ISOLATION OF A NEUTRALIZING HUMAN RSV ANTIBODY FROM A DOMINANT, NONNEUTRALIZING IMMUNE REPERTOIRE BY EPITOPE-BLOCKED PANNING

We isolated a large panel of human Abs directed against the respirator y syncytial virus (RSV) Ag from combinatorial phage display libraries. Following initial differentiation of the Fabs by BstNI restriction pa tterns, DNA sequence analysis revealed 10 different classes of V-H pai red with more than 35 different V-L genes. All the Fabs bound with hig h affinity to the F Ag. However, most Fabs competed with the binding o f a representative member of this group, suggesting that the Fabs reco gnized a common epitope on the F Ag, and none of them neutralized viru s in vitro. To suppress repetitive isolation of these non-neutralizing Abs, a representative Fab was included during panning to block this c ommon epitope on the F Ag. By this ''epitope-blocked panning'' approac h, two novel Fabs, encoded by unique V-H and V-L genes, were isolated from a previously screened library. Competition binding analysis confi rmed that the Fabs recognized epitopes distinct from that of the previ ously isolated Fabs. One of these Fabs, 516, neutralized RSV in cell c ulture. These activities of Fab-516 were retained upon its genetic con version to a mAb (IgG1) and expression in mammalian cells. Our results suggest that the RSV F glycoprotein presents a dominant, non-neutrali zing epitope to the human immune system, which may serve in evasion of host defenses. However, less prevalent, fusion-inhibiting Abs were re vealed by blockade of this epitope during the panning process.