Myeloma class switch variant X63.2aRI‐16 secretes IgG 2a and IgG 1 with identical VDJ sequence

Abstract The myeloma line X63.2aRI‐16 isolated in vitro as spontaneous tertiary class switch variant (γ 1 → γ 2b → γ 2a → γ 2a ,γ 1 ) of X63 (IgG 1 , 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 γ 1 heavy chains and IgG 1 , 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 γ 1 , respectively. The identities of the two VDJ sequences expressed in X63.2aRI‐16 cells suggest that the reverse class switch event to γ 1 cannot simply be explained on the basis of a deletion model involving only a single C H locus.