The COVID-19 pandemic has illustrated the potential for monoclonal antibody (mAb) therapeutics as prophylactic and therapeutic agents against pandemic viruses. However, no such therapeutics currently exist for other human coronaviruses. NL63 is a human alphacoronavirus that uses the same receptor (ACE2) as the highly pathogenic SARS-CoV and SARS-CoV-2, but with a generally mild clinical course.
We characterised antibody and B cell responses against NL63 spike within a cohort of healthy Australian adults. While NL63 spike and receptor binding domain (RBD)-specific binding antibodies and neutralisation activity could be detected in plasma of all subjects, memory B cells against NL63 spike were variable and relatively low in frequency compared to that against SARS-CoV-2 spike.
Based on B cell binding to NL63 spike or RBD, 9 human mAbs were isolated and assessed for breadth of recognition against a panel of alpha- and beta-coronaviruses, and neutralisation using a live virus neutralisation assay. Five highly potent neutralising mAbs were identified, with 4 targeting the RBD and 1 targeting the S2 domain of spike. Generation of viral escape mutants revealed that neutralising RBD mAbs expectedly targeted the receptor binding motif and blocked ACE2 receptor engagement. Interestingly, a mAb targeting the S2 domain of NL63 spike displayed highly potent neutralisation (IC50 of 8ng/ml), in contrast to reported S2 neutralising antibodies for SARS-CoV-2 that tended to neutralise with reduced potency (IC50 >10µg/ml). This mAb binds to a unique membrane proximal heptad repeat 2 (HR2) region of S2 that is absent in betacoronaviruses, suggesting neutralisation occurs via interfering with fusion of the viral and cellular membranes. Promisingly, the 5 neutralising mAbs were able to neutralise 3 clinical isolates of NL63 that were grown from PCR+ nasal swabs.
A deeper understanding of human antibody immunity against NL63 can guide vaccine and therapeutic development against future alphacoronavirus threats.