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Chronic Bacterial and Viral Infections in Neurodegenerative and Neurobehavioral Diseases

Garth L. Nicolson, PhD
Lab Med. 2008;39(5):291-299.

Often, patients with neurodegenerative or neurobehavioral diseases have chronic, neuropathic infections that could be important in disease inception, disease progression, or increasing the types or severities of signs and symptoms. Although controversial, the majority of patients with various neurodegenerative or neurobehavioral conditions, such as amyotrophic lateral sclerosis, multiple sclerosis, Alzheimer's disease, Parkinson's disease, and autistic spectrum disorders, show evidence of central nervous system or systemic bacterial and viral infections. For example, using serology or polymerase chain reaction evidence of Chlamydia pneumoniae, Borrelia burgdorferi, Mycoplasma species, human herpesvirus-1 and -6, and other bacterial and viral infections revealed high infection rates that were not found in control subjects. Although chronic infections were not found in some studies, and the specific role of chronic infections in neurological disease pathogenesis has not been determined or is inconclusive, the data suggest that chronic bacterial or viral infections could be common features of progressive neurodegenerative and neurobehavioral diseases.

Neurodegenerative diseases are chronic degenerative diseases of the central nervous system (CNS) that cause dementia. For the most part, the causes of these brain diseases remain largely unknown.[1] They are characterized by molecular and genetic changes in nerve cells that result in nerve cell degeneration and ultimately nerve dysfunction and death, resulting in neurological signs and symptoms and dementia.[1,2] In addition to neurodegenerative diseases, there are also neurobehavioral diseases that mainly, but not exclusively, appear in the young, such as autistic spectrum disorders (ASD) that encompass autism, attention deficit disorder, Asperger's syndrome, and other disorders.[3]

There appear to be genetic links to neurodegenerative and neurobehavioral diseases, but the genetic changes that occur and the changes in gene expression that have been found in these diseases are complex and not directly related to simple genetic alterations.1,4 In addition, it is thought that nutritional deficiencies, environmental toxins, heavy metals, chronic bacterial and viral infections, autoimmune immunological responses, vascular diseases, head trauma and accumulation of fluid in the brain, changes in neurotransmitter concentrations, among others, are involved in the pathogenesis of various neurodegenerative and neurobehavioral diseases.[1-3,5-8] One of the biochemical changes found in essentially all neurological, neurodegenerative, and neurobehavioral diseases is the overexpression of oxidative free radical compounds (oxidative stress) that cause lipid, protein, and genetic structural changes.[5-9]

Oxidative stress can be caused by a variety of environmental toxic insults, and when combined with genetic factors, pathogenic processes could result.[10] An attractive hypothesis for the causation or promotion of neurological disease involves chronic bacterial or viral toxic products, which result in the presence of excess reactive oxygen species and culminate in pathologic changes.[11,12]

Infectious agents may enter the CNS within infected migratory macrophages, they may gain access by transcytosis across the blood-brain barrier, or enter by intraneuronal transfer from peripheral nerves.[11] Cell-wall-deficient bacteria, principally species of Chlamydia (Chlamydophila), Borrelia, Brucella (among others), bacteria without cell walls, such as Mycoplasma species, and various viruses are candidate infectious agents that may play important roles in neurodegenerative and neurobehavoral diseases.[12-14] Since they are usually systemic, such infections can affect the immune system and other organ systems, resulting in a variety of systemic signs and symptoms.[15-18]

Amyotrophic lateral sclerosis (ALS) is an adult-onset, idopathic, progressive neurodegenerative disease affecting both central and peripheral motor neurons. Patients with ALS show gradual progressive weakness and paralysis of muscles due to destruction of upper motor neurons in the motor cortex and lower motor neurons in the brain stem and spinal cord, ultimately resulting in death, usually by respiratory failure.[19,20] The overall clinical picture of ALS can vary, depending on the location and progression of pathological changes found in nervous tissue.[21]

