MCDB 436 (10): T&B Cell Cooperation

how CD8 T cells expand without direct APC infection

- viral proteins taken up by virally-infected cells - then, viral proteins are degraded inside the DC and presented by class II and class I via cross-presentation - thus: Ag peptides cross from class II to class I

phagosome-to-cytosol pathway

a form of MHC I cross-presentation Ag within the phagosome escapes to the cytosol either by membrane disruption or through an active translocation mechanism - cytosolic Ag then loaded onto MHC I through regular pathway

vacuolar pathway

a form of MHC I cross-presentation - Ag cleaved by proteases within the endocytic vesicle - MHC I molecules containing vesicles recruited from the plasma membrane or ER - vesicles fuse, the peptide is loaded onto MHC I in the endosome, fuse with the cytoplasmic membrane

B cell -> IgM

CD4 (with CD40L and cytokines) IL-21

B cell -> IgG subclasses

CD4 (with CD40L and cytokines) IFN-gamma

B cell -> IgE

CD4 (with CD40L and cytokines) IL-4

B cell -> IgA

CD4 (with CD40L and cytokines) mucosal tissues; cytokines, such as TGF-beta

CD4 help - types

Tfh: B cell proliferation + memory development Th: isotype switching needed in spleen, lymph nodes, MALT

TD antigens

T cell dependent - B cell require T cell help for activation and proliferation

TI antigens

T cell-independent - B cell does not require T cell help for activation and proliferation - TI-1 and TI-2 (TI-1 not really antigens; non-specific proliferation factors)

TI-2 antigens

have highly repetitive structures (ex: bacterial capsular polysaccharides) - do not contain intrinsic B-cell-stim activity - multiple cross linking of BcR by TI-2 can lead to IgM Ab production activate by 1) PRR-MAMP binding 2) BcR binding by multiple cross linking

____ is the driver of all isotype switching in B cells

Tfh

3 signals for B cell activation by TD Ags

1) BCR ligation; strengthened by co-receptors CD19 & CD21. but the BCR epitope is NOT the same as the MHC II epitope 2) activated CD4 T cells; strengthened by CD40L (T-cell) interaction with CD40 (B-cell) 3) cytokines; may be the CD4 Tfh cell involved in immune synapse with the B cell, or they may be produced by other nearby cells (innate or adaptive)

immune synapse's role for T cells

provides a structure for directed secretion of T cell cytokines

naive T cells, after encountering a DC, can become: (2)

1) an effector TH1, TH2, or TH17 cell 2) a pre-Tfh cell

formation of a Tfh cell

naive T cell -> DC interaction -> pre-Tfh cell -> engage with B cell -> Tfh cell

CD40/CD40L

CD40L: co-stimulatory on Tfh CD40: co-stimulatory on B cell

Tfh cell - cytokines

the cytokines produced depends on the environment it's in; involved in type 1, 2, 3/Th17 responses - IFN-gamma -> activate B cells -> produce IgG Abs (type 1) - IL-4 -> B cell differentiation -> produce IgE -> mast cells, basophils in type 2 response - IL-17 -> Abs against type 3

C3d's role in TD Ags

binds to CD21 in receptor-mediated endocytosis

linked recognition

TD Ab responses require B-cell activation by T-cells responding to the same Ag as B cell

T/F: TD Ags stimulate Ag-specific T cell and B cell responses to the SAME Ag but different epitope

true

B-cell T-cell interactions - location

between the B cell and T cell zone

follicular dendritic cells (FDCs)

dendritic in morphology; unrelated to DCs - secrete CXCL13, driving the organization of B cells into follicles and recruiting B cells from circulation goal: increase surface area to be covered with complement receptors/FcRs - decorated with Ags derived from the body - a little remains to maintain constant low-level stimulation of immune response

opsonized Ags in B-cell follicles

C3b trapped by CR1 C3d trapped by CR2 on surface of FDCs

fate of activated B cells

1) move back into follicle to form a germinal center 2) migrate to form Ab-secreting plasmablasts in spleen or lymph nodes

germinal center

site of sustained B-cell proliferation and differentiation

centroblasts

germinal center B cells that express CXCR4/5 and form the dark zone of the germinal center

