Immunoglobulin messenger RNAs in murine cell lines that have characteristics of immature B lymphocytes

Abstract

The immunoglobulin-encoding messenger RNAs in a pre-B cell-like lymphoma line, 70Z/3, were characterized according to size, concentration in nuclear and cytoplasmic compartments and response to stimulation by the B cell mitogen, lipopolysaccharide. The μ heavy chain and κ light chain mRNAs produced by these cells were size-fractionated on methylmercuryhydroxide-agarose gels, transferred to diazotized cellulose paper and identified by hybridization with radioactive cloned probes containing C μ or C κ sequences. Unstimulated cells, which produce only intracellular μ chains, contain negligible amounts of κ mRNA and a few hundred molecules of μ mRNA per cell. The ratio of nuclear to cytoplasmic heavy chain mRNA sequences in these cells is about 50 times greater than in an immunoglobulin-secreting plasma cell. Stimulation of 70Z/ 3 cells by lipopolysaccharide, which causes the cells to express surface IgM, results in a greater than 10 fold increase in the concentration of κ mRNA, but does not markedly affect the level of μ mRNA. Four distinct μ mRNA components ranging in size from approximately 2.1 to 3.0 kb were observed in 70Z/3 cells as well as in a cloned derivative, 70Z/3–12. Two of these components, which were found in the membrane-bound polyribosomes, may code for the membrane and secretory forms of the μ heavy chain. The other two mRNAs, which were confined to the cytoplasmic fraction containing free polyribosomes and nonribosome-associated mRNP, may code for intracellular μ chains. The two membrane-bound μ mRNAs were observed in another immature B cell line, WEHI 231. In contrast, mature IgM-secreting plasma cells produce a single μ mRNA component of uniform size. The multiple species of μ mRNA may arise from different arrangements of the C μ genetic elements on allelic chromosomes and/or from alternative processing of C μ-containing transcripts.