Abstract
Genetic transfer of neutralizing antibodies (Abs) has been shown to confer strong and persistent protection against bacterial and viral infectious agents. Although it is well established that for many exogenous neutralizing Abs increased antigen affinity correlates with protection, the effect of antigen affinity on Abs produced in situ after adenoviral gene transfer has not been examined. The mouse IgG2b monoclonal Ab, 2C12.4, recognizes the Yersinia pestis type III secretion apparatus protein, LcrV (V antigen), and confers protection in mice when administered as an IgG intraperitoneally or after genetic immunization with engineered, replication-defective serotype 5 human adenovirus (Ad). The 2C12.4 Ab was expressed as a single-chain variable fragment (scFv) in Escherichia coli and was shown to display an equilibrium dissociation constant (K(D))=3.5 nM by surface plasmon resonance analysis. The 2C12.4 scFv was subjected to random mutagenesis, and variants with increased affinity were isolated by flow cytometry using the anchored periplasmic expression bacterial display system. After a single round of mutagenesis, variants displaying up to 35-fold lower K(D) values (H8, K(D)=100 pM) were isolated. The variable domains of the H8 scFv were used to replace those of the parental 2C12.4 IgG encoded in the Ad vector, AdalphaV, giving rise to AdalphaV.H8. The two adenoviral vectors resulted in similar titers of anti-V antigen Abs 3 days after immunization, with 10(9), 10(10) or 10(11) particle units (pu). After intranasal challenge with 363 LD(50) (lethal dose, 50%) of Y. pestis CO92, 54% of the mice immunized with 10(10) pu of AdalphaV.H8 survived through the 14 day end point compared with only 15% survivors for the group immunized with AdalphaV expressing the lower-affinity 2C12.4 (P<0.04; AdalphaV versus AdalphaV.H8). These results indicate that affinity maturation of a neutralizing Ab delivered by genetic transfer may confer increased protection not only for Y. pestis challenge but also possibly for other pathogens.