Microheterogeneous monoclonal antibody subspecies with differential hepatitis C virus core antigen binding properties...

A monoclonal antibody directed against the core protein of hepatitis C virus was characterized for its utility in a sandwich antigen immunoassay wherein the mAb was used as the conjugate. Analysis of unconjugated and acridinium-conjugated monoclonal IgG using a silica-based HPLC size exclusion column revealed the existence of a single, symmetrical peak. Subsequent analysis of unconjugated IgG using a methacrylate-based HPLC size exclusion column revealed the presence of two species of IgG, but only by using a low ionic strength mobile phase buffer. Independent conjugation and testing of the two species showed significant differential reactivity towards HCV core antigen. Isoelectric focusing gels indicated subtle differences in the subspecies composition. Measurement of target peptide dissociation constants using fluorescence correlation spectroscopy indicated that the two HPLC column fractions exhibited a two-fold difference in Kd in low salt buffer that disappeared in high salt buffer. ESI-MS analysis of the fractionated IgG peaks revealed a reduction sensitive modification of the IgG and F(ab')2 of approximately 674 Da. In addition, both IgG and F(ab')2 contained two major heavy chain subspecies differing by about 1216 Da that was reduction insensitive. These modifications were present in only the one of the two SEC-HPLC peaks. These results suggest that this monoclonal antibody consists of microheterogeneous subspecies that exhibit different antigen binding properties associated with differences in post-translational modification of the heavy chain variable region. The choice of size exclusion column matrix and buffer composition was critical to the identification of these monoclonal IgG subspecies.