Neuroinflammation in Long Covid

[Pyrrhus]

And here are two news articles about the possibility of the novel coronavirus infecting the brain and what it might mean for those with Long Covid:


The Conversation: Does coronavirus linger in the body? What we know about how viruses in general hang on in the brain and testicles
https://theconversation.com/amp/doe...ral-hang-on-in-the-brain-and-testicles-142878
Excerpt:

As millions of people are recovering from COVID-19, an unanswered question is the extent to which the virus can “hide out” in seemingly recovered individuals. If it does, could this explain some of the lingering symptoms of COVID-19 or pose a risk for transmission of infection to others even after recovery?

I am a physician-scientist of infectious diseases at the University of Virginia, where I care for patients with infections and conduct research on COVID-19. Here I will briefly review what is known today about chronic or persistent COVID-19.

Harvard Gazette: Early details of brain damage in COVID-19 patients
https://news.harvard.edu/gazette/st...details-of-brain-damage-in-covid-19-patients/
Excerpt:

Specialized scanning furthers understanding of the virus’ potential effects on the brain

While it is primarily a respiratory disease, COVID-19 infection affects other organs, including the brain.

One of the first spectroscopic imaging-based studies of neurological injury in COVID-19 patients has been reported by researchers at Harvard-affiliated Massachusetts General Hospital (MGH) in the American Journal of Neuroradiology. Looking at six patients using a specialized magnetic resonance (MR) technique, they found that COVID-19 patients with neurological symptoms show some of the same metabolic disturbances in the brain as patients who have suffered oxygen deprivation (hypoxia) from other causes, but there are also notable differences.

 

[Pyrrhus]

How does the virus enter the brain?

Here are four research publications that consider pathways for the virus to infect the brain. They suggest that the virus appears to move directly from cell-to-cell along cranial nerves, such as the olfactory nerve or the vagus nerve, which conveniently bypass the blood-brain-barrier and the cerebrospinal fluid.


Brainstem neuropathology in two cases of COVID-19: SARS-CoV-2 trafficking between brain and lung (Bulfamonte et al., 2021)
https://link.springer.com/article/10.1007/s00415-021-10604-8
Main points:

• In two patients, damage was seen in the medulla oblongata part of the brainstem.
• Coronavirus proteins were found in neurons in the brainstem and cranial nerves such as the vagus nerve.
• The presence of the coronavirus in the vagus nerve and medulla oblongata suggests that the virus might travel inside the vagus nerve from the lung to the brainstem.

SARS-CoV-2 might spread through the nervous system, reaching respiratory centers in the brainstem. [...] Autopsies showed normal gross brainstem anatomy. Histopathological examination demonstrated increased neuronal and [corpora amylacea (CA)] damage in Covid-19 patients’ medulla oblongata. Immunohistochemistry disclosed SARS-CoV-2 NP in brainstem neurons and glial cells, and in cranial nerves. Glial elements also exhibited a widespread increase in Iba-1 expression. Sars-Co-V2 was immunohistochemically detected in the vagus nerve fibers. [...] Sars-Co-V2 detection in the vagus nerve argues for viral trafficking between brainstem and lung.

SARS-CoV-2 Dissemination Through Peripheral Nerves Explains Multiple Organ Injury (Fenrich et al. 2020)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7419602/
Main points:

1. Most COVID patients do not display virus in blood, despite displaying the virus in other parts of the body.
2. The virus might first infect the brainstem through cranial nerves.
3. The virus might then move from the brain to other organs in the body inside peripheral nerves.

SARS-CoV-2 is reported to be able to infect the lungs, the intestines, blood vessels, the bile ducts, the conjunctiva, macrophages, T lymphocytes, the heart, liver, kidneys, and brain. More than a third of cases displayed neurological involvement, and many severely ill patients developed multiple organ infection and injury. However, less than 1% of patients had a detectable level of SARS-CoV-2 in the blood, raising a question of how the virus spreads throughout the body.

We propose that nerve terminals in the orofacial mucosa, eyes, and olfactory neuroepithelium act as entry points for the brain invasion, allowing SARS-CoV-2 to infect the brainstem.

By exploiting the subcellular membrane compartments of infected cells, a feature common to all coronaviruses, SARS-CoV-2 is capable to disseminate from the brain to periphery via vesicular axonal transport and passive diffusion through axonal endoplasmic reticula, causing multiple organ injury independently of an underlying respiratory infection.

The proposed model clarifies a wide range of clinically observed phenomena in CoVID-19 patients, such as neurological symptoms unassociated with lung pathology, protracted presence of the virus in samples obtained from recovered patients, exaggerated immune response, and multiple organ failure in severe cases with variable course and dynamics of the disease.

