Inflammation

Normal oesophagus Normal palatine tonsils Normal skin

Candida infection Streptococcal infection Staphylococcalinfection

Leukocyte Migration and Inflammation

• The IS relies upon the continual circulation of leukocytes through the body– For the Innate IR – a variety of lymphocytes,

granulocytes, and monocytes can respond

– For the Adaptive IR – lymphocytes must contact Ag in either tissue, lymph, or blood

Lymphocyte re-circulation

• Lymphocytes constantly re-circulate from blood to spleen, lymph nodes, and 3° lymphoid tissues

• Continual circulation provides systemic protection• A complete circuit can be performed 1-2X per day• ~1 in 105 lymphocytes can recognize a specific Ag

therefore, constant circulation increases chance of lympho contacting Ag

How do leukocytes transit the bloodstream?

They must bind to an endothelial cell first!

• Endothelial cells exhibit ‘cell adhesion molecules’ – CAM’s

• Lympho’s, granulo’s, and mono’s form receptors which bind to CAM’s

From Here to There?

• Lymphoid stem cell migrate to central lymphoid organs

• Mature lymphocyte migrate to peripheral lymphoid organs

• Recirculation of lymphocytes• Lymphocyte migrate to the sites of

inflammation

Post capillary venules in 2º lymphoid tissue HIGH ENDOTHELIAL VENULES.Specialised to allow lymphocytes and nothing else into the lymph node

HEV

High endothelial venules

Post capillary venules in other tissues are lined by simple squamous epithelium

Non-lymphoid cells

Pass through the blood vessels in the lymph node and continue arterio-venous circulation

Recirculation

HEV

HEV

Naïve lymphoid cells

Adhere to and squeeze between High Endothelial Venules (HEV), then percolate through the lymph node and exit via the efferent lymphatic vessel

Role of endothelial cells in trafficking and recirculation

Endothelial are involved in:Vasomotor tone, vascular permeability, regulation of coagulation, immune modulation and lymphocyte extravasation

High endothelial venules

Constitutively present insecondary lymphoid tissue

Need to allow egress ofnaïve cells from the circulation

Post-capillary venules

Present in non-lymphoid tissues

Molecules expressed by endothelial cells regulate traffickingand recirculation through lymphoid and non-lymphoid tissues

Antigen Collection

• Spleen - collects antigen from the bloodstream; • Mucosa-associated lymphoid tissues (MALT) - collects

antigen from the respiratory, gastrointestinal and urogenital tracts and are particularly well organized in the small intestine, in structures known as Peyer’s patches.

• The lymph nodes are connected to the tissues and the bloodstream by a system of lymphatic vessels.

• Afferent lymphatics drain extracellular fluid (lymph) from the tissues, including mucosal tissues, into the lymph nodes.

• Efferent lymphatics carry the lymph out of the secondary lymphoid tissues and ultimately into a collecting vessel known as the thoracic duct (or for lymph nodes in the neck, the cervical duct), and thence through the heart and into the bloodstream

draining lymph from tissue

The four types of CAM’s• Selectins – resp. for intial

contact between leukocytes and endothelial cells– Bind to specific CHO

groups (i.e., mucins)• Mucins – glycosylated

proteins– Bind to selectins on

endothelium– Bind to other mucins

(CD34 and glyCAM) on endothelium of lymph nodes

The four types of CAM’s• Integrins – heterodimer

proteins formed by all leukocytes– Bind to ICAM’s along vasc.

endothelium

• ICAM’s – CAM’s with Ig domains on vasc. endothelia– Bind to integrins at Ig domain– MadCAM’s – have both IG

and mucin-like domains; found on mucosal endothelia

• Bind to integrins on lymphocytes

SELECTINSLeucocytes inc. Naive T cells: L SELECTINEndothelial cells: P SELECTIN & E SELECTINP selectin: Weibel-Palade bodies. E selectin: TNF & IL-1 induced

A common core with different extracellular C type lectin domains that bind carbohydrates in a Ca2+ dependent manner.Each selectin binds to specific carbohydrates and is able to transduces signals into the cell

Selectins & addressins

VASCULAR ADDRESSINSOn high endothelial venules in lymphoid tissue:

Carbohydrates that “decorate” CD34 and GlyCAM-1Sialyl LewisX molecules

Peripheral Node addressins (PNAd)

