Expression of 2 immunoglobulin isotypes, IgM and IgA, with identical idiotype in the B cell lymphoma 1.29

The 1.29 tumor originated in I strain mice as a B cell lymphoma. In its ascitic form, the cells express Ia 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 that 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 subpopulations are consistently observed. The proportion of cells binding fluorescein-labeled anti-μ and rhodamine-labeled anti-α simultaneously in the cytoplasm is slightly higher (+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 IgM ^-IgA ⁺ phenotype. Implantation of 1 such cell line in nude mice produced a tumor that retained this phenotype in successive passages.