Lymphocytes

Question 1

What are the 4 properties of the adaptive immune system?

Answer 1

1. Improves the efficacy of the innate immune response
2. Focuses a response on the site of infection and the organism responsible
3. Has memory
4. Needs time to develop

Question 2

What is immunological memory and what is it based on?

Answer 2

Once the immune system has recognised and responded to an antigen, it exhibits “memory” as a consequence of clonal selection. Antigen-specific lymphocytes (B + T) are the cellular basis. Memory responses are characterised by a more rapid and heightened immune reaction that serves to eliminate pathogens fast and prevent diseases. Can confer life-long immunity to many infections and is the basis for vaccines.

Question 3

What are the 2 types of adaptive immune response?

Answer 3

Cell-mediated immunity uses T-cells where cytokines and killing cells are the effector molecule and function respectively. Humoral immunity uses B-cells where antibodies are the effector molecules.

Question 4

How are antigens recognised by different lymphocytes?

Answer 4

T-cells recognise linear epitopes in the context of MHC while B-cells recognise structural epitopes.
Antigens: Molecules that act induce an adaptive immune response (mostly protein)
Epitope: The region of an antigen which the receptor binds to.

Question 5

What is the process of clonal selection?

Answer 5

1. Each lymphocyte bears a single, unique receptor
2. Interaction between a foreign molecule and that receptor leads to activation
3. Clonal Selection//Expansion
4. Differentiated effector cells of that lineage will bear the same receptor

Self-specific receptors are deleted early in development

Question 6

How is diversity of antibodies produced?

Answer 6

Functional genes for antigen receptors do not exist until they are generated during lymphocyte development. Each BCR receptor chain (kappa, lambda and heavy chain genes) is encoded by separate multigene families on different chromosomes. During B cell maturation these gene segments are rearranged and brought together. This process is called Immunoglobulin gene rearrangement.

Question 7

Describe the TCR

Answer 7

The T cell receptor part of a complex of proteins on the cell surface. The variable region made by gene reassortment (1015 – 1020). Recognizes antigen fragments presented by other cells.

Question 8

What is the MHC?

Answer 8

The Major Histocompatibility Complex (MHC) plays a central role in defining self and not self. Encoded by HLA genes in humans. Presents antigens to T cells. Critical in surgery- and donor matching.

Question 9

Describe MHC gene expression

Answer 9

The MHC is polygenic: several class I and class II loci. Expression is co-dominant (maternal and paternal genes both expressed) 
MHC class I: all nucleated cells, although at various levels: levels may be altered during infection, or by cytokines 
MHC class II: normally only on “professional” antigen presenting cells: may be regulated by cytokines

Question 10

Which cells bind to the different MHC molecules?

Answer 10

There are two types of T-cells: CD4 (helper) and CD8 (killer). CD4 cells bind MHC II while CD8 binds MHC I - defined by cell surface molecules. MHC is encoded by HLA genes: genes controlling MHC I are A,B and C. Genes controlling MHC II are DP, DQ and DR. Thus, with three MHC class I genes and a possible three sets of MHC class II genes on each chromosome 6, a human typically expresses six different MHC class I molecules and eight different MHC class II molecules on his or her cells.

Question 11

What are the differences in the distribution of MHC I and MHC II?

Answer 11

MHC I is present on almost all nucleated cells including platelets. MHC II has a restricted tissue distribution and are chiefly found on macrophages, dendritic cells, B cells, and other antigen presenting cells only.

Question 12

What are the differences in the encoding genes of MHC I and MHC II?

Answer 12

MHC class I proteins are encoded by the HLA-A, HLA-B, and HLA-C genes. MHC Class II proteins are encoded by the genes of HLA-D region.

Question 13

What are the differences in the nature of presented antigens of MHC I and MHC II?

Answer 13

Antigens presented by MHC class I molecules are of endogenous origin. Antigens presented by MHC class II molecules are derived from extracellular proteins.

Question 14

What are the differences in the presented antigens of MHC I and MHC II?

Answer 14

MHC I presents cytosolic proteins; they sample peptides generated within the cell or those that may enter cytosol from phagosomes. Class II molecules sample peptides outside the cell such as lysosomal proteins mostly internalized from extracellular environment.

