The potential of B lymphocytes for isotype expression.

The isotypes secreted by the progeny of individual antigenically stimulated murine B cells were studied to determine the extent to which B cells exhibit class commitment. The study includes an analysis of B lymphocytes at different maturational states as well as an analysis of B cells selected on the basis of their surface isotype. All B cell subsets were analyzed under optimal stimulation conditions by the T-dependent splenic focus assay. From these studies it was found that the majority of B cells are not committed to a particular isotype that on antigenic stimulation would give rise to all progeny that secrete solely the one immunoglobulin class. The results indicate that the major influence attributable to the B cells themselves with regard to isotype commitment stems from the maturational state of the B cell. Thus, immature B cells from fetal liver and spleen give rise to clones secreting IgM and/or IgA, but not IgG; secondary B cells that lack surface mu appear unable to produce IgM-secreting clones on stimulation. However, most mature B cells (both primary and secondary) have the potential to give rise to clones secreting multiple immunoglobulin classes including IgE. Therefore, the predominant form of regulation regarding isotype expression is more likely to be controlled at the T cell level rather than separate lineages of B cells committed to a particular isotype prior to antigenic exposure.