Chlamydia pneumoniae: An Etiologic Agent for Late-Onset Dementia

The Chlamydia pneumoniae effector SemD exploits its host's endocytic machinery by structural and functional mimicry

To enter epithelial cells, the obligate intracellular pathogen Chlamydia pneumoniae secretes early effector proteins, which bind to and modulate the host-cell's plasma membrane and recruit several pivotal endocytic host proteins. Here, we present the high-resolution structure of an entry-related chlamydial effector protein, SemD. Co-crystallisation of SemD with its host binding partners demonstrates that SemD co-opts the Cdc42 binding site to activate the actin cytoskeleton regulator N-WASP, making active, GTP-bound Cdc42 superfluous. While SemD binds N-WASP much more strongly than Cdc42 does, it does not bind the Cdc42 effector protein FMNL2, indicating effector protein specificity. Furthermore, by identifying flexible and structured domains, we show that SemD can simultaneously interact with the membrane, the endocytic protein SNX9, and N-WASP. Here, we show at the structural level how a single effector protein can hijack central components of the host's endocytic system for efficient internalization.

Chlamydia pneumoniae in Alzheimer's disease pathology

While recent advances in diagnostics and therapeutics offer promising new approaches for Alzheimer's disease (AD) diagnosis and treatment, there is still an unmet need for an effective remedy, suggesting new avenues of research are required. Besides many plausible etiologies for AD pathogenesis, mounting evidence supports a possible role for microbial infections. Various microbes have been identified in the postmortem brain tissues of human AD patients. Among bacterial pathogens in AD, Chlamydia pneumoniae (Cp) has been well characterized in human AD brains and is a leading candidate for an infectious involvement. However, no definitive studies have been performed proving or disproving Cp's role as a causative or accelerating agent in AD pathology and cognitive decline. In this review, we discuss recent updates for the role of Cp in human AD brains as well as experimental models of AD. Furthermore, based on the current literature, we have compiled a list of potential mechanistic pathways which may connect Cp with AD pathology.

Clinically apparent Helicobacter pylori infection and the risk of incident Alzheimer's disease: A population-based nested case-control study

INTRODUCTION

Our population-based study assessed whether clinically apparent Helicobacter pylori infection (CAHPI) is associated with the risk of Alzheimer's disease (AD).

METHODS

We assembled a population-based cohort of all dementia-free subjects in the United Kingdom's Clinical Practice Research Datalink (UK CPRD), aged ≥50 years (1988-2017). Using a nested case-control approach, we matched each newly developed case of AD with 40 controls. Conditional logistic regression estimated odds ratios (ORs) with 95% confidence intervals (CIs) of AD associated with CAHPI compared with no CAHPI during ≥2 years before the index date. We also used salmonellosis as a negative control exposure.

RESULTS

Among 4,262,092 dementia-free subjects, 40,455 developed AD after a mean 11 years of follow-up. CAHPI was associated with an increased risk of AD (OR, 1.11; 95% CI, 1.01-1.21) compared with no CAHPI. Salmonellosis was not associated with the risk of AD (OR, 1.03; 95% CI, 0.82-1.29).

DISCUSSION

CAHPI was associated with a moderately increased risk of AD.

HIGHLIGHTS

CAHPI was associated with an 11% increased risk of AD in subjects aged ≥50 years. The increase in the risk of AD reached a peak of 24% a decade after CAHPI onset. There was no major effect modification by age or sex. Sensitivity analyses addressing several potential biases led to consistent results.

New approaches for understanding the potential role of microbes in Alzheimer's disease

Alzheimer's disease (AD) involves a complex pathological process that evolves over years, and its etiology is understood as a classic example of gene-environment interaction. The notion that exposure to microbial organisms may play some role in AD pathology has been proposed and debated for decades. New evidence from model organisms and -omic studies, as well as epidemiological data from the recent COVID-19 pandemic and widespread use of vaccines, offers new insights into the "germ hypothesis" of AD. To review new evidence and identify key research questions, the Duke/University of North Carolina (Duke/UNC) Alzheimer's Disease Research Center hosted a virtual symposium and workshop: "New Approaches for Understanding the Potential Role of Microbes in Alzheimer's disease." Discussion centered around the antimicrobial protection hypothesis of amyloid accumulation, and other mechanisms by which microbes could influence AD pathology including immune cell activation, changes in blood-brain barrier, or direct neurotoxicity. This summary of proceedings reviews the content presented in the symposium and provides a summary of major topics and key questions discussed in the workshop.

Analysis of the Association Between Pathogen Exposure and the Risk of Dementia

Background

Previous research has suggested that pathogen infections may serve as potential contributors to dementia.

Objective

Consequently, the study aimed to evaluate whether pathogen exposure heightens the risk of dementia.

Methods

Between 2006 and 2010, a total of 8,144 individuals from the UK Biobank had data on pathogen antibodies and were included in the baseline assessment. Cox proportional hazard models were employed for the analysis.

Results

Out of the 8,144 participants, 107 eventually developed dementia, while 55 participants were diagnosed with Alzheimer's disease (AD). Multivariate Cox regression analysis revealed that the levels of pathogen antibody titers of EBV and C. trachomatis were associated with an increased risk of dementia/AD. The highest quartile of EBV EBNA-1 and EBV VCA p18, and the second quartile of H. pylori VacA significantly increased the risk of dementia compared lower quartile (EBV EBNA-1: HR = 1.938, p = 0.018; EBV VCA p18: HR = 1.824, p = 0.040; H. pylori VacA: HR = 1.890, p = 0.033). Besides, the highest quartile of EBV VCA p18 had a higher risk of AD compared lower quartile (HR = 2.755, p = 0.029).

Conclusions

The study demonstrated that exposure to EBV, H. pylori, and C. trachomatis substantially elevated the risk of dementia/AD. Despite the relatively widespread occurrence of EBV infection in the population, elevated pathogen antibody titers were still found to increase the risk of dementia/AD. Besides, since C. trachomatis and C. pneumoniae are quite homologous, this study found that trachomatis (C. trachomatis/C. pneumoniae) may be significantly associated with the risk of AD/dementia.

Vaccines and Dementia: Part II. Efficacy of BCG and Other Vaccines Against Dementia

 There is growing awareness that infections may contribute to the development of senile dementia including Alzheimer's disease (AD), and that immunopotentiation is therefore a legitimate target in the management of diseases of the elderly including AD. In Part I of this work, we provided a historical and molecular background to how vaccines, adjuvants, and their component molecules can elicit broad-spectrum protective effects against diverse agents, culminating in the development of the tuberculosis vaccine strain Bacille Calmette-Guérin (BCG) as a treatment for some types of cancer as well as a prophylactic against infections of the elderly such as pneumonia. In Part II, we critically review studies that BCG and other vaccines may offer a measure of protection against dementia development. Five studies to date have determined that intravesicular BCG administration, the standard of care for bladder cancer, is followed by a mean ∼45% reduction in subsequent AD development in these patients. Although this could potentially be ascribed to confounding factors, the finding that other routine vaccines such as against shingles (herpes zoster virus) and influenza (influenza A virus), among others, also offer a degree of protection against AD (mean 29% over multiple studies) underlines the plausibility that the protective effects are real. We highlight clinical trials that are planned or underway and discuss whether BCG could be replaced by key components of the mycobacterial cell wall such as muramyl dipeptide. We conclude that BCG and similar agents merit far wider consideration as prophylactic agents against dementia.

A reappraisal on amyloid cascade hypothesis: the role of chronic infection in Alzheimer's disease

Alzheimer disease (AD) is a progressive neurological disorder that accounted for the most common cause of dementia in the elderly population. Lately, 'infection hypothesis' has been proposed where the infection of microbes can lead to the pathogenesis of AD. Among different types of microbes, human immunodeficiency virus-1 (HIV-1), herpes simplex virus-1 (HSV-1), Chlamydia pneumonia, Spirochetes and Candida albicans are frequently detected in the brain of AD patients. Amyloid-beta protein has demonstrated to exhibit antimicrobial properties upon encountering these pathogens. It can bind to microglial cells and astrocytes to activate immune response and neuroinflammation. Nevertheless, HIV-1 and HSV-1 can develop into latency whereas Chlamydia pneumonia, Spirochetes and Candida albicans can cause chronic infections. At this stage, the DNA of microbes remains undetectable yet active. This can act as the prolonged pathogenic stimulus that over-triggers the expression of Aβ-related genes, which subsequently lead to overproduction and deposition of Aβ plaque. This review will highlight the pathogenesis of each of the stated microbial infection, their association in AD pathogenesis as well as the effect of chronic infection in AD progression. Potential therapies for AD by modulating the microbiome have also been suggested. This review will aid in understanding the infectious manifestations of AD.

