Identification of a Pre-BCR Lacking Surrogate Light Chain

SLP-65^−/− pre-B cells show a high proliferation rate in vitro. We have shown previously that λ5 expression and consequently a conventional pre-B cell receptor (pre-BCR) are essential for this proliferation. Here, we show that pre-B cells express a novel receptor complex that contains a μ heavy chain (μHC) but lacks any surrogate (SL) or conventional light chain (LC). This SL-deficient pre-BCR (SL⁻pre-BCR) requires Ig-α for expression on the cell surface. Anti-μ treatment of pre-B cells expressing the SL⁻pre-BCR induces tyrosine phosphorylation of substrate proteins and a strong calcium (Ca²⁺) release. Further, the expression of the SL⁻pre-BCR is associated with a high differentiation rate toward κLC-positive cells. Given that B cell development is only partially blocked and allelic exclusion is unaffected in SL-deficient mice, we propose that the SL⁻pre-BCR is involved in these processes and therefore shares important functions with the conventional pre-BCR.