The stages of B-cell development are, in the order they occur, early pro-B cell, late pro-B cell, large pre-B cell, small pre-B cell, immature B cell, and mature B cell (Fig. 8.4). Rearrangement of the heavy-chain locus is initiated in the pro-B cell when E2A and EBF induce the expression of several key proteins that enable gene rearrangement to occur, including the RAG-1 and RAG-2 components of the V(D)J recombinase (see Chapter 5). Only one gene locus is rearranged at a time, in a fixed sequence. The first rearrangement to take place is the joining of a D gene segment to a J segment at the immunoglobulin heavy-chain (IgH) locus. D to JH rearrangement takes place mostly in the early pro-B-cell stage, but can be seen as early as the common lymphoid progenitor. In the absence of E2A or EBF this initial rearrangement event fails to occur. Another key protein induced by E2A and EBF is the transcription factor Pax5, one isoform of which is known as the B-cell activator protein (BSAP) (see Fig. 8.3). Among the targets of Pax5 are the gene for the B-cell co-receptor component CD19 and the gene for Igα, a signaling component of both the pre-B-cell receptor and the B-cell receptor (see Section 7-7). In the absence of Pax5, pro-B cells fail to develop further down the B-cell pathway but can be induced to give rise to T cells and myeloid cell types, indicating that Pax5 is required for commitment of the pro-B cell to the B-cell lineage. Pax5 also induces the expression of the B-cell linker protein (BLNK), an SH2-containing scaffold protein that is required for further development of the pro-B cell and for signaling from the mature B-cell antigen receptor lgD Stem cell Early pro-B cell Late pro-B cell Immature B cell Mature B cell Small pre-B cell Large pre-B cell L-chain genes Surface Ig Absent Absent Absent Intracellular ˜ chain IgM expressed on cell surface Germline Germline GermlineGermline VJ rearranged VJ rearrangedV–J rearranging H-chain genes Germline D–J rearranging V–DJ rearranging VDJ rearranged VDJ rearranged VDJ rearranged VDJ rearranged lgM lgMpre-B receptor BP-1 Amino-peptidase CD19 Unknown CD43 CD24 Growth factor receptorIL-7R Kit TdT ˛5 Ig˝ VpreB Ig˙ CD45R Btk RAG-1 RAG-2 N-nucleotide addition Protein Function Surrogate light-chain components Lymphoid-speciÿc recombinase Signal transduction ˜ chain transiently at surface as part of pre-B-cell receptor. Mainly intracellular IgD and IgM made from alternatively spliced H-chain transcripts