Differential binding of histidine-rich glycoprotein (HRG) to human IgG subclasses and IgG molecules containing κ and λ light chains

In previous studies we showed that the plasma protein histidine-rich glycoprotein (HRG) binds strongly to pooled human IgG. In the present work myeloma proteins consisting of different human IgG subclasses were examined for their ability to interact with human HRG. Using an IAsys optical biosensor we found initially that IgG subclasses differ substantially in their affinity of interaction with HRG. However, the most striking finding was the observation that the kinetics of the HRG interaction was dramatically affected by whether the IgG subclasses contained the κ or λ light (L)- chains. Thus, the on-rate for the binding of HRG to the κ L-chain containing IgG1 and IgG2 (IgG1κ and IgG2κ) was ~4- and ~10-fold faster than that for the binding of HRG to λ L-chain containing IgG1 and IgG2 (IgG1λ and IgG2λ), respectively, with the dissociation constants (K(d)) in the range 3- 5 nM and 112-189 nM for the κ and λ isoforms, respectively. In contrast, the on-rate for the binding of HRG to IgG3κ and IgG4κ was found to be 9- and 20-fold slower than that for the binding of HRG to IgG3λ and IgG4λ, respectively, with the K(d) in the range 147-268 nM and 96-109 nM for the κ and λ isoforms, respectively. The binding of HRG to immunoglobulins containing the λ L-chain (particularly IgG1κ) was generally potentiated in the presence of a physiological concentration (20 μM) of Zn²⁺ (K(d) decreased to 0.60 ± 0.01 for IgG1κ), but Zn²⁺ had no effect or slightly inhibited the binding of HRG to immobilized IgG subclasses possessing the λ L-chain. Interestingly, HRG also bound differentially to Bence Jones (BJ) proteins containing κ and λ L-chains, with HRG having a 14-fold lower K(d) for BJK than for BJλ when 20 μM Zn²⁺ was present. HRG also bound to IgM (IgMκ), but the affinity of this interaction (K(d) ~1.99 ± 0.05 μM) was markedly lower than the interaction with IgG, and the affinity was actually decreased 4-fold in the presence of Zn²⁺. The results demonstrate that both the heavy (H)- and L-chain type have a profound effect on the binding of HRG to different IgG subclasses and provide the first evidence of a functional difference between the κ and λ L-chains of immunoglobulins.