Wk 1 Antigen Presentation & Lymphocyte Development Flashcards
What can B cell receptors bind?
Soluble, free-floating antigens
What do T cell receptors bind?
Only small peptide antigens (8-20 aa) presented by MHC on the cell surface of APCs
What are the two classes of MHC?
- Class I on APC binds CD8+ T cells
- Class II on APC binds CD4+ T cells
What are the 3 MHC Class I genes?
HLA-A,
HLA-B,
HLA-C
HLA = human leukocyte antigen
HLA = MHC
What are the 3 MHC Class II genes?
HLA-DP
HLA-DQ
HLA-DR
What is MHC inheritance?
Inherited w/ co-dominant expression - 2 alleles of each
-all nucleated cells express MHC Class I
-professional APCs (dendritic cells, macrophages, B cells) also express MHC Class II
-Histocompatibility genes are inherited as a group (haplotype), one from each parent. Thus, HLA genes are co-dominantly expressed in each individual. A heterozygous human inherits one paternal and one maternal haplotype, each containing three class I (A, B, and C) and three class II (DP, DQ, and DR) loci
What is the difference b/w MHC Class I and II molecular structures?
MHC Class I: Single a chain forms peptide binding cleft, surface expression stabilized by a small molecule called b2-microglobulin.
MHC Class II: a and b chains that come together to form peptide binding cleft.
What are the T cell co-receptors for CD4 and CD8?
CD4 -> MHC Class II
CD8 -> MHC Class I
What are MHC most of in the human genome?
The most polymorphic
-each can bind a different set of peptides
-gives us as many opportunities as possible to bind variety of pathogens
How do MHC molecules behave under non-infectious conditions?
MHC molecules are not stable unless bound to something, so bind to self peptides
What do MHC do under infectious conditions?
MHC presents foreign peptides derived from pathogens to T cells
TCR always need to recognize self and foreign antigen when binding to MHC
What are MHC Class I peptides derived from?
cytosolic antigens (think viruses)
What do MHC Class II present?
Peptides from extracellular antigens that have been phagocytosed
Where are MHC Class I formed?
in ER
How do many viruses evade T cell responses?
By interfering w/ antigen presentation on MHC Class I
-turn off fxns
MHC Class II antigen processing
-assembled in ER by complexing w/ invariant chain (Li) and blocking the binding of peptides
MHC Class II Ag processing summary
What is cross-presentation?
Process whereby antigens derived from phagolysosomes can be exported into cytosol and can enter into MHC Class II molecule presentations
-a specialized fxn of dendritic cells
Important
Dendritic cells are required for activation of naïve T cells.
* Activated CD8+ T cells can target infected nucleated cells, as they all express MHC I.
* Activated CD4+ T cells interact with MHC II-expressing macrophages and B cells to enhance their function.
What’s the difference b/w B cell receptors and antibodies?
BCR is attached to membrane
Antibodies are free floating
Explain the structure of the antibody
–2 identical heavy chains
–2 identical light chains
=> bivalent
–immunoglobulin domains
–Variable region mediates antigen recognition
–Hinge region allows flexible multi- valent binding
–Constant (Fc) region mediates effector functions:
*Complement activation
*Opsonization
*Cellular activation (Fc receptors)
What are the 3 roles of the Constant (Fc) region?
- complement activation
- opsonization
- cellular activation (Fc receptors)
How many antigen receptor types does each naive or resting B cell express?
1 cell = 1 specificity
-1 antigen receptor type that’s either membrane-bound or secreted
-activated B cells can differentiate into antibody-producing plasma cells
How many antigen receptors does each naive or resting B cell express?
1 cell = 1 specificity
-1 antigen receptor type that’s either membrane-bound or secreted
-activated B cells can differentiate into antibody-producing plasma cells
What switches on B cells to change antibody isotypes?
Constant region
5 antibody types
What antibody type is found in the gut?
IgA
What antibody is the most abundant?
IgG
What B cell region binds to antigens?
The variable region (which is composed of hypervariable aa sequences that determine specificity) and is made of the heavy and light chains
Where are TCRs?
