Here, we describe a human monoclonal antibody (mAb), designated S2X259, recognizing a highly conserved cryptic receptor-binding domain (RBD) epitope and cross-reacting with spikes from all sarbecovirus clades. S2X259 broadly neutralizes spike-mediated entry of SARS-CoV-2 including the B.1.1.7, B.1.351, P.1, B.1.427/B.1.429 VOC, as well as a wide spectrum of human and potentially zoonotic sarbecoviruses through inhibition of ACE2 binding to the RBD. Furthermore, deep-mutational scanning and in vitro escape selection experiments demonstrate that S2X259 possesses an escape profile limited to the single substitution G504D. We show that prophylactic and therapeutic administration of S2X259 protects Syrian hamsters against challenge with the prototypic SARS-CoV-2 and the B.1.351 VOC, suggesting this mAb is a promising candidate for the prevention and treatment of emergent variants and zoonotic infections. Our data unveil a key antigenic site targeted by broadly-neutralizing antibodies and will guide the design of pan-sarbecovirus vaccines.

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