Modification of Antigen Impacts on Memory Quality after Adenovirus Vaccination.

The establishment of robust T cell memory is critical for the development of novel vaccines for infections and cancers. Classical memory generated by CD8(+)T cells is characterized by contracted populations homing to lymphoid organs. T cell memory inflation, as seen for example after CMV infection, is the maintenance of expanded, functional, tissue-associated effector memory cell pools. Such memory pools may also be induced after adenovirus vaccination, and we recently defined common transcriptional and phenotypic features of these populations in mice and humans. However, the rules that govern which epitopes drive memory inflation compared with classical memory are not fully defined, and thus it is not currently possible to direct this process. We used our adenoviral model of memory inflation to first investigate the role of the promoter and then the role of the epitope context in determining memory formation. Specifically, we tested the hypothesis that conventional memory could be converted to inflationary memory by simple presentation of the Ag in the form of minigene vectors. When epitopes from LacZ and murine CMV that normally induce classical memory responses were presented as minigenes, they induced clear memory inflation. These data demonstrate that, regardless of the transgene promoter, the polypeptide context of a CD8(+)T cell epitope may determine whether classical or inflating memory responses are induced. The ability to direct this process by the use of minigenes is relevant to the design of vaccines and understanding of immune responses to pathogens.

Differential immunogenicity between HAdV-5 and chimpanzee adenovirus vector ChAdOx1 is independent of fiber and penton RGD loop sequences in mice.

Replication defective adenoviruses are promising vectors for the delivery of vaccine antigens. However, the potential of a vector to elicit transgene-specific adaptive immune responses is largely dependent on the viral serotype used. HAdV-5 (Human adenovirus C) vectors are more immunogenic than chimpanzee adenovirus vectors from species Human adenovirus E (ChAdOx1 and AdC68) in mice, though the mechanisms responsible for these differences in immunogenicity remain poorly understood. In this study, superior immunogenicity was associated with markedly higher levels of transgene expression in vivo, particularly within draining lymph nodes. To investigate the viral factors contributing to these phenotypes, we generated recombinant ChAdOx1 vectors by exchanging components of the viral capsid reported to be principally involved in cell entry with the corresponding sequences from HAdV-5. Remarkably, pseudotyping with the HAdV-5 fiber and/or penton RGD loop had little to no effect on in vivo transgene expression or transgene-specific adaptive immune responses despite considerable species-specific sequence heterogeneity in these components. Our results suggest that mechanisms governing vector transduction after intramuscular administration in mice may be different from those described in vitro.

The relative magnitude of transgene-specific adaptive immune responses induced by human and chimpanzee adenovirus vectors differs between laboratory animals and a target species.

Adenovirus vaccine vectors generated from new viral serotypes are routinely screened in pre-clinical laboratory animal models to identify the most immunogenic and efficacious candidates for further evaluation in clinical human and veterinary settings. Here, we show that studies in a laboratory species do not necessarily predict the hierarchy of vector performance in other mammals. In mice, after intramuscular immunization, HAdV-5 (Human adenovirus C) based vectors elicited cellular and humoral adaptive responses of higher magnitudes compared to the chimpanzee adenovirus vectors ChAdOx1 and AdC68 from species Human adenovirus E. After HAdV-5 vaccination, transgene specific IFN-γ(+) CD8(+) T cell responses reached peak magnitude later than after ChAdOx1 and AdC68 vaccination, and exhibited a slower contraction to a memory phenotype. In cattle, cellular and humoral immune responses were at least equivalent, if not higher, in magnitude after ChAdOx1 vaccination compared to HAdV-5. Though we have not tested protective efficacy in a disease model, these findings have important implications for the selection of candidate vectors for further evaluation. We propose that vaccines based on ChAdOx1 or other Human adenovirus E serotypes could be at least as immunogenic as current licensed bovine vaccines based on HAdV-5.

4-1BBL enhances CD8+ T cell responses induced by vectored vaccines in mice but fails to improve immunogenicity in rhesus macaques.

