The unique and immunoglobulin-like regions of surrogate light chain component lambda5 differentially interrogate immunoglobulin heavy-chain structure.

PreBCR signaling is critical for B cell development and normally depends on the association of a nascent, component Ig H chain with the surrogate L chain (SLC), which helps ensure that only B cells that synthesize structurally sound antibody can develop. How the invariant and lambda-like SLC vets billions of unique V(H) domains for compatibility with polymorphic kappa and lambda L chains is unclear, because the SLC is composed of not only the Ig domains of VpreB and lambda5, but also the unique regions (URs) that reside at what would be the L chain CDR3. We evaluated the contribution of the Ig and UR domains of lambda5 to H chain screening by evaluating the preBCR-forming capability of lambda5 mutants with a diverse panel of H chains. Using transformed mouse B cells, we demonstrate that the Ig domain of lambda5 was sufficient and its UR dispensable for the rejection of V(H)Q52 and V(H)10 SLC-incompatible H chains. In contrast, the lambda5 UR was necessary to discriminate between SLC-incompatible and -compatible V(H)81X H chains. Substituting the Ig domains of lambda5 with equivalent kappa sequences impaired the SLC's ability to escort all H chains to the surface. Two SLC-incompatible H chains were able to form surface BCRs with two kappa L chains, indicating that the SLC's ability to predict the L chain compatibility of a H chain is not absolute. In sum, lambda5 differentially relies on the lambda-like Ig and UR to probe H chain structure to best accommodate diversity among H chains.

Multiple levels of selection responsive to immunoglobulin light chain and heavy chain structures impede the development of Dmu-expressing B cells.

The truncated/V(H)-less mouse H chain Dmu forms precursor B cell receptors with the surrogate L chain complex that promotes allelic exclusion but not other aspects of pre-B cell development, causing most progenitor B cells expressing this H chain to be eliminated at the pre-B cell checkpoint. However, there is evidence that Dmu-lambda1 complexes can be made and are positively selected during fetal life but cannot sustain adult B lymphopoiesis. How surrogate and conventional L chains interpret Dmu's unusual structure and how that affects signaling outcome are unclear. Using nonlymphoid and primary mouse B cells, we show that secretion-competent lambda1 L chains could associate with both full-length H chains and Dmu, whereas secretion-incompetent lambda1 L chains could only do so with full-length H chains. In contrast, Dmu could not form receptors with a panel of kappa L chains irrespective of their secretion properties. This was due to an incompatibility of Dmu with the kappa-joining and constant regions. Finally, the Dmu-lambda1 receptor was less active than the full-length mouse mu-lambda1 receptor in promoting growth under conditions of limiting IL-7. Thus, multiple receptor-dependent mechanisms operating at all stages of B cell development limit the contribution of B cells with Dmu H chain alleles to the repertoire.

The unique region of surrogate light chain component lambda5 is a heavy chain-specific regulator of precursor B cell receptor signaling.

Signals transduced by precursor-BCRs (pre-BCRs) composed of Ig mu heavy chains (HCs) and the surrogate L chain components lambda5 and VpreB are critical for B cell development. A conserved unique region (UR) of lambda5 was shown to activate pre-BCR complexes in transformed cells and to engage putative ligands, but its contribution to pre-B cell development is not known. It is also not clear why the lambda-like sequences in lambda5 are used to select HCs that will associate mainly with kappa L chains. In this study, we show that, in transformed and primary mouse B cell progenitors, receptors containing full-length HCs and lacking the lambda5UR were expressed at higher surface levels, but exhibited reduced activity compared with normal pre-BCRs in supporting developmental changes that accompany the progenitor to pre-B cell transition in primary cell culture systems and in the bone marrow in vivo. In contrast, deletion of the lambda5UR did not change net signaling output by the Dmu-pre-BCR, a developmentally defective receptor that exhibited impaired activity in the primary cell culture system. Moreover, the lambda-like sequences in lambda5 were more accommodating than kappa in supporting surface expression and signaling by the different HCs. These results show that the lambda5UR is important, although not essential, for surrogate L chain-dependent receptor signaling in primary cells, and furthermore may help allow discrimination of signaling competency between normal and Dmu-pre-BCRs. That the lambda-like portion of lambda5 in the absence of the UR was nondiscriminatory suggests that the lambda5UR focuses pre-BCR-dependent selection on the HC V region.