This traffic of lymphocytes throughout the body enables these antigen‐specific cells to seek “their” antigen and to be deployed to sites at which a response is required. When antigen reaches a lymph node in a primed animal, there is a dramatic fall in the output of cells in the efferent lymphatics, a phenomenon described variously as “cell shutdown” or “lymphocyte trapping.” This process involves a reduced responsiveness of lymphocytes to sphingosine 1‐phosphate (S1P), a molecule that signals lymphocytes to exit the lymph node. The shutdown phase is followed by an output of activated cells that peaks at around 80 hours.
Naive lymphocytes home to lymph nodes
Naive lymphocytes can enter
a lymph node either in the lymphatic fluid (lymph) draining into the node via
the afferent lymphatics or by exiting from the bloodstream across the
specialized high‐walled endothelium of the postcapillary venules (HEV)
(Figure 6.13). If arriving via the HEV, their entry is determined by a series
of homing receptors on the lymphocyte that include the integrin
superfamily member LFA‐1 (αLβ2, Table 6.1), the
selectin family member L‐selectin, and the chemokine receptor CCR7. Their
ligands on the endothelium act as vascular addressins. Thus, L‐selectin
recognizes LewisX oligosaccharide structures present on a variety of
glycoproteins on the HEVs (including GlyCAM‐1 and CD34) and generally referred
to as peripheral node addressins (PNAd) (Figure 6.3). Chemokines presented by vascular endothelium play a key role in triggering lymphocyte arrest, the
chemokine receptors on the lymphocyte being involved both in binding to their
ligand and in the functional activation of integrins.
Figure 6.13 High‐walled endothelial venule (HEV). Scanning electronic micrograph of rat mesenteric lymph node showing loosely packed lymphocytes around an HEV (blue) and lymphatic vessels (yellow). (The black hole at the top of the HEV is an artifact where a tributary of the HEV was lost during preparation.) The lymph node was sliced with a vibratome that removes many of the free lymphocytes from the lumen of the HEV and the lymphatic vessels. Scale bar = 20 μm.
Therefore, naive lymphocytes, and also dendritic cells, by expressing the CCR7 chemokine receptor are directed into peripheral lymph nodes because the HEVs in the nodes display the chemokines CCL19 and CCL21 (see Table 8.2) on their luminal surface. While CCL21 is produced by the endothelial cells themselves, CCL19 is secreted by the network of fibroblastic reticular cells (FRCs) within the lymph node and subsequently transferred to the HEV. The plt/plt mouse, which lacks expression of both of these chemokines, not unsurprisingly exhibits defective T‐cell migration into peripheral lymph nodes. Chemokine activation of integrins occurs as a result of the chemokine signals facilitating their lateral mobility in the cell membrane and also by inducing structural changes in the integrins that increases their affinity for their ligands.
Passage through the HEV into the lymph node occurs in
Stage 1: Tethering and rolling
In order for the lymphocyte
to become attached to the HEV, it has to overcome the shear forces created by
the blood flow. This is effected by the forces of attraction between the homing
receptors and their ligands on the vessel wall that operates through microvilli
on the leukocyte surface (Figure 6.14). After this tethering process, the
lymphocyte rolls along the HEV, with L‐selectin and other adhesion molecules on
the lymphocyte binding to their ligands on the endothelium. The selectins
generally terminate in a lectin domain (hence “selectin”), as might be expected
given the oligosaccharide nature of the ligands.
Figure 6.14 Homing and transmigration of lymphocytes into peripheral
lymph nodes. Fast‐moving lymphocytes are
tethered (Stage 1) to the vessel walls of the tissue they are being guided to
enter through an interaction between specific homing receptors, such as L‐selectin (green dots) located on the microvilli of the
lymphocyte, and its peripheral node addressin (PNAd) ligands on the vessel
wall. PNAd comprises several molecules, including CD34 and GlyCAM‐1, which possess fucosylated, sulfated and sialylated
LewisX structures. Various chemokine receptors (red dots) are also
present on these T‐ and B‐cells. After rolling along the surface of the
endothelial cells, activation of the lymphocyte LFA‐1 integrin (blue dots) (see Table 6.1) occurs (Stage
2) in response to stimulation by chemokines. For T‐cells this stage is mainly regulated by CCL19 and
CCL21 binding to CCR7 as shown, whereas for B‐cells
CXCL13 binding to CXCR5 provides additional signals. Note that, because LFA‐1 is absent from the microvilli, firm binding occurs
by the body of the lymphocyte to its ligands, ICAM‐1/2, on the endothelium. This process results in cell
arrest and flattening followed by migration of the lymphocyte between adjacent
endothelial cells, a process referred to as diapedesis (Stage 3), which
involves LFA‐1 binding not only to ICAM‐1/2 but additionally to the junctional adhesion molecule‐1 (JAM‐1), which is present
between the endothelial cells.
Stage 2: LFA‐1 activation resulting in firm adhesion
The process of tethering and
rolling leads to the recruitment of the LFA‐1 integrin to the nonvillous
surface of the lymphocyte. This integrin undergoes structural activation in
response to chemokine signals, resulting in very strong binding to ICAM‐1 and ‐2
on the endothelial cell. The intimate contact causes the lymphocyte rolling to
be arrested and a flattening of the lymphocyte.
Stage 3: Diapedesis
The flattened lymphocyte now
uses the LFA‐1 to additionally bind to junctional adhesion molecule‐1 (JAM‐1)
on the endothelial cells in order to elbow its way between the endothelial
cells and into the tissue in response to chemotactic signals.
Lymphocyte homing to other tissues
Homing of activated and
memory lymphocytes to other tissues, such as the liver, involves a similar
process but with different receptors and ligands involved (Figure 6.3).
Dendritic cells from the appropriate tissue play an important role in
selectively imprinting the correct address code during their activation of
naive T‐cells. Cells concerned in mucosal immunity are imprinted to enter Peyer’s
patches by binding to HEVs in this location. In other cases involving migration
into normal and inflamed tissues, the lymphocytes bind to and cross
nonspecialized flatter endothelia.
It is essential that once
activated in secondary lymphoid tissues the lymphocytes of appropriate antigen
specificity can rapidly be deployed to the site of the infection. The
upregulated expression of the VLA‐4 and LFA‐1 integrins on these activated
antigen‐specific cells permits them to detect, respectively, the VCAM‐1 and
ICAM‐1 cell adhesion molecules that become expressed on vascular endothelium in
response to IL‐1 production in inflamed tissues.