(emphasis and spacing added)
To learn more about the "vesicular axonal transport" mentioned in this paper, see:
https://forums.phoenixrising.me/thr...-as-stealth-spheres.75937/page-3#post-2382334


Olfactory transmucosal SARS-CoV-2 invasion as a port of central nervous system entry in individuals with COVID-19 (Meinhardt et al., 2020)
https://www.nature.com/articles/s41593-020-00758-5
Main points:

1. The novel coronavirus can infect the brain via the olfactory nerve in the nose.
2. The virus also finds its way to the medulla oblongata, where it interferes with autonomic control of breathing and the heart.

The newly identified severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes COVID-19, a pandemic respiratory disease. Moreover, thromboembolic events throughout the body, including in the CNS, have been described. Given the neurological symptoms observed in a large majority of individuals with COVID-19, SARS-CoV-2 penetrance of the CNS is likely.

By various means, we demonstrate the presence of SARS-CoV-2 RNA and protein in anatomically distinct regions of the nasopharynx and brain. Furthermore, we describe the morphological changes associated with infection such as thromboembolic ischemic infarction of the CNS and present evidence of SARS-CoV-2 neurotropism.

SARS-CoV-2 can enter the nervous system by crossing the neural–mucosal interface in olfactory mucosa, exploiting the close vicinity of olfactory mucosal, endothelial and nervous tissue, including delicate olfactory and sensory nerve endings. Subsequently, SARS-CoV-2 appears to follow neuroanatomical structures, penetrating defined neuroanatomical areas including the primary respiratory and cardiovascular control center in the medulla oblongata.

(emphasis and spacing added)


COVID-19–related anosmia is associated with viral persistence and inflammation in human olfactory epithelium and brain infection in hamsters (De Melo et al., 2021)
Pre-print: https://www.biorxiv.org/content/10.1101/2020.11.18.388819v1.full
Final publication: https://www.science.org/doi/10.1126/scitranslmed.abf8396
Main point:

• Meticulous evidence for infection of the brain via the olfactory nerve

Olfactory and taste dysfunction are common in COVID-19, especially in mildly symptomatic patients. Here, we conducted a virologic, molecular, and cellular study of the olfactory neuroepithelium of seven patients with COVID-19 presenting with acute loss of smell.

We report evidence that the olfactory neuroepithelium is a major site of SARS-CoV2 infection with multiple cell types, including olfactory sensory neurons, support cells, and immune cells, becoming infected. SARS-CoV-2 replication in the olfactory neuroepithelium was associated with local inflammation.

Furthermore, we showed that SARS-CoV-2 induced acute anosmia and ageusia in golden Syrian hamsters, lasting as long as the virus remained in the olfactory epithelium and the olfactory bulb. Last, olfactory mucosa sampling from patients showing long-term persistence of COVID-19–associated anosmia revealed the presence of virus transcripts and of SARS-CoV-2–infected cells, together with protracted inflammation.
[...]
Because loss of smell is a hallmark of COVID-19 and several respiratory viruses (influenza, endemic human CoVs, and SARS-CoV-1) invade the CNS through the olfactory mucosa via a retrograde route, we hypothesized that SARS-CoV-2 might be neurotropic and capable of invading the CNS through [olfactory sensory neurons (OSNs)].
[...]
Syrian golden hamsters (both sexes) were intranasally inoculated with 6 × 104 PFU of SARS-CoV-2 and followed up between 24 hours and 14 days post-inoculation (dpi). [...] Viral RNA was also detected from 2 dpi and onward in various parts of the brain, including the olfactory bulb, cerebral cortex, brain stem (diencephalon, midbrain, pons, and medulla oblongata), and cerebellum (Fig. 3D). In addition, we were able to isolate infectious viral particles from the nasal turbinates, the lung, and different brain areas (olfactory bulb, cerebral cortex, brain stem, and cerebellum), indicative of the replication and production of SARS-CoV-2 in the CNS of hamsters (Fig. 3E).
[...]
Having shown that SARS-CoV-2 infects OSNs and that SARS-CoV-2–infected hamsters exhibit signs of anosmia and ageusia, we wondered whether SARS-CoV-2 invades the CNS via a retrograde route from the olfactory system. SARS-CoV-2 was detected in olfactory nerve bundles close to the neuroepithelium, as demonstrated by the colocalization of SARS-CoV-2 NP antigen and OMP+ sensory neuron axons reaching the olfactory bulb (Fig. 6, E, K, and L), consistent with a retrograde infection of axons.
[...]
The high viral RNA loads in the nasal turbinates and in the olfactory bulb, together with the observation of viral antigens along the entire route from the olfactory sensory organ to the bulb, suggest that SARS-CoV-2 enters the brain through the olfactory system, although this finding does not rule out other ports of CNS entry in patients with COVID-19.
[...]
We therefore confirm that SARS-CoV-2 has a tropism for the olfactory mucosa, and we demonstrate that it can persist locally, not only a few weeks after general symptoms resolution but also several months in [olfactory sensory neurons (OSNs)]. [...] Although this does not rule out other routes, our study indicates that SARS-CoV-2 indeed does invade the CNS via the retrograde olfactory pathway.

