A potent protective bispecific nanobody targeting Herpes simplex virus gD reveals vulnerable epitope for neutralizing.

Herpes simplex virus (HSV) causes significant health burden worldwide. Currently used antiviral drugs are effective but resistance can occur. Here, we report two high-affinity neutralizing nanobodies, namely Nb14 and Nb32, that target non-overlapping epitopes in HSV gD. Nb14 binds a neutralization epitope located in the N-A' interloop, which prevents the interaction between gD and gH/gL during the second step of conformational changes during membrane fusion after virus attachment. The bispecific nanobody dimer (Nb14-32-Fc) exhibits high potency in vitro and in vivo. Mechanistically, Nb14-32-Fc neutralizes HSVs at both the pre-and post-attachment stages and prevents cell-to-cell spread in vitro. Administration of Nb14-32-Fc at low dosage of 1 mg/kg provides 100% protection in an HSV-1 infection male mouse model and an HSV-2 infection female mouse model. Our results demonstrate that Nb14-32-Fc could serve as a promising drug candidate for treatment of HSV infection, especially in the cases of antiviral drug resistance and severe herpes encephalitis.