A chemically induced leukemia of BDF1 mice, designated 70Z/3, was adapted to tissue culture in our laboratory. A minority of these cells displayed surface immunoglobulin (sIg) as detected by immunofluorescence with rhodamine-labeled class (IgM)-specific antibodies. Addition of the B cell mitogen, LPS, to the cultures induced sIg expression on all of these cells. The kinetics of this transition were dependent on the dose of LPS. As little as 0.1 µg/ml induced sIg on >97% of the cells within 36 hr. DxS also induced sIg whereas other mitogens (Con A, PPD) or inducing agents (DMSO, dimethyl formamide, butyric acid), tested over a wide range of concentrations, failed to induce sIg expression.
The cells bear H-2 antigens but lack IgD, IgG, IgA, Ia, and Thy 1.2. Exposure to LPS had no effect on the presence or absence of these structures. A small percentage of cells possessed receptors for complement and for the Fc portion of immunoglobulin. Cytoplasmic IgM was detectable within all of the cells, and constitutive production of small quantities of IgM was confirmed by SDS-polyacrylamide gel electrophoresis of serologic precipitates of cell lysates after pulsing with radioactive amino acids. Using similar methods, we failed to detect active secretion of immunoglobulin.
Thus, this cell line has properties similar to cytoplasmic Ig+, surface Ig- cells found in immature tissues and in bone marrow of mice and humans that are thought to be immediate precursors of sIg+ B lymphocytes. It may provide a model for studying the mechanism of LPS activation and the molecular events associated with externalization of cell surface receptors.