Infection with severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) causes coronavirus disease 2019 (COVID‐19), which presents with respiratory symptoms including fever, sore throat, cough, and congestion, suggesting that infection of the oral cavity and nasopharynx (ONP) is an obligate step in its pathogenesis. Consistent with this role in the initial infection, oral and nasopharyngeal epithelial cells express the SARS‐CoV‐2 spike protein receptor, angiotensin‐converting enzyme 2 (ACE2) (Sakaguchi et al., 2020), and RT‐PCR based testing can routinely detect SARS‐CoV‐2 RNA within both the nasal and buccal tissues. Local innate and adaptive immune responses in the oral epithelia are almost certainly involved in the defense against SARS‐CoV‐2 infection, yet little is known about these defenses.

In an initial approach to elucidate the antiviral defense in the oral cavity, we identified the expression of the antiviral type I interferon (IFN) and IFN‐stimulated genes (ISGs) expressed in oral epithelial cells (Brice et al., 2019). In addition, we demonstrated the vitamin D‐mediated induction of cathelicidin (McMahon et al., 2011; Menzel et al., 2019) in the oral epithelium. Cathelicidin is a host defense peptide that inactivates another enveloped, orally transmitted virus through membrane disruption (Brice et al., 2018). However, our understanding of the dynamic antiviral defense mechanisms of the oral cavity is poorly developed. In order to help combat this major public health emergency, we must devote a significant effort to characterization of the antiviral immune defenses in the oral cavity.

A sample of gingival epithelium has been examined by single cell RNA sequencing (scRNAseq), and shows the wide variety of cell types, including several that play important roles in antiviral defense, including plasmacytoid dendritic cells (pDC), innate lymphoid cells (ILC), T cells and NK cells (Byrd2020). An analysis of this database indicates the expression of numerous genes, including ACE2 and proinflammatory cytokine genes in several cell types. This supports the hypothesis that SARS‐CoV‐2 can infect the oral epithelium as part of an initial infection, and that antiviral defenses are found in the oral cavity.

To examine the response of oral tissues to SARS‐CoV‐2 infection, we obtained saliva and buccal samples from patients with COVID‐19 and uninfected control individuals (details on subject recruitment, demographics and sample isolation are provided in Supplementary Information). Quantification of ACE2 mRNA by QRT‐PCR shows that there is a significant reduction in mean ACE2 mRNA levels in the COVID‐19 patients compared to uninfected controls (Figure 1a). The related virus, SARS‐CoV, also utilizes ACE2 as its receptor, and infection of epithelial cells with this virus leads to a downregulation of ACE2 mRNA (Kuba et al., 2005). Our result suggests that SARS‐CoV‐2 acts similarly in infected oral tissues, to downregulate ACE2 expression. This could negatively affect the body's ability to regulate the renin‐angiotensin pathway, and lead to more severe disease.

We also examined the expression of the pro‐inflammatory cytokine IL‐6, and discovered that surprisingly, its expression was reduced in COVID‐19 patients (Figure 1b). However, Chen et al. have recently shown that SARS‐CoV‐2 infection inhibits the JAK‐STAT pathway that regulates IL‐6 production (Chen et al., 2020). These results from clinical samples confirm the prior experimental results, and further demonstrate that we can observe the virus‐mediated inhibition of innate antiviral defense mechanisms in oral tissues.

Together, our results show that the oral cavity represents an important interface for the infection of SARS‐CoV‐2, and easily obtained buccal swabs and saliva samples can be used to study the host response to its infection. We urge the oral microbiology and immunology community to partner with infectious disease clinicians and other medical professionals as we have done, and utilize these important resources to assist the broader scientific community in its effort to battle this public health emergency. Essential studies include (but are not limited to) changes in the oral microbiota in infection, the function of innate antiviral immune mediators in the oral cavity, and the effect of infection on oral pathogenic conditions.

