Abstract:
Mouse immunoglobulin (Ig) molecules have previously been shown to bind to the surface of CD5⁺ B cells
from patients with B-cell chronic lymphocytic leukemia (B-CLL). The results indicated that surface IgM
was involved in the interaction and suggested the phenomenon was an example of the polyreactive binding
capacity of the surface Ig (sIg) expressed by these malignant cells. This article describes the further
characterization of the interaction between human IgM and mouse Ig molecules and subunits. Mouse Ig
molecules of both kappa and lambda light chain classes bound to the B-CLL cell surface. The dissociation
constant for the interaction of mouse IgGI (KI21) with the B-CLL cell surface was 3.6 x 10-⁷ M. To confirm
the involvement of the human IgM expressed by the B-CLL cells in the interaction, the malignant cells were
stimulated in vitro to induce secretion of human IgM. Enzyme immunoassay was used to show that secreted
human IgM bound to intact mouse Ig, as occurred with the cell surface analysis. The mouse Ig epitope
recognized by the purified secreted human IgM was shown by Western blot analysis to be located on the
light chain of the mouse Ig molecule and to be conformationally dependent. KI21light chain was cloned and
expressed in E. coli and the recombinant light chain bound to the surface of CLL B cells. The results confirm
that human IgM is the reactive ligand in the interaction with mouse Ig and indicate that the interaction of
polyreactive IgM with mouse IgG occurs via the light chain component of IgG.
