Introduction

June Kinoshita, with Brian Balin, Denah Appelt, Joseph Lyons, Ruth Itzhaki, and Curtis Dobson, led this live discussion on 1 July 2004. Readers are invited to submit additional comments by using our Comments form at the bottom of the page.
 

• Itzhaki RF, Wozniak MA, Appelt DM, Balin BJ. Infiltration of the brain by pathogens causes Alzheimer’s disease. Neurobiol Aging 25(4);619-627. [View .pdf]
• Little CS, Hammond CJ, MacIntyre A, Balin BJ, Appelt DM. Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice. Neurobiol Aging 2004;25(4);419-429. [View .pdf]
• Robinson, SR, Dobson, C, Lyons J. Challenges and directions for the pathogen hypothesis of Alzheimer's disease. Neurobiol Aging 2004; 629-637. [View .pdf]

June Kinoshita, with Brian Balin, Denah Appelt, Joseph Lyons, Ruth Itzhaki, and Curtis Dobson, led this live discussion on 1 July 2004. Readers are invited to submit additional comments by using our Comments form at the bottom of the page.

Transcript:
The Pathogen Hypothesis

Participants: Bryan Bolinger, Philadelphia College of Osteopathic Medicine; June Kinoshita, Alzforum; Joseph Lyons, City of Hope National Medical Center; Brian Balin, Philadelphia College of Osteopathic Medicine; Ruth Itzhaki, University of Manchester Institute of Science & Technology (UMIST); Allen Bain, affiliated with Immune Network; Hollie Schmidt, Boston Cure Project; Keith Crutcher, Cincinnati, Ohio; Marie Sanchirico; Leslie E. Webb III, Pensacola Beach Florida; Edmond Klaussner, Sinai School of Medicine, NYC.

Note: The transcript has been edited for clarity and accuracy.

June Kinoshita
Hello, everyone! Thanks for being here today.

Brian Balin
Hello, all; it's our pleasure to be here.

Joseph Lyons
Likewise, a different venue but a common interest that connects us.

Edmond Klaussner
Hi, I worked as a clinician for over 30 years with AD patients at Mt. Sinai in NYC. I have a longstanding interest in this topic, a broad interest in molecular biology, and am a beginner, but learning fast.

Joseph Lyons
Hello, everyone, and good to be connected with all of you again. Brian, congratulations on your publication of the model; it is certainly a start in the direction that the results of your human study made a necessity. Still requires some controls, which we discussed at the debate, which I hope you are considering.

June Kinoshita
Thanks, Joe. Brian, could you give everyone here a quick summary of your mouse findings?

Brian Balin
Yes, Joe, we have started using the AR39 strain of Cpn [Chlamydophila pneumoniae] in other animals and have begun analyzing the data. Early indications show some commonalities to the 9641 brain strain reported in the paper (you can download a copy of this and other relevant papers). The mouse findings were that infection through the intranasal route by the brain strain of Cpn resulted in amyloid plaques appearing in the mouse hippocampus, and other cortical areas.

Joseph Lyons
Brian, have you included a non-Cpn strain of chlamydia as a control?

Brian Balin
The brain strain was isolated from an AD brain a number of years ago. No, we have not used a non-Cpn strain yet, but hope to do this some time in near future.

June Kinoshita
Brian, in addition to plaques, do you see neurodegenerative changes or inflammation?

Brian Balin
We saw some inflammation with regard to astroglial responses, as determined by labeling for activated astrocytes. These appeared around areas of amyloid deposition, including around some blood vessels. We have not observed neurofibrillary tangles at this time. We believe that longer infections and/or infection in older animals may result in tangle formation.

June Kinoshita
Brian, any ideas as to what Cpn does once it enters the brains of the mice? Does it infect specific cells?

Brian Balin
We have found that Cpn was present in olfactory bulbs in the animals. In humans, Cpn shows up in glial cells, endothelial cells, and more recently we have found this in neuronal cells. Alan Hudson has also found this to be the case in a separate analysis. Both the olfactory pathways and the vascular route appear to be involved with brain infection. Probably the monocytes are taking the organism from lung to blood to the brain; a recent publication by Gieffers et al. showed this.

Joseph Lyons
Brian, have you cultured the organism at later time points when elementary bodies (which are the mature and infectious form of this agent and a form not often seen in chronic/persistent infection) are histologically present, and if not, what do you think is going on?

Brian Balin
Joe, we have cultured the organism from mouse olfactory bulbs after four months post-infection. The organism does appear at times to change antigenic expression. This may also reflect some change in gene expression during persistent versus acute infection. Joe, we also have been doing much in-vitro infection with Cpn and have been finding some really interesting results in neuroblastoma cells. Apoptosis is affected (i.e., delayed and inhibited by infection); also, some changes in cytoskeleton (tau protein) have been noted at 10 days post-infection.

Ruth Itzhaki
Is that published yet, Brian? I'd be very interested to have details.

Brian Balin
Ruth, the in-vitro data has not been published yet, but you will see this in Philly during the International AD meeting (see poster P3-189 from that meeting). Ruth, you may want to say something about familial AD and the recent herpes reactivation observations. This seems to tie infection to familial disease as well, something that we also have been considering but have not done at this point in time.

Ruth Itzhaki
It is rather a small number, so we can't tie it in with ApoE, but it is still very intriguing; also, Mori et al. detected viral proteins, so maybe there's a persistent very low-level infection, at least in some brains, rather than true latency.

Brian Balin
Ruth, this is very interesting. Do you think that ApoE is a more general feature of many infections in both brain and elsewhere, because we also have evidence that ApoE could be involved with Cpn infection?

