The control of membrane and secreted heavy chain biosynthesis varies in different immunoglobulin isotypes produced by a monoclonal B cell lymphoma

The control of production of the membrane (m) vs secreted (s) forms of immunoglobulin heavy chains was investigated in a panel of cell lines expressing different heavy chain classes but identical light chains (λ) and variable regions. These cell lines could be induced towards Ig secretion by mitogen treatment. During this process a shift from m to s heavy chain production takes place. Here we show that, similarly to IgA- and IgE-producing B cells, in IgG_2a-producing I.29 cells the γ_m-γ_s shift was accompanied by a shift in the corresponding mRNAs, with a decrease of γ_m mRNA and an increase of the γ_s mRNA in LPS-stimulated cells. By contrast, the μ_m mRNA was increased in LPS-stimulated IgM-producing cells, albeit these cells synthesized reduced amounts of μ_m polypeptides. The utilization of the translational level in the early steps of B lymphocyte maturation thus distinguishes the mode of regulation of μ chains from those of the other isotypes. In addition, in B cells a post-translational event blocks the secretion of IgM but not of IgG or IgE.