Spike Protein Inhibition Assay (SPIA)

The Spike Protein Inhibition Assay (SPIA) is a competitive immunoenzymaticcolorimetric method to detect neutralising antibodies against SARS-CoV-2 in human serum.

The 2019 novel coronavirus, named SARS-CoV-2 is identified as the causative agent of an outbreak of a viral pneumonia or SARS (Severe Acute Respiratory Syndrome) which the World Health Organisation (WHO) subsequently named COVID-19¹. The viral infection causes a series of respiratory illness including severe respiratory syndrome, indicating the virus most likely infects respiratory epithelial cells and spreads mainly via respiratory tract from human to human². SARS-CoV-2 belongs to the β-coronavirus genus and is similar to the SARS outbreak in 2003 and MERS (Middle East Respiratory Syndrome) in 2012. The genome of coronavirus encodes four structural proteins including Spike (S), Envelope (E), Membrane (M) and Nucleocapsid (N) proteins. The virus infects and replicates itself within Type II pneumocytes cells and is transmitted via primarily droplet³. Receptor-mediated endocytosis is the main process of virus entry to the host cells: S protein contains a receptor binding domain (RBD) involved in the binding of the angiotensin-converting enzyme 2 (ACE2)⁴. ACE2 is a cell-surface receptor that is present in the kidney, blood vessels, heart and importantly, in the lung alveolar type II (AT2) cells which are respiratory tract epithelial cells. Once the virus gains access inside the target cell, the host immune system recognises the whole virus or its surface epitopes, eliciting an immune response, characterised by elevations in specific antibodies. Those antibodies that are able to bind to the spike protein and prevent it from entering cells by inhibition of the RBD-ACE2 interaction are known as neutralising antibodies (nAb) and the presence of these have been shown to be associated with protection against infection in disease models⁵. While nAb are believed to be important for protection, the level required is not defined. As such a wide range of nAb titres have been reported following SARS-CoV-2 infection and these vary depending on the length of time from infection and the severity of the disease.

1.https://www.who.int/emergencies/diseases/novel-coronavirus-2019/technical-guidance/naming-the-coronavirus-disease-(covid-2019)-and-the-virus-that-causes-it
2. Hu, B., Guo, H., Zhou, P.et al.Characteristics of SARS-CoV-2 and COVID-19.Nat Rev Microbiol19,141–154 (2021).
3. Tai W, He L, Zhang X, Pu J, Voronin D, Jiang S, Zhou Y, Du L. Characterization of the receptor-binding domain (RBD) of 2019 novel coronavirus: implication for development of RBD protein as a viral attachment inhibitor and vaccine. Cell Mol Immunol. 2020 Jun;17(6):613-620.
4. Jiang S, Zhang X, Yang Y, Hotez PJ, Du L. Neutralizing antibodies for the treatment of COVID-19. Nat Biomed Eng. 2020 Dec;4(12):1134-1139.
5. Rogers TF, Zhao F, Huang D, et al. Isolation of potent SARS-CoV-2 neutralizing antibodies and protection from diseasein a small animal model. Science. 2020 Aug 21;369(6506):956-963.