Only membrane bound
What is the structure of TCR like?
- TCR: a/b (conventional) or g/d
- Structurally related to Ig.
- Does not recognize free antigen.
- Peptide antigens presented by the
major histocompatibility complex
(MHC) molecules. - No Fc region: effector function
mediated by the T cell, not the TCR
-will either secrete cytokines or kill target
What is the defining marker of T cells?
CD3
Where does lymphocyte activation occur?
In secondary lymphoid organs, incl spleen and lymph nodes
4 steps in lymphocyte development
- cytokine-driven lymphoid development: survival and expansion
- selection: successful antigen receptor rearrangement
- (-) selection: elemination of reactivity to self-antigens
- Egress to secondary lymphoid tissues
What is the role of IL-7?
Lymphoid lineage commitment (lymphopoiesis)
-impaired IL-7 -> signaling defective T and B cell development -> SCID
How are diverse antigen receptors generated?
DNA recombination
How does DNA recombination create diverse antigen receptors?
- Recombination-activating gene (RAG) recombinase - cuts and pastes regions - randomly recombines VDJ
- TdT = terminal deoxynucleotide transferase - randomly inserts nucleotides into sites of recombination to enhance diversity - messy process, most don’t give rise to coding region
- RNA splicing determines constant regions - either mu or delta (IgM or IgD)
What happens during positive selection?
- combination of germline gene segments (VDJ) by RAG recombinase
- Addition of extra nucleotides by TdT at recombination sites
- combination of 2 chains
- each B and T cell has its own unique antigen receptor
BCR vs TCR
4 steps of B cell positive selection
- productive rearrangment results in intracellular signal to cell (+ signal). W/o signal, cell -> apoptosis
- 2 chances for productive heavy chain rearrangement (1 locus, 2 alleles)
- 4 chances for productive light chain rearrangement (2 loci, 2 alleles each)
- Once functioning antigen receptor is expressed, no more rearranging
What is X-linked agammaglobulinemia?
A defect in positive selection
-Bruton’s Tyrosine Kinase (Btk) (IMP kinase in + selection process)
- X-chromosome encodes Bruton’s tyrosine kinase (Btk) found in pre-B cells.
- Btk signals cell that a productive antigen receptor is present at the cell surface.
- Btk deficiency leads to lack of Bcells: X-linked agammaglobulinemia.
- Autosomal recessive versions as well.
Explain B cell negative selection
What is a thymocyte?
committed T cell progenitors that have migrated from the bone marrow to the thymus
-majority of cells in thymus
What is a thymocyte?
committed T cell progenitors that have migrated from the bone marrow to the thymus
-majority of cells in thymus
What occurs in thymus?
Site of T cell (+) and (-) selection
-thymic epithelial cells assist selection
-genetic loss = DiGeorge syndrome can -> severe immunodeficiency
What are the 3 processes in thymus?
- Positive selection part I: Productive TCR rearrangement and expression (cortex)
- Positive selection part II: recognition of host MHC by new TCRs (cortex)
- Negative selection: removal of auto-reactive thymocytes (medulla)
What is positive selection of T cells?
Recognition of self MHC in thymic cortex
What and where are double positive thymocytes?
In thymus
CD4+ and CD8+
Explain negative selection of T cells
In medulla
= removal of auto-reactive T cells
Di George Syndrome
=a defect in thymic development -> partial or complete lack of thymic development
* Other defects: structural defects in face and pharynx, hypoparathyroidism, cardiovascular, neurological.
* “Complete”DiGeorge=nothymic development, severe lack of T cells. Only 1% of cases. Susceptible to opportunistic infections, death in the first couple of years. Treatment is thymic transplant.
What is L-selectin?
Preferentially attracts naive lymphocytes
-located in high endothelial venule (HEV) in lymph node
-Naïve T and B cells express high levels of L-selectin and traffic to secondary lymphoid organs via High Endothelial Venules.
Trafficking to T cell and B cell zones
IL-7 drive lymphoid commitment and survival signals
BAFF = B cell activating factor
What is TACI?
