Identification of B cell epitopes enhanced by protein unfolding and aggregation

Aggregation of therapeutic proteins is a key factor in the generation of unwanted immunogenicity, and can result in reduced serum half-life, neutralization of function and adverse health effects. There is currently little information regarding how aggregates interact with B-cell receptors or cognate antibodies at the protein sequence level, or whether non-native, aggregate-induced epitopes predominate in these interactions. Using an antibody fragment (single chain antibody variable fragment; scFv) that forms aggregates readily at low temperature, anti-scFv IgG antibody responses were generated by intraperitoneal injection of BALB/c strain mice with monomer or aggregate preparations. Aggregate-specific immunosignatures were identified by oligo-peptide microarray fine epitope mapping, using overlapping 15mer peptides based on the linear sequence of scFv, printed onto glass slides. IgG antibodies from mice immunized with aggregated scFv preferentially recognized a patch of overlapping peptides. This region mapped to a β-strand located at the interface between the V_H and V_L domains. Molecular dynamics simulations indicated that the V_L domain is less stable than the V_H domain, suggesting the interface region between the two domains becomes exposed during partial unfolding of the scFv during aggregate formation. These data are consistent with the hypothesis that epitopes from partially unfolded states are revealed, or are more fully exposed, in the aggregated state, and that this can augment the IgG antibody response. This observation offers the theoretical possibility that epitopes preferentially associated with aggregates can be identified from the anti-drug antibody serum IgG response which may, in turn, lead to better methods for detection of anti-drug antibody responses, and improved design of therapeutic proteins to control immunogenicity.