Adaptive Antigen Recognition in the Immune System Flashcards
How are B Cells activated?
B Cells do not require an APC to become activated (note: b cells are APCs)
B cells recognize soluble or cell-surface associated Ags, Recognition of Ag promotes B cell activation
- NOTE: B cells, unlike T cells, can recognize non-protein Ags (fats, nucleic acids, carbohydrates)
Once B Cell is activated, it transforms into either a Plasma Cell that secretes Antibodies or a Memory Cell
Plasma cells are primarily found in lymphoid organs, not in plasma
How do T Cells become activated?
T cells are MHC restricted
T Cell is activated when TCR recognizes Ag presented by an APC within MHC
T cells recognize LINEAR PEPTIDE FRAGMENTS presented on MHCs from APCs only
What is a BCR?
B Cell Receptor
It is a Immunoglobulin (Y-shaped) molecule composed of two heavy chains (VH) and two light chains (VL)
The N terminus end of the light and heavy chains (the open end of the molecule) are highly variable
Carboxyl end of the heavy chains are within the cytosol
In a single B Cell, all the BCRs expressed on the cell surface are identical
Signaling cascade is started through the Ig-alpha and Ig-beta subunits
What is a TCR?
T Cell Receptor
N terminus of the the Alpha and Beta chains of the TCR are Highly Variable
The C Terminus end has limited variability and it is located in the cytosol
In a single T Cell, every TCR expressed on the cell surface is identical
Signaling cascade is initiated through the CD3 and Zeta chain subunits
What are some effector functions of immunoglobulins?
- Have direct anti-bacterial activity
- Reduce the damage to host from inflammatory response
- Participate in opsonization
- Neutralize toxins
- Activate the Classical Complement Pathway
Describe the structure of antibodies
Abs have a basic unit of 4 polypeptide chains
- 2 Heavy chains
- 2 light chains
Chains are held together via covalent disulfide bridges and non-covalent interactions
Both Heavy and Light chains are divided into V (variable) and C (constant) regions
V-region contains the Ag-binding site
C-Region determines the fate of the Ag
Abs can be cleaved to yield 3 fragments:
- 2 Fab fragments (Fragment Ag-binding, Fab)
- 1 Fc fragments (Fragment crystallized, Fc) - which are responsible for the effector functions
What is papain?
Papain is an enzyme responsible for proteolysis of IgG
It breaks apart IgG into 3 fragments; 2 Fab fragments, and 1 Fc fragment
Additionally, it separates the binding of Fc Receptors to Fc fragments as well as Complement Receptors from Fc fragments
What is pepsin?
Pepsin is an enzyme responsible for proteolysis of IgG
It generates a single bivalent antigen-binding fragment, F(ab)2
It does this by separating Fc fragment from the conjoined Fab fragments and destroying the Fc fragment
Essentially turns Y into V
What are the various types of immunoglobulins, and what determines their identity?
IgM, IgD, IgA, IgG, and IgE
The identity of the Heavy chain determines the identity of the immunoglobulin.
There are 5 different heavy chains that correspond to the 5 different immunoglobulins listed above
They are mu (IgM), delta (IgD), alpha (IgA), gamma (IgG), and epsilon (IgE)
What are the types of light chains?
The light chains are either kappa or lambda
Each antibody unit can have only kappa light chains or lambda light chains.
It cannot have both
What are immunoglobulin domains?
The heavy and light chains of the immunoglobulin each contain INTRAchain disulfide bridges that create polypeptide loops, aka DOMAINS
Each domain is labelled.
The Light chains have a VL (variable; light chain) domain and a CL (constant; light chain) domain
The heavy chains has a VH (variable; heavy chain) domain, followed by a series of CH chains that are increasingly numbered as they head toward the C-terminus
- CH1 (constant; heavy chain 1) domain
- CH2 (constant; heavy chain 2) domain
- etc
What is the difference between the Secreted IgG and the Membrane-Bound IgM?
Secreted IgG
- Has gamma heave chain
- Has 3 CH domains (constant; heavy chain)
- Heavy chain C-terminus ends are capped by Tail Pieces
- Has Complement and Fc binding sites on the constant heavy chain domains
Membrane IgM
- Has Mu Heavy chain
- Has 4 CH domains (constant; heavy chain)
- Has C-terminal transmembrane and cytoplasmic portions that anchor the molecule in the plasma membrane
What are hinge regions and where are they found?
Hinge regions provide flexibility to immunoglobulins. The two variable regions on the immunoglobulin are able to bind to two antigens simultaneously.
Sometimes the Ags are farther apart, sometimes theyare close together. The Hinge region allows the Ig to bind to the Ags regardless.