(spacing and emphasis added)

EDIT: Added new study and included link to final published version of the pre-print

 

[Pyrrhus]

How it affects patients and their lives

Here's an interesting news story by ME patient Ryan Prior:

Covid-19's effects include seizures and movement disorders -- even in some moderate cases, study finds
https://edition.cnn.com/2020/12/10/health/effects-on-the-brain-covid-19-wellness/index.html
Excerpt:

Covid-19 can lead to neurological complications, including strokes, seizures and movement disorders, researchers have found.

The complications, which go well beyond cognitive impairment, can occur even in moderate cases, according to a study published Wednesday in the journal Neurology: Clinical Practice.
"These particular complications of Covid, and neurological disorders more generally, are about your ability to interact meaningfully with the world," said lead study author Dr. Pria Anand, an assistant professor of neurology at Boston University School of Medicine. "I think that's one of the unique and devastating things about this (virus)."

And another news story from MSN:

Coronavirus: How COVID-19 could damage the brain
https://www.msn.com/en-ca/health/medical/coronavirus-how-covid-19-could-damage-the-brain/ar-BB1bS1qo
Excerpt:

When Erica Taylor got COVID-19, her brain seemed to shut off completely.

“I couldn’t think. Suddenly I couldn’t hold onto any thoughts,” she said, adding “I didn’t know who I was, where I was. I don’t know how long I was like that.”

At the time, she was 31 years old and showing the usual first signs of infection, dry cough and nausea.

What ended up happening was far from typical. She started suffering from debilitating neurological symptoms that have now taken over her life. Her doctors don’t know why.

 

[bensmith]

Think covid has damaged my brain. Think its why my cfs is so severe and why its so brainy. I cant do stuff with my brain without pem. Like thinking or imagination or watching tv etc.

 

[Pyrrhus]

Think covid has damaged my brain.

But remember that neuroinflammation does not necessarily mean permanent damage. If the cause of the neuroinflammation can be fixed, one might expect the neuroinflammatory symptoms to disappear...

 

[bensmith]

Thats good : )

i prob need to get some covid stuff like ivermectin going. Esp after being reinfected.

i wonder if long covid stuff will help me. Since most long covid people dont seem to have pem
I dont think so but one can hope. At least that it seems most dont have pem in my time with other long haulers. And if they donit seems to go away for some reason.

 

[Pyrrhus]

Here's a Letter to the editor in the New England Journal of Medicine by NIH researchers, including Avi Nath:
(not peer reviewed)

The researchers used an impressive 11.7 Tesla MRI machine to look in detail at the olfactory bulb and the brainstem in patients who appeared to have died of COVID. They also used conventional histological examination to find direct evidence of neuroinflammation in some patients, in the olfactory bulb, substantia nigra of the brainstem/basal ganglia, dorsal motor nucleus of the vagus nerve in the brainstem, and the pre-Bötzinger complex in the medulla oblongata part of the brainstem.

I'm a former student of biomedical imaging and I've never even heard of an MRI machine that strong, with such detailed resolution! 25 micron resolution! If only it would be safe to use these machines on living humans...

However, this report, which was not peer-reviewed, has been criticized for failing to be able to find viral RNA in the brains of patients when almost every other research group that looked for the virus in the brains of patients successfully found viral RNA or proteins.

Microvascular Injury in the Brains of Patients with Covid-19 (Lee et al., 2020)
https://www.nejm.org/doi/full/10.1056/NEJMc2033369

We conducted postmortem high-resolution magnetic resonance imaging (magnetic resonance microscopy) of the brains of patients with coronavirus disease 2019 (Covid-19) (median age, 50 years) and histopathological examination that focused on microvascular changes in the olfactory bulb and brain stem.
[...]
Images were obtained from the brains of 13 patients with the use of an 11.7-Tesla scanner at a resolution of 25 μm for the olfactory bulb and at a resolution of 100 μm for the brain. Abnormalities were seen in the brains of 10 patients.
[...]
Magnetic resonance microscopy showed punctate hyperintensities in 9 patients, which represented areas of microvascular injury and fibrinogen leakage.
[...]
Punctate hypointensities on imaging in 10 patients corresponded to congested blood vessels (Figure 1C) with surrounding areas of fibrinogen leakage (Figure 1D and Fig. S1) and relatively intact vasculature (Figure 1E).
[...]
Activated microglia were found adjacent to neurons in 5 patients, which is suggestive of neuronophagia in the olfactory bulb, substantia nigra, dorsal motor nucleus of the vagal nerve, and the pre-Bötzinger complex in the medulla, which is involved in the generation of spontaneous rhythmic breathing (Figure 1K through 1N and Fig. S3).
[...]
Because of the limited clinical information that was available, no conclusions can be drawn in relation to neurologic features of Covid-19.

