Eduardo López-Granados, Francisco Sánchez-Madrid, Sofia R. Gardeta, Arantzazu Alfranca, José María Frade, Celia López-Sanz, Daniel Fernández-Soto, José M. Casasnovas, Pedro Martínez-Fleta, Hugh T. Reyburn, Gloria Esteso, Salomé Prat, Mar Valés-Gómez, Yaiza Cáceres-Martell, Ligia Gabrie, Isidoro González-Álvaro, Tamara Mateu-Albero, Consejo Superior de Investigaciones Científicas (España), Comunidad de Madrid, Ministerio de Ciencia, Innovación y Universidades (España), Agencia Estatal de Investigación (España), European Commission, Instituto de Salud Carlos III, Fundación 'la Caixa', Banco Santander, Martinez-Fleta, Pedro [0000-0001-6107-0262], Alfranca, Arántzazu [0000-0002-3732-5816], González-Álvaro, Isidoro [0000-0001-9614-5199], Casasnovas, José María [0000-0002-2873-6410], Fernández-Soto, Daniel [0000-0002-7325-8614], Cáceres-Martell, Yaiza [0000-0002-7814-9409], López-Sanz, Celia [0000-0001-7203-0290], Prat, Salomé [0000-0003-2684-5485], Sánchez-Madrid, Francisco [0000-0001-5303-0762], Reyburn, H. T. [0000-0003-2855-1595], Rodríguez-Frade, José Miguel [0000-0002-7753-1462], Valés-Gómez, Mar [0000-0001-7424-3206], Martinez-Fleta, Pedro, Alfranca, Arántzazu, González-Álvaro, Isidoro, Casasnovas, José María, Fernández-Soto, Daniel, Cáceres-Martell, Yaiza, López-Sanz, Celia, Prat, Salomé, Sánchez-Madrid, Francisco, Reyburn, H. T., Rodríguez-Frade, José Miguel, and Valés-Gómez, Mar
Currently, there is a need for reliable tests that allow identification of individuals that have been infected with SARS-CoV-2 even if the infection was asymptomatic. To date, the vast majority of the serological tests for SARS-CoV-2–specific Abs are based on serum detection of Abs to either the viral spike glycoprotein (the major target for neutralizing Abs) or the viral nucleocapsid protein that is known to be highly immunogenic in other coronaviruses. Conceivably, exposure of Ags released from infected cells could stimulate Ab responses that might correlate with tissue damage and, hence, they may have some value as a prognostic indicator. We addressed whether other nonstructural viral proteins, not incorporated into the infectious viral particle, specifically the viral cysteine-like protease, might also be potent immunogens. Using ELISA tests, coating several SARS-CoV-2 proteins produced in vitro, we describe that COVID-19 patients make high titer IgG, IgM, and IgA Ab responses to the Cys-like protease from SARS-CoV-2, also known as 3CLpro or Mpro, and it can be used to identify individuals with positive serology against the coronavirus. Higher Ab titers in these assays associated with more-severe disease, and no cross-reactive Abs against prior betacoronavirus were found. Remarkably, IgG Abs specific for Mpro and other SARS-CoV-2 Ags can also be detected in saliva. In conclusion, Mpro is a potent Ag in infected patients that can be used in serological tests, and its detection in saliva could be the basis for a rapid, noninvasive test for COVID-19 seropositivity., This work was supported by the Spanish National Research Council (Project 202020E079) and grants from the Madrid Regional Government (IMMUNOTHERCAN S2017/BMD-3733-2 [to M.V.-G.]), the Spanish Ministry of Science and Innovation (MCIU/AEI/FEDER, EU: RTI2018-093569-B-I00 [to M.V.-G.], SAF2017-82940-R [to J.M.R.F.], SAF2017-83265-R [to H.T.R.], and SAF2017-82886-R [to F.S.-M.]), and RETICS Program of Instituto de Salud Carlos III (RD16/0012/0006; RIER [to J.M.R.F.], RD16/0011/0012 and PI18/0371 [to I.G.-A.], and PI19/00549 [to A.A.]). The study was also funded by La Caixa Banking Foundation (HR17-00016 to F.S.-M.) and Banco Santander Supera COVID to F.S.-M. This work has been cofunded by grant Covid 2019 from the Madrid Regional Government to Health Institute “La Princesa.”