These hinge regions are located between the CH1 and CH2 domains
What is the function of IgA?
Mucosal immunity
What is the function of IgD?
Naive B Cell Antigen Receptor
What is the function of IgG?
- Opsonization
- Complement Activation
- Antibody-dependent cell-mediated cytotoxicity
- Neonatal immunity
- Feedback inhibition of B Cells
What is the function of IgE?
- Defense against helminthic parasites
- Immediate hypersensitivity (anaphylaxis, type I hypersensitivity)
What is the function of IgM?
- Naive B Cell antigen receptor
- Complement activation
Which immunoglobulin has the greatest serum concentration and greatest half life?
IgG
Describe the secreted form of IgA
Can be a monomer, dimer, or a trimer
Each unit of IgA is connected to the others (in dimeric and trimeric molecules) by a J Chain
Secreted IgA unit has 3 constant, heavy chain domains on each heavy chain
Which immunoglobulin has the lowest serum concentration?
IgD
Describe the secreted form of IgG
Monomer
Contains 3 constant heavy chain domains on each heavy chain
Describe the secreted form of IgE
Monomer
Contains 4 constant heavy chain domains on each heavy chain
Describe the secreted form of IgM
Pentamer
Each of the five units have their C-termini connected to a single J chain molecule, giving it a snowflake-like appearance.
Contains 4 constant heavy chain domains on each heavy chain
How does antibody affinity for its target antigen differ during the primary response vs the memory response?
Antibody affinity during the primary response (first time body is exposed to the antigen), the affinity for the antigen is generally low
During the memory response (exposure to the antigen for the second time, or third time, or fourth time, etc) the antibody affinity for the antigen is higher (binding constant is 1000x greater)
What is antibody valency?
Antibody valency determines the number of antigenic determinants the antibody can react with
Ex: IgG has two Fab regions and can bind two molecules of Ag or two identical sites on the same particle
- Thus IgG has a valence of 2
Valence is important for binding affinity
- Having two or more binding sites can greatly increase the tightness of binding of Ab to Ags
IgM has the greatest valency of the immunoglobulens, being a pentameric molecule
- Has low affinity, but high valency
What is antibody avidity?
Antibody avidity gives a measure of the overall strength of an Ab-Ag complex
It is dependent on:
- Affinity of the Ab for the epitope
- Valence of both the Ab and the Ag
Even though IgG has a greater affinity than IgM, IgM has a greater Avidity due to being a pentameric molecule with 10 possible binding sites (compared to IgG’s 2 binding sites)
What are complementary-determining regions (CDR)?
Complementary-determining regions make up 3 portions of the Alpha Variable region and the Beta Variable Region of the TCR
There are also 3 CDRs in each VL and VH domains on the BCR
The complementary-determining regions are Hypervariable
What structures make up the TCR complex?
TCR complex consists of:
- Alpha/Beta TCR
- CD3
- Zeta proteins
All three covalently linked
What is the function of CD3 and the zeta proteins?
The TCR has very small cytoplasmic tails that cannot transduce signals into the T cell
The CD3 and Zeta proteins serve as the SIGNAL TRANSDUCING SUBUNITS of the TCR complex
What are the the TCR co-receptors and what is ther difference in their structure?
CD4 and CD8
CD4 is a monomer that has 4 extracellular Ig-like domains
CD8 is a heterodimer, composed of 1 CD8-alpha chain and 1 CD8-beta chain
- Both chains have a single extracellular Ig-like domain
What are the steps in Ig expression in B cell development?
1) Stem Cell
- No Ig expression
2) Pre-B Cell
- Cytoplasmic “mu” heavy chain
- Pre-B receptor
3) Immature B Cell
- Membrane IgM
4) Mature B Cell
- Membrane IgM
- Membrane IgD
5) Activated B Cell
- Low rate of Ig secretion
- Heavy chain isotope switching
- Affinity maturation
6) Antibody-secreting Cell
- High rate of Ig secretion
- Reduced membrane Ig
What are the steps in Maturation of B and T Lymphocytes?
B and T Lymphocytes follow the same maturation steps
1) Commitment of hematopoietic progenitors to either B or T cell lineage
2) Proliferation of these progenitors
3) The rearrangement and expression of Ag receptor genes
- Failure to express pre-antigen receptors or normal antigen receptor results in cell death
4) Selection events to preserve and expand cells that express potentially useful antigen receptors
- Developing lymphocytes are presented with self antigens
- If cell has a poor recognition of the presented self-antigen, the cell is allowed to live
- If cell has a strong recognition of the presented self-antigen, the cell is destroyed (Negative Selection)
On which chromosomes are the genes for the BCR Heavy and Light chains?
