, the effect of immunization appears to be limited to the periphery in this model, and further experiments will be needed to address whether or not a neurological benefit can be achieved. Generating antibodies against particular epitopes of huntingtin or using a different type of vaccine might be part of a paradigm required to achieve neurological benefits [3Lecerf J.-M. et al.Human single-chain Fv intrabodies counteract in situ huningtin aggregation in cellular models of Huntington's disease.Proc. Natl. Acad. Sci. USA. 2001; 98: 4764-4769Crossref PubMed Scopus (184) Google Scholar, 4Khoshnan A. et al.Effects of intracellular expression of anti-huntingtin antibodies of various specificities on mutant huntingtin aggregation and toxicity.Proc. Natl. Acad. Sci. USA. 2002; 99: 1002-1007Crossref PubMed Scopus (132) Google Scholar, 6Weiner H.L. Selkoe D.J. Inflammation and therapeutic vaccination in CNS diseases.Nature. 2002; 420: 879-884Crossref PubMed Scopus (221) Google Scholar, 7Shenk D. Amyloid-β immunotherapy for Alzheimer's disease: the end of the beginning.Nat. Rev. Neurosci. 2002; 3: 825-828Crossref Google Scholar]. In addition to possible limitations of an HD vaccine's effectiveness, the possibility for serious side effects must be considered. Studies in Alzheimer's disease reinforce this caution. A phase Ila clinical trial of an active Ap42 vaccine was halted last year because of serious side effects in 15 patients. Of those who received immunizations, 5% developed encephalitis [6]. The recent analysis of one brain from a treated patient who exhibited this adverse response showed extensive infiltration of CD8-/CD4+ T lymphocytes into the leptomeninges [9Nicoll J.A.R. et al.Neuropathology of human Alzheimer disease after immunization with amyloid-β peptide: a case report.Nat. Med. 2003; 9 (advance online publication)https://doi.org/10.1038/nm840Crossref PubMed Scopus (1300) Google Scholar]. It is believed that pro-inflammatory T-cell effects are a major cause of adverse responses to immunization, and such effects have also been observed in response to multiple sclerosis vaccines [6Weiner H.L. Selkoe D.J. Inflammation and therapeutic vaccination in CNS diseases.Nature. 2002; 420: 879-884Crossref PubMed Scopus (221) Google Scholar]. The brain of the immunized Alzheimer's patient also showed infiltration of cerebral white matter by macrophages. Given these serious side effects, we must remain guarded in our optimism that vaccines against endogenous neurodegenerative disease proteins can be administered safely. It is hoped that new generations of therapeutic vaccines may increase their efficacy while decreasing their potential for toxicity [6Weiner H.L. Selkoe D.J. Inflammation and therapeutic vaccination in CNS diseases.Nature. 2002; 420: 879-884Crossref PubMed Scopus (221) Google Scholar, 7Shenk D. Amyloid-β immunotherapy for Alzheimer's disease: the end of the beginning.Nat. Rev. Neurosci. 2002; 3: 825-828Crossref Google Scholar]. The next phases of clinical and preclinical trials should yield important information about whether a safe and effective HD vaccine is feasible. HD is a particularly compelling application for a therapeutic vaccine because its administration can be limited to individuals who are at risk for the disease as identified by genetic testing or positive family history, and because no other treatments are currently available. Although much remains to be learned about both the safety and effectiveness of this strategy, the results of the present study offer some hope for vaccination therapy in the treatment of HD.