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To demonstrate the utility of phage display in generating highly specific antibodies, affinity selections were conducted on 20 related Src Homology 2 (SH2) domains (ABL1, ABL2, BTK, BCAR3, CRK, FYN, GRB2, GRAP2, LYN, LCK, NCK1, PTPN11 C, PIK3R1 C, PLCgamma1 C, RASA1 C, SHC1, SH2D1A, SYK N, VAV1 and the tandem domains of ZAP70). The domains were expressed in Escherichia coli, purified and used in affinity selection experiments. In total, 1292/3800 of the resultant antibodies were shown to bind the target antigen. Of the 695 further evaluated in specificity ELISAs against all 20 SH2 domains, 379 antibodies were identified with unique specificity (i.e. monospecific). Sequence analysis revealed that there were at least 150 different clones with 1-19 different antibodies/antigen. This includes antibodies that distinguish between ABL1 and ABL2, despite their 89% sequence identity. Specificity was confirmed for many on protein arrays fabricated with 432 different proteins. Thus, even though the SH2 domains share a common three-dimensional structure and 20-89% identity at the primary structure level, we were able to isolate antibodies with exquisite specificity within this family of structurally related domains.

The relationship between the efficiency of the neutralization process and the affinity of five monoclonal IgG antibodies specific for the haemagglutinin of type A influenza virus has been investigated by determining their neutralization rate constants (Kneut.) and affinities (Kdissoc.). We addressed the hypothesis that if antibody affinity alone determined the efficiency of neutralization, then the Kneut.:Kdissoc. ratio would be the same for all antibodies. However, we found that the Kneut.:Kdissoc. ratio varied by up to 125-fold, suggesting that properties unique to the epitope are of major importance in determining the efficiency of neutralization. These data suggest that vaccines should preferentially stimulate antibodies to epitopes that mediate the most efficient neutralization, and that a high Kdissoc. should be an important but secondary consideration.

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.