Ruth Itzhaki
Yes, it seems to affect the response in several diseases caused by very diverse pathogens including cold sores (which are definitely caused by HSV1), liver disease caused by hepatitis C virus and malaria [preliminary result]. All of these pathogens bind, like ApoE, to heparin sulfate proteoglycans (HSPG) and some enter cells via ApoE receptors. Regarding herpes simplex keratitis (HSK), there was an effect but not statistically significant, and we attribute these differences to differences in cell type involved. This would take several sentences to explain: See our Trends in Microbiology review.

June Kinoshita
Hi, Ruth, is that in humans who had HSK (I presume this occurs in people and not just mice…)?

Ruth Itzhaki
Yes, we studied humans with herpes simplex keratitis.

June Kinoshita
Marie, is ApoE genotype something you looked at with HSK?

Marie Sanchirico
No, until we started some work on atherosclerosis and AD in the lab, we never worried about ApoE.

June Kinoshita
Brian, do you think that all types of cells in the brain are equally vulnerable to Cpn, or is there selective vulnerability?

Brian Balin
June, that's a really good question. The mode of infection or uptake of the organism may differ from one cell type to another. Interestingly, and this crosses with Ruth's herpes work, Cpn seems to use heparin sulfate proteoglycans as a mechanism for uptake in some cells. We also think there may be more specific receptor-mediated uptake.

June Kinoshita
I'm also curious about the anatomical distribution of plaques in the mice. Are plaques produced only by some cell populations in response to Cpn infection? Or is the anatomical pattern related more to the route of infection and the presence of Cpn?

Brian Balin
June, plaques in the mice seem to arise in areas that are connected to olfaction, but not exclusively. We see plaques also in areas not directly related to olfaction, but at times in areas such as cerebellum, midbrain, etc.

Joseph Lyons
Everyone, I am attracted to any model of AD initiation and progression that includes the nose, as it is both a common portal of entry and is constantly exposed to all of life's many insults, and the olfactory bulb and associated nervous system connections may play a role in β-amyloid clearance. Thus, Brian's model is significant as an infectious cause of disruption of normal activity in this area, but might be nothing more than an example of temporary bulbectomy, which was recently shown to result in β-amyloid deposits and associated mental impairment in mice (see Aleksandrova et al., 2004). Thus the need for many more controls to establish specificity in the murine model. Comments, anyone?

June Kinoshita
Joe, that's interesting about the bulbectomy. So one possibility is that Aβ plaques are a generalized response to injury/infection.

Joseph Lyons
June, the authors found that six weeks after bilateral olfactory bulbectomy, a peptide with molecular weight of 4 kD was revealed in extracts of the neocortex and hippocampus from mice. Using monoclonal antibodies 4G8, this peptide was identified as beta-amyloid. Its level was significantly higher in the bulbectomized animals than in sham-operated mice. The bulbectomized mice displayed sharp impairment in spatial memory when tested in the Morris water maze. The results suggest that bulbectomy initiates in the brain a pathological process similar to human Alzheimer's disease in location, biochemistry, and behavioral manifestations.

Brian Balin
The infection seems to be isolated to the olfactory bulbs in the mice, but the pathology is found in the brain proper. Also, in other studies of toxin delivery to the olfactory system of dogs, neurodegenerative changes have been observed in deeper brain structures.

June Kinoshita
I think an important question is whether in the pathogen hypothesis, the development of AD is pathogen-specific or part of a more general response to insults and stresses. Marie has worked on a model where the neurodegeneration results from a pathogen-specific mechanism. Is everyone familiar with the work on HSK, or would it be helpful to have Marie provide a summary?

Brian Balin
The issue of generalized response may have some validity because, as we all know, amyloid can develop in the brain following different types of insult, such as brain injury, and with other mediators triggering an inflammatory response. Marie, could you please provide a synopsis?

Marie Sanchirico
HSK is a viral-induced autoimmune disease in humans. There is a terrific mouse model for this disease (HSV1 strain-specific) in which you can infect the eye with HSV1 and within two weeks mice develop HSK, very similar to humans. We have shown that it's CD4+-mediated, and also that a specific viral epitope makes the response more aggressive (mimicry). In HSK, we focused very much on the immune response rather than the infection. Infection is not persistent in mice, and is gone by seven days.

Brian Balin
I think that one of the key issues with regard to the pathogen hypothesis is that different pathogens, depending on their ability to enter the brain and/or trigger inflammatory responses, can result in activation of pathways that coincide with neurodegenerative responses such as amyloid formation and tangle development.

Keith Crutcher
In support of a suggested role for pathogens in amyloidosis beyond AD, there is a recent review of familial amyloidotic polyneuropathy [FAP, caused by transthyretin amyloid; see ARF related news story on transthyretin amyloidoses] that reports that transgenic mice carrying the relevant mutation show a lower incidence of the phenotype when raised in a pathogen-free environment (Joao Saraiva et al.).

Brian Balin
Marie, in your system, do the viruses become latent?

Marie Sanchirico
I'm not sure with KOS (a designation for one of the available infective lab strains) if it does become latent.

Ruth Itzhaki
Message from Curtis [Dobson], who's here, to Marie: Is there any evidence of changes in APP or amyloid in the mice?

Marie Sanchirico
I never checked APP or amyloid in these mice. Along those lines, one experiment I talked about with Ruth was to infect PDAPP and TgCRND8 mice with HSV1 and look to see if the pathology more resembles human AD. Unfortunately, I never got to do those experiments before leaving Harvey Cantor's lab.

