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Infection and Immunity, May 2004, p. 2989-2994, Vol. 72, No. 5
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.5.2989-2994.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Tick Saliva Reduces Adherence and Area of Human Neutrophils
Ruth R. Montgomery,1* Denise Lusitani,1 Anne de Boisfleury Chevance,2 and Stephen E. Malawista1
Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06520,1
Centre d'Ecologie Cellulaire, Hôpital de la Salpétrière, Paris, France2
Received 12 December 2003/
Returned for modification 26 January 2004/
Accepted 9 February 2004

ABSTRACT
During natural infection with the agent of Lyme disease,
Borrelia burgdorferi, spirochetes are delivered with vector saliva, which
contains anti-inflammatory and antihemostatic activities. We
show here that the saliva of ixodid ticks reduces polymorphonuclear
leukocyte (PMN) adhesion via downregulation of ß2-integrins
and decreases the efficiency of PMN in the uptake and killing
of spirochetes. Inhibition of integrin adhesion and signaling
reduces anti-inflammatory functions of PMN. These effects may
favor the initial survival of spirochetes in vivo.

INTRODUCTION
Lyme disease is caused by infection with the spirochete
Borrelia burgdorferi and is characterized by the skin lesion erythema
migrans, which may be followed by carditis, neurological symptoms,
and arthritis (
12). In a natural infection, spirochetes are
delivered via an ixodid tick vector in the presence of saliva
and are contained in a cement plug at the inoculation site (
1).
Arthropod saliva contains potent anti-inflammatory and antihemostatic
activities and promotes a higher spirochete burden in the host
(
7,
21,
29).
Polymorphonuclear leukocytes (PMN) are the initial responding cells after a tick bite (2). They are effective in eliminating spirochetes by a variety of pathways but are more efficient in the presence of opsonizing antisera (10, 11, 17). The saliva of Ixodes ticks is known to inhibit in some way both phagocytosis and superoxide production by PMN in vitro (24). Ixodes tick saliva also inhibits T-cell proliferation (27), perhaps due to an interleukin-2 binding protein (6); inhibits complement lysis of erythrocytes (28); and reduces the production of cytokines and nitric oxide (NO) and the killing of spirochetes by macrophages (9, 20, 27). We undertook the present study to determine whether saliva inhibits PMN chemotaxis and adherence, basic functions that are likely to be important for the clearance of spirochetes.

MATERIALS AND METHODS
Cell isolation and imaging.
PMN were isolated from the heparinized blood of healthy volunteers
by 3% dextran sedimentation, and red blood cells were removed
by hypotonic lysis (
10). Informed consent was obtained from
all blood donors in accordance with the guidelines of the Human
Investigations Committee of Yale University. For immunofluorescent
staining, PMN were plated at a concentration of 5
x 10
5 cells
per 12-mm round glass coverslip in Krebs Ringer phosphate buffer
with 5.4 mM glucose (KRPG) containing 10% heat-inactivated pooled
human serum. Adherent PMN samples were incubated with spirochetes
at a 100:1 ratio in KRPG containing 10% heat-inactivated pooled
normal human serum as described previously (
17). At times up
to 60 min, samples were fixed in 4% paraformaldehyde and blocked
in phosphate-buffered saline (PBS) containing 10% goat serum
and 0.01% saponin. PMN were labeled with primary antibodies
directed against the PMN azurophilic granule component myeloperoxidase
(DAKO Corp., Carpinteria, Calif.). Rabbit polyclonal anti-
B. burgdorferi serum, a kind gift from Fred Kantor, Yale University
School of Medicine, was used to label spirochetes (
18). Antibodies
were detected with appropriate fluorescein isothiocyanate- or
tetramethyl rhodamine isocyanate-conjugated secondary antibodies
(Tago Immunologicals, Biosource Intl., Camarillo, Calif.), and
samples were imaged by using multitracking with a Zeiss LSM
510 scanning laser confocal microscope equipped with an argon-krypton
laser (
17).
For measurement of cell area, freshly isolated PMN in PBS containing Ca2+, Mg2+, and 0.1% bovine serum albumin (BSA) were plated at a concentration of 1 x 105 cells per glass coverslip and incubated for 1 h at 37°C to allow adherence. Adherent cells were then incubated for an additional 1 h in a humidified chamber either alone or in dilutions of saliva (1:10, 1:20, and 1:100). Cells were fixed in 4% paraformaldehyde in PBS, mounted in Mowiol, and examined with a Zeiss Axiovert 200 M microscope (Carl Zeiss Microimaging, Inc., Thornwood, N.Y.). Digital images were taken from 100 to 200 cells under each condition at a magnification of x40, and the diameter of each cell (in arbitrary units) was determined from a print of the digital image.
For videomicroscopy, 5-µl portions of cells in PBS-2% BSA were sealed under glass coverslips with a mean final thickness of 5.7 µm so that the cells were compressed between the slides and coverslips (13, 15). PMN were examined by phase-contrast microscopy by using a x40 objective on a 33°C stage. The orientation and trajectory of PMN was recorded before, during, and after their response to a chemotactic stimulus created by ruby laser microbeam destruction of an erythrocyte (wavelength, 694.3 min; duration of flash, 0.5 ms) (13). Freeze-frame images were collected with a Hamamatsu C2400 microscope video camera (Hamamatsu Photonics K.K., Hamamatsu City, Japan) and a Panasonic AG6720 time-lapse video recorder (Matsushita Electric Industrial Co., Osaka, Japan). Reagents for treatment of cells were obtained from Sigma-Aldrich Fine Chemicals (St. Louis, Mo.)
Spirochete culture.
A low-passage clonal isolate of B. burgdorferi strain N40 was cultivated in Barbour-Stoenner-Kelly (BSK) II medium at 33°C as described previously (10). Spirochetes were opsonized for 30 min with 1 to 10% heat-inactivated serum from a well-characterized Lyme disease patient or a normal volunteer in PBS; donor serum reactivities were documented by an enzyme-linked immunosorbent assay and Western blotting against B. burgdorferi lysate.
Saliva preparation.
Adult Ixodes scapularis ticks were allowed to feed on naïve rabbits for 5 to 7 days until they were engorged. Saliva was harvested from the ticks following pilocarpine stimulation as described previously (28) and was stored at 80°C until it was used.
Detection of killing of spirochetes.
B. burgdorferi was quantified by using a modified regrowth assay which measures spirochete uptake of [3H]adenine over 48 h (10). PMN in these experiments did incorporate some [3H]adenine, but as they did not divide, it was a small amount. Briefly, 5 x 106 B. burgdorferi cells per ml were incubated with PMN for 1 h at 37°C with agitation. Triplicate aliquots (50 µl) plated in 96-well plates in the presence of 200 µl of BSK II medium containing 5 µCi of [3H]adenine were incubated for 48 h at 33°C. Spirochetes and cells were harvested with a semiautomated cell harvester (Skatron Instruments, Inc., Sterling, Va.), and the incorporated [3H]adenine was counted with an LKB/Wallac 1205 Betaplate liquid scintillation counter (LKB/Wallac, Gaithersburg, Md.). Viability was determined by measuring incorporation of [3H]adenine by B. burgdorferi alone compared to incorporation of [3H]adenine by B. burgdorferi incubated with PMN. The results were expressed as the increase in the percentage of viable spirochetes recovered after incubation with saliva-treated PMN compared with the results obtained with control PMN.
FACS analysis of PMN.
Freshly isolated PMN in PBS containing Ca2+, Mg2+, and 0.1% BSA were preincubated in dilutions of saliva before stimulation with tumor necrosis factor alpha (TNF-
) (15 ng/ml; R & D Systems, Minneapolis, Minn.). Cells were labeled with specific antibodies (DAKO Corp.) in PBS-BSA for 1 h at 4°C, washed, fixed in 0.5% paraformaldehyde in PBS, and stored at 4°C until fluorescence-activated cell sorter (FACS) analysis. Fluorescein isothiocyanate-conjugated CD4 (T-cell marker) served as a negative control. Pilocarpine treatment did not change integrin expression.
Statistical analysis.
Significance was assessed by analysis of variance and paired, one-tailed Student's t tests.

