COVID-19 Severity Is Associated with Differential Antibody Fc-Mediated Innate Immune Functions

Abstract

Beyond neutralization, antibodies binding to their Fc receptors elicit several innate immune functions including antibody-dependent complement deposition (ADCD), antibody-dependent cell-mediated phagocytosis (ADCP), and antibody-dependent cell-mediated cytotoxicity (ADCC). These functions are beneficial, as they contribute to pathogen clearance; however, they also can induce inflammation. We tested the possibility that qualitative differences in SARS-CoV-2-specific antibody-mediated innate immune functions contribute to coronavirus disease 2019 (COVID-19) severity. We found that anti-S1 and anti-RBD antibodies from hospitalized COVID-19 patients elicited higher ADCD but lower ADCP compared to antibodies from nonhospitalized COVID-19 patients. Consistently, higher ADCD was associated with higher systemic inflammation, whereas higher ADCP was associated with lower systemic inflammation during COVID-19. Our study points to qualitative, differential features of anti-SARS-CoV-2 specific antibodies as potential contributors to COVID-19 severity. Understanding these qualitative features of natural and vaccine-induced antibodies will be important in achieving optimal efficacy and safety of SARS-CoV-2 vaccines and/or COVID-19 therapeutics.IMPORTANCE A state of hyperinflammation and increased complement activation has been associated with coronavirus disease 2019 (COVID-19) severity. However, the pathophysiological mechanisms that contribute to this phenomenon remain mostly unknown. Our data point to a qualitative, rather than quantitative, difference in SARS-CoV-2-specific antibodies' ability to elicit Fc-mediated innate immune functions as a potential contributor to COVID-19 severity and associated inflammation. These data highlight the need for further studies to understand these qualitative features and their potential contribution to COVID-19 severity. This understanding could be essential to develop antibody-based COVID-19 therapeutics and SARS-CoV-2 vaccines with an optimal balance between efficacy and safety.

Keywords: COVID-19; Fc-mediated functions; SARS-CoV-2; antibody; inflammation.

FIG 1

Hospitalized COVID-19 is associated with differential antibody Fc-mediated innate immune functions. (a and b) The ability of anti-S1 (a) and anti-RBD (b) antibodies to elicit antibody-dependent complement deposition (ADCD) was measured as the ability of 1 mg/ml of IgG to fix complement on target cells. The Kruskal-Wallis test was used for statistical analysis. Horizontal dotted lines indicate the maximum values from the SARS-CoV-2 negative-control group. (c and d) The ability of anti-S1 (c) and anti-RBD (d) antibodies to elicit antibody-dependent cellular phagocytosis (ADCP) was measured as the ability of 1 mg/ml of IgG to mediate phagocytosis of S1 and RBD antigen-coated beads. The Kruskal-Wallis test was used for statistical analysis. Horizontal dotted lines indicate the maximum values from the SARS-CoV-2 negative-control group. (e) Comparison between the fraction of nonhospitalized and hospitalized COVID-19 patients whose antibodies elicit anti-RBD-specific ADCP with values higher than background (maximum value from the SARS-CoV-2 negative-control group). The chi-square test was used for statistical analysis. (f to i) The ability of anti-S1 (f and g) and anti-RBD (h and i) antibodies to elicit antibody-dependent cell-mediated cytotoxicity (ADCC) was estimated by NK cell degranulation and intracellular cytokine (TNF-α [f and h] or IFN-γ [g and i]) production after exposure of 100 μg/ml of IgG to S1- and RBD-pulsed target cells. The Kruskal-Wallis test was used for statistical analysis. Horizontal dotted lines indicate the maximum values from the SARS-CoV-2 negative-control group. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; ns, not significant.

FIG 2

Antibody titers do not correlate with ability to elicit ADCD, which associates with higher systemic inflammation. (a) S1-specific antibody titers of the indicated groups. The Kruskal-Wallis test was used for statistical analysis. The horizontal dotted line indicates the maximum value from the SARS-CoV-2 negative-control group. (b to d) S1-specific antibody titers in individuals whose antibodies can elicit S1-specific ADCD (b), ADCP (c), or ADCC (d) higher (+ symbol) or lower (− symbol) than background (maximum value from the SARS-CoV-2 negative-control group). The Mann-Whitney U test was used in statistical analysis. (e) RBD-specific antibody titers of the indicated groups. The Kruskal-Wallis test was used for statistical analysis. The horizontal dotted line indicates the maximum value from the SARS-CoV-2 negative-control group. (f to h) RBD-specific antibody titers in individuals whose antibodies can elicit RBD-specific ADCD (f), ADCP (g), or ADCC (h) higher (+ symbol) or lower (− symbol) than background (maximum value from the SARS-CoV-2 negative-control group). The Mann-Whitney U test was used in statistical analysis. (i) The ability of purified IgG to neutralize SARS-CoV-2 pseudovirus. Total IgG concentrations were normalized across samples and ranged from 100 μg/ml to 0.045 μg/ml to yield an IC₅₀. Any sample that yielded no measured neutralization ability was given a value of 110 μg/ml for graphing purposes. The Kruskal-Wallis test was used for statistical analysis. (j) Spearman's correlation heat map showing associations between Fc-mediated innate immune functions in rows and levels of plasma inflammatory markers in columns during COVID-19 (n = 60). The size and color of circles represent the strength of the correlation, with blue shades representing negative correlations and red shades representing positive correlations. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001; ns, not significant.