Immunological Basics of Lymphoma

Lymphocyte Development and Antigen Receptor Generation

• General Principles of Maturation:     - The process generates a vast number of cells, each possessing a unique antigen receptor.     - The system preserves cells with useful receptors and eliminates those that are ineffective or dangerous.     - Receptor Diversity Generation: The molecular process involved in creating millions of receptors is independent of antigen recognition. Recognition only occurs after the receptors are generated and expressed on the cell surface.     - Selection Processes:         - Positive Selection: Promotes the survival of lymphocytes with receptors that can recognize antigens (e.g., microbial antigens).         - Negative Selection: Eliminates cells that cannot recognize any antigens or those that recognize "self-antigens" with high enough affinity to pose a risk of autoimmune disease.

• B-Cell and T-Cell Receptors (BCRs and TCRs):     - B Lymphocytes: Express membrane-bound antibodies known as B-Cell Receptors ($BCRs$).     - T Lymphocytes: Express T-Cell Receptors ($TCRs$).     - Core Function: Detect external stimuli and trigger cellular responses.     - Clonal Distribution: Antigen receptors are clonally distributed; each lymphocyte clone is specific for a distinct antigen and possesses a unique receptor different from all other clones.     - Ligand Recognition:         - BCRs: Can recognize a broad range of chemical structures, including proteins, lipids, carbohydrates, and nucleic acids.         - TCRs: Recognize only peptides bound to Major Histocompatibility Complex ($MHC$) molecules.

Structural Organization of Antigen Receptors

• Receptor Regions:     - Variable (V) Regions: The antigen-recognizing domains where sequence variation is concentrated.     - Constant (C) Regions: The conserved portions of the receptor.     - Hypervariable Regions / Complementarity-Determining Regions (CDRs): Short stretches within the V region where variation is most extreme. These CDRs form the part of the receptor that physically binds to the antigen (complementary to the antigen's shape).     - Invariant Membrane Proteins: Associated with receptor chains; these deliver intracellular signals to the cytosol and nucleus following antigen recognition, leading to division, differentiation, or effector functions.

• Detailed B-Cell Receptor (BCR) Structure:     - Assembly: Formed by four chains in a Y-shaped molecule.     - Chain Connectivity: Each light chain is attached to one heavy chain; the two heavy chains are connected to each other via disulfide bonds.     - Domains:         - Light Chain: One V and one C domain.         - Heavy Chain: One V and three or four C domains.     - Antigen-Binding Site: Composed of the V regions of both the heavy ($V_H$) and light ($V_L$) chains. Each antibody structure contains two identical antigen-binding sites.     - CDRs: Each V region ($V_H$ or $V_L$) contains three hypervariable regions; $CDR3$ is known to be the most variable.

Immunoglobulin Isotypes and Heavy/Light Chains

• Heavy Chain Isotypes: Determining the class (isotype) of the antibody is based on the sequence of the constant region of the heavy chain.     - Types of Heavy Chains:         - $\mu$ ($IgM$): Functions as a B-cell receptor.         - $\delta$ ($IgD$): Functions as a B-cell receptor.         - $\gamma1, \gamma2, \gamma3, \gamma4$ ($IgG1, IgG2, IgG3, IgG4$).         - $\alpha1, \alpha2$ ($IgA1, IgA2$).         - $\epsilon$ ($IgE$).

• Light Chains: There are two types, $\kappa$ and $\lambda$, which differ in their C regions. An individual antibody has either $\kappa$ or $\lambda$ light chains, never both.

• Naive B Lymphocytes: Mature B cells that have not yet encountered an antigen; they co-express membrane-bound $IgM$ and $IgD$.

• Heavy-Chain Class (Isotype) Switching: After stimulation by antigens and helper T cells, B-cell clones may expand and switch from producing $IgM/IgD$ to other classes (e.g., $IgG, IgA, IgE$).

