Biomedical Engineering Reference
In-Depth Information
and indeed, if chemoattractants are present in high enough concentrations,
leukocytes lose their ability to orientate, but still remain able to respond to
novel chemoattractants (100). Migrational models under agarose have
suggested that neutrophils are able to respond as if to the vector sum of
two or more different signals, integrating directional signals from chemoat-
tractant sources (101). If two sources of the same agonist are used, migra-
tion towards the second attractant is poor (most probably due to receptor
saturation, desensitization and
=
or receptor sequestration) (100); therefore,
two distinct agonists are required for targeting specificity. These findings
may account for the observation that activated cells characteristically
secrete multiple induced chemoattractants concurrently. Cells can regain
their prior sensitivity, but this process takes time, requiring recycling of
receptors. For example, neutrophils, preincubated with low chemotactic
levels of LTB4, showed a significant reduction in chemotaxis to LTB4,
which improved after 10min (100) (Fig. 3).
B. The Migrational ''Stop Signal''
How cells remain at a specific target is less well understood. Studies have
demonstrated that exposing the leukocytes to some chemoattractants
diminishes their ability to respond to others (102,103), and a hierarchy of
chemoattractants and their receptors has been suggested. Responses to
CXCL8 and LTB4 can be suppressed in the presence of even low concentra-
tions of C5a and fMLP (102). Once stimulated, fMLP and C5a receptors
induce phosphorylation of the CXCL8 receptor causing functional uncou-
pling from its signaling apparatus, rendering it inactive (104,105). This
prevents neutrophils from migrating away from the source of C5a or fMLP
towards other chemoattractants (100), which is logical, as these molecules
define some physiologic end targets for neutrophils, because bacteria produce
formyl peptides and immune complexes that fix complement generate C5a.
C. Transmigration
Neutrophil proteinases are released during migration through extracellular
matrix (99), but it has been difficult to ascertain whether proteinases are
necessary for neutrophil migration. However, chemotaxis through artificial
substrates in response to fMLP can be inhibited by 50% by
a
1
-AT (106) and
in similar assays, CG antibodies, synthetic inhibitors of CG,
a
1
-AT, and
a
1
-antichymotrypsin (ACT), also reduce neutrophil migration (107). In
addition, fMLP-stimulated migration across an artificial basement
membrane is also reduced by inhibitors of both NE and MMP-9 (108).
On the other hand, it has been shown consistently that proteinase inhi-
bitors are ineffective at stopping neutrophil migration through intact
endothelial cell monolayers and basement membrane matrices in vitro
(109,110) although they reduce degradation of basement membrane
