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Chapter 6: The Receptor Genes of Lymphocyte and Expression Organization

Introduction

This chapter provides an in-depth analysis of the structural organization and expression of receptor genes, focusing primarily on lymphocytes, which are key players in the immune response. It references a significant publication pertaining to the immunoglobulin loci during the critical stages of lymphocyte development.

Understanding Immunoglobulin Gene Structure

Major Questions:

1. A vast array of different B-cell receptors (BCRs) have been generated, exceeding 10^7 in their diversity.

2. This situation raises pivotal questions regarding the nature of the variable regions of BCRs:

   • Is it feasible for a finite quantity of genetic information to encode an extensive range of antigen-binding sites?

   • How can identical sections of the sequence coexist alongside remarkable variability within others?

3. Notable experiments conducted in the 1970s have shed light on the mechanisms utilized by B cells to produce such diverse receptors.

Theories Explaining Antibody Diversity

Two Competing Theories:

• Gene segments: Suggests that BCR diversity arises from a set of discrete genetic segments.

• Mutation: Proposes that mutations occurring in the variable regions after gene rearrangement contribute to diversity.

1. Germ-line segments: These segments serve as the foundation, encoding various parts of the antibody's variable region.

2. Segment rearrangement: Different naïve B cells rearrange these segments uniquely, leading to diversity.

3. Post-antigen exposure mechanisms: Additional diversification occurs upon the recognition of antigens, enhancing BCR variability.

Recombination Mechanisms in B Cells

Gene Segment Recombination:

B cells employ recombination of specific gene segments to craft diverse antibodies. There are four primary categories of gene segments:

• Variable (V)

• Diversity (D)

• Joining (J)

• Constant (C)

Notably, D segments are explicitly tied to the heavy chains of antibodies.

Immunoglobulin Gene Organization

Multigene Organization:

• Kappa light chain genes are organized on chromosome 2 (in humans) and chromosome 6 (in mice).

• Lambda light chain genes are found on chromosome 22 (in humans) and chromosome 16 (in mice).

• Heavy chain genes are located on chromosome 14 (in humans) and chromosome 12 (in mice).

Details of Gene Segmentation

1. Kappa Light Chain Genes:

   • Comprises Variable (V), Joining (J), and Constant (C) segments.

   • In mice, there are 18 Vκ gene families, whereas human arrangements differ in quantity.

2. Lambda Light Chain Genes:

   • Include paired J and C segments; 5% of mouse immunoglobulins possess lambda chains.

   • Humans utilize approximately 33 Vλ genes, comprising a series of functional Jλ-Cλ pairs.

3. Heavy Chain Gene Organization:

   • Involves VH, DH, JH, and CH segments, with multiple functional regions based on species.

   • Contains a significant regulatory gap situated between the VH and DH regions.

Antibody Diversity Mechanisms

Recombination of Antibody Genes:

The process of recombination involving V region gene segments generates a rich repertoire of functional antibody sites. Estimates show:

• Heavy chain combinations: 45 V x 23 D x 6 J = 6,210 possible combinations.

• Kappa light chains: 41 V x 5 J = 205 possible combinations.

• Lambda light chains: 33 V x 5 J = 165 combinations.

Mechanism of V(D)J Recombination

Recombination Steps:

1. Initiated by RAG1/2 proteins binding to recombination signal sequences (RSS).

2. Formation of hairpin coding ends, which are subsequently cleaved by the Artemis protein.

3. The process sees the sequential addition of P nucleotides and other nucleotide modifications.

4. Utilizes non-homologous end joining (NHEJ) repair mechanisms during the final assembly of gene segments.

Regulatory Components of Recombination

Chromatin Alterations:

RAG1/2 activity is significantly influenced by epigenetic modifications occurring on histones within chromatin. The structure of chromatin plays a crucial role in determining the accessibility for recombination events to take place effectively.

B-cell Receptor Expression and Development

Recombination and Allelic Exclusion:

Each mature B cell synthesizes one heavy chain and one light chain. The formation of a Pre-B Cell Receptor (Pre-BCR) occurs through the pairing of the heavy chain with a surrogate light chain, which is essential for B cell maturation. Nonproductive gene arrangements lead to programmed cell death (apoptosis) during early developmental stages.

Receptor Editing and Self-tolerance

Light Chain Editing:

Mechanisms exist that enable immature B cells to edit their potentially autoreactive receptors through further genetic rearrangements, thereby enhancing self-tolerance and preventing autoimmune responses.

mRNA Splicing and Antibody Production

IgM and IgD Production:

Due to the presence of polycistronic primary mRNA transcripts, various splicing mechanisms ultimately dictate which type of antibody is expressed, predominantly IgM and IgD.

T-cell Receptor Genes and Expression

TCR Structure:

The T-cell receptor (TCR) is structured as a heterodimer, consisting of α and β chains. It undergoes a recombination process similar to that of immunoglobulin genes, yielding structural diversity in response to different antigens.

Summary of Advances in Immunology

The advancements made in understanding the intricate gene arrangements and recombination mechanisms have critically enhanced the knowledge regarding the functions of B and T lymphocytes. This deepened understanding also influences therapeutic strategies against normal and abnormal immune responses, unveiling many possibilities for clinical applications.