Humoral Immunity Flashcards
Immunoglobulin (antibody) molecules functions in the humoral immune response
1) antigen recognition
Membrane assocated antibody IgM is the receptor that B cells use to repscifically recognise antigens. The antibodies that B cells produce have the same specificities as the receptors on that B cell
2) Effector function
Soluable antobodies are secreted by antigen-activated B cells and plasma cells. Antibodies are the major effector molecules of humoral immunity. Different antibody types are specialized for different locations and functions
Antibodies are defined by what
Isotype and idiotype
Isotype
defined by the constant domain (ie IgG, IgM, IgE, ect) This domain determines the effector function of Ig molecule
What domain determines the effector function of the Ig molecule
The constant domeain
ie isotype
Idiotype
determined by the variable and hypervariable domains of the Ig molecule. The idiotype determines the specificity of the anitobody molecule
What determines the specificity of the antibody molecule
hypervsariable and variable domains
ie idiotype
B cell development occurs in two destinct stages
Antigen intependent phase
Antigen dependent phase
Antigen independent phase of B cell development
occurs in bone marrow
Diverse B cell receptors generated (>10^9 specificities)
Test for. functional BCR (b cell receptor)
Select out self reactive cells
not dependent on specific interactions with other immune cells
Antigen dependent phase of B cell development
Occurs in peripheral lymphoid tissues
Selecting and amplifying responses
avoid responding to peripheral selft antigen
increase affinity to antigen
select Ig subclass, which influcences effector function
generate memory cells for recell responses
critical role of CD4+ T helper cells
Review of B lymphocyte maturation
Pro B cell expands. THen has pre B cell heavy chain. If failed receptor goes to apoptosis. If immature B cell IgM+ reacts to self antigen it will either receptor edit or apoptosis
Then goes to periphery
MAture B cell IgM+, IgD+ responds to antigen
it secretes antibody and establishes memory
Overview of B cell responses in the periphery
Naive B cell recognises microbial antigen
T cell helps with antigen presentation and B cell is activated
Clonal expansion
Differentiation
You can wither have class switch or affinity maturation
Also make memory B cells IgG+
Role of peripheral lymphoid organs in B cell development
1) Where B cells encounter concentrated antigen
2) Where B cells interact with othet immune cells
- T helper cells: critical for clonal B cell ploliferation and maturation
- Folicular dendritic calls and Tfh: mediate affinity maturation
3) Provides the appropriate microenvironment to influence effector function (through class switching)
Peripheral lymphoid organ
Parts of peripheral lymphoid organ
Primary follicles (B cell zone)- Where B cells encounter antigen
Paracortex (T cell zone)- Where T cells encounter antigen presenting cells (APCs)
(dendritic cells travel here when they have found antigen)
Secondary follicle (germinal center)- consequence of the interaction between B and CD4+ T cells
How do B cells find antigen?
Antigen enters the nodes via the afferent lymphatics and is either trapped in the subcapsular sinus by specialized macrophages or is delivered to the follicles via conduits
Recirculating B cells migrate from the blood into the follicles of the lymph node
Upon Ag engagement via the B cell receptor, the B cells move towards ther paracortex (where the T cells hang out)
Without Ag engagement, the B cell leaves the node through the efferent lymph and recirculate between and through secondary lymphoid tissues
How do B cells find antigen in my words and in picture
Antigen enters node in afferent lymph
Ag trapping cells in subcapsular sinus trap antigen
B cell follicle- Recirculatnig B cell migrates from blood to follicular mantle
Engages Ag via BCR, inducing interaction with primed T cells
Engages Ag via +CR2 and trandports it to FDC
If no Ag involvement, B cell leaves node in lymph
Microbial Antigen Recognition
Microbe binds to antigen receptor (which has no signaling capability) However, Igalpha and Igbeta have ITAM which is a signaling pathway ans helps them to form signal cascade ultimately leading to transcroption
Features of antigens that promote B cell responses
1) bound complement
B cell signaling is enhances by complement bound to antigen
When a pathogen is coated with C3d, engagement of the CD21/CD19 complex by C3d synergizes with signaling through the B cell receptor
Features of antigens that promote B cell responses
2) pathogen multivalency
Highly multivalent antigens induce stronger B cell responses
Some highly multivalent angigens can induce T-cell independent responses
Features of antigens that promote B cell responses
3) pathogen-associated molecular patterns (PAMPs)
Most pathogens contain PAMP that can activate PRR receptors (such as TLRs) expressed by B cells
Signaling through toll-like (and other) receptors synergizes with BCR signaling
These lignds acan also induce polyclonal activation of B cells
What are the functional cconsequences of antigen binding to the B cell receptor ?
