14. B cell subsets, Fc-receptors, NK cells, Memory Flashcards
centrocytes vs centroblasts?
dark zone contains rapidly proliferating cells called CENTROBLASTS undergoing somatic hypermutation
light zone contains cells called CENTROCYTES which are testing their mutated antigen
FDCs function in GCs?
antigen bound on their surface provides the life-saving signals to B cells that express mutated receptors (after somatic hypermutation of Ig V genes)
there is selection of high affinity B cells and death of B cells that do not bind antigen
how do FDCs change when a primary follicle becomes a secondary follicle in the immune response?
increase receptors and change under influence of cytokines
FDCs are born in the follicle and always stay there (not even hamatopoietically derived)
AID (activation induced cytidine deaminase) is required for both somatic hypermutation and class switch recombination - what happens when it is lacking/deficient?
this occurs in type 2 IgM hypergammaglobulinemia
there is no IgG (or any other isotypes) but still form B/T cells and germinal centers (unlike CD40L deficient people who don’t form germinal centers)
pts experience repeated bacterial pneumonia, otitis, lymphadenopathy
treatment = I.V. Ig
what happens in Type 1 HIGM? (which is x-linked or autosomal)
deficient/mutated CD40 or CD40L, so the B cells don’t get a signal from T cells to go back into the follicle and thus do not form germinal centers
Pts get repeated bacterial pneumonia, otitis, pneumocystis infections
Normal B/T-cells, no leukocytosis w/infection, hyper IgM, no other Ig isotypes
treatment: I.V. Ig
induction of primary response vs secondary response (re; antigen)?
primary response induced by all immunogens
secondary response only induced by protein antigens because you need helper T cells
required immunization vs. primary response and secondary response?
primary response: required immunization is relatively high doses of antigens, optimally with adjuvants (for protein antigens)
secondary response: low doses of antigens, adjuvants may not be necessary (w/ lower dose of antigen, then select for highest affinity clones!)
when centroblasts divide and mutate in the GC dark zone, mutations are targeted to which part of the antibodies?
V regions, and supertargeted to hotspots
centrocytes re-expressing the BCR and the GC response leads to what?
memory and long-lived plasma cells (w/bone marrow homing)
T follicular helper cells - please explain.
TFH are T cells that get stimulated and upregulate CXCR5, then go all the way into the light zone of the follicle.
After the T-B interaction and B cells go back into the follicle to undergo somatic hypermutation and class switching, there is a chance that the antibody could mutate to recognize self antigen (instead of the intendeded higher affinity for the same antigen to which it was originally expressed)
Thus, Tfh cells interact with GC B cells as a “checkpoint” - if the B cell expresses a self-antigen in the context of class II, then the Tfh won’t respond to it and give help to the Bcell, and the B cell then doesn’t become a memory or plasma cell
Is B7 constitutively expressed on B cells or induced?
induced - binds to CD28 on T cells (which is constitutively expressed)
is CD40L constitutively expressed on T cells or induced?
induced - binds to CD40 on B cells (which is constitutively expressed)
is CD28 constitutively expressed on T cells or induced?
constitutively - binds to B7 on B cells (which is induced)
is CD40 constitutively expressed on B cells or induced?
constitutively - binds to CD40L on T cells (Which is induced)
is ICOS constitutively expressed on T cells or induced?
induced - binds to ICOSL on B cells (also induced)
are cytokines constitutively released from T cells or induced?
induced - bind to cytokine recepotrs on B cells (also induced)…
cytokines mediate the effector functions of the B cell: antibody production (type, opsonizing, neutralizing, etc) - different cytokines induce different responses!
Tfh are required for the germinal center reaction, with production of which cytokine?
IL-21 (but can also express different cytokines, which is new in research - dual-phenotype of these T cells)
what does IL-21 do in the GC reaction?
saves B cells
can T-independent antigens mediate class switch?
yes, but only IgG
what type of B cells respond to T-independent antigens?
B1 cells - important for first line of defense against bacteria and are found in the peritoneal cavity and in the mucosal surfaces because involved in first line of defense (analogous to gamma/delta T cells)
MZ B cells (marginal zone) are found in the spleen. They act as sentinels and respond rapidly to infection - positioned in an area of spleen that filters blood
There is no memory/affinity maturation for ^^ cell types
B2 (conventional B cell) can make T-independent responses if their receptors are heavily crosslinked
what type of B cells have no memory or no affinity maturation?
