Human Fc receptor-like 5 distinguishes IgG2 disulfide isoforms and deamidated charge variants

Human Fc receptor-like 5 (FCRL5) is a novel IgG receptor. We reported that IgG2 samples display a thousand-fold range affinity for FCRL5, indicating that attributes beyond the isotype affect binding. We hypothesized that the complex interaction could be exploited to identify distinct changes in the IgG2 molecule. We investigated using surface plasmon resonance two factors that might affect the interaction between IgG2 and FCRL5; heterogeneity related to disulfide isoforms and charge variants. We found that panitumumab and denosumab samples enriched for the more flexible A disulfide isoform bound FCRL5 with two-fold and 82-fold higher apparent affinity, respectively, than the B isoform. We next assessed whether FCRL5 binding can distinguish panitumumab charge variants which increase during storage, using two approaches. First, samples were stored at 40°C to promote acidic variants. Heat stressed panitumumab had up to four-fold higher apparent affinity for FCRL5. Next, we used conditions that promoted deamidation, a common cause of acidic variants. We found that deamidated panitumumab had up to 14-fold higher apparent affinity for FCRL5, indicating that deamidation promotes the interaction. Statistical analyses of kinetic parameters and similarity scores obtained from sensogram comparisons indicated that IgG2 disulfide isoforms, heat stressed and deamidated samples each bind FCRL5 differently. We conclude that based on FCRL5 binding, we can discern distinct changes in the IgG2 molecule, including the disulfide isoform structure and charge variants related to deamidation. Since both IgG2 deamidation and conversion of disulfide isoforms occur in vivo, these findings elucidate the biological FCRL5 ligand.