Hybrid immunity elicits potent cross-variant ADCC against SARS-CoV-2 through a combination of anti-S1 and S2 antibodies

Antibodies capable of neutralising SARS-CoV-2 have been well studied, but the Fc receptor-dependent antibody activities that also significantly impact the course of infection have not been studied in such depth. SARS-CoV-2 infection induces antibody-dependent NK cell responses targeting multiple antigens, however, as most vaccines induce only anti-spike antibodies, we investigated spike-specific antibody-dependent cellular cytotoxicity (ADCC). Vaccination produced antibodies that only weakly induced ADCC, however, antibodies from individuals who were infected prior to vaccination (‘hybrid’ immunity) elicited much stronger anti-spike ADCC. Quantitative and qualitative aspects of humoral immunity contributed to this capability, with infection skewing IgG antibody production towards S2, vaccination skewing towards S1 and hybrid immunity evoking strong responses against both domains. The capacity for hybrid immunity to provide superior spike-directed ADCC was associated with selectively increased antibody responses against epitopes within both S1 and S2. Antibodies targeting both spike domains were important for strong antibody-dependent NK cell activation, with three regions of antibody reactivity outside the receptor-binding domain (RBD) corresponding with potent anti-spike ADCC. Consequently, ADCC induced by hybrid immunity with ancestral antigen was conserved against variants containing neutralisation escape mutations in the RBD [Delta and Omicron (BA.1)]. Induction of antibodies recognizing a broad range of spike epitopes and eliciting strong and durable ADCC may partially explain why hybrid immunity provides superior protection against infection and disease than vaccination alone, and demonstrates that spike-only subunit vaccines would benefit from strategies to induce a combination of S1- and S2-specific antibody responses. Significance Neutralising antibodies prevent the entry of cell-free virus, however, antibodies that promote Fc-dependent activities such as ADCC are critical to control cell-associated virus. Although current SARS-CoV-2 vaccines induce potent neutralising antibodies, they fail to induce robust ADCC. Our demonstration that hybrid immunity induces superior ADCC with pan-variant activity may partially explain why hybrid immunity offers enhanced protection against reinfection. It also highlights that vaccine strategies based on expression of the spike subunit alone should not focus solely on inducing antibody responses targeting the receptor binding domain. ### Competing Interest Statement The authors have declared no competing interest.