Myeloma class switch variant X63.2aRI-16 secretes IgG2a and IgG1 with identical VDJ sequence

The myeloma line X63.2aRI-16 isolated in vitro as spontaneous tertiary class switch variant (γ₁ → γ_2b → γ_2a → γ_2a,γ₁) of X63 (IgG₁,x) by fluorescence-activated cell sorting using class-specific antisera expresses two heavy chains. X63.2aRI-16 secretes IgG_2a,x as does the parental cell line X63.2a-25, a hybrid molecule containing γ_2a and γ₁ heavy chains and IgG₁,x. The chain of the latter protein is the product of a reverse class switch in regard to the embryonic order of the C_H genes. We purified the immunoglobulins of X63.2aRI-16, isolated the CNBr fragments of both heavy chains and determined the complete amino acid sequence of the V_H domains and of the C_H domains up to the first subclass-specific residues. The remaining CNBr fragments of the C_H domains were characterized by amino acid analyses. It was found that both heavy chains of the double producer possess identical V_H domains and C_H domains characteristic for the subclasses γ_2a and γ₁, respectively. The identities of the two VDJ sequences expressed in X63.2aRI-16 cells suggest that the reverse class switch event to γ₁ cannot simply be explained on the basis of a deletion model involving only a single C_H locus.