Olfactory transmucosal SARSCoV-2 invasion as port of central nervous system entry in COVID-19

Introduction: Coronavirus disease 2019 (COVID-19) is a pandemic respiratory disease which is accompanied by a broad spectrum of neurological manifestations in more than one-third of COVID-19 cases. For the latter, olfactory and gustatory disturbances such as anosmia and ageusia, are often leading symptoms of SARS-CoV-2 infection. Given the close proximity of neuronal and supporting cells the olfactory mucosa as a neural-mucosal interface may represent a potential port of CNS entry for SARS-CoV-2. Objectives: Identifying potential sites of SARS-COV-2 CNS entry and morphological changes associated with SARS-CoV-2 infection. Methods: We systematically investigated postmortem tissue of the CNS and nasopharynx from 75 individuals with COVID-19. Based on the correlation of clinical data and (neuro-) pathological examinations, SARSCoV- 2-specific morphological changes were determined. Using quantitative real-time PCR, RNAscope in situ hybridization, immunohistochemistry and electron microscopy, we characterized the CNS tropism of SARSCoV- 2 and the consequences thereof. Results: Acute thromboembolic ischemic infarcts (n = 10/64) and a strong innate immune response, mediated by HLA-DR+ microglia with a linked increase in proinflammatory mediators in the cerebrospinal fluid, are leading alterations in the CNS. Besides, a distinct immunoreactivity for SARSCoV Spike protein was found in the olfactory epithelium - here co-localizing with neural/neuronal cells - and cerebral endothelial cells. We were also able to illustrate intact coronavirus particles in the olfactory mucosa ultrastructurally. Conclusion: SARSCoV- 2 can enter the nervous system by crossing the neural-mucosal interface in the olfactory mucosa. SARSCoV- 2 infection results in an innate immune response with activation of HLA-DR+ microglia and increased levels of inflammatory mediators in the CNS.