Establishment of a consensus protocol to explore the brain pathobiome in patients with mild cognitive impairment and Alzheimer's disease: Research outline and call for collaboration

Microbial infections of the brain can lead to dementia, and for many decades microbial infections have been implicated in Alzheimer's disease (AD) pathology. However, a causal role for infection in AD remains contentious, and the lack of standardized detection methodologies has led to inconsistent detection/identification of microbes in AD brains. There is a need for a consensus methodology; the Alzheimer's Pathobiome Initiative aims to perform comparative molecular analyses of microbes in post mortem brains versus cerebrospinal fluid, blood, olfactory neuroepithelium, oral/nasopharyngeal tissue, bronchoalveolar, urinary, and gut/stool samples. Diverse extraction methodologies, polymerase chain reaction and sequencing techniques, and bioinformatic tools will be evaluated, in addition to direct microbial culture and metabolomic techniques. The goal is to provide a roadmap for detecting infectious agents in patients with mild cognitive impairment or AD. Positive findings would then prompt tailoring of antimicrobial treatments that might attenuate or remit mounting clinical deficits in a subset of patients.

Neuroinflammation in Alzheimer's Disease: A Potential Role of Nose-Picking in Pathogen Entry via the Olfactory System?

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by progressive cognitive decline and memory impairment. Many possible factors might contribute to the development of AD, including amyloid peptide and tau deposition, but more recent evidence suggests that neuroinflammation may also play an-at least partial-role in its pathogenesis. In recent years, emerging research has explored the possible involvement of external, invading pathogens in starting or accelerating the neuroinflammatory processes in AD. In this narrative review, we advance the hypothesis that neuroinflammation in AD might be partially caused by viral, bacterial, and fungal pathogens entering the brain through the nose and the olfactory system. The olfactory system represents a plausible route for pathogen entry, given its direct anatomical connection to the brain and its involvement in the early stages of AD. We discuss the potential mechanisms through which pathogens may exploit the olfactory pathway to initiate neuroinflammation, one of them being accidental exposure of the olfactory mucosa to hands contaminated with soil and feces when picking one's nose.

Friends and Foes in Alzheimer's Disease

Alzheimer's disease (AD) is a disabling neurodegenerative disease. The prognosis is poor, and currently there are no proven effective therapies. Most likely, the etiology is related to cerebral inflammatory processes that cause neuronal damage, resulting in dysfunction and apoptosis of nerve cells. Pathogens that evoke a neuroinflammatory response, collectively activate astrocytes and microglia, which contributes to the secretion of pro-inflammatory cytokines. This leads to the deposit of clustered fragments of beta-amyloid and misfolded tau proteins which do not elicit an adequate immune reaction. Apart from the function of astrocytes and microglia, molecular entities such as TREM2, SYK, C22, and C33 play a role in the physiopathology of AD. Furthermore, bacteria and viruses may trigger an overactive inflammatory response in the brain. Pathogens like Helicobacter pylori, Chlamydia pneumonia, and Porphyromonas gingivalis (known for low-grade infection in the oral cavity) can release gingipains, which are enzymes that can damage and destroy neurons. Chronic infection with Borrelia burgdorferi (the causative agent of Lyme disease) can co-localize with tau tangles and amyloid deposits. As for viral infections, herpes simplex virus 1, cytomegalovirus, and Epstein-Barr virus can play a role in the pathogenesis of AD. Present investigations have resulted in the development of antibodies that can clear the brain of beta-amyloid plaques. Trials with humanized aducanumab, lecanemab, and donanemab revealed limited success in AD patients. However, AD should be considered as a continuum in which the initial preclinical phase may take 10 or even 20 years. It is generally thought that this phase offers a window for efficacious treatment. Therefore, research is also focused on the identification of biomarkers for early AD detection. In this respect, the plasma measurement of neurofilament light chain in patients treated with hydromethylthionine mesylate may well open a new way to prevent the formation of tau tangles and represents the first treatment for AD at its roots.

Clinical evidence of human pathogens implicated in Alzheimer's disease pathology and the therapeutic efficacy of antimicrobials: an overview

A wealth of pre-clinical reports and data derived from human subjects and brain autopsies suggest that microbial infections are relevant to Alzheimer's disease (AD). This has inspired the hypothesis that microbial infections increase the risk or even trigger the onset of AD. Multiple models have been developed to explain the increase in pathogenic microbes in AD patients. Although this hypothesis is well accepted in the field, it is not yet clear whether microbial neuroinvasion is a cause of AD or a consequence of the pathological changes experienced by the demented brain. Along the same line, the gut microbiome has also been proposed as a modulator of AD. In this review, we focus on human-based evidence demonstrating the elevated abundance of microbes and microbe-derived molecules in AD hosts as well as their interactions with AD hallmarks. Further, the direct-purpose and potential off-target effects underpinning the efficacy of anti-microbial treatments in AD are also addressed.

Viruses - a major cause of amyloid deposition in the brain

INTRODUCTION

Clinically, Alzheimer's disease (AD) is a syndrome with a spectrum of various cognitive disorders. There is a complete dissociation between the pathology and the clinical presentation. Therefore, we need a disruptive new approach to be able to prevent and treat AD.

AREAS COVERED

In this review, the authors extensively discuss the evidence why the amyloid beta is not the pathological cause of AD which makes therefore the amyloid hypothesis not sustainable anymore. They review the experimental evidence underlying the role of microbes, especially that of viruses, as a trigger/cause for the production of amyloid beta leading to the establishment of a chronic neuroinflammation as the mediator manifesting decades later by AD as a clinical spectrum. In this context, the emergence and consequences of the infection/antimicrobial protection hypothesis are described. The epidemiological and clinical data supporting this hypothesis are also analyzed.

EXPERT OPINION

For decades, we have known that viruses are involved in the pathogenesis of AD. This discovery was ignored and discarded for a long time. Now we should accept this fact, which is not a hypothesis anymore, and stimulate the research community to come up with new ideas, new treatments, and new concepts.

AGE-RAGE axis culminates into multiple pathogenic processes: a central road to neurodegeneration

Advanced glycation end-products (AGEs; e.g., glyoxal, methylglyoxal or carboxymethyl-lysine) are heterogenous group of toxic compounds synthesized in the body through both exogenous and endogenous pathways. AGEs are known to covalently modify proteins bringing about loss of functional alteration in the proteins. AGEs also interact with their receptor, receptor for AGE (RAGE) and such interactions influence different biological processes including oxidative stress and apoptosis. Previously, AGE-RAGE axis has long been considered to be the maligning factor for various human diseases including, diabetes, obesity, cardiovascular, aging, etc. Recent developments have revealed the involvement of AGE-RAGE axis in different pathological consequences associated with the onset of neurodegeneration including, disruption of blood brain barrier, neuroinflammation, remodeling of extracellular matrix, dysregulation of polyol pathway and antioxidant enzymes, etc. In the present article, we attempted to describe a new avenue that AGE-RAGE axis culminates to different pathological consequences in brain and therefore, is a central instigating component to several neurodegenerative diseases (NGDs). We also invoke that specific inhibitors of TIR domains of TLR or RAGE receptors are crucial molecules for the therapeutic intervention of NGDs. Clinical perspectives have also been appropriately discussed.