Ig Heavy Chain is located on chromosome 14
Kappa Ig Light Chain is located on chromosome 2
Lambda Ig Light Chain is located on chromosome 22
On which chromosomes are the genes for the TCR located?
TCR Alpha Chain is located on chromosome 14
TCR Beta Chain is located on chromosome 7
What is allelic exclusion in regard to BCR and TCR?
Individuals CODOMINANTLY inherit maternal and parental sets of alleles for Light and Heavy chains of Ig molecule (BCR)
However, ONLY ONE light chain (VLCL) and heavy chain (VHCH) alleles (either maternal or parental) is expressed in a single B cell
This is called allelic exclusion
Allelic exclusion also governs the expression of TCR
Overall, since there are many B and T Cells, the maternal and paternal allotypes are expressed equally
How is that BCRs and TCRs are so diverse and are able to recognize such a large variety of antigens?
DNA chromosonal rearrangement is the major mechanism of epitope-specific diversity of BCR and TCR
The process of rearrangement includes deletion of DNA/RNA nucleotides and reannealing gene segments
What are the 3 mechanisms of chromosomal rearrangement?
Somatic Recombination
mRNA splicing
Junctional Diversity
What are RAG1 and RAG2?
RAG1 and RAG2 are proteins that initiate recombination by introducing double-stranded breaks in the DNA molecules
Non-homologous end joining (NHEJ) is conducted to fix the break
What is the mechanism behind recombination?
Recombination starts with the HEAVY CHAIN first
Then if the heavy chain is functional, the rearrangement of the LIGHT CHAIN (either kappa or lambda) occurs
RAG1 1 and RAG2 (Recombination-Activating Genes 1 and 2) encode for enzymes performing recombination of BCR and TCR during the process of VDJ recombination
VDJ are three segments of the Heavy Chain DNA
RAG1 and RAG2 expression are restricted to B and T cells during the development stages
How is BCR diversity achieved?
Immature B Cells have 4 segments of DNA that are part of the BCR gene (VDJC)
There are several copies of VDJ in the immature B Cell
During the maturation process, recombination occurs, causing all copies to be deleted except 1 each for V, D, and J
- First D and J are chosen and DNA between them is DELETED
- Second a V segment is chosen and DNA between V and DJ is DELETED
- Third, a C is chosen and DNA between VDJ and C is deleted
C segment codes for the constant portions of the heavy chains
What is the mechanism for recombination of the Light Chain?
If the VH rearrangement is productive, then:
- B Cell proliferates for a while and “takes care” of the VL
- Kappa and Lambda light chains do not have D segments like the heavy chains
- The rest of the recombination process is the same as the heavy chains
- J is chosen first
- V is chosen second, DNA between V and J is deleted
- C is chosen third, DNA between VJ and C is deleted
C segments code for the constant portions of the Light Chain
What is the final result of recombination?
Final result is each B cell produces ONLY ONE KIND OF VH and VL
Because the number of combinations possible is very big, B cells produced can recognize any Ag possible
What is the difference in recombination of TCR and BCR?
The DNA
TCR DNA codes for the alpha and beta chains
- Alpha chain has V, J, and C segments
- One copy of V, J, and C are selected, other copies are deleted
- Beta chain has V, J,D, and C segments
- One copy of V, J, D, and C are selected, other copies are deleted
BCR DNA codes for the Heavy Chains and the Light Chains
- Heavy Chain has V, J, D, and C segments
- One copy of V, J, D, and C are selected, other copies are deleted
- Light Chain has V, J, and C segments
- One copy of V, J, and C are selected, other copies are deleted
What is Junctional Diversity?
The process of VDJ recombination generates a repertoire of different TCR and BCR molecules for recognition of various microbial antigens
However, the VDJ recombination alone is NOT ENOUGH to produce a huge variety of TCRs and BCRs which would recognize all antigens possible
The inaccuracies of joining is achieved by JUNCTIONAL DIVERSITY mediated by Terminal Deoxynucleotidyl Transferase (TdT)
- Further increases the diversity of TCRs and BCRs
* TdT adds or removes nucleotides to the exposed ends of the VDJ genes before they are reunited*
Junctional Diversity is generated at the points between the joining genes
- Loss of nucleotides via Exonucleases
- Addition of N and P nucleotides
Opening of hairpin loops of DNA produces short, self-complementary sing stranded extensions that can be incorporated into junctions, or can be removed via an exonuclease activity
Where does maturation of B cells occur?
Bone Marrow
How do the stages of B cell maturation match up with the stages of recombination and Ig expression?
