Selection of membrane IgM- variants from a mIgM+ murine B lymphoma cell: problems and solutions

We have used antibody-mediated complement killing to isolate membrane IgM-negative (mIgM-) variants from the mIgM+ murine B cell lymphoma, WEHI 279.1. This procedure has been used previously to select variants which lack expression of other cell-surface antigens on lymphoid cells. In those experiments, multiple rounds of selection have often been required for selection of the negative variants. We found that many cycles of selection produced very few variants and that those isolated had reduced, but still measurable, levels of mIgM. We were able to select large numbers of stable mIgM- variants by subjecting the populations with reduced levels of mIgM to two rounds of immunoselection within one cell cycle. These variants are stable and exhibit a variety of defects which are all expressed as a failure to display IgM on their external surface. Analysis of these variant clones at the biochemical level will begin to define the requirements for proper display of mIgM on the cell membrane of B lymphoma cells.