Electrophoretic characteristics of monoclonal immunoglobulin G of different subclasses

Monoclonal IgG are commonly observed in various B cell disorders, of which multiple myeloma is the most clinically relevant. In a series of serum samples, we identified by immunofixation 73 monoclonal IgG, including 63 IgG1, 4 IgG2, 5 IgG3, and 1 IgG4. The light chains were of κ type in 45 cases, and of λ type in 28 cases. These monoclonal IgG were further characterized by high resolution two-dimensional polyacrylamide gel electrophoresis (2-DE) in various isoelectric focusing conditions, as well as by 3-DE (2-DE of the proteins extracted from agarose after serum protein agarose electrophoresis). After 2-DE, 38 out of 73 monoclonal γ chains (52%) were visualized using immobilized pH 3–10 gradients for isoelectric focusing. In 6 cases (8%), γ chains were only detected using alkaline immobilized pH 6–11 gradients. In 3 cases (4%), 3-DE revealed monoclonal γ chains hidden by polyclonal γ chains. Finally, in 26 cases (36%), no monoclonal γ chains were clearly visualized. Sixty-one monoclonal light chains (84%) were detected using immobilized pH 3–10 gradients, whereas 12 (16%) were not. Monoclonal γ chains and light chains were highly heterogeneous in terms of pI and Mr. However, a statistically significant correlation (P<0.05) was observed between the position of the monoclonal IgG in agarose gel and the pI of their heavy and light chains (R=0.733, multiple linear regression). Because of the extreme diversity of their heavy and light chains, it appears that a classification of monoclonal IgG based only on their electrophoretic properties is not possible.