B1/marginal zone B cells because they respond to carb moieties and do not interact with helper T cells
the switch to IgG1 and IgG3, the “inflammatory isotypes” capable of opsonization and C’ fixation, is under the influence of which cytokine?
IFN-gamma - the major Th1 cytokine
IgG4 is a “______” isotype. Does it induce inflammation?
neutralizing; does not induce inflammation when it binds to antigen, so it is often used in therapeutic antibodies
IgG4 is not bivalent like other IgG isotypes, but it undergoes what process after secrection (and what does this process do)?
undergoes “arm-exchange” after secretion from plasma cells, so that all of the molecules in plasma are chimeric and monovlaent
which IgG FcR promote or activate the immune response?
CD16 - NK receptor that activates ADCC
CD64 - opsonization FcR found on phagocytes
both associate with ITAM-containing molecules
CD32 - what is it, what does it do, and how does it impact IVIg treatment of autoimmune patients.
It is the inhibitory FcR on B cells and APC
it shuts down B cell responses and induces apoptosis in plasma cells when it is crosslinked by immune complexes (helps to clean out bone marrow of older plasma cells during an ongoing immune response)
autoimmune patients treated with IVIg improve because of dilution of autoantibodies AND because of interaction of infused IgG with inhibitory FcR CD32 (calms inflammatory reactions)
FcR CD32 has an _____ motif to shut down the response when there is a lot of antibody around (mainly shuts down the naive cells, so over time if you keep immunizing people, you wind up with getting a lot of the same old Abs made because you stimulate this feedback mechanism to naive B cells)
ITIM motif
VDJ recombination and class switch recombination can lead to chromosome translocations and what clinical problem?
lymphoid malignancies
acute lymphoblastic leukemia (ALL) arise at which stage of B cell development?
lymphoid progenitory (in bone marrow and blood, IgV genes unmutated)
chronic lymphocytic leukemia arises at which stage of B cell development?
activated or memory B cells (in the periphery, IgV usually unmutated)
Burkitt’s lymphoma arises at which stage of B cell development?
mature, memory B cell (resembles germinal center B cell) (in the periphery, mutated IgV genes with intraclonal variability)
hodgkin’s lymphoma arises at which stage of B cell development?
germinal center B cell (in the periphery, mutated IgV gene with -/- intraclonal variability)
multiple myeloma arises at which stage of B cell development?
plasma cells of various isotypes (in the bone marrow, mutated IgV gene with no variability within clones)
The B1 cell binds bacterial capsular polysaccharide or cell-wall components ad receives a signal (IL-5) from accessory cells….what are the next steps in fighting this pathogen?
B1 cell secretes IgM anti-polysaccharide antibody
IgM binds polysaccharide capsule
Activation of complement and removal of bacteria
which Igs activate complement?
IgM, IgG3, and IgG1
which is the strongest Ig opsonizer?
IgG1
which Ig is really good at being transported across the epithelium?
IgA
what is the major Ig transported across the placenta?
IgG1
In order to block C5 in PNH, ______ is used as the Fc region in eculizumab.
IgG4
why is FcRn often used to enhance Abs?
it enhances the lifetime - this molecule takes them up and then releases them later
Basic functions of IgG FcR’s on the following:
- plasma cell
- B cell
- monocyte or macrophage
- DC
- plasma cell = apoptosis
- B cell = modulation of cell activation, and shaping of B cell repertoire
- monocyte/macrophage = oxidative burst, cytotoxicity, release of pro-inflamm mediators
- DC = modulation of cell activation, antigen presentation, regulation of peripheral tolerance
The NK cell is a lymphocyte that can kill virally-infected cells via the _____ pathway, just like CTL.
perforin and granzyme B
One way the NK cells kill is by ADCC - explain?
antibody-dependent cellular cytotoxicity
NK cells bear Fc receptors (CD16) for IgG molecs…if the target cell has an antigen recognized by these antibodies, the NK cell can bind and lyse it (ADCC is the major cause of transfusion reactions)
Besides ADCC, how do NK cells kill?
can kill when MHC molecules are not present on the surface of the cell. Viruses have evolved elaborate mechanisms to downregulate class I expression. This is then recognized by NK cells.
NK cells vs CDL, which is for eradicatino of established infection and which is for protection vs infection?
NK cells - protection vs infection
CTL - eradication of est infection
Do NK cells make cytokines?
