Variations in the neutralizing and haemagglutination-inhibiting activities of five influenza A virus-specific IgGs and their antibody fragments.

Neutralization and haemagglutination-inhibition (HI) of a type A influenza virus by a panel of five monoclonal IgGs, their F(ab')2s, Fabs and Fabs+ anti-mouse Fab were compared. The MAbs were specific for antigenic sites A, B and D of the haemagglutinin. Activities of the IgGs varied by up to 6-fold on a molar basis, apart from the HI activity of HC58 which was > 100-fold lower. This was not due to low functional affinity as HC58 had the second highest value (nM) as determined by an equilibrium method with whole virions. Conversion to the F(ab')2 reduced neutralization and HI by only 2- to 6-fold, indicating that the Fc region had little involvement in these processes. However, all Fabs had low neutralization and HI activity compared with their IgGs, neutralization being reduced by 86 to > 1912-fold, and HI by 13 to > 69-fold. Although decreased, their affinities remained high, in the nM range. Neutralization and HI by three of the Fabs (HC2, HC3W and HC61) were restored by the addition of anti-Fab IgG; however, HC10 Fab+anti-Fab IgG still had no detectable neutralization activity but gave HI, and HC58 Fab+anti-Fab IgG had no detectable HI activity but neutralized to the same extent as its IgG. The different properties of the antibodies are discussed in the light of their known mechanisms of action: HI by steric blocking of attachment of virus to the red cell receptor, and neutralization by the inhibition of post-attachment events (HC2, HC10 and HC61). The data demonstrate just how variable are the antiviral properties of individual IgGs.