Saliva After Toothbrushing Enhances the Inhibitory Effect on the Interaction Between SARS‐CoV‐2 Spike Protein and Human ACE2 In Vitro

ABSTRACT Aim Saliva can inhibit viral infections; however, it is not clear whether oral hygiene influences this inhibitory effect. We aimed to investigate the inhibitory effect of saliva sampled before and after toothbrushing on the interaction between severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) spike protein and angiotensin‐converting enzyme 2 (ACE2). Methods Saliva samples were collected from 16 participants. First, resting saliva and mouth‐rinsed water were sampled, and then, the participants were requested to brush their teeth for 5 min using toothpaste. The resting saliva and mouth‐rinsed water were collected 5 min and 1 h thereafter. Oral cleanliness, a factor of oral hygiene was assessed based on ammonia level using the Salivary Multi Test. The interaction between SARS‐CoV‐2 spike protein and ACE2 was examined, and total secretory immunoglobulin A (sIgA) in saliva and sIgA with binding potential for SARS‐CoV‐2 spike protein were measured. Results The ammonia level in mouth‐rinsed water decreased 5 min and 1 h after brushing compared with the pre‐brushing level. The inhibitory effect of saliva on the interaction between SARS‐CoV‐2 spike protein and ACE2 significantly increased from before brushing to 1 h after brushing. Additionally, the sIgA ratio available to bind with SARS‐CoV‐2 spike protein to total sIgA level significantly increased 1 h after brushing. This ratio exhibited a positive correlation with the inhibitory effect of saliva on the interaction between the SARS‐CoV‐2 spike protein S1 and human ACE2. Conclusions These results may inform strategies to prevent SARS‐CoV‐2 and other viral infections via the salivary route.