BCR vs. TCR Organization

BCR vs. TCR Organization

• The cell effect or function dictates the organization and development of BCRs and TCRs.
• BCRs have a membrane-bound immunoglobulin (IG) molecule composed of two identical light and heavy chains.
• BCRs use two hypervariable regions for antigen recognition.
• Additional surface proteins are needed to stabilize the BCR and facilitate signal transduction.
• TCRs are smaller molecules, but still require assessment molecules for stability.
• The single chains of the TCR resemble the heavy and light chains of IG and BCR molecules.
• TCR alpha and beta chains are the most dominant T cells.
• Gamma Delta T cells are less abundant, thought to play a role in innate immune recognition.
• TCR variable regions contact the ligand, which is the antigen, MHC, class one or two complexes.
• The TCR Variable Region alpha is organized like the light chain of the BCR
• The TCR beta chain is organized like the heavy chain of the BCR
• BCR variable regions contact the ligand, antigen directly, or antigen, MHC class II complex.
• The BCR heavy chain undergoes class switching to change the IG class, without changing the epitope binding region

Diversity Mechanisms of BCRs and TCRs

• Both BCR and TCR diversity mechanisms use VDJ rearrangement for light and heavy chains.
• Gene rearrangement varies depending on structural differences between BCRs, TCRs, and IG molecules.
• BCR and TCR gene rearrangement occur early in cell differentiation, prior to antigen exposure.
• Recombination enzymes are used at the DNA level to splice and rejoin gene segments into a single mRNA transcript.
• This process provides an area for further junctional diversity

Antigen Receptors: BCR and TCR

• The B cell receptor (BCR) is essentially a membrane-bound antibody with a transmembrane part attached to the B cell surface.
• Similar to antibodies, the BCR has a heavy and a light chain.
• The fragment antigen binding (Fab) region of the BCR binds the antigen, and each BCR has two Fab regions.
• The fragment constant (Fc) region determines the B cell receptor class (e.g., IgM, IgD) and whether it remains membrane-bound or is secreted as a free-floating antibody.
• The T cell receptor (TCR) has two chains: an alpha chain (like the B cell light chain) and a beta chain (like the B cell heavy chain).
• TCR alpha and beta chains form a transmembrane receptor and do not get secreted; there are no different classes of TCRs.
• TCRs have only one antigen-binding site.

VDJ Rearrangement

• VDJ rearrangement affects the portion of the protein that contacts the antigen for both BCRs and TCRs, known as the variable (V) region.
• The V region is supported by a constant (C) region.
• The variable regions of BCRs and TCRs are highly diverse across the cell population, ensuring each T cell or B cell recognizes a unique antigen.
• Variability is concentrated in hypervariable regions, also known as complementarity-determining regions (CDRs), within the V region.
• When the two chains of the TCR or BCR come together, these hypervariable regions form one hypervariable site at the tip of each arm of the BCR or the tip of the TCR, which is the antigen-binding site.

The Process of VDJ Rearrangement

• The first step of VDJ rearrangement happens before birth.
• Individuals inherit multiple V, D, and J gene segments from each parent.
• For example, a person has 44 V gene segments, 27 D segments, and 6 J segments for B cell heavy chains; there are more V, D, and J segments for the T cell receptor beta chain and more V and J segments for the B cell light chain and T cell Alpha chain.
• Gene segments are lined up on the DNA similarly to boxcars on a train.
• V gene segments are located farthest from the constant gene segments, while J segments are closer.
• D gene segments sit between the V and J segments on chromosomes containing heavy or beta chain genes.
• Gene segments reassort randomly in B and T cells to generate diversity.

Recombination Signal Sequences

• The rearrangement of exons is accomplished by recombination signal sequences, which are non-coding DNA sequences that flank the DNA that needs to be cut and rearranged.
• Each recombination signal sequence contains three parts: a seven-nucleotide sequence (heptamer), a stretch of either 12 or 23 nucleotides, and a nine-nucleotide sequence (nonamer).
• During VDJ rearrangement, DNA loops to bring together two recombination signal sequences in a mirror-image orientation.
• Enzymes called RAG1 and RAG2 cut the DNA at these sequences.
• Repair proteins then reattach and recombine the DNA, creating an extra chromosomal, circular piece of DNA that is lost when the cell divides.
• Daughter cells will have the same VDJ sequence as the original cell.

