Plant derived natural products as novel fusion inhibitors against Herpes simplex virus type 1 (HSV-1)

Herpesvirus infection and spread can be specifically blocked by preventing the fusion between the virion and the host cell membrane. The core fusion machinery of HSV-1 consists of the glycoproteins gD, gH, gL and gB. While gD mediates the interaction with various host cell receptors, gB executes the fusion of viral with cellular membrane after activation by a reaction cascade between the glycoproteins. Based on this mechanism a virus-free in vitro screening assay was developed for direct identification of antiviral compounds with fusion-inhibiting capability. Vero cells are transfected with gD, gH, gL and gB (effector-cells) and seeded on untransfected Vero cells (target-cells). The formation of syncytia and thereby the amount of fusion is visualized through mCherry-labelled gB. To quantify fusion activity, effector cells transfer a transactivator into the target cells, which in turn switches on a reporter gene, e.g. luciferase [1]. The use of Tet-On 3G as transactivator reduced cytotoxicity, widened the measureable window and allowed selective induction of reporter gene expression. Docosanol (5 mg/mL), a known entry inhibitor of enveloped viruses, and α-gB-2c, a neutralising, gB-specific monoclonal antibody, served as positive controls. Aescin from Aesculus hippocastanum was identified as a potent fusion inhibitor against HSV-1. Two different batches, characterized in detail by LC-MS, showed IC 50 between 5 and 10 µM, depending on incubation time and serum concentration in the cultivation media. Aescin reduces also HSV-1 plaque formation. Within a broader screening of saponins as fusion inhibitors we identified 2 active oleanan glycosides: hederacoside C (IC 50 about 200 µM) from Hedera helix , and esculentoside A (IC 50 about 150 µM) from Phytolacca esculenta . References: [1] PE Pertel, A Fridberg, ML Parish, PG Spear. Cell fusion induced by herpes simplex virus glycoproteins gB, gD, and gH-gL requires a gD receptor but not necessarily heparan sulfate. Virology 2001; 279: 313 – 24