Question 15

What are the differences in the enzymes involved in peptide generation of MHC I and MHC II?

Answer 15

MHC I uses cytosolic proteasome. MHC II uses endosomal and lysosomal proteases.

Question 16

What are the differences in where peptides are loaded and the peptide loading complex of MHC I and MHC II?

Answer 16

Peptide loading of MHC I occurs in the ER. Peptide loading complex includes the ER transporter associated with antigen processing (TAP1/2), tapasin, the oxidoreductase ERp57, and the chaperone protein calreticulin. Peptide loading of MHC II occurs in a specialized vesicular compartment. Peptide loading complex includes chaperones in ER; invariant chain in ER, Golgi and MHC Class II compartment/Class II vesicle.

Question 17

What are the differences in the recognising co-receptors of MHC I and MHC II?

Answer 17

MHC I is recognized by CD8 co-receptors through the MHC Class I β2 subunit. MHC II is recognized by CD4 co-receptors through β1 and β2 subunits.

Question 18

What are the differences in the structure and amino acids of MHC I and MHC II?

Answer 18

MHC class I molecules consist of one membrane-spanning α chain produced by MHC genes, and one β chain produced by the β2-microglobulin gene. Possess 8-10 amino acids. MHC class II molecules consist of two membrane-spanning chains, α and β both produced by MHC genes. Possess 13-18 amino acids.

Question 19

What are the differences in the peptide binding domains and invariant chains of MHC I and MHC II?

Answer 19

α1 and α2 are peptide binding domains in MHC I molecules. Has no invariant chain. α1 and β1 are peptide binding domains in MHC II molecules. Has a variant chain.

Question 20

What are the differences in the functional effect and detection method of MHC I and MHC II?

Answer 20

Presence of abundant antigens detected by MHC I targets cell for destruction. Detection method is serology. Presences of foreign antigens detected by MHC II induces antibody production. Detected by serology and mixed lymphocyte reaction.

Question 21

What are the differences in processing of intracellular pathogen vs Extracellular pathogen

Answer 21

IC pathogen is processed in the cytosol and presented on MHC I molecules to CD8 T-cells. CD8 binds the alpha-3 domain of MHC Class I on the target cell. EC pathogen is processed in endosomes and presented on MHC II molecules to CD4 T-cells. CD4 binds the beta-2 domain of MHC Class II on the target cell.

Question 22

Provide an overview of how CD8 T-cells act?

Answer 22

Cytotoxic T cells (CTL) kill their targets by programmed cell death. Apoptosis is characterized by fragmentation of nuclear DNA. CTL store perforin, granzymes, granulysin in cytotoxic granules released after target recognition. Perforin molecules polymerise, form pores

Question 23

What are the three major functions CD8+ T-cells carry out post activation?

Answer 23

The first is secretion of cytokines, primarily TNF-α and IFN-γ, which have anti-tumour and anti-viral microbial effects. The second is the production and release of cytotoxic granules. These granules, also found in NK cells, contain two families of proteins, perforin, and granzymes. The third major function of CD8+ T cell destruction of infected cells is via Fas/FasL interactions.

Question 24

Describe the role of perforin and granzymes

Answer 24

Perforin forms a pore in the membrane of the target cell, similar to the membrane attack complex of complement. This pore allows the granzymes also contained in the cytotoxic granules to enter the infected or malignant cell.
Granzymes are serine proteases which cleave the proteins inside the cell, shutting down the production of viral proteins and ultimately resulting in apoptosis of the target cell. The cytotoxic granules are released only in the direction of the target cell, aligned along the immune synapse, to avoid non-specific bystander damage to healthy surrounding tissue. CD8+ T cells are able to release their granules, kill an infected cell, then move to a new target and kill again, often referred to as serial killing.

Question 25

How do Fas/FasL interactions destroy infected cells?

