Immunological Basics of Lymphoma

Lymphocyte Development and Selection Principles

• Nature of Lymphocyte Maturation:
    - Generates a vast number of cells, each possessing a unique antigen receptor.
    - The goal is to preserve cells that possess useful receptors for immune defense.

• Molecular Generation of Receptors:
    - This process creates millions of receptors through a molecular mechanism that cannot be influenced by the specific antigens they will eventually recognize.
    - Recognition only occurs after the receptors have been generated and expressed on the cell surface.

• The Selection Process:
    - Selection occurs once antigen receptors are expressed on developing lymphocytes.
    - Positive Selection: Promotes the survival of cells with receptors capable of recognizing useful antigens (e.g., microbial antigens).
    - Negative Selection: Eliminates cells that either cannot recognize any antigens or recognize self-antigens with high enough affinity to pose a risk of autoimmune disease.

Anatomy and Structure of Antigen Receptors

• General Receptor Types:
    - B Lymphocytes: Express membrane-bound antibodies known as B-cell receptors (BCRs).
    - T Lymphocytes: Express T-cell receptors (TCRs).

• Primary Function:
    - Detecting external stimuli and triggering the intracellular responses of the expressing lymphocyte.

• Clonal Distribution:
    - Antigen receptors are distributed clonally; every lymphocyte clone is specific for a unique antigen and possesses a distinct receptor from all other clones.

• Molecular Architecture:
    - Variable (V) Regions: The antigen-recognizing domains of the receptors.
    - Constant (C) Regions: The conserved portions of the receptor.
    - Hypervariable Regions / Complementarity-Determining Regions (CDRs): Short stretches of concentrated variation within the V regions. These form the parts of the receptor that physically bind to antigens by matching their shapes.

• Signal Transduction:
    - Antigen receptor chains associate with invariant membrane proteins.
    - These invariant proteins deliver intracellular signals to the cytosol and nucleus upon antigen recognition.
    - These signals can cause a lymphocyte to divide, differentiate, or perform specific effector functions.

B-Cell Receptor (BCR) and Immunoglobulin Structure

• Molecular Assembly:
    - Assembled as a Y-shaped molecule consisting of four chains.
    - Consists of two light chains and two heavy chains. One light chain is attached to one heavy chain, and the two heavy chain complexes are linked via disulfide bonds.
    - Light Chain Structure: Includes one variable (V) domain and one constant (C) domain.
    - Heavy Chain Structure: Includes one V domain and three or four C domains.

• Antigen-Binding Site:
    - Formed by the interaction of the V regions of both the heavy ($V_H$) and light ($V_L$) chains.
    - A core antibody structure contains two identical antigen-binding sites.
    - Each V region has three hypervariable regions (CDRs), with CDR3 identified as the most variable.

• Antigen Recognition Range:
    - BCRs can recognize a wide variety of chemical structures, including proteins, lipids, carbohydrates, and nucleic acids.

• Isotypes (Classes) of Immunoglobulins:
    - Classes are determined by the sequence of the constant region of the heavy chain.
    - There are five types of heavy chains:
  , 1, 3, 4, 5 and $6$.
    - Heavy Chain Gene Names and Corresponding Isotypes:
      -
  : IgM (Initial BCR)
      - $2$: IgD (Initial BCR)
      - $3, 5, 7, 5$: IgG1, IgG2, IgG3, IgG4
      - $6, 6$: IgA1, IgA2
      - $7$: IgE

• Naive B-Cell Receptors:
    - Mature B cells that have not encountered an antigen express membrane-bound IgM and IgD.

• Heavy-Chain Class (Isotype) Switching:
    - Upon stimulation by antigens and helper T cells, an IgM/IgD expressing clone expands.
    - Progeny may switch to producing other heavy-chain classes (IgG, IgA, or IgE) while maintaining the same antigen specificity.

• Light Chain Types:
    - There are two types:  (kappa) and $$ (lambda).
    - An antibody will have only  or only $$ chains, never both.
    - All antibodies produced by a specific B cell contain the same light-chain type.

Genetic Mechanisms of BCR Rearrangement

• Genomic Loci Locations:
    - Ig Heavy chain locus: Chromosome 14.
    - Ig  chain locus: Chromosome 2.
    - Ig $$ chain locus: Chromosome 22.

• Gene Segments:
    - Variable (V):
  segments for heavy chain, $2$ for , $3$ for $$.
    - Diversity (D): Only found in Ig heavy chain and TCR $\beta$ chain loci. Heavy chain has
  segments.
    - Joining (J): 6 segments for heavy chain, 5 for $8$, $9$ for $$.
    - Constant (C): Consists of exons like C_
  , $C_2$, $C_3$, etc.

