Surrogate light chain (vLC) genes are transcriptionally active in progenitor B (pro-B) cells before immunoglobulin genes are rearranged. Current hypothetical models suggest that the vLC proteins may couple with surrogate or conventional heavy chain proteins to form cell surface receptors that signal the progressive differentiation of pro-B, precursor B (pre-B), and immature Bcells. Monoclonal antibodies were produced and used to examine the synthesis, expression, intermolecular interaction, and function of vLC during B cell differentiation. The results indicate that, while vLC production spans several developmental stages, cell surface expression isconfined to a relatively late stage in normal pre-B cell differentiation, during which receptor cross-linkage does not impede cell growth or B cell differentiation. introduction

The progressive differentiation of hematopoietic progenitor cells along the B cell pathway is marked by rearrangement of variable (V), diversity (D), and joining (J) gene elements before expression of immunoglobulin heavy chain (HC) and light chain (LC) genes (Tonegawa, 1983). The usual order of this rearrangement process is DJH, then V-DJH, to allow c~ HC expression (Alt et al., 1992). VJL rearrangement then occurs in the K or X loci to allow LC expression. The progenitor B (pro-B) cells thus lack both HC and LC proteins, while daughter precursor B (pre-B) cells produce pHCs (Cooper and Burrows, 1990) that are largely retained in the endoplasmic reticulum by a protein called BiP (Haas and Wabl, 1983). Following LC gene rearrangement and expression, the pre-B cells are converted into B cells as their KLC or 1LC displaces BiP to allow cell surface immunoglobulin M (IgM) expression (Hendershot et al., 1987). The IgM receptors on newly formed B cells consist of covalently bonded pairs of KLC or ILC and bHC, which in turn are noncovalently associated with two transmembrane proteins called a and p to form a complex signal transduction unit (Reth, 1992).