感染严重急性呼吸系统综合症冠状病毒 2 (SARS-CoV-2) 引起冠状病毒病 2019 (COVID-19)，表现为发热、咽痛、咳嗽和充血等呼吸道症状，提示口腔和鼻咽部感染(ONP) 是其发病机制中的必要步骤。与初始感染中的这种作用一致，口腔和鼻咽上皮细胞表达 SARS-CoV-2 刺突蛋白受体、血管紧张素转换酶 2 (ACE2)（Sakaguchi 等人，2020 年），基于 RT-PCR 的检测可以 常规进行检测鼻腔和口腔组织中的 SARS-CoV-2 RNA。口腔上皮细胞中的局部先天性和适应性免疫反应几乎肯定参与了对 SARS-CoV-2 感染的防御，但对这些防御知之甚少。

在阐明口腔抗病毒防御的初步方法中，我们确定了口腔上皮细胞中表达的抗病毒 I 型干扰素 (IFN) 和 IFN 刺激基因 (ISG) 的表达（Brice 等人，2019 年 ）。此外，我们证明了维生素 D 介导的 口腔上皮中导管素的诱导作用（McMahon 等人， 2011 年；Menzel 等人，  2019 年）。Cathelicidin 是一种宿主防御肽，可通过膜破坏使另一种包膜、口服传播的病毒失活（Brice 等人，  2018 年）). 然而，我们对口腔动态抗病毒防御机制的了解还很薄弱。为了帮助应对这一重大突发公共卫生事件，我们必须投入大量精力来表征口腔中的抗病毒免疫防御系统。

牙龈上皮样本已通过单细胞 RNA 测序 (scRNAseq) 进行检测，并显示出多种细胞类型，包括在抗病毒防御中发挥重要作用的几种细胞，包括浆细胞样树突状细胞 (pDC)、先天性淋巴样细胞 (ILC) 、T 细胞和 NK 细胞 (Byrd 2020 )。对该数据库的分析表明，许多基因在几种细胞类型中表达，包括 ACE2 和促炎细胞因子基因。这支持了这样的假设，即 SARS-CoV-2 可以作为初始感染的一部分感染口腔上皮细胞，并且在口腔中发现了抗病毒防御。

为了检查口腔组织对 SARS-CoV-2 感染的反应，我们从 COVID-19 患者和未感染的对照个体中获取了唾液和口腔样本（补充信息中提供了受试者招募、人口统计和样本分离的详细信息）。通过 QRT-PCR 对 ACE2 mRNA 的定量表明，与未感染的对照组相比，COVID-19 患者的平均 ACE2 mRNA 水平显着降低（图 1a）。相关病毒 SARS-CoV 也利用 ACE2 作为其受体，用这种病毒感染上皮细胞会导致 ACE2 mRNA 的下调（Kuba 等人，2005 年 ）). 我们的结果表明，SARS-CoV-2 在受感染的口腔组织中的作用相似，可下调 ACE2 的表达。这可能会对身体调节肾素-血管紧张素通路的能力产生负面影响，并导致更严重的疾病。

我们还检查了促炎细胞因子 IL-6 的表达，令人惊讶地发现，它在 COVID-19 患者中的表达降低了（图 1b）。然而，陈等人。最近表明，SARS-CoV-2 感染会抑制调节 IL-6 产生的 JAK-STAT 通路（Chen 等人，  2020 年）。这些来自临床样本的结果证实了先前的实验结果，并进一步证明我们可以观察到病毒介导的口腔组织先天抗病毒防御机制的抑制。

总之，我们的结果表明，口腔代表了 SARS-CoV-2 感染的重要界面，容易获得的口腔拭子和唾液样本可用于研究宿主对其感染的反应。我们敦促口腔微生物学和免疫学界像我们一样与传染病临床医生和其他医疗专业人员合作，并利用这些重要资源来协助更广泛的科学界努力应对这一突发公共卫生事件。基本研究包括（但不限于）感染时口腔微生物群的变化、先天抗病毒免疫介质在口腔中的功能，以及感染对口腔病原体的影响。