Surrogate light chain-mediated interaction of a soluble pre-B cell receptor with adherent cell lines

Signals initiated by the precursor B cell receptor (pre-BCR) are critical f or B cell progenitors to mature into precursor B cells. The pre-BCR consist s of a homodimer of muH chains, the covalently associated surrogate L (SL) chain composed of VpreB and lambda5, and the transmembrane signal molecules Ig alpha and Ig beta. One way to explain how maturation signals are initia ted in late progenitor B cells is that the pre-BCR is transported to the ce ll surface and interacts from there with a ligand on stroma cells. To addre ss this hypothesis, we first produced soluble Fab-like pre-BCR and BCR frag ments, as well as SL chain, in baculovirus-infected insect cells. Flow cyto metry revealed that, in contrast to Fab-like BCR fragments, the soluble pre -BCR binds to the surface of stroma and several other adherent cell lines, but not to B and T lymphoid suspension cells. The specific binding of the s oluble pre-BCR to stroma cells is saturable, sensitive to trypsin digestion , and not dependent on bivalent cations. The binding of pre-BCR seems to be independent of the H chain of IgM (muH chain), because SL chain alone was able to interact with stroma cells. Finally, soluble pre-BCR specifically p recipitated a 135-kDa protein from ST2 cells. These findings not only demon strate for the first time the capacity of a pre-BCR to specifically bind to a structure on the surface of adherent cells, but also suggest that the pr e-BCR interacts via its SL chain with a putative ligand on stroma cells.