(emphasis added)

 

[dave11]

Letter to the editor in the New England Journal of Medicine

There is some additional information relating to the New England Journal article, and blood vessels leaking fibrinogen into the brain causing an immune reaction, here:

https://www.thailandmedical.news/ne...hows-severe-brain-damage-in-covid-19-patients

 

[Pyrrhus]

There is some additional information relating to the New England Journal article, and blood vessels leaking fibrinogen into the brain causing an immune reaction, here:

https://www.thailandmedical.news/ne...hows-severe-brain-damage-in-covid-19-patients

Thanks @dave11 !

There may, however, be some problems with the way that piece has been portrayed in the media. This twitter thread discusses whether Avi Nath's report really supports his theory of "microvascular injury without virus":

https://twitter.com/i/web/status/1345849750000570368

EDIT:
Avi Nath's theory of "microvascular injury without virus" has also been disputed in a subsequent review article:

What can cerebrospinal fluid testing and brain autopsies tell us about viral neuroinvasion of SARS-CoV-2 (Li et al., 2021)
https://doi.org/10.1002/jmv.26943

Microglial activation is a common pathological feature during neuronal injury induced by a variety of insults. [...] The detection rate of SARS-CoV-2 is much higher in the brain regions with [microglial activation] and/or lymphocytic infiltrations, relative to those with hypoxic brain injury or vascular [injury]. [...] These findings indicate that the inflammatory responses in some specific brain areas cannot be attributed to only the hypoxemia or vascular [injury] in critical patients with COVID-19.

 

[Pyrrhus]

...And for anyone still on the fence about whether the novel coronavirus infects the brain, there is this fairly definitive study from the team of Akiko Iwasaki:


Neuroinvasion of SARS-CoV-2 in human and mouse brain (Song et al., 2021)
https://rupress.org/jem/article/218/3/e20202135/211674/Neuroinvasion-of-SARS-CoV-2-in-human-and-mouse

This publication provides both a standard written abstract, as well as a "Graphical Abstract", which is an excellent idea for quickly communicating the essence of a paper to busy readers.


Graphical Abstract:

Written Abstract:

Although COVID-19 is considered to be primarily a respiratory disease, SARS-CoV-2 affects multiple organ systems including the central nervous system (CNS). Yet, there is no consensus on the consequences of CNS infections. Here, we used three independent approaches to probe the capacity of SARS-CoV-2 to infect the brain.

First, using human brain organoids, we observed clear evidence of infection with accompanying metabolic changes in infected and neighboring neurons. However, no evidence for type I interferon responses was detected. We demonstrate that neuronal infection can be prevented by blocking ACE2 with antibodies or by administering cerebrospinal fluid from a COVID-19 patient.

Second, using mice overexpressing human ACE2, we demonstrate SARS-CoV-2 neuroinvasion in vivo.

Finally, in autopsies from patients who died of COVID-19, we detect SARS-CoV-2 in cortical neurons and note pathological features associated with infection with minimal immune cell infiltrates.

These results provide evidence for the neuroinvasive capacity of SARS-CoV-2 and an unexpected consequence of direct infection of neurons by SARS-CoV-2.

(emphasis and spacing added for readability)

 

[Pyrrhus]

...And for anyone still on the fence about whether the novel coronavirus infects the brain, there is this new, fairly definitive, study from the team of Akiko Iwasaki:

This paper has been seen to contradict Avi Nath's theory about "microvascular injury without virus":

https://twitter.com/i/web/status/1354671311377883141

 

[Pyrrhus]

Multiple lines of in vitro evidence

...But the team of Akiko Iwasaki are not the only ones to use in vitro evidence to show that the novel coronavirus can infect neurons.

Here are two publications that had also described in vitro evidence pointing to a direct infection of the brain:


SARS-CoV-2 targets neurons of 3D human brain organoids (Ramani et al., September 2020)
https://doi.org/10.15252/embj.2020106230
Main point:

• The authors showed that the novel coronavirus could infect an in vitro model of the brain called a "brain organoid".

Unexpectedly, emerging clinical reports indicate that neurological symptoms continue to rise, suggesting detrimental effects of SARS-CoV-2 on the central nervous system (CNS). Here, we show that a Düsseldorf isolate of SARS-CoV-2 enters 3D human brain organoids within 2 days of exposure. We identified that SARS-CoV-2 preferably targets neurons of brain organoids. [...] While an increase in viral RNA was detected in the supernatants of Vero cells, no apparent increase was identified in brain organoid supernatants.