1) Stem Cell
- Germline DNA
- No Ig Expression
2) Pro-B Cell
- Germline DNA
- No Ig Expression
3) Pre-B Cell
- Recombined Heavy Chain gene (VDJ)
- Mu heavy chain mRNA
- Cytoplasmic Mu heavy chain and pre-B receptor-associated Mu heavy chain
4) Immature B Cell
- Recombined Heavy chain gene
- Recombined kappa or lambda light chain genes
- Mu heavy chain RNA and kappa or lambda light chain mRNA
- Membrane IgM expression (mu + kappa or lambda chain)
5) Mature B Cell
- Alternative splicing of primary transcript to form C(mu) and C(delta) mRNA
- Membrane IgM and IgD
How does the selection process work?
During lymphocyte development, the cells go through numerous checkpoints at which the developing cells are tested and continue to mature only if a preceding step in the process has been successfully completed
Checkpoint 1:
- After the production of the FIRST polypeptide chain of the two-chain Ag receptor is completed
- For B-cells - after the development of the Ig mu Heavy Chain
- For T-Cells - after the development of the TCR Beta Chain
Checkpoint 2:
- After the production of the SECOND polypeptide chain of the Ag receptor is completed
- For B-Cells - after the development of the Ig kappa or lambda Light Chain
- For T-Cells - after the development of the TCR Alpha Chain
Checkpoints ensure that only lymphocytes that have successfully completed Ag receptor gene rearrangement processes are selected to mature
The selection eliminates potentially harmful self-reactive lymphocytes
What are Pre-AgRs?
Pre-Antigen Receptors, called Pre-BCRs in B cells and Pre-TCRs in T Cells
Pre-Ag Receptors contain only ONE polypeptide chain present in a mature Ag receptor and SURROGATE RECEPTOR CHAIN
- Pre-BCRs contain the Ig mu heavy chain
- Pre-TCRs contain the TCR Beta Chain
What are In-Frame and out-Frame Rearrangements?
In-Frame Rearrangements of an Ag receptor gene occurs when the rearrangement results in the ability to produce a protein
Out-of-Frame Rearrangements of an Ag receptor gene occurs when the rearrangement makes the cell unable to produce a protein
30% of developing B and T cells make productive In-Frame Rearrangements, and as a result receive survival signals (causing the cells to proliferate)
The remaining 70% of developing B and T cells that make Out-of-Frame rearrangements do not receive survival signals and they undergo apoptosis
What is Positive Selection?
After Pre-B and Pre-T cells with FUNCTIONAL Pre-Ag receptors proceed past the First Checkpoint, they express genes encoding the second chain of BCR or TCR
If the cells make PRODUCTIVE rearrangements of the 2nd chain, they express the complete Ag receptor while they are still immature
Lymphocytes that express useful Ag receptors are preserved by a process called POSITIVE SELECTION
Positive Selection of T cells ensures that maturation of CD8 or CD4 T cells whose receptors DONT RECOGNIZE self-antigens but can RECOGNIZE MHC molecuels (Class I or Class II)
T Cells positively selected by self MHC molecules in the thymus are able to RECOGNIZE FOREIGN ANTIGEN displayed by the same self MHC molecules on APCs in peripheral tissues
What is negative selection?
Negative selection of immature lymphocytes is an important mechanism for maintaining the CENTRAL TOLERANCE to many self antigens
Negative selection occurs shortly after Ag receptors are first expressed on developing B and T cells
Negative selection ELIMINATES harmful T cells and ALTERS harmful B cells whose Ag receptors bind strongly to self Ags present in the thymus or bone marrow
Developing harmful T cells with a high affinity for self AGs are eliminated by APOPTOSIS (clonal deletion)
Developing harmful B cells with a high affinity for Self Ags undergo the second attempt in Ig gene rearrangement
- a process called RECEPTOR EDITING
- If receptor editing fails, the self-reactive B cells also die (clonal deletion)
What happens to T cells and B Cells when negatively selected?
Immature T cells that strongly bind to self Ags are eliminated by apoptosis
Immature B cells that strongly bind to self Ags are given a second chance at first. They undergo a process called Receptor Editing, in an attempt to fix the receptor so it no longer binds strongly to self Ags
- If Receptor Editing fails, then the Immature B Cell dies
What type of B cells develop from the Fetal liver?
Fetal Liver-derived stem cells differentiate into the B-1 Cell Lineage
What type of B cells develop from Bone Marrow?
Bone marrow precursors after birth arise to the B-2 Cell lineage
What are the features of B-1 Cells?
