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