Download The importance of the immune system

Functional Anatomy of the
Adaptive Immune Response
Jason Cyster
• Describe the principal functions of spleen,
lymph nodes, tonsil
• Explain how lymphocytes get from the
blood into a lymph node
• Describe the mechanism of dendritic cell
antigen transport
• Understand how B cells encounter antigen
• Describe changes in effector T cells that
permit migration to sites of inflammation
Lymphatics
(thin walled)
One-way valves
Lymph is filtered by
Lymph Nodes before
returning to circulation
(liters per day)
Lymph contains T and B
cells and dendritic cells
Anatomy of a
Lymph Node
afferent lymphatic vessels
radial sinus
germinal center
(secondary follicle)
Paracortex (T zone)
mainly T-cells
capsule
subcapsular sinus
]
Primary follicles
(B zones)
Cortex (T+B zone)
medullary sinus
Filter antigens from the lymph
- for recognition by T and B cells
- for destruction by macrophages to
prevent systemic spread
medulla
efferent
lymphatic vessel
Spleen - A filter of the blood
• Two main functions of the spleen carried out in two major
regions
1) White-pulp is where immune responses against blood-borne antigens
occur
2) Red-pulp is responsible for monitoring and removing old or damaged
RBC
• Red-pulp consists of thin walled splenic (or venous) sinuses
and dense collections of blood cells (including numerous
macrophages) that form red-pulp cords (or cords of Billroth)
• Blood supply: branches of central arteries open directly into
red-pulp cords, adjacent to the splenic sinuses (open
circulation)
– Released RBC must cross the sinus walls; interendothelial slits are a
major mechanical barrier and only the most supple, mechanically
resilient RBC survive; old and damaged cells are removed by
macrophages
Anatomy of
the Spleen
Follicle Red Pulp
(B zone)
cord
Splenic (venous)
sinus
White-Pulp cord
PALS or T zone
(periarterial lymphoid
sheath)
Pulp
vein
Capsule
Trabecular vein
Trabecular artery
SECONDARY LYMPHOID ORGANS
Splenic White-Pulp
Cord
Lymph Node
lymph fluid
Mucosal lymphoid
tissue (e.g. Tonsil, Appendix)
intestinal contents
B cell
follicle
hev
ca
blood
T cell
zone
hev
red-pulp
- filter antigens from body fluids
- bring together antigen presenting cells and lymphocytes
- help bring together antigen reactive B and T cells
Lymphocytes traverse HEVs to enter lymph nodes and
then compartmentalize in B cell follicles and T cell zones
Follicle or B zone
- B cells
- FDCs
HEV
(High
Endothelial
Venule)
T cell zone (paracortex)
- T cells
- DCs
LN section stained with:
B cell marker
L-selectin ligand
The cascade (multistep) model of leukocyte
extravasation
Selectins
• lectins are sugar (carbohydrate)-binding proteins;
selectins are a specialized type of lectin that bind
appropriately glycosylated membrane proteins
• Three types:
– L-selectin: restricted to lymphocytes
• ligands on lymphoid tissue HEVs
– P-selectin: made by platelets and activated (inflammed)
endothelium
– E-selectin: made by activated (inflammed) endothelium
• E- and P-selectin ligands expressed on neutrophils,
monocytes, activated T lymphocytes
Chemoattractant-Cytokines or “Chemokines”
>40, small secreted proteins
Four families: C, CC, CXC, CX3C
chemokine
outside
chemokine receptor
cytoplasm
Cells with the appropriate receptor migrate (chemotax) up chemokine gradient
Chemokines also promote cell adhesion to endothelium
Lymphoid chemokines – help direct the homeostatic trafficking of cells through
lymphoid tissues (e.g. CCR7 / CCL21; CXCR5 / CXCL13)
Inflammatory chemokines – induced at sites of inflammation; can be expressed by
many cell types; help recruit cells to these sites (e.g. CXCR2 / IL-8; CCR2/MCP1)
Integrins
• heterodimers of  and  polypeptide chains
• can be in inactive (low affinity for ligand) and active states
• intracellular signals can cause ‘inside-out’ signaling in the integrin,
converting it from an inactive to an active state
• chemokine and antigen receptor signaling can activate integrins
• bind extracellular matrix proteins (e.g.  binds fibronectin)
or transmembrane proteins
(e.g.