June Kinoshita
What Marie says about the infection being gone before the neurodegeneration sets in is so interesting; i.e., even in the absence of acute infection, and possibly even after the pathogen has been cleared, it leaves an immune system memory that leads to a neurodegenerative response. So the fact that there are inconsistent findings regarding the presence of HSV1 or Cpn in AD tissue may be irrelevant.

Ruth Itzhaki
That makes it even more difficult to show that a pathogen is involved, though!

Brian Balin
With any chronic disease process, there will be difficulty establishing gold standards for detection as well as sampling issues. In addition, the hit-and-run hypothesis of infection [suggests that a pathogen] could trigger [a disease process] without having to remain around. For example, the initiation of an autoimmune response due to antigenic mimicry, etc.

June Kinoshita
I would love to see the results of that experiment, HSV1 in an APP overexpressing mouse. Is it possible to infect the mice without killing them outright?

Marie Sanchirico
It should be easy—need to bug Harvey. Yes, try to give them mild non-lethal encephalitis and see if the APP Tg mice develop a human-like AD. Ruth was starting to advise me on this; I don't remember who you said was the expert in this type of experiment?

Ruth Itzhaki
Mike Pappolla suggested our collaborating on HSV-infected APP Tg mice but he moved his lab before we could start anything.

Brian Balin
Ruth, do you think that the mice need to be double transgenics (Tg) or have a specific ApoE genotype?

Ruth Itzhaki
Brian, I would expect more relevant effects in doubly Tg or triply Tg mice. Wouldn't you?

Brian Balin
Yes; Ruth, the multiple transgenics would seem to be more relevant.

June Kinoshita
Ruth, I'm not sure. It might be more important to know whether the Aβ produced in the model mimics the pathogenic epitope.

Ruth Itzhaki
June, you may be right if such a change in amyloid occurs.

Allen Bain
Is there a model where aggregated Aβ can be tested for its ability to activate a latent HSV or Cpn infection? Or is this already known?

Keith Crutcher
I tried to test the hypothesis that AD is a disease of response to infection by looking at antiviral activity against HSV1 using brain homogenates. Interestingly, there was greater antiviral activity in the AD tissue compared to control.

June Kinoshita
Keith, that's interesting. Will you be publishing that or presenting anything on it at a meeting?

Keith Crutcher
June, I actually presented the data at Neuroscience a couple of years ago…then got funded for other work and haven't had time to follow up. But I would like to.

Joseph Lyons
At the City of Hope an investigator named Ed Canton uses a similar model and has not observed any neurological signs following recovery. In his hands, a latent infection that can be stress reactivated is established, and he has shown sex and mouse strain differences in susceptibility and course of disease. I have attempted to get him interested in this connection without success. Marie, is there any olfactory involvement in your model?

Marie Sanchirico
We infect the eye and never looked further. One other important consideration is the strain of HSV1 used. Especially in mice, different viral strains are significantly more or less encephalitic in the same line of mice.

Ruth Itzhaki
Seems also to vary amongst mice of the same strain, i.e., some mice get HSE and die but others survive after suffering from a much milder disease.

Brian Balin
Ruth, is there an age effect on who gets HSE?

Ruth Itzhaki
In humans, I think it affects the middle-aged, but I think there are two peaks; sorry, I can't remember the details. Some say bimodal—with peaks in those less than 20 and in those over 50, and others say in middle-aged and the elderly—while others say merely in those in middle-age groups! Does Marie know?

Marie Sanchirico
I was hoping you did—no.

June Kinoshita
Marie, is there published information on which viral strains produce encephalitis in which mouse strains?

Marie Sanchirico
Not sure. I know RE (another HSV1 strain designation) is much more encephalitic than KOS. I'm sure David Knipe or other HSV1 biologists would have a good idea of this.

June Kinoshita
Speaking of which, would anyone like to address the molecular mimicry question?

Marie Sanchirico
I'm not sure I understand the mimicry question; did I miss something?

June Kinoshita
That was my shorthand way of asking whether Cpn and HSV1 proteins have molecular mimics in the nervous system, e.g., Aβ or some other proteins.

Brian Balin
The molecular mimicry issue is big in the Cpn field in that Hsp60, for one, has been shown to be highly homologous to eukaryotic Hsp60. Thus, presentation of the prokaryotic form may elicit responses that cross with the eukaryotic form. Also, Bachmaier et al., 1999, showed some autoimmune phenomenon in mice following Chlamydial infection—cardiac myosin was attacked.

Marie Sanchirico
I think mimicry would only be an issue in AD if we start to think autoimmunity is involved or there is mimicry on the innate level (signaling through the Toll pathway). As Brian mentioned, in the cardiac model mimicry is suggested, but that is looking at an autoimmune reaction.

June Kinoshita
Marie, there is a paper that just appeared in PubMed (Mruthinti et al.): "Autoimmunity in Alzheimer's disease: increased levels of circulating IgGs binding Aβ and RAGE peptides."

Marie Sanchirico
Also, June, I believe they have found autoimmunity in the patients that developed encephalomyelitis in the Elan trials—possibly boosted already existing immunity (see related news on the trials and tribulations of the Elan trial and debates about apparent benefits of the vaccine therapy).

June Kinoshita
Ruth has been following the Elan study with great interest!

Joseph Lyons
We concluded our article with a suggestion that multiple infections over the course of a lifetime might be a better way of understanding the chronic processes of AD; hence, might I suggest merging two models of disease such as HSV and Cpn and throw in an episode or two of endotoxemia, this latter being perhaps a valuable tool to assess reactivity at an established site of deposition and microglial accumulation?

