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From the Departments of 1 Ophthalmology, 2 Medicine, and 3 Cell and Developmental Biology, Casey Eye Institute, Oregon Health Sciences University, Portland, Oregon.
| Abstract |
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METHODS. Two hundred fifty nanograms of E. coli endotoxin was injected into the vitreous of knockout mIL-8Rh-/- (n = 7) mice or heterozygous littermate mIL-8Rh+/- controls (n = 7). Intravital microscopic examination of iris microvasculature was performed at baseline and 6 and 24 hours after endotoxin injection. The numbers of rolling (cells/mm2 endothelial surface/min), sticking (cells/mm2 endothelial surface), and infiltrating cells (cells/mm2 iris tissue) were evaluated by digital off-line quantification.
RESULTS. The number of infiltrating cells was significantly reduced in mIL-8Rh-/- mice: 406 ± 77 cells/mm2 at 6 hours and 242 ± 50 cells/mm2 at 24 hours in mIL-8Rh+/- mice versus 14 ± 4 cells/mm2 at 6 hours and 38 ± 11 cells/mm2 at 24 hours in mIL-8Rh-/- mice (P < 0.001). In contrast, the absence of the IL-8 receptor homologue did not reduce rolling or sticking.
CONCLUSIONS. Iris rhodamine angiography allows precise quantification of leukocyteendothelial dynamics in the absence of surgical trauma. IL-8 and its homologues are known to be potent signals for leukocyte migration. Although IL-8 has previously been implicated in cell adhesion, video imaging in vivo demonstrated that deletion of the IL-8 receptor homologue had minimal effect on rolling or arrest in this model of inflammation.
| Introduction |
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Recruitment is achieved through a multistep paradigm that includes margination, selectin-mediated rolling, ß2-integrinmediated firm adhesion, and migration of leukocytes into the site of inflammation.4 Binding of neutrophils to inflamed endothelium initiates an orchestrated series of events in which neutrophils shed L-selectin and engage a second set of adhesion-promoting glycoproteins, the leukocyte ß2-integrins.5 6
Human IL-8 receptors are members of a family of G proteincoupled receptors and are abundantly expressed on neutrophils, the primary target for CXC chemokines. There are at least two different IL-8 receptor types with 77% identical amino acid sequences. The type-1 receptor (IL-8R1, CXCR1) specifically binds IL-8 and granulocyte-chemoattractant protein (GCP)-2 (reviewed by Baggiolini and co-workers.2 7 ) The type-2 receptor (IL-8R2, CXCR2) also binds additional neutrophil-attracting CXC chemokines: MGSA (melanoma growth stimulatory activity), growth related oncogene (GRO), MIP (macrophage inflammatory protein)-2, and NAP (neutrophil-activating protein)-2. IL-8 receptors are also found on monocytes, basophils, and eosinophils, but the responses of these cells to IL-8 are much weaker than those of neutrophils.2 CXCR1 and CXCR2 are present in similar numbers on all neutrophils and monocytes but only on a minority of lymphocytes.
No true homologue for human IL-8 has been found in mice. Mouse
KC has been accepted as the closest murine homologue of human
GRO-
. In analogy, the KC receptor is an IL-8 type-2 receptor
homologue capable of binding KC, MIP-2, and
GCP-2/liposaccharide-induced CXC chemokine (LIX) with high
affinity. Each of these ligands may activate mouse neutrophils. It has
been shown that KC is biologically active on human neutrophils and
competes with 125IIL-8 binding to IL-8R2 but
not IL-8R1.8
9
So far, only one receptor for this family of chemokines (mIL-8Rh, KC-receptor) has been described in the mouse.8 The mIL-8Rh binds at least three chemokines that activate neutrophils (KC, MIP-2, and GCP-2/LIX) and is structurally and functionally homologous to CXCR2.
Although IL-8 and its homologues are clearly involved in leukocyte migration, the role of the IL-8 family of chemokines is less clearly understood with regard to leukocyte rolling and arrest. Experiments in vitro suggest that IL-8 released from stimulated endothelial cells facilitates the transendothelial migration of neutrophils1 by both regulating L-selectin and ß2-integrin expression and forming a transendothelial cell chemotactic gradient. In contrast, IL-8 does not affect the expression of intercellular adhesion molecule (ICAM)-1 or E-selectin on the endothelial cell surface.1 IL-8 activation of neutrophils in the microvascular compartment results in the concomitant shedding of L-selectin and the expression of the ß2-integrin CD11b/CD18 on the surface of the neutrophil, which leads to firm ICAM-1dependent intravascular adhesion of neutrophils to the endothelium. Transgenic mice overexpressing the human gene for IL-8 have downregulated the expression of L-selectin.10 Evidence for the ß2-integrin pathway has been given by a complete inhibition of IL-8induced neutrophil transmigration by monoclonal antibodies to Mac-1.11
Because IL-8 regulates expression of both P-selectin and ß2-integrins, molecules implicated in rolling and arrest, respectively,4 it seems logical to hypothesize that both rolling and arrest would be impaired in mIL-8Rhdeficient mice. Testing this hypothesis requires an experimental system that can measure dynamic interactions between cells. We are aware of only two such studies. One of them implicated IL-8 in arrest of isolated monocytes rolling on cultured human umbilical cord endothelial cells expressing E-selectin that had been transduced with an adenovirus vector.12 A brief communication reported that the absence of the IL-8 receptor homologue resulted in reduced arrest in response to MIP-2 in cremasteric vessels, whereas the adhesion response to fMLP (formylmethionylleucylphenylalanine) did not differ between knockouts and controls.13 Neither of these studies correlated the numbers of free-flowing, rolling, adherent, and infiltrating cells. To clarify the importance of mIL-8Rh in leukocyte rolling, sticking, and diapedesis, we studied leukocyte migration using intravital microscopy of iris microvasculature in receptor-knockout mice.
| Methods |
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Microscopy
The technique of digital live-tissue video microscopy of iris
vessels in mice during endotoxin-induced uveitis has been described in
detail elsewhere.16
Therefore, we give here only a
brief description.
