IgG Fc glycosylation: A marker for severe COVID-19

By Dr. Prajakta Tambe, Ph.DReviewed by Benedette Cuffari, M.Sc.Mar 24 2022

In a recent eBioMedicine study, researchers suggest that low anti-spike immunoglobulin G1 (IgG1) glycosylation, sialyation, and high bisection are markers of severe coronavirus disease 2019 (COVID-19).

Study: Immunoglobulin G1 Fc glycosylation as an early hallmark of severe COVID-19. Image Credit: Vector-3D / Shutterstock.com

Background

As of March 25, 2022, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has infected over 477 million people around the world and caused more than 6.1 million deaths. Most patients infected with SARS-CoV-2 will experience mild symptoms; however, a significant minority of patients will experience severe and even life-threatening symptoms.

IgG plays a vital role in combatting SARS-CoV-2 infection by binding directly to the viral spike (S) protein through effector functions. These effector functions arise from the activation of complement or fragment crystallizable (Fc) gamma receptors (FcgR) on immune cells and can also be mediated through N-glycan moieties present on the C_H2 domains of antibodies.

About the study

In the current prospective, observational single-center cohort study, a total of 159 polymerase-chain reaction (PCR)-confirmed COVID-19 positive and vaccinated patients were included. Moreover, the study cohort consisted of 39 females,119 males, and one unknown gender, all of whom provided plasma samples between the first and second wave of the SARS-CoV-2 pandemic.

Sampling was done longitudinally during admission in the hospital, which was required when the patients’ oxygen saturation was below 92%. One convalescent sample was taken after the six weeks of discharge in an individual who was not dependent on oxygen supplementation.

IgG Fc glycosylation was analyzed by first capturing IgG from plasma samples and then detecting glycopeptides and assigning IgG1 glycoforms based on mass and migration position through high-performance liquid chromatography (HPLC) attached to a mass spectrometer. Samples collected pre-COVID-19 were used as negative controls.

Serum cytokine and chemokine levels were detected, and pools of four hospitalized patients were used as reference. Moreover, SARS-CoV-2 anti-nucleocapsid IgG, IgG antibodies against the S1 and S2 viral spike proteins, and IgM against the anti-receptor binding domain (RBD) were detected.

The severity score was calculated based on the 4C mortality score, wherein daily oxygen flow was considered for non-intensive care unit (ICU) patients (L/min) and p/f ratio (kPa) and FiO2 (%) were used for ICU patients. For this, the patients were divided into three groups based on severity including those with a score 0-5 (low), 6-11 (intermediate), and 12-17 (high).

Study findings

IgG Fc glycosylation analyzed by liquid chromatography-mass spectrometry (LC-MS) detected 14 glycoforms that corresponded to the anti-S IgGl glycosylation. The results showed lower fucosylation of anti-S than total IgGI, with 56 patients exhibiting less than 85% fucosylation of the S protein and several patients exhibiting less than 66%. Bisection was also found to be lower than total IgGI, whereas sialylation and anti-S glycosylation were higher than IgGI.

When the dynamic nature of glycosylation was studied, both anti-S galactosylation and total IgG1 were found to change with time and disease course. Glycosylation traits were found to be transient and varied with each patient.

While fucosylation and bisection rapidly elevated after the disease onset until 60 days, the galactosylation and sialylation decreased; however, the levels of these two events remained slightly higher than IgG1. Outpatient sampling after six weeks also showed similar results.

Further experiments involved determining whether Fc glycosylation is associated with ICU admissions and disease severity level. ICU admitted patients showed lower bisection, as well as higher galactosylation and sialyation during hospitalization as compared to non-ICU patients, all of which were more prominent in patients who experienced severe infection. However, fucosylation was higher during high severity but remained constant throughout hospitalization.

Moreover, increased bisection and decreased galactosylation and sialyation were observed during high disease severity at the time of hospitalization. The changes in the galactosylation and sialyation showed changes in anti-S IgG1 glycosylation, whereas changes in bisection were attributed to alterations in total IgG1 levels. Bisection was also found higher in those at baseline.

IgG Fc glycosylation was also associated with the expression of various chemokines, cytokines, and other mediators. The results showed a negative correlation of inflammatory markers with galactosylation and sialylation, while a positive correlation was observed for bisection and fucosylation. Fucosylation proinflammatory markers are also reduced over the course of the disease, which indicates a higher anti-inflammatory Fc glycosylation profile.

Conclusions

The researchers investigated the severity marker potential of anti-S IgG1 glycosylation in severe and mild hospitalized COVID-19 cases and correlated these findings with numerous inflammatory and clinical markers. Decreased galactosylation and sialylation with a higher bisection of anti-S IgG1 corresponded to reduced COVID-19 severity and no ICU admission.

Although the current study provides more information on IgG Fc glycosylation, several limitations including the context of a single-center study, unknown parameters, and lack of mechanistic data warrant further investigations.