In ALS, the role of chronic infections has attracted attention with the finding of enterovirus sequences in a majority of spinal cord samples by polymerase chain reaction (PCR).[22,23] This finding is not without controversy, since others failed to detect enterovirus sequences in spinal cord samples from patients with or without ALS.[24,25] Nonetheless, infectious agents that penetrate the CNS may play a role in the etiology of ALS, although evidence for a transmission of an infectious agent or transfer of an ALS-like disease from man-to-man or man-to-animal has not been demonstrated.[26]

The presence of systemic mycoplasmal infections in ALS patients has been investigated with PCR methods.[27,28] Our studies indicated that 100% of Gulf War veterans diagnosed with ALS (N=8 from 3 different nations) had systemic mycoplasmal infections.[27] All but 1 patient had Mycoplasma fermentans, and 1 veteran from Australia had a systemic M. genitalium infection. In approximately 80% of nonmilitary (unrelated to military patients) ALS patients from the United States, Canada, and Great Britain (N=28), blood mycoplasmal infections were also found.[27] Of the mycoplasma-positive civilian patients who were further tested for M. penetrans, M. fermentans, M. hominis, and M. pneumoniae, most were positive for M. fermentans (59%), but other Mycoplasma species, such as M. hominis (31%) and M. pneumoniae infections (9%), were also found. Some of the civilian ALS patients had multiple mycoplasmal infections; however, multiple mycoplasmal infections were not found in the military patients with ALS.[27] In another study in Mexico, 10 of 20 ALS patients showed evidence of systemic Mycoplasma species by analysis of their blood by PCR.[28]

Another chronic infection that is commonly found in ALS patients who live in certain areas is Borrelia burgdorferi, the principal etiologic agent of Lyme disease (LD). For example, ALS patients who live in New York, an LD-intense area, were examined for B. burgdorferi infections, and over one-half were found to be seropositive for Lyme Borrelia compared with 10% of matched controls.[29] In addition, some patients diagnosed with ALS were subsequently diagnosed with neuroborreliosis.[30] A survey of the literature indicates that spirochetal forms have been observed for some time in the CNS tissue of ALS and other neurodegenerative diseases.[31] Thus, a byproduct of LD may be progression to ALS, but this is probably only possible in some LD patients who have the genetic susceptibility genes for the neurodegenerative disease and who have other toxic exposures.[32,33]

Amyotrophic lateral sclerosis patients also have other chronic infections, including human herpesvirus-6 (HHV6), Chlamydia pneumoniae, and other infections.[34,35] A suggestion that retroviruses might be involved in ALS and other motorneuron diseases[36] prompted McCormick and colleagues[37] to look for reverse transcriptase activity in serum and cerebrospinal fluid (CSF) of ALS and non-ALS patients. They found reverse transcriptase serum activity in one-half of ALS cases but in only 7% of controls (P <0.008). Interestingly, only 1 of 25 ALS CSF samples contained reverse transcriptase activity.[37]

The exact role that infections play in the pathogenesis or progression of ALS is not known. They could be cofactors in ALS pathogenesis, or they could simply be opportunistic infections that cause morbidity in ALS patients, such as the respiratory and rheumatic symptoms and other problems often found in ALS patients. They could also be involved in the progression of ALS rather than in its inception. Although the exact cause of ALS remains unknown, there are several hypotheses on its pathogenesis: (a) accumulation of glutamate causing excitotoxicity; (b) autoimmune reactions against motor neurons; (c) deficiency of nerve growth factor; (d) dysfunction of superoxide dismutase due to mutations; and (e) chronic infection(s).[22-24,27-29,31-34] None of these hypotheses is exclusive, and ALS may have a complex pathogenesis involving multiple factors.[34] Future studies should determine more precisely the role of chronic bacterial and viral infections in the pathogenesis and progression of ALS.