T cells play a central role in the immune response to many of the world's major infectious diseases. In this study we investigated the tumour necrosis factor receptor superfamily costimulatory molecule, 4-1BBL (CD137L, TNFSF9), for its ability to increase T cell immunogenicity induced by a variety of recombinant vectored vaccines. To efficiently test this hypothesis, we assessed a number of promoters and developed a stable bi-cistronic vector expressing both the antigen and adjuvant. Co-expression of 4-1BBL, together with our model antigen TIP, was shown to increase the frequency of murine antigen-specific IFN-γ secreting CD8(+) T cells in three vector platforms examined. Enhancement of the response was not limited by co-expression with the antigen, as an increase in CD8(+) immunogenicity was also observed by co-administration of two vectors each expressing only the antigen or adjuvant. However, when this regimen was tested in non-human primates using a clinical malaria vaccine candidate, no adjuvant effect of 4-1BBL was observed limiting its potential use as a single adjuvant for translation into a clinical vaccine.

Enhanced vaccine-induced CD8+ T cell responses to malaria antigen ME-TRAP by fusion to MHC class ii invariant chain.

The orthodox role of the invariant chain (CD74; Ii) is in antigen presentation to CD4+ T cells, but enhanced CD8+ T cells responses have been reported after vaccination with vectored viral vaccines encoding a fusion of Ii to the antigen of interest. In this study we assessed whether fusion of the malarial antigen, ME-TRAP, to Ii could increase the vaccine-induced CD8+ T cell response. Following single or heterologous prime-boost vaccination of mice with a recombinant chimpanzee adenovirus vector, ChAd63, or recombinant modified vaccinia virus Ankara (MVA), higher frequencies of antigen-specific CD4+ and CD8+ T cells were observed, with the largest increases observed following a ChAd63-MVA heterologous prime-boost regimen. Studies in non-human primates confirmed the ability of Ii-fusion to augment the T cell response, where a 4-fold increase was maintained up to 11 weeks after the MVA boost. Of the numerous different approaches explored to increase vectored vaccine induced immunogenicity over the years, fusion to the invariant chain showed a consistent enhancement in CD8+ T cell responses across different animal species and may therefore find application in the development of vaccines against human malaria and other diseases where high levels of cell-mediated immunity are required.

Immunogenicity and efficacy of a chimpanzee adenovirus-vectored Rift Valley fever vaccine in mice.

BACKGROUND: Rift Valley Fever (RVF) is a viral zoonosis that historically affects livestock production and human health in sub-Saharan Africa, though epizootics have also occurred in the Arabian Peninsula. Whilst an effective live-attenuated vaccine is available for livestock, there is currently no licensed human RVF vaccine. Replication-deficient chimpanzee adenovirus (ChAd) vectors are an ideal platform for development of a human RVF vaccine, given the low prevalence of neutralizing antibodies against them in the human population, and their excellent safety and immunogenicity profile in human clinical trials of vaccines against a wide range of pathogens. METHODS: Here, in BALB/c mice, we evaluated the immunogenicity and efficacy of a replication-deficient chimpanzee adenovirus vector, ChAdOx1, encoding the RVF virus envelope glycoproteins, Gn and Gc, which are targets of virus neutralizing antibodies. The ChAdOx1-GnGc vaccine was assessed in comparison to a replication-deficient human adenovirus type 5 vector encoding Gn and Gc (HAdV5-GnGc), a strategy previously shown to confer protective immunity against RVF in mice. RESULTS: A single immunization with either of the vaccines conferred protection against RVF virus challenge eight weeks post-immunization. Both vaccines elicited RVF virus neutralizing antibody and a robust CD8+ T cell response. CONCLUSIONS: Together the results support further development of RVF vaccines based on replication-deficient adenovirus vectors, with ChAdOx1-GnGc being a potential candidate for use in future human clinical trials.

Utilizing poxviral vectored vaccines for antibody induction-progress and prospects.

Over the last decade, poxviral vectors emerged as a mainstay approach for the induction of T cell-mediated immunity by vaccination, and their suitability for human use has led to widespread clinical testing of candidate vectors against infectious intracellular pathogens and cancer. In contrast, poxviruses have been widely perceived in the vaccine field as a poor choice of vector for the induction of humoral immunity. However, a growing body of data, from both animal models and recent clinical trials, now suggests that these vectors can be successfully utilized to prime and boost B cells and effective antibody responses. Significant progress has been made in the context of heterologous prime-boost immunization regimes, whereby poxviruses are able to boost responses primed by other vectors, leading to the induction of high-titre antigen-specific antibody responses. In other cases, poxviral vectors have been shown to stimulate humoral immunity against both themselves and encoded transgenes, in particular viral surface proteins such as influenza haemagglutinin. In the veterinary field, recombinant poxviral vectors have made a significant impact with numerous vectors licensed for use against a variety of animal viruses. On-going studies continue to explore the potential of poxviral vectors to modulate qualitative aspects of the humoral response, as well as their amenability to adjuvantation seeking to improve quantitative antibody immunogenicity. Nevertheless, the underlying mechanisms of B cell induction by recombinant poxviruses remain poorly defined, and further work is necessary to help guide the rational optimization of future poxviral vaccine candidates aiming to induce antibodies.

