IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 8. The Genetic Basis of Antibody Diversity

REGULATION OF IMMUNOGLOBULIN SYNTHESIS

The intracellular regulation of immunoglobulin synthesis is remarkably complex. In the initial stage, a D Gene segment is brought into proximity with a J segment, followed by the joining of a V segment. In pre-B Cells, fully assembled μ chains can be detected in the Cytoplasm. These newly synthesized heavy chains remain non-covalently bound to BiP/GRP78 chaperone Proteins in The Endoplasmic reticulum until they associate with light chains. Pre-B cells do not produce light chains, but instead synthesize small amounts of structurally similar surrogate light chains (λ5 and V_preB in mice, 14.1 and V_preB in humans), which pair with μ chains. As a result, a small number of IgM-like molecules are transported to The Cell surface, although their exact function remains elusive. It is known, however, that mice lacking surrogate light chains exhibit a substantial reduction in B-cell numbers. The cytoplasmic μ chains in pre-B cells trigger V-J recombination at the κ-chain locus and subsequently (if required) at the λ-chain locus. It is thought that recombination is attempted iteratively at these loci on maternal and paternal Chromosomes until either a functional V-region light-chain gene is successfully assembled, the genetic material is exhausted, or the B cell undergoes elimination. Aside from potential somatic hypermutation, recombinant light-chain V genes undergo little subsequent alteration. Nevertheless, the B cell must still undergo Class switch recombination of C_H genes to express different immunoglobulin isotypes and, upon activation, transition to The production of the secreted antibody form. The facts and hypotheses concerning immunoglobulin Biosynthesis are summarized in Fig. 8.23.

Fig. 8.23. Stages of immunoglobulin formation in B cells. In pre-B cells, recombination attempts between D and J segments, and subsequently the V segment, are initiated iteratively to generate a functional heavy-chain V-D-J gene, which is transcribed alongside the μ and δ genes and translated into membrane-bound Ig μ and δ chains. In turn, μ chains induce recombination events at the light-chain loci. The cell attempts to generate functional chains utilizing both maternal and paternal chromosomal loci until success is achieved or the pool of un rearranged gene segments is depleted. Failure to synthesize a functional immunoglobulin leads to the elimination of that B-cell clone. Mature B cells express membrane-bound IgM and IgD. Following primary antigenic stimulation, these cells can synthesize secreted IgM; upon immunization with a T-dependent antigen, B-cell class switching to IgG, IgA, or IgE production can occur with T-cell assistance. Somatic Mutations in V genes frequently arise at this stage.

Immunoglobulin chains are translated on Ribosomes and translocated across the endoplasmic reticulum membrane

Before antibody synthesis can begin, the primary RNA transcript must be freed of introns via splicing. At the 5' and 3' ends of each intron lie distinct nucleotide sequences known as the donor and acceptor splice sites. These sites are thought to interact with each other and with nuclear ribonucleoproteins, resulting in the precise excision of introns and the ligation of the mRNA ends to restore a continuous reading frame. Naturally, maintaining absolute precision in this process is critical to avoid reading frame shifts.

Immunoglobulin mRNA is translated on ribosomes, and the newly synthesized Polypeptides are directed into the endoplasmic reticulum, where light and heavy chains assemble into an antibody molecule (Fig. 8.24). Membrane-bound and secretory forms of IMMUNOGLOBULINS undergo differential Processing to ensure proper intracellular sorting—a mechanism not yet fully elucidated—and to reach their final destinations.

Fig. 8.24. The mRNA encoding the heavy chain of the secreted Ig form leaves The Nucleus and enters the cytoplasm, where it binds to a ribosome (1). The translated leader (signal) sequence (L) of the nascent polypeptide binds to a signal recognition particle (SRP), which transiently arrests further Translation (2). The SRP-ribosome complex then migrates to the endoplasmic reticulum (ER), where SRP interacts with a docking protein on the membrane (3). Translation resumes, and the elongating chain is translocated across the membrane into the ER lumen (4). Within the ER lumen, the leader sequence is cleaved, and the completed chain assembles with other H and L chains into an immunoglobulin subunit (5). A set of Enzymes (E₁) attaches CARBOHYDRATES (shown in blue) to the immunoglobulin while a transport vesicle buds off from the ER (6). In the Golgi apparatus, another group of enzymes (E₂) modifies the carbohydrate moieties, and the fully assembled molecule is subsequently secreted from the cell via reverse pinocytosis (7).