High Endothelial Venules Are Sites of Lymphocyte Extravasation

Some regions of vascular endothelium in postcapillary venules of various lymphoid organs are composed of specialized cells with a plump, cuboidal ("high") shape; such regions are called high-endothelial venules, or HEVs (Figure 15-4a, b). Their cells contrast sharply in appearance with the flattened endothelial cells that line the rest of the capillary. Each of the secondary lymphoid organs, with the exception of the spleen, contains HEVs. When frozen sections of lymph nodes, Peyer's patches, or tonsils are incubated with lymphocytes and washed to remove unbound cells, over 85% of the

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Leukocyte Extravasation

Some interactions between cell-adhesion molecules implicated in leukocyte extravasation*

Receptor on cells

Expression

Ligands on endothelium

Step involving interaction1

Main function

CLA or ESL-1 Effector T cells E-selectin Tethering/rolling Homing to skin and migration into inflamed tissue

L-selectin All leukocytes GlyCAM-1, Tethering/rolling Lymphocyte recirculation

CD34, via HEVs to peripheral lymph

MAdCAM-1 nodes and migration into inflamed tertiary sites

LFA-1 (aLß2) Leukocyte ICAM-1, 2, 3 Adhesion/arrest General role in lymphocyte subsets extravasation via HEVs and leukocyte migration into inflamed tissue

LPAM-1 (a4ß7) Effector T cells, MAdCAM-1, Rolling/adhesion Homing of T cells to gut via monocytes VCAM-1 mucosal HEV; migration into inflamed tissue

Mac-1 (aMß2) Monocytes VCAM-1 — Monocyte migration into inflamed tissue

PSGL-1 Neutrophils E- and Tethering/rolling Neutrophil migration into

P-selectin inflamed tissue

VLA-4 (a4ß1) Neutrophils, VCAM-1 Rolling/adhesion General role in leukocyte

T cells, MAdCAM-1, migration into inflamed tissue monocytes fibronectin

VLA-6 (a6ß1) T cells Laminin — Homing of progenitor T cells to thymus; possible role in T-cell homing to nonmucosal sites

*Most endothelial and leukocyte CAMs belong to four groups of proteins as shown in Figure 15-2. In general, molecules in the integrin family bind to Ig-superfamily

CAMs, and molecules in the selectin family bind to mucin-like CAMs. Members of the selectin and mucin-like families can be expressed on both leukocytes and endothelial cells, whereas integrins are expressed only on leukocytes, and Ig-superfamily CAMs are expressed only on endothelium.

See Figures 15-3a and 15-7 for an illustration of steps in the extravasation process.

bound cells are found adhering to HEVs, even though HEVs account for only 1%-2% of the total area of the frozen section (Figure 15-4c).

It has been estimated that as many as 1.4 X 104 lymphocytes extravasate every second through HEVs into a single lymph node. The development and maintenance of HEVs in lymphoid organs is influenced by cytokines produced in response to antigen capture. For example, HEVs fail to develop in animals raised in a germ-free environment. The role of antigenic activation of lymphocytes in the maintenance of HEVs has been demonstrated by surgically blocking the afferent lymphatic vasculature to a node, so that antigen entry to the node is blocked. Within a short period of time, the HEVs show impaired function and eventually revert to a more flattened morphology.

High-endothelial venules express a variety of cell-adhesion molecules. Like other vascular endothelial cells, HEVs express CAMs of the selectin family (E- and P-selectin), the mucin-

like family (GlyCAM-1 and CD34), and the immunoglobulin superfamily (ICAM-1, ICAM-2, ICAM-3, VCAM-1, and MAdCAM-1). Some of these adhesion molecules are distributed in a tissue-specific manner. These tissue-specific adhesion molecules have been called vascular addressins (VAs) because they serve to direct the extravasation of different populations of recirculating lymphocytes to particular lym-phoid organs.

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