1) entry into the cell cycle
2) Low level IgM secretion (short lived Plasma cells)
3) Expression of molecules that are important in subsequent interactions with T helper cells
These include:
Co-stimulatory molecules
Cytokine receptors
Presentation og Ag on MHC II
4) Migration out of the follicle toward the T rich zone (CCR7 up arrow)
What happens when CD4+ T cells respons to antigen
Antigen presentatio by DC to helper T cell
Activation of CD40 ligand, cytokine secretion
Entry into cell cycle (proliferation)
differentiation into effector T cells
- Secretion of cytokines and expression of surface molecules
- Migration towards B cell zone (lymphoid follicle)
- Enter circulation and migrate to the site of infection
Overview of B and T cell interaction in the Lymph Node
B cells amd T cells are segregated in the lymph node
B cells encounter antigen in primary follicles
T cells encounter antigen presenting ccells in the T rich zone
Upon antigen recognition, both cell types modulate CCR7 expression and migrate towards eachother
STEPS
Antigen presentation:T cell activation
CCR7 down and CXCR5 up and migration of activated T cekks to edge of follicle
B cell presents antigen to activated helper T cells
Antigen uptake and precessing; B cell activation
B cells increase CCR7 and migration of activated B cells into folicle
The specificity of the B/T cell interaction in mediated by MHC class II presentation of internalized antigen
B cell interacts with microbial protein antigen
Receptor mediated endocytosis of antigen happens
antigen processing and presentation happens on class II MHC peptide complex
T cell recognition of antigen (activted CD4+ T cell)
B cell antigen presentation and the concept of linked T helper epitopes
MHC can only display proteins
If you have a sigar specific B cell will do receptor-mediated antigen internalization, proteolysis, and presentation of peptides
Will then display protein on cell and present it ot a protein specific T cell
Conjugate vaccines
A technique for proviting T helper responses to antigens that do not have T cell epitopes
Polysaccharide (PS): contains B cell epitopes but lacks T cell epitopes
Tetanus Toxoid (TT): contains both B and T cell epitopes
Immunize with PS
B cells PS specific
Th cells: NONE
IgG response: NONE
Immunize with TT-PS conjugate
B cells: PS-specific and TT specific
TH cells: TT specific
IgG response: anti PS IgG and anti-TT IgG
Key components of T cell help
Activation of T cell; expression of CD40 ligand, cytokine secretion
Activation of B cell and by cytokines and CD40 ligation
B cell proliferation and differentiation
The specificity of this interaction is maintained by:
1) Cognate interaction mediated by cell-cell contacts
2) Interaction occurs in a specific location at the boundary of the T-rich zone and the primary B cell follicle
X-linked Hyper IgM syndrome is due to a mutation in CD40L; B cells in these patients do not class switch
What are the consequences of the T:B cell interaction
1) Clonal expansion of B cells
2) Establishment of germinal centers
3) Class switching
4) Affinity maturation
5) Differentiation to memory cells
In general, all of these events are predicted on B cells recieving help from CD4+ T cells
The germinal center reaction
Upon activation by CD4+ T cells, B cells begin to proliferate and enter what is known as secondary follicles
Secondary follicles are home to germinal center
Each follicle is mainly clonal, and the germinal center contains two regions, the light zone and the dark zone
In the dark zone, B cells undergo rapid cell division and turn on the machinery that leads to:
1) somatic hypermutation of the variable regions of the B cell receptor
2) Class switching
The light zone of the germinal center is rich with follicular dendritic cells (FDCs) and CD4+ T cells (TFH cells). B cell survival is dependent on interaction with FDC-bound Ag and presentaion of Ag to T cells
Antibody affinity selection
Affinity selection is the process that leads to increqased affinity of antibodies for a particular antigen as a T cell dependent humoral immune response progress
Mutations and affinities were measured after vaccinations. Shaded regions are CDRs (complementary determining regions) The mutations are found here
This shows that the antibodies are getting mutations to increase binding
Affinity selection is mediated by 3 factors
(affinity maturation)
1) Rapid division
After interacting with CD4+ T cells, B cells rapidly proliferate (divide every 6-12 hours)
2) high mutation rate
Expression of the protein AID ( activation induces deaminase) leads to hypermutation of the V domain of the BCR (1 error per 1000 bases per cell division)
3) Selection
B cells compete for binding of limiting amounts of antigen displayed by folicular dendritic cells. Only cells taht successfully bund to antigen and then recieve help from Tfh are selected sa
Affinity selection: Evolution in a germinal center
