B-chronic lymphocytic leukemias can undergo isotype switching in vivo and can be induced to differentiate and switch in vitro.

B-chronic lymphocytic leukemias (B-CLL) represent expanded clones of resting B lymphocytes that mostly express surface IgM (sIgM). The present study shows that B-CLL cells, freshly isolated from two patients, were sIgM+, sIgG-, and sIgA- but expressed IgG and IgA transcripts. cDNA cloning and sequencing showed that the VDJ segments associated to gamma and alpha heavy chain transcripts are identical to those from mu transcripts, thus showing that B lymphocytes giving rise to CLL cells have undergone isotype switching in vivo. Stimulation of these B-CLL cells through surface CD40 in the presence of interleukin-10 induced them to secrete IgG and IgA, proving that they can also differentiate into Ig-secreting cells. Finally, CD40-stimulated B-CLL cells were induced to switch towards IgE in response to interleukin-4, as shown by the presence of specific VDJ-C epsilon transcripts and the secretion of IgE. Therefore, B-CLL cells tested herein can undergo isotype switching in vivo and can be induced to undergo further isotype switching and differentiation in vitro.