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COVID-19-neutralizing antibodies predict disease severity and survival

Our take —

This study of 113 COVID-19 patents at Massachusetts General Hospital found that patients with severe COVID-19 disease (intubated or deceased) had higher titers of neutralizing antibodies compared to those with less severe disease (both non-hospitalized and hospitalized). Despite this finding, their neutralizing potency was lower compared to non-severe COVID-19 patients, suggesting a presence of excess non-neutralizing antibodies in sera of severe COVID-19 patients. Those with low neutralizing antibody potency (NT50/IgPC) <100 had a significantly higher risk of death compared to those with higher potency levels (NT50/IgPC>100). Patients with severe COVID-19 illness also had higher levels of CRP, D-dimer, ferritin, LDH and troponin-T, were more likely to be diabetic, hypertensive, male, and older in age compared to those with less severe disease. These results suggest that patients with severe COVID-19 disease tend to have exuberant humoral antibody responses but with lower neutralizing potency and higher levels of inflammatory markers than those with less severe disease. Given the cross-sectional nature of the study, with testing conducted at only one point in time, it is not possible to determine whether this type of antibody response is de-novo or occurs as a result of high cytokine levels in patients with severe disease. The study was limited by a relatively small sample size, incomplete adjustment of confounders, and measurement of neutralizing antibodies and cytokines at a single time point, which precludes the ability to make any causal assertions.

Study design

Cross-Sectional

Study population and setting

The study examined the effect of antibody response to SARS-CoV-2 virus among 113 COVID-19 patients at Massachusetts General Hospital, who were categorized into 5 groups: non-hospitalized, hospitalized, intubated, deceased, and immunocompromised due to medications or medical conditions. Blood samples were collected between 14 to 42 days from the time of symptom onset. Sera from 1275 subjects who were infected with non-SARS-CoV-2 viruses from the pre-pandemic era and samples from 78 healthy donors served as study controls. ELISA tests were used to measure anti-receptor binding domain (RBD) and anti-spike antibodies against SARS-CoV-2 (IgG, IgM, and IgA). A pseudo-lentivirus assay with 50% neutralizing activity against RBD and spike antigens of SARS-CoV-2 virus, was used as a primary tool in measuring the neutralizing antibodies in COVID-19 patients (NT50).

The association between COVID-19 severity and patient demographics, treatment against SARS-CoV-2, inflammatory markers (D-dimer, CRP, IL-6, lactate dehydrogenase, lymphopenia and elevated Troponin-T), and the level of NT50 was evaluated using Pearson correlation, analysis of variance, and multivariate regression. Due to variability between isotypes in the level of neutralizing antibodies among each COVID-19 patient, especially among those with severe disease (intubated and deceased), a composite antibody measure (IgPC) was created by weighting the sum of neutralizing antibody of each isotype in each COVID-19 patient using principal component analysis (PCA). This index was categorized as high (>=100) or low (<=100). Data pertaining to patient’s age, pre-existing medical conditions (diabetes, hypertension), level of inflammatory marker,s and treatment directed against SARS-CoV-2 (corticosteroids, Tocilizumab, remdesivir, hydroxychloroquine and azithromycin) in addition to the level of neutralizing antibodies (NT50, anti-RBD IgG and IgA, anti-spike IgG, and IgA) were collected for each patient and then clustered using a PCA.

Summary of Main Findings

Patients who were hospitalized were significantly older and more likely to be male compared to non-hospitalized COVID-19 patients (median age of 63 years vs. 28 years, 74% vs. 51%). Compared to 37 healthy controls, patients with COVID-19 had early isotype switches to IgG, with IgG antibodies lasting for 72 days while IgA and IgM antibodies waned at 42 days after symptoms onset. Anti-RBD IgG antibodies highly correlated with neutralization (R^2=0.78) compared to anti-spike IgG antibodies and had higher specificity in predicting neutralization (100% vs. 92%). On the other hand, anti-spike IgG antibodies were more sensitive in predicting neutralization (98% vs. 78%). The weighted sum of each isotope of neutralizing antibodies (IgPC) was highly correlated with neutralization (R^2=0.84) compared to each isotype alone. Patients with severe COVID-19 illness (intubated and deceased) had significantly higher IgG and IgA, but not IgM, anti-SARS-CoV-2 RBD and spike antibodies compared to non-hospitalized COVID-19 patients. Also, patients with severe disease had significantly higher-level neutralizing antibodies (IgPC), NT50, D-dimer, ferritin, LDH, CRP, and Troponin-T compared to those with non-severe disease. Immunocompromised patients (none deceased) had blunted neutralizing antibody response (IgPC). Hypertension, diabetes, advanced age, elevated inflammatory markers and IgPC were clustered in patients with severe disease, per the PCA.

A neutralizing potency index (NT50/IgPC) was created due to variability in the level of neutralizing antibodies (IgPC) in each disease category particularly among those with severe disease. Severely ill COVID-19 patients (intubated and deceased) had significantly lower measures of neutralizing potency (NT50/IgPC) compared to those with less severe disease. Patients with low (NT50/IgPC) had 87% survival compared to 100% survival among those with high (NT50/IgPC). In analyses adjusting for age, sex, preferred language, and days between symptom onset and sample collection, a 10 unit decrease in (NT50/IgPC) was associated with a 3.7 times higher hazard of death.

GM-CSF and IL-33 were significantly correlated with neutralizing antibody potency (NT50/IgPC) among patients with non-severe COVID-19 illness (non-hospitalized and hospitalized), while TNF-alpha and sCD40L were negatively correlated. IL6 was negatively correlated with neutralizing antibody potency (NT50/IgPC) among patients with severe disease but had weak and positive correlation among those with less severe disease.

Sera from COVID-19 patients was shown to cross-neutralize the emerging D614G variant, however, not against WIV1-CoV (a zoonotic coronavirus), SARS-CoV, nor MERS-CoV.

Study Strengths

Blood samples were collected within the same predefined period among patients with variable severity of COVID-19 illness, strengthening the study comparisons and internal validity. Neutralizing antibody titers were measured using ELISA and pseudo-lentivirus with well-defined cutoffs that have high sensitivity and specificity. The study incorporated detailed data pertaining to COVID-19 treatment, patient demographics, and individual immune responses (production of neutralizing antibodies and the level of antibody neutralizing potency) to describe the immunological basis behind the efficacy of corticosteroids and tocilizumab in COVID-19 patients. Finally, the study added valuable information about the ability of anti-SARS-CoV-2 neutralizing antibodies to neutralize D614G variants but not WIV1-CoV (a zoonotic coronavirus), SARS-CoV, nor MERS.

Limitations

The relatively small sample size limited the ability to incorporate patient demographics and laboratory and inflammatory markers in multivariate analysis. The negative correlation between Tocilizumab and neutralizing antibody potency almost disappeared after incorporating other potential confounder variables. Therefore, it was unclear whether this association was real, or the study was inadequately powered to evaluate treatment effect. Also, the levels of neutralizing antibodies and cytokines among SARS-CoV-2 infected patients were measured at only one point in time, precluding exploration of longitudinal changes in antibody response over time.

Value added

The study added valuable information regarding the type and the titer of antibody response among patients with variable severities of COVID-19 disease and insight about the neutralizing potency of sera from COVID-19 patients relative to sera from pre-pandemic patients against other viral infections and healthy controls. Also, the study revealed the ability of anti-SARS-CoV-2 neutralizing antibodies to neutralize D614G variants but not WIV1-CoV (a zoonotic coronavirus), SARS-CoV, nor MERS.

This review was posted on: 26 March 2021