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 Aß deposition.

Burkholderia pseudomallei invades the olfactory nerve and bulb after epithelial injury in mice and causes the formation of multinucleated giant glial cells in vitro.

The infectious disease melioidosis is caused by the bacterium Burkholderia pseudomallei. Melioidosis is characterised by high mortality and morbidity and can involve the central nervous system (CNS). We have previously discovered that B. pseudomallei can infect the CNS via the olfactory and trigeminal nerves in mice. We have shown that the nerve path is dependent on mouse strain, with outbred mice showing resistance to olfactory nerve infection. Damage to the nasal epithelium by environmental factors is common, and we hypothesised that injury to the olfactory epithelium may increase the vulnerability of the olfactory nerve to microbial insult. We therefore investigated this, using outbred mice that were intranasally inoculated with B. pseudomallei, with or without methimazole-induced injury to the olfactory neuroepithelium. Methimazole-mediated injury resulted in increased B. pseudomallei invasion of the olfactory epithelium, and only in pre-injured animals were bacteria found in the olfactory nerve and bulb. In vitro assays demonstrated that B. pseudomallei readily infected glial cells isolated from the olfactory and trigeminal nerves (olfactory ensheathing cells and trigeminal Schwann cells, respectively). Bacteria were degraded by some cells but persisted in other cells, which led to the formation of multinucleated giant cells (MNGCs), with olfactory ensheathing cells less likely to form MNGCs than Schwann cells. Double Cap mutant bacteria, lacking the protein BimA, did not form MNGCs. These data suggest that injuries to the olfactory epithelium expose the primary olfactory nervous system to bacterial invasion, which can then result in CNS infection with potential pathogenic consequences for the glial cells.

Pathogens penetrating the central nervous system: infection pathways and the cellular and molecular mechanisms of invasion.

The brain is well protected against microbial invasion by cellular barriers, such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). In addition, cells within the central nervous system (CNS) are capable of producing an immune response against invading pathogens. Nonetheless, a range of pathogenic microbes make their way to the CNS, and the resulting infections can cause significant morbidity and mortality. Bacteria, amoebae, fungi, and viruses are capable of CNS invasion, with the latter using axonal transport as a common route of infection. In this review, we compare the mechanisms by which bacterial pathogens reach the CNS and infect the brain. In particular, we focus on recent data regarding mechanisms of bacterial translocation from the nasal mucosa to the brain, which represents a little explored pathway of bacterial invasion but has been proposed as being particularly important in explaining how infection with Burkholderia pseudomallei can result in melioidosis encephalomyelitis.

Burkholderia pseudomallei penetrates the brain via destruction of the olfactory and trigeminal nerves: implications for the pathogenesis of neurological melioidosis.

ABSTRACT Melioidosis is a potentially fatal disease that is endemic to tropical northern Australia and Southeast Asia, with a mortality rate of 14 to 50%. The bacterium Burkholderia pseudomallei is the causative agent which infects numerous parts of the human body, including the brain, which results in the neurological manifestation of melioidosis. The olfactory nerve constitutes a direct conduit from the nasal cavity into the brain, and we have previously reported that B. pseudomallei can colonize this nerve in mice. We have now investigated in detail the mechanism by which the bacteria penetrate the olfactory and trigeminal nerves within the nasal cavity and infect the brain. We found that the olfactory epithelium responded to intranasal B. pseudomallei infection by widespread crenellation followed by disintegration of the neuronal layer to expose the underlying basal layer, which the bacteria then colonized. With the loss of the neuronal cell bodies, olfactory axons also degenerated, and the bacteria then migrated through the now-open conduit of the olfactory nerves. Using immunohistochemistry, we demonstrated that B. pseudomallei migrated through the cribriform plate via the olfactory nerves to enter the outer layer of the olfactory bulb in the brain within 24 h. We also found that the bacteria colonized the thin respiratory epithelium in the nasal cavity and then rapidly migrated along the underlying trigeminal nerve to penetrate the cranial cavity. These results demonstrate that B. pseudomallei invasion of the nerves of the nasal cavity leads to direct infection of the brain and bypasses the blood-brain barrier. IMPORTANCE Melioidosis is a potentially fatal tropical disease that is endemic to northern Australia and Southeast Asia. It is caused by the bacterium Burkholderia pseudomallei, which can infect many organs of the body, including the brain, and results in neurological symptoms. The pathway by which the bacteria can penetrate the brain is unknown, and we have investigated the ability of the bacteria to migrate along nerves that innervate the nasal cavity and enter the frontal region of the brain by using a mouse model of infection. By generating a mutant strain of B. pseudomallei which is unable to survive in the blood, we show that the bacteria rapidly penetrate the cranial cavity using the olfactory (smell) nerve and the trigeminal (sensory) nerve that line the nasal cavity.

