PRESENCE OF IMMUNOGLOBULIN (IG)-M AND IGG DOUBLE ISOTYPE BEARING CELLS AND DEFECT OF SWITCH RECOMBINATION IN HYPER IGM IMMUNODEFICIENCY

We established a transformed B cell line expressing both IgM and IgG on the cell surface from a patient with hyper IgM immunodeficiency using Epstein-Barr viruses. DNA and RNA of the cells were analyzed. DNA rearrangements of Ig J(H) gene loci were observed on both chromosomes. Cloning and DNA sequence analyses showed that one has a V(H)D(H)J(H) structure while the other has a D(H)J(H) structure. Southern hybridization with 5'-S(μ) and S(γ) region-containing probes indicated germline configuration in the switch regions of μ and γ genes on both chromosomes. To test expression of μ and γ chains in the transformed cells at the mRNA-level, we used the polymerase chain reaction with three kinds of synthetic oligonucleotides as primers, one of which was part of the V(H) gene, while the other two were complementary to parts of C(μ) and C(γ) genes. Sequence analysis of the amplified products showed that the same V(H)D(H)J(H) sequence is directly connected with either the C(μ) or the C(γ) sequence in the mRNAs. This is direct evidence showing that in double isotype-bearing cells one V(H)D(H)J(H) exon in the transcript is alternatively spliced to C(μ) or C(γ) without DNA rearrangement. The defect in this disease could be at the S-S recombination stage.