Antibody-dependent cellular cytotoxicity against SARS-CoV-2 Omicron sub-lineages is reduced in convalescent sera regardless of infecting variant

Abstract

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron BA.4 and BA.5 variants caused major waves of infections. Here, we assess the sensitivity of BA.4 to binding, neutralization, and antibody-dependent cellular cytotoxicity (ADCC) potential, measured by FcγRIIIa signaling, in convalescent donors infected with four previous variants of SARS-CoV-2, as well as in post-vaccination breakthrough infections (BTIs) caused by Delta or BA.1. We confirm that BA.4 shows high-level neutralization resistance regardless of the infecting variant. However, BTIs retain activity against BA.4, albeit at reduced titers. BA.4 sensitivity to ADCC is reduced compared with other variants but with smaller fold losses compared with neutralization and similar patterns of cross-reactivity. Overall, the high neutralization resistance of BA.4, even to antibodies from BA.1 infection, provides an immunological mechanism for the rapid spread of BA.4 immediately after a BA.1-dominated wave. Furthermore, although ADCC potential against BA.4 is reduced, residual activity may contribute to observed protection from severe disease.

Keywords: COVID-19; Fc effector function; Omicron BA.4; SARS-CoV-2; VOC; antibody-dependent cellular cytotoxicity; breakthrough infection; neutralization.

Graphical abstract

Figure 1

BA.4 neutralization escape varies by the infecting variant in unvaccinated convalescent individuals Pseudovirus neutralization titer (ID₅₀) in convalescent plasma from unvaccinated donors infected with (A) D614G, (B) Beta, (C) Delta, and (D) Omicron BA.1. Plasma was tested against D614G, Beta, Delta, and Omicron BA.1, BA.2, and BA.4. Lines indicate geometric mean titer (GMT) also represented below the plot with fold decrease and knockout (KO) of detectable activity for other variants as a percentage relative to the infecting strain. Dotted lines indicate the limit of detection of the assay. Statistical significance across variants is shown by Friedman test with Dunn’s correction. ∗p < 0.05, ∗∗p < 0.01, and ∗∗∗p < 0.001. All data are representative of two independent experiments containing a minimum of two biological replicates.

Figure 2

Breakthrough infections show reduced neutralization activity against BA.4 despite high titers against other VOCs (A and B) Pseudovirus neutralization titer (ID₅₀) in convalescent plasma from vaccinated donors subsequently infected with (A) Delta and (B) Omicron BA.1. Plasma were tested against D614G, Beta, Delta, and Omicron BA.1, BA.2, and BA.4. Lines indicate GMT also represented below the plot with fold decrease and KO of activity for other variants as a percentage relative to the infecting variant. Dotted lines indicate the limit of detection of the assay. (C and D) Fold decrease in neutralization for each VOC represented as a ratio of the titer to the infecting variant Delta (C) or BA.1 (D) for infections in unvaccinated individuals (green for Delta and orange for BA.1) and BTIs (black). Dots represent mean fold decrease while error bars represent standard deviation of the mean. Statistical significance across variants is shown by Friedman test with Dunn’s correction. ∗∗p < 0.01, ∗∗∗p < 0.001, and ∗∗∗∗p < 0.0001. All data are representative of two independent experiments containing a minimum of two biological replicates.

Figure 3

FcγRIIIa signaling (ADCC) against BA.4 is reduced but preserved in convalescent plasma from previously unvaccinated individuals regardless of the infecting variant Antibody-dependent cellular cytotoxicity (ADCC) in unvaccinated individuals infected with (A) D614G, (B) Beta, (C) Delta, and (D) Omicron BA.1. The ability of plasma to cross-link spike expressed on the surface of HEK293T cells and activate FcγRIIIa represented as normalized relative light units (RLU) with background as determined in the absence of antibody are shown. All data are representative of two independent experiments containing a minimum of two biological replicates. Lines indicate geometric mean (GM) RLUs, also represented below the plot with fold decrease of activity for other variants relative to the infecting variant. Statistical significance across variants is shown by Friedman test with Dunn’s correction. ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < 0.001, and ∗∗∗∗p < 0.0001.

Figure 4

FcγRIIIa signaling (ADCC) against BA.4 is reduced in BTIs caused by Delta or BA.1 to a similar extent as in convalescent plasma (A and B) ADCC of vaccinated donors subsequently infected with (A) Delta and (B) Omicron BA.1. The ability of plasma to cross-link spike expressed on the surface of HEK293T cells and activate FcγRIIIa represented as normalized RLUs without background as determined in the absence of antibody are shown. Lines indicate GM RLUs, also represented below the plot with fold decrease of activity for other variants relative to the infecting variant. Positivity threshold is indicated with a dotted line as determined by pre-pandemic SARS-CoV-2-negative controls. (C and D) Fold decrease in ADCC for each VOC represented as a ratio of ADCC activity to the infecting variant Delta (C) or BA.1 (D) for infections in unvaccinated individuals (green for Delta and orange for BA.1) and BTIs (black). Dots represent mean fold decrease while error bars represent standard deviation of the mean. Statistical significance across variants is shown by Friedman test with Dunn’s correction. ∗p < 0.05 and ∗∗p < 0.01. All data are representative of two independent experiments containing a minimum of two biological replicates.