Immunological studies of the functions of paramyxovirus glycoproteins

The HN and F glycoproteins of the paramyxovirus SV5 were purified and monospecific antibodies to each prepared. The effects of bivalent (IgG) and monovalent (Fab) forms of these antibodies on the biological activities of the glycoproteins were determined. Anti-HN antibodies inhibited adsorption of virus to erythrocytes, hemagglutination, and hemolysis. Inhibition of hemolysis was shown to be secondary to the inhibition of adsorption; anti-HN antibodies added after virus adsorption did not affect hemolysis. Anti-HN antibodies inhibited neuraminidase activity, and this inhibition was independent of substrate size in experiments in which virus and antibodies were allowed to react and substrates of different size added, i.e., neuraminlactose and fetuin. This result is in contrast to previous findings with influenza virus in which antibodies inhibited the action of the viral enzyme on the macromolecular substrate only. Thus with SV5, the antibodies appear to inhibit the hydrolytic process directly, rather than sterically hindering the access of the enzyme to large substrates, as in the case in influenza virus. Anti-F antibodies had no effect on virus adsorption or neuraminidase activity, but inhibited hemolysis when added either before or after absorption, confirming the direct involvement of the F protein in the hemolytic process. The inhibition of virus adsorption, neuraminidase, and hemolytic activities by the respective antibodies was accomplished by Fab fragments as well as IgG, indicating that in each case the inhibitory activity was a consequence of a direct effect on individual glycoprotein molecules rather than crosslinking of glycoproteins or aggregation of virions. Both anti-HN and anti-F antibodies neutralized virus infectivity in MDBK and CV-1 cells. Anti-F IgG and Fab, and anti-HN IgG caused essentially complete neutralization in both cell types; however, anti-HN Fab caused only partial neutralization in CV-1 cells.