Immunoglobulin Double Isotype-Producing Hybridomas Isolated from an Autoimmune (NZB × NZW)F1 Mouse

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Abstract
In the sera of (NZB × NZW)F1 (B/W) mice that develop a lupus-like syndrome, increased levels of IgG antibodies (Ab) reacting with TNP have been detected before the appearance of IgG anti-DNA Ab and clinical symptoms. A single injection of trinitrophenyl-bovine serum albumin (TNP/BSA) in physiological saline into a young B/W mouse (3 months old), followed by fusion of its splenocytes 3 days later, gave rise to hybridomas simultaneously secreting IgM and IgG Ab with anti-TNP reactivity. Both μ and γ chains were detected in culture supernatants by ELISA, and double isotype-producing cells were labeled by immunofluorescence. Molecular analysis of two of these double isotype-producing hybridomas showed the presence of mRNA coding for both μ and γ chains of Ig, and this γ mRNA could be translated in vitro into a γ heavy (H) chain. Comparison of the H chain variable-region sequences of IgM and IgG revealed 100% homology between μ and γ VH genes in one clone, while μ and γ VH genes showed only 80% homology in the other clone. Both clones produced a single κ light (L) chain. These two hybridomas, isolated from a B/W mouse, thus represent two different mechanisms of double isotype expression: the first one corresponds to an IgM to IgG switch, while the second one reflects a lack of allelic exclusion.