Increasing Diversity

• Recombinational inaccuracy and terminal deoxynucleotide transferase (TDT) increase diversity at this point in the process.
• TDT randomly adds and removes nucleotides (N nucleotides).
• Cells with the same V, D, and J regions may differ due to the action of TDT.
• Additional changes alter the antigen specificity of the receptor.
• DNA ends are held together and joined by enzymes, including DNA ligase IV, creating a new coding joint with the rearranged V, D, and J regions.

Immunoglobulin Switching

• Immunoglobulin switching from IgM to IgG is known as class switch recombination, occurring in B cells.
• Class switch recombination allows the immune system to produce antibodies with different functions but the same pathogen recognition.
• Mature B cells encountering antigen differentiate into plasma or memory B cells.
• Plasma cells initially secrete IgM, the default antibody.
• Class switch recombination occurs in germinal centers of secondary lymphoid tissues after B cell activation by the antigen and T helper cells.
• T helper cells recognize the antigen presented on the B cell’s MHC II molecules, providing signals for the B cell to switch from IgM to IgG.
• DNA recombination occurs in the heavy chain's constant region, which determines the antibody class.
• Enzymes introduce breaks in the DNA at switch regions, looping out and deleting intervening DNA.
• The VDJ region then links to the IgG constant region.
• B cells produce IgG antibodies with the same antigen specificity (same VDJ region) but with IgG effector functions.
• IgG opsonizes bacteria for easier engulfment and destruction by macrophages and neutrophils.
• Memory B cells with IgG specificity are generated, providing faster and stronger responses upon re-exposure.
• Class Switch Recombination (CSR) occurs in the constant region (C region) of the immunoglobulin heavy chain gene.
• The heavy chain is composed of segments for each antibody class (IgM, IgG, IgA, IgE).
• During CSR, rearrangement of the DNA sequence occurs in the heavy chain, replacing one constant region with another.
• Facilitated by enzymes and regulatory elements, it results in antibodies with a different heavy chain while maintaining the same antigen-binding variable region.
• In the context of an H. influenzae type B infection, B cells may switch to produce IgG antibodies.

Function of Antibody Types in Immunoglobulin Switching

• IgG antibodies have distinct effector functions compared to IgM, including enhanced antigen-binding affinity.
• IgM Antibodies are initially produced and are effective at neutralizing pathogens.
• IgG Antibodies are produced after class switching and their properties include the ability to opsonize pathogens, activate complement, and participate in antibody-dependent cellular cytotoxicity (ADCC).

IgA with a Truncated Cytoplasmic Domain

• B cells expressing IgA with a truncated cytoplasmic domain (lacking ITAMs) express both IgM and IgD but cannot be activated by an antigen.
• ITAM sequences are crucial for initiating the signaling when the B cell receptor (BCR) is engaged by antigen, enabling B cell activation.
• The B cell cannot signal properly without the ITAM sequences in the cytoplasmic tail of IgA, meaning the B cell receptor involving IgA can't signal properly.
• However, the B cell can still express other forms of the receptor, such as IgM and IgD, which are also expressed on the surface of B cells during their development.
• The B cell does not undergo the necessary signaling for activation and response, which is a critical step in mounting an immune response.
• A truncated cytoplasmic domain affects signaling.
• The issue is that the truncation of IgA (not IgB) does not allow optimal signaling for activation when the B cell encounters an antigen.

Light Chain Rescue

• Light Chain Rescue results from multiple V-J joining events on a single chromosome until productive rearrangement of the light chain occurs or all J segments have been recombined.
• Light Chain Rescue is the process that when a developing B cell has failed to produce a functional light chain in one of its two chromosomes.
• B cell continues developing by attempting multiple attempts at V-J recombination on a single chromosome.
• Light Chain Rescue allows the developing B cell attempts another recombination on the second light chain chromosome.
• If no productive rearrangement occurs after all J segments have been used, the B cell undergoes death by apoptosis.
• Light Chain Rescue involves multiple attempts of V-J recombination on the same chromosome, trying different combinations of V and J segments until a productive rearrangement is achieved.
• Light Chain Rescue is about retrying recombination if the first attempt fails, not specifically related to self-specific B cells nor on self-reactivity.
• The surrogate light chain binds to the pre-BCR and helps during the heavy chain rearrangement stage.
• The IgaIgb complex signals during pre-BCR signaling, but Light Chain Rescue happens after heavy chain rearrangement.