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Functions of B cells and antibodies

Kinetics of B Cell and Antibody Responses

  • Primary Response:
    • Involves activation of naïve B cells.
    • Characterized by unswitched Ig (immunoglobulin).
    • Results in short-lived plasma cells (PCs).
    • Leads to the generation of memory B cells.
  • Re-call (Memory) Response:
    • Involves activation of memory B cells.
    • Characterized by switched Ig.
    • Faster time to peak antibody (Ab) level.
    • Higher affinity due to post-germinal center (GC) activity.
    • Greater magnitude, producing higher levels of antibodies.
    • Homeostatic contraction occurs after each response.

Molecular Structure of Immunoglobulins

  • Antibodies consist of fragments:
    • Fragment antigen-binding (Fab): Binds to the antigen.
    • Crystallisable fragment (Fc): Involved in innate cell activation, transcytosis/transport, stability (half-life), and complement binding.
    • Adjoining hinge region: Provides flexibility.
    • Complementarity-determining regions (CDRs) on Fab bind antigen.
    • The site on the antibody that binds to the antigen is known as the paratope.
    • The site on the antigen that is bound by the antibody is known as the epitope.
    • Fc fragment functions to:
      • Bind Fc receptors (innate cell activation)
      • Facilitate transcytosis/transport
      • Mediate stability (half-life)
      • Bind complement

Antibody Binding to Antigen

  • Non-covalent (reversible) binding:
    • Involves hydrogen bonds, van der Waals forces, and hydrophobic interactions.
  • Affinity:
    • The strength of binding between a single paratope and an epitope.
  • Avidity:
    • The combined strength of binding between antibodies and antigens through multiple points of contact.
    • Allows antibodies with low affinity to bind effectively to pathogens.
    • IgM (primary response)

Structural Forms of Human Immunoglobulin Isotypes

  • Immunoglobulins comprise 5 isotypes: IgM, IgD, IgG, IgA, and IgE.
  • IgA and IgG have additional subclasses: IgA1, IgA2, IgG1, IgG2, IgG3, IgG4.
  • IgA can assume monomeric, dimeric, or trimeric forms and is involved in mucosal immunity.
  • IgM can form pentamers.
  • IgE is monomeric and involved in immunity to helminth parasites.
  • IgG can be monomeric (IgG2 can be dimeric) and provides immunity to a wide range of pathogens in the periphery.

Transcytosis of IgA and IgM

  • The polymeric Ig receptor facilitates transcytosis of dimeric IgA and pentameric IgM through epithelial barriers.
  • Binding of the poly-Ig receptor results in endocytosis of antibodies at the mucosal end and release of the antibodies at the luminal surface.

Transcytosis of IgG

  • The neonatal Fc receptor (FcRn) facilitates transcytosis of IgG from:
    • Maternal circulation to fetal circulation (across syncytiotrophoblasts).
    • Lamina propria to the lumen of the gut (across enterocytes).
    • Upper airway to blood (across airway epithelium).
  • FcRn is also expressed on endothelial cells and macrophages, where it rescues IgG from degradation, extending its circulating half-life (approximately 23 days), except for IgG3 (half-life = 7 days).

Antibodies and Vaccines

  • Antibodies underlie the success of most vaccines.

Antibody and Complement Mechanisms

  • Neutralization:
    • Of microbes and toxins.
  • Opsonization and phagocytosis:
    • Of microbes.
  • Complement activation:
    • Leads to phagocytosis of microbes opsonized with complement fragments (e.g., C3b).
    • Induces inflammation and lysis of microbes.

Antibody Activation of Effector Cells Through Fc Receptors

  • Various cell types express different Fc receptors, which are activated by specific Ig isotypes.
    • Natural Killer cells: FcγRIIIa, FcγRIIc (in ~45% of individuals) for IgG1 and IgG3.
    • Monocytes: FcγRI, FcγRIIa (FcγRIIc) for IgG1, IgG2, and IgG3.
    • Dendritic cells: FcγRIIa for IgG1 and IgG2.
    • Basophils/Mast cells: FcεRI for IgE.

Classification of FcγRs

  • Activating vs. Inhibitory receptors:
    • Activating receptors contain Immunoreceptor tyrosine-based activation motifs (ITAM) or ITAM-containing adaptor molecules.
    • Inhibitory receptors contain Immunoreceptor tyrosine-based inhibitory motifs (ITIM).
  • High vs. Low affinity:
    • High-affinity receptors bind monomeric IgG.
    • Low-affinity receptors bind immune-complexed IgG.
    • FcγRI is likely saturated with IgG at steady state and still requires cross-linking with antigen for receptor activation.