centrocytes

express only CXCR5

secondary follicles

follicles in which germinal centers have formed

SHM of a B cell

introduces mutations into the rearranged Ig V regions of B cells, improving Ag binding in secondary, tertiary etc responses - mutations accumulate during clonal expansion of B cells - mainly single base substitutions - 10^-5 to 10^-3 mutations per base pair per generation

three processes that modify the rearranged Ig gene

- primary Ig formation - somatic hypermutation - class switch

Ab binding to FDC

binds to FDC's FcRs - initial IgM binds, then Abs with enhanced affinity for Ag

Tfh cell's role in affinity selection

B cells mutate Ab genes in dark zone -> B cells with high affinity capture and process antigen for MHC II -> B cells that present Ags to Tfh cells receive survival and mitogenic signals via CD40 and cytokines -> B cells that receive help from Tfh cells can reenter the dark zone to undergo add'l mutations

B cell mutation affinity selection

- mutations that increase Ab affinity: enhance FDC-B and B-Tfh interactions -> positive selection -> proliferation - mutations that decrease Ab affinity: decrease FDC-B and B-Tfh interactions -> negative selection -> death

________ and _________ increase with repeated immunization

affinity :: Ab amount

___, ___, and ____ in B cells are driven by CD4 Th cells

Ab amount :: affinity :: isotype switching

activation-induced cytidine deaminase (AID)

initiator of mutations in SMH and isotype switching - expressed only B cells - cytosine -> uridine conversion - either mismatch repair + error-prone polymerase or base-excision repair

how AID works

AID -> C to U -> A:T point mutations -> SMH UNG -> U to abasic -> C:G point mutations -> SMH APE1 -> double-strand breaks -> class switch recombination

switch regions

location where double-stranded DNA breaks occur - downstream switching: loop out and lose intervening DNA, cannot return to other isotypes

resting B cell intrinsic and Ag properties

- surface IG: high - surface MHC II - no Ig secretion inducible by Ag stimulation - growth - SMH - class switch

plasma cell intrinsic and Ag properties

- surface Ig: very low/absent - surface MHC II - high Ig secretion do not proliferate anymore; not inducible by Ag stimulation

Ig isotypes selective distribution in the body

- IgG: predominate in blood, major Abs of ECF - IgA: major Abs of ECF, predominate in secretions across epithelia - IgE: mainly with mast cells beneath epithelial surfaces (GI, respiratory, skin) - no Igs in the brain usually

dimeric IgA

the major class of Ab in the lumen; highest # Ig in the body - binds to bacterial microbiota; preventing from invading mucus and adhering to epithelium - pIgR - polymeric Ig receptor (binds dimeric IgA and pentameric IgM) - secretory component generated by cleavage of pIgR

ADCC

antibody-dependent cell-mediated cytotoxicity - get specificity from binding to Ab - have CD16 on surface - encounter IgG1 or IgG3 -> rapidly kill

mechanisms against intestinal parasites

B cells - make Ab, IgE Th2 cells - initiate eosinophil and mucus production IgE - mast cell activation mast cells - kill worm eosinophils

mechanisms against allergic responses

B cells - make Ab, IgE Th2 cells - initiate eosinophil and mucus production IgE - mast cell activation mast cells - kill infectious agent eosinophils same as parasitic

IL-13 effects

- epithelial cell repair - goblet cells -> mucus - contractility of smooth muscle cells - M2 macrophage products (TH2 product)

IL-5 effects

- M2 macrophage products - eosinophils -> MBP -> kills parasites; also mediate ADCC (TH2 product)

IL-9 effects

mast cell products - histamine, TNF-a, MMCP (TH2 product)

IL-3 effects

mast cell products - histamine, TNF-a, MMCP (TH2 product)

IFN-gamma effects

bind to macrophage's IFN-gamma receptor -> become highly microbicidal (TH1 product)

FasL and LT-beta effects

induce apoptosis of bacteria-laden macrophages - mediates *extrinsic* pathway of apoptosis (TH1 product)

IL-2 effects

act on naive CD4 and CD8 T cells - CD4: favor TH1 over Tfh - CD8: support expansion (TH1 product)

IL-3 and GM-CSF effects

stimulate production of monocytes by bone marrow (TH1 product)

CCL2

chemoattractant for monocytes (TH1 product)

granuloma

form when an intracellular pathogen or its constituents cannot be completely eliminated - central core of infected macrophages + epitheloid outer layer - central core surrounded by CD4 TH1 cells (primarily)