A single chlamydial protein reshapes the plasma membrane and serves as recruiting platform for central endocytic effector proteins

Uptake of obligate intracellular bacterial pathogens into mammalian epithelial cells is critically dependent on modulation of the host's endocytic machinery. It is an open question how the invading pathogens generate a membrane-bound vesicle appropriate to their size. This requires extensive deformation of the host plasma membrane itself by pathogen-derived membrane-binding proteins, accompanied by substantial F-actin-based forces to further expand and finally pinch off the vesicle. Here we show that upon adhesion to the host cell, the human pathogenic bacterium Chlamydia pneumoniae secretes the scaffolding effector protein CPn0677, which binds to the inner leaflet of the invaginating host's PM, induces inwardly directed, negative membrane curvature, and forms a recruiting platform for the membrane-deforming BAR-domain containing proteins Pacsin and SNX9. In addition, while bound to the membrane, CPn0677 recruits monomeric G-actin, and its C-terminal region binds and activates N-WASP, which initiates branching actin polymerization via the Arp2/3 complex. Together, these membrane-bound processes enable the developing endocytic vesicle to engulf the infectious elementary body, while the associated actin network generates the forces required to reshape and detach the nascent vesicle from the PM. Thus, Cpn0677 (now renamed SemD) acts as recruiting platform for central components of the endocytic machinery during uptake of chlamydia.

Programmed ageing: decline of stem cell renewal, immunosenescence, and Alzheimer's disease

The characteristic maximum lifespan varies enormously across animal species from a few hours to hundreds of years. This argues that maximum lifespan, and the ageing process that itself dictates lifespan, are to a large extent genetically determined. Although controversial, this is supported by firm evidence that semelparous species display evolutionarily programmed ageing in response to reproductive and environmental cues. Parabiosis experiments reveal that ageing is orchestrated systemically through the circulation, accompanied by programmed changes in hormone levels across a lifetime. This implies that, like the circadian and circannual clocks, there is a master 'clock of age' (circavital clock) located in the limbic brain of mammals that modulates systemic changes in growth factor and hormone secretion over the lifespan, as well as systemic alterations in gene expression as revealed by genomic methylation analysis. Studies on accelerated ageing in mice, as well as human longevity genes, converge on evolutionarily conserved fibroblast growth factors (FGFs) and their receptors, including KLOTHO, as well as insulin-like growth factors (IGFs) and steroid hormones, as key players mediating the systemic effects of ageing. Age-related changes in these and multiple other factors are inferred to cause a progressive decline in tissue maintenance through failure of stem cell replenishment. This most severely affects the immune system, which requires constant renewal from bone marrow stem cells. Age-related immune decline increases risk of infection whereas lifespan can be extended in germfree animals. This and other evidence suggests that infection is the major cause of death in higher organisms. Immune decline is also associated with age-related diseases. Taking the example of Alzheimer's disease (AD), we assess the evidence that AD is caused by immunosenescence and infection. The signature protein of AD brain, Aβ, is now known to be an antimicrobial peptide, and Aβ deposits in AD brain may be a response to infection rather than a cause of disease. Because some cognitively normal elderly individuals show extensive neuropathology, we argue that the location of the pathology is crucial - specifically, lesions to limbic brain are likely to accentuate immunosenescence, and could thus underlie a vicious cycle of accelerated immune decline and microbial proliferation that culminates in AD. This general model may extend to other age-related diseases, and we propose a general paradigm of organismal senescence in which declining stem cell proliferation leads to programmed immunosenescence and mortality.

The electronic tree of life (eToL): a net of long probes to characterize the microbiome from RNA-seq data

BACKGROUND

Microbiome analysis generally requires PCR-based or metagenomic shotgun sequencing, sophisticated programs, and large volumes of data. Alternative approaches based on widely available RNA-seq data are constrained because of sequence similarities between the transcriptomes of microbes/viruses and those of the host, compounded by the extreme abundance of host sequences in such libraries. Current approaches are also limited to specific microbial groups. There is a need for alternative methods of microbiome analysis that encompass the entire tree of life.

RESULTS

We report a method to specifically retrieve non-human sequences in human tissue RNA-seq data. For cellular microbes we used a bioinformatic 'net', based on filtered 64-mer sequences designed from small subunit ribosomal RNA (rRNA) sequences across the Tree of Life (the 'electronic tree of life', eToL), to comprehensively (98%) entrap all non-human rRNA sequences present in the target tissue. Using brain as a model, retrieval of matching reads, re-exclusion of human-related sequences, followed by contig building and species identification, is followed by confirmation of the abundance and identity of the corresponding species groups. We provide methods to automate this analysis. The method reduces the computation time versus metagenomics by a factor of >1000. A variant approach is necessary for viruses. Again, because of significant matches between viral and human sequences, a 'stripping' approach is essential. Contamination during workup is a potential problem, and we discuss strategies to circumvent this issue. To illustrate the versatility of the method we report the use of the eToL methodology to unambiguously identify exogenous microbial and viral sequences in human tissue RNA-seq data across the entire tree of life including Archaea, Bacteria, Chloroplastida, basal Eukaryota, Fungi, and Holozoa/Metazoa, and discuss the technical and bioinformatic challenges involved.

CONCLUSIONS

This generic methodology is likely to find wide application in microbiome analysis including diagnostics.

High resolution 16S rRNA gene Next Generation Sequencing study of brain areas associated with Alzheimer's and Parkinson's disease

Introduction

Alzheimer's (AD) and Parkinson's disease (PD) are neurodegenerative conditions characterized by incremental deposition of β-amyloid (Aβ) and α-synuclein in AD and PD brain, respectively, in relatively conserved patterns. Both are associated with neuroinflammation, with a proposed microbial component for disease initiation and/or progression. Notably, Aβ and α-synuclein have been shown to possess antimicrobial properties. There is evidence for bacterial presence within the brain, including the oral pathobiont Porphyromonas gingivalis, with cognitive impairment and brain pathology being linked to periodontal (gum) disease and gut dysbiosis.

Methods

Here, we use high resolution 16S rRNA PCR-based Next Generation Sequencing (16SNGS) to characterize bacterial composition in brain areas associated with the early, intermediate and late-stage of the diseases.

Results and discussion

This study reveals the widespread presence of bacteria in areas of the brain associated with AD and PD pathology, with distinctly different bacterial profiles in blood and brain. Brain area profiles were overall somewhat similar, predominantly oral, with some bacteria subgingival and oronasal in origin, and relatively comparable profiles in AD and PD brain. However, brain areas associated with early disease development, such as the locus coeruleus, were substantially different in bacterial DNA content compared to areas affected later in disease etiology.

Infectious origin of Alzheimer’s disease: Amyloid beta as a component of brain antimicrobial immunity

The amyloid cascade hypothesis, focusing on pathological proteins aggregation, has so far failed to uncover the root cause of Alzheimer’s disease (AD), or to provide an effective therapy. This traditional paradigm essentially explains a mechanism involved in the development of sporadic AD rather than its cause. The failure of an overwhelming majority of clinical studies (99.6%) demonstrates that a breakthrough in therapy would be difficult if not impossible without understanding the etiology of AD. It becomes more and more apparent that the AD pathology might originate from brain infection. In this review, we discuss a potential role of bacteria, viruses, fungi, and eukaryotic parasites as triggers of AD pathology. We show evidence from the current literature that amyloid beta, traditionally viewed as pathological, actually acts as an antimicrobial peptide, protecting the brain against pathogens. However, in case of a prolonged or excessive activation of a senescent immune system, amyloid beta accumulation and aggregation becomes damaging and supports runaway neurodegenerative processes in AD. This is paralleled by the recent study by Alam and colleagues (2022) who showed that alpha-synuclein, the protein accumulating in synucleinopathies, also plays a critical physiological role in immune reactions and inflammation, showing an unforeseen link between the 2 unrelated classes of neurodegenerative disorders. The multiplication of the amyloid precursor protein gene, recently described by Lee and collegues (2018), and possible reactivation of human endogenous retroviruses by pathogens fits well into the same picture. We discuss these new findings from the viewpoint of the infection hypothesis of AD and offer suggestions for future research.