integrin LFA1 binds ICAM1;
integrin 
binds VCAM1)


Cascade Model of Lymphocyte Recruitment
into Lymph Nodes
HEV
blood flow
STEP 1: Tethering and Rolling: L-Selectinglycosylated HEV ligands
STEP 2: Integrin Triggering: Chemokines (e.g. CCR7- CCL21)
STEP 3: Firm Adherence: LFA1/ICAM1
STEP 4: Transmigration
HEV
Lymphoid Tissue
Multi-step cascade of lymphocyte migration to site of infection/inflammation:
same logic, different ‘area code’
• Inhibitor of integrin (tysabri) in clinical use for treatment of Multiple Sclerosis
Schematic view of a lymph node
CXCL13
Follicle
Paracortex
CCL21
Medulla
CXCL13 -> CXCR5
CCL21 -> CCR7
B zones produce a B cell attracting chemokine
T zone produces T cell and DC attracting chemokines
DCs migrate from periphery to lymphoid
organ T zone bearing Ag
• immature ‘sentinel’ DCs are present in most tissues,
continually sampling their microenvironment for antigen
– by pinocytosis, phagocytosis and engulfment of apoptotic cells
• detection of ‘danger signals’ (e.g. LPS, dsRNA,
bacterial DNA, necrotic cells, TNF, IL-1) causes the
cells to mature
– decrease adhesion to local tissue cells (e.g. keratinocytes)
– increase expression of receptors (CCR7) for chemokines
made by lymphatic endothelial cells and lymphoid organ T
zones
– upregulate MHC and costimulatory molecules
• migrate into lymphoid T zone
• present antigen to T cells
Immature (sentinel) DCs in tracheal epithelium
longitudinal
section
tangential
section
DCs migrate from periphery to lymphoid organ T zone
bearing antigen
Skin draining Lymph Node
Follicle
T zone
DC
LC = Langerhans’ Cell, the immature DC of the skin
B cell antigen encounter
antigen
lymph fluid
Sinus Macrophage
FDC
hev
• B cells bind intact antigen through their surface Ig / B cell receptor (BCR)
• Antigen that enters via blood or lymph reaches the follicle and can be captured
directly by B cells
• Follicular dendritic cells (FDCs – tissue resident cells related to fibroblasts) can
display antigen on their surface in an intact form for long periods
Lymph node egress occurs in response to a circulatory
lipid (sphingosine-1-phosphate, S1P)
MEDULLARY SINUSES
S1P
Diagram courtesy of Ted Yednock
-Lymphocytes express a
receptor (S1PR1) for S1P
- Egress involves migrating
to S1P that is high in
blood/lymph and low in the
tissue
Lymphocytes express S1PR1 and exit lymphoid
organs in response to S1P
lymph node, Peyer’s patch
thymus, spleen
S1PR1
S1PR1
S1P lyase
S1P lyase
Immunosuppressant
RBC
S1P
blood
S1P
FTY720
efferent lymph
•
S1PR1 is required for T cell egress from thymus and for T and B cell egress
from spleen, lymph nodes, tonsil
•
Activated lymphocytes transiently down-regulate S1PR1 and are retained in the
responding lymphoid tissue until they become effectors
•
FTY720 (Fingolimod) inhibits egress and is in clinical development as an
immunosuppressant (FDA approved in 2010 for treatment of multiple sclerosis)
Effector T cell Trafficking
• Activated T cells exit lymphoid tissue ->
circulation
– upregulate S1PR1
– ability to re-enter lymphoid tissue is reduced
(decrease in CCR7, L-selectin)
• Increased ability to enter inflammed tissue
due to increased expression of:
– ligands for E- and P- selectins
– receptors for inflammatory chemokines (e.g. CXCR3)
– adhesion molecules (e.g. integrin )