Abeta Epitope DNA and Peptide Vaccination: Bridging the ‘Therapeutic Threshold’ for Cognitive Aging and Alzheimer’s Disease

Recent years have seen both substantial progress, and significant frustration, in the preferred regenerative engineering approach to the treatment and prevention of Alzheimer’s disease (AD), and the eventual regeneration of genuinely youthful cognitive function: immunotherapeutic clearance of beta-amyloid (AmyloSENS). Phase IIa trials in patients with advanced AD administered AD1792 (the first active beta-amyloid protein (Aß) vaccine(1-3)) reported substantial Aß plaque clearance in autopsied subjects who had mounted a robust immunological response to the vaccine. But this number constituted only ~19% of treated subjects, perhaps because of immunosescence (mean age = 74.9 ± 7.32 yr). But despite the thoroughgoing purging of Aß plaque, vaccination did not appear to have any effect on the associated  neurofibrillary pathology, which is widely believed to be most directly responsible for neuronal loss in AD. Moreover, both AN1792 and bapineuzumab (the first-out-of-the-gate passive vaccine)(4) have exhibited ambiguous clinical efficacy and significant safety hurdles in this setting.

These results appear to many to commend an earlier window of opportunity for intervention, before concomitant NFT damage and neuronal losses have made the removal of Aß alone insufficient for cognitive rescue. Early intervention might also maximize the therapeutic window for vaccination,  preventing the burden of Aß neuropathology from ever reaching levels so high as to interact with other forms of aging damage  in already frail and immunosenescent, impeding therapy and increasing vulnerability to adverse reactions.

But if safety concerns prove a significant hurdle to those Aß vaccines currently well-advanced into the clinical pipeline, they are unlikely to be tested in mild cognitive impairment (MCI), high-risk subjects, or “normal” cognitive aging. And early intervention — or early intervention alone — would not likely address some other limitations to existing approaches to Aß immunotherapy. As summarized by Dr. David Cribbs, a researcher with the the Institute for Molecular Medicine and the UC Irvine Institute for Brain Aging and Dementia,

Because at this point we cannot readily identify individuals in the preclinical or prodromal stages of AD pathogenesis, passive immunotherapy is reserved for those that already have clinical symptoms. … [But m]onoclonal antibodies [for passive Aß vaccination] are expensive and require repeated dosing to maintain therapeutic levels of the antibodies in the patient. However in the event of an adverse response to the passive therapy antibody delivery can simply be halted, which may provide a resolution to the problem.

On the other hand, active immunization has several significant advantages, including lower cost, and the typical immunization protocol should be much less intrusive … [I]n the advent of … adverse events the patients will have to receive immuno-suppressive therapy for an extended period until the anti-Aß antibody levels drop naturally as the effects of the vaccine decays over time.(5)

Dr. Cribbs and others at these Institutes have therefore been working for some time on a novel immunotherapeutic strategy that might escape these limits: DNA- and peptide-based epitope vaccines. Because mapping studies  have found that the of B- and T-cell epitopes of Aß42 peptides are distinct, vaccines can be designed in which the self-Aß T-helper (Th)-cell epitope can be replaced with a non-self Th-cell epitope, while retaining the self-B-cell epitope intact. In principle, therefore, vaccines based on targeting these epitopes should avoid a strongly Th1-based, inflammatory response in favor of a primarily anti-inflammatory, Th2-based resonse, potentially reducing or eliminating the risk of encephalitis observed in a minority of AN1792 patients.

Their initial work had been with peptide vaccines, using first the B-cell epitope from Aß(1-15) (the immunodominant region of Aß42), and later 2-3 copies of Aß(1-11), conjoined with pan-HLA DR-binding peptide (PADRE), a synthetic, foreign-promiscuous T-cell epitope that amplifies the specific immune response against a wide range of antigens to which it is combined. In the course of their work, the group has tested such vaccines with range of different adjuvants.(6-9) Some of these experiments have proven quite successful in animal models of AD, inducing strong humoral responses without engendering autoreactive T-cells as with AN1792, and clearing Aß plaque without mobilizing soluble Aß into the brain (a concern with some vaccines, as it appears to lead to to microhaemorrhages due to iatrogenic cerebral amyloid angiopathy (CAA)).(8) But any translation to human therapies was expected to be impeded by the technical difficulty and high cost of scaling up the synthesis of sufficient quantities of such complex epitope vaccines as would be required for clinical trials, let alone ultimate distribution of an approved therapy. Moreover, these vaccines required the coadministration of potent adjuvants to be effective, but when tested in AD model mice, the sole adjuvant  approved for human use (alum) failed to elicit a sufficiently robust anti-Aß response.(9)

To overcome these hurdles, the affiliated researchers have now advanced to work with DNA epitope vaccines,(10) which have features that are highly desirable for an early Aß immunotherapy strategy. Their efficacy against a wide range of pathogen, autoimmune, and tumor antigens has been demonstrated in preclinical models, and the antigens are produced endogenously, so that antigen processing can occur directly in target transfected cells. Moreover, ongoing expression of the antigen leads to the maintenance of an active immunological response, potentially removing much of the need for repeated dosing that clearly poses a challenge to monoclonal antibody vaccines and even conventional active vaccination.

