Complexes of µ heavy chain, surrogate light chains, and the signal-transducing proteins Igα and Igβ form the preantigen receptor of the B lineage, known as the pre-BCR. The µ heavy chain associates with the λ5 and Vpre–B proteins, also called surrogate light chains because they are structurally homologous to conventional (κ or λ) light chains but are invariant (i.e., they are identical in all pre–B cells) and are synthesized only in pro–B and pre–B cells (Fig. 1A). This receptor associates with the signaling molecules Igα and Igβ (also known as CD79a and CD79b) to form the pre–B-cell receptor complex, similar to the BCR complex in mature B cells. The developing B-lineage cells that make proper in-frame rearrangements at the Ig heavy-chain locus express the pre-BCR and receive pre-BCR signals, which allow the cells to survive and be now identified as pre–B cells. Signals from the pre-BCR are also responsible for the largest proliferative expansion of B-lineage cells during B-cell development. During this proliferation, synthesis of RAG proteins is transiently shut off, so Ig gene rearrangement is temporarily halted. It is not known if the pre-BCR recognizes any ligand; the consensus view is that this receptor functions in a ligand-independent manner and that it is activated by the process of assembly. The importance of pre-BCRs is illustrated by studies of knockout mice and rare cases of human deficiencies of these receptors. For instance, in mice, engineered deletion of the gene encoding the µ chain or one of the surrogate light chains results in markedly reduced numbers of mature B cells because development is blocked at the pro–B stage.
Fig1. Pre–B-cell and pre–T-cell receptors (pre-BCR and pre-TCR). The pre–B-cell receptor (A) and the pre-TCR (B) are expressed during the pre–B-cell and pre–T-cell stages of maturation, respectively, and both receptors share similar structures and functions. The pre–B-cell receptor is composed of the µ heavy chain and an invariant surrogate light chain. The surrogate light chain is composed of two proteins: the V pre–B protein, which is homologous to a light-chain V domain, and a λ5 protein that is covalently attached to the µ heavy chain by a disulfide bond. The pre-TCR is composed of the TCR β chain and the invariant pre-T α (pTα) chain. The pre–B-cell receptor is associated with the immunoglobulin α(Igα) and Igβ signaling molecules that are also part of the BCR complex in mature B cells (see Chapter 9), and the pre-TCR associates with the CD3 and ζ proteins that are also part of the TCR complex in mature T cells.
The expression of the pre-BCR is the first checkpoint in B-cell maturation. Numerous signaling molecules linked to the pre BCR (and to the BCR in mature B cells) are required for cells to successfully negotiate the pre–BCR-mediated checkpoint at the pro–B to pre–B-cell transition. A kinase called Bruton's tyrosine kinase (BTK) is activated downstream of the pre-BCR and is required for delivery of signals from this receptor that mediate survival, proliferation, and maturation at and beyond the pre B-cell stage. In humans, mutations in the BTK gene result in the disease called X-linked agammaglobulinemia (XLA), which is characterized by a failure of B-cell maturation. In a mouse strain called Xid (for X-linked immunodeficiency), mutations in btk result in a less severe B-cell defect because murine pre–B cells express a second Btk-like kinase called TEC that partially compensates for the defective Btk. Other molecules upstream and downstream of the BTK signaling pathway that are required at this checkpoint include the µ heavy-chain gene, the λ5 gene, Igα, Igβ, SYK, the BLNK/SLP65 signaling adaptor, and the p85 subunit of PI3-kinase. Mutations of these genes are the causes of rare cases of autosomal recessive agammaglobulinemia.
The pre-BCR regulates further rearrangement of Ig genes in two ways. First, if a µ protein is produced from the recombined heavy-chain locus on one chromosome and forms a pre-BCR, this receptor signals to irreversibly inhibit rearrangement of the Ig heavy-chain locus on the other chromosome. If the first rearrangement is nonproductive, the heavy-chain allele on the other chromosome can complete VDJ rearrangement at the IgH locus. Thus, in any B-cell clone, one heavy-chain allele is productively rearranged and expressed and the other is either retained in the germline configuration or nonproductively rearranged. As a result, an individual B cell can express an Ig heavy-chain protein encoded by only one of the two inherited alleles. This phenomenon is called allelic exclusion, and it ensures that every B cell will express a single antigen receptor, thus maintaining clonal specificity. Ig heavy chain allelic exclusion involves changes in chromatin structure in the heavy-chain locus that limit accessibility to the V(D)J recombinase. If both alleles undergo nonproductive IgH gene rearrangements, the developing cell cannot produce Ig heavy chains, cannot generate a pre-BCR–dependent survival signal, and undergoes programmed cell death.
The second way in which the pre-BCR regulates the production of the antigen receptor is by stimulating κ light-chain gene rearrangement. Pre–B cells proliferate first as large pre–B cells, and then shut off surrogate light-chain gene expression and become nondividing small pre–B cells that express the µ heavy chain intracellularly. These nondividing cells synthesize RAG proteins and are thus able to rearrange their κ light-chain genes. Pre-BCR signals con tribute to making the κ light-chain locus available to the enzymes that mediate V(D)J recombination. If an in-frame rearrangement occurs at the κ locus, the cell will produce a κ light-chain protein, which associates with the previously synthesized µ chain to produce a complete IgM protein. If the κ light chain contributes to a self-reactive BCR or if the κ locus is not productively rearranged, the cell can rearrange the λ locus and again produce a complete IgM molecule.
