B Cells: Function in Adaptive Immune Response, University of Portsmouth

Learning Objectives

On completion of this session you should be able to

1. Describe the function of B cells in the adaptive immune response, including antibody production and antigen presentation.
2. Explain the clonal selection theory.
3. Understand the mechanisms underlying the generation of antibody diversity.

Adaptive Response

1. Commences ~96 h after initial infection
2. Initiated if innate system fails to contain and eliminate infectious agents
3. Mediated by B & T lymphocytes
4. B cells - mediate adaptive humoral response
5. B cells -> antibody (plasma cells)
6. T cells - mediate adaptive cell mediated response
7. T cells -> T., Th1, Th2, T, FH, Th17
8. Activation of adaptive immunity occurs in secondary lymphatic tissue (Lymph nodes, spleen, MALT)

Adaptive Response - Clonal Selection Theory

1. Each lymphocyte bears a single type of receptor of unique specificity
2. Interaction between foreign antigen and lymphocyte receptor leads to lymphocyte activation
3. Activated lymphocytes proliferate clonally and differentiate into effector cells which bear antigen receptors of identical specificity to those of the parent cell
4. Lymphocytes bearing receptors specific for self molecules are deleted at an early stage in lymphoid cell development and are absent from the repertoire of mature lymphocytes

B Cells

1. B lymphocytes function in the humoral immunity component of adaptive immune system producing antibodies (plasma cells)
2. Each kind of B cell produce one kind of antibody
3. Memory (memory cells)
4. Antigen specificity
5. Discrimination between self and non-self

B Cell Types

1. 3 types:
   • B1 cells - found in the peritoneal and pleural cavities
   • Marginal zone B cells - found in marginal zone of spleen
   • Follicular B cells - found in B cell follicles in lymphatic tissue

Generation of Diversity

1. Each B cell synthesises antibody with unique specificity
2. The collection of antibody specificities is called the antibody repertoire and is approx 1011 in humans
3. For each specificity to be encoded by a separate gene is inefficient
4. Diversity arises due to processes of somatic gene recombination and somatic hypermutation

Somatic Gene Recombination

1. DNA encoding V and C regions is separated in all cells except B cells
2. During development, coding segments are brought together so a functional antibody molecule can be produced
3. This occurs for both H and L chain genes

Light Chain Genes

1. Two light chain genes encoding k and l chains
2. k Light chain genes are found on chromosome 2

Somatic Gene Recombination on Chromosome 22

1. 2 Light chain genes are found on chromosome 22
2. The light chain V region is encoded by DNA formed from the recombination of two gene segments
   • a V (variable) gene segment
   • a J (joining) gene segment
3. The V gene segment encodes the first 95 - 101 amino acids and the J gene segment encodes for the remainder of the domain (~13 residues)

Somatic Gene Recombination Details

1. The V region DNA is separated from the C region DNA by non-coding sequences that are removed by RNA splicing to give the mature RNA transcript
2. For k light chain genes there are ~38 V gene segments and 5 J gene segments
3. Potentially 190 different Vx regions can be produced
4. For 2 light chain genes there are ~30 V gene segments and 4 J gene segments
5. Potentially 120 possible V, regions can be produced

Number of functional gene segments in human immunoglobulin loci

Light chains Heavy chain Segment K H Variable (V) 34-38 29-33 38-46 Diversity (D) 0 0 23 Joining (J) 5 4-5 6 Constant (C) 1 4-5 9 Figure 5.2 Janeway's Immunobiology, 8ed. ( Garland Science 2012)

Somatic Gene Recombination and Antibody Production

1. C region coding sequences are located downstream and joined by RNA splicing
2. Heavy chain genes comprise ~40 V gene segments, 23 D gene segments and 6 J gene segments
3. Potentially ~5,520 different V_ regions can be produced
4. Combining H and L chains, ~1.7 x 106 different antibody specificities can be produced

Light chain Heavy chain V 1 C LV D J Germline DNA Somatic recombination V C DNA D-J rearranged DNA joined Somatic recombination V C L D V-J or V-DJ joined rearranged DNA El Transcription C L C D Primary transcript RNA -AAA -I AA RNA Splicing C mRNA -AAA AAA Translation V CH3 CHZ Polypeptide chain Protein VH CH1 Figure 5.1 Janeway's Immunobiology, 8ed. (O Garland Science 2012) C L C L V L V DJ LVIC

Additional Mechanisms for Diversity

1. Imprecise joining of segments:
   • Nucleotides can be added or deleted during the joining of segments in a random manner
   • B cells with non-functional rearrangements are deleted
2. Somatic Hypermutation:
   • Occurs in secondary lymphoid organs
   • Point mutations are introduced in V-regions of rearranged H- and L-chain genes
   • This results in the production of high affinity antibody during an immune response

Summary of B Cell Function

1. B cells are key players in the adaptive immune response, fulfilling crucial roles in antibody production and antigen presentation.
2. They are central to the clonal selection theory, which proposes that antigen-specific B cells are selectively activated and expanded upon encountering their cognate antigen. This theory underpins our understanding of how the immune system responds to diverse pathogens.
3. The generation of antibody diversity is facilitated by mechanisms such as somatic hypermutation, ensuring a broad repertoire of antibodies capable of recognising various antigens.