The progress of other DNA vaccines from preclinical models and past Phase I/II testing in humans has been frustrated by consistently inadequate immunological response in human testing. But the Institute researchers have now presented results which suggest they may have eliminated this problem with inclusion of molecular adjuvants in their constructs:  proteins expressed from genes included in the vaccine plasmid and that that serve an adjuvant function. Some tested constructs proved unacceptable in preclinical testing: eg, an Aß DNA vaccine with a mannan molecular adjuvant was found effective as a preventive therapy against plaque deposition, but led to cerebral microhaemorrhages as have some other vaccines.(11) But vaccines with the same core construct in combination other molecular adjuvants, such as complement component C3d (12) and macrophage-derived chemokines (MDC),(13) exhibited all the favorable properties that made the earlier peptide-based epitope vaccines so promising, generating strong non-self Th responses and high anti-Aß antibody titers while avoiding the induction of autoreactive T-cells, without requiring the use of pharmacological adjuvants.

More impressive results were seen when one of these DNA vaccines was tested in  the triple-transgenic (3xTg-AD) Alzheimer’s model mice created in Dr. Frank LaFerla’s lab. These animals express disease-associated human mutant genes for amyloid precursor protein, presenillin-1, and tau, and are a closer model of the human AD disease process than more commonly-used lines: they accumulate soluble intraneuronal Aß, develop neuritic plaques and neurofibrillary pathology, and exhibit synaptic dysfunction including deficits in long-term potentiation associated with intraneuronal Aß rather than plaque burden.  A plasmid construct composed of 3 copies genes encoding Aß(1–11) fused in frame with PADRE and MDC (pMDC-3Aß(1–11)-PADRE) exhibits potent early neuropathological and behavioral efficacy when tested in 3-4 mo old 3xTg-AD mice, eliciting a robust humoral Aß response, and treatment greatly reduces  age-associated plaque accumulation and glial activation while protecting against behavioral deficits, without contributing to the burden of cerebral microhaemorrhage.(13)

To further increase the likelihood of human clinical translatability, these investigators have now moved into work with an enhanced vaccination protocol for the pMDC-3Aß(1–11)-PADRE vaccine. This involves so-called “heterologous immunization,” in which the immune system is first primed with the relatively mild pMDC-3Aß1–11-PADRE DNA epitope vaccine, and then followed up with a boost comprising its homologous recombinant protein. Previous work with heterologous immunization has found that such protocols increase humoral titer response while also improving the avidity of the resulting antibodies, possibly by activating different B-cell subsets.

When tested in wild-type mice, this heterologous vaccine protocol proved superior to control experiments with the 2 possible homologous protocols: DNA prime/DNA boost, or protein prime/protein boost. On each tested variable, the heterologous protocol demonstrated equal or better responses than either alternative: higher anti-Aß(1-11) titers, that remained in active circulation longer due to a slower decay rate; increased immunoglobulins Aß avidity, without alteration of their cell-mediated-inducing vs. humoral-inducing isotype pattern; and higher anti-PADRE T-cell proliferation. Importantly, these robust responses to the protein vaccine boost occurred irrespective of the strength of the antibody response to the DNA vaccine primer, promising good efficacy even in relatively immunosenescent subjects. And the anti-Aß antibodies bound not only to synthetic Aß, but to beta-amyloid plaques in human brain tissue — binding that was blocked in the presence of added “decoy” Aß42 peptide.(14)

The results are especially promising considering earlier results in the 3xTg-AD model, in which active immunization markedly inhibited the formation, and slowed the progression, of early tau pathology.(15-17) In this setting, such reductions continue to be important and even necessary to cognitive rescue in even late stages of disease.(16) This ability would be expected to substantially increase the benefit of a vaccine which could be used in recipients who are only in the early stages of brain aging or prodromal AD, forestalling the onset of significant neurofibrillary pathology and likely the associated neuronal death. The issue is especially important as, parallel to the rodent results,(17) autopsy studies suggest that late-stage intervention with AN1792  does not appear to remove neurofibrillary pathology in responders, despite marked clearance of Aß plaque.(18-20) 

If these results can indeed be translated to human use, the DNA Aß vaccine would prove an extremely valuable tool in the rejuvenation engineering armamentarium. An intervention that can safely be administered over long periods of time to persons who have not yet reached the threshold of significant neuropathological damage, clearing Aß pathology and subtantially retarding the accumulation of downstream aggregated tau pathology (and thus, evidence suggests, abrogate neuronal loss) in in late middle, the possible therapeutic dilemma Cribbs notes could be escaped, and the chances for rapid entry into “longevity escape velocity” (LEV)(21) maximized. Results to date do appear to justify Cribbs’ optimism:

Ultimately, we believe that the further refinement of our AD DNA epitope vaccines, possibly combined with a prime boost regime, will facilitate translation to human clinical trials in either very early AD, or preferably in preclinical stage individuals identified by validated AD biomarkers.(5)