The diversity of the B-cell antigen-receptor repertoire is enhanced at this stage by the enzyme terminal deoxynucleotidyl transferase (TdT). TdT is expressed by the pro-B cell and adds nontemplated nucleotides (N-nucleotides) at the joints between rearranged gene segments (see Section 5-8). In adult humans, it is expressed in pro-B cells during heavy-chain gene rearrangement, but its expression declines at the pre-B-cell stage during light-chain gene rearrangement. This explains why N-nucleotides are found in the V–D and D–J joints of nearly all heavy-chain genes but only in about a quarter of human light-chain joints. N-nucleotides are rarely found in mouse light-chain V–J joints, showing that TdT is switched off slightly earlier in the development of mouse B cells. In fetal development, when the peripheral immune system is first being supplied with T and B lymphocytes, TdT is expressed only at low levels, if at all.
8-3 The pre-B-cell receptor tests for successful production of a complete heavy chain and signals for the transition from the pro-B cell to the pre-B cell stage.
The imprecise nature of V(D)J recombination is a double-edged sword. Although it produces increased diversity in the antibody repertoire, it also results in many unsuccessful rearrangements. Pro-B cells therefore need a way of testing whether a potentially functional heavy chain has been produced. They do this by incorporating a functional heavy chain into a receptor that can signal its successful production. This test takes place in the absence of light chains, whose loci have not yet rearranged. Instead, pro-B cells make two invariant ‘surrogate’ proteins that together have a structural resemblance to the light chain and can pair with the μ chain to form the pre-B-cell receptor (pre-BCR) (see Fig. 8.5). The assembly of a pre-B-cell receptor signals to the B cell that a productive rearrangement has been made, and the cell is then considered a pre-B cell.
The surrogate chains are encoded by nonrearranging genes separate from the antigen-receptor loci, and their expression is induced by E2A and EBF (see Fig. 8.4). One is called λ5 because of its close resemblance to the C domain of the λ light chain; the other, called VpreB, resembles a light-chain V domain but has an extra region at the amino-terminal end. Pro-B cells and pre-B cells also express the invariant proteins Igα and Igβ, introduced in Chapter 7 as the signaling components of the B-cell receptor complex on mature B cells. As components of the pre-B-cell receptor, Igα and Igβ transduce signals by interacting with intracellular tyrosine kinases through their cytoplasmic tails, just
Amino-terminal tails on VpreB and ˜5 in adjacent pre-B-cell receptor molecules bind each other and cross-link the receptors, inducing clustering and signaling as they function to transduce signals through the antigen receptor on mature B cells (see Section 7-7).