Humoral Immune Responses Flashcards
(35 cards)
Mature Naive B Cells
Resting mature naive B cells express
BCR: IgM, IgD, Iga &IgB Co-BCR: CD19, CD81 & CR2 (CD21) HLA-Class II CD40 CD45R(A) CD20
B cells can be divided into two major subsets
B1 and B2
B2 cells can also be divided into two major subsets
-Follicular B cells are re-circulating B cells (majority)
-Marginal B cells reside in the spleen blood-borne polysaccharide Ags
Migration of Naive B cells
Travel to Secondary Lymphoid Tissue (SLT) - Primary lymphoid follicles -spleen: enter via blood -Lymph nodes: enter via Lymphatics Passage through SLT -enter through HEV - no Ag, migrate to primary follicle (CXCR5) -receive signal to survive (FDCs) -exit through efferent lymphatic vessel
Competition for Survival Signals
- took nay B cells, not enough FDCs to provide survival signals
- naive B cells die within weeks in absence of Ag
B Cell Activation and Expansion Ag dependent phase
- Response initiated by recognition of Ag (epitope) by B cell specific for that Ag (idiotope)
- Ag binds to mIg on naive cells and activates these cells
- Activation can occur in a T dependent or T-independent manner
- -Complete activation requires 2 signals
B Cell Activation: First Signal
-Ag recognition by miss
-Must crosslink 2 or more BCR
-Signaling occurs through Iga and IgB cytoplasmic tails
—the IC signaling steps in B-cell activation are identical to those of T cells. The only differences lie in the SRC-family kinases involved in the initial IC signaling steps
-Prepares cell for interaction with 2nd signal
—process Ag
—Biochemical signaling
Ag binds to BCR —> turns on ITAM —> Fyn, Lyn, Blk phosphorylates Iga and IgB —> Syk comes and binds to IgB —> PLCy activation —> inositol triphostae —> increased cytosolic Ca2+ —> ca2+ dependent enzymes —> Myc and NFAT
PLCy activation —> diacylglycerol (DAG) —> PKC —> NFAT and NFkB
Blk, Lyn, Fyn phospharyaltes Syk —> Syk phosphorylates adapter proteins —> GTP/GDP exchange on Ras, Rac —> Ras GTP, RAC GTP —> ERK, JNK —> AP-1
B Cell Activation: First Signal cont
- Cross-linking of BCR by Ag generates a signal that is necessary but NOT sufficient to activate naive B cells
- Ag with bound C3d recognized by miGs & CR2
- CR2 provides cross-linkage for signaling
- Signaling occurs through Iga and IgB CR2, &CD19 cytoplasmic tails (BCR co-receptor complex)
- If C3d is attached to protein Ag Ag is 1000x fold more immunogenic
- TLR signaling through cytoplasmic domains
Microbial Ag binds to BCR and bound CD3 —> BCR signaling and enhancement of BCR signaling —> proliferation of differentiation
Microbial Ag binds to BCR —> PAMP from microbe activates TLR —> TLR signaling along with BCR signaling —> proliferation and differentiation
Outcomes of First Signal
Ag binding to cross-linking of membrane Ig —> Changes in activated B cells
—> expression of proteins that promote survival and cell cycling —> increased survival proliferation
—> Ag presentation increased B7 expression—> Interaction with helper T cells
—> Increased expression of cytokine receptors —> Responsiveness to cytokines
—>increased expression of CCR7. Secretion of IgM —> Migration from follicle to T cell areas
Migration of Activated B Cells
- After activation by Ag in the follicular area, B cells change their chemokine receptor expression and migrate to the edge of the follicular zone
- Activated B cells secrete low levels of IgM and increase expression of co-stimulators molecules and cytokine receptors
Ag presentation, T cell activation —> CCR7 decreases and CXCR5 increases (B cell chemokine, allows them to move towards B cell) ) and migration of activated T cells to edge of follicle—> B cells present Ag to activated helper T cells —> Ag uptake and processing, B cell activation, increase CCR7 (allows them to move towards T cells) and migration of activated B cells to edge of follicle —> B cells present Ag to activated helper T cells —> Ag uptake and processing, B cell activation, CCR7 (allows them to move towards T cells) increases and migration of activated B cells to edge of follicle
Second Signal
TI-1 Ag (B cell)
-mitogen
TD Ag (B cell)
