Functional Anatomy of the Adaptive Immune Response

• 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

Jason Cyster

Lymphatics

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

(thin walled)

]

afferent lymphatic vessels

efferent lymphatic vessel

capsule

Cortex (T+B zone)

medulla

Primary follicles(B zones)

germinal center(secondary follicle)

Paracortex (T zone)mainly T-cells

Anatomy of a Lymph Node

medullary sinus

subcapsular sinus

radial sinus

Filter antigens from the lymph- for recognition by T and B cells- for destruction by macrophages to prevent systemic spread

Spleen - A filter of the blood• Two main functions of the spleen carried out in two major

regions1) White-pulp is where immune responses against blood-borne

antigens occur2) 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

Trabecular arteryTrabecular vein

Capsule

Pulpvein

Anatomy of the Spleen

PALS or T zone(periarterial lymphoid sheath)

Follicle(B zone)

Red Pulp cord Splenic (venous)

sinus

White-Pulp cord

lymph fluid

blood

intestinal contents

ca hev hev

SECONDARY LYMPHOID ORGANS

B cellfollicle

T cellzone

red-pulp

Splenic White-Pulp Cord Lymph Node Mucosal lymphoid tissue (e.g. Tonsil, Appendix)

- 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

LN section stained with:B cell markerL-selectin ligand

T cell zone (paracortex)- T cells- DCs

HEV(High Endothelial Venule)

Follicle or B zone- B cells - FDCs

The cascade (multistep) model of leukocyte extravasation

1) Tethering and Rolling

2) Triggering/Activation of Integrins

3) Firm Adhesion 4) Diapedesis

EndothelialCells

Selectins

IntegrinsChemokines

Blood Flow

• 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

Selectins

>40, small secreted proteins

Four families: C, CC, CXC, CX3C

chemokine

chemokine receptor

Chemoattractant-Cytokines or “Chemokines”

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)

outside

cytoplasm

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

HEV

STEP 1: Tethering and Rolling: L-Selectin-glycosylated HEV ligands

STEP 2: Integrin Triggering: Chemokines (e.g. CCR7- CCL21)

STEP 3: Firm Adherence: LFA1/ICAM1

STEP 4: Transmigration

blood flow

Lymphoid Tissue

• Inhibitor of integrin (tysabri) in clinical use for treatment of Multiple Sclerosis

Multi-step cascade of lymphocyte migration to site of infection/inflammation: same logic, different ‘area code’

Schematic view of a lymph node

AfferentLymphatic

dendriticcell

T zone

B zoneHEV

TT

T

T**

**

B

B

EfferentLymphatic

T

ArteryVein

T

CXCL13

CCL21

B zones produce a B cell attracting chemokineT zone produces T cell and DC attracting chemokines

CXCL13 -> CXCR5 CCL21 -> CCR7

Follicle

Paracortex

Medulla

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

T zone

Follicle

DC

Skin draining Lymph Node

DCs migrate from periphery to lymphoid organ T zone bearing antigen

LC = Langerhans’ Cell, the immature DC of the skin

lymph fluid

hev

B cell antigen encounterantigen

FDC

Sinus Macrophage

• 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

Dia

gram

cou

rtes

y of

Ted

Yed

nock

-Lymphocytes express a receptor (S1PR1) for S1P- Egress involves migrating to S1P that is high in blood/lymph and low in the tissue

S1P

blood

thymus, spleen

S1PR1

S1P

S1P lyase

RBC

efferent lymph

lymph node, Peyer’s patch

S1PR1

S1P

S1P lyase

Lymphocytes express S1PR1 and exit lymphoid organs in response to S1P

• 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)

FTY720

Immunosuppressant

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 )