• Somatic Recombination Process:
    - Commitment to a B-cell lineage involves randomly selecting and fusing gene segments.
    - Step 1: One D segment joins one J segment to form a DJ complex.
    - Step 2: A V segment rearranges to the fused DJ complex to form a VDJ exon.

• Transcription and Splicing:
    - The VDJ exon is transcribed. In the primary RNA transcript, it is spliced to the C-region exons of the
  chain (the 5’ C region).
    - This results in
  messenger RNA (mRNA).
    - The mRNA is translated into the
  heavy chain, the first Ig protein synthesized in B-cell maturation.

Mechanisms of V(D)J Recombination

• VDJ Recombinase Enzyme:
    - Composed of Recombination-Activating Gene 1 and 2 (RAG-1 and RAG-2) proteins.
    - Expressed only in immature B and T lymphocytes.
    - Recognizes DNA sequences flanking all V, D, and J segments.
    - Brings segments into proximity and cleaves DNA at specific sites.

• DNA Repair:
    - Additional enzymes (non-lymphocyte specific) repair the double-stranded DNA breaks introduced by the recombinase.

Maturation Stages of B Lymphocytes

• Location: Primarily the bone marrow.

• Pro-B Cells: committed progenitors that proliferate and begin rearranging Ig heavy-chain genes.

• Pre-B Cells:
    - Large Pre-B Cells: Selected if they successfully rearrange a heavy-chain gene and express a functional
  heavy chain. This is the first checkpoint.
    - Pre-BCR Complex: Consists of a membrane
  protein associated with two surrogate light chains.
    - Allelic Exclusion: Signal from the pre-BCR shuts off recombination of heavy-chain genes on the second chromosome. This ensures the B cell expresses a receptor of only a single specificity from only one parental allele.
    - Small Pre-B Cells: Initiate Ig light-chain rearrangement.

• Immature B Cells:
    - Assemble a complete membrane IgM receptor.
    - 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 V-J recombination to change its antigen specificity.

• Mature B Cells:
    - Co-express IgM and IgD using the same V regions and specificity.
    - This occurs via alternative splicing of the primary RNA transcript into
  or $2$ mRNA.
    - These are the $IgM^+ IgD^+$ cells found in peripheral lymphoid tissues.

T-Cell Receptor (TCR) and Maturation

• TCR Structure:
    - A membrane-bound heterodimer composed of an
  chain and a  chain.
    - Each chain has one V region (containing 3 CDRs) and one C region.
    - Recognizes only peptide antigens bound to Major Histocompatibility Complex (MHC) molecules.

• TCR Genetics:
    - TCR-  chain loci contain D gene segments;
  chain loci do not.

• Thymic Development:
    - Pro-T Cells (Double Negative): Do not express CD4 or CD8.
    - Pre-T Cells: Successful VDJ recombination of the  chain allows its expression with an invariant pre-T
  protein to form the pre-TCR complex.
    - Checkpoints: Failure to express an
  chain or complete TCR leads to cell death.
    - Double-Positive T cells: Cells that survive and express complete
  eta TCR and both CD4 and CD8 coreceptors.
    - Single-Positive T cells: Final development based on selection; become either CD8+ (Class I MHC-restricted) or CD4+ (Class II MHC-restricted).

• Thymic Selection:
    - Positive Selection: Immature T cells must recognize a self-MHC molecule with low/moderate affinity to survive.
    - Negative Selection: Elimination of strongly self-reactive T cells (recognizing self-peptides/self-MHC with high affinity) to prevent autoimmunity.

Immunological Diversity Statistics

• Combinatorial Diversity:
    - Possible V(D)J combinations: B cells (Ig) $ext{ extasciitilde }3 imes 10^6$; T cells (TCR) $ext{ extasciitilde }6 imes 10^6$.

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

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

Lymph Node Organization

• Physical Channels: Lymph and antigens enter the node sinus via afferent lymphatics.

• Cortex:
    - Primarily a B-cell area.
    - Organized into primary or secondary follicles (containing germinal centers).

• Paracortex: Primarily a T-cell area.

• Anatomical Zones (B-cell concentration):
    - Secondary follicle.
    - Germinal Centre (GC).
    - Mantle Zone (Mn).
    - Subcapsular Zone (SC) - between the capsule and the follicle.

Lymphoma: Characteristics and Classification

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

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

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

• Basis of Classification: Morphologic, immunophenotypic, genetic, and clinical features.

