A unique role for the λ5 nonimmunoglobulin tail in early B lymphocyte development

Precursor BCR (pre-BCR) signaling governs proliferation and differentiation of pre-B cells during B lymphocyte development. However, it is controversial as to which parts of the pre-BCR, which is composed of 1g mu H chain, surrogate L chain (SLC), and Ig alpha-Ig beta, are important for signal initiation. Here, we show in transgenic mice that the N-terminal non-Ig-like (unique) tail of the surrogate L chain component lambda 5 is critical for enhancing pre-BCR-induced proliferation signals. Pre-BCRs with a mutated lambda 5 unique tail are still transported to the cell surface, but they deliver only basal signals that trigger survival and differentiation of pre-B cells. Further, we demonstrate that the positively charged residues of the lambda 5 unique tail, which are required for pre-BCR self-oligomerization, can also mediate binding to stroma cell-associated self-Ags, such as heparan sulfate. These findings establish the lambda 5 unique tail as a pre-BCR-specific autoreactive signaling motif that could increase the size of the primary Ab repertoire by selectively expanding pre-B cells with functional Ig mu H chains.