More than a century after its discovery, Alzheimer’s disease (AD) remains incurable and mysterious. The dominant hypothesis of amyloid cascade has succeeded in explaining the key pathological mechanism, but not its trigger. Amyloid beta has been traditionally considered a pathological peptide, and its physiological functions remain poorly known. These knowledge gaps have contributed to repeated failures of clinical studies. The emerging infectious hypothesis of AD considers central nervous system (CNS) infection the primary trigger of sporadic AD. A closely connected hypothesis claims that amyloid beta is an antimicrobial peptide. In this review, we discuss the available evidence for the involvement of infections in AD, coming from epidemiological studies, post mortem analyses of brain tissue, and experiments in vitro and in vivo. We argue there is no unique “Alzheimer’s germ,” instead, AD is a general reaction of the CNS to chronic infections, in the milieu of an aged immune system. The pathology may become self-sustained even without continuous presence of microbes in the brain. Importantly, the infectious hypothesis leads to testable predictions. Targeting amyloid beta should be ineffective, unless the triggering pathogen and inflammatory response are addressed as well. Meticulous control of selected infections might be the best near-term strategy for AD prevention.

Circulating Chlamydia Trachomatis Antigens in Subjects With Alzheimer's Disease

BACKGROUND/AIM

Chlamydia pneumoniae (C. pneumoniae) is implicated in the pathogenesis of Alzheimer's disease (AD). Chlamydial elementary and reticulate bodies have been identified in tissues from afflicted AD brain regions by electron and immunoelectron microscopy, whereas similar tests of non-AD brains were negative for the bacterium. Studies in mice have shown that C. pneumoniae can rapidly penetrate the central nervous system by entering glia and causing beta amyloid deposition via the nerves between the nasal cavity and the brain, which serve as invasion pathways.

MATERIALS AND METHODS

We used data from the UK Biobank (UKBB) to assess the relationship of chlamydia and AD. Circulating C. pneumoniae antigen measurements were not available, but UKBB data field 23037 held measurements of PorB antigen for Chlamydia trachomatis (C. trachomatis). We used C. trachomatis as a surrogate for C. pneumoniae since serum cross-reactivity to C. trachomatis and C. pneumoniae antigens occurs in patients with documented infection and in healthy children as revealed by microimmunofluorescence and immunoblotting techniques. Single nucleotide polymorphism (SNP) data for rs429358 and rs7412 were used to impute ApoE genotypes.

RESULTS

PorB antigen levels for C. trachomatis were significantly higher in subjects with AD (p=0.007). PorB antigen levels were not related to ApoE genotype (e3e3, e3e4, e4e4) p=0.783. To control for the effects of age, sex, educational level, and apoE genotype, logistic regression analysis was performed. AD was the dependent variable. Independent variables were sqrt PorB antigen for C. trachomatis, age, sex, educational level, apoE genotype. AD odds ratio (OR) increased 1.156 for each unit increase of sqrt PorB antigen for C. trachomatis and the effect was significant (p=0.004).

CONCLUSION

PorB antigens for C. trachomatis being significantly higher in subjects with AD, corroborates previous studies demonstrating that C. pneumoniae inflammation appears to play a role in AD development. AD may result from the reactivation of embryologic processes and pathways silenced at birth. A trigger for the reactivation may be bacterial or viral infections. Further studies are warranted.

Increased occurrence of Treponema spp. and double-species infections in patients with Alzheimer's disease

Although the link between microbial infections and Alzheimer's disease (AD) has been demonstrated in multiple studies, the involvement of pathogens in the development of AD remains unclear. Here, we investigated the frequency of the 10 most commonly cited viral (HSV-1, EBV, HHV-6, HHV-7, and CMV) and bacterial (Chlamydia pneumoniae, Helicobacter pylori, Borrelia burgdorferi, Porphyromonas gingivalis, and Treponema spp.) pathogens in serum, cerebrospinal fluid (CSF) and brain tissues of AD patients. We have used an in-house multiplex PCR kit for simultaneous detection of five bacterial and five viral pathogens in serum and CSF samples from 50 AD patients and 53 healthy controls (CTRL). We observed a significantly higher frequency rate of AD patients who tested positive for Treponema spp. compared to controls (AD: 62.2 %; CTRL: 30.3 %; p-value = 0.007). Furthermore, we confirmed a significantly higher occurrence of cases with two or more simultaneous infections in AD patients compared to controls (AD: 24 %; CTRL 7.5 %; p-value = 0.029). The studied pathogens were detected with comparable frequency in serum and CSF. In contrast, Borrelia burgdorferi, human herpesvirus 7, and human cytomegalovirus were not detected in any of the studied samples. This study provides further evidence of the association between microbial infections and AD and shows that paralleled analysis of multiple sample specimens provides complementary information and is advisable for future studies.

Does Dementia Have a Microbial Cause?

The potential contribution of pathogenic microbes to dementia-inducing disease is a subject of considerable importance. Alzheimer’s disease (AD) is a neurocognitive disease that slowly destroys brain function, leading to cognitive decline and behavioral and psychiatric disorders. The histopathology of AD is associated with neuronal loss and progressive synaptic dysfunction, accompanied by the deposition of amyloid-β (Aβ) peptide in the form of parenchymal plaques and abnormal aggregated tau protein in the form of neurofibrillary tangles. Observational, epidemiological, experimental, and pathological studies have generated evidence for the complexity and possible polymicrobial causality in dementia-inducing diseases. The AD pathogen hypothesis states that pathogens and microbes act as triggers, interacting with genetic factors to initiate the accumulation of Aβ, hyperphosphorylated tau protein (p-tau), and inflammation in the brain. Evidence indicates that Borrelia sp., HSV-1, VZV (HHV-2), HHV-6/7, oral pathogens, Chlamydophila pneumoniae, and Candida albicans can infect the central nervous system (CNS), evade the immune system, and consequently prevail in the AD brain. Researchers have made significant progress in understanding the multifactorial and overlapping factors that are thought to take part in the etiopathogenesis of dementia; however, the cause of AD remains unclear.

Streptococcus agalactiae Infects Glial Cells and Invades the Central Nervous System via the Olfactory and Trigeminal Nerves

Streptococcus agalactiae causes neonatal meningitis and can also infect the adult central nervous system (CNS). S. agalactiae can cross the blood-brain barrier but may also reach the CNS via other paths. Several species of bacteria can directly invade the CNS via the olfactory and trigeminal nerves, which extend between the nasal cavity and brain and injury to the nasal epithelium can increase the risk/severity of infection. Preterm birth is associated with increased risk of S. agalactiae infection and with nasogastric tube feeding. The tubes, also used in adults, can cause nasal injuries and may be contaminated with bacteria, including S. agalactiae. We here investigated whether S. agalactiae could invade the CNS after intranasal inoculation in mice. S. agalactiae rapidly infected the olfactory nerve and brain. Methimazole-mediated model of nasal epithelial injury led to increased bacterial load in these tissues, as well as trigeminal nerve infection. S. agalactiae infected and survived intracellularly in cultured olfactory/trigeminal nerve- and brain-derived glia, resulting in cytokine production, with some differences between glial types. Furthermore, a non-capsulated S. agalactiae was used to understand the role of capsule on glial cells interaction. Interestingly, we found that the S. agalactiae capsule significantly altered cytokine and chemokine responses and affected intracellular survival in trigeminal glia. In summary, this study shows that S. agalactiae can infect the CNS via the nose-to-brain path with increased load after epithelial injury, and that the bacteria can survive in glia.

Chlamydia pneumoniae can infect the central nervous system via the olfactory and trigeminal nerves and contributes to Alzheimer's disease risk

Chlamydia pneumoniae is a respiratory tract pathogen but can also infect the central nervous system (CNS). Recently, the link between C. pneumoniae CNS infection and late-onset dementia has become increasingly evident. In mice, CNS infection has been shown to occur weeks to months after intranasal inoculation. By isolating live C. pneumoniae from tissues and using immunohistochemistry, we show that C. pneumoniae can infect the olfactory and trigeminal nerves, olfactory bulb and brain within 72 h in mice. C. pneumoniae infection also resulted in dysregulation of key pathways involved in Alzheimer’s disease pathogenesis at 7 and 28 days after inoculation. Interestingly, amyloid beta accumulations were also detected adjacent to the C. pneumoniae inclusions in the olfactory system. Furthermore, injury to the nasal epithelium resulted in increased peripheral nerve and olfactory bulb infection, but did not alter general CNS infection. In vitro, C. pneumoniae was able to infect peripheral nerve and CNS glia. In summary, the nerves extending between the nasal cavity and the brain constitute invasion paths by which C. pneumoniae can rapidly invade the CNS likely by surviving in glia and leading to Aβ deposition.

