Humoral Immune Responses Notes

Humoral Immune Responses

B Cell Biology

  • Adaptive Humoral Immunity: Mediated by B lymphocytes.

  • B Lymphocytes: Adaptive immune cells that develop in the bone marrow; morphologically similar to T lymphocytes.

  • Effector B Cells: Activated B cells differentiate into plasma cells.

  • Plasma Cells: Secrete soluble form of the B Cell Receptor (BCR), known as antibody, which has potent anti-microbial activity.

Key Concepts

  • Innate vs. Adaptive Immunity: The innate immune system is critical early in infection. If overwhelmed, the adaptive immune system takes over. Lymphocytes (and their products; antibodies) are the critical components of the adaptive immune system.

  • Types of Lymphocytes:

    • B lymphocytes (bone marrow-dependent): humoral immunity.

    • T lymphocytes (thymus-dependent): cellular immunity.

  • Lymphocyte Receptors: Each lymphocyte clone expresses a unique receptor with exquisite specificity for antigen.

Clonal Selection and Memory

  • Clonal Selection: A vast array of lymphocyte clones with unique specificities is generated in development (independent of foreign antigen). In an immune response, antigen selects and expands clones of the appropriate specificity.

  • Immunological Memory: Adaptive immune system remembers antigen, leading to bigger and faster secondary responses.

B Cell Receptor (BCR) and Antibody

  • BCR vs. TCR: The antigen receptors of B cells and T cells are distinct.

  • BCR Structure: The B cell receptor is surface-bound antibody. The BCR of Naive B cells are IgM and IgD. Antibody has two identical heavy chains and two identical light chains.

  • Antigen Recognition: Antibodies recognize “free” antigen via the CDRs of the heavy and light chains.

  • Antibody Classes (Isotypes): B cells produce 5 different classes of antibody (IgA, IgD, IgE, IgG, IgM), each with specific functions.

Antibody Production and Uses

  • Monoclonal Antibodies: We can generate monoclonal antibody and produce unlimited amounts of antibody with a single defined specificity.

  • Importance of Antibodies:

    • Protect us from infection with extracellular microbes.

    • Neutralize viruses.

    • Are important in viral infection.

    • Protect us from helminth infestation.

    • Most vaccines work by eliciting long-lived plasma cells and memory B cells.

    • Antibody immunodeficiencies are often fatal if untreated.

    • Monoclonal antibodies against T cell checkpoint inhibitory molecules (CTLA-4 and PD-1) promote cellular immunity in cancer patients.

    • Monoclonal antibodies against cancer antigens are used to kill cancers (eg Rituximab).

    • In some autoimmune diseases B cells are autoreactive and antibodies are pathogenic.

    • In some autoimmune diseases monoclonal antibodies against proinflammatory cytokines (eg Infliximab is aTNF) are used to treat patients.

    • Antibodies are used in diagnosis (eg ELISA to detect prostate-specific antigen in a blood test).

    • Antisera are used to neutralize venoms.

    • Antibodies can provoke allergic (hypersensitive) reactions.

  • Vaccines: Elicit long-lived plasma cells and memory B cells.

B Cell Signaling

  • Key Cellular Events: Primary vs. secondary antibody responses.

  • Phases of Humoral Immune Response:

    • IgM production (if no T cell help).

    • IgG, IgA & IgE Production (With T cell help).
      *Signaling through the B cell receptor is very similar to TCR signaling
      *Enzymes and second messengers
      *Signalling through the B cell receptor is very similar to TCR signaling

B Cell Receptor Signaling

  • Similarity to TCR Signaling: Signaling through the B cell receptor is very similar to TCR signaling.

  • Membrane Proximal Events: Tyrosine phosphorylation activates enzymes and creates docking sites for adaptor proteins, leading to the formation of multimolecular signaling complexes.

  • Second Messengers: Common biochemical second messengers amplify the cell surface signal and transduce it to the nucleus.

  • Transcription Factors: Activation of transcription factors leading to gene transcription.

  • BCR Structure and ITAM Motifs: The B cell receptor has a tiny cytoplasmic domain that is too tiny to signal, therefore it associates with two signaling chains Iga and Igb. Iga and Igb have ITAM motifs in their cytoplasmic domain.

  • Src Family Kinases: There are 3 src family kinases that can phosphorylate the ITAMS of Iga and Igb – Fyn, Lyn, Blk.