Olfactory nerve--a novel invasion route of Neisseria meningitidis to reach the meninges.

Neisseria meningitidis is a human-specific pathogen with capacity to cause septic shock and meningitis. It has been hypothesized that invasion of the central nervous system (CNS) is a complication of a bacteremic condition. In this study, we aimed to characterize the invasion route of N. meningitidis to the CNS. Using an intranasally challenged mouse disease model, we found that twenty percent of the mice developed lethal meningitis even though no bacteria could be detected in blood. Upon bacterial infection, epithelial lesions and redistribution of intracellular junction protein N-cadherin were observed at the nasal epithelial mucosa, especially at the olfactory epithelium, which is functionally and anatomically connected to the CNS. Bacteria were detected in the submucosa of the olfactory epithelium, along olfactory nerves in the cribriform plate, at the olfactory bulb and subsequently at the meninges and subarachnoid space. Furthermore, our data suggest that a threshold level of bacteremia is required for the development of meningococcal sepsis. Taken together, N. meningitidis is able to pass directly from nasopharynx to meninges through the olfactory nerve system. This study enhances our understanding how N. meningitidis invades the meninges. The nasal olfactory nerve system may be a novel target for disease prevention that can improve outcome and survival.

Bacteria and PAMPs activate nuclear factor kappaB and Gro production in a subset of olfactory ensheathing cells and astrocytes but not in Schwann cells.

The primary olfactory nerves provide uninterrupted conduits for neurotropic pathogens to access the brain from the nasal cavity, yet infection via this route is uncommon. It is conceivable that olfactory ensheathing cells (OECs), which envelope the olfactory nerves along their entire length, provide a degree of immunological protection against such infections. We hypothesized that cultured OECs would be able to mount a biologically significant response to bacteria and pathogen-associated molecular patterns (PAMPs). The response of OECs to Escherichia coli (E. coli) and various PAMPs was compared to that of Schwann cells (SCs), astrocytes (ACs), and microglia (MG). A subset of OECs displayed nuclear localization of nuclear factor kappaB), an inflammatory transcription factor, after treatment with E. coli (20% +/- 5%), lipopolysacchride (33% +/- 9%), and Poly I:C (25% +/- 5%), but not with peptidoglycan or CpG oligonucleotides. ACs displayed a similar level of activation to these treatments, and in addition responded to peptidoglycan. The activation of OECs and ACs was enhanced by coculture with MG (56% +/- 16% and 85% +/- 13%, respectively). In contrast, SCs did not respond to any treatment or to costimulation by MG. Immunostaining for the chemokine Gro demonstrated a functional response that was consistent with NF kappaB activation. OECs expressed mRNA for Toll-like receptors (TLRs) 2 and 4, but only TLR4 protein was detected by Western blotting and immunohistochemistry. The results demonstrate that OECs possess the cellular machinery that permits them to respond to certain bacterial ligands, and may have an innate immune function in protecting the CNS against infection.

Olfactory pathways in three patients with cryptococcal meningitis and acquired immune deficiency syndrome.

The olfactory mucosa, bulbs and tracts were examined for the presence of Cryptococcus neoformans in 3 patients with the acquired immune deficiency syndrome (AIDS) and cryptococcal meningitis. Two of them had antibodies against HIV-1 and one had positive serology for HIV-2. Cryptococci were seen in the subarachnoid space around olfactory tracts and bulbs and in the submucosal olfactory nerve fascicles. In one case, olfactory nerve fascicles from the lamina propria were also affected. Olfactory epithelium and respiratory mucosa were not involved. We suggest that Cryptococcus reached the olfactory nerve fascicles through the olfactory pathways for cerebrospinal fluid drainage which might serve as a source of latent cryptococcal infection.

The olfactory nerve and not the trigeminal nerve is the major site of CNS entry for mouse hepatitis virus, strain JHM.