B-1 cells are developed in the fetal liver
They express a limited BCR diversity since TdT is not expressed in the Fetal Liver (aka NO JUNCTIONAL DIVERSITY)
Found as a self-renewing population in the peritoneum and mucosal sites
B-1 cells spontaneously secrete IgM Abs that often react with microbial polysaccharides and lipids as well as oxidized lipids
These Abs are sometimes called NATURAL ANTIBODIES because they are present in individuals without overt immunization
B-1 cells contribute most of the serum IgM during the EARLY PHASES of infection
Participate in INNATE IMMUNITY (act as a transitional type of lymphocyte)
What are the features of B-2 Cells?
B-2 Cells develop in the Bone Marrow
Following rearrangement of their BCR chain genes and removal of autoreactive cells via central tolerance, IMMATURE B-2 CELLS relocate to the SPLEEN
IMMATURE B-2 cells differentiate into either Marginal zone MZ B-2 Cells or Mature Follicular FO B-2 Cells
The affinity of the BCRs for Self Ags may contribute to differentiation of B-2 cells into:
- FO B-2 Cells are recirculating lymphocytes
- MZ B-2 cells are abundant in the spleen and also the lymph nodes
MZ B-2 Cells localize to the splenic marginal zone and respond to blood-borne Ags
What are FO B-2 Cells?
FO B-2 Cells = Mature Follicular B-2 Cells
FO B-2 Cells are recirculating lymphocytes
They require constant replenishment from the Bone Marrow
They respond to PROTEIN Ags in a T-Cell Dependent manner, an progressively undergo immunoglobulin ISOTYPE SWITCHING and AFFINITY MATURATION
Only mature FO B-2 cells upon T-cell dependent activation develop into long-lived PLASMA CELLS or MEMMORY CELLS
What are MZ B-2 Cells?
Marginal Zone B-2 Cells
MZ B-2 Cells are abundant in the spleen and the Lymph nodes
They localize in the splenic marginal zone and respond to blood-borne Ags
MZ B-2 Cells are predominantly self-renewing
Most responses of MZ B-2 CElls are independent of T cell help
MZ B-2 Cells are similar to B-1 Cells in that they:
- Have LIMITED DIVERSITY
- Respond to polysaccharide Ags
- Generate Natural Abs (IgM antibodies present in individuals without overt immunization)
MZ B-2 Cells respond rapidly to BLOOD BORNE MICROBES and differentiate into short-lived IgM-secreting PLASMA CELLS
What are the two main cell types of the thymus epithelium?
Cortical thymic epithelial cells (cTECs)
Medullary thymic epithelial cells (mTECs)
What are the phenotypically distinct stages of T Cell development?
Double Negative (DN) cells (express neither CD4 nor CD8)
Double Positive (DP) cells (express both CD4 and CD8)
Single Positive (SP) cells (express either CD4 or CD8)
How does DP Thymocyte Positive Selection occur?
Positive Selection determines the fate of DP cells and depends on the ability of its TCR to interact in a self-peptide-MHC complex
DP thymocytes are PROGRAMMED TO UNDERGO APOPTOSIS unless a “rescue signal” is delivered via TCR self-peptide-MHC interaction
cTECs present MHCs with Cortex-sepcific peptide to the DP thymocytes.
Vast majority (95%) of DP cells fail to demonstrate any specificity for an MHC ligand, and these immunologically useless thymocytes undergo death by neglect or apoptosis
A small proportion of DP cells express TCR that are able to bind an MHC ligand with MILD AVIDITY, and the resulting signaling cascade diverts those thymocytes from apoptosis
TCR signaling also produces DP maturation to the SP thymocytes (CD4+ or CD8+ T Cells)
POSITIVE SELECTION OCCURS WITHIN THE THYMIC CORTEX
How does SP Thymocyte Negative Selection occur?
NEGATIVE SELECTION, also referred to as clonal deletion, is the process by which high-avidity TCR self-peptide MHC ligation induces cell death
Negative selection for self-reactive T-cells appears to occur in the THYMIC MEDULLA
DP/SP thymocytes undergo apoptosis if both TCR and CD28 are engaged in the binding
mTECs express both self peptide-MHC and costimulatory molecules CD80 and CD86 (interact with CD28)
CD80/CD86 expression is significantly HIGHER IN THE MEDULLA than in the cortex, thus suggesting the medulla as the more favorable environment for negative selection
The successful elimination of potentially self-reactive thymocytes through CLONAL DELETION requires a complete representation of tissue specific Ags (TSAs) within the thymus
Control of TSA expression appears to be controlled by the genes such as the AUTOIMMUNE REGULATOR (AIRE)
