Rearrangements and deletions of immunoglobulin heavy chain genes in the double-producing B cell lymphoma I.29.

The B cell lymphoma I.29 consists of a mixture of cells expressing membrane-bound immunoglobulin M (IgM) (lambda) and IgA (lambda) of identical idiotypes. Whereas most of the cells express either IgM or IgA alone, 1 to 5% of the cells in this tumor express IgM and IgA simultaneously within the cytoplasm and on the cell membrane (R. Sitia et al., J. Immunol. 127:1388-1394, 1981; R. Sitia, unpublished data). When IgM+ cells are purified from the lymphoma and passaged in mice or cultured, a portion of the cells convert to IgA+. These properties suggest that some cells of the I.29 lymphoma may undergo immunoglobulin heavy chain switching, although it is also possible that the mixed population was derived by a prior switching event in a clone of cells. We performed Southern blotting experiments on genomic DNAs isolated from populations of I.29 cells containing variable proportions of IgM+ and IgA+ cells and on a number of cell lines derived from the lymphoma. The results were consistent with the deletion model for heavy chain switching, as the IgM+ cells contained rearranged mu genes and alpha genes in the germ line configuration on both the expressed and nonexpressed heavy chain chromosomes, whereas the IgA+ cells had deleted both mu genes and contained one rearranged and one germ line alpha gene. In addition, segments of DNA located within the intervening sequence 5' to the mu gene, near the site of switch recombination, were deleted from both the expressed and the nonexpressed chromosomes. Although mu genes were deleted from both chromosomes in the IgA+ cells, the sites of DNA recombination differed on the two chromosomes. On the expressed chromosome, Smu sequences were recombined with S alpha sequences, whereas on the nonexpressed chromosome, Smu sequences were recombined with S gamma 3 sequences.     

Structure of germ line immunoglobulin alpha heavy-chain RNA and its location on polysomes.

We describe the structure of the major germ line RNA transcribed from unrearranged immunoglobulin alpha heavy-chain genes in immunoglobulin M-expressing cells of the I.29 mu B-cell lymphoma, a cell line capable of switching to immunoglobulin A expression upon lipopolysaccharide treatment. This germ line alpha RNA has a small open reading frame that does not include the C alpha domain, and this RNA appears to be present on polysomes in I.29 mu cells.     

Mutations, duplication, and deletion of recombined switch regions suggest a role for DNA replication in the immunoglobulin heavy-chain switch.

The heavy-chain switch from immunoglobulin M (IgM) expression to IgA expression is mediated by a recombination event between segments of DNA called switch regions. The switch regions lie two to six kilobases upstream of the mu and alpha constant region coding segments. Switch recombination to IgA expression results in a recombinant mu-alpha switch region upstream of the expressed alpha constant region gene. We have characterized the products of switch recombination by a lymphoma cell line, I.29. Two sets of molecular clones represent the expected products of simple mu to alpha switches. Five members of a third set of molecular clones share the same recombination site in both the mu and the alpha switch regions, implying that the five molecular clones were derived from a single switch recombination event. Surprisingly, the five clones fall into two sets of sequences, which differ from each other by several point mutations and small deletions. Duplication of switch region sequences are also found in these five molecular clones. An explanation for these data is that switch recombination involves DNA synthesis, which results in nucleotide substitutions, small deletions, and duplications.     

The roles of IL-4, TGF-beta and LPS in IgA switching.

CH12.LX B cells have been used as a lymphoma model of MHC restricted, antigen-dependent B cell differentiation. These B cells express surface IgM and secrete IgM. Most recently we have demonstrated that CH12.LX is a model of cytokine driven IgA differentiation. Recently, transforming growth factor beta (TGF-beta) has been shown to be a probable switch factor for IgA in LPS-stimulated mouse lymphocytes, therefore we chose CH12.LX B cells to study the effect of IL-4, TGF-beta and LPS in IgA isotype switching. Adding TGF-beta to the monoclonal cell line CH12.LX results in induction of mIgA expression but no enhancement of IgA secretion similar to the effect of IL-4. The addition of LPS serves as a non-specific stimulus to enhance the secretion of the expressed immunoglobulin, but has no IgA specific activity of its own. IL-4 and TGF-beta together are synergistic for mIgA expression. Pretreatment studies show that TGF-beta added after IL-4 is the same as TGF-beta alone whereas the converse is the same as adding both cytokines together. TGF-beta acts by increasing the steady state levels of alpha message, whereas northern analysis indicates that IL-4 does not induce alpha message the way TGF-beta does. These data confirm that TGF-beta by itself is an isotype switch factor for IgA. In addition, IL-4 and TGF-beta cause mIgA expression through different mechanisms. CH12.LX B cells serve as a valuable model to study the role of multiple signals required for mIgA expression and IgA secretion.