A series of four mannose(Man)-, three N-acetylglucosamine (GlcNAc)n-, ten N-acetylgalactosamine/galactose(GalNAc/Gal)-,
one 5-acetylneuraminic acid (alpha-2,3-Gal/GalNAc)- and one 5-acetylneuroaminic acid(alpha-2,6-Gal/Gal-NAc)-specific
plant agglutinins were evaluated for their antiviral activity in vitro. the mannose-specific
lectins from the orchid species Cymbidium hybrid (CA), Epipactis helleborine (EHA)
and Listera ovata (LOA) were highly inhibitory to human immunodeficiency virus type
1 (HIV-1) and type 2 (HIV-2) in MT-4, and showed a marked anti-human cytomegalovirus
(CMV), respiratory syncytial virus (RSV) and influenza A virus activity in HEL, HeLa
and MDCK cells, respectively. The 50% effective concentration (EC50) of CA and EHA
for HIV ranged from 0.04 to 0.08 micrograms/ml, that is about 3 orders of magnitude
below their toxicity threshold (50% inhibitory concentration for MT-4 cell growth:
54 to 60 micrograms/ml). Also, the (GlcNAc)n-specific lectin from Urtica dioica (UDA)
was inhibitory to HIV-1-, HIV-2-, CMV-, RSV- and influenza A virus-induced cytopathicity
at an EC50 ranging from 0.3 to 9 micrograms/ml. The GalNAc/Gal-, alpha-2,3-Gal/GalNAc-
or alpha-2,6-Gal/GalNAc-specific lectins were not inhibitory to HIV or CMV at non-toxic
concentrations. CA, EHA and UDA proved to be potent inhibitors of syncytium formation
between persistently HIV-1- and HIV-2-infected HUT-78 cells and CD4+ Molt/4 (clone
8) cells (EC50: 0.2-2 micrograms/ml). Unlike dextran sulfate, the plant lectins CA,
EHA and UDA did not interfere with HIV-1 adsorption to MT-4 cells and RSV- and influenza
A virus adsorption to HeLa and MDCK cells, respectively. They presumably interact
at the level of virion fusion with the target cell.