Lecture 19: B Lymphocytes and Antibodies Flashcards
Name the different groups of Cytokines
1) Innate (Immune response) Cytokines
2) Adaptive (Immune response) Cytokines
3) Chemokines
4) Haematopoietic Cytokines
What are the different types of innate immune response cytokines?
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Cellular response to viral infection involves the Type I interferons, including
- IFN-α
- IFN-β.
- Response to bacterial infection involves “proinflammatory” cytokines, including
- IL-1, IL-6
- TNF-α.
Describe Innate (Immune Response) Cytokines
(types and how they work)
Innate cytokines are important in viral and bacterial infections.
1) I_nterferon-α (IFN-α_), i_nterferon-β (IFN-β)_, interferon-γ (IFN-γ) inhibit viral infections of other cells and prevents spread of infections.
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Transient Virus Resistance
- If cells are infected by viruses, viruses use the host cell’s ribosomes to produce their protein.
- Our cells detect that the ribosomes are being used for abnormal purposes.
- They can upregulate production and release of Interferon into the near area
- These interferon proteins affect other cells nearby that haven’t been affected and cause change in these cells to go into Transient Anti-viral state where they upregulate certain genes that are attached to ribosomes, that make them difficult to be taken over by Viral messages.
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Natural killer cells
- Activate natural killer cells to recognise virus infected targets and kill them
2) Tumour necrosis factor-α (TNF-α), interleukin-1 and 6 (IL-1 and IL-6) are pro-inflammatory cytokines (esp. in bacterial infections).
- Affect
- Wound healing and tissue repair
- Fibroblast proliferation
- Bone resorption
- Prostaglandin and collagenase synthesis
- Neuroendocrine effect
- Wound healing and tissue repair
Name the different Adaptive Immune Response Cytokines
Adaptive Immune Response Cytokines
- IL-1 is a second messenger to activate T cells following contact with antigen.
- IL-2 stimulates clonal proliferation of antigen-specific T cells.
- IFN-γ causes activation of macrophages, promotes HLA expression, activates NK cells.
- IL-4, IL-5 and IL-6 are important in stimulating the growth and differentiation of B cells.
Describe action of Adaptive (Immune Response) Cytokines
(Don’t need to know all the details)
(Don’t need to know all the details)
Following presentation of antigen by APCs to T cells, IL-1 is secreted by APC as a second messenger for T cell activation. It also involves in B cell activation.
This in turn stimulates secretion of IL-2 by helper T cells which stimulates T cell proliferation. IL-2 is also an important stimulator of NK cell activity and may act with other factors to stimulate B cell growth.
Interferon-γ (IFN-γ) has a_nti-viral_ and anti-proliferative effects in common with IFN-α and IFN-β. IFN-γ also activates macrophages to synthesize enzymes which mediate the respiratory burst allowing macrophages to kill phagocytosed microbes, and activates NK cell activity.
Interleukin-4 (IL-4) promotes B cell growth, activation and clonal proliferation. Functions in switching antibody production from IgM to antibodies of other isotypes such as IgE therefore important in allergy.
Interleukin-6 (IL-6) promotes differentiation of activated B cells to cause them to differentiate into plasma cells. (Transgenic mice who over-express the IL-6 gene develop massive polyclonal proliferation of plasma cells.)
Describe Chemokines
(Don’t need to know details)
Cytokines that function as chemotactic factors are chemokines.
- C-X-C chemokines (e.g. IL-8) are produced at sites of inflammation promoting chemotaxis of neutrophils (previously called neutrophil chemotactic factor).
- Neutrophil migration
- C-C chemokines (e.g. MCP-1 (monocyte chemotactic protein) and MIP-1α (macrophage inflammatory protein)) play an important role in allergic inflammation by stimulating basophils to release histamine.
- Allergic Inflammation
Describe Haematopoietic Cytokines
(Don’t need to know details of each one- just know general things)
Haematopoietic Cytokines
Haematopoietic cytokines act primarily to promote growth and differentiation of various lineages of haematopoietic cells. Often termed colony stimulating factors (CSF).
- G-CSF, M-CSF and GM-CSF stimulate proliferation of granulocytes, monocytes (lineage-specific control).
- Interleukin-3 (IL-3) is also known as multi-CSF as it promotes proliferation of all lineages of haematopoietic cells.
- Interleukin-5 (IL-5) stimulates eosinophil growth and activation and probably plays an important role in allergic responses.
- Interleukin-7 (IL-7) stimulates erythroblast and megakaryocyte growth and involved in B cell development (ontogeny) in bone marrow.
Describe the Antibody Structure
Antibodies or immunoglobulins (Ig) constitute approximately 1% of blood proteins. Four polypeptide chains held together by disulphide bonds and non-covalent interactions.
- Light chains can be one of two types, include a constant and variable regions.
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Heavy chains are made up of variable and constant chains
- There are also hinge regions- give flexibility
- The variable regions in the light and heavy chains make up the epitope/antigen binding site (two identical binding sites on each arm)
- The two regions in the heavy chain form the c_omplement bidning region._
- Bottom part of the heavy chain is the Fc region.