Mucosal endothelium:MAdCAM-1

Guides lymphocyte entry into lymphoid tissues

Adhesion molecules participate in lymphocyte homing

Lymphocyte homing receptor Addressins

Adhesion molecules

Functions Adhesion molecules

Functions

L-selectin

Lymphocyte homing to peripheral immune organs

PNAd 　 Addressin of peripheral high venous endothelial cell

CLA Receptor on memory T cell surface for directionally homing to skin

E-selectin

Express on vascular endothelial cell of inflammation portion of skin

LFA-1 Participate in many lymphocytes for homing

ICAM-1ICAM-2

Participate in many lymphocytes for homing

Adhesion molecules participate in lymphocyte homing

Lymphocyte homing receptor Addressins

Adhesion molecules

Functions Adhesion molecules

Functions

VLA-4 Receptor of lymphocyte homing

VCAM－ 1

Express on vascular endothelial cell of inflammation portion

CD44 Participate in many lymphocytes homing

MAd Intestinal lymphatic tissue and lamina propria

integrin Receptor of lymphocyte for directionally migrate to Peyre's Patch

MAd 　 Addressin of vascular endothelial cell

α4β7

Neutrophil extravasation in inflammation

Blood flow

Cell adhesions of neutrophils

RollingActivation Adhesion

Lymphocyte extravasation

• Involves same 4 steps as neutrophils

• Small % of endothelial cells w/i lymphoid organs exhibit “high-endothelial venules” (HEV’s) which contain many CAM’s

• CAM’s function in “Homing” and “Trafficking” of lymphocytes

Memory and naïve T cells

Naïve

Activated

+

-

L-s

elec

tin

+ + -+ + -

VL

A-4CD

45R

A

CD

45R

O

CD

2

LFA

-1

CD

44

++ ++ ++ +++-

Associates with TcR and CD4 - phosphatase activity reduces threshold of T cell signalling

Naïve cells need to access lymphoid tissue to become stimulated Memory cells need to access sites of inflammation

Initial contact of Naïve lymphocytes

High endothelial venule cell

Steps of extravasation of naïve T cell

Trafficking, homing and adhesion

Trafficking:Non-random movement of cells from tissues, blood or lymph.Includes migration to and from sites of lymphocyte maturation as well as homing.

Adhesion:Binding of cells to other cells or extracellular matrix

Homing: Tendency of lymphocytes activated in a particular region of the body to preferentially return to the same region Includes localisation of cells in distinct regions of lymphoid tissue.

Why is lymphocyte homing necessary?

Tendency of lymphocytes activated in a particular region of the body to preferentially return to the same region.

Gut

Gut pathogene.g. rotavirus

Anti-rotavirusT cells activated

Response resolves, lymphocytes non-randomly redistributed

Anti-rotavirusT cells will neverbe needed in the skin

Anti-rotavirusT cells willbe neededin the gut

Pro-T cell migrate to thymus

• Homing receptor: CD44 and L-selectin expressed by pro-T cell

• Adressin: EA1 molecule expressed by thymus vascular endothelial cell

• And integrin α6β1 、 α6β4 play an important role in adhesion of pro-T cell

Lymphocyte migrate to peripheral lymphoid organs

Lymphocytes migrate to lymph node

• Homing receptor: L-selectin on lymphocyte• Adressin ： peripheral lymphonode vascular

addressin (PNAd)

• LFA-1/ICAM-1 、 ICAM-2 and CD44/Mad molecules participate in the adhesion and penetration

Lymphocyte migrate to Peyer’s Patch

• Homing receptor: integrin α4β7 molecule ； CD44 and LFA-1 molecules

• Adressin: Vascular endothelial cell of peyre’s patch specifically highly express mucosal vascular addressin (Mad); ICAM-1 、 ICAM-2

• Peyre’s patch means the aggregated lymphoid nodule in small intestine.

Lymphocyte migrate to peripheral lymphoid organs

Lymphocyte homing

Initial contacts of effector T cells

Quantitative aspects of lymphocyte migration

Traffic between lymphoid/non-lymphoid tissues involves~ 5 x 1011 cells per day

Only ~2% (1 x 1010) of these cells are in the blood at any one time

Lymphocytes only stay in the blood for ~30 minutes

Circulating blood pool of lymphocytes is exchanged 48 times a day

However……

Less than 10% of blood lymphocytes migrate into lymph nodes, tonsils & Peyer’s patches.

~90% of lymphocytes leave the blood to enter organs such as the liver, lung spleen and bone marrow.Traffic is 5 times faster than traffic through lymphoid tissue