A Prefusion Form of Herpes Simplex Virus 1 gB has a Distinct Antigenic Signature

Herpes simplex virus (HSV) fusion and entry is mediated by a cascade of interactions among gB, gD and gH/gL. The gB homotrimer undergoes a conformational transition from a metastable prefusion state to a more stable postfusion form, driving fusion of the viral envelope and a host cell membrane. An H516P mutation in gB domain III restricts formation of the extended core alpha helix and constrains gB in a prefusion state. Several prefusion gB structures have been determined that contain this mutation. We assessed the antigenic reactivity of gB H516P by quantitative immunodotblot using a panel of gB-reactive monoclonal antibodies All antibodies tested bound to both prefusion (H516P) gB and wild type gB. Antibodies tested to gB domains II, IV and V exhibited differential binding to H516P gB compared to wild type gB. The results suggest that gB H516P has a distinct antigenic profile. The antigenic signature of H516P may be useful as a rapid indicator of prefusion forms of gB. The low pH environment of the cellular endosome is a cell-specific factor for HSV entry and triggers antigenic changes in gB. The step at which low pH impacts gB refolding to execute fusion is not well-understood. The results suggest that gB H516P undergoes wild-type-like conformational changes in gB domains I and V triggered by low pH. We propose that pH acts on an early stage of gB fusion function, prior to extension of the domain III core helix.