Multiple sclerosis (MS) is the most common demyelinating disease of the CNS, and it can occur in young as well as older people. In MS, inflammation and the presence of autoimmune antibodies against myelin and other nerve cell antigens are thought to cause the myelin sheath to break down, resulting in decrease or loss of electrical impulses along the nerves.[38,39] In the progressive subset of MS, neurological damage occurs additionally by the deposition of plaques on the nerve cells to the point where nerve cell death occurs. In addition, breakdown of the blood-brain barrier in MS is associated with local inflammation caused by glial cells.[38,39] The clinical manifestations of demyelinization, plaque damage, and blood-brain barrier disruptions are variable but usually include impaired vision, alterations in motor, sensory, and coordination systems, and cognitive dysfunction. Often these are cyclic (relapsing-remitting subset) over time, but a substantial MS subset progresses without remitting.[39]

There is strong evidence for a genetic component in MS.[40,41] Although it has been established that there is a genetic susceptibility component to MS, epidemiological and twin studies suggest that MS is an acquired, rather than an inherited, disease.[42]

The possibility that MS is linked to chronic infections has attracted attention.[43,44] In fact, MS patients show immunological and cytokine elevations consistent with chronic infections.[44-46] A possible infectious cause for MS has been under investigation for approximately the last decade, and patients have been examined for various viral and bacterial infections.[44,47] One of the most common findings in MS patients is the presence of antibodies and DNA of C. pneumoniae in their CSF.[47-49] For example, Sriram and colleagues[48] examined relapsing-remitting (N=17) and progressive (N=20) MS patients for the presence of C. pneumoniae in CSF by culture, PCR, and immunoglobulin reactivity with C. pneumoniae elementary body antigens. They were able to isolate C. pneumoniae from 64% of MS patients' CSF versus 11% of patients with other neurological diseases. High rates of PCR-positive (MOMP gene) patients (97% MS-positive versus 18% with other neurological diseases) as well as serology-positive patients (86% MS-positive, confirmed by enzyme-linked immunosorbant assays [ELISA] and Western blot analysis) were found in MS.[48] Further examination of MS patients for oligoclonal antibodies against C. pneumoniae revealed that 14 of 17 patients were positive, whereas none of the control non-MS patients had antibodies that were absorbed by C. pneumoniae elemental body antigens.[49]

Other studies have also found evidence for the presence of C. pneumoniae in MS patients but at lower incidence. Fainardi and colleagues[50] used ELISA techniques and found that high-affinity antibodies against C. pneumoniae were present in the CSF of 17% of 71 MS cases compared with 2% of 52 patients with noninflammatory neurological disorders. They found that the majority of the progressive forms of MS were positive compared with patients with remitting-relapsing MS. The presence of C. pneumoniae antibodies were also found in other inflammatory neurological disorders (N=51), and thus it was not specific to MS.[50] Using immunohistochemistry, Sriram and colleagues[51] performed a study of formalin-fixed CNS tissue from MS and non-MS neurological disease controls and found that in a subset (7 of 20) of MS patients, chlamydial antigens were localized to ependymal surfaces and pariventricular regions. Staining was not found in 17 CNS tissue samples from other neurological diseases. Frozen tissues were available in some of these MS cases, and PCR amplification of C. pneumoniae genes was accomplished in 5 of 8 CNS tissue samples from MS patients but none in 17 frozen CNS tissues from other neurological diseases. In addition, they examined CSF sediment by immuno-gold-labeled staining for chlamydial antigens and found by electron microscopy that the electron-dense bodies resembling bacterial structures correlated with PCR-positive results in 10 of 11 MS cases.[51] The same group also used different nested PCR methods to examine additional C. pneumoniae gene sequences in the CSF of 72 MS patients and linked these results to MRI evidence of MS-associated lesions.[52] Similarly, Grimaldi and colleagues[53] linked the presence of C. pneumoniae infection with abnormal MRI results in 23 of 107 MS patients with more progressive disease. In addition, a higher rate of C. pneumoniae transcription was found by Dong-Si and colleagues[54] in the CSF of 84 MS patients. The above, among other data,[55-57] support the presence of C. pneumoniae in the CNS of MS patients, at least in a subset of more progressed patients that are most likely the progressive forms of MS.

