ABSTRACT
Signals initiated by the precursor B cell receptor (pre-BCR) are critical for B cell progenitors to mature into precursor B cells. The pre-BCR consists of a homodimer of microH chains, the covalently associated surrogate L (SL) chain composed of VpreB and lambda5, and the transmembrane signal molecules Ig(alpha) and Igbeta. One way to explain how maturation signals are initiated in late progenitor B cells is that the pre-BCR is transported to the cell surface and interacts from there with a ligand on stroma cells. To address this hypothesis, we first produced soluble Fab-like pre-BCR and BCR fragments, as well as SL chain, in baculovirus-infected insect cells. Flow cytometry 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 soluble 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 (microH chain), because SL chain alone was able to interact with stroma cells. Finally, soluble pre-BCR specifically precipitated a 135-kDa protein from ST2 cells. These findings not only demonstrate 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 pre-BCR interacts via its SL chain with a putative ligand on stroma cells.