Marie Sanchirico
Joe, this definitely could be true and is basically what I was saying about MS. One infection early in life predisposes you immunologically and a second CNS localized infection initiates the chronicity.

Brian Balin
Joe, your suggestion is one that we also have been considering. This would fit with a polymicrobial approach to this disease as well as with the influence of inflammation (i.e., body's response) in contributing to AD.

Joseph Lyons
Brian, you have the model for initial deposition that could certainly be exploited in this way.

Allen Bain
Joseph, given that many postmortem brains from patients without AD symptoms show lots of plaques, could it be that a latent infection and a presence of plaques due to previous acute infections may be the two hits necessary for rapid decline seen in AD, i.e., not necessarily two infections?

Joseph Lyons
That is the hypothesis I most favor for the role of infection in progression.

June Kinoshita
Re: the two-hit model: Is there some way to analyze stored blood samples from a large epidemiological study, such as the Framingham study, to see whether exposure to Cpn or HSV1 earlier in life increases the risk of later developing AD?

Ruth Itzhaki
June, at least in those at a low socioeconomic level, HSV1 infection occurs in infancy. I think the same is true for Cpn, isn't it, Brian?

June Kinoshita
Ruth, are you saying these infections occur in 100 percent of these populations?

Brian Balin
Blood analysis will probably not help in determining later risk because with late age, most of the population already shows exposure. Cpn seems to occur more across the board as community acquired.

Ruth Itzhaki
Yes, eventually; by early adulthood, in such people, the infection level is almost 100 percent (in the PNS, of course).

Joseph Lyons
June, Cpn epidemics do occur in institutional settings and ultimately, as Brian mentioned, no one escapes exposure, thus making it the interesting candidate agent Brian is so endeared to.

Brian Balin
Thank you, Joe.

Joseph Lyons
Brian, do you see any vascular involvement/damage in your model?

Brian Balin
Yes, Joe, we do see some vascular involvement in our model. Thus, we believe this route also is vulnerable in this infection.

Allen Bain
Brian, are you planning pharmacologic interference in upcoming studies with your model, such as antiinflammatories, myeloperoxidase (MPO) blockers, other immunomodulatory drugs, or even anti-chlamydial drugs?

Brian Balin
Yes, Allen, we want to try antibiotics at various times following infection, or prior to infection, to determine how they affect plaque initiation.

Joseph Lyons
The problem with many intervention strategies is that they could in themselves "chase" Cpn into a persistent state which in the end may be the most pathologically destructive form of infection.

Keith Crutcher
Joe/Brian, do you think it is just coincidence that most AD patients officially die of pneumonia?

Joseph Lyons
Keith, the inclusion of that remark was made somewhat tongue-in-cheek, but given the almost pathognomic association of Cpn with death revealed by the two studies used to establish the relationship between AD and Cpn, it begs the question: During the throes of terminal degeneration does a reactivation of Cpn infection at a distant site seed the brain via already compromised vascular lesions and act as the final death-inducing insult?

Keith Crutcher
No tongue-in-cheek here. It was a serious question and I think your answer is intriguing!

Ruth Itzhaki
The HSV1 idea needs careful repeat studies by others of human brain, plus investigation of mechanisms of HSV1-ApoE interactions, ApoE Tg mice studies, plus (what a hope!), clinical trials of antivirals.

Brian Balin
We need to focus on aging as a susceptibility factor with these infections, immunosenescence, influence of infection on the cells following persistency, etc.

June Kinoshita
It seems animal models could play an important role in advancing this hypothesis. What needs to be done in this area? Brian's made a great beginning with his Cpn mice. In addition to carrying out more control experiments, what do you want to do?

Brian Balin
Did you happen to see the Loeb at al. study with antibiotics in AD?

June Kinoshita
Refresh our memories, Brian.

Brian Balin
Doxycycline and rifampin led to some improvement in some cognitive tests and memory for AD patients. With comparison to cholinesterase inhibitors, the data was just as strong on the delay of progression issue (see Loeb et al., 2004).

Ruth Itzhaki
Comment from Curtis: It might be interesting to see whether HSV1 infected mice have serum that cross-reacts with human amyloid. This could support June's autoimmune hypothesis (remember that the HSV1 protein gB shows homology with amyloid).

Brian Balin
We would also like to proceed by using different animal models, including transgenics, to try to understand cofactors in the infection process, and the result of infection.

June Kinoshita
Ruth and I have exchanged some e-mails about carrying out multi-lab studies using a reference set of tissue. Does that seem like a high-priority next step?

Ruth Itzhaki
That is, PCR of tissue and of extracted DNA.

Brian Balin
Do we really have a gold standard for detection and how should we proceed, use real-time PCR or culturing of samples?

Ruth Itzhaki
We don't have a real-time PCR machine. Wish we had.

Brian Balin
Well, we're getting one, so we'll have to talk at the meeting.

Ruth Itzhaki
Good, hope we can all meet there, perhaps in the evening for this purpose?

Brian Balin
Ruth, that would be great.

June Kinoshita
Brian, re: gold standard, it might be worth some effort to establish a consensus.

Brian Balin
This is now in the works for Cpn, looking at genes for both active acute and persistent infections.

June Kinoshita
I'd love to see this discussion develop further via e-mails in the style of Open Peer Commentaries. We have one going on now on cell death in AD, and it has been very successful. If you are all willing, I'll send you some e-mails to get it going.

Ruth Itzhaki
Thanks, June.