Not all studies have obtained evidence, however, for the presence of C. pneumoniae [58,59] or other bacteria[60] in the CNS of MS patients. Hammerschlag and colleagues[61] used nested PCR and culture to examine 12 frozen brain samples from MS patients but could not find C. pneumoniae in any of the tissue samples. Alternatively, in one study, C. pneumoniae was found at similar incidence in MS and other neurological diseases, but only MS patients had C. pneumoniae in their CSF.[59] Swanborg and colleagues[62] have reviewed the evidence linking C. pneumoniae infection with MS and have concluded that they are equivocal due to negative reports, and they also speculated that specific genetic changes may be necessary to fulfill the role of such infections in the etiology of MS.

Another possible reason for the equivocal evidence linking MS etiology with infection, such as C. pneumoniae, is that multiple coinfections could be involved. In addition to C. pneumoniae found in most studies, MS patients could also have Mycoplasma species, B. burgdorferi, and other bacterial infections as well as viral infections.[63] When multiple infections are considered, it is likely that >80% of MS patients have obligate intracellular bacterial infections caused by Chlamydia (Chlamydophila) or other bacteria that can be intracellular, such as Mycoplasma, Borrelia, and other infections. These infections were found only singly and at very low incidence in age-matched subjects.[63] In spite of these findings, others did not find evidence of Mycoplasma species in brain tissue (N=30), CSF, or peripheral blood (N=57) of MS patients.[64]

Viruses have also been associated with MS. Certain viruses have been found in MS patients, such as HHV6, but these viruses have also been found at lower incidence in control samples.[62] Sanders and colleagues[65] used PCR to examine postmortem brain tissue (N=37) and controls (N=61) for the presence of neurotrophic viruses. They found that 57% of MS cases and 43% of non-MS neurological disease controls were positive for HHV6, whereas 37% and 28%, respectively, were positive for herpes simplex virus (HSV1 and HSV2) and 43% and 32%, respectively, were positive for varicella zoster virus; however, these differences did not achieve significance, and the authors concluded that "an etiologic association to the MS disease process [is] uncertain." They also found that 32% of the MS active plaques and 17% of the inactive plaque areas were positive for HHV6.[65] Challoner and colleagues[66] used sequence difference analysis to search for pathogens in 86 MS brain specimens. Using PCR, they found that >70% of the MS specimens were positive. They also used immunocytochemistry and found staining around MS plaques more frequently than around white matter; nuclear staining of oligodendrocytes was also seen in MS samples but not in controls.[66] Using immunofluorescent and PCR methods, HHV6 DNA has also been found in peripheral leukocytes in the systemic circulation of MS patients.[67,68] However, using PCR methods, others did not find herpesviruses in the peripheral blood or CSF of MS patients.[69,70]

Although significant information (reviewed in[43,44,70]) points to an infectious process in MS, this remains a controversial concept. As evidence emerges of new possible pathogens in MS, such as a new putative retrovirus,[71] these reports must be intensively examined and further studies initiated. Since most studies have found that the progressive form of MS, rather than relapsing-remitting forms of MS, were associated with chronic infections, infections might be more important in MS progression than in its inception. Various infections may also nonspecifically stimulate the immune system.[43] As in other neurodegenerative diseases, multiple factors appear to be involved in the pathogenesis of MS. Thus, like ALS, MS progression may turn out to be more likely linked to chronic infections, rather than its inception.

Alzheimer's disease (AD), the most common cause of dementia, is a collection of brain disorders usually found in aged patients. The disease is characterized by slow, progressive loss of brain function, especially notable by lapses in memory, disorientation, confusion, mood swings, changes in personality, language problems (such as difficulty in finding the right words for everyday objects), loss of behavioral inhibitions, loss of motivation, and paranoia. The prognosis and course of AD varies widely, and the duration of illness can range from a few years to over 20 years. During this time, the parts of the brain that control memory and thinking are among the first affected, followed by other brain changes that ultimately result in brain cell death.[72]

