Inducible Ig heavy chain switching in an IgM+ Ly-1 B cell line. Evidence for a state of switch commitment

We have analyzed the pattern of immunoglobulin (Ig) heavy chain isotype secretion in AJ9, a cloned, IgM+ murine B lymphocyte cell line. Upon induction by a variety of lymphokines and polyclonal B-cell activators, AJ9 cells express multiple subclasses of IgG and IgA in addition to IgM. In certain cases, mature isotype is restricted--e.g., IL-5 predominantly elicits production of IgG2 and IgA, a restriction also observed in short-term lymphocyte cultures. In other cases (e.g., anti-IgM plus 8-mercaptoguanosine, a polyclonal B-cell activator) production of mature isotypes is unrestricted. Under optimal conditions, only a low abundance of secreted Ig and low frequency of secreting cells (less than 0.5%) were detected. A serial cloning assay was devised to define the pattern of isotype switching in induced cells and their progeny. We expected to observe a progressive limitation of progeny to expression of single mature isotypes. Surprisingly, nearly all subclones of the induced cells were found to produce a range of mature isotypes. Sequential cloning in basal medium revealed that this induced phenotype persisted for more than a month (greater than 40 generations). Throughout this period, the abundance of mature isotype production remained low, and membrane Ig was exclusively of the IgM isotype. We interpret this induced response to reflect an intermediate state of B-cell differentiation, in which cells become committed to the switching process, but are not adequately stimulated to efficiently complete the process required for expression of mature isotypes. These findings are discussed in regard to the control of the switching process, and their possible relevance to the memory state of B cells.