Brian Balin
That would be fantastic. There is just so much to talk about. Thanks, June.

Keith Crutcher
Great idea, June. I must say that after reading Plague Time, I came away convinced that this is the right track. Good luck to all of you!

June Kinoshita
We're at the end of our hour. It would be great if we could wrap by proposing key experiments to move this area of inquiry forward.

Joseph Lyons
I have made some suggestions throughout the discussion and, time permitting, I would like to enter the field more actively. I have enjoyed the experience and wish everyone success in your independent and collective efforts to establish the links between infection and AD. Take care.

June Kinoshita
Thank you all for taking part in today's discussion. To be continued!

Background

Moderator’s Summary: Pathogens as a Cause of Alzheimer’s Disease
By June Kinoshita

The notion that microbes such as herpes simplex virus 1 (HSV1) and Chlamydophila pneumoniae (Cp) could be a causal factor in Alzheimer’s diseases would probably be viewed by the main stream of AD researchers as being beyond the pale. Although a small body of recent findings has reported strikingly strong associations between these pathogens and AD [1,7], subsequent attempts to replicate the findings have met with mixed results (discussed in [10]). At this juncture, it might be convenient to dismiss the hypothesis, but as both sides of this debate session agreed, there are plausible reasons for these discrepancies that deserve to be resolved through further research. While opinions diverged on the strength of evidence for and against the hypothesis, there was a consensus that the possibility of common infectious agents causing such a widespread scourge of old age is one that is too important to ignore.

It may be of interest to step back from the specific merits and future challenges of the pathogen hypothesis (which the participants cover thoroughly in this issue) to ponder changes in the medical culture that may have contributed to the willingness of the debate participants and audience alike to weigh the evidence dispassionately rather than to dismiss the whole idea as being implausible on the face of it. Most researchers today grew up in an era when microbes were presumed to have been brought under human control. The 19th and 20th centuries saw the microbial perpetrators of the great killer diseases tracked down one by one and vanquished with drugs and vaccines. Events over the past two decades have rudely awakened medical science to the reality that we have not, after all, advanced into the post-infectious era. The AIDS pandemic and emergence of drug-resistant tuberculosis, malaria and other scourges shocked us into realizing that microorganisms have been far from conquered. These devastating setbacks have driven home the fact that we are engaged in an evolutionary arms race in which our science and wits are pitted against the ability of microbes to adapt to our most clever weapons.

In the same period, a microbe, Helicobacter pylori, came to be accepted as causing duodenal ulcers and gastric cancers [3]. Previously, ulcers were viewed as a classic degenerative condition, the result of some toxic combination of stress, chemical irritants and bad genes. The discovery of a bacterial origin was greeted initially with hostility, but was eventually hailed as marking a paradigm shift in the pathogenesis of chronic diseases. More recently, another microbe, C. pneumoniae, has come under suspicion for playing a role not only in AD, but in atherosclerosis [2,5], the preeminent chronic killer disease.

These may not be flukes, argues biologist Paul Ewald. Ewald has championed a theory, first suggested by physicist Gregory Cochran, that most, if not all, of the chronic degenerative diseases of aging are microbial in origin [4]. While a great deal of effort is currently being invested in pinpointing genes for late-onset Alzheimer disease, the evolutionary argument holds that deleterious genetic mutations, even those that are expressed late in life, cannot persist in a population. Pathogens, in contrast, can persist indefinitely because the host’s ability to evolve resistance to pathogens is matched by the pathogens’ ability to keep shifting their strategy for living off the host. Thus, Ewald writes, “If we see chronic diseases that have commonly been causing damage for a long time, the best bet is that they have infectious causes” [6].

Evolutionary theory has yet to make inroads into the thinking of most Alzheimer researchers. For the majority, it is safe to assume that the idea that microorganisms can cause Alzheimer’s stretches credulity. If pathogens were responsible, one might well wonder how the culprits could have escaped the scrutiny of generations of pathologists. Microbes, however, are capable of astounding stealth. They can insinuate themselves into host cells and genomes, where they may lie latent and be very challenging to detect. Microbes can also leave a trace in the host’s immune memory, exerting lethal effects not through acute infection but by triggering autoimmune responses through molecular mimicry between microbial proteins and host proteins [12].

While these are some compelling theoretical arguments for taking the pathogen hypothesis seriously, the burden lies with proponents to prove the theory, rather than with the indifferent majority to disprove it. In this issue of Neurobiology of Aging, Itzhaki et al. [7], present arguments in support of the pathogen hypothesis, reviewing not only the positive and negative studies that have sought evidence for HSV1 and Cp in Alzheimer brains, but also discussing how pathogens might interact with other known risk factors for AD, such as APOE-α 4 genotype [8], aging, the immune system and trafficking of pathogens into the central nervous system. The companion article by Robinson et al. [10], points out some important discrepancies in these studies and discusses major arguments that could be made against the hypothesis, such as whether it is compatible with the existence of inherited forms of AD. The authors also make constructive suggestions regarding future research. Clinical trials of antibiotics or antiviral drugs, for example, could test whether removing a putative pathogen has any effect on disease progression.

As these articles make clear, these are still early days for the pathogen hypothesis, and the proponents have their work cut out for them. Both HSV1 and Cp are highly challenging to detect, and disputes over their association with AD are clouded by methodological issues. A rigorous effort to test the hypothesis would profit from standardizing methods, for example by distributing a uniform set of tissues with positive and negative controls to determine whether all of the laboratories involved are achieving equal levels of sensitivity. The standard protocol should also require multiple testing of each brain [11], preferably using diverse methods.

