L2: The Immune Responses----Specificity and Memory

Learning Objectives

  • Understand the development of immune responses including the origin of immune cells and their functions

  • Define the specificity and memory of immune responses

  • Understand the principles of monoclonal antibody and yeast antigen display

Essential Cells for Immune Response

Lymphocytes

  • mediators of adaptive immune responses

  • only cells with specific receptors

Naive

  • mature lymphocytes that have not previously encountered antigens

  • Function: recognise antigen

  • → lymph nodes: concentrated with antigens for immune response

Effector

  • activated lymphocytes: eliminate microbes ✔︎ (effector function)

  • Effector T cell

    • Helper cells: secrete cytokines

    • Killer cells: eradicate infected cells

  • Plasma cells: secrete antibody

  • other leukocytes: granulocytes (neutrophils and eosinophils) and macrophages

Memory

  • long lived

  • functionally silent cells

  • secondary response: rapid response to antigen

Antigen-presenting Cells (APCs)

Characteristics

  • different cell type in innate immunity linking to adaptive immunity

  • activate T helper cells

  • rich in MHC II molecule

  • specialised to capture, concentrate and display antigens for recognition by lymphocytes

Dendritic cell

  • uptake antigens by macropinocytosis and phagocytosis / viral infection

  • MHC expression: low on tissue DC; high on DC in lymphoid tissue

  • co-stimulator delivery: constitutive by mature, nonphagocytic lymphoid DC

  • locate in T cell area in lymph node (around GC)

Macrophage

  • bacterial infection

  • uptake antigens by phagocytosis

  • MHC expression induced by bacteria & cytokines

  • locate throughout lymph nodes

  • co-stimulator delivery: inducible

B cell

  • microbial toxin on antibodies

  • uptake antigens by antigen-specific receptor (IgM)

  • MHC expressed after activation

  • locate in follicle of GCs in lymph nodes

  • co-stimulator delivery: inducible

CD Nomenclature

  • Cluster of differentiation: structurally defined leukocytes surface molecule expressed on cells of a particular lineage and recognised by a group of cell-specific antibodies

  • function of antibodies against CD molecules

    • identify and isolate leucoyte subpopulation

    • study functions of leukocytes

    • eliminate particular cell population

Major distinguishing CD markers

  • B cells

    • CD19/CD20

    • CD40

  • CTL

    • (CD3)

    • CD8

  • Th cells

    • (CD3)

    • CD4

    • CD69

    • CD25

  • Memory T cell

    • CD44

    • CD127

    • CXCR4

Specificity

Antigen

Characteristics

  • molecules that the body recognise as foreign → attack

  • include components of bacterial cell wall, capsule, pili, flagella and proteins of virus, fungi and protozoa

  • food and dust: antigenic particles

Epitope

  • also called antigenic determinants

  • three-dimentional regions on antigens to be recognised

  • source

    • exogenous (from extra cellular microbes)

    • endogenous (intracellular virus)

    • autoantigens (tumour cell)

Receptor

  • receptors on B and T cells are unique for a particular antigenic determinant on antigens

  • TCR: monovalent

  • BCR: divalent (but recognise the same Ag)

Origin

  • Instructionist hypothesis

    • single AgR germline

    • ╳ explain self vs non-self

  • Clonal selection hypothesis

    • AgR pre-formed on B and T cells

    • Ag select clones with correct receptors

    • well explained features of immune responses

      • specificity

      • signal required for activation

      • lag in adaptive immune response

      • discrimination between self and non-slf

Clonal selection

  • Clone: population of cells derived from a single progenitor cell

Principles

  • each lymphocyte has a single type of AgR

  • high affinity between antigen and AgR → activation

  • differentiated effector cell: same AgR as the parental lymphocytes (clone)

  • lymphocytes with AgR for self molecules: deleted early and absent from repertoire

Phases of adaptive immune response

  • Antigen recognition: APCs present antigens to naive B and T cells → clonal expansion and differentiation

  • lymphocyte activation: plasma cell and CTL

  • antigen elimination

    • humoral immunity by antibodies produced by plasma cells

    • cell-mediated immunity by CTL

  • contraction: apoptosis of effector lymphocytes

  • memory: memory cells survive

Memory

Types of acquired immunity

  • Naturally acquired: immune response against antigens encountered in daily life

  • Artificially acquired: respond to antigen introduced via vaccine

  • Active response:

    • via humoral or cell-mediated response

    • long-lasting protection

    • ✔︎ lag time

  • Passive response:

    • receive Abs from another individual

    • rapid protection

    • short duration

    • no memory

  • Examples

    • active + natural: infection

    • active + artificial: vaccine

    • passive + natural: IgG via placenta aor IgA from breast milk

    • passive + artificial: antisera and antitoxin

B cell

Activation

  • naive B cell encounter antigen → lymphoblast

  • clonal expanstion / proliferation

  • differentiation into effector B lymphocytes and memory B cells

  • B cells with low affinity→ apoptosis

Memory response

  • primary response

    • lag period

    • low conc of IgM

    • class switching → IgG with higher concentration

  • secondary response

    • no lag period

    • IgM: low conc; IgG high conc

Memory T cell

Development

  • antigen presentation

    • APC presents epitope to TCR of Th cell and inactive Tc cell

    • APC releases IL-12 → Th

  • Th differentation → release IL-2

  • Clonal expansion: IL-2 binds to IL-2R on inactive Tc cell to trigger prolifereation and differentiation to memory T cell and active Tc cells

  • self stimulation: more IL-2 is released to activate Tc cells themselves to further proliferate

Important application

Monoclonal antibody

Strategy to generate

  • introduce antigen with particular epitopes to a mouse

  • isolate spleen cells to collect plasma cells

  • hybridise with myeloma cells to form hybridomas (keep the specificity with immortality)

  • select hybridomas specific to different epitopes and cloning

  • produce monclonal antibodies

Cloning hybridomas from fusion

  • plate hybridomas at limiting dilution in 96 well plate

  • allow clones to expland and select the positive well

  • further expand the positive well and test for production of Ab of desired specificity in culture supernatant

Phage display

  • assemble desired gene selected from patient or gene library into phagemid and introduce it to the phage library

  • phage expresses antibody fragment with the coat protein

  • binding to immobilised antigen

  • isolate high-affinity binder

Yeast display

  • Gene Assembly: Desired genes are assembled into expression vectors suitable for yeast cells.

  • Transformation: The expression vectors are introduced into yeast cells, allowing them to express the protein or peptide on their surface.

  • Binding: The yeast display the proteins or peptides, which can interact with specific antigens or targets.

  • Selection: Yeast cells that bind to the target antigen are selected

  • Screening and Amplification: The selected yeast clones are further screened for binding affinity, and successful clones can be grown in larger cultures to produce the desired proteins.