TACI inhibits B cell expansion and promotes the differentiation and survival of plasma cells
=Transmembrane activator calcium modulator
-~10% deficiency in CVID (common variable immunodeficiency)
Key points
Key points
What types of molecules can B cells respond to?
Many - lipids, sugars, nucleic acids, etc
-protein antigens induce best Ab responses since they can mobilize Th cells too
What does direct antigen-mediated activation of B cells induce?
Short-lived IgM responses from short-lived plasma cells w/ limited clonal expansion
What can provide costimulatory signals to B cells?
Toll-like receptor ligands (ie LPS) and complement fragments
What happens to antigens before they’re expressed to T follicular helper cells (Tfh)?
They’re phagocytosed, then expressed on the APC w/ CD40
What happens to antigens before they’re expressed to T follicular helper cells (Tfh)?
They’re phagocytosed, then expressed on the APC (B cell) MHC Class II molecules w/ CD40
What are 2 things the Tfh cells do to help mediate the activation of B cells?
- Express CD40L, which binds to CD40 on surface of B cells as costim pathway
- secrete cytokines (differ based on Tfh response) - can include IL-4, IFNgamma, IL-21 —–cytokines drive isotype switching
5 steps of T cell help to B cell clonal expansion
- B cells internalize antigens that bind B cell receptors
- Antigens are degraded in endosomes and presented on MHC Class II
- Tfh effector cells specific for the same antigen bind to MHC Class II
- T cells induce B cell activation via the CD40 on B cells /CD40L on T cells costimulatory pathway
- T cells induce B cell activation through cytokine production -> B cell clonal expansion and formation of the Germinal Center
What is AID?
=activation-induced deaminase
-activated by CD40
-goes to nucleus and mediates:
1. isotype class switching of Ig constant region
2. somatic hypermutation of the VDJ region
What are the 2 critical processes that AID initiates in the germinal center?
- isotype class switching of the Ig constant region by initiating DNA recombination
- somatic hypermutation of the VDJ region
How does isotype class switching happen?
AID initiates DNA recombination to switch from one constant region to another
-selection of constant regions is driven by cytokines
-one way event: upstream constant regions are excised from the genome
-IgM -> IgG next, IgA, IgE
What is somatic hypermutation?
DNA mutagenesis within the VDJ region of the immunoglobulin genes
-mediated by AID
-most mutations disrupt antigen binding affinity and -> apoptosis
-some mutations may increase affinity -> proliferation
What is affinity maturation?
Multiple rounds of somatic hypermutation and subsequent selection of higher affinity variants -> the emergence of high affinity B cell clones
What role do follicular dendritic cells play in affinity maturation?
They promote affinity maturation by retaining antigens in the germinal center for weeks or months after pathogen clearance
-prolongs process of affinity maturation
-very high affinity IgG can take months to fully mature
What is the result of somatic hypermutation and affinity maturation?
Formation of long-lived plasma cells and memory B cells
What isotype/class are memory B cells?
Mostly IgG, IgA, IgE
What are 3 characteristics of long-lived plasma cells?
- persist for years
- reside in BM
- continuous antibody secretion
Compare a single exposure w/ no T cell help, single exposure w/ T cell help and second exposure to antigens
T cell and B cell formation summary
4 antibody effector mechanisms
- Neutralization (all isotypes) - prevent infection, prevent colonization and block bacterial toxins
- opsonization (IgG - b/c many phagocytes express receptors called Fc gamma receptors for the constant region of IgG antibodies coating the pathogens
- complement activation by IgG and IgM, which -> MAC formation -> lysis of target bacteria AND/OR complement opsonizes the pathogen for enhanced phagocytosis
-complement fragments (eg C5a) -> vasodilation and inflammation - Mast cell activation by IgE upon 2nd exposure -> degranulation -> histamines and leukotriene release -> allergic response
- ADCC (antibody-dependent cell-mediated cytotoxicity) mediated by IgG - occurs when NK cells express Fc gamma receptors that bind to constant region of IgG molecules & can then recognize the viral GPs on infected cells and release Granzyme B and Perforin
Summary