Recombinant MVA vaccines: dispelling the myths.

Diseases such as HIV/AIDS, tuberculosis, malaria and cancer are prime targets for prophylactic or therapeutic vaccination, but have proven partially or wholly resistant to traditional approaches to vaccine design. New vaccines based on recombinant viral vectors expressing a foreign antigen are under intense development for these and other indications. One of the most advanced and most promising vectors is the attenuated, non-replicating poxvirus MVA (modified vaccinia virus Ankara), a safer derivative of the uniquely successful smallpox vaccine. Despite the ability of recombinant MVA to induce potent humoral and cellular immune responses against transgenic antigen in humans, especially when used as the latter element of a heterologous prime-boost regimen, doubts are occasionally expressed about the ultimate feasibility of this approach. In this review, five common misconceptions over recombinant MVA are discussed, and evidence is cited to show that recombinant MVA is at least sufficiently genetically stable, manufacturable, safe, and immunogenic (even in the face of prior anti-vector immunity) to warrant reasonable hope over the feasibility of large-scale deployment, should useful levels of protection against target pathogens, or therapeutic benefit for cancer, be demonstrated in efficacy trials.

T cell responses induced by adenoviral vectored vaccines can be adjuvanted by fusion of antigen to the oligomerization domain of C4b-binding protein.

Viral vectored vaccines have been shown to induce both T cell and antibody responses in animals and humans. However, the induction of even higher level T cell responses may be crucial in achieving vaccine efficacy against difficult disease targets, especially in humans. Here we investigate the oligomerization domain of the α-chain of C4b-binding protein (C4 bp) as a candidate T cell "molecular adjuvant" when fused to malaria antigens expressed by human adenovirus serotype 5 (AdHu5) vectored vaccines in BALB/c mice. We demonstrate that i) C-terminal fusion of an oligomerization domain can enhance the quantity of antigen-specific CD4(+) and CD8(+) T cell responses induced in mice after only a single immunization of recombinant AdHu5, and that the T cells maintain similar functional cytokine profiles; ii) an adjuvant effect is observed for AdHu5 vectors expressing either the 42 kDa C-terminal domain of Plasmodium yoelii merozoite surface protein 1 (PyMSP1(42)) or the 83 kDa ectodomain of P. falciparum strain 3D7 apical membrane antigen 1 (PfAMA1), but not a candidate 128kDa P. falciparum MSP1 biallelic fusion antigen; iii) following two homologous immunizations of AdHu5 vaccines, antigen-specific T cell responses are further enhanced, however, in both BALB/c mice and New Zealand White rabbits no enhancement of functional antibody responses is observed; and iv) that the T cell adjuvant activity of C4 bp is not dependent on a functional Fc-receptor γ-chain in the host, but is associated with the oligomerization of small (<80 kDa) antigens expressed by recombinant AdHu5. The oligomerization domain of C4 bp can thus adjuvant T cell responses induced by AdHu5 vectors against selected antigens and its clinical utility as well as mechanism of action warrant further investigation.

A novel chimpanzee adenovirus vector with low human seroprevalence: improved systems for vector derivation and comparative immunogenicity.

Recombinant adenoviruses are among the most promising tools for vaccine antigen delivery. Recently, the development of new vectors has focused on serotypes to which the human population is less exposed in order to circumvent pre-existing anti vector immunity. This study describes the derivation of a new vaccine vector based on a chimpanzee adenovirus, Y25, together with a comparative assessment of its potential to elicit transgene product specific immune responses in mice. The vector was constructed in a bacterial artificial chromosome to facilitate genetic manipulation of genomic clones. In order to conduct a fair head-to-head immunological comparison of multiple adenoviral vectors, we optimised a method for accurate determination of infectious titre, since this parameter exhibits profound natural variability and can confound immunogenicity studies when doses are based on viral particle estimation. Cellular immunogenicity of recombinant E1 E3-deleted vector ChAdY25 was comparable to that of other species E derived chimpanzee adenovirus vectors including ChAd63, the first simian adenovirus vector to enter clinical trials in humans. Furthermore, the prevalence of virus neutralizing antibodies (titre >1:200) against ChAdY25 in serum samples collected from two human populations in the UK and Gambia was particularly low compared to published data for other chimpanzee adenoviruses. These findings support the continued development of new chimpanzee adenovirus vectors, including ChAdY25, for clinical use.

