Limited life span of extensively proliferating B cells: no evidence for a continuous class or subclass switch.

The polyclonal B cell activators (PBA) dextran sulphate (DxS) and lipopolysaccharide (LPS) induce a marked synergistic response in mouse splenic B cells. Such activated B cell blasts could be maintained in culture for a total period of 15 to 20 days, provided the cells were subcultured repeatedly in fresh medium containing the PBA. After this time period activation could no longer be achieved. The B cell blasts responded at all times better to LPS plus DxS than to either PBA added alone. Furthermore, LPS-induced B cell blasts also responded better to LPS plus DxS than to LPS alone. Activated cells secreted IgM and switched to IgG3 and IgG2b secretion. At the end of the culture period, approximately one cell out of four secreted IgG. The kinetics of IgG3 and IgG2b production were similar, and the fraction of cells secreting these subclasses increased at similar rates throughout the culture period. We conclude that high-rate secreting and proliferating B cells have a limited life span. Furthermore, most activated B cells do not seem to go through a continuous subclass switch but keep on secreting the subclass they originally switched to.