An animal model that develops AD pathology and behavioral deficits upon exposure to pathogens would help establish the credibility of the hypothesis. The Balin laboratory presented a mouse model that develops amyloid-beta deposits in the brain following intranasal infection with Cp [9]. That study awaits publication and independently replication. Finally, the hypothesis might be more readily accepted if its advocates could clarify whether the two pathogens implicated to date are acting through independent pathways, or are involved in a common mechanism.

At the end of the day, one might ask, so what? Suppose that microbes cause Alzheimer’s, how will that change strategies for treating the disease? Proponents of the hypothesis suggest that antimicrobial drugs or vaccines can be marshaled to nip the disease in the bud. However, if microbes turn out to work harm through amyloid-beta or non-specific inflammatory responses, would not these remain the better therapeutic targets? These questions probably cannot be answered until more is known about whether and how pathogens contribute to AD. Readers of Robinson et al., and Itzhaki et al., in this issue, are invited to consider the facts and issues and decide whether it is worth enlarging their imaginations to include microbes as potential players in causing Alzheimer’s disease.

Note on Permissions: The background text for this live discussion, "Moderator’s summary: Pathogens as a cause of Alzheimer’s disease," by June Kinoshita, originally appeared in Neurobiology of Aging 25 (2004) 639–640. (See Science Direct). Articles by Robinson et al., Itzhaki et al., and Little et al. reprinted from Neurobiol Aging, 25, with permission from Elsevier.

More Recent Reference

Wozniak M, Mee A, Itzhaki R. Herpes simplex virus type 1 DNA is located within Alzheimer's disease amyloid plaques. J Pathol. 2008 Sep 18. Abstract

References

[1] Balin BJ, Gerard HC, Arking EJ, Appelt DM, Branigan PJ, Abrams JT, et al. Identification and localization of Chlamydia pneumoniae in the Alzheimer’s brain. Med Microbiol Immunol (Berl) 1998;187:23– 42. Abstract

[2] Campbell LA, Kuo CC. Chlamydia pneumoniae and atherosclerosis. Semin Respir Infect 2003;18(March (1)):48–54. Abstract

[3] Cilley RE, Brighton VK. The significance of Helicobacter pylori colonization of the stomach. Semin Pediatr Surg 1995;4(November (4)):221–7. Abstract

[4] Cochran GM, Ewald PW, Cochran KD. Infectious causation of disease: an evolutionary perspective. Perspect Biol Med 2000;43 (Spring (3)):406–48. Abstract

[5] de Boer OJ, van der Wal AC, Becker AE. Atherosclerosis, inflammation, and infection. J Pathol 2000;190(3):237–43. Abstract

[6] Ewald P. Plague time. Anchor Books; 2002. p. 56.

[7] Itzhaki RF, Wozniak MA, Appelt DM, Balin BJ. Infiltration of the brain by pathogens causes Alzheimer’s disease. Neurobiol Aging 2004; this issue. See .pdf above.

[8] Itzhaki RF, Dobson CB, Lin WR, Wozniak MA. Association of HSV1 and apolipoprotein E-varepsilon4 in Alzheimer’s disease. J Neurovirol 2001;7(December (6)):570–1. Abstract

[9] Little CS, Hammond CJ, MacIntyre A, Balin BJ, Appelt DM. Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice. Neurobiol Aging 2004;25(4), in press. See .pdf above.

[10] Robinson SR, Dobson C, Lyons J. Challenges and directions for the pathogen hypothesis of Alzheimer’s disease. Neurobiol Aging 2004; this issue. See .pdf above.

[11] Smieja M, Mahony JB, Goldsmith CH, Chong S, Petrich A, Chernesky M. Replicate PCR testing and probit analysis for detection and quantitation of Chlamydia pneumoniae in clinical specimens. J Clin Microbiol 2001;39(May (5)):1796–801. Abstract

[12] Zhao ZS, Granucci F, Yeh L, Schaffer PA, Cantor H. Molecular mimicry by herpes simplex virus-type 1: autoimmune disease after viral infection. Science 1998;279(February (5355)):1305. Abstract

Additional References
Macdonald AB. Alzheimer's neuroborreliosis with trans-synaptic spread of infection and neurofibrillary tangles derived from intraneuronal spirochetes. Med Hypotheses. 2006 Oct 19. Abstract

Higuchi Md et al. Trypanosoma cruzi trans-sialidase as a new therapeutic tool in the treatment of chronic inflammatory diseases: possible action against mycoplasma and chlamydia. Med Hypotheses. 2004 Jan 1;63(4):616-23. Abstract

Hill JM, Steiner I, Matthews KE, Trahan SG, Foster TP, Ball MJ. Statins lower the risk of developing Alzheimer's disease by limiting lipid raft endocytosis and decreasing the neuronal spread of Herpes simplex virus type 1. Med Hypotheses. 2005;64(1):53-58. Abstract

Mori I, Kimura Y, Naiki H, Matsubara R, Takeuchi T, Yokochi T, Nishiyama Y. Reactivation of HSV-1 in the brain of patients with familial Alzheimer's disease. J Med Virol. 2004 Aug ; 73(4):605-11. Abstract

Perry VH. The influence of systemic inflammation on inflammation in the brain: implications for chronic neurodegenerative disease. Brain Behav Immun. 2004 Sep 1;18(5):407-13. Abstract

See also Forum Discussion: The Pathogen Hypothesis—Challenging the Primacy of Genetics in Late-Onset Alzheimer Disease

Question from Joy K.—Posted 20 July 2004

Has anyone tested the use of antibiotics for Alzheimer's patients? My mother was diagnosed with the disease more than seven years ago. Although she quit after the diagnosis, she was a heavy smoker most of her life, which resulted in congestion problems. Over the last seven years she was given antibiotics several times. Each time her condition improved dramatically. When she stopped the medication she reverted back to the way she was before. She is now in the last stages of her disease and refuses to eat or drink. She was sent to the emergency room and not expected to survive the night. They gave her and antibiotic drip and by the next day she was fighting to go home. She recognized us, was able to put three words together, and understood and responded to everything we said to her. She even played a little joke on my sister, pretending to be dead and then jump up laughing because she scared her.