  • Syk Kinase: Syk is the tyrosine kinase that binds to the phosphorylated tyrosines of Iga and Igb. Fyn, Lyn or Blk then phosphorylate Syk. Syk then phosphorylates adaptor proteins which in turn activate 2nd messenger pathways.

B Cell Co-receptor Complex

  • Components: The B cell co-receptor complex is comprised of CR2 (Complement receptor, aka CD21), CD19, and CD81.

  • Function: If complement is bound to antigen then CD21 signals via CD19 to lower the threshold of signaling required for B cell activation.
    *Downstream consequences of B cell receptor signalling
    *increased survival, proliferation, antigen presentation and interaction with helper T cells
    *increased expression of cytokine receptors, responsiveness to cytokines
    *increased expression of CCR7, T cell zone Chemokines migration from follicle to T cell zone
    *Generation of plasma cells and IgM, antibody secretion

T Cell Dependent vs. Independent Antibody Responses

  • T-dependent:

    • Antigen: Protein antigen.

    • Cells Involved: Helper T cell, Follicular B cells.

    • Antibody: Isotype-switched, high-affinity antibodies; memory B cells, long-lived plasma cells.

  • T-independent:

    • Antigen: Polysaccharide antigen.

    • Cells Involved: B-1 cells, marginal zone B cells.

    • Antibody: mainly IgM, low-affinity antibodies; short-lived plasma cells

T Cell Independent Antibody Responses

  • Importance: Can be very important in immunity to capsulated bacteria, as it targets the bacteria for phagocytosis.

  • Polyvalent Antigens: Antigens that promote T independent antibody responses are often polyvalent (e.g., polysaccharides). They can cross-link the B cell receptor and induce B cell activation without the need for T cell help.

Pattern Recognition in B Cells

  • PRRs Expression: B cells express pattern recognition receptors.

  • Mechanism: 1 way that T cell-independent antibody responses can occur is that PAMPs, attached to antigens can costimulate B cells. Ie 2 signals synergise – Signal 1 is the BCR – Signal 2 is the PRR.

T Cell Dependent Antibody Responses

*Produce high affinity, isotype switched antibodies, memory and long lived plasma cells
*occur in secondary lymphoid organs
*B cells are organized into B cell follicles
*T cells are in the T cell zones

  • Kinetics of a Humoral T-dependent Immune Response: ISOTYPE SWITCHING - First antibody produced is IgM, this peaks around day 7, Then IgM diminishes as B cells switch to other isotypes (eg IgG).

  • Kinetics of a Humoral T-dependent Immune Response: IMMUNOLOGICAL MEMORY – Primary response peaks around day 14, When re-exposed to the same antigen the secondary response is greater and quicker

  • Kinetics of a Humoral T-dependent Immune Response: AFFINITY MATURATION – Early in the immune response the affinity of antibody produced is low, This improves with time, The affinity of antibody produced in a secondary immune response is generally higher than that produced in the primary immune response

Germinal Centre Reaction

*Site of intense B cell proliferation and differentiation that occurs within the B cell follicles of secondary lymphoid organs during an adaptive immune response
*A feature of T cell dependent antibody responses
*In addition to B cells two other types of immune cell play a role within germinal centres, Follicular Dendritic cells (FDC), T Follicular Helper cells (TFH)

Follicular Dendritic Cells

*They may have the right shape, but they are not conventional Dendritic cells, They arent bone marrow derived but are mesenchymal, They don’t process antigen and present it on MHC
*They act as an antigen depot, intact antigen is stuck on the FDC surface via Fc and complement receptors
*Ag specific B cells acquire antigen from FDC-displayed Ag via the BCR

T Follicular Helper Cells

*Another type of Thelper cell
*Found within B cell follicles
*Specialised in driving B cell proliferation, isotype switching and affinity maturation

Affinity Maturation

*Low affinity antibody is produced early in an immune response
*During the germinal centre reaction random, the enzyme AICD (Activastion Induced Cytidine Deaminase) introduces somatic point mutations occur in the Ig V genes, This process is known as somatic hypermutation
*B cell clones that display high affinity mutations are selected in the germinal centre reaction

Isotype Switching

*T cell cytokines drive isotype switching
*IgM is the default isotype; without T cell help (and cytokines), B cells will produce IgM
*IFNg drives (some subclasses of) IgG
*IL-4 drives IgE
*TGFb drives IgA