SARS-CoV-2 infects human neural progenitor cells and brain organoids (Zhang et al., August 2020)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7399356/
Main points:

1. The authors had previously shown that the novel coronavirus could infect neuron-like cells.
2. The authors had previously shown that the novel coronavirus could infect hamster neurons.
3. The authors showed that the novel coronavirus could infect and kill neuronal stem cells.
4. The authors showed that the novel coronavirus could infect an in vitro model of the brain called a "neurosphere".
5. The authors showed that the novel coronavirus could infect another in vitro model of the brain called a "brain organoid".

Importantly, a recent study in Germany demonstrated that SARS-CoV-2 RNA could be detected in brain biopsies in 36.4% (8/22) of fatal COVID-19 cases, which highlights the potential for viral infections in the human brain. To date, there has been no direct experimental evidence of SARS-CoV-2 infection in the human central nervous system (CNS). We recently demonstrated that SARS-CoV-2 could infect and replicate in cells of neuronal origin. In line with this finding, we showed that SARS-CoV-2 could infect and damage the olfactory sensory neurons of hamsters. In addition, angiotensin-converting enzyme 2 (ACE2), the entry receptor of SARS-CoV-2, is widely detected in the brain and is highly concentrated in a number of locations including substantia nigra, middle temporal gyrus, and posterior cingulate cortex. Together, these findings suggest that the human brain might be an extra-pulmonary target of SARS-CoV-2 infection.

To explore the direct involvement of SARS-CoV-2 in the CNS in physiologically relevant models, we assessed SARS-CoV-2 infection in induced pluripotent stem cells (iPSCs)-derived human neural progenitor cells (hNPCs) [...] Importantly, SARS-CoV-2 infection significantly reduced the viability of hNPCs to 4.7% (P < 0.0001) and 2.5% (P < 0.0001) of that of the mock-infected hNPCs at 72 and 120 hpi, respectively (Fig. 1a).
[...]
Next, we challenged 3D neurospheres with SARS-CoV-2 and harvested supernatant samples from the infected neurospheres at 0, 24, 48, and 72 hpi for virus replication assessment. We found the SARS-CoV-2 RNA-dependent RNA polymerase (RdRp) copy number significantly increased in a time-dependent manner (Fig. 1b, left). In addition, a significant amount of infectious virus particles were released from the infected neurospheres as determined by plaque assays (Fig. 1b, right). In parallel, SARS-CoV-2-infected neurospheres were cryosectioned and immunostained for viral antigen assessment. Importantly, SARS-CoV-2 nucleocapsid (N) protein was readily detected across the infected neurospheres but no positive signals were detected in the mock-infected neurospheres (Fig. 1c). Furthermore, electron microscopy detected extensive viral particles in vacuoles within the double-membrane structures, which may represent sites of viral particle formation (Fig. 1d). These findings indicate that neurospheres were permissive to SARS-CoV-2 infection and supported productive virus replication.

Next, we examined whether SARS-CoV-2 could infect 3D human brain organoids. We generated iPSC-derived human brain organoids using previously described protocols. The 35-day-old brain organoids showed self-organizing internal morphology with fluid-filled ventricular-like structures resembling that of developing cerebral cortex (Fig. 1e). [...] Importantly, extensive SARS-CoV-2 antigen was detected in the infected samples at 72 hpi (Fig. 1f), indicating that SARS-CoV-2 directly infected the brain organoids. Immunofluorescence staining and confocal microscopy revealed SARS-CoV-2-N signals in the peripheral regions (Fig. 1f, arrows) and in deeper regions of the brain organoids (Fig. 1f, white arrowheads). In addition, cell-cell fusion was readily detected in regions with robust SARS-CoV-2 infection (Fig. 1f, yellow arrowheads). No SARS-CoV-2-N signals were detected in the mock-infected brain organoids (Fig. 1f). We next analyzed supernatant samples from infected brain organoids to evaluate SARS-CoV-2 virus particle release. The results demonstrated SARS-CoV-2 RdRp gene copy number increased in a time-dependent manner, suggesting active release of progeny virus particles from infected brain organoids (Fig. 1g, left). [...] These findings unambiguously demonstrated that SARS-CoV-2 can productively infect brain organoids with release of viral particles (Fig. 1g, right). Remarkably, double immunostaining demonstrated that SARS-CoV-2-N was colocalized with neuronal marker TUJ1 and NPC marker NESTIN, suggesting that SARS-CoV-2 can directly infect cortical neurons and NPCs in brain organoids (Fig. 1h, i).

(emphasis added)

 

[Pyrrhus]

Evidence from animal models

...And the team of Akiko Iwasaki are not the only ones to use animal models to show that the novel coronavirus can infect the brain. Here are three studies that also used animal models.