Alzheimer's disease is characterized by distinct neuropathological changes in the brain. Among the most notable are the appearance of plaques and tangles of neurofibrils within brain nerves that affect nerve synapses and nerve-nerve cell communication. Both of these structural alterations involve the deposition of altered amyloid (Aß) proteins.[73,74] Although the cause of AD is not known with any certainty, the formation of the amyloid plaques and neurofiber tangles may be due to genetic defects and resulting changes in the structure of Aß proteins, neurotox-icity caused by chemicals or other toxic events, inflammatory responses, oxidative stress and increases in reactive oxygen species, loss of nerve trophic factors important in nerve physiology, and loss of nerve cell transmission.[73-77]

Brain infections in AD have only recently become an important topic.[78-80] One pathogen that has attracted considerable attention is C. pneumoniae.[81,82] As mentioned above, this intracellular bacterium has a tropism for neural tissue,[81] and it has been found at high incidence in the brains of AD patients (17 of 19 patients in brain areas of typical AD-related pathology) by PCR and immunohistochemistry methods.[82] Chlamydia pneumoniae has also been found in nerve cells in close proximity to neurofibrillary tangles.[82,83] This microorganism can invade endothelial cells and promote the transmigration of monocytes through human brain endothelial cells into the brain parenchyma.[84] Although C. pneumoniae has been found in the brains of most AD patients studied,[77,81] and this infection results in amyloid beta (Aß) plaque formation in mice injected with C. pneumoniae,[85] some investigators have not found an association of C. pneumoniae infection with AD using PCR or immunohistochemistry.[86,87]

In addition to C. pneumoniae, evidence has been forthcoming that AD patients also have other bacterial infections, such as B. burgdorferi.[88] This infection has been examined in AD cases by serology, culture, Western blot, and immunofluorenscence;[89,90] however, others could not find evidence of B. burgdorferi in AD patients.[91,92] The presence of intracellular infections, like B. burgdorferi in AD patients, has been hypothesized to be a primary event in the formation of AD amyloid plaques by forming "congophilic cores" that attract amyloid materials.[93] Multiple reports show that AD nerve cells are often positive for B. burgdorferi.[88-90,93,94]

In addition to the hypothesis that intracellular microorganisms may provide "cores" for the attraction of amyloid materials, the induction of reactive oxygen species, lipid peroxidation, and the breakdown of the lysosomal membranes releasing lysosomal hydrolases are also thought to be important in amyloid deposition.[94] Although the possibility that infections may be important in AD pathogenesis is attractive, some negative reports where investigators have not confirmed the presence of infections, such as B. burgdorferi, in AD patients, indicate that this is still controversial (reviewed in[91,94]).

Herpes simplex virus infections have also been found in AD, and an interesting relationship has developed between the presence of HSV1 in AD.[95] It had been noted previously that HSV1 but not a related neurotrophic virus, varicella zoster virus, was found often in AD brains and may be linked to patients who have the AD risk factor ApoE e4 allele.[96,97] In AD, HSV1 is thought to be involved in the abnormal aggregation of beta amyloid or Aß fragments within the brain by reducing the amount of full length amyloid precursor protein and increasing the amount of the Aß fragment from this precursor.[98] Recently, Wozniak and colleagues[99] showed that HSV1 infection of cultured glial and neuronal cells results in a dramatic increase in the intracellular levels of beta amyloid forms, whereas the levels of native amyloid precursor protein decreased. This is similar to what has been found in mice infected with HSV1, indicating that HSV1 is probably involved directly in the development of senile-associated plaques. Another herpesvirus, HHV6, has also been found in AD patients, but it is thought that this virus is not directly involved in AD pathogenesis, but it may exacerbate the effects of HSV1 in ApoE e4 carriers.[100]

In spite of the evidence that AD has been associated with, for example, C. pneumoniae, HSV1, or other infections, Robinson and colleagues[101] have stressed caution in concluding that infections act as a trigger or cofactor in AD. In particular, there is a paucity of experimental evidence that pathogens can elicit the neuropathological changes and cognitive deficits that characterize AD. They also stress that there is a need for consideration of systemic infections as potential contributors to the pathogenesis of AD.

Read more: http://www.medscape.com/viewarticle/574944_4