Antibody Neutralization

  • Neutralization by antibodies includes:
    • Inactivation of pathogenic effects of toxins (e.g., Tetanus and Diphtheria toxins).
    • Inhibition of an intracellular pathogen from entering a susceptible cell.
    • Inhibition of pathogens from crossing mucosal barriers.
  • Antibodies inhibit entry of pathogens into a cell by:
    • Binding critical sites necessary for pathogen-host membrane fusion.
    • Steric hindrance (occupancy).

Antibody-Dependent Cellular Cytotoxicity (ADCC)

  • ADCC occurs when antibodies opsonize whole cells and direct Natural Killer cell-mediated lysis.
    • Cells expressing tumor antigens.
    • Cells expressing pathogen-derived antigens.
    • Cells with attached pathogens on the surface.
  • NK cells mediate ADCC through FcγRIIIa and (in 45% of individuals) through FcγRIIc.
  • IgE-opsonised parasites are lysed by eosinophils.
  • Eosinophils recognise complexed IgE via FcεRII.

Antibody-Mediated Phagocytosis

  • Antibodies opsonise pathogens, directing immune cells to phagocytose through either FcγRI and FcγRIIa.
    • Monocytes/macrophages (MΦ), dendritic cells (cDC and pDC), neutrophils.
  • Phagocytosis results in:
    • Uptake and degradation of microbes (reactive oxygen species).
    • Pro-inflammatory cytokine production (TNF, IL-6), type I interferon (pDC).
    • Presentation of antigens to prime naïve T cells (DC).
  • Opsonophagocytic antibodies are important in immunity against encapsulated bacteria (e.g., Streptococcus pneumoniae), primarily mediated by IgG2 antibodies.

Complement-Mediated Phagocytosis and Complement Fixation by Antibodies

  • Complement molecules are also opsonins.
  • C3b and C4b opsonisation of microbes leads to phagocytosis through complement receptor 1 (CR1).
  • Pentameric IgM and immune-complexed IgG1/IgG3 arranged in a hexameric structure bind C1q.

Activation of the Classical Complement Cascade Initiates with Antibody Recruitment of C1q

  • The classical pathway, lectin pathway, and alternative pathway converge to activate complement, leading to the cleavage of C3 and C5, resulting in opsonisation, inflammation, and lysis of microbes.

Antibody Effector Mechanisms

  • Most effector functions require complexed antibodies to be initiated.
  • Antibody subclasses exhibit differing binding characteristics to each FcγR, which are differentially expressed across innate cells and some lymphocytes.
  • Effector function is fine-tuned by class-switch recombination.
  • Antibody effector mechanisms are not mutually exclusive; a neutralising antibody can also exhibit opsonophagocytic and ADCC activity.
  • Antibodies provide specificity to innate cells.

Regulation of B Cell and Antibody Responses by FcγRIIb

  • FcγRIIb is the only known inhibitory Fc receptor in humans.
  • In B cells, FcγRIIb ITIM signalling overcomes activation signals of the BCR.
  • In plasma cells, FcγRIIb ligation by immune complexes induces apoptosis.

Intravenous Immunoglobulin (IVIg) in the Treatment of Autoimmunity

  • Serum IgG fractions pooled from thousands of healthy donors.
  • Used in treatment of some autoimmune diseases:
    • Alloimmune thrombocytopenia.
    • Kawasaki’s disease.
    • Guillain-Barre syndrome.
  • Reduces the half-life of pathological antibodies by competing for FcRn.
  • Interferes with binding between activating FcγRs and pathological antibodies.
  • Binds FcγRIIb and upregulates its expression.

Summary: B Cell Function

  • There are two phases of B cell activation: primary and re-call.
  • The primary response occurs through the activation of naïve B cells; the re-call response occurs through the activation of memory B cells.
  • The re-call response exhibits heightened kinetics, affinity, and magnitude compared to the primary response.
  • Antibodies are secreted in mucosal sites and transported by specific receptors.

Summary: Antibody Function

  • Antibodies consist of isotypes and subclasses.
  • Antibody subclasses have unique binding characteristics to Fc receptors, which are differentially expressed across immune cells.
  • Protective mechanisms of antibodies include neutralisation, ADCC, opsonophagocytosis, and complement fixation.
  • FcγRIIb facilitates a negative feedback loop between antibody levels and B cell activation/plasma cell survival.