Yes, IFN-gamma (when stimualted by IL-12 from macrophage with phagocytosed microbes, NK cells release IFN-gamma to kill the phagocytosed microbes)
If you wipe out the NK cells in a person and then infect them with a virus - what happens to them?
they die. The NK cells give the immune system enough time to respond so SUPER IMPORTANT
NK receptors - what are the 2 types and how do they interact?
activating with ITAM motif, and inhibiting with ITIM motif.
Usually, the inhibitory receptors are dominant over the activating receptors, so when MHC Class I is expressed on an opposing cell, NK cells remain quiescent.
When MHC expression is blocked, the inhibitory signal is not provided and the activating receptors like NKG2D bind cell (via a variety of ligands that effect lysis).
what is the NK cell repertoire?
NK receptors are not generated by rearragnement, but 2-4 of 10-20 receptor genes are expressed on each NK cell, and all of the NK cells in the body make up the repertoire of these different combinations
KIRS vs KIRL
KIRS have short cytosolic tail a and a receptor that associates with ITAM-containing adaptors (activating)
KIRL have long cytoplasmic tails containing ITIM mofits - inhibitory
what receptor is clonally expressed in abT cells vs NK cells?
TCR clonally expressed vs KIR clonally expressed
what are the activating receptors on NK cells?
NKG2-D = recognizes SOS molecules on the surface of cells who have been infected and can’t apoptose - they are not MHC molecules
KIRS
what are the inhibitory receptors on NK cells?
KIRL
on which part of the chromosome are KIRs encoded?
LRC (leukocyte receptor complex) or Chrom 19
on which part of the chromosome are NKG2 receptors encoded?
NKC (NK cell receptor complex) on chromosome 12
how do NK cells complement CTL?
when MHC class I is downregulated, inhibitory receptors bearing inhibitory ITIMS on NK cells (KIRL) are not engaged and activating receptors (KIRS) are freed up to kill the targets = MISSING SELF HYPOTHESIS
NK cells can also kill when stressed cells express SOS molecules - which are so-called non-classical class I molecs that do not bind peptides like MICA and ULBPS - what is the receptor for these molecules?
NKG2D
functional characteristics of Memory cells - name 3?
- enhanced affinity or avidity for primary stimulus (lower doses required)
- enhanced frequency of reactive cells (extended life-span, lower turnover)
- decreased requirements for activation (responses to formerly recognized antigen)
what are the 2 types of memory T cells?
effector memory and central memory
phenotype of effector T cell?
CD45RA
phenotype of memory T cell?
CD45RO
where are effector T cells found?
non-lymphoid tissue
phenotype of central memory T cell vs effector memory T cell?
central: CD62L (l-selectin) hi and CCR7 hi
effector: CD62L lo and CCR7 lo
function kinetics of central memory T cell and Effector Memory T cell?
central: rapid proliferation and release of effector cytokines
effector: rapid release of effector cytokines and immediate killing
T cell exhaustion?
Activated effector T cells keep releasing inflammatory molecules like IL-2. IFN-gamma, and TNF and keeps putting up receptors that push the cell towards apoptosis - especially CTLA4 and PD1 (the EXHAUSTION MARKERS ON T CELLS)
Effector memory cells may not be very long lasting - but what is their general function?
to kill an infection if it returns in a certain amount of time without costimulation
why do B cells make a lot of IgG much faster in secondary immune response?
- precursor frequency is higher for the antigen (and memory T cells too)
- affinity is higher
- transmembrane form of IgG is special in that the cytoplasmic tail is long and contains signaling motifs lacking on the IgM/IgD tails - this enhanced signaling causes a burst of proliferation after antigen exposure and rapid differentiation into plasmablasts
Fab has how many binding sites? It can be separated out from an antibody by which proteolytic enzyme?
1 binding site; papain
F(ab’)2 hsa how many binding sites? it can be separated out from antibody by which proteolytic enzyme?
2 (so has avidity effect still); pepsin
why would you use Fab vs F(ab’)2?
to block something, high affinity Fab is good because it wont cross link and generate inflammation
if you do want inflammation, maybe use F(ab’)2 and put on it an Fc to recognize IgG1 or IgG2 to generate inflammation
CARs are what?
chimeric antigen receptors - a new class of molecules used for tumor therapy (lego antigen receptors) - engineer molecules good at binding to antigen and signaling for the appropriate response in order to flush out all of the tumor cells in a patient - esp used for B cell lymphomas
