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 (gamma 1 leads to gamma 2b leads to gamma 2a,gamma 1) of X63 (IgG1, chi) by fluorescence-activated cell sorting using class-specific antisera expresses two heavy chains. X63.2aRI-16 secretes IgG2a,chi as does the parental cell line X63.2a-25, a hybrid molecule containing gamma 2a and gamma 1 heavy chains and IgG1,chi. The chain of the latter protein is the product of a reverse class switch in regard to the embryonic order of the CH 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 VH domains and of the CH domains up to the first subclass-specific residues. The remaining CNBr fragments of the CH domains were characterized by amino acid analyses. It was found that both heavy chains of the double producer possess identical VH domains and CH domains characteristic for the subclasses gamma 2a and gamma 1, respectively. The identities of the two VDJ sequences expressed in X63.2aRI-16 cells suggest that the reverse class switch event to gamma 1 cannot simply be explained on the basis of a deletion model involving only a single CH locus.