Here's a study that found that the novel coronavirus can infect a monkey brain:

SARS-CoV-2 causes brain inflammation and induces Lewy body formation in macaques (Philippens et al., 2021)
https://www.biorxiv.org/content/10.1101/2021.02.23.432474v2

Here we show that SARS-CoV-2 infection causes brain inflammation in the macaque model. [...] Post-mortem analysis demonstrated infiltration of T-cells and activated microglia in the brain, and viral RNA was detected in brain tissues from one animal.

And here are two studies where they genetically engineered mice to have the same coronavirus receptors as humans, and then administered the novel coronavirus into the nose of the mice:

Neuroinvasion and Encephalitis Following Intranasal Inoculation of SARS-CoV-2 in K18-hACE2 Mice (Kumari et al., 2021)
https://www.mdpi.com/1999-4915/13/1/132
Main points:

• All animals showed coronavirus in the brain.
• The virus in the brain resulted in encephalitis-like pathology.

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection can cause neurological disease in humans, but little is known about the pathogenesis of SARS-CoV-2 infection in the central nervous system (CNS).

Herein, using K18-hACE2 mice, we demonstrate that SARS-CoV-2 neuroinvasion and encephalitis is associated with mortality in these mice. Intranasal infection of K18-hACE2 mice with 105 plaque-forming units of SARS-CoV-2 resulted in 100% mortality by day 6 after infection.

The highest virus titers in the lungs were observed on day 3 and declined on days 5 and 6 after infection. By contrast, very high levels of infectious virus were uniformly detected in the brains of all the animals on days 5 and 6. Onset of severe disease in infected mice correlated with peak viral levels in the brain.

SARS-CoV-2-infected mice exhibited encephalitis hallmarks characterized by production of cytokines and chemokines, leukocyte infiltration, hemorrhage and neuronal cell death. SARS-CoV-2 was also found to productively infect cells within the nasal turbinate, eye and olfactory bulb, suggesting SARS-CoV-2 entry into the brain by this route after intranasal infection.

Our data indicate that direct infection of CNS cells together with the induced inflammatory response in the brain resulted in the severe disease observed in SARS-CoV-2-infected K18-hACE2 mice.

(emphasis and spacing added)


Microglia do not restrict SARS-CoV-2 replication following infection of the central nervous system of K18-hACE2 transgenic mice (Olivarria et al., 2021)
https://doi.org/10.1128/jvi.01969-21
Main points:

• Half of the mice developed a coronavirus infection of the brain.
• The tissue-resident immune cells of the brain (microglia) appeared powerless to stop the brain infection.

We demonstrate SARS-CoV-2 can infect and replicate in human iPSC-derived neurons and that infection shows limited anti-viral and inflammatory responses. [...] Intranasal infection of K18 human ACE2 transgenic mice (K18-hACE2) with SARS-CoV-2 resulted in lung pathology associated with viral replication and immune cell infiltration.

In addition, ∼50% of infected mice exhibited CNS infection characterized by wide-spread viral replication in neurons accompanied by increased expression of chemokine (Cxcl9, Cxcl10, Ccl2, Ccl5 and Ccl19) and cytokine (Ifn-λ and Tnf-α) transcripts associated with microgliosis and a neuroinflammatory response consisting primarily of monocytes/macrophages.

Microglia depletion via administration of colony-stimulating factor 1 receptor inhibitor, PLX5622, in SARS-CoV-2 infected mice did not affect survival or viral replication but did result in dampened expression of proinflammatory cytokine/chemokine transcripts and a reduction in monocyte/macrophage infiltration. These results argue that microglia are dispensable in terms of controlling SARS-CoV-2 replication in in the K18-hACE2 model but do contribute to an inflammatory response through expression of pro-inflammatory genes.

(emphasis and spacing added)

EDIT: Added two more relevant studies

 

[Pyrrhus]

The eyes are the window to the brain...


At the beginning of this discussion, the connection between the eye and the brain was discussed:

Here's a new, fascinating paper that detected inflammatory cells in the eye in Long Covid patients 2-3 months after infection. Since the optic nerve at the back of the eye is part of the nervous system, and the optic nerve is a major route for drainage of brain lymph, inflammatory cells in the eye is sometimes a marker of inflammation in the nervous system:

Subclinical ocular inflammation in persons recovered from ambulatory COVID-19 (Bakhoum et al., September 2020)
https://www.medrxiv.org/content/10.1101/2020.09.22.20128140v2

Now here is a study that found eye nerve damage at the front of the eye in patients with neurological symptoms a month after acute COVID, but not in post-COVID patients without persisting neurological symptoms:

Corneal confocal microscopy identifies corneal nerve fibre loss and increased dendritic cells in patients with long COVID (Bitirgen et al., July 2021)
https://bjo.bmj.com/content/early/2021/07/08/bjophthalmol-2021-319450.info

Patients with neurological symptoms 4 weeks after acute COVID-19 had a lower [corneal nerve fiber density] (p=0.032), [corneal nerve branch density] (p=0.020), and [corneal nerve fiber length] (p=0.012), and increased [immune dendritic cell] density (p=0.046) compared with controls, while patients without neurological symptoms had comparable corneal nerve parameters, but increased [immune dendritic cell] density (p=0.003).

