Study finds human surfactant protein A can inhibit SARS-CoV-2 infectivity

In a recent study posted to the bioRxiv* preprint server, researchers in the United States assess the efficacy of the human surfactant protein A (SP-A) against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infectivity.

Study: Human Surfactant Protein A Alleviates SARS-CoV-2 Infectivity in Human Lung Epithelial Cells. Image Credit: Filip Fuxa / Shutterstock.com

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

What is human SP-A?

Human SP-A is a hydrophilic protein found on mucosal epithelial surfaces of the upper airway and lungs. This protein can bind to pathogen-associated molecular patterns (PAMPs) of invading microbes.

In silico analysis suggests that SP-A may have a similar affinity to the SARS-CoV-2 spike protein as the angiotensin-converting enzyme 2 (ACE2)-spike interaction, which could potentially impact the development of SARS-CoV-2 infection. Efforts are being made globally to develop and enhance immunomodulatory and antiviral agents due to the continuous evolution of SARS-CoV-2 variants that threaten the effectiveness of novel vaccines and therapeutics for coronavirus disease 2019 (COVID-19).

About the study

In the present study, researchers investigate whether SP-A can interact with SARS-CoV-2 spike, receptor-binding domain (RBD), and ACE2 in human cells

Herein, the competitive inhibition of SP-A recognition of RBD and S protein by sugars was examined by incubating SP-A with immobilized RBD and S protein with sugars. The team assessed whether there was a direct interaction between SP-A and ACE2. Furthermore, the impact of SP-A on RBD-human ACE2 (hACE2) interaction was examined by incubating them simultaneously on RBD-immobilized enzyme-linked immunosorbent assay (ELISA) plates.

The potential of SP-A to hinder the infectivity of SARS-CoV-2 was assessed using pseudotyped particles as well as the infectious SARS-CoV-2 Delta variant. The ability of SP-A to bind S protein and RBD and subsequently inhibit viral entry was also assessed.

Assays were conducted to confirm the involvement of SP-A in the infectivity, binding, and entry of the SARS-CoV-2 Delta variant. Additionally, SP-A levels in the salivary samples of COVID-19 patients who were hospitalized with different levels of disease severity were also determined.

Results

SP-A showed dose-dependent binding to the SARS-CoV-2 S protein in the presence of calcium. However, the presence of ethylenediaminetetraacetic acid (EDTA) reduced the interaction between SP-A and S protein by 46%. This suggests that while SP-A and S protein binding was somewhat calcium-dependent, other non-calcium-dependent regions may also be involved in their interaction since EDTA was not able to completely eliminate SP-A and S protein binding.

SP-A also showed dose-dependent binding to immobilized SARS-CoV-2 RBD. EDTA exposure resulted in a 59% reduction in SP-A and RBD binding, thereby indicating that the SP-A and RBD interaction involves the SP-A carbohydrate recognition domain (CRD) along with other domains.

Mannose and maltose decreased SP-A and S binding, while galactose and GlcNAc did not show significant inhibition. All tested sugars inhibited the recognition of RBD by SP-A.

Higher levels of maltose did not eliminate the interaction between SP-A and S protein and RBD. SP-A was also found to bind glycoconjugates on SARS-CoV-2 S protein and RBD. The observed interaction between proteins may occur irrespective of the presence or dosage of sugars, thus suggesting the possibility of other types of protein-protein interaction.

There was an elevation in the dose-dependent binding of SP-A to biotinylated hACE2. The binding of SP-A to hACE2 was reduced when exposed to EDTA, thus suggesting that calcium is necessary for their optimal interaction.

Mannose, which is a preferred ligand for SP-A, did not strongly inhibit the interaction between SP-A and hACE2. Thus, the interaction between SP-A and hACE2 may involve glycoconjugate and non-glycoconjugate binding.

SP-A was also found to decrease hACE2 binding to RBD, whereas bovine serum albumin (BSA) did not affect the interaction between RBD and hACE2. Thus, SP-A could disrupt the binding of RBD and hACE2.

COVID-19 patients had higher levels of total protein and SP-A in their saliva samples in comparison to healthy individuals. Furthermore, severe COVID-19 patients had significantly lower levels of SP-A, despite having higher total protein levels. Thus, maintaining a comparatively preserved SP-A level in the salivary mucosal samples of SARS-CoV-2-infected patients is crucial in alleviating severe COVID-19 symptoms.

Conclusions

SP-A has significant antiviral impacts on SARS-CoV-2 infectivity in lung epithelial cells that are achieved by the interaction between SP-A and the S protein, which inhibits viral binding, entry, and viral titers in susceptible cells. Thus, the study findings contribute to ongoing efforts in developing new surfactant-based treatments for COVID-19.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Bhavana Kunkalikar

Bhavana Kunkalikar is a medical writer based in Goa, India. Her academic background is in Pharmaceutical sciences and she holds a Bachelor's degree in Pharmacy. Her educational background allowed her to foster an interest in anatomical and physiological sciences. Her college project work based on ‘The manifestations and causes of sickle cell anemia’ formed the stepping stone to a life-long fascination with human pathophysiology.