The Brain-Nose Interface: A Potential Cerebrospinal Fluid Clearance Site in Humans

The human brain functions at the center of a network of systems aimed at providing a structural and immunological layer of protection. The cerebrospinal fluid (CSF) maintains a physiological homeostasis that is of paramount importance to proper neurological activity. CSF is largely produced in the choroid plexus where it is continuous with the brain extracellular fluid and circulates through the ventricles. CSF movement through the central nervous system has been extensively explored. Across numerous animal species, the involvement of various drainage pathways in CSF, including arachnoid granulations, cranial nerves, perivascular pathways, and meningeal lymphatics, has been studied. Among these, there is a proposed CSF clearance route spanning the olfactory nerve and exiting the brain at the cribriform plate and entering lymphatics. While this pathway has been demonstrated in multiple animal species, evidence of a similar CSF egress mechanism involving the nasal cavity in humans remains poorly consolidated. This review will synthesize contemporary evidence surrounding CSF clearance at the nose-brain interface, examining across species this anatomical pathway, and its possible significance to human neurodegenerative disease. Our discussion of a bidirectional nasal pathway includes examination of the immune surveillance in the olfactory region protecting the brain. Overall, we expect that an expanded discussion of the brain-nose pathway and interactions with the environment will contribute to an improved understanding of neurodegenerative and infectious diseases, and potentially to novel prevention and treatment considerations.

The role of Aβ in Alzheimer's Disease as an Evolutionary Outcome of Optimized Innate Immune Defense

Alzheimer's Disease is a progressive manifestation of aging associated with accumulated Amyloid β. It remains frustratingly unclear why this protein accumulates and how it contributes to Alzheimer's Disease pathology. In one recent hypothesis, Amyloid β is suggested to function as an antimicrobial peptide in innate immune defense within the brain, where Amyloid β gains toxicity when it becomes abundant. This essay proposes an evolutionary explanation for why Amyloid β expression is regulated at an optimum based on its function as a defense and how this leads to disease. Among its potential physiological functions, Amyloid β confers benefits to reduce direct pathogen damage while this simultaneously entails cellular cost of defense. Optimal Amyloid β expression occurs when the gain in fitness from an incremental increase is balanced by the marginal cost of this increase. It proposes that natural selection acting upon the young favored systems to maintain Amyloid β at an optimal level through mechanisms that induce the defense and repress its expression. With age, the force of natural selection declines and permits mechanisms of negative feedback repression to degenerate. Consequently, Amyloid β is expressed beyond its optimum. Age also elevates cumulative pathogen exposure, reduces pathogen barriers and reactivates latent pathogens. The net effect is elevated, chronic induction of Amyloid β in the brain. The model recommends attention to innate immune negative regulation in the brain to discover ways to restore these functions toward a youthful state in the elderly.

Neuroimmune contributions to Alzheimer's disease: a focus on human data

The past decade has seen the convergence of a series of new insights that arose from genetic and systems analyses of Alzheimer's disease (AD) with a wealth of epidemiological data from a variety of fields; this resulted in renewed interest in immune responses as important, potentially causal components of AD. Here, we focus primarily on a review of human data which has recently yielded a set of robust, reproducible results that exist in a much larger universe of conflicting reports stemming from small studies with important limitations in their study design. Thus, we are at an important crossroads in efforts to first understand at which step of the long, multiphasic course of AD a given immune response may play a causal role and then modulate this response to slow or block the pathophysiology of AD. We have a wealth of new experimental tools, analysis methods, and capacity to sample human participants at large scale longitudinally; these resources, when coupled to a foundation of reproducible results and novel study designs, will enable us to monitor human immune function in the CNS at the level of complexity that is required while simultaneously capturing the state of the peripheral immune system. This integration of peripheral and central perturbations in immune responses results in pathologic responses in the central nervous system parenchyma where specialized cellular microenvironments composed of multiple cell subtypes respond to these immune perturbations as well as to environmental exposures, comorbidities and the impact of the advancing life course. Here, we offer an overview that seeks to illustrate the large number of interconnecting factors that ultimately yield the neuroimmune component of AD.

Gram-negative bacteria and their lipopolysaccharides in Alzheimer's disease: pathologic roles and therapeutic implications

Alzheimer's disease (AD) is the most serious age-related neurodegenerative disease and causes destructive and irreversible cognitive decline. Failures in the development of therapeutics targeting amyloid-β (Aβ) and tau, principal proteins inducing pathology in AD, suggest a paradigm shift towards the development of new therapeutic targets. The gram-negative bacteria and lipopolysaccharides (LPS) are attractive new targets for AD treatment. Surprisingly, an altered distribution of gram-negative bacteria and their LPS has been reported in AD patients. Moreover, gram-negative bacteria and their LPS have been shown to affect a variety of AD-related pathologies, such as Aβ homeostasis, tau pathology, neuroinflammation, and neurodegeneration. Moreover, therapeutic approaches targeting gram-negative bacteria or gram-negative bacterial molecules have significantly alleviated AD-related pathology and cognitive dysfunction. Despite multiple evidence showing that the gram-negative bacteria and their LPS play a crucial role in AD pathogenesis, the pathogenic mechanisms of gram-negative bacteria and their LPS have not been clarified. Here, we summarize the roles and pathomechanisms of gram-negative bacteria and LPS in AD. Furthermore, we discuss the possibility of using gram-negative bacteria and gram-negative bacterial molecules as novel therapeutic targets and new pathological characteristics for AD.

Borrelia burgdorferi Co-Localizing with Amyloid Markers in Alzheimer's Disease Brain Tissues

BACKGROUND

Infections by bacterial or viral agents have been hypothesized to influence the etiology of neurodegenerative diseases.

OBJECTIVE

This study examined the potential presence of Borrelia burgdorferi spirochete, the causative agent of Lyme disease, in brain autopsy tissue of patients diagnosed with either Alzheimer's (AD) or Parkinson's diseases.

METHODS

Brain tissue sections from patients with age-matched controls were evaluated for antigen and DNA presence of B. burgdorferi using various methods. Positive Borrelia structures were evaluated for co-localization with biofilm and AD markers such as amyloid and phospho-tau (p-Tau) using immunohistochemical methods.

RESULTS

The results showed the presence of B. burgdorferi antigen and DNA in patients with AD pathology and among those, one of them was previously diagnosed with Lyme disease. Interestingly, a significant number of Borrelia-positive aggregates with a known biofilm marker, alginate, were found along with the spirochetal structures. Our immunohistochemical data also showed that Borrelia-positive aggregates co-localized with amyloid and anti-phospho-tau markers. To further prove the potential relationship of B. burgdorferi and amyloids, we infected two mammalian cell lines with B. burgdorferi which resulted in a significant increase in the expression of amyloid-β and p-Tau proteins in both cells lines post-infection.

CONCLUSION

These results indicate that B. burgdorferi can be found in AD brain tissues, not just in spirochete but a known antibiotics resistant biofilm form, and its co-localized amyloid markers. In summary, this study provides evidence for a likely association between B. burgdorferi infections and biofilm formation, AD pathology, and chronic neurodegenerative diseases.

The Role of Chronic Infection in Alzheimer's Disease: Instigators, Co-conspirators, or Bystanders?

PURPOSE OF REVIEW

Herein, we provide a critical review of the clinical and translational research examining the relationship between viral and bacterial pathogens and Alzheimer's disease. In addition, we provide an overview of the biological pathways through which chronic infection may contribute to Alzheimer's disease.

RECENT FINDINGS

Dementia due to Alzheimer's disease is a leading cause of disability among older adults in developed countries, yet knowledge of the causative factors that promote Alzheimer's disease pathogenesis remains incomplete. Over the past several decades, numerous studies have demonstrated an association of chronic viral and bacterial infection with Alzheimer's disease. Implicated infectious agents include numerous herpesviruses (HSV-1, HHV-6, HHV-7) and various gastric, enteric, and oral bacterial species, as well as Chlamydia pneumonia and multiple spirochetes.