Genetic Mechanisms of BCR Rearrangement

• Chromosomal Loci:     - Ig Heavy chain locus: Located on Chromosome 14. Contains approx. $45$ V segments, $23$ D segments, and $6$ J segments.     - Ig $\kappa$ chain locus: Located on Chromosome 2. Contains approx. $35$ V segments and $5$ J segments.     - Ig $\lambda$ chain locus: Located on Chromosome 22. Contains approx. $30$ V segments and $4$ J segments.

• V(D)J Recombination Process:     - Somatic Recombination: Mediated by the $VDJ$ recombinase, comprising Recombination-Activating Gene 1 and 2 ($RAG-1$ and $RAG-2$) proteins.     - Mechanism: The recombinase recognizes flanking DNA sequences, brings segments into proximity, and cleaves DNA at specific sites. This occurs only in immature B and T lymphocytes.     - Steps in Heavy Chain Rearrangement:         1. One $D$ segment joins with one $J$ segment ($DJ$ complex).         2. One $V$ segment joins the fused $DJ$ complex to form a recombined $VDJ$ exon.         3. The $VDJ$ exon is transcribed and spliced to the $C$-region exons of the $\mu$ chain (the 5' C region) to form $\mu$ mRNA.         4. Translation produces the $\mu$ heavy chain, the first Ig protein synthesized in B-cell maturation.

Maturation and Checkpoints of B Lymphocytes

• Location: Maturation occurs primarily in the bone marrow.

• Sequential Steps:     1. Pro-B Cells: Begin rearranging Ig heavy-chain genes.     2. Pre-B Cells: Formed if heavy-chain rearrangement is successful. The cell expresses a Pre-BCR, consisting of a $\mu$ heavy chain and two surrogate light chains. This is the first checkpoint to select for functional $\mu$ chains.     3. Allelic Exclusion: The Pre-BCR signals the cell to shut off recombination of Ig heavy-chain genes on the second chromosome. This ensures the B cell expresses only one receptor specificity.     4. Small Pre-B Cells: Initiate Ig light-chain gene rearrangement.     5. Immature B Cells: Assemble a complete membrane $IgM$ receptor.     6. Mature B Cells: Co-express $IgM$ and $IgD$. This occurs via alternative splicing of the primary RNA transcript (the VDJ unit is spliced to either $C_{\mu}$ or $C_{\delta}$ exons).

• Receptor Editing: If an immature B cell binds self-antigen with high affinity in the bone marrow, it may re-express $VDJ$ recombinase to undergo additional light-chain recombination, changing the receptor's specificity.

T-Cell Receptor (TCR) Maturation

• TCR Structure: Heterodimeric protein composed of an $\alpha$ chain and a $\beta$ chain ($\alpha\beta$ TCR). Each chain has one V region (with 3 hypervariable CDRs) and one C region.

• TCR Loci Statistics:     - $\alpha$ chain: $45$ V segments, $0$ D segments, $50$ J segments.     - $\beta$ chain: $48$ V segments, $2$ D segments, $12$ J segments.

• Maturation Steps in the Thymus:     1. Pro-T Cells (Double-Negative): Do not express $CD4$ or $CD8$.     2. Pre-T Cells: Formed if $\beta$ chain rearrangement is successful. The $\beta$ chain associates with an invariant protein called $pre-T\alpha$ to form the Pre-TCR complex.     3. Double-Positive T Cells: Surviving cells express the complete $\alpha\beta$ TCR and both $CD4$ and $CD8$ coreceptors.     4. Selection:         - Positive Selection: T cells representing low/moderate affinity for self-MHC survive to ensure they can recognize peptides on APCs.         - Negative Selection: Elimination of strongly self-reactive T cells to prevent autoimmunity.     5. Single-Positive T Cells: T cells become either $CD8+$ (MHC class I restricted) or $CD4+$ (MHC class II restricted).

Repertoire Diversity Quantities

• Combinatorial Diversity:     - Ig: Approx. $3 \times 10^6$ possible V(D)J combinations.     - TCR: Approx. $6 \times 10^6$ possible V(D)J combinations.

• Junctional Diversity: Caused by the removal or addition of nucleotides (N-region or P-nucleotides).

