The advent of recombinant DNA technology provided a powerful new approach to address many of the unresolved issues of B cell differentiation and antibody diversification. The molecular immunology era began in 1976 with the finding by Nobumichi Hozumi and Susumu Tonagawa59 that immunoglobulin light chain variable and constant region gene segments undergo somatic rearrangement during B cell differentiation. It was then shown that creation of the light chain variable region exon involves rearrangement of two gene segments — V (variable) and J (joining) — and that this same process at the heavy chain locus involves three gene segments (V, D (diversity) and J)60,61. Recombination signal sequences flanking the V, D and J gene segments were shown to guide the assembly process62,59. The sequential recombinatorial assembly of the heavy chain gene first and then the light chain gene can explain the orderly expression of μ-heavy chains by pre-B cells and the subsequent expression of IgM by newly formed B cells58. The switch from transmembrane IgM receptors on B cells to secreted IgM antibodies was found to be due to alternative splicing of the heavy chain gene transcripts62. Antibody class switching was shown to involve deletion of the constant region gene segment of the μ-heavy chain and its replacement by one of the other downstream constant region gene segments63,64,65. The antigen-selected evolution of cumulative somatic mutations proved to be the underlying mechanism for affinity maturation of antibodies66 and this process was shown to occur primarily in the germinal centres67.

These studies provided insight into the genetic basis for the orderly expression of clonally diverse IgM heavy and then light chain genes, the subsequent switches in constant region genes to enable B cells to express different antibody classes and the concomitant antigen-mediated selection of somatic mutations (reviewed in Ref. 68). However, several more years of research were required to define the composition of the BCR for antigens and that of the pre-BCR (Fig. 3) as essential first steps in understanding the signalling pathways that are triggered by antigen activation of B cells, the analysis of which is still ongoing.

Figure 3: The composition and the expression of pre-B cell and B cell receptors. The elucidation of the antibody genes and the differential splicing of the heavy chain transcripts provided insight into the transmembrane expression of IgM and IgD (not shown here) as B cell receptors (BCRs) for antigen recognition on B cells. Pre-B cells, which have not yet rearranged their light chain genes, instead express surrogate light chain genes that encode λ5 and VpreB, in combination with the μ-heavy chain83. For both the pre-BCR and the BCR, the short intracytoplasmic portions of the μ-heavy chains are inadequate for initiation of signal transduction. The solution to this problem came with the identification of the associated transmembrane proteins Igα and Igβ, which have cytoplasmic motifs that undergo phosphorylation after antigen engagement of the receptor complex to trigger the signalling cascades that are responsible for activation of normal and abnormal B cells (reviewed in Refs 82,84,85). Full size image Download PowerPoint slide

Many other important discoveries deserve mention in this brief historical sketch of B cells. One is the discovery of the paired recombination-activating gene 1 (RAG1) and RAG2, which encode enzymes that are essential for initiating V(D)J recombination69,70. Another is the discovery of the activation-induced deaminase gene, which encodes an enzyme that is essential for initiating heavy chain class switching and somatic mutation of V regions to promote the affinity maturation of antibodies71. Moreover, the gene conversion mechanism that was shown to be essential for generating antibody diversity in the avian bursa of Fabricius72,73 is also mediated by this enzyme74. With regard to the question of how T cells 'see' antigens, the discovery that cytotoxic T cells kill virus-infected cells only in the context of self-MHC class I molecules provided pivotal insight into this issue75. The subsequent identification of the T cell receptor (TCR) genes76,77 paved the way to understanding how the MHC class I and class II proteins present peptides to T cells. Investigations of the two major T cell developmental pathways, in which lymphocytes express either a γδ TCR or an αβ TCR, and their many sublineages are ongoing.