SUMMARY

Evidence supports the association between multiple pathogens and Alzheimer's disease risk. Whether these pathogens play a causal role in Alzheimer's pathophysiology remains an open question. We propose that the host immune response to active or latent infection in the periphery or in the brain triggers or accelerates the Alzheimer's disease processes, including the accumulation of amyloid-ß and pathogenic tau, and neuroinflammation. While recent research suggests that such theories are plausible, additional longitudinal studies linking microorganisms to Aß and phospho-tau development, neuroinflammation, and clinically defined Alzheimer's dementia are needed.

Neuroprotective effects of some epigenetic modifying drugs' on Chlamydia pneumoniae-induced neuroinflammation: A novel model

Chlamydia pneumoniae (Cpn) is a gram-negative intracellular pathogen that causes a variety of pulmonary diseases, and there is growing evidence that it may play a role in Alzheimer's disease (AD) pathogenesis. Cpn can interact functionally with host histones, altering the host's epigenetic regulatory system by introducing bacterial products into the host tissue and inducing a persistent inflammatory response. Because Cpn is difficult to propagate, isolate, and detect, a modified LPS-like neuroinflammation model was established using lyophilized cell free supernatant (CFS) obtained from infected cell cultures, and the effects of CFS were compared to LPS. The neuroprotective effects of Trichostatin A (TSA), givinostat, and RG108, which are effective on epigenetic mechanisms, and the antibiotic rifampin, were studied in this newly introduced model and in the presence of amyloid beta (Aβ) 1-42. The neuroprotective effects of the drugs, as well as the effects of CFS and LPS, were evaluated in Aβ-induced neurotoxicity using a real-time cell analysis system, total ROS, and apoptotic impact. TSA, RG108, givinostat, and rifampin all demonstrated neuroprotective effects in both this novel model and Aβ-induced neurotoxicity. The findings are expected to provide early evidence on neuroprotective actions against Cpn-induced neuroinflammation and Aβ-induced neurotoxicity, which could represent a new treatment option for AD, for which there are currently few treatment options.

Chlamydia pneumoniae and Helicobacter pylori infections and immunological profile of community-dwelling older adults

Chronic bacterial infections are associated with changes in the immunosenescence process and immunological biomarkers can assist in monitoring these changes. The identification of this immunological profile is important because Chlamydia pneumoniae (C. pneumoniae) and Helicobacter pylori (H. pylori) infections are important factors of morbidity and mortality among the older adults. This study aimed to identify changes in the immunological profile in the presence of C. pneumoniae and H. pylori infections among community-dwelling older adults. This is a cross-sectional study that used data from 1432 participants from the Bambuí Cohort Study of Aging, Minas Gerais, Brazil. The presence of immunoglobulin G (IgG) for C. pneumoniae and H. pylori was considered a dependent variable and assessed in the participants' serum using the enzyme-linked immunosorbent assay (ELISA). In assessing the immunological profile, the following inflammatory markers were considered: CXCL8, CXCL9, CXCL10, CCL2, CCL5, IL-1β, IL-6, IL-10, IL-12, TNF, and CRP. Associations were assessed by logistic regression, estimating odds ratios and confidence intervals (95%) using the Stata® V.13.1 software. The seroprevalence of anti-C. pneumoniae and anti-H. pylori antibodies was 55.9% and 70.3%, respectively. While high levels of anti-C. pneumoniae antibodies were associated with higher concentrations of CXCL10 and IL-10, higher levels of IL-1β and IL-6 were inversely associated with the titration of anti-H. pylori antibodies. The results characterize immunological profiles associated with these chronic infections and reinforce the potential effects of biomarkers on infections by these bacteria and on the immunosenescence process.

Association Between Antibiotic Treatment of Chlamydia pneumoniae and Reduced Risk of Alzheimer Dementia: A Nationwide Cohort Study in Taiwan

Background: Chlamydia pneumoniae (CPn) is a common community-acquired pneumonia. In the literature, CPn infection is demonstrated to exhibit an association with Alzheimer dementia (AD). We executed the present nationwide, population-based research with the goal of probing the association of CPn infection and antibiotic therapy with AD risk. Methods: We conducted a cohort study using a database extracted from Taiwan's National Health Insurance Research Database (NHIRD). All medical conditions for each enrolled individuals were categorized using the International Classification of Diseases, ninth Revision classifications. Hazard ratios (HRs) and 95% confidence intervals (CIs) for associations between CPn pneumonia-associated hospitalizations and AD were estimated using Fine and Gray's survival analysis and adjusted for comorbidities. The effects of the antibiotics on the HRs for AD in the patients with CPn pneumonia-associated hospitalization were also analyzed. Results: Our analyses included 6,628 individuals, including 1,657 CPn-infected patients, as well as 4,971 controls matched by age, index date, and sex (1:3). In this study, patients hospitalized for CPn pneumonia exhibited a significantly higher AD risk (adjusted HR = 1.599, 95% CI = 1.284-1.971, p < 0.001). We also noted an association of macrolide use (≥15 days) and fluoroquinolone use (≥15 days) with decreased AD risk. Conclusions: We determined CPn pneumonia to be associated with a relatively high AD risk. The result in this study confirmed the findings from previous literatures, by using a large, nationwide, population-based database. Appropriate macrolide and fluoroquinolone treatment may attenuate this risk.

Association between dementia and hepatitis B and C virus infection

ABSTRACT

Several viral infections are known to increase the risk of dementia through brain cell damage and systemic infection. The association between hepatitis B and C virus (HBV and HCV) infections and dementia was evaluated using a national sample cohort from South Korea. Using the national cohort study from the Korean National Health Insurance Service, we extracted data for patients with HBV or HCV infection and for matched control participants. The controls were matched to the patients according to age, sex, income, region of residence, and past medical histories. The incidence of HCV infection was higher in the dementia group (1.0% [113/11,228]) than in the control group (0.8% [364/44,912], P = .043). However, there was no difference in the incidence of HBV infection in the dementia and control groups. The adjusted odds ratio (OR) for HCV infection was 1.25 (95% confidence interval [CI] = 1.01-1.54, P = .043) in the dementia group. According to the subgroup analysis by sex, the adjusted ORs for HCV infection were 1.04 (95% CI = 072-1.49, P = .851) in men and 1.38 (95% CI = 1.06-1.79, P = .016) in women. We concluded that the incidence of HCV infection was higher (with a higher OR) in women with dementia than in matched control participants in South Korea.

Oxidative Stress and Inflammation in SARS-CoV-2- and Chlamydia pneumoniae-Associated Cardiovascular Diseases

Throughout the years, a growing number of studies have provided evidence that oxidative stress and inflammation may be involved in the pathogenesis of infectious agent-related cardiovascular diseases. Amongst the numerous respiratory pathogens, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a novel coronavirus responsible for the global ongoing pandemic, and Chlamydia pneumoniae, a widely known intracellular obligate bacteria, seem to have an essential role in promoting reactive oxygen species and cytokine production. The present review highlights the common oxidative and inflammatory molecular pathways underlying the cardiovascular diseases associated with SARS-CoV-2 or C. pneumoniae infections. The main therapeutic and preventive approaches using natural antioxidant compounds will be also discussed.

Hygiene hypothesis and autoimmune diseases: A narrative review of clinical evidences and mechanisms

Since the start of the "modern era", characterized by the increase in urbanization, a progressive attention to hygiene and autoimmune conditions has considerably grown. Although these diseases are often multifactorial, it was demonstrated that environment factors such as pollution, diet and lifestyles may play a crucial role together with genetic signature. Our research, based on the newest and most significant literature of this topic, highlights that the progressive depletion of microbes and parasites due to increased socioeconomic improvement, may lead to a derangement of immunoregulatory mechanisms. Moreover, special attention was given to the complex interplay between microbial agents, as gut microbiome, diet and vitamin D supplementation with the aim of identifying promising future therapeutic options. In conclusion, autoimmunity cannot be limited to hygiene-hypothesis, but from the point of view of precision medicine, this theory represents a fundamental element together with the study of genomics, the microbiome and proteomics, in order to understand the complex functioning of the immune system.