Several viruses, including mouse hepatitis virus strain JHM (MHV-JHM), enter the brain after intranasal inoculation and spread transneuronally to other parts of the central nervous system (CNS). Both the olfactory and trigeminal nerves innervate the nasal cavity and are potential portals of virus entry into the CNS. To evaluate the relative importance of each nerve for MHV infection, mice were infected under conditions that discriminated between trigeminal and olfactory nerve entry. When olfactory nerve entry was selectively eliminated by surgical removal of both olfactory bulbs or by chemical destruction of the olfactory epithelium, MHV-JHM spread into the CNS was completely prevented. On the other hand, direct inoculation into the olfactory bulb, which eliminates all entry via the trigeminal nerve, had no effect on the pattern of virus infection. Thus MHV-JHM enters the CNS via the olfactory nerve after intranasal inoculation while entry via the trigeminal nerve is an insignificant part of this process.

Immunohistological demonstration of spread of Aujeszky's disease virus via the olfactory pathway in HPCD pigs.

The spread of Aujeszky's disease virus (ADV) from nasal mucosa via the olfactory pathway was studied in HPCD pigs. ADV antigen was detected in the epithelial cells, nasal gland cells, olfactory nerve cells and peripheral nerve fibres in the nasal cavity and in neuroglial cells in the olfactory bulb. Results indicate that the olfactory pathway is one of the most important neuronal pathways of ADV infection in pigs.

Spread of a neurotropic murine coronavirus into the CNS via the trigeminal and olfactory nerves.

The route of entry into the central nervous system (CNS) of most neurtropic viruses has not been established. The coronavirus, mouse hepatitis virus strain JHM (MHV-JHM), causes acute encephalomyelitis and acute and chronic demyelinating diseases and is an important model system for virus-induced neurological disease. Suckling C57BL/6 mice infected intranasally with MHV-JHM develop either the acute encephalomyelitis or a late onset, symptomatic demyelinating encephalomyelitis, depending on whether they are nursed by unimmunized or immunized dams. Analysis by in situ hybridization was used to determine the route of entry of MHV-JHM into the CNS in these mice. At early times, viral RNA was detected only in the trigeminal and olfactory nerves and in their immediate connections in all mice. A few days later, MHV-JHM RNA was found throughout the brain in mice dying of the acute encephalomyelitis, but remained confined to the entry sites in mice which did not develop acute disease. These results suggest that MHV-JHM enters the CNS via an interneuronal route in all mice, but that the presence of maternal antibody prevents the dissemination of virus via extracellular fluid. In addition, MHV-JHM may establish low-level persistence in the trigeminal or olfactory nerve or in one of its connections in mice that do not develop acute encephalomyelitis.

Selective infections of olfactory and respiratory epithelium by vesicular stomatitis and Sendai viruses.

Following intranasal instillation of vesicular stomatitis virus (VSV) in mice there was an extensive infection of the olfactory epithelium in contrast to a minimal involvement of the respiratory epithelium. Sendai virus (SV), on the other hand, caused an extensive infection of the respiratory epithelium and only minimal infection of the olfactory mucous membrane. VSV budded from basolateral surfaces of supporting cells and olfactory neurons, but not from their apical surfaces or the ciliated bulbous endings of the olfactory neuron dendrites. This asymmetric release of VSV favoured neuroinvasion. The virus spread along the olfactory nerves to the glomeruli in the olfactory bulbs after which it propagated transneuronally into the rest of the brain. SV budded only from the apical surface of respiratory epithelial cells, was released into the air passages, and there were no signs of invasion into the olfactory bulbs. Inoculation of the olfactory mucous membrane is a useful procedure for studies on selectivity of attack on peripheral neurons by viruses and on mechanisms of virus invasion of the nervous system in vivo.

[Nasal infection of the domestic cat with feline herpesvirus as an animal model of experimental viral labyrinthitis]. / Nasale Infektion der Hauskatze mit felinem Herpesvirus als Tierversuchsmodell der experimentellen viralen Labyrinthitis.

Looking for a suitable animal model of experimental labyrinthitis we performed in vivo infections of the domestic cat with feline rhinotracheitis virus. After instillation of the virus suspension into the nose of the anaesthetized cat we could find the complete virus particles in so-called virus factories in the nasal mucosa four days later by means of electron-microscopy. The adjacent ganglia of the olfactory nerve were stained by immunohistochemical methods and showed positive viral antigen. After embedding of the cochlea for electron microscopy virus particles could be demonstrated in the inflammatory cell material in the scala tympani. Our experimental model seems to be suitable for further studies of the rhinogenic way of infection in experimental viral labyrinthitis.

Localization of herpes simplex virus in the trigeminal and olfactory systems of the mouse central nervous system during acute and latent infections by in situ hybridization.

