Potent neutralising monoclonal antibodies targeting the spike of NL63 coronavirus

NL63 is an alphacoronavirus that uses the same ACE2 receptor as SARS-CoV and SARS-CoV-2, but generally causes mild respiratory illness. In a cohort of healthy adults, we characterised humoral responses against NL63 spike and isolated a panel of human monoclonal antibodies (mAbs), including five with potent viral neutralising activity. Four neutralising mAbs blocked ACE2 receptor engagement and were found to target the receptor binding motif. A single mAb targeting the S2 subunit displayed potent neutralisation activity comparable to those directly blocking receptor engagement. The S2 mAb targets a membrane proximal heptad repeat 2 (HR2) region in spike that is absent in betacoronaviruses, potentially revealing a site of vulnerability unique to alphacoronaviruses. For all neutralising mAbs, putative epitopes were highly conserved in over 200 NL63 sequences, including recent clinical isolates. A deeper understanding of the recognition of alphacoronavirus spike by human antibodies will guide vaccine and therapeutic development against alphacoronavirus threats.