Secondary lymphoid organs, homing, GCs Flashcards
Name 3 different APCs in the lymph node.
DCs
macrophages
B cells
How do lymphocytes move?
amoeboid movement after extravasation
passive transport via bloodstream
Lymphocyte homing is a combinatorial decision process with the sequential engagement of adhesion and signalling receptors. Describe this process in detail.
leukocytes travel towards the gradient of the chemokine they have receptors for:
- CCR7 for CCL19/21 from the lymph node
- alpha4:beta7 integrin for MAdCAM-1 in the gut
- CLA for E selectin in the skin
cells at the site of extravasation express P and E-selectin on their surface, which slow down the passing lymphocytes by binding oligosaccharydes on their surface -> rolling
at the site of extravasation, cytokines (e.g. CXCL8) are caught in the ECM -> lymphocytes bind it with the receptors on their surface -> triggers conformational change of LFA-1 to high-affinity state -> can bind I-CAM1
LFA-1:ICAM-1 interaction also used for squeezing out the vessel through the endothelial cells
Ž - Describe anatomy of the lymph node.
cells come in through afferent vessel and exit by passing between HEV cells
major areas: subcapsular sinus B cell zone (= where B cell follicles will form -> GCs) T cell zone medulla efferent vessel
multiple locations in the body
Ž - Describe anatomy of the spleen.
red pulp = site of red blood cell destruction
white pulp = where lymphocytes are
around the arteriole going into the spleen, there is periarteriolar lymphoid sheath (PALS) = T cells
next to it = B cell zone -> germinal center with B cell corona and marinal zone around it
both T and B zones surrounded with perifolliuclar zone that separates them from the red pulp
spleen found above pancreas/stomach
Ž - Describe the anatomy of Peyer’s patch.
contains numerous B cell follicles with GCs, surrounded by T cell dependent areas
surface between surface epithelium and follicles = subepithelial dome
has no afferent lymphatics, but antigens enter directly from the gut via M cells
lymphocytes enter through HEV
in gut mucosa
Ž - Antigen recognition can be thymus dependent or independent. What are the properties of each type of antigens? Provide an example for each.
Thymus dependent = T cell help needed for recognition
leads to isotype switching and somatic hypermutation in GCs
e.g. Rh factor
Thymus independent = no T cell help (epitopes are repetitive and lead to strong BCR signaling), DCs might help (via BAFF)
isotope switching is very limited (only BAFF -> IgG switch), somatic hyper mutation does not happen
2 types: TI-1 and TI-2 (type 1 does not need a repetitive epitope)
e.g. AB0 system, LPS
Ž - Thymus independent antigens trigger B cells to produce ABs upon recognizing them. Describe how this response is dependent on the dosage of the TI-1 antigen present.
low concentration of TI Ag: only B cells with high affinity for this Ag will recognize it and start proliferating etc.
high concentration of TI Ag: a lot of cells will recognize
Natali:
TI1-ANTIGEN-SPECIFIC AB RESPONSE vs. NONSPECIFIC AB RESPONSE (POLYCLONAL B-CELL ACTIVATION)
Ž - Which cytokines/ligands are needed from B cell survival, differentiation into plasma cells, SHM, class switching?
IL-4
IL-21
CD40
(there’s more, but I think these are essential)
Ž - When T and B cells interact in secondary lymphoid tissues, they still move around even though they are interacting with each other. Which cells lead the way?
B
Ž - Describe the formation (formation only) of the germinal center in the lymph node.
before recognizing their cognate Ag:
CXCR5+ B cells reside in B cell follicles where CXCL13 is
CCR7+ CD4 T cells are in T cell zone where CCL19 and CCL21 are
- already activated by DCs upon MHC:p recognition -> Tfh!
after Ag recognition (displayed on fDCs or subcapsular sinus macrophages):
B cells induce CCR7, T cells induce CXCR5
= both migrate towards the border where they meet and interact
- B cells also expresses EBI2
activation of B cells by Tfh:
a) some go to the outer follicle and establish primary focus
b) most go to the primary lymphoid follicle with their associated Tfh and form a germinal center (there: EBI2 downregulation)
Ž - Describe what happens in the germinal center. Which cells are found there?
cells: mainly proliferating B cells, up to 10% of Ag specific Tfh cells, fDCs
GC is an area of active cell division
proliferating B cells displace the resting B cells into a mantle zone in the periphery of the follicle
active B cells are in light and dark zone
cells circlate between these two zones:
CXCL13 in light zone = CXCR5+ cells there (downregulated CXCR4)
CXCL12 in dark zone = CXCR4+ (&CXCR5+) cells there
light zone = less proliferation than in the dark zone, T cells and fDCs also present for chcking the affinity maturation of rearranged receptors
dark zone = SHM taking place, only B cells
Ž - What is the biggest difference between the result of primary focus and germinal center reaction?
quality of the antibody they produce
PF: IgM, no improving GC: SHM, affinity maturation, class switching
Ž - Which cells in lymph nodes support B cell survival and differentiation after Ag recognition?
fDCs
stromal
epithelial
express BAFF, APRIL
Ž - What is the role of the macrophages in subcapsular sinus of the lymph node?
opsonized pathogens are bound by complement receptors on the surface of these macrophages
these macrophages have low endocytic and degradative activity, so they preserve the antigens trapped on their surfaces
= allow B cells to recognize it or fDCs to take it and present inside the follicles
Ž - Which molecule in the cytoplasm of Tfh cells causes prologned contact between activated Tfh and C cell?
SAP
Ž - What are fDCs?
follicular dendritic cells
not from HSC (not classical DCs)
have long dendrites, form dense network in GC
present AG IN A NATIVE FORM
via Fc and compement receptor
can retain Ag for months
Ž - When selecting higher affinity BCR in germinal centers, what is the limiting factor?
competition for T cell help
not competition for Ag
Ž - Which enzyme is required for affinity maturation processes in B cells in GCs? Which regions of DNA are mutated by this enzyme?
AID
mtuations in V region, where it recognizes specific DNA motifs (WRC)
Ž - What is the result of GC reaction, which 2 cell types are produced?
long-lived plasma cells
memory B cells
Ž - In GC reaction, long-lived plasma cells are produced. What are their properties?
produce large amounts of high-affinity ABs
relocalize to BM after leaving the GC, because that is specialized niche for B cell long-term survival (except mucosal plasma cells)
have abundant rER, well developed Golgi
(also short-lived exist in medullary cords and red pulp)
Ž - In GC reaction, memory B cells are produced. What are their properties?
long-lived
express surface Ig, but do not secrete ABs
recirculate the body in resting state, but are activated and rapidly differentiated in plasma cells upon Ag recognition
= prevent re-infection with already known Ag
Ž - GC response is mainly mediated by expressing different TFs that regulate cell fates and expression of proteins. What else besides proteins has an important regulatory role in the cells?
micro RNAs
Ž - Describe S1PR1 signaling and its effect on lymphocyte migration.
S1PR1 recognizes S1P = high concentration in blood
naive mature CD4 T cells upregulate S1PR1
= go in blood stream
S1P binding downregulates S1PR1
= T cells will downregulate the receptor and exit at the first possible site
= recognizing HEV in lymph nodes
when in lymph node: upregulation of receptor again
a) if no Ag recognized, return to blood stream
b) if Ag recognized: activation marker CD69 upregulation
= CD69 inactivates S1PR1
= T cell is temporarily retained in lymph node (because after a while, CD69 gets downregulated, but in the meanwhile, T cell clonally expanded in the LN)
= eventally, T cell with S1PR1 upregulated returns to blood and has effector function now