Expression and cellular immunogenicity of a transgenic antigen driven by endogenous poxviral early promoters at their authentic loci in MVA.

CD8(+) T cell responses to vaccinia virus are directed almost exclusively against early gene products. The attenuated strain modified vaccinia virus Ankara (MVA) is under evaluation in clinical trials of new vaccines designed to elicit cellular immune responses against pathogens including Plasmodium spp., M. tuberculosis and HIV-1. All of these recombinant MVAs (rMVA) utilize the well-established method of linking the gene of interest to a cloned poxviral promoter prior to insertion into the viral genome at a suitable locus by homologous recombination in infected cells. Using BAC recombineering, we show that potent early promoters that drive expression of non-functional or non-essential MVA open reading frames (ORFs) can be harnessed for immunogenic expression of recombinant antigen. Precise replacement of the MVA orthologs of C11R, F11L, A44L and B8R with a model antigen positioned to use the same translation initiation codon allowed early transgene expression similar to or slightly greater than that achieved by the commonly-used p7.5 or short synthetic promoters. The frequency of antigen-specific CD8(+) T cells induced in mice by single shot or adenovirus-prime, rMVA-boost vaccination were similarly equal or marginally enhanced using endogenous promoters at their authentic genomic loci compared to the traditional constructs. The enhancement in immunogenicity observed using the C11R or F11L promoters compared with p7.5 was similar to that obtained with the mH5 promoter compared with p7.5. Furthermore, the growth rates of the viruses were unimpaired and the insertions were genetically stable. Insertion of a transgenic ORF in place of a viral ORF by BAC recombineering can thus provide not only a potent promoter, but also, concomitantly, a suitable insertion site, potentially facilitating development of MVA vaccines expressing multiple recombinant antigens.

Mixed vector immunization with recombinant adenovirus and MVA can improve vaccine efficacy while decreasing antivector immunity.

Substantial protection can be provided against the pre-erythrocytic stages of malaria by vaccination first with an adenoviral and then with an modified vaccinia virus Ankara (MVA) poxviral vector encoding the same ME.TRAP transgene. We investigated whether the two vaccine components adenovirus (Ad) and MVA could be coinjected as a mixture to enhance protection against malaria. A single-shot mixture at specific ratios of Ad and MVA (Ad+MVA) enhanced CD8(+) T cell-dependant protection of mice against challenge with Plasmodium berghei. Moreover, the degree of protection could be enhanced after homologous boosting with the same Ad+MVA mixture to levels comparable with classic heterologous Ad prime-MVA boost regimes. The mixture increased transgene-specific responses while decreasing the CD8(+) T cell antivector immunity compared to each vector used alone, particularly against the MVA backbone. Mixed vector immunization led to increased early circulating interferon-γ (IFN-γ) response levels and altered transcriptional microarray profiles. Furthermore, we found that sequential immunizations with the Ad+MVA mixture led to consistent boosting of the transgene-specific CD8(+) response for up to three mixture immunizations, whereas each vector used alone elicited progressively lower responses. Our findings offer the possibility of simplifying the deployment of viral vectors as a single mixture product rather than in heterologous prime-boost regimens.

Preventing spontaneous genetic rearrangements in the transgene cassettes of adenovirus vectors.