IL-17 effects

- induce epithelial AMPs - stimulate stromal + myeloid cell -> G-CSF -> bone marrow -> neutrophil production; + -> chemokines -> neutrophil recruitment (TH17 product)

IL-22 effects

- induce epithelial AMPs - increase epithelial cell turnover

CCL20

produced by TH17; chemoattractant for other TH17 cells

Treg cell

- prevent autoreactive immune responses by making immuno-supressatory cytokines

iTreg cell

prevention of inflammatory immune responses to the commensal microbiota, particularly microbes resident in mucosal tissues such as the intestines. ~ “prevent inflammatory & allergic diseases” (i for inflammatory)

nTreg cell

develop from potentially self-reactive T cells that express conventional α:β T-cell receptors and are selected in the thymus by high-affinity binding to MHC molecules containing self-peptides. ~ “prevent autoimmunity” (n for NO autoimmunity!)

TGF-b effects

- inhibit T-cell proliferation (Treg product)

IL-10 effects

- inhibit expression of MHC + co-stim molecules by APCs - inhibits production of pro-inflammatory cytokines by APCs (Treg product)

immune responses are tuned to the ___ (5) of the response

specificity, effector, pathways, robustness, duration

CD4, CD8, and B cells can undergo primary and secondary responses because:

- clonal expansion - development of memory cells

smallpox eradication

completely eradicated because out of transmission and humans were the only vector - recall responses measured in people who were vaccinated for smallpox can be taken to represent true memory in the absence of reinfection

primary vs secondary response

- primary: usually Abs made by plasma cells derived from more diverse precursor B cells; relatively low affinity - secondary: frequency of Ag-specific B cells increases up to 1000 fold; higher affinity

immunological memory

- long-lived Ag-specific lymphocytes persist until a second encounter with the pathogen - maintained by cytokines (IL-7, IL-15) produced either constitutively or during Ag-specific immune responses - # of memory cells is highly regulated; long half-life if pre-existing antibody can completely neutralize the pathogen, a secondary immune response may not occur

memory B cells

largely responsible for secondary response; almost immediate production of IgG - can arise from the germinal center rxn during a primary response - can undergo isotype switching and SMH at the T-B cell zone interface - recirculate through the same secondary lymphoid compartments as naive B cells memory B cells are in limbo. once they encounter the Ag, they go back into the lymphoid organ and differentiate

undifferentiated vs differentiated memory B cells

- undifferentiated: migrate into the follicle -> germinal center B cells -> SMH -> plasma cells - differentiated: produce Abs

B cell affinity

- higher affinity: capture Ag on FDC, process and present on MHC II -> CD40L and IL-21 help -> survival and proliferation - lower affinity: fail to capture/present Ag

how secondary Ab response differs

- earlier, vigorous generation of plasma cells -> abundant production of IgG - higher MHCII and co-stim -> increases Tfh cytokine expression

memory T cells

most die after Ag clearing, but some remain. applies for BOTH CD4 and CD8 - arise from effector T cells - IMPORTANT: IL-7Ra - central-memory T cells: recirculate between 2ndary lymph organs and blood - effector-memory T cells: circulate through blood and home to inflamed tissue - tissue-resident memory T cells: retained in tissues

IL-7Ra

naive T cells express it, but not expressed on effector T - IL-7 critical for memory cell maintenance (prevent apoptosis)

IL-7 effects

provides continuous signals to resting naive and memory T cells to prevent apoptosis (stromal cell product)

central memory T cells

- express L-selectin and CCR7 (for movement to lymphoid tissue) - primarily migrate from the blood -> secondary lymphoid organs -> lymphatic system -> blood

effector memory T cells

- no CCR7; have B1 and B2 integrins -> rapidly enter inflamed tissues - express inflammatory chemokine receptors -> can release effector cytokines after maturing - migrate from blood -> peripheral nonlymphoid tissues -> afferent lymphatic system -> secondary lymphoid tissues

tissue-resident memory T cells

- do not migrate; take up long-term residency in mucosal sites - no CCR7; express other chemokine receptors for migration

mRNA vaccines

induce strong CD8 responses - either fuses -> uncoats or receptor-mediated endocytosis - fed proteins through exogenous pathway or cross-presentation -> activate MHCs