• Total Potential Repertoire:     - Ig: Approx. $10^{11}$.     - TCR: Approx. $10^{16}$.

Lymph Node Architecture

• Afferent Lymphatics: Channel through which lymphocytes and antigens from tissue spaces enter the lymph node sinus.

• Cortex: Primarily a B-cell area. B cells are organized into primary or secondary follicles (containing a germinal center).

• Paracortex: Contains mainly T cells.

• B-Cell Zones (Immunohistochemical Staining):     - Secondary follicle.     - Germinal Centre ($GC$).     - Mantle Zone ($Mn$).     - Subcapsular Zone ($SC$) (between the capsule and the follicle).

Lymphoma and Lymphoid Malignancies

• Definition: Malignant proliferation of lymphoid cells in lymph nodes or the spleen.

• Broad Classification: Hodgkin's and Non-Hodgkin's Lymphoma ($NHL$).

• Clinical Presentation:     - Painless lymphadenopathy.     - B symptoms: Unexplained fever, night sweats, weight loss.     - Itching.     - Severe/recurrent opportunistic infections (e.g., Herpes simplex, Varicella-zoster).

• Monoclonality: Malignancies originate from a single lymphocyte or plasma cell. All cells in the malignant population share identical immune receptor gene rearrangements and express identical Ig or TCR molecules.

• Pathogenesis:     - Chromosomal Translocations: Errors during gene recombination or class switching.     - Specific Oncogenes:         - MYC: Chromosomal translocation to the Ig heavy chain gene. Stimulates unrestrained lymphocyte proliferation.         - BCL2: Translocation to the Ig heavy chain gene. Protects against apoptosis, allowing lymphocyte survival.

• Viral Pathogenesis:     - Epstein-Barr Virus (EBV): Produces proteins that immortalize B cells, drive proliferation, and inhibit apoptosis. Linked to Hodgkin's and Non-Hodgkin's lymphoma.     - Burkitt Lymphoma: Especially common in malaria-endemic regions; results from polyclonal B-cell activation by both malaria and EBV.     - HTLV1: A retrovirus causing rare T-cell malignancies. Encodes the Tax protein, which functions similarly to the T-cell survival cytokine $IL-2$.

Characteristics of Specific Malignancies

Idiotype and Personalized Immunotherapy

• Idiotype: The unique variable region sequences of antibodies or TCRs produced by a single lymphocyte clone. It serves as a molecular "fingerprint."

• Concepts:     - Ab1 (Idiotype): The primary BCR or antibody recognizing a specific antigen. Its variable region contains a unique "idiotope."     - Ab2 (Anti-idiotype): An antibody produced against the variable region of $Ab1$. It acts as a "mirror image."

• Clinical Application in Lymphoma:     - Since lymphoma is monoclonal, every cancer cell in a patient expresses the same unique idiotype (a "tumor-specific antigen").     - Personalized Immunotherapy Steps:         1. Tumor Biopsy: Isolate the unique idiotype from the patient's cells.         2. Vaccine Production: Produced in a lab and conjugated to a carrier protein for visibility.         3. Injection: Induces the patient's body to produce anti-idiotype antibodies and T cells to target the specific malignant clone.     - Note: These vaccines have struggled in Phase III trials to show consistent long-term efficacy.

Treatment of Lymphoma

• Modalities: Chemotherapy, immunotherapy, and radiation.

• Monoclonal Antibodies:     - Rituximab: Attaches to CD20 found on B cells. Used for NHL.     - Brentuximab vedotin: An Antibody-Drug Conjugate ($ADC$). It targets CD30 on Hodgkin's lymphoma cells and delivers a chemotherapeutic drug directly to the cell.     - Radio-immunotherapy: Antibodies attached to small radioactive particles to deliver targeted radiation.

• CAR T-Cell Therapy: Chimeric Antigen Receptor T-cells. Designed with an extracellular antibody-like portion (heavy and light chain variable domains) to target specific tumor antigens like CD19, CD20, and CD30.