She has not been this responsive in close to a year! I attribute it to the antibiotic drip. In the past when she took antibiotics orally she significantly improved but the drip seemed to really make a huge difference. I hope something can be done to research this. I am trying to tell everyone I can. Please let me know if this has been researched.

Reply from Brian Balin, Ph.D., Philadelphia College of Osteopathic Medicine—Posted 20 July 2004

Remarkably, this is something that has been recognized by clinicians for many, many years. I have innumerable accounts from individuals who have reported on exactly the same response. There have been reports back to me of individuals who have not spoken for years that have "recovered" this ability following antibiotic therapy. Is the response specific to treating an infection systemically or in the brain, or does it have to do with an anti-inflammatory action of the antibiotics? We just don't have the answers to these questions at this time. In my estimation, there has to be a mandate in this for performing clinical trials based on the antibiotic approach. Hopefully, we can convince the NIH or big pharma that these trials would be worthwhile.

Question from Allen Cox—Posted 30 July 2004

Several Alzheimer's patients have had postmortem studies done and the Lyme spirochete has been found in the brain embedded in neurons.

Comments

1. • Stephen Robinson
     RMIT University
   • Posted: 01 Jul 2004

   When will the pathogen hypothesis catch on?

   The idea that Alzheimer's disease is caused by a pathogen which invades the brain has been around for decades, but this notion has never attracted serious attention from mainstream researchers. It is often dismissed because "if AD was really caused by a virus or bacteria, they would have found it by now, and in any case everyone knows that AD is not contagious—it only affects the aged". This reasoning overlooks the fact that the vast majority of diseases known to humanity are caused by pathogens, including quite a few that affect cognitive function, either directly (eg. HIV-1) or indirectly (eg. hepatitis). That the pathogen has not yet been identified is hardly surprising. After all, it took thousands of researchers, two decades and many billions of dollars to reach the conclusion that amyloid deposition does not cause AD. The marginalization of the pathogen hypothesis has stymied research in this area, and much of the supporting data which exists was generated on a pauper's budget, doing credit to the tenacity of proponents such as Ruth Itzhaki, Brain Balin and Mel Ball.

   Since the leading proponents of the amyloid deposition hypothesis capitulated (Hardy and Selkoe, 2002), the Alzheimer's field has been left in a vacuum. Sure we still have the 'oligomeric amyloid hypothesis' the 'inflammation hypothesis' and the 'oxidative stress hypothesis' but when one looks beyond the hype it is clear that they merely describe a facet of AD, not its cause. They cannot explain for example, the spatiotemporal spread of plaques and tangles, why certain neurotransmitter types are preferentially affected, or why particular pathways in the brain are selectively targeted. They cannot account for the non-cognitive behavioural disturbances (eg sundowning), or the predilection for old age, and they struggle to explain why ApoE4 is the major genetic risk factor.

   The pathogen hypothesis by contrast, offers explanations for all facets of AD, and for this reason it deserves serious consideration. The pathogen hypothesis was showcased in a debate at the second Challenging Views of Alzheimer's Disease conference in July, 2003. As an 'outsider' I was astounded to discover that despite three decades of publications by its proponents, not a single skeptic had taken the pathogen hypothesis seriously enough to write a critique. With colleagues Curtis Dobson and Joseph Lyons, we have now written that critique (Robinson et al., 2004). It is clear to us that much research remains to be done before a strong case can be established, yet it is equally evident that many important questions and issues are ripe for investigation. Certainly there are enough indirect observations to pique the interest of any objective researcher, including reports in the past few months of HSV-1 in the brain tissue of AD patients (Denaro et al., 2003; Mori et al., 2004).

   Who knows, perhaps one day we will be able to immunize against AD!

   References:

   Denaro FJ, Staub P, Colmer J, Freed DM. Coexistence of Alzheimer disease neuropathology with herpes simplex encephalitis. Cell Mol Biol (Noisy-le-grand). 2003 Dec;49(8):1233-40. PubMed.

   Hardy J, Selkoe DJ. The amyloid hypothesis of Alzheimer's disease: progress and problems on the road to therapeutics. Science. 2002 Jul 19;297(5580):353-6. PubMed.

   Mori I, Kimura Y, Naiki H, Matsubara R, Takeuchi T, Yokochi T, Nishiyama Y. Reactivation of HSV-1 in the brain of patients with familial Alzheimer's disease. J Med Virol. 2004 Aug;73(4):605-11. PubMed.

   Robinson SR, Dobson C, Lyons J. Challenges and directions for the pathogen hypothesis of Alzheimer's disease. Neurobiol Aging. 2004 May-Jun;25(5):629-37. PubMed.