- T cell dependent to activate, has to be a protein Ag —> only thing T cells recognize
- contact dependent
Cellular Sequence of B cell Activation
- APC (DC) HLA class II to interact w/T-Cell (CD4) (MHC binds with TCR)
- B7 of DC binds with CD28 of Th Cell
- CD40 of DC binds with CD40L (provides 2nd signal)
- All of these make up the immune synapse —> proliferation and expansion
T Dependent Immune Synapse
- The costimulatory signals are generated through the interactions of CD40:CD40L and adhesion molecules
- Th cytokine modulated class switching
- Inuced expression of activation-induced delaminates (AID) enzyme
- Affinity maturation (somatic hyermutation) of secreted Abs (the last 2 happen at the same time)
- -Class switching and affinity maturation often occur at the same time
TCR of T cell binds to MHC class II of B cell —> CD28 of T cell binds to B7 of B cell —> CD40L of T cell binds to CD40
Migration of Activated B Cells
Had the meet and greet w/T cell, now is fully activated
After receiving T cell help, B cells change their chemokine receptor expression and migrate to the follicular area and establish germinal centers in the follicules
Activated B cells begin cytokine modulated class switching and affinity maturation of receptors
Successful re-arrangements are selected/supported by Tfh and follicular dendritic cells (IgM is coming out)
- DC binds with naive CD4+ T cell —> T cell activation —> helper T cells —> Initial T-B interaction —> short lived plasma cells (extrafollicular focus) —> germinal center reaction
OR
- B cell activation —> initial T-B interaction —> short-lived plasma cells —> (extrafollicular helper T cells) —-> germinal center reaction (follicular dendritic cell and follicular helper T cell)
T Follicular Helpers
Tfh
- CD4+/low levels of CD25 expression (been activated by DC)
- ICOS/ICOS-L essential for germinal center reaction
- Secrete IL-2L facilitates differentiation from B cell to plasma blasts
- Provides IFN-y and IL-4 for cytokine switching
- DC binds naive CD4+ T cell —> activates T cell (expresses CXCR5) —> Tfh cell expresses ICOS binds with ICOS-L activated B cell —> germinal center B cell bound with follicular dendritic cell
Tfh cell —> follicular helper T cell (while bound with B cell at TCR-MHC II and CD4-;CD40L, follicular helper T cell (secretes IL-21)
Class Switching in the Germinal Center
- Cytokines released by Th cells promote two general functions
- The first is to induce H chain class switching
- The second function is to augment C cell differentiation and proliferation
- There is great redundancy in this system as many cytokines have overlapping functions
-Each cytokines has multiple effects and acts on multiple cell types
Helper T cell (binds with activated B cell with CD40:CD40L and TCR-MHC II)
—> IgM B cell —> IgM - complement activation
—> ? —> IgG subclasses (IgG1, IgG3) - opsonization and phagocytosis; complement activation; neonatal immunity (placental transfer)
—> IL-4 —> IgE and IgG4 — Immunity against helminths, mast cell degranulation (immediate hypersensitivity)
—> (mucosal tissues, cytokines (e.g. TGF-B, APRIL, BAFF, others) —> IgA — mucosal immunity (transport of IgA through epithelia)
Switch Recombination
CD40:CD40L ligation and cytokines trigger isotype switching and affinity maturation by:
- modulation of switch regions
- Increasing the accessibility of the DNA at a specific C region
- T-dependent Ag
- Expression of activation-induced delaminates (AID)
Rearranged VDJ gene segment recombined with a downstream C region gene and the intervening DNA is deleted
*Cannot go back after you have spliced past gene, AID aids in the splicing (like RAG)
Affinity Maturation
-Introduction of point mutations in the switch regions of the variable areas of the Ig genes resulting in an expansion of the Ab repertoire to generate high-affinity Ag-specific antibodies
Somatic Hypermutation: 10^3 to. 10^4 times higher than normal spontaneous mutation rates
Key enzyme: AID, converts Cs to Us allowing APE I endonuclease to relate double-stranded breaks in the DNA —> thereby causing a low-affinity Ab to become a high-affinity Ab by creating point mutations
