Positive selection of precursor (pre-) B cells by Ig membrane μ H chains (μm HC) and counterselection mediated by the truncated HC Dμ depend on the ability of each HC to form a pre-B cell receptor (pre-BCR) signaling complex with the surrogate L chain (SLC) components λ5 and Vpre-B. To better understand how pre-BCR signaling output is determined by its Ig components and the SLC, we investigated the regulation of pre-BCR surface expression and HC secretory maturation in a new nonlymphoid system. We took this approach as a means to distinguish B-lineage-specific effects from pre-BCR-intrinsic properties that may influence these aspects of pre-BCR homeostasis necessary for signaling. As in pre-B cells, the SLC in nonlymphoid cells supported only a limited degree of μm HC maturation and low pre-BCR surface expression levels compared with conventional LCs, indicating that this was due to an intrinsic property of the SLC. We identified the non-Ig region of λ5 as harboring the restrictive activity responsible for this phenotype. This property of λ5 was also evident with Dμ, but the overall SLC- and L chain-dependent requirements for Dμ maturation and surface expression were markedly different from those for μm. Surprisingly, Dμ was modified in an unusual manner that was only dependent on Vpre-B. These results establish a novel function of λ5 in limiting surface pre-BCR levels and reveal biochemical properties of Ig molecules that may underlie the diverse consequences of pre-BCR signaling in vivo by different HCs.

Signals transduced by precursor-BCRs (pre-BCRs) composed of Ig μ heavy chains (HCs) and the surrogate L chain components λ5 and VpreB are critical for B cell development. A conserved unique region (UR) of λ5 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 λ-like sequences in λ5 are used to select HCs that will associate mainly with κ L chains. In this study, we show that, in transformed and primary mouse B cell progenitors, receptors containing full-length HCs and lacking the λ5UR 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 λ5UR did not change net signaling output by the Dμ-pre-BCR, a developmentally defective receptor that exhibited impaired activity in the primary cell culture system. Moreover, the λ-like sequences in λ5 were more accommodating than κ in supporting surface expression and signaling by the different HCs. These results show that the λ5UR 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 Dμ-pre-BCRs. That the λ-like portion of λ5 in the absence of the UR was nondiscriminatory suggests that the λ5UR focuses pre-BCR-dependent selection on the HC V region.