B cell development and activation

Learning Objectives for B Cell Development

On completion of this session you should be able to

1. Explain the stages of B cell development from hematopoietic stem cells in the bone marrow to mature B cells in peripheral lymphoid organs.
2. Describe the process of B cell activation upon encountering antigens.
3. Explain the formation of germinal centres within secondary lymphoid organs, and understand the role of germinal centres in facilitating B cell proliferation, affinity maturation, and the generation of high-affinity antibody-producing plasma cells and memory B cells.
4. Define class switching as the process by which B cells change the class of antibody they produce (e.g., switching from IgM to IgG, IgA, or IgE).

B Cell Development Stages

1. Occurs in the bone marrow
2. B cells develop from a common haemopoietic stem cell
3. 4 broad stages are recognised:
   1. Pro B cell - earliest B cell progenitor. During this stage heavy chain genes rearrange (DJ then VDJ rearrangements)
   2. Pre B cell - expresses surface u chain in association with surrogate light chain. Light chain genes rearrange
   3. Immature B cell - expresses surface IgM
   4. Mature B cell (naïve B cell) - expresses surface IgM and IgD and B cell co-receptor

B Cell Development Process

Early pro-B cell Late pro-B cell Pre-B cell Immature B cell H-chain gene rearrangement H-chain gene rearrangement L-chain gene rearrangement Rearrangement ceases D-J rearrangements on both chromosomes + V-DJ rearrangement on first chromosome Cell expresses p/k Rearrange « gene on first chromosome + V-DJ rearrangement on second chromosome IgM Rearrange « gene on second chromosome +) Apoptosis Rearrange À gene on first chromosome + Cell expresses p/À productive rearrangement Rearrange & gene on second chromosome + IgM unproductive rearrangement Apoptosis Figure 4-7 The Immune System, 2/e ( Garland Science 2005)

B Cell Selection and Maturation

1. B Cell Selection
2. Immature B cells enter peripheral circulation (transitional B cells)
3. Mature into B1, marginal zone or follicular B cells in several days
4. If recognise self-antigen they will undergo apoptosis
5. If encounter antigen in secondary lymphoid tissue, they will differentiate into plasma cells and memory cells and undergo class switching

B Cell Activation Mechanism

1. Membrane bound antibody have short cytoplasmic tails
2. Too short to generate signal by associating with tyrosine kinases and G proteins
3. Membrane Ig must be associated with B-cell receptor
4. lg-a/lg-ß have ITAMs (Immunoreceptor Tyrosine Activation Motifs)

mlg lg-a/lg-B B-cell membrane Cytoplasm ITAM Y XXL/IX ... X(6-9 amino acids) Y X X L/I ITAM sequence Figure 11-4

B Cell Coreceptor and Regulation

1. ITIM (immunoreceptor tyrosine inhibitory motif)
2. Associated with CD22
3. Functions to deactivate B cells- negative regulation
4. Important in preventing autoimmunity

B-cell coreceptor C3d Inhibitory B-cell coreceptor Antigen CD22 -CR2 (CD21) mlgM CD19 TAPA-1 (CD81) lg-a/lg-B Activation 0 H P ITIM SHP.1 P + - 1 Deactivating phosphatase Accessory activation factors Activation signals Activation signals Figure 11-11 Naby IMMUNOLOGY, Sinth Edition 6 2003 W.H.Freeman and Company P

B Cell Activation Types

1. Thymus-dependent (TD) antigens
   • B cell required direct contact with TH cells
2. Thymus-independent antigens (TI)
   • These antigens activate B cells by different means
   • Type 1 (TI-1) - lipopolysaccharide
   • Type 2 (TI-2) - highly repetitious molecules (bacterial flagella)

(a) TI-1 antigen 12 B cell (b) TD antigen TH cell 1 2 O CD40/CD40L B cell Figure 11-7

Properties of Antigens

TABLE 11-2 Properties of thymus-dependent and thymus-independent antigens TI antigens Property TD antigens Type 1 Type 2 Chemical nature Soluble protein Bacterial cell- wall components (e.g., LPS) Polymeric protein antigens; capsular polysaccharides Humoral response Isotype switching Yes No Limited Affinity maturation Yes No No Immunologic memory Yes No No Polyclonal activation No Yes (high doses) No Table 11-2 Kuby IMMUNOLOGY, Sixth Edition O 2007 W.H. Freeman and Company

Activation of B cells in TD Response

1. Naïve B cells recognise antigen in lymphatics via BCR
2. Receive co-stimulation from TEL cell in T cells zone
3. B cells proliferate in T cell zone or red pulp of spleen and produce IgM of early response
4. Other B cells migrate to B cell follicles to form a germinal centre