-Can be useful, sometimes not
Somatic Hypermutation
Day 7 of primary response - heavy chains and light chains only show one point mutation
Day 14 of primary
-Both chains show more point mutations
Secondary
-about 3x as many point mutations in both regions
Tertiary
-Many more point mutations in both regions
By the time you get the tertiary response the affinity is very high, gets “better, better, and better” diversity also increases
Cytokine Influence of Class Switching
Naive B cell or Naive B cell +LPS —> no isotpye switching
Naive B cell+LPS+IL-4 —> isotype switching to epsilon + gamma
Naive B cell+LPS+TGF-B —> alpha and gamma 2
*switch to one or another, not both
Selection Checkpoint
Checking to make sure the point mutations still bind with the epitope- otherwise they will be deleted
- Selective survival of the B cells producing the highest affinity Abs occurs in the germinal center(s)
- FDC and Tfh are interactions with high affinity B cells is necessary for survival-if not they will die
- FDCs provide intact Ag in the form of complexes called immune complexes
T-Independent Ag B cell Activation
B-1 cells respond to T independent responses (non-protein) Ag in mucosal tissues
-Marginal zone B cells (B-2) in spleen recognize blood-borne polysaccharides
T dependent
Follicular B cells binds with protein Ag —> Helper T cell binds with B cell —-> isotype-switched; high affinity Ab, memory B cells, long lived plasma cells+IgG, IgA, and IgE
T independent
B1 cells, marginal zone B cells binds with polysaccharide Ag —> other signals i.e. complement protein, microbial product —> Mainly IgM, low affinity Ab, short lived plasma cells+IgM
-Secrete natural Ab—> IgM in response to mucosal biome (commensal in gut) that interact with blood—> why you react with other blood
Plasma Cells
- Terminally differentiated effector B cell
- Short versus long-lived
- Cell markers- decrease CD19 and CD20, HLA class I, increase CD27, sIg class dependent
- Secrete Abs at rates ranging from hundreds to thousands of Ab per second per cell!
- Expansion of ER in cytoplasm
- Abs: effector molecules of humoral immunity
Memory B cells
- only for PROTEIN Ag
- T DEPENDENT process
- Survive for long periods of time without additional Ag stimulation
- Express high levels of the anti-apoptotic protein Bcl-2, which contributes to their long life span
- Surface markers: CD27 and CD45R(0)
- sIg (=BCR) class DEPENDENT to what they switched to
- Capable of mounting a rapid response to subsequent exposure to same Ag (secondary immune response)
Antibody feedback
-turn it down
- Control mechanism triggered by secreted AB that blocks further AB production
- IgG ONLY
- Ab excess
- Bind to Fc -> ITIM (blocks further activation)
(FYI ITAM turns it on —> interact with Syk instead of ZAP70)
A. BCR signaling leads to PIP3 formation, which binds other signaling molecules, leading to activation
Polyvalent Ag —> Syk binds —> PI3K —-> leads to phosphorylation PIP2 to PIP3 —> BTK, PLCy, PDK1
B. Fc receptor-associated phosphates, SHIP, converts PIP3 to PIP2 in B cell - receptor complex, blocking downstream signaling
Polyvalent Ag makes Ab-antigen complex —> SHIP turned on, turns on ITIM —> PIP3 to PIP2 —> Block in B cell receptor signaling, no activation
Clinical Blue Box-DM
3yo, high temperature, high RR, low O2 saturation, enlarged neck and axillae LN
—> from this we know there’s an infection- acute inflammation response (innate)
What do you want to know?
- sick contacts?
- happened before?
- travel
PMH- 10 previous episodes of otitis media, had pneumonia once before, received DTap vaccine
Now what more info do you want?
-FH
No sibs, no relevant med history in family
-Doesn’t tell us much
Clinical Blue Box- DM
Labs
High WBC
High IgM
Low IgG
IgA and IgE- undetectable
Blood- positive for streptococcus pneumoniae
Now what do you think? -she’s not class switching
Has proper response to IgM titter, IgG was undetectable- not class switching She had no specific IgG Ab against tetanus toxoid -This tells us she’s not responding to protein Ag (TD-Ag, T cell activation needed for class switching) She is responding to TI-Ag —> polysaccharide
