Humoral response to spike S1 and S2 and nucleocapsid proteins on microarray after SARS-CoV-2 infection.

Many aspects of the humoral immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), such as its role in protection after natural infection, are still unclear. We evaluated IgA and IgG response to spike subunits 1 and 2 (S1 and S2) and Nucleocapsid proteins of SARS-COV-2 in serum samples of 109 volunteers with viral RNA detected or seroconversion with different clinical evolution (asymptomatic, mild, moderate, and severe coronavirus disease 2019), using the ViraChip® Test Kit. We observed that the quantification of antibodies to all antigens had a positive correlation to disease severity, which was strongly associated with the presence of comorbidities. Seroreversion was not uncommon even during the short (median of 77 days) observation, occurring in 15% of mild-asymptomatic cases at a median of 55 days for IgG and 46 days for IgA. The time to reach the maximal antibody response did not differ significantly among recovered and deceased volunteers. Our study illustrated the dynamic of anti-S1, anti-N, and anti-S2 IgA and IgG antibodies, and suggests that high production of IgG and IgA does not guarantee protection to disease severity and that functional responses that have been studied by other groups, such as antibody avidity, need further attention.

Assessment of avidity related to IgG subclasses in SARS-CoV-2 Brazilian infected patients.

SARS-CoV-2 is considered a global emergency, resulting in an exacerbated crisis in the health public in the world. Although there are advances in vaccine development, it is still limited for many countries. On the other hand, an immunological response that mediates protective immunity or indicates that predict disease outcome in SARS-CoV-2 infection remains undefined. This work aimed to assess the antibody levels, avidity, and subclasses of IgG to RBD protein, in symptomatic patients with severe and mild forms of COVID-19 in Brazil using an adapted in-house RBD-IgG ELISA. The RBD IgG-ELISA showed 100% of specificity and 94.3% of sensibility on detecting antibodies in the sera of hospitalized patients. Patients who presented severe COVID-19 had higher anti-RBD IgG levels compared to patients with mild disease. Additionally, most patients analyzed displayed low antibody avidity, with 64.4% of the samples of patients who recovered from the disease and 84.6% of those who died in this avidity range. Our data also reveals an increase of IgG1 and IgG3 levels since the 8th day after symptoms onset, while IgG4 levels maintained less detectable during the study period. Surprisingly, patients who died during 8-14 and 15-21 days also showed higher anti-RBD IgG4 levels in comparison with the recovered (P < 0.05), suggesting that some life-threatening patients can elicit IgG4 to RBD antibody response in the first weeks of symptoms onset. Our findings constitute the effort to clarify IgG antibodies' kinetics, avidity, and subclasses against SARS-CoV-2 RBD in symptomatic patients with COVID-19 in Brazil, highlighting the importance of IgG antibody avidity in association with IgG4 detection as tool laboratory in the follow-up of hospitalized patients with more significant potential for life-threatening.