COVID-19 microglia encephalopathy: Immunohistological assessment

Aims: Activation of microglial cells represents the most common neu-ropathological change in fatal cases of COVID-19 with particular prominence in the brainstem. However, detailed assessments are lacking. Here, we assessed reactive microglia in COVID-19 tissue and tested for disease-specific activation patterns. Methods: We used an early-(Iba1) and a late-stage (CD68) immuno-histochemistry marker for microglial activation in human post-mortem brainstem and frontal lobe tissues in eight fatal COVID-19 cases, seven septic controls and six non-septic controls. We quantified the level of microglial activation employing a Qu-Path-based automated approach. Using a mixed three-way ANOVA, we tested for effects and interactions of brain region, microglial marker and group. Results: Reactive microglia were detected in all cases across brain regions and antibodies. However, COVID-19 brains exhibited significantly higher levels of microglial activation than septic and control brains, especially of late-stage microglia (CD68+). Irrespective of disease, microglia activation was significantly more pronounced and further progressed (CD68+) in brainstem tissues, particularly the medulla, than in the frontal lobe. Whilst survival time from admission marginally significantly correlated with the level of reactive microglia in COVID-19, no associations were found between neuroinflammation and either gender or age at death. Conclusions: Whilst the brainstem demonstrates a disease-independent high susceptibility to inflammation, microglial activation in COVID-19 (COVID-19 microglia encephalopathy) is specific and of importance for understanding the involvement of the CNS in this disease.

Detection of SARS-CoV-2 within enteric neurons and in brain

SARS-CoV-2, the causative agent of COVID-19, typically manifests as a respiratory illness although extrapulmonary involvement, such as in the gastrointestinal tract and nervous system, are increasingly recognised. Through immunohistochemistry against the SARS-CoV-2 nucleocapsid protein (NP), we aimed to characterise the multisystem viral tropism of SARS-CoV-2. FFPE tissue was obtained from 16 PCR-confirmed post-mortem COVID cases. Of these cases, 10 were full-body, 5 were brain only and 1 was a brain biopsy. Brain regions studied included frontal cortex, medulla, cerebellum, pons and olfactory bulb. Neurological symptoms featured in the cohort included brainstem encephalitis, acute disseminated encephalomyelitis (ADEM) and brain infarction. Immunohistochemistry of digestive system tissues revealed presence of SARS-CoV-2 NP in neurons of the myenteric plexus, a site of high ACE-2 expression, the entry receptor for SARS-CoV-2 and one of the earliest affected cells in Parkinson's disease (PD). Within the brain, staining was widespread in all sampled regions but limited to endothelial cells only (including in the olfactory bulb). Furthermore, in the full-body post-mortem cases, positivity in brain endothelia was restricted to cases exhibiting multiorgan tissue positivity (3/9 cases). The average time from symptom onset to time of death was shorter in positively versus negatively stained postmortem cases (mean = 10.3 days vs mean = 20.3 days, p = 0.0416) suggesting NP detection was confined to the infectious period. Together, our findings provide evidence for enteric nervous system but not brain neuroinvasion of SARS-CoV-2 as well as potential insights into long-term complications of COVID-19 and PD pathogenesis.

COVID-19 encephalitis: The pathological evidence

Introduction: SARS-COV2 may cause a wide range of neurological manifestations in about 30% of covid 19 patients. Cases of encephalitis have been described in patients presented with severe respiratory distress with estimated incidence of 58/100,000 patients. However, the correlated pathological changes of these cases of encephalitis and its pathogenesis are poorly understood. Methods: We have examined brains from 23 randomly referred cases of patients dying with COVID-19, with spectrum of neuropathological findings such as hypoxic-ischemic changes, infarcts, hemorrhages and necrotizing encephalopathy and mild to moderate increase in number of activated microglia cells. There are 5 cases of intense infiltration of brainstem structures with microglia cells and mild T lymphocyte infiltration which raises the possibility of what is called "brainstem encephalitis" although with poorly correlated neurological manifestations. In these cases, there is intense activation of microglia cells with microglia nodules formation and mild infiltration with T lymphocytes but there is no significant demyelination, necrosis or axonal damage. The immunohistochemistry and in-situ hybridization failed to demonstrate SARS-COV2 virus or other viruses. On the other hand, we found only mild increase in number of activated microglia cells but without microglial nodules in the brainstem of 3/6 patients that died from non-COVID related septicemia and multi-organ failure. Results: These results suggest that the inflammatory changes in the brainstem structures are most likely related to peculiar substantial systemic inflammatory reaction and massive release of cytokines (cytokines storm) in COVID-19 patient causing activation of innate immunity and alteration of blood brain barrier (BBB) in brainstem structures. Therefore, this pathology should be termed as "COVID- 19 encephalopathy" rather than encephalitis. Conclusion: This and other works highlighted the importance for more research into the role of microglia cells in brain pathology in acute and long Covid19 diseases including psychiatric manifestations.

Invited Review: The spectrum of neuropathology in COVID-19.

There is increasing evidence that patients with Coronavirus disease 19 (COVID-19) present with neurological and psychiatric symptoms. Anosmia, hypogeusia, headache, nausea and altered consciousness are commonly described, although there are emerging clinical reports of more serious and specific conditions such as acute cerebrovascular accident, encephalitis and demyelinating disease. Whether these presentations are directly due to viral invasion of the central nervous system (CNS) or caused by indirect mechanisms has yet to be established. Neuropathological examination of brain tissue at autopsy will be essential to establish the neuro-invasive potential of the SARS-CoV-2 virus but, to date, there have been few detailed studies. The pathological changes in the brain probably represent a combination of direct cytopathic effects mediated by SARS-CoV-2 replication or indirect effects due to respiratory failure, injurious cytokine reaction, reduced immune response and cerebrovascular accidents induced by viral infection. Further large-scale molecular and cellular investigations are warranted to clarify the neuropathological correlates of the neurological and psychiatric features seen clinically in COVID-19. In this review, we summarize the current reports of neuropathological examination in COVID-19 patients, in addition to our own experience, and discuss their contribution to the understanding of CNS involvement in this disease.