Answer 25

Activated CD8+ T cells express FasL on the cell surface, which binds to its receptor, Fas, on the surface of the target cell. This binding causes the Fas molecules on the surface of the target cell to trimerise, which pulls together signalling molecules.
These signalling molecules result in the activation of the caspase cascade, which also results in apoptosis of the target cell. Because CD8+ T cells can express both molecules, Fas/FasL interactions are a mechanism by which CD8+ T cells can kill each other, called fratricide, to eliminate immune effector cells during the contraction phase at the end of an immune response.

Question 26

What do T helper cells produce?

Answer 26

T Helper cells produce cytokines (a family of inflammatory mediators). Cytokines have diverse actions on a wide range of cells. Cytokines influence the outcome of the immune response.

Question 27

What are the different CD4 cell classes, the cytokines they produce and their function?

Answer 27

Tfh: Pro-Antibody
IL-21
Th17: Pro-Inflammatory, Control bacterial and fungal infection
IL-17, IL-23, IL-6
Th1: Pro-Inflammatory; Boost cellular immune response
Interferon gamma, tumour necrosis factor(TNF), IL-12
Th2: Pro-Allergic; boost multicellular response
IL-4, IL-5, IL-13
Treg (Th0): Anti-inflammatory; limit the immune response
IL-10, TGF-beta

Question 28

What can uncommitted CD4 cells do?

Answer 28

Uncommitted CD4+ T-cells can differentiate into Th1 or Th2 cells, based on the prevailing pro-inflammatory /anti-inflammatory environment, and that these activated Th1 and Th2 cells had distinct cytokine production patterns and functions. Uncontrolled Th1 responses were implicated in autoimmunity and aberrant Th2 responses were associated with allergy and asthma development

Question 29

What are the different T-cell classes activated by?

Answer 29

Th1 is activated by IL-12 from APCs and NKs . Th2 is activated by IL-5 and IL-13. Th17 is activated by IL-23 from APCs. Tregs is activated by IL-2 from APCs. Tfh is activated by IL-6, ICOS-L.

Question 30

What cytokines do the different T-cell classes secrete?

Answer 30

Th1 secretes TNF, IFN-γ, IL-12. Th2 secretes IL-4, IL-5, IL-13. Th17 secretes IL-17. Tregs secrete IL-10, IL-35, TGF-ß. Tfh secretes IL-21.

Question 31

What are the targets of the different T-cell classes?

Answer 31

Th1 targets Macrophages, CTL, NKs. Th2 targets B cells
mast cells, eosinophils, goblet cells, SMC. Th17 targets neutrophils. Tregs target T-cells and macrophages. Tfh targets B cells.

Question 32

What is the effect of the different T-cell classes?

Answer 32

Th1 provides protection against intracellular pathogens Th2 provides protection against extracellular pathogens. Th17 provides protection against protection against extracellular pathogens; autoimmunity. Tregs are anti-inflammatory. Tfh is pro-antibody (direct the formation of germinal centres)

Question 33

What are the 3 core protective roles of antibodies?

Answer 33

1. Neutralisation - antibody prevents bacterial adherence.
2. Opsonisation - antibody promotes phagocytosis
3. Complement activation - antibody activates complement, which enhances opsonisation and lyses some bacteria.

Question 34

Where do B-cells come from?

Answer 34

Derived from stem cells in the bone marrow where their generation and maturation occurs in the absence of antigen. Migrate into the circulation (blood, lymphatic system) and into lymphoid tissues.

Question 35

When can a naive lymphocyte be activated?

Answer 35

Naïve antigen-specific lymphocytes (B or T) cannot be activated by antigen alone. Naïve B cells require accessory signal:
1. Directly from microbial constituents
2. From a T helper cell
These two pathways are either thymus-dependent or thymus-independent

Question 36

What is the pathway for thymus-independent antigens?

Answer 36

Directly activate B cells without the help of T cells. Often bacterial/ polysachharide, needs to be repetitive structure. The second signal required is either provided by the microbial constituent or by an accessory cell. Only IgM antibodies involved.

Question 37

What is the pathway for thymus-dependent antigens?

Answer 37

1. The membrane bound BCR recognises antigen
2. The receptor-bound antigen is internalised and degraded into peptides
3. Peptides associate with “self” molecules (MHC class II) and is expressed at the cell surface
4. This complex is recognised by matched CD4 T helper cell
5. B cell activated