Production of sterile transcripts of Cγ genes in an IgM-producing human neoplastic B cell line that switches to IgG-producing cells

A human neoplastic B cell line SSK41 that expresses IgM on its surface switches spontaneously to IgG-producing cells. The SSK41 line contains a single immunoglobulin heavy-chain locus, the constant region (C) genes of which retain the germline configuration. The IgG-producing SSK41 line was purified by sorting, and shown to have undergone S-S recombination with deletion of the C, gene. This line produced secretory and membrane-bound forms of γ-chain mRNA. From cDNA libraries of a mixed popuiation of IgM+/lgG+ SSK41 cells, we have isolated cDNA clones encoding the mature membrane-bound and secretory forms of the μ and γ1, heavy chains, all of which share the same variable region sequence. cDNA clones containing the mature γ3, chain were identified as well. We also isolated cDNA clones containing Cγ1, and Cγ3, sterile transcripts from the SSK41 line. These sterlle transcripts contained additional exon sequences designated 'I' which were locallzed upstream of the Cγ1 and Cγ3, switch regions and homologous to murine counterparts. The I sequences were precisely spliced to the 5' ends of the corresponding Cγ exon sequences. These features of gennllne CH transcripts, i.e. the isotype specificity to class switching, location of exons, and sequences per se, are highly conserved between man and mouse.