Cellular Events: Leukocyte Recruitment
and Activation
As mentioned earlier, an important function of the inflammatory
response is to deliver leukocytes to the site of injury
and to activate them. Leukocytes ingest offending agents,
kill bacteria and other microbes, and eliminate necrotic
tissue and foreign substances. A price that is paid for the
defensive potency of leukocytes is that once activated, they
may induce tissue damage and prolong inflammation,
since the leukocyte products that destroy microbes can also
injure normal host tissues. Therefore, host defense mechanisms
include checks and balances that ensure that leukocytes
are recruited and activated only when and where
they are needed (i.e., in response to foreign invaders and
dead tissues). Systemic activation of leukocytes can, in
fact, have detrimental consequences, as in septic shock.Leukocyte Recruitment
Leukocytes normally flow rapidly in the blood, and in inflammation,
they have to be stopped and brought to the offending agent
or the site of tissue damage, which are typically outside the
vessels. The sequence of events in the recruitment of leukocytes
from the vascular lumen to the extravascular space
consists of (1) margination and rolling along the vessel
wall; (2) firm adhesion to the endothelium; (3) transmigration
between endothelial cells; and (4) migration in interstitial
tissues toward a chemotactic stimulus (Fig. 2–5).
Rolling, adhesion, and transmigration are mediated by the
interactions of adhesion molecules on leukocytes and
endothelial surfaces (see later on). Chemical mediators—
chemoattractants and certain cytokines—affect these processes
by modulating the surface expression and binding
affinity of the adhesion molecules and by stimulating directional
movement of the leukocytes.
Margination and Rolling. As blood flows from capillaries
into postcapillary venules, circulating cells are swept by
laminar flow against the vessel wall. Because the smaller
red cells tend to move faster than the larger white cells,
leukocytes are pushed out of the central axial column and
thus have a better opportunity to interact with lining
endothelial cells, especially as stasis sets in. This process
of leukocyte accumulation at the periphery of vessels is
called margination. If the endothelial cells are activated by
cytokines and other mediators produced locally, they
express adhesion molecules to which the leukocytes
attach loosely. These cells bind and detach and thus begin
to tumble on the endothelial surface, a process called
rolling.The weak and transient interactions involved in rolling
are mediated by the selectin family of adhesion molecules
(Table 2–2). Selectins are receptors expressed on leukocytes
and endothelium that contain an extracellular domain that
binds sugars (hence the lectin part of the name). The three
members of this family are E-selectin (also called CD62E),
expressed on endothelial cells; P-selectin (CD62P), present
on platelets and endothelium; and L-selectin (CD62L), on
the surface of most leukocytes. Selectins bind sialylated
oligosaccharides (e.g., sialyl–Lewis X on leukocytes) that
are attached to mucin-like glycoproteins on various cells.
The endothelial selectins are typically expressed at low
levels or are not present at all on unactivated endothelium,
and are up-regulated after stimulation by cytokines and
other mediators. Therefore, binding of leukocytes is largely
restricted to endothelium at sites of infection or tissue
injury (where the mediators are produced). For example,
in unactivated endothelial cells, P-selectin is found primarily
in intracellular Weibel-Palade bodies; however, within
minutes of exposure to mediators such as histamine or
thrombin, P-selectin is distributed to the cell surface, where
it can facilitate leukocyte binding. Similarly, E-selectin and
the ligand for L-selectin, which are not expressed on normal
endothelium, are induced after stimulation by the cytokines
IL-1 and TNF.
Adhesion. The rolling leukocytes are able to sense changes
in the endothelium that initiate the next step in the reaction
of leukocytes, which is firm adhesion to endothelial surfaces.
This adhesion is mediated by integrins expressed on
leukocyte cell surfaces interacting with their ligands on
endothelial cells (Fig. 2–5 and Table 2–2). Integrins are transmembrane heterodimeric glycoproteins that mediate
the adhesion of leukocytes to endothelium and of various
cells to the extracellular matrix. They are normally
expressed on leukocyte plasma membranes in a lowaffinity
form and do not adhere to their specific ligands
until the leukocytes are activated by chemokines.
Chemokines are chemoattractant cytokines that are
secreted by many cells at sites of inflammation and
are displayed on the endothelial surface. (Cytokines are
described later in the chapter.) When the adherent leukocytes
encounter the displayed chemokines, the cells are
activated, and their integrins undergo conformational
changes and cluster together, thus converting to a highaffinity
form. At the same time, other cytokines, notably
TNF and IL-1 (also secreted at sites of infection and injury),
activate endothelial cells to increase their expression of
ligands for integrins. These ligands include intercellular
adhesion molecule-1 (ICAM-1), which binds to the integrins
leukocyte function–associated antigen-1 (LFA-1) (also
called CD11a/CD18) and macrophage-1 antigen (Mac-1)
(i.e., CD11b/CD18), and vascular cell adhesion molecule-1
(VCAM-1), which binds to the integrin very late antigen-4
(VLA-4) (Table 2–2). Engagement of integrins by their
ligands delivers signals to the leukocytes that lead to cytoskeletal
changes that mediate firm attachment to the substrate.