- __receptors on various cells- particuarly in the neutrophils bind to Fc regions. This allows them to bind and ingest what the antibodies bind to
What are the different isotypes of the light and heavy chains?
- Light chains can be one of two types, include κ (kappa) or λ (lambda).
- Heavy chains are one of 5 isotypes, which defines antibody class.
- They are γ (gamma), μ (mu), α (alpha), ε (epsilon) and δ (delta), giving rise to IgG, IgM, IgA, IgE & IgD respectively.
- Class type (i.e. heavy chain) governs other effector functions of antibodies.
We make ____ classes of antibodies.
The differences between them are….
5 (IgG, IgM, IgA, IgE & IgD)
They all have the same fundamental structure, but the differences lie in the amino acid sequences in the constant region.
They all bind to antigens, what they do with the antigens depend on the amino acid seuqences
A 10 month old NZ European infant girl attends the paediatric outpatient department because the Plunket nurse and her mother are concerned that she is not gaining enough weight. Her weight at the age of 6 months was on the 25th percentile and now is on the 2nd percentile.
Started to develop recurrent infections, including pneumonia, otitis media (inflammation of middle ear), erysipelas (streptococcal skin infection)
Serum immunoglobulin tests shows:
- IgG: _8g/l (n_ormal 60 - 150)
- IgA: 0 g/l (normal 5 - 12.5)
- IgM: 1g/l (normal 7.5 - 15)
Few detectable B cells and no tonsils
1) What are some possible diagnoses?
2) Why no tonsils?
3) Why was she well for 10 months?
4) Treatment?
1) Possible diagnosis is X-linked agammaglobulinaemia
2) Genetic defect is stopping her from producing B cells. B cells form a lot of the lymphoid tissue in the tonsils. Therefore without these, the tonsils are smaller.
3) As we’re developing as fetus, maternal IgG passes via the planceta. When we’re born, they decay over a period of time, (but can get some more during breastfeeding)- passively transferred immunity. (pic). So in the first 10 months, this defect may not be clinically seen.
4) Give her antibodies from donor serum (cover lots of spectrum) or give her a bone marrow transplant (without the genetic defect)
Describe the Clonal selection of antigen-sensitive B lymphocytes
Surface immunoglobulin allows B cells to bind and internalize antigens.
Internalized antigen is processed and antigenic fragments expressed on surface by class II MHC, where they can be recognized by helper T cells.
This then leads to T helper cell (co-stimulator CD40 and CD40-L) produce cytokines (e.g. IL-4) necessary for activation of B cell to generate antibody-forming cells (or plasma cells) or memory cells
Name the functions of the antibodies/effects
Functional properties of antibodies include
- blocking and direct neutralization,
- agglutination,
- opsonization,
- antibody-dependent cell-mediated cytotoxicity,
- complement activation.
Describe how viruses undergo Blocking and Direct Neutralization
It prevents attachment and entry (viruses, parasites, etc.). Main protective effect of secretory IgA.
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Attachment and Entry
- Viral infection initially requires attachment of virus particles to its target cells through specialized protein or glycoprotein structures on surface of virus particle, which are able to recognize and interact with specific cell-surface components (such as hormone receptors, for example) on cells of target tissue. Antiviral antibodies can physically prevent adsorption to target cells and thus effectively neutralize the virus.
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Toxins
- Similarly, action of bacterial and plant toxins is usually dependent on their attachment to specific sites on susceptible cells and subsequent transport into these cells. Antitoxin (tetanus, diphtheria) antibodies can inhibit this attachment.
- It can also _immobilize bacterial fla_gella.
Describe Agglutination- effect of antibodies
Phagocytes are not very efficient at clearing very small foreign substances on their own, but can be greatly assisted if antibodies agglutinate small particles into larger complexes (assist phagocytosis).
(right pic)- shows that the size of the clumps depend on the ratio of the antigens-antibodies. If there are too many antibodies relative to antigens, there’ll be reduced agglutination because all of the antigen binding sites are saturated with 1 or 2 antigens.
Sometimes these clumps can get lodged in our capillaries and cause damage
This is especially effective for IgM.
Describe Opsonisation
Granulocytes and macrophages have surfaces receptors for Fc region of immunoglobulin molecules and for C3b component of complement.
Through these receptors, phagocytes are able to recognize and bind to antigen-antibody complexes with high affinity. As a result, their phagocytic efficiency is enhanced.
IgG
Describe Antibody-dependent cellular cytotoxicity- effect of antibodies
A small population of lymphocytes in blood which do not have classical B or T cell characteristics and are referred to as K cells.
They have Fc receptors and C3b receptors on their surface.
- K cells are not phagocytic, but instead kill cellular material to which they become bound through antibody or C3b mediator by delivering short-range cytotoxicity factors.
- ADCC killing is a property of large granular leukocytes (LGL) (CD16, CD56, CD57).