The precise anatomical location of latent herpes simplex virus (HSV) infection of the mouse central nervous system (CNS) has been identified by application of a 3H-labeled HSV-specific probe to deparaffinized sections of mouse brain tissue in situ. At times after corneal inoculation with HSV type 1 (HSV-1), strain F, representing the acute and latent phases of infection, BALB/c mice were perfused with a fixative containing sodium m-periodate, lysine, and paraformaldehyde and their CNS tissues and trigeminal ganglia embedded in paraffin, sectioned, and and subjected to hybridization. During the acute phase, HSV-1 was localized to neurons and some small supporting cells in the sensory portion of the 5th cranial nerve including the trigeminal ganglia and nerve root, principal sensory nucleus, mesencephalic nucleus, descending tract and nuclei, and cerebral cortex. During the latent phase, HSV-1 was found only in neurons located primarily in the descending nuclei and mesencephalic nucleus. Evidence was also obtained that implicated the olfactory tract as an additional route of entry into the CNS, in that positive hybridization was found in the olfactory bulb, the entorhinal cortex, and adjacent cerebral cortex. Additionally, HSV-1 established latent infections in neurons of the olfactory system. HSV-1-specific RNA was detected in ganglionic and CNS neurons throughout the acute and latent phases of infection, whereas HSV-1-specific DNA was detected only during the acute phase, indicating that the relationship between HSV and latently infected CNS and ganglionic neurons involves limited transcription of the viral genome.

Mode of entry of a neurotropic arbovirus into the central nervous system. Reinvestigation of an old controversy.

The mechanism by which neurotropic arboviruses gain access to the central nervous system remains uncertain, although it is generally assumed that viremic infection results in growth across or passive diffusion through brain capillaries. In contrast to the natural reservoir hosts of these arboviruses, clinical hosts (e.g., horses, humans) have viremias of very brief duration and low magnitude. We investigated the question of neuroinvasion in 5- to 6-week-old Syrian hamsters infected with St. Louis encephalitis virus (strain TBH-28). This model shares with the human disease low or undetectable viremia and many clinical and pathoanatomical features. The mortality rate after intraperitoneal inoculation of a moderate viral dose was 88%. No viremia was detectable by a sensitive assay in 31% of the animals. In the remaining hamsters, the mean peak viremia was 1.0 log10 plaque-forming units/0.05 ml and the mean duration 1 to 2 days. There was no correlation between viremia and outcome of infection, length of incubation period, or brain virus titer. Tissue infectivity studies showed a rise in titer in the olfactory neuroepithelium on day 4 postinoculation, then in the olfactory bulbs (day 5 postinoculation), and finally in the remainder of the brain (day 6 postinoculation). Specific immunofluorescence was demonstrated in the bipolar neurons of the olfactory epithelium, their dendrites, and in axon bundles of the olfactory nerves in the submucosa. By electron microscopy, virus particles and associated tubular structures were demonstrated within dendrites, perikarya, and axons of olfactory neurons, and to a lesser extent in macrophages and Bowman's gland cells in the lamina propria. In cells of Bowman's glands large numbers of virions were sequestered within secretory granules. Virus was recovered from nasal washings on day 4 postinoculation. Similar findings were obtained in weanling mice inoculated intraperitoneally with another (mouse-virulent) St. Louis encephalitis viral strain (77V-12908). These data taken together indicate that the olfactory pathway is the principal route of viral entry into the central nervous system. After peripheral inoculation a low-level viremia results in infection of highly susceptible cells in the olfactory neuroepithelium, allowing centripetal axonal transport of virus to the olfactory bulb, whence spread is unimpeded throughout the neuropil of the central nervous system. Infection of Bowman's gland cells in the olfactory mucosa and shedding of virus in nasal mucus may be an adaptation for nonarthropod-borne transmission, a feature of many flaviviruses.

Necropsy study of the olfactory portal of entry in herpes simplex encephalitis.

Two fatal cases of sporadic herpes simplex virus (HSV) encephalitis were seen and diagnosed at the Sir Charles Gairdner Hospital, Perth, between 1979 and 1981. Necropsy studies were directed to the possible portal of entry of HSV into the central nervous system. Viruses were present in the left olfactory nerve in one patient. No viruses were found in the Gasserian ganglia of either patient. The finding of HSV within an olfactory nerve provides further evidence of an olfactory portal of entry to the central nervous system in sporadic human HSV encephalitis.

Fatal herpes simplex encephalitis with demonstration of virus in the olfactory pathway.

A fatal case of herpes simplex encephalitis contained viruses within the olfactory pathway but not in the trigeminal ganglia. This finding supports the theory that the olfactory nerves are the portal of entry in adult herpes simplex encephalitis.