Thus, the net result of cytokine-stimulated increased
integrin affinity and increased expression of integrin
ligands is stable attachment of leukocytes to endothelial
cells at sites of inflammation.
Transmigration. After being arrested on the endothelial
surface, leukocytes migrate through the vessel wall primarily
by squeezing between cells at intercellular junctions.
This extravasation of leukocytes, called diapedesis, occurs
mainly in the venules of the systemic vasculature; it has
also been noted in capillaries in the pulmonary circulation.
Migration of leukocytes is driven by chemokines produced
in extravascular tissues, which stimulate movement of the
leukocytes toward their chemical gradient. In addition,
platelet endothelial cell adhesion molecule-1 (PECAM-1)
(also called CD31), a cellular adhesion molecule expressed
on leukocytes and endothelial cells, mediates the binding
events needed for leukocytes to traverse the endothelium.
After passing through the endothelium, leukocytes secrete collagenases that enable them to pass through the vascular
basement membrane.
Chemotaxis. After extravasating from the blood, leukocytes
move toward sites of infection or injury along a chemical
gradient by a process called chemotaxis. Both exogenous
and endogenous substances can be chemotactic for leukocytes,
including the following:
• Bacterial products, particularly peptides with N-formylmethionine
termini
• Cytokines, especially those of the chemokine family
• Components of the complement system, particularly C5
• Products of the lipoxygenase pathway of arachidonic
acid (AA) metabolism, particularly leukotriene B4
(LTB4)
These mediators, which are described in more detail later,
are produced in response to infections and tissue damage
and during immunologic reactions. Leukocyte infiltration
in all of these situations results from the actions of various
combinations of mediators.
Chemotactic molecules bind to specific cell surface
receptors, which triggers the assembly of cytoskeletal contractile
elements necessary for movement. Leukocytes
move by extending pseudopods that anchor to the ECM
and then pull the cell in the direction of the extension. The
direction of such movement is specified by a higher density
of chemokine receptors at the leading edge of the cell.
Thus, leukocytes move to and are retained at the site where
they are needed.
The type of emigrating leukocyte varies with the age of
the inflammatory response and with the type of stimulus.
In most forms of acute inflammation, neutrophils predominate
in the inflammatory infiltrate during the first 6 to 24 hours
and are replaced by monocytes in 24 to 48 hours (Fig. 2–6).
Several factors account for this early abundance of neutrophils:
These cells are the most numerous leukocytes in the
blood, they respond more rapidly to chemokines, and they
may attach more firmly to the adhesion molecules that
are rapidly induced on endothelial cells, such as P- and
E-selectins. In addition, after entering tissues, neutrophils
are short-lived—they die by apoptosis and disappear within
24 to 48 hours—while monocytes survive longer. There are
exceptions to this pattern of cellular infiltration, however,in certain infections (e.g., those caused by Pseudomonas
organisms), the cellular infiltrate is dominated by continuously
recruited neutrophils for several days; in viral infections,
lymphocytes may be the first cells to arrive; and in
some hypersensitivity reactions, eosinophils may be the
main cell type.
Figure 2–5 Mechanisms of leukocyte migration through blood vessels. The leukocytes (neutrophils shown here) first roll, then become activated
and adhere to endothelium, then transmigrate across the endothelium, pierce the basement membrane, and migrate toward chemoattractants emanating
from the source of injury. Different molecules play predominant roles in different steps of this process: selectins in rolling; chemokines (usually
displayed bound to proteoglycans) in activating the neutrophils to increase avidity of integrins; integrins in firm adhesion; and CD31 (PECAM-1) in
transmigration. ICAM-1, intercellular adhesion molecule-1; IL-1, interleukin-1; PECAM-1, platelet endothelial cell adhesion molecule-1; TNF, tumor
necrosis factor.
Table 2–2 Endothelial and Leukocyte Adhesion Molecules
Figure 2–6 Nature of leukocyte infiltrates in inflammatory reactions. The photomicrographs show an inflammatory reaction in the myocardium after
ischemic necrosis (infarction). A, Early (neutrophilic) infiltrates and congested blood vessels. B, Later (mononuclear) cellular infiltrates. C, The approximate
kinetics of edema and cellular infiltration. For sake of simplicity, edema is shown as an acute transient response, although secondary waves of