Phagocytosis by Peripheral Glia: Importance for Nervous System Functions and Implications in Injury and Disease

The central nervous system (CNS) has very limited capacity to regenerate after traumatic injury or disease. In contrast, the peripheral nervous system (PNS) has far greater capacity for regeneration. This difference can be partly attributed to variances in glial-mediated functions, such as axon guidance, structural support, secretion of growth factors and phagocytic activity. Due to their growth-promoting characteristic, transplantation of PNS glia has been trialed for neural repair. After peripheral nerve injuries, Schwann cells (SCs, the main PNS glia) phagocytose myelin debris and attract macrophages to the injury site to aid in debris clearance. One peripheral nerve, the olfactory nerve, is unique in that it continuously regenerates throughout life. The olfactory nerve glia, olfactory ensheathing cells (OECs), are the primary phagocytes within this nerve, continuously clearing axonal debris arising from the normal regeneration of the nerve and after injury. In contrast to SCs, OECs do not appear to attract macrophages. SCs and OECs also respond to and phagocytose bacteria, a function likely critical for tackling microbial invasion of the CNS via peripheral nerves. However, phagocytosis is not always effective; inflammation, aging and/or genetic factors may contribute to compromised phagocytic activity. Here, we highlight the diverse roles of SCs and OECs with the focus on their phagocytic activity under physiological and pathological conditions. We also explore why understanding the contribution of peripheral glia phagocytosis may provide us with translational strategies for achieving axonal regeneration of the injured nervous system and potentially for the treatment of certain neurological diseases.

The Role of Inflammation and Infection in Age-Related Neurodegenerative Diseases: Lessons From Bacterial Meningitis Applied to Alzheimer Disease and Age-Related Macular Degeneration

Age-related neurodegenerative diseases, such as Alzheimer disease (AD) and age-related macular degeneration (AMD), are multifactorial and have diverse genetic and environmental risk factors. Despite the complex nature of the diseases, there is long-standing, and growing, evidence linking microbial infection to the development of AD dementia, which we summarize in this article. Also, we highlight emerging research findings that support a role for parainfection in the pathophysiology of AMD, a disease of the neurosensory retina that has been shown to share risk factors and pathological features with AD. Acute neurological infections, such as Bacterial Meningitis (BM), trigger inflammatory events that permanently change how the brain functions, leading to lasting cognitive impairment. Neuroinflammation likewise is a known pathological event that occurs in the early stages of chronic age-related neurodegenerative diseases AD and AMD and might be triggered as a parainfectious event. To date, at least 16 microbial pathogens have been linked to the development of AD; on the other hand, investigation of a microbe-AMD relationship is in its infancy. This mini-review article provides a synthesis of existing evidence indicating a contribution of parainfection in the aetiology of AD and of emerging findings that support a similar process in AMD. Subsequently, it describes the major immunopathological mechanisms that are common to BM and AD/AMD. Together, this evidence leads to our proposal that both AD and AMD may have an infectious aetiology that operates through a dysregulated inflammatory response, leading to deleterious outcomes. Last, it draws fresh insights from the existing literature about potential therapeutic options for BM that might alleviate neurological disruption associated with infections, and which could, by extension, be explored in the context of AD and AMD.

The role of microbial infection in the pathogenesis of Alzheimer's disease and the opportunity for protection by anti-microbial peptides

Alzheimer's disease (AD) is the most common cause of dementia. Its pathology is primarily characterized by extracellular deposits of amyloid β peptide and intracellular neurofibrillary tangles. Current rationales to explain the pathogenesis of AD include amyloid cascade, inflammation, infection defense and anti-microbial protection hypotheses. This review focuses on recent advances in the infection hypothesis, in particular on those pathogenic microbes that act systemically, via periodontal and gastro-intestinal infection routes. It is proposed that the evidence convincingly supports that pathogenic microbial infection is associated with, and is likely a causative trigger for, AD pathology. Microbes can drive AD pathology by two main pathways: either by directly infecting the brain and stimulating amyloid-mediated defence (causative trigger) or indirectly, by stimulating the pro-inflammatory effects of infection. In this context, it follows that anti-microbial/anti-infection therapies could be effective for regulating the pathology and symptoms of AD, depending on the stage of disease. As long-term administration of traditional antibiotic therapy is not recommended, alternative antibiotic agents such as anti-microbial peptides (AMPs), could be preferred for intervention and disease management of AD.

Chlamydia muridarum Can Invade the Central Nervous System via the Olfactory and Trigeminal Nerves and Infect Peripheral Nerve Glial Cells

Chlamydia pneumoniae can infect the brain and has been linked to late-onset dementia. Chlamydia muridarum, which infects mice, is often used to model human chlamydial infections. While it has been suggested to be also important for modelling brain infection, nervous system infection by C. muridarum has not been reported in the literature. C. pneumoniae has been shown to infect the olfactory bulb in mice after intranasal inoculation, and has therefore been suggested to invade the brain via the olfactory nerve; however, nerve infection has not been shown to date. Another path by which certain bacteria can reach the brain is via the trigeminal nerve, but it remains unknown whether Chlamydia species can infect this nerve. Other bacteria that can invade the brain via the olfactory and/or trigeminal nerve can do so rapidly, however, whether Chlamydia spp. can reach the brain earlier than one-week post inoculation remains unknown. In the current study, we showed that C. muridarum can within 48 h invade the brain via the olfactory nerve, in addition to infecting the trigeminal nerve. We also cultured the glial cells of the olfactory and trigeminal nerves and showed that C. muridarum readily infected the cells, constituting a possible cellular mechanism explaining how the bacteria can invade the nerves without being eliminated by glial immune functions. Further, we demonstrated that olfactory and trigeminal glia differed in their responses to C. muridarum, with olfactory glia showing less infection and stronger immune response than trigeminal glia.

Targeting Impaired Antimicrobial Immunity in the Brain for the Treatment of Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia and aging is the most common risk factor for developing the disease. The etiology of AD is not known but AD may be considered as a clinical syndrome with multiple causal pathways contributing to it. The amyloid cascade hypothesis, claiming that excess production or reduced clearance of amyloid-beta (Aβ) and its aggregation into amyloid plaques, was accepted for a long time as the main cause of AD. However, many studies showed that Aβ is a frequent consequence of many challenges/pathologic processes occurring in the brain for decades. A key factor, sustained by experimental data, is that low-grade infection leading to production and deposition of Aβ, which has antimicrobial activity, precedes the development of clinically apparent AD. This infection is chronic, low grade, largely clinically silent for decades because of a nearly efficient antimicrobial immune response in the brain. A chronic inflammatory state is induced that results in neurodegeneration. Interventions that appear to prevent, retard or mitigate the development of AD also appear to modify the disease. In this review, we conceptualize further that the changes in the brain antimicrobial immune response during aging and especially in AD sufferers serve as a foundation that could lead to improved treatment strategies for preventing or decreasing the progression of AD in a disease-modifying treatment.

Herpes Labialis, Chlamydophila pneumoniae, Helicobacter pylori, and Cytomegalovirus Infections and Risk of Dementia: The Framingham Heart Study

BACKGROUND

An association between chronic infectious diseases and development of dementia has been suspected for decades, based on the finding of pathogens in postmortem brain tissue and on serological evidence. However, questions remain regarding confounders, reverse causality, and how accurate, reproducible and generalizable those findings are.

OBJECTIVE

Investigate whether exposure to Herpes simplex (manifested as herpes labialis), Chlamydophila pneumoniae (C. pneumoniae), Helicobacter pylori (H. pylori), and cytomegalovirus (CMV) modifies the risk of dementia in a populational cohort.

METHODS

Questionnaires regarding incidence of herpes infections were administered to Original Framingham Study participants (n = 2,632). Serologies for C. pneumoniae, H. pylori, and CMV were obtained in Original (n = 2,351) and Offspring cohort (n = 3,687) participants. Participants are under continuous dementia surveillance. Brain MRI and neuropsychological batteries were administered to Offspring participants from 1999-2005. The association between each infection and incident dementia was tested with Cox models. Linear models were used to investigate associations between MRI or neuropsychological parameters and serologies.

RESULTS

There was no association between infection serologies and dementia incidence, total brain volume, and white matter hyperintensities. Herpes labialis was associated with reduced 10-year dementia risk (HR 0.66, CI 0.46-0.97), but not for the duration of follow-up. H. pylori antibodies were associated with worse global cognition (β -0.14, CI -0.22, -0.05).