Describe the Complement Activation
Blood contains a group of approximately 20 proteins (collectively called complement), which can be activated by:
- Either _antigen-antibody complexes (_immune complexes);
- Or in antigen-independent manner by components of some bacterial cell walls and yeasts.
Complement activation results in a cascade of highly-specific proteolytic enzymes which, in turn, cleave other complement proteins converting them to active forms, eventually resulting in formation of membrane-attack complex C3b
The activation of the complement system is closely analogous to the proteolytic cascade involved in blood clotting.
Describe the different complement proteins
Complement Proteins
Central and most abundant complement protein is C3.
Cleavage of C3 into C3a and C3b stimulates sequential events, which lead to formation of a membrane-attack complex and to other biological effects.
- Small peptide C3a is a potent anaphylatoxin, stimulating mast cells to release histamine, which causes increased v_ascular permeability a_nd vasodilation.
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C3b is a very reactive molecule, which has a capacity to bond covalently to other molecules or to cell surfaces in its immediate vicinity.
- Short half-life of C3b intermediate ensures that C3b is only deposited near site of complement activation. This provides an important control of a process that can potentially damage or kill cells.
- C3b can be recognized by receptors on phagocytic cells resulting in enhanced efficiency of phagocytosis (opsonisation). C3b also form a focus for assembly of late complement components leading to cell lysis.
C3 can be depleted in chronic infection, and individuals’ unfortunate enough to be born with a genetic C3 deficiency suffer recurrent infections.
What are the 4 effects of Complement Activation?
1) lysis
2) Opsonisation
3) Chemotaxis
4) Anaphylaxis
What is the difference between Classical and Alternative pathway of the complement cascade?
Classical Pathway of Complement Cascade
- Classical pathway triggered exclusively by CH2 immunoglobulin domain of antigen-antibody complexes (immune complexes).
- C1q, C1r and C1s then bind to this domain to form an active protease, C1-bar, which initiates the activation process of classical pathway. (By convention, active complement components have a bar superscript.)
- C1-bar then cleaves C2 (when bound to C4) giving another protease C42-bar, which is C3 convertase of classical pathway.
Alternative Pathway of Complement Cascade
- Another complement components that recognise certain pathogen surfaces which forms C3 convertase.
- Alternative pathway is most important in early stages of infection with some bacteria and yeast. C3 convertase of alternative pathway is Mg2+-dependent complex of two proteins C3b and factor B, activated by a protease called factor D.
- This reaction is initiated by presence of trace amounts of C3b formed by small amounts of spontaneous C3 breakdown (called C3 ‘tickover’).
Describe antibody class switching
IgM is the first class of antibodies to be synthesized in a primary antibody response. But after a time, IgM synthesis begins to decrease and is replaced by production of IgG.
- Individual activated B cell clones begin by producing IgM but subsequently switch to IgG production.
- But because any particular B lymphocyte is committed to only one type of antigen-binding specificity, this does not change appreciably during immunoglobulin class switch.
- IgG synthesis rather than IgM that is dramatically increased in _secondary response_s because memory B cells preferentially make IgG.
- IgM component of secondary response is mostly due to activation of _new naive antigen-sensitive B cells s_eeded to secondary lymphoid organs from bone marrow after the initial contact with antigen.
Describe IgM
- Pentamer (largest antibody molecule) (10 antigen binding sites)
- Confined to bloodstream and lymphatics, does not cross placenta.
- 10% total serum antibody.
- _Produced first i_n primary antibody responses
- Very effective agglutinator and c_omplement activator._
- Important defence against blood-borne spread of infectious organisms such as bacteria
Describe IgG
- Small monomer, diffuses easily out of blood into extravascular areas, crosses placenta.
- 70-75% total serum antibody.
- Major class in secondary responses
- Good complement activator,
- Ppsonisation and Fc receptor-mediated effector mechanisms (strongly bound by phagocytic cells).
- Potent antitoxin antibody,
- Effective barrier against v_irus infections._
Describe IgA
- Dimmer.
- 15-20% total serum antibody
- Predominant class in _sero-mucus secretion_s
- Protection of external body surfaces; surface protection of gut, respiratory and genitourinary tracts.
Describe IgD
- Monomer.
- Trace amounts in blood and other body fluids.
- Highly sensitive to proteolysis.
- Found on surface of antigen-sensitive naïve (virgin) B lymphocytes; receptor for antigen binding to activated naïve B lymphocytes.
Describe how Antibodies can be used in diagnostic and therapeutic ways
Antiserum is a blood serum containing antibodies against s_pecific antigens,_ injected to treat or protect against specific diseases.
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Monoclonal antibodies (mAb or moAb) are antibodies that are made by _identical immune cell_s that are all clones of a unique parent cell.
- Single epitope recognised and single affinity
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Polyclonal antibodies (pAbs) are antibodies that are secreted by different B cell lineages within the body (whereas monoclonal antibodies come from a single cell lineage).
- Problem: Range of B cells are stimulated- range of epitopes are recognised in range of affiniities
