The website of SARS-CoV-2 entry and replication critically impacts strategies for COVID-19 diagnosis, transmission mitigation, and treatment. substantial damage in the lung , suggesting that the airway is the principal entry and target of SARS-CoV-2. However, analysis of multiple single cell RNA-seq datasets reveal overall low ACE2 RNA transcription in nasal airway epithelium, with further reduced expression in lower airway club cells and rare expression in alveolar epithelial type II cells . This pattern of ACE2 expression provides evidence that the upper, rather than the lower, airway is the initial site of SARS-CoV-2 infection. There is growing interest in a presentation of SARS-CoV-2 infection characterized by olfactory loss without concomitant nasal inflammatory symptoms. Disturbances in the sense of smell have been widely reported in COVID-19 patients internationally, with a reported prevalence as high as 85% in a large, multicenter European survey . These reports show, importantly, that some COVID-19 patients manifest olfactory loss as their initial or only symptom. As this presentation is largely not recognized or thought to mandate isolation, this patient group may be a source of continued viral spread and a target population for early intervention and mitigation. The loss of the sense of smell suggests the possibility of direct targeting by SARS-CoV-2 of the olfactory system. However, the cellular location of ACE2 protein in the olfactory epithelium has not been previously demonstrated. Results Within the nasal cavity, the specialized olfactory neuroepithelium has an apical surface consisting mainly of sustentacular cells, which support neuronal dendritic projections containing the odor-sensing cilia. We performed immunohistological analysis to determine the location of ACE2 protein in human nasal and trachea biopsies. Confocal images demonstrated that the majority of ACE2 staining is localized to the apical surface of Krt18+ sustentacular cells in the olfactory neuroepithelium (Figure 1A and ?andB).B). This distribution of ACE2 protein is similar to that reported in bat nasal epithelium . We further quantified olfactory ACE2 expression and Demethoxycurcumin found the number of ACE2 positive cells to be comparable between healthy controls and chronic rhinosinusitis (CRS), a common inflammatory disease of the nasal mucosa (Figure 1A, ?,C,C, and ?andH)H) and affects the olfactory mucosa . ACE2 is Mouse monoclonal to HK1 not present in olfactory neurons, demonstrated by co-staining with the immature and mature olfactory neuron marker DCX (Figure 1E and ?andF)F) and PGP9.5 (Figure 1G), respectively. Open in a separate window Figure 1. Cellular location of ACE2 in human olfactory epithelium.(A and B) Confocal image of ACE2 (red) and Krt18 (green) immunostaining in the healthy control olfactory neuroepithelium. ACE2 is localized to Demethoxycurcumin the apical surface of Krt18 positive sustentacular Demethoxycurcumin cells in the olfactory epithelium. (C and D) Representative image of ACE2 and Krt18 immunostaining in the olfactory neuroepithelium of a CRS patient. The Boxed area in Panel A and C was highlighted in B and D, respectively. (E and F) The location of ACE2 and DCX positive immature olfactory sensory neurons in control (E) and CRS biopsy (F). (G) The location of ACE2 and PGP9.5 positive mature olfactory sensory neurons in control. (H) Quantification of ACE2 positive cells in olfactory epithelium. Data are represented as mean SEM. value was calculated by unpaired two-tailed Students test (H). Scale bars, 20 Demethoxycurcumin m. High intensity Demethoxycurcumin ACE2 staining was detected in all 11 olfactory mucosal biopsies. In addition, ACE2 is frequently observed in Bowmans glands (Figure 2A). In the adjacent nasal respiratory epithelium, ACE2 is also located on the apical surface (Figure 2B and ?andC),C), with a significantly lower level of expression than the olfactory epithelium (Figure 2D). As shown in Figure 2E in the adjacent area, intensive ACE2 expressed in PGP9.5+ olfactory region but can barely be detected in PGP9.5? respiratory epithelium. Only 47.4% of nasal respiratory epithelial biopsies (9 in 19) contained ACE2 positive epithelial cells. The intensity of ACE2 fluorescence was 200C700-fold higher in the olfactory epithelium (Figure 2F). Open in a separate window Figure 2. Elevated ACE2 in olfactory relative to respiratory epithelium.(A-C) Expression of ACE2 in glands (A) and nasal respiratory epithelium (B and C). The Boxed area in Panel B was highlighted.