Expression of 2 immunoglobulin isotypes, IgM and IgA, with identical idiotype in the B cell lymphoma I.29.

Abstract

The 1.29 tumor originated in I strain mice as a B cell lymphoma. In its ascitic form, the cells express la and Lyb antigens and carry the Fc receptor. The tumor cells synthesize 2 immunoglobulins, IgM and IgA, which have λ-chains and heavy chain idiotypic determinants in common. Double labeling of viable cells in immunofluorescence demonstrates that a majority of cells have only IgM (µ+), a proportion has only IgA (α+), and a small population (usually less than 5%) expresses both immunoglobulin isotypes (µ+α+). Although the relative proportions of µ+, α+, and µ+α+ cells fluctuate quite markedly in the serial passages of the tumor, the 3 different sub-populations are consistently observed. The proportion of cells binding fluorescein-labeled anti-µ and rhodamine-labeled anti-α simultaneously in the cytoplasm is slightly higher (<10%). The appearance of cytoplasmic staining as brightly fluoresceinating granules suggests sequestration of immunoglobulin, possibly into vesicles. In vitro culture of 1.29 cells rapidly leads to preferential expression of IgA only. This conversion from IgM- to IgA-producing cells was also observed when cultures were initiated with highly purified IgM+IgA− 1.29 cells, suggesting differentiation rather than selective growth as the cause of the change. Cloning of 1.29 cells by the limiting dilution technique yielded only cell lines of lgM−lgA+ phenotype. Implantation of 1 such cell line in nude mice produced a tumor that retained this phenotype in successive passages.