First-generation, E1/E3-deleted adenoviral vectors with diverse transgenes are produced routinely in laboratories worldwide for development of novel prophylactics and therapies for a variety of applications, including candidate vaccines against important infectious diseases, such as HIV/AIDS, tuberculosis, and malaria. Here, we show, for two different transgenes (both encoding malarial antigens) inserted at the E1 locus, that rare viruses containing a transgene-inactivating mutation exhibit a selective growth advantage during propagation in E1-complementing HEK293 cells, such that they rapidly become the major or sole species in the viral population. For one of these transgenes, we demonstrate that viral yield and cytopathic effect are enhanced by repression of transgene expression in the producer cell line, using the tetracycline repressor system. In addition to these transgene-inactivating mutations, one of which occurred during propagation of the pre-viral genomic clone in bacteria, and the other after viral reconstitution in HEK293 cells, we describe two other types of mutation, a small deletion and a gross rearranging duplication, in one of the transgenes studied. These were of uncertain origin, and the effects on transgene expression and viral growth were not fully characterized. We demonstrate that, together with minor protocol modifications, repression of transgene expression in HEK293 cells during viral propagation enables production of a genetically stable chimpanzee adenovirus vector expressing a malarial antigen which had previously been impossible to derive. These results have important implications for basic and pre-clinical studies using adenoviral vectors and for derivation of adenoviral vector products destined for large-scale amplification during biomanufacture.

Transgene optimization, immunogenicity and in vitro efficacy of viral vectored vaccines expressing two alleles of Plasmodium falciparum AMA1.

BACKGROUND: Apical membrane antigen 1 (AMA1) is a leading candidate vaccine antigen against blood-stage malaria, although to date numerous clinical trials using mainly protein-in-adjuvant vaccines have shown limited success. Here we describe the pre-clinical development and optimization of recombinant human and simian adenoviral (AdHu5 and ChAd63) and orthopoxviral (MVA) vectors encoding transgene inserts for Plasmodium falciparum AMA1 (PfAMA1). METHODOLOGY/PRINCIPAL FINDINGS: AdHu5-MVA prime-boost vaccination in mice and rabbits using these vectors encoding the 3D7 allele of PfAMA1 induced cellular immune responses as well as high-titer antibodies that showed growth inhibitory activity (GIA) against the homologous but not heterologous parasite strains. In an effort to overcome the issues of PfAMA1 antigenic polymorphism and pre-existing immunity to AdHu5, a simian adenoviral (ChAd63) vector and MVA encoding two alleles of PfAMA1 were developed. This antigen, composed of the 3D7 and FVO alleles of PfAMA1 fused in tandem and with expression driven by a single promoter, was optimized for antigen secretion and transmembrane expression. These bi-allelic PfAMA1 vaccines, when administered to mice and rabbits, demonstrated comparable immunogenicity to the mono-allelic vaccines and purified serum IgG now showed GIA against the two divergent strains of P. falciparum encoded in the vaccine. CD8(+) and CD4(+) T cell responses against epitopes that were both common and unique to the two alleles of PfAMA1 were also measured in mice. CONCLUSIONS/SIGNIFICANCE: Optimized transgene inserts encoding two divergent alleles of the same antigen can be successfully inserted into adeno- and pox-viral vaccine vectors. Adenovirus-MVA immunization leads to the induction of T cell responses common to both alleles, as well as functional antibody responses that are effective against both of the encoded strains of P. falciparum in vitro. These data support the further clinical development of these vaccine candidates in Phase I/IIa clinical trials.

Viral vectors as vaccine platforms: deployment in sight.

A little more than a decade after the explosion of research into recombinant live-attenuated or replication-deficient viruses as vaccine platforms, many viral vector-based vaccines have been licensed for animals. Progress has been slower for humans but 2011 will see the licensure of the first viral-vectored vaccine for humans, against Japanese Encephalitis. In addition a vaccine with a viral-vectored component showed efficacy against HIV infection in humans. Viral-based vaccines have an excellent safety profile but must deal with the potential problem of pre-existing anti-vector immunity. Recent successes reflect diverse improvements such as development of new adenovirus serotypes and better prime-boost approaches, suggesting that many viral vectors are approaching their final years as vaccine 'candidates' rather than vaccines.

Long-term thermostabilization of live poxviral and adenoviral vaccine vectors at supraphysiological temperatures in carbohydrate glass.