And here is a study that found that 35.7% of COVID patients had coronavirus antibodies in the eye, suggesting infection of the eye. But if the novel coronavirus infects the eye, is the eye always infected from the air, or could it be infected via the optic nerve or corneal nerve from the brain?

Anti-SARS-Cov-2 IgA Response in Tears of COVID-19 Patients (Caselli et al., 2020)
https://www.mdpi.com/2079-7737/9/11/374/htm

Simple Summary
SARS-CoV-2 can enter the body via the eye but the local antiviral response is still poorly known, we thus analyzed the presence of mucosal antibodies in the tears of COVID-19 patients. The results show that 35.7% of COVID-19 subjects have specific antiviral secretory antibodies in the eye. Their detection may be extremely helpful in clarifying, at this level, virus pathology and epidemiology.

Abstract
The pandemic virus SARS-CoV-2 has been reported to be able to enter the body via the eye conjunctiva, but the presence of antiviral response in the eye remains poorly known. Our study was thus aimed to analyze the presence of secretory mucosal anti-SARS-CoV-2 type A immunoglobulins (IgA) in the conjunctival fluid of COVID-19 patients. The tears of 28 COVID-19 patients and 20 uninfected controls were collected by the Schirmer test and analyzed by a specific ELISA assay detecting anti-spike (S1) virus protein IgA. The results showed that 35.7% of COVID-19 subjects have specific antiviral IgA at the ocular level, persisting till 48 days post disease onset. Most of the IgA positive subjects presented mild symptoms. The collected data indicate a prolonged persistence of anti-SARS-CoV-2 IgA at the eye level and suggest that IgA detection may be extremely helpful in clarifying virus pathology and epidemiology.

EDIT: added a relevant study

 

[andyguitar]

Here is a study that found that 35.7% of COVID patients had coronavirus antibodies in the eye, suggesting infection of the eye:

The probability of the virus getting in via the eye was proposed very early on in the pandemic. But when you see how few health care workers use eye protection it seems to have been ignored.

 

[Pyrrhus]

Now that it seems established that the novel coronavirus can infect the brain and cause inflammation, let's turn our attention back to Long Covid, and ask:


Can the novel coronavirus persist in the brain of Long Covid patients?


First, we should note that the brain is traditionally considered to be an "immune-privileged" organ, since blood-borne immune cells are, in most cases, restricted from entering the brain to fight brain infections:

Immune privileged organs were operationally defined as sites in the body where foreign tissue grafts can survive for extended, often indefinite periods of time, whereas similar grafts placed at regular sites in the body are acutely rejected (Medawar, 1948). These organs include the eye and the brain, as well as the pregnant uterus, testis, and several others (Streilein, 2003b; Niederkorn, 2006). Such immune privilege is thought to be an evolutionary adaptation to protect tissues that are indispensable, yet have limited regeneration capacity, like the brain and the eye, from the potentially damaging effects of an uncontrolled inflammatory immune response. Thus, immune privileged organs were considered as ones to which immune cell entry is forbidden; leukocytes were believed to be excluded from these vital organs by the presence of specialized physical barriers, the blood–tissue barriers.

Source: https://www.frontiersin.org/articles/10.3389/fimmu.2012.00296/full


Second, we should note that other RNA viruses are known to persist in the brain indefinitely, causing problems:

Genomic material from many neurotropic RNA viruses (e.g., measles virus [MV], West Nile virus [WNV], Sindbis virus [SV], rabies virus [RV], and influenza A virus [IAV]) remains detectable in the mouse brain parenchyma long after resolution of the acute infection. [...] Here we show that MV RNA remains detectable in permissive mouse neurons long after challenge with MV and, moreover, that immunosuppression can cause RNA and protein synthesis to rebound, triggering neuropathogenesis months after acute viral control. [...] These results illuminate the potential consequences of noncytolytic, immune-mediated viral control in the [central nervous system] and demonstrate that what were once considered "resolved" RNA viral infections may, in fact, induce diseases later in life that are distinct from those caused by acute infection.

Source: https://pubmed.ncbi.nlm.nih.gov/31270232/


Lastly, we should note that previous coronaviruses have been found to persist in the human body:

We report the in situ hybridization detection of coronavirus RNA in 12 of 22 [multiple sclerosis (MS)] brain samples using cloned coronavirus cDNA probes. In addition, tissue was screened for coronavirus antigen by immunohistochemical methods; antigen was detected in two patients with rapidly progressive MS. Significant amounts of coronavirus antigen and RNA were observed in active demyelinating plaques from these two patients. These findings show that coronaviruses can infect the human central nervous system and raise the possibility that these viruses may contribute to the pathogenesis of MS in some patients.

Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7159714/#idm140073824761296title

https://twitter.com/i/web/status/1345451224829722626

 

[vision blue]

But remember that neuroinflammation does not mean permanent damage. If the cause of the neuroinflammation can be fixed, one should expect the neuroinflammatory symptoms to disappear...

But what if you've had it for a long tine? or many repeated episodes? I think i have now for past 4 years at least

 

[Violeta]

But remember that neuroinflammation does not mean permanent damage. If the cause of the neuroinflammation can be fixed, one should expect the neuroinflammatory symptoms to disappear...

What does the virus do to the terrain that it leaves behind inflammation?

And then, how do we fix it?

 

[Pyrrhus]

You can't biopsy a brain from a living Long Covid patient

Scientists face a fundamental problem in trying to determine whether or not the coronavirus persists in the brain of Long Covid patients: There is simply no way to directly look at viruses inside the brain of a living person.

Therefore, you must use an experimental technique that looks at indirect evidence of brain infection.

Unfortunately, all of these indirect techniques come with serious limitations:

1. You can try to use non-invasive imaging to find "hints" of a viral infection in Long Covid.
   • You can perform an MRI to look for brain damage, but persistent viral brain infections generally don't leave behind any noticeable brain damage, since the viruses persist inside the nerve cells without killing the cells. Furthermore, MRIs can not detect the subtle neuroinflammation associated with these persistent viral brain infections - they can only detect classical inflammation, as seen in acute encephalitis.
   • You can perform an FDG-PET to look for regions of hypermetabolism or hypometabolism in the brain. Some situations, such as neuroinflammation, might initially increase metabolism - unless critical nutrients in that part of the brain happen to have been depleted by long-term neuroinflammation, in which case you might see decreased metabolism. Loss of blood flow to a region of the brain might also result in hypometabolism. Finally, a simple lack of cognitive activity might also decrease metabolism. So any findings of hypermetabolism or hypometabolism could have multiple interpretations.
2. You can try to get a less-invasive biopsy from somewhere close to the brain. But often, close is just not good enough, so you would have to actually get inside the brain if you want to know what's really happening in the brain.
   • An easy way to get close to the brain is to look in the cerebrospinal fluid. However, the brain and the cerebrospinal fluid are separated by a part of the blood-brain-barrier, so this technique will only work if the blood-brain-barrier is damaged or compromised, as in acute encephalitis. For this reason, viruses that can directly target the brain, such as poliovirus, have evolved ways to bypass the blood-brain-barrier without damaging it, and are almost never found in the cerebrospinal fluid.
3. You can try to develop an animal model of Long Covid, just as people develop animal models of diseases in order to test out experimental treatments. However, in order to develop an animal model of a disease you must first have a clear understanding of the underlying pathology of the disease that you are trying to model. If you don't, then the animal model may turn out to be useless, or even worse - misleading. (Unfortunately, there are many examples of this in the literature.) And it's probably safe to say that we don't yet have a clear understanding of the underlying pathology of Long Covid, so we don't yet know how to develop an animal model of Long Covid.

So let's take a look at the ways that people have tried these three indirect techniques to study Long Covid...

 

[Pyrrhus]

Non-invasive imaging in Long Covid

Here are two FDG-PET studies that looked at brain metabolism in Long Covid. Both found hypometabolism in multiple parts of the brain.


18F-FDG brain PET hypometabolism in patients with long COVID (Guedj et al., 2021)
https://link.springer.com/article/10.1007/s00259-021-05215-4

In comparison to healthy subjects, patients with long COVID exhibited bilateral hypometabolism in the bilateral rectal/orbital gyrus, including the olfactory gyrus; the right temporal lobe, including the amygdala and the hippocampus, extending to the right thalamus; the bilateral pons/medulla brainstem; the bilateral cerebellum (p-voxel < 0.001 uncorrected, p-cluster < 0.05 FWE-corrected). These metabolic clusters were highly discriminant to distinguish patients and healthy subjects (100% correct classification). These clusters of hypometabolism were significantly associated with more numerous functional complaints (brainstem and cerebellar clusters), and all associated with the occurrence of certain symptoms (hyposmia/anosmia, memory/cognitive impairment, pain and insomnia) (p < 0.05).

Long COVID hallmarks on [18F]FDG-PET/CT: a case-control study (Sollini et al., 2021)
https://link.springer.com/article/10.1007/s00259-021-05294-3

Long COVID patients exhibited brain hypometabolism in the right parahippocampal gyrus and thalamus (uncorrected p < 0.001 at voxel level). Specific area(s) of hypometabolism characterised patients with persistent anosmia/ageusia, fatigue, and vascular uptake (uncorrected p < 0.005 at voxel level).