Antigen is taken up by FDCs in the form of immune complexes and held in a non-degraded form for month after an infection. FDCs + antigen + Tfh + CD40L provide survival signal to B cells
Taken together, this process selects for mutations that lead to stronger binding of antibody to antigen
B cell class switching
Class switching- a biological mechanism that changes a B Cells production of antibody from one class (isotype) to another
Different antibody types have different effector functions
Class switching is mediated by cytokine environment
cytokines are largely produced by Th cells. Different Th subsets produce different cytokines and promote switching to different isotypes
Th1 produces INF-gamma which mediates class swithcing to IgG
Th2 produces IL-4, IL-5, IL-13 which mediates class switching to IgE
Th1 produces ____ which mediates class switching to ___
INF-gamma
IgG
Th2 produces ____ which mediates class switching to ___
IL-4, IL-5, IL-13
There are 5 different antibody classes based on different
heavy chains
IgM and IgD
Expressed on naive B cells. Both are expressed as membrane receptors, but only IgM is expressed as soluable antibody
IgG, IgE, and IgA
Expressed on ACTIVATED or MEMORY B CELLS that have undergone class switching. All 3 molecules can be expressed as membrane receptors or as soluable antibody
How do cytokines mediate class switching?
1) Cytokines induce transcription from specific promoters. (For example, IL-4 induces transcription from Ie)
2) Transcription triggers a recombination events that result in the deletion of intervening constant (C) genes
3) The resulting DNA can be transcribed and translated
4) This process results in an antibody with the same specificity, but a switched constant domain
IgM effector function
produced early upon B cell activation
activates the classical complement pathway
IgG effector function
the major Ig isotype in the blood
Transferred across the placenta
Activated the classical complement pathway
mediated neutralization and opsonization of pathogens
mediated antibody-dependent cellular cytotoxicity
IgA isotype
Produced in mucosal tissues and secreted into the lumens of the GI and respiratory tracts
Mediates pathogen neutralization
Can be transferred to infants via breast milk
IgE effector function
defense against helminths
involved in allergic responses
Effector function os antibodies
Some of them are mediated by Fc receptors
Memory B cells
Generated in germinal centers
therefore we only have humoral memory to T dependent antigens
Circulate through lymphoid organs
Express the affinity matured BCR on their surface
Typically isotype switched (eg IgG+ or IgA+)
Very long lived
persistence of memory B cells after an immune response ensures that we have increased numbers of B cells specific for the antigen and ready to rapidly expand and differentiate into plasma cells and secrete antibody upon reexposure to antigen
Provide the basis for secondary GC reactions
Plasma cells
arise predominantly from germinal ceners (in primary responses) or from memory B cells (in secondary responses)
Are very long lived (several months or longer)
Often home to bone marrow, gut, lactating mammary gland
Are predominantly isotype switched
Features of primary and secondary antibody responses to a T DEPENDENT ANTIGEN
Notice that hte first response takes much longer
Antibody levels at peak are also much higher in second exposure
In primary response, you make IgM and IgG and in secondary response we only see IgG
Do you have IgM respsonse with a polysaccharide T independent response?
YES
Some antigens can provoke B cells responses without the requirement for T help
With T independent antigen first and second response
ARE THE SAME
Which of the following events does NOT occur in the germinal centers
somatic mutation of Ig V genes
Ig gene V(D)G recombination
B cell proliferation
T folicular helper cell iteractions with B cells
generation of memory B cells
Ig gene V(D)J recombination
this is an early event that happens in the bone marrow
Previously healthy young european woman who had not been immunized against measels virus became infected with the virus while traveling to disneyland developed a rash, high fever, cough, and runny nose, and then fully recovered after 4 days. A blood test performed 2 months later, after her return to europe, showed the presence of high affinity IgG antibodies specific for measels virus antigens. Which of the following proteins was NOT necessary for the production of these high affinity IgG antibodies?
CD40 ligand
Activation induced deaminase
Class II MHC
CXCR5
Fc gamma receptor
Fc Gamma receptor- helps regulate but not produce
CD40 ligand- expressed on T cells. important for class switching
activation induced deaminase (AID) - important in class switch and affinity maturation
Class II MHC- B and T cells come together
CXCR5- chemokine to go to zone
Distinct B cell subsets mediate different types of antibody responses
