Deregulated cellular circuits driving immunoglobulins and complement consumption associate with the severity of COVID-19 patients

SARS-CoV-2 infection causes an abrupt response by the host immune system, which is largely responsible for the outcome of COVID-19. We investigated whether the specific immune responses in the peripheral blood of 276 patients were associated with the severity and progression of COVID-19. At admission, dramatic lymphopenia of T, B, and NK cells is associated with severity. Conversely, the proportion of B cells, plasmablasts, circulating follicular helper T cells (cTfh) and CD56–CD16+ NK-cells increased. Regarding humoral immunity, levels of IgM, IgA, and IgG were unaffected, but when degrees of severity were considered, IgG was lower in severe patients. Compared to healthy donors, complement C3 and C4 protein levels were higher in mild and moderate, but not in severe patients, while the activation peptide of C5 (C5a) increased from the admission in every patient, regardless of their severity. Moreover, total IgG, the IgG1 and IgG3 isotypes, and C4 decreased from day 0 to day 10 in patients who were hospitalized for more than two weeks, but not in patients who were discharged earlier. Our study provides important clues to understand the immune response observed in COVID-19 patients, associating severity with an imbalanced humoral response, and identifying new targets for therapeutic interventionThe study was funded by grants SAF2017- 82886-R to FS-M from the Ministerio de Economía y Competitividad, and from “La Caixa Banking Foundation” (HR17-00016) to FS-M. Grant PI018/01163 to CMC and grant PI19/00549 to AA were funded by Fondo de Investigaciones Sanitarias, Ministerio de Sanidad y Consumo, Spain. SAF2017-82886-R, PI018/01163 and PI19/00549 grants were also co-funded by European Regional Development Fund, ERDF/FEDER. This work has been funded by grants Fondo Supera COVID (CRUE-Banco de Santander) to FSM, and “Ayuda Covid 2019” from Comunidad de Madri

SARS-CoV-2 protease antibodies in serum and saliva

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 antibodies are based on serum detection of antibodies to either the viral spike glycoprotein (the major target for neutralising antibodies) or the viral nucleocapsid protein that are known to be highly immunogenic in other coronaviruses. Conceivably, exposure of antigens released from infected cells could stimulate antibody responses that might correlate with tissue damage and, hence, they may have some value as a prognostic indicator. We addressed whether other non-structural 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 titre IgG, IgM and IgA antibody 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 antibody titres in these assays associated with more severe disease and no cross-reactive antibodies against prior betacoronavirus were found. Remarkably, IgG antibodies specific for Mpro and other SARS-CoV-2 antigens can also be detected in saliva. In conclusion, Mpro is a potent antigen in infected patients that can be used in serological tests and its detection in saliva could be the basis for a rapid, non-invasive test for COVID-19 seropositivity.This work was supported by the Spanish National Research Council (CSIC, project number 202020E079) and grants from Madrid Regional Government IMMUNOTHERCAN [S2017/BMD-3733-2 (MVG)]; the Spanish Ministry of Science and Innovation [(MCIU/AEI/FEDER, EU): RTI2018-093569-B-I00 (MVG), SAF2017-82940-R (JMRF), SAF2017-83265-R (HTR); SAF2017-82886-R (FSM)]; RETICS Program of ISCIII [RD16/0012/0006; RIER (JMRF); RD16/0011/0012, PI18/0371 (IGA), PI19/00549 (AA)]. The study was also funded by La Caixa Banking Foundation (HR17-00016 to FSM) and Fondo Supera COVID (CRUE-Banco de Santander) to FSMN

SARS-CoV-2 Cysteine-like Protease Antibodies Can Be Detected in Serum and Saliva of COVID-19–Seropositive Individuals

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.”Peer reviewe