Live recombinant viral vectors based on adenoviruses and poxviruses are among the most promising platforms for development of new vaccines against diseases such as malaria, tuberculosis, and HIV-AIDS. Vaccines based on live viruses must remain infectious to be effective, so therefore need continuous refrigeration to maintain stability and viability, a requirement that can be costly and difficult, especially in developing countries. The sugars sucrose and trehalose are commonly used as stabilizing agents and cryoprotectants for biological products. Here, we have exploited the ability of these sugars to vitrify on desiccation to develop a thermostabilization technique for live viral vaccine vectors. By slowly drying vaccines suspended in solutions of these disaccharide stabilizers onto a filter-like support membrane at ambient temperature, an ultrathin glass is deposited on the fibers of the inert matrix. Immobilization of two recombinant vaccine vectors-E1/E3-deleted human adenovirus type 5 and modified vaccinia virus Ankara-in this glass on the membranes enabled complete recovery of viral titer and immunogenicity after storage at up to 45 degrees C for 6 months and even longer with minimal losses. Furthermore, the membrane carrying the stabilized vaccine can be incorporated into a holder attached to a syringe for almost simultaneous reconstitution and injection at point of use. The technology may potentially be developed for the deployment of viral vector-based biopharmaceuticals in resource-poor settings.

Different levels of immunogenicity of two strains of Fowlpox virus as recombinant vaccine vectors eliciting T-cell responses in heterologous prime-boost vaccination strategies.

The FP9 strain of F has been described as a more immunogenic recombinant vaccine vector than the Webster FPV-M (FPW) strain (R. J. Anderson et al., J. Immunol. 172:3094-3100, 2004). This study expands the comparison to include two separate recombinant antigens and multiple, rather than single, independent viral clones derived from the two strains. Dual-poxvirus heterologous prime-boost vaccination regimens using individual clones of recombinant FP9 or FPW in combination with recombinant modified V Ankara expressing the same antigen were evaluated for their ability to elicit T-cell responses against recombinant antigens from Plasmodium berghei (circumsporozoite protein) or human immunodeficiency virus type 1 (a Gag-Pol-Nef fusion protein). Gamma interferon enzyme-linked immunospot assay and fluorescence-activated cell sorting assays of the responses to specific epitopes confirmed the approximately twofold-greater cellular immunogenicity of FP9 compared to FPW, when given as the priming or boosting immunization. Equality of transgene expression in mouse cells infected with the two strains in vitro was verified by Western blotting. Directed partial sequence analysis and PCR analysis of FPW and comparison to available whole-genome sequences revealed that many loci that are mutated in the highly attenuated and culture-adapted FP9 strain are wild type in FPW, including the seven multikilobase deletions. These "passage-specific" alterations are hypothesized to be involved in determining the immunogenicity of fowlpox virus as a recombinant vaccine vector.

Fowlpox virus as a recombinant vaccine vector for use in mammals and poultry.

Live vaccines against fowlpox virus, which causes moderate pathology in poultry and is the type species of the Avipoxvirus genus, were developed in the 1920s. Development of recombinant fowlpox virus vector vaccines began in the 1980s, for use not only in poultry, but also in mammals including humans. In common with other avipoxviruses, such as canarypox virus, fowlpox virus enters mammalian cells and expresses proteins, but replicates abortively. The use of fowlpox virus as a safe vehicle for expression of foreign antigens and host immunomodulators, is being evaluated in numerous clinical trials of vaccines against cancer, malaria, tuberculosis and AIDS, notably in heterologous prime-boost regimens. In this article, technical approaches to, and issues surrounding, the use of fowlpox virus as a recombinant vaccine vector in poultry and mammals are reviewed.

Amyloid fibril formation by a synthetic peptide from a region of human acetylcholinesterase that is homologous to the Alzheimer's amyloid-beta peptide.

A region near the C-terminus of human acetylcholinesterase (AChE) is weakly homologous with the N-terminus of the Alzheimer's disease amyloid-beta peptide. We report that a 14-amino acid synthetic polypeptide whose sequence corresponds to residues 586-599 of the human synaptic or T form of AChE assembles into amyloid fibrils under physiological conditions. The fibrils have all the classical characteristics of amyloid: they have a diameter of 6-7 nm and bind both Congo red and thioflavin-T. Furthermore, the kinetics of assembly indicate that fibril formation proceeds via a two-step nucleation-dependent polymerization pathway, and a transition in the peptide conformation from random coil to beta-sheet is observed during fibril formation using far-UV circular dichroism spectroscopy. We also show that the peptide in aggregated fibrillar form has a toxic effect upon PC-12 cells in vitro. AChE normally resides mainly on cholinergic neuronal membranes, but is abnormally localized to senile plaques in Alzheimer's disease. Recently, an in vitro interaction between AChE and A beta, the principal constituent of the amyloid fibrils in senile plaques, has been documented. The presence of a fibrillogenic region within AChE may be relevant to the interaction of AChE with amyloid fibrils formed by Abeta.