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A new study published on the prepress server bioRxiv * in December 2020 indicates the ability of anti-SARS-CoV-2 antibody titers and induction time to differentiate between severe and mild COVID-19 and could lead to the development of a prognostic biomarker for use early in the course of the illness.
With the pandemic showing no signs of regressing anytime soon, researchers are investigating the response of antiviral antibodies to diagnose infection, predict disease severity, and understand the nature of the immunity it elicits. Preliminary evidence suggests that severe COVID-19 is related to high levels of antibodies and B cell responses.
Unconventional Antibody Responses in COVID-19
To better understand this, the researchers behind the current study conducted a multiple antigen immunoassay to discover the various antibody responses and their correlation with clinical severity. They examined COVID-19 plasma samples from 52 acutely infected and 69 convalescent patients, comparing them to 106 and 137 healthy adults before and after the pandemic, respectively. They measured the titers of immunoglobulin M (IgM), immunoglobulin G (IgG) and immunoglobulin A (IgA) against the nucleocapsid (N), the S1 peak subunit, the receptor-binding domain (RBD) and the N-terminal domain S1. (S1). -NTD).
Antibody responses to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) do not always follow the canonical pattern, in which IgM antibodies are produced first, followed by IgG. Many patients showed an increase in IgM and IgG at the same time. Again, the neutralizing activity was correlated with anti-RBD S1 antibodies, but antibodies are also formed against the S1 and S2 subunits of the peak protein, S1-NTD, and protein N. Antigens include epitopes that are They bind both neutralizing and non-neutralizing antibodies and can be used in antibody-based diagnostics.
Previous research indicated an increase in specific antibodies in severe disease compared to milder forms, but the degree of overlap between these categories made it useless for prognostic purposes.
High predictive value of the multiple test
Therefore, the current study began with the development of a sensitive antibody assay. Using this, the researchers found that a reliable diagnosis can be made in the acute phase (<30 days from the onset of symptoms) using the combined IgA, IgG and IgM values, or IgG values alone, against the S1 and antigens. RBD, and N Antigen, to a slightly lesser degree.
In general, in acute infection, IgA targeting the N, S1, and S1-RBD antigens was also potentially useful, as was specific IgM for S1 and S1-RBD. This indicates that IgA, IgG, or IgM could be helpful in diagnosing an infection early in the acute phase.
However, if the two classes of patients are analyzed separately from day 6 to day 30 after the onset of symptoms, patients with severe or critical illness have higher levels of anti-S1, anti-S1-RBD and anti-N IgG, IgA or combined IgG, IgA and IgM, compared to those with mild or moderate infections.
IgM values were poorly predictive of infection in the convalescence period, especially those targeting IgM S1-NTD and N. IgA had a stronger predictive value for S1 and RBD. The highest diagnostic value was observed with IgG against N, S1 and S1-RBD, with the latter classes showing high specificity and sensitivity for patients in the convalescent period.
False negatives due to high titers
Once again, severe or critical illness was associated with a strong prozone effect in serum. There was a bell-shaped binding curve for the severely and critically ill patient groups, indicating the occurrence of false negatives for antibody interactions with N, S1, and RBD. To prevent this from happening, the researchers used samples diluted 1: 500 to ensure the inclusion of as many patients as possible from the mild / moderate group, with a low antibody titer, while minimizing the impact of the prozone effect in critically ill patients. or critical. group.
Serum from severely / critically ill patients had a high neutralizing capacity, being able to inhibit ACE2 receptor binding activity by 50% even when diluted several hundred times. This was not observed in the mild-moderate group. Anti-S1 IgG titers were measured at titers up to 45 μg / mL and 100-400 μg / mL in the mild-moderate and severe-critical groups. There was no significant difference between measurements taken from serum or plasma.
Slower and lower kinetics in mild disease
Finally, mild or moderate disease was associated with a slower and lower response in the form of IgG anti-N and anti-S1 antibodies, in relation to severe or critical illness. In severe disease, the antibody titer increased early and to a higher level for almost all isotypes and for all antigens, except only IgM targeting S1-NTD. Both classes of patients had comparable anti-RBD IgG and neutralizing antibodies at 2-4 months.
In the convalescence stage, the antibodies of the mild-moderate group almost rise to those of the severe-critical group, which registers a slight drop. However, IgG, IgA and IgM against N and S1 quickly fall into the latter group, which explains their lack of utility in the diagnosis of the convalescent stage.
What are the implications?
In summary, the diagnosis of infection in both the acute and convalescent stages can be made using the combination of antibodies or IgG alone, against RBD, confirmed by anti-N and anti-S1 titers. Differentiation of mild-moderate and severe critical cases was best done by combined isotype measurements or anti-RBD, anti-S1, or anti-N assays. The best in both classes was anti-S1-RBD.
The high expansion of ASC in severe / critical illness in early infection is related to higher levels of antibodies, suggesting that B cells and ASC have different sources in mild-moderate and severe-critical infections. The early response in the latter is vigorous and comprises mainly IgG and IgA, while in the former it conforms to the classical model and takes 4 to 6 weeks to develop. Starting with extrafollicular activation that proceeds to transient IgM production, then shifts to germinal center activation, leading to the generation of memory B cells and plasma cells and high affinity specific antibodies with isotype switching.
However, extrafollicular reactions that proceed directly to the isotype change phase also occur in this infection, probably explaining the high production of IgG and IgA in severe critical cases. The germinal centers are altered in these patients. Subsequent antibody profiles will vary with the degree of disruption and restoration of germinal center function.
The researchers comment: “The surprising early response produced by critically ill / critically ill patients may allow early detection of patients most likely to experience poor outcomes.. “Second, they point out,”The kinetics of virus-specific antibodies vary according to the severity of the disease and we speculate that the specific viral antigens will have implications for long-term humoral protection for reinfection by SARS-CoV-2.. “
And finally, “This unexpected difference in the speed and magnitude of humoral responses may reflect important differences in immune response and provide early signs for patients that could benefit from targeted immunomodulatory interventions.. “
*Important news
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice / health-related behavior, or be treated as established information.