• Monoclonality:
    - Malignancies originate from a single progenitor.
    - All cells in the malignant population possess identical antigen receptor gene rearrangements and express identical BCRs or TCRs.

Comparative Characteristics of Lymphoid Malignancies

• Acute Lymphoblastic Leukemia (ALL):
    - Cell Type: Immature pre-B cells or B cells.
    - Features: Rare; affects young people; very aggressive; replaces bone marrow and invades tissues (brain).
    - Diagnosis: Cells in blood/marrow; flow cytometry to distinguish from AML.

• Chronic Lymphocytic Leukemia (CLL):
    - Cell Type: Mature B cells.
    - Features: Common in elderly; often nonaggressive; cells are present in blood.
    - Diagnosis: Blood film lymphocytosis; specific surface molecules via flow cytometry.

• Lymphoma (General Mature):
    - Cell Type: Mature B cells.
    - Features: Tends to cause solid lesions in lymph nodes or MALT; associated with EBV or chromosomal translocations.
    - Diagnosis: Biopsy of affected tissue.

• Multiple Myeloma:
    - Cell Type: Slow-growing plasma cells.
    - Features: Common in elderly; produces monoclonal immunoglobulin.
    - Diagnosis: Monoclonal Ig in blood or light chains in urine; plasma cells in marrow; osteolytic bone lesions on radiography.

• T-cell Malignancy:
    - Cell Type: T cells.
    - Features: Rare (T cells are less vulnerable to EBV and more prone to apoptosis); caused by HTLV1 infection. Can behave as leukemia or lymphoma.

Pathogenesis of Lymphoma

• Genetic Translocations:
    - Chromosomal breaks/translocations during gene recombination or class switching may not be correctly repaired.
    - MYC Oncogene: Translocation to the Ig heavy chain gene stimulates lymphocyte proliferation.
    - BCL2 Oncogene: Translocation protects against apoptosis, allows unrestrained proliferation.

• Viral Causes:
    - Epstein-Barr Virus (EBV): Immortalizes B cells by driving proliferation and inhibiting apoptosis. Contributes to Hodgkin and Non-Hodgkin types. In malaria-endemic regions, Burkitt lymphoma arises from polyclonal B-cell activation by both malaria and EBV.
    - Human T-lymphotropic virus 1 (HTLV1): Causes rare T-cell malignancies. This retrovirus encodes the Tax protein, which mimics the effects of IL-2 (a T-cell survival cytokine).

Idiotype and Personalized Immunotherapy

• Definition of Idiotype:
    - The shared structural features of antibodies or TCRs of the same specificity produced by a single clone.
    - Represents the unique variable region amino acid sequences (the B/T cell's "fingerprint").

• Idiotype (Ab1) vs. Anti-idiotype (Ab2):
    - Ab1 (Idiotype): The primary BCR/antibody recognizing an antigen; contains the unique "idiotope."
    - Ab2 (Anti-idiotype): An antibody produced against the variable region of Ab1 (the "mirror image").

• Targeted Therapy Logic:
    - Every lymphoma cell in a patient expresses the same unique idiotype (a tumor-specific antigen) not found on healthy B cells.
    - Personalized vaccines involve isolating the unique idiotype from a tumor biopsy, producing it in a lab, and conjugating it to a carrier protein.
    - Injection back into the patient induces anti-idiotype antibodies and T cells to eliminate the malignant clone.
    - Constraint: Idiotype vaccines have struggled in Phase III clinical trials to show consistent long-term efficacy.

Advanced Treatments and Pharmacotherapy

• Standard Modalities: Chemotherapy, immunotherapy, and radiation.

• Monoclonal Antibodies (mAbs):
    - Rituximab: Targets the CD20 protein found on B cells; used for Non-Hodgkin lymphoma.
    - Brentuximab vedotin: An Antibody-Drug Conjugate (ADC). Attaches a chemotherapeutic drug to an antibody targeting CD30 on Hodgkin’s lymphoma cells.

• Radioimmunotherapy: Monoclonal antibodies attached to small radioactive particles to deliver radiation directly to cancer cells.

• CAR T-Cell Therapy (Chimeric Antigen Receptor):
    - Utilizes a single-chain antibody-like extracellular portion ($V_H$ and $V_L$ domains) to bind tumor antigens.
    - Targets commonly include CD19, CD20, and CD30 antigens of lymphomas.

Reference Material

• Content based on Basic Immunology: Functions and Disorders of the Immune System (7th Edition) by Abul K. Abbas, Andrew H. Lichtman, and Shiv Pillai.