Human Surfactant Protein D Binds Spike Protein and Acts as an Entry Inhibitor of SARS-CoV-2 Pseudotyped Viral Particles

Abstract

Human SP-D is a potent innate immune molecule whose presence at pulmonary mucosal surfaces allows its role in immune surveillance against pathogens. Higher levels of serum SP-D have been reported in the patients with severe acute respiratory syndrome coronavirus (SARS-CoV). Studies have suggested the ability of human SP-D to recognise spike glycoprotein of SARS-CoV; its interaction with HCoV-229E strain leads to viral inhibition in human bronchial epithelial (16HBE) cells. Previous studies have reported that a recombinant fragment of human SP-D (rfhSP-D) composed of 8 Gly-X-Y repeats, neck and CRD region, can act against a range of viral pathogens including influenza A Virus and Respiratory Syncytial Virus in vitro, in vivo and ex vivo. In this context, this study was aimed at examining the likely protective role of rfhSP-D against SARS-CoV-2 infection. rfhSP-D showed a dose-responsive binding to S1 spike protein of SARS-CoV-2 and its receptor binding domain. Importantly, rfhSP-D inhibited interaction of S1 protein with the HEK293T cells overexpressing human angiotensin converting enzyme 2 (hACE2). The protective role of rfhSP-D against SARS-CoV-2 infection as an entry inhibitor was further validated by the use of pseudotyped lentiviral particles expressing SARS-CoV-2 S1 protein; ~0.5 RLU fold reduction in viral entry was seen following treatment with rfhSP-D (10 µg/ml). These results highlight the therapeutic potential of rfhSP-D in SARS-CoV-2 infection and merit pre-clinical studies in animal models.

Full-Length Recombinant hSP-D Binds and Inhibits SARS-CoV-2

“…A second mechanism to explain rhSP-D inhibition of virus replication, which could cooperate with the first mechanism proposed, would be the potential aggregation of SARS-CoV-2 induced by rhSP-D by reducing the number of viral molecules available to interact with the host cell, however, further experiments are needed to confirm this second hypothesis. Interestingly, it has been recently published that the trimeric fragment rfhSP-D containing only the carbohydrate-binding domain and neck regions of SP-D inhibited viral entry in cells using pseudotyped lentiviral particles expressing SARS-CoV-2 S1 protein [41]. One could speculate that the same or even stronger effect should be expected when using full length rhSP-D, testing this hypothesis in lung epithelial cells is very interesting and will be done as part of future work in this project.…”

Pattern Recognition Proteins: First Line of Defense Against Coronaviruses

“…The ability of SP-D and recombinant fragment of human SP-D to bind SARS-CoV-2 S-protein suggest that they are capable of dampening the “cytokine storm” by rapid clearance of the virus-infected cells and strengthening the lung capacity by restoring homeostasis. Both, SP-A and SP-D have antiviral and immunomodulatory properties against SARS-CoV-2 ( 70 , 71 ). SP-D participates in pulmonary viral neutralization, agglutination and clearance; reduction of the inflammatory response upon infection; enhancement of dendritic cell phagocytosis; enhancement of macrophage-mediated pathogen killing, inflammatory cytokine modulation and chemotaxis; modulation of intrapulmonary T-cell response; promotion of clearance of apoptotic cells to prevent necrosis and inflammation; and binding of neutrophil/eosinophil extracellular traps preventing degranulation and modulating cytokine production ( 70 ).…”

“…SP-D participates in pulmonary viral neutralization, agglutination and clearance; reduction of the inflammatory response upon infection; enhancement of dendritic cell phagocytosis; enhancement of macrophage-mediated pathogen killing, inflammatory cytokine modulation and chemotaxis; modulation of intrapulmonary T-cell response; promotion of clearance of apoptotic cells to prevent necrosis and inflammation; and binding of neutrophil/eosinophil extracellular traps preventing degranulation and modulating cytokine production ( 70 ). SP-D recognizes SARS-CoV-2 S-protein, and recombinant SP-D bound to S-protein has been shown to inhibit interaction of the S-protein with ACE2 receptors ( 71 ). The importance of SP-A and SP-D for lung protection against viral infections has been demonstrated by the increased susceptibility of SP-A and SP-D knockout mice to influenza A and respiratory syncytial virus infections and viral-mediated inflammation ( 70 ).…”

“…The introduction of recombinant human SP-A and SP-D suggest that pattern recognition proteins may be used as anti-infective/anti-inflammatory therapies ( 66 , 69 , 312 , 313 ) ( Figure 12 ). SP-A and SP-D ( 70 , 71 ) are structurally like the viral fusion proteins they specifically bind and could be used to develop new anti-infective, and immune-modulatory therapies ( 70 , 71 ). Recombinant forms of SP-D recently developed could be used therapeutically ( 60 , 66 , 67 , 69 , 312 – 314 ) to reduce inflammatory processes in pulmonary diseases associated with viral infections like COVID-19.…”

The Role of Pulmonary Surfactants in the Treatment of Acute Respiratory Distress Syndrome in COVID-19

“…The recombinant fragment of human lung surfactant protein D (rfhSP-D) is reported to be more potent than remdesivir, an antiviral, in inhibiting the replication and infectivity of SARS-CoV-2 and the activity is found to mediated through down regulation of RdRp gene expression ( Hsieh et al, 2021 ; Madan et al, 2021 ).…”