CONCLUSION

We found no association between measures of chronic infection and incident dementia, except for a reduction in 10-year dementia risk for patients with herpes labialis. This unexpected result requires confirmation and further characterization, concerning antiviral treatment effects and capture of episodes.

Alzheimer's disease - the 'microbial hypothesis' from a clinical and neuroimaging perspective

The etiology of Alzheimer's disease (AD) is under debate since its first description in 1906. Extracellular senile plaques composed of beta-amyloid peptide (Aβ) and intracellular neurofibrillary tangles composed of tau protein characterize the histopathology of the disease. The 'amyloid cascade hypothesis' summarizes the molecular mechanisms leading to deposition of these proteins. However, treatments derived from this hypothesis have been unsuccessful. An infectious etiology for AD has been repeatedly proposed. Neurotropic viruses, gut and lung bacteriae, and Bovine Meat and Milk Factors have been implicated in neurodegenerative disorders including AD. These pathogens may act directly or as a trigger or co-factor for inducing neurodegeneration in AD. The antimicrobial properties of beta-amyloid have shifted the discussion of the etiological origin of AD towards an interaction hypothesis. Neuroimaging studies have added to the understanding of mechanisms involved in neurodegeneration. Antiviral agents and a bacterial protease inhibitor targeting Porphyromonas gingivalis toxins are currently tested in clinical trials. Further clinical studies are needed to test if strategies directly derived from the 'microbial hypothesis' or combination strategies including antimicrobial agents may be beneficial for patients with Alzheimer's disease.

A Disintegrin and Metalloproteinase-Control Elements in Infectious Diseases

Despite recent advances in treatment strategies, infectious diseases are still under the leading causes of death worldwide. Although the activation of the inflammatory cascade is one prerequisite of defense, persistent and exuberant immune response, however, may lead to chronicity of inflammation predisposing to a temporal or permanent tissue damage not only of the site of infection but also among different body organs. The initial response to invading pathogens is mediated by the recognition through various pattern-recognition receptors along with cellular engulfment resulting in a coordinated release of soluble effector molecules and cytokines aiming to terminate the external stimuli. Members of the ‘a disintegrin and metalloproteinase’ (ADAM) family have the capability to proteolytically cleave transmembrane molecules close to the plasma membrane, a process called ectodomain shedding. In fact, in infectious diseases dysregulation of numerous ADAM substrates such as junction molecules (e.g., E-cadherin, VE-cadherin, JAM-A), adhesion molecules (e.g., ICAM-1, VCAM-1, L-selectin), and chemokines and cytokines (e.g., CXCL16, TNF-α) has been observed. The alpha-cleavage by ADAM proteases represents a rate limiting step for downstream regulated intramembrane proteolysis (RIPing) of several substrates, which influence cellular differentiation, cell signaling pathways and immune modulation. Both the substrates mentioned above and RIPing crucially contribute to a systematic damage in cardiovascular, endocrine, and/or gastrointestinal systems. This review will summarize the current knowledge of ADAM function and the subsequent RIPing in infectious diseases (e.g., pathogen recognition and clearance) and discuss the potential long-term effect on pathophysiological changes such as cardiovascular diseases.

Human Herpesviruses 6A and 6B in Brain Diseases: Association versus Causation

Human herpesvirus 6A (HHV-6A) and human herpesvirus 6B (HHV-6B), collectively termed HHV-6A/B, are neurotropic viruses that permanently infect most humans from an early age. Although most people infected with these viruses appear to suffer no ill effects, the viruses are a well-established cause of encephalitis in immunocompromised patients. In this review, we summarize the evidence that the viruses may also be one trigger for febrile seizures (including febrile status epilepticus) in immunocompetent infants and children, mesial temporal lobe epilepsy, multiple sclerosis (MS), and, possibly, Alzheimer's disease. We propose criteria for linking ubiquitous infectious agents capable of producing lifelong infection to any neurologic disease, and then we examine to what extent these criteria have been met for these viruses and these diseases.

Can Toxoplasma gondii Pave the Road for Dementia?

Dementia is an ominous neurological disease. Scientists proposed a link between its occurrence and the presence of Toxoplasma gondii (T. gondii). The long-term sequels of anti-Toxoplasma premunition, chiefly dominated by TNF-α, on the neurons and their receptors as the insulin-like growth factor-1 receptor (IGF-1R), which is tangled in cognition and synaptic plasticity, are still not clear. IGF-1R mediates its action via IGF-1, and its depletion is incorporated in the pathogenesis of dementia. The activated TNF-α signaling pathway induces NF-κβ that may induce or inhibit neurogenesis. This study speculates the potential impact of anti-Toxoplasma immune response on the expression of IGF-1R in chronic cerebral toxoplasmosis. The distributive pattern of T. gondii cysts was studied in association with TNF-α serum levels, the in situ expression of NF-κβ, and IGF-1R in mice using the low virulent ME-49 T. gondii strain. There was an elevation of the TNF-α serum level (p value ≤ 0.004) and significant upsurge in NF-κβ whereas IGF-1R was of low abundance (p value < 0.05) compared to the controls. TNF-α had a strong positive correlation with the intracerebral expression of NF-κβ (r value ≈ 0.943, p value ≈ 0.005) and a strong negative correlation to IGF-1R (r value -0.584 and -0.725 for area% and O.D., respectively). This activated TNF-α/NF-κβ keeps T. gondii under control at the expense of IGF-1R expression, depriving neurons of the effect of IGF-1, the receptor's ligand. We therefore deduce that T. gondii immunopathological reaction may be a road paver for developing dementia.

Perspectives on the Intracellular Bacterium Chlamydia pneumoniae in Late-Onset Dementia

Purpose of Review

Chronic diseases remain a daunting challenge for clinicians and researchers alike. While difficult to completely understand, most chronic diseases, including late-onset dementias, are thought to arise as an interplay between host genetic factors and environmental insults. One of the most diverse and ubiquitous environmental insults centers on infectious agents. Associations of infectious agents with late-onset dementia have taken on heightened importance, including our investigations of infection by the intracellular respiratory bacterium, Chlamydia pneumoniae (Cpn), in late-onset dementia of the Alzheimer’s type.

Recent Findings

Over the last two decades, the relationship of this infection to pathogenesis in late-onset dementia has become much clearer. This clarity has resulted from applying contemporary molecular genetic, biochemical, immunochemical, and cell culture techniques to analysis of human brains, animal models, and relevant in vitro cell culture systems. Data from these studies, taken in aggregate form, now can be applied to evaluation of proof of concept for causation of this infection with late-onset disease. In this evaluation, modifications to the original Koch postulates can be useful for elucidating causation.

Summary

All such relevant studies are outlined and summarized in this review, and they demonstrate the utility of applying modified Koch postulates to the etiology of late-onset dementia of the Alzheimer’s type. Regardless, it is clear that even with strong observational evidence, in combination with application of modifications of Koch’s postulates, we will not be able to conclusively state that Cpn infection is causative for disease pathogenesis in late-onset dementia. Moreover, this conclusion obtains as well for the putative causation of this condition by other pathogens, including herpes simplex virus type 1, Borrelia burgdorferi, and Porphyromonas gingivalis.

From conifers to cognition: Microbes, brain and behavior

A diversity of bacteria, protozoans and viruses ("endozoites") were recently uncovered within healthy tissues including the human brain. By contrast, it was already recognized a century ago that healthy plants tissues contain abundant endogenous microbes ("endophytes"). Taking endophytes as an informative precedent, we overview the nature, prevalence, and role of endozoites in mammalian tissues, centrally focusing on the brain, concluding that endozoites are ubiquitous in diverse tissues. These passengers often remain subclinical, but they are not silent. We address their routes of entry, mechanisms of persistence, tissue specificity, and potential to cause long-term behavioral changes and/or immunosuppression in mammals, where rabies virus is the exemplar. We extend the discussion to Herpesviridae, Coronaviridae, and Toxoplasma, as well as to diverse bacteria and yeasts, and debate the advantages and disadvantages that endozoite infection might afford to the host and to the ecosystem. We provide a clinical perspective in which endozoites are implicated in neurodegenerative disease, anxiety/depression, and schizophrenia. We conclude that endozoites are instrumental in the delicate balance between health and disease, including age-related brain disease, and that endozoites have played an important role in the evolution of brain function and human behavior.