CELL-DIVISION NUMBER REGULATES IGG1 AND IGE SWITCHING OF B-CELLS FOLLOWING STIMULATION BY CD40 LIGAND AND IL-4

CD40 ligand (CD40L) and IL-4 are sufficient to induce resting murine B cells to divide and switch isotypes from IgM and IgD to IgG1 and IgE. Tracking of cell division following (5-and 6) carboxyfluorescein diac etate succinimidyl ester (CFSE) labeling revealed that B cells express ed IgG1 after three cell divisions, and IgE after five. The probabilit y of isotype switching at each division was independent of both time a fter stimulation and of the dose of CD40L. IL-4 concentration regulate d the number of divisions that preceded isotype switching. Loss of sur face IgM and IgD was also related to cell division and appeared to be differentially regulated. B cell proliferation was typically asynchron ous with the proportion of cells in consecutive divisions being marked ly affected by the concentration of CD40L and IL-4. Simultaneous (5-br omo)-2' -deoxyuridine labeling and CFSE staining revealed that B cells in each division cycle were dividing at the same rate. Therefore, div ision cycle asynchrony resulted from dose-dependent variation in the t ime taken to enter the first division cycle. These results suggest tha t T-dependent B cell expansion is linked to predictable functional cha nges that may, in part, explain why IgE is produced in response to pro longed antigenic stimulation.