Structural basis for galectin-1-dependent pre-B cell receptor (pre-BCR) activation.

During B cell differentiation in the bone marrow, the expression and activation of the pre-B cell receptor (pre-BCR) constitute crucial checkpoints for B cell development. Both constitutive and ligand-dependent pre-BCR activation modes have been described. The pre-BCR constitutes an immunoglobulin heavy chain (Igµ) and a surrogate light chain composed of the invariant λ5 and VpreB proteins. We previously showed that galectin-1 (GAL1), produced by bone marrow stromal cells, is a pre-BCR ligand that induces receptor clustering, leading to efficient pre-BII cell proliferation and differentiation. GAL1 interacts with the pre-BCR via the unique region of λ5 (λ5-UR). Here, we investigated the solution structure of a minimal λ5-UR motif that interacts with GAL1. This motif adopts a stable helical conformation that docks onto a GAL1 hydrophobic surface adjacent to its carbohydrate binding site. We identified key hydrophobic residues from the λ5-UR as crucial for the interaction with GAL1 and for pre-BCR clustering. These residues involved in GAL1-induced pre-BCR activation are different from those essential for autonomous receptor activation. Overall, our results indicate that constitutive and ligand-induced pre-BCR activation could occur in a complementary manner.

All oligosaccharide moieties of the µ chains in the pre-BCR are of the high-mannose type.

Although it is well established that pre-BCR signaling governs proliferation and differentiation during B cell development, the components of the pre-BCR that are important for signaling are a matter of controversy. It has been suggested that signaling by the µ heavy chains of the pre-BCR induces survival and differentiation of pre-B cells, while the λ5 part of the pre-BCR is essential for proliferation and clonal expansion. However, the mechanism by which pre-BCR µ chains initiate differentiation signals is not clear. Using two variants of a murine B-lymphocyte cell line that differ only in surface expression of either BCR or pre-BCR, we demonstrated that surface µ chains in the pre-BCR are of the high-mannose type only, while those in the BCR are of the complex type. It is hypothesized that mannose-specific lectin-like molecules on accessory cells or in solution may function as the non-antigen ligand that triggers the pre-BCR.