   View all comments by Stephen Robinson

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References

Webinar Citations

1. The Pathogen Hypothesis—Challenging the Primacy of Genetics in Late-Onset Alzheimer Disease. 22 Jan 2006
2. The Pathogen Hypothesis 24 Nov 2008

News Citations

1. NSAIDs and Derivatives Keep Transthyretin in Its Place 31 Jan 2003
2. Trials and Tribulations—Autopsy Reveals Pros and Cons of AD Vaccine 21 Mar 2003
3. Exchange of Letters Asks if Silver Lining in Vaccine Trial Was Real 5 Feb 2004

Paper Citations

1. Wozniak MA, Mee AP, Itzhaki RF. Herpes simplex virus type 1 DNA is located within Alzheimer's disease amyloid plaques. J Pathol. 2009 Jan;217(1):131-8. PubMed.
2. Balin BJ, Gérard HC, Arking EJ, Appelt DM, Branigan PJ, Abrams JT, Whittum-Hudson JA, Hudson AP. Identification and localization of Chlamydia pneumoniae in the Alzheimer's brain. Med Microbiol Immunol. 1998 Jun;187(1):23-42. PubMed.
3. Campbell LA, Kuo CC. Chlamydia pneumoniae and atherosclerosis. Semin Respir Infect. 2003 Mar;18(1):48-54. PubMed.
4. Cilley RE, Brighton VK. The significance of Helicobacter pylori colonization of the stomach. Semin Pediatr Surg. 1995 Nov;4(4):221-7. PubMed.
5. Cochran GM, Ewald PW, Cochran KD. Infectious causation of disease: an evolutionary perspective. Perspect Biol Med. 2000 Spring;43(3):406-48. PubMed.
6. de Boer OJ, van der Wal AC, Becker AE. Atherosclerosis, inflammation, and infection. J Pathol. 2000 Feb;190(3):237-43. PubMed.
7. Itzhaki RF, Dobson CB, Lin WR, Wozniak MA. Association of HSV1 and apolipoprotein E-varepsilon4 in Alzheimer's disease. J Neurovirol. 2001 Dec;7(6):570-1. PubMed.
8. Smieja M, Mahony JB, Goldsmith CH, Chong S, Petrich A, Chernesky M. Replicate PCR testing and probit analysis for detection and quantitation of Chlamydia pneumoniae in clinical specimens. J Clin Microbiol. 2001 May;39(5):1796-801. PubMed.
9. Zhao ZS, Granucci F, Yeh L, Schaffer PA, Cantor H. Molecular mimicry by herpes simplex virus-type 1: autoimmune disease after viral infection. Science. 1998 Feb 27;279(5355):1344-7. PubMed.
10. MacDonald AB. Alzheimer's neuroborreliosis with trans-synaptic spread of infection and neurofibrillary tangles derived from intraneuronal spirochetes. Med Hypotheses. 2007;68(4):822-5. PubMed.
11. Hill JM, Steiner I, Matthews KE, Trahan SG, Foster TP, Ball MJ. Statins lower the risk of developing Alzheimer's disease by limiting lipid raft endocytosis and decreasing the neuronal spread of Herpes simplex virus type 1. Med Hypotheses. 2005;64(1):53-8. PubMed.
12. Mori I, Kimura Y, Naiki H, Matsubara R, Takeuchi T, Yokochi T, Nishiyama Y. Reactivation of HSV-1 in the brain of patients with familial Alzheimer's disease. J Med Virol. 2004 Aug;73(4):605-11. PubMed.
13. Perry VH. The influence of systemic inflammation on inflammation in the brain: implications for chronic neurodegenerative disease. Brain Behav Immun. 2004 Sep;18(5):407-13. PubMed.
14. Gieffers J, van Zandbergen G, Rupp J, Sayk F, Krüger S, Ehlers S, Solbach W, Maass M. Phagocytes transmit Chlamydia pneumoniae from the lungs to the vasculature. Eur Respir J. 2004 Apr;23(4):506-10. PubMed.
15. Dobson CB, Wozniak MA, Itzhaki RF. Do infectious agents play a role in dementia?. Trends Microbiol. 2003 Jul;11(7):312-7. PubMed.
16. Aleksandrova IY, Kuvichkin VV, Kashparov IA, Medvinskaya NI, Nesterova IV, Lunin SM, Samokhin AN, Bobkova NV. Increased level of beta-amyloid in the brain of bulbectomized mice. Biochemistry (Mosc). 2004 Feb;69(2):176-80. PubMed.
17. João Saraiva M, Mendes Sousa M, Cardoso I, Fernandes R. Familial amyloidotic polyneuropathy: protein aggregation in the peripheral nervous system. J Mol Neurosci. 2004;23(1-2):35-40. PubMed.
18. Bachmaier K, Neu N, de la Maza LM, Pal S, Hessel A, Penninger JM. Chlamydia infections and heart disease linked through antigenic mimicry. Science. 1999 Feb 26;283(5406):1335-9. PubMed.
19. Mruthinti S, Buccafusco JJ, Hill WD, Waller JL, Jackson TW, Zamrini EY, Schade RF. Autoimmunity in Alzheimer's disease: increased levels of circulating IgGs binding Abeta and RAGE peptides. Neurobiol Aging. 2004 Sep;25(8):1023-32. PubMed.
20. Loeb MB, Molloy DW, Smieja M, Standish T, Goldsmith CH, Mahony J, Smith S, Borrie M, Decoteau E, Davidson W, McDougall A, Gnarpe J, O'DONNell M, Chernesky M. A randomized, controlled trial of doxycycline and rifampin for patients with Alzheimer's disease. J Am Geriatr Soc. 2004 Mar;52(3):381-7. PubMed.

Other Citations

1. Abstract

External Citations

1. (See Science Direct
2. poster P3-189

Further Reading