Human Coronavirus HKU1 Neutralizing Monoclonal Antibodies Target Diverse Epitopes Within and Around the TMPRSS2 Receptor Binding Site.

Endemic human coronaviruses (HCoVs), like HCoV-HKU1, account for ~30% of common cold/year and can cause serious upper and lower respiratory infections, yet no licensed vaccines target HCoVs. In fact, little is known about HCoV-HKU1's antigenic landscape. Thus, we characterized key interactions between HCoV-HKU1 spike (S) with monoclonal antibodies (mAbs) isolated from pre-pandemic HCoV-HKU1 convalescent PBMCs. We isolated 14 mAbs, which bound distinct S regions: receptor binding domain (RBD), N-terminal domain (NTD), and S2 subunit. Structural and functional studies revealed three groups of RBD-specific mAbs targeting diverse footprints within and around the TMPRSS2 receptor binding site, exemplified by: (1) The most potently neutralizing mAb, H501-022 (IC50 = 0.01 μg/mL), which recognizes the TMPRSS2 binding motif, thereby blocking receptor engagement; (2) mAb H501-008 (IC50 = 0.05 μg/mL) that binds a conserved, cross-reactive epitope outside of the TMPRSS2 binding site that is shared with HCoV-OC43; and (3) H501-018 (IC50 = 0.28 μg/mL) that recognizes both "up" and "down" RBD conformations at a distinct, non-overlapping site outside of the TMPRSS2 binding motif, distinguishing itself from H501-022 and H501-008, which bind exclusively to the "up" RBD conformation. These mAbs represent the first type-specific HCoV-HKU1 mAbs isolated from a convalescent donor. Our findings provide molecular insight into HCoV-HKU1 antibody recognition and neutralization mechanisms, importantly highlighting antigenic differences comparing HCoVs and pandemic CoVs - a critical step towards advancing universal CoV vaccine design.