To visualize leukocytes, animals were intravenously injected with rhodamine 6G 10 minutes before experiments (35 mg/kg; Sigma Chemical, St. Louis, MO). Animals were anesthetized with isoflurane 1.7% in O2 (1 l/min). The microcirculation of iris vessels was recorded with digital video microscopy at three different times (baseline and 6 and 24 hours after induction of inflammation) with an epifluorescence intravital microscope (modified Orthoplan; Leica, Wetzlar, Germany). Iris vessels were selected as described in detail elsewhere.16 Only venules in the right eye of knockout (n = number of animals/total number of venules = 7/57, respectively) and heterozygous control animals (n = 7/53, respectively) were examined. Diameter and length of each vessel segment (in micrometers) and leukocyte behavior (free-flowing, rolling, sticking, and infiltrating) were analyzed off-line with image-analysis software. Free-flowing cells are expressed as number of cells passing a vessel segment per minute. Infiltrating cells are given as number of cells per square millimeter of iris tissue. For the calculation of rolling and sticking leukocytes, a cylindrical tube was assumed from the diameter and the length of each vessel segment. The number of rolling cells was given as number of cells per square millimeter of endothelial surface per minute. The number of sticking cells was given as number of cells per square millimeter of endothelial surface.
Data Analysis
To describe leukocyte recruitment quantitatively and to consider
the different sizes of the circulating leukocyte pool in homozygous and
heterozygous animals, we normalized the above-mentioned parameters
according to a recent proposal from Jung and coworkers.17
Rolling efficiency is equivalent to rolling leukocyte flux (percent of
rolling cells of all free-flowing and rolling cells). The adhesion
efficiency is calculated as the number of sticking cells per square
millimeter divided by the total number of rolling and sticking cells
per square millimeter per minute. This parameter relates the number of
sticking cells to the rolling leukocyte pool available in the same
vessel. Probability values were calculated using a MannWhitney rank
sum test.
Induction of Inflammation
Uveitis was induced by intravitreal injection of 2 µl saline
containing 250 ng LPS from Escherichia coli 055:B5 (List
Biological Laboratories, Campbell, CA) and 0.25% human serum albumin
(Baxter Healthcare, Glendale, CA).15
Wholemount Iris Immunohistology
Immunostaining of wholemount iris preparations has been described
in detail elsewhere.16
Irises were dissected and cut into
3 pieces. All pieces were stained with
anti-platelet-endothelial cell adhesion molecule (PECAM)
antibody to visualize vessels. In addition, 1 piece was labeled with
anti-CD45 (leukocyte common antigen; PharMingen, San Diego, CA) to
detect all leukocytes in the iris.
Availability of Video Images
Because leukocyte rolling is a dynamic variable, this parameter is
best appreciated with video rather than static photography. Video
images related to this study can be viewed by visiting our web site at
http://www.ohsu.edu/cei/Iris/Iris-Homepage.html.
| Results |
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| Discussion |
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We believe that this is the first report describing the use of intravital microscopy of the eye in a knockout animal. The iris offers a rare opportunity for studying leukocyte adhesion and migration physiologically, without artifacts from surgical trauma. The transparency of the cornea and the absence of pigmentation in the iris of albino mice enable visualization of infiltrating leukocytes at the light-microscopic level. Additionally, the inherent thinness of the mouse iris facilitates a very precise quantification of cells that have migrated across the vascular endothelium into the tissue stroma. The infiltrating cells reside essentially in only two dimensions before they leave the iris and enter the aqueous humor. This allows infiltrating leukocytes to be counted and expressed as numbers of cells per square millimeter of iris tissue. Digitalization of the images facilitates quantification and enhances image quality. Traditional histologic methods cannot quantitate rolling or allow a study of the same vessel over multiple time points.
Our observations clearly demonstrate that mice lacking the IL-8 receptor homologue have a severe impairment in leukocyte migration without a comparable defect in rolling or arrest. Our study shows the advantage of dynamic imaging of the immune response in vivo compared with histology as a means to study the stepwise process of inflammation. On the basis of our study, we predict that the clinical utility of an IL-8 receptor antagonist would be excellent for blocking neutrophil accumulation outside a vessel as in inflammation, but the same antagonist would be ineffective in blocking neutrophil arrest as might be the goal in limiting tissue damage due to ischemia.
In a complex milieu induced by LPS, signals other than those that activate CXCR2 are sufficient to support leukocyte adhesion in vivo. However, these signals appear insufficient for subsequent extravasation into surrounding tissue.
| Footnotes |
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Submitted for publication December 29, 1999; accepted February 10, 2000.
Commercial relationships policy: N.
Corresponding author: Matthias D. Becker, Casey Eye Institute, Oregon Health Sciences University, 3375 SW Terwilliger Boulevard, Portland, OR 97201. beckemat{at}ohsu.edu
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