Class switch recombination is IgG1 specific on active and inactive IgH loci of IgG1-secreting B-cell blasts.

Mouse B lymphocytes can be activated polyclonally by bacterial lipopolysaccharide (LPS) to secrete Ig and perform Ig class switch. In the presence of the T-cell lymphokine B-cell differentiation factor, the frequency of IgG1-secreting cells is drastically enhanced. We show here that IgG1-secreting B cells isolated from such cultures have undergone a similar DNA rearrangement of the switch regions (S mu, S gamma 1) of the Ig heavy chain constant region genes C mu and C gamma 1 on both active and inactive IgH loci. This result argues against a stochastic model of class switch recombination and suggests programmed class-specific switch recombination in the case of the switch to IgG1. In accord with this notion, cells expressing IgM but not IgG on the surface have not deleted or rearranged C mu or S gamma 1 on either chromosome.