RESULTS
Saliva treatment reduces attachment of B. burgdorferi by PMN.
We have recently shown that PMN are inefficient at binding spirochetes
in the absence of specific antibody (
17). In our examination
of PMN efficiency, we noted that saliva-treated PMN appeared
to be less well attached to coverslips, which suggested that
they might have an impaired ability to bind spirochetes. When
control PMN on coverslips were incubated with opsonized spirochetes,
a procedure that generally results in efficient binding, we
observed well-spread cells with numerous attached spirochetes
(Fig.
1A). In contrast, saliva-treated PMN were significantly
less well spread and bound fewer spirochetes, even when the
spirochetes had been opsonized with specific antibody (Fig.
1B). A fivefold reduction in the number of PMN with attached
B. burgdorferi cells was noted when saliva-treated PMN were
compared to control cells (
P = 0.09;
n = 3) (Fig.
1C). Reduced
binding was observed both in the presence of serum and under
serum-free conditions (data not shown).
Saliva reduces the cellular area of treated PMN.
When we examined coverslips with adherent PMN that had been
incubated for 1 h in the presence of saliva, we observed an
apparent reduction in the number of cells. While counts of cells
on grid coverslips indicated that there was a modest reduction
in the number of cells after saliva treatment (data not shown),
there was a dramatic difference in the area of the remaining
cells (Fig.
2). We quantified the cellular areas of untreated
PMN and PMN incubated with dilutions of saliva (1:10, 1:20,
and 1:100) from printed digital images of 100 to 200 fixed cells
per condition (
n = 2). As shown in Table
1, there was a dose-dependent
reduction in the cellular area after treatment of PMN with saliva
for 1 h; the difference is statistically significant (
P = 0.0001).
We found a similar reduction in the cellular area of PMN treated
with saliva in suspension for 1 h prior to plating on glass
coverslips (data not shown).
Saliva reduces expression of PMN ß2-integrins.
The reduced attachment and spreading of PMN in the presence
of saliva suggested that there was altered expression of PMN
ß2-integrins, the molecules that mediate adhesion
of the cells. To examine this, we incubated PMN with saliva
and quantified expression of several surface antigens. As expected,
PMN stimulated in suspension with 15 ng of TNF-

per ml for 30
min showed significant upregulation of CD18 integrin expression
(Table
1) (
16). In contrast, a dramatic decrease in CD18 (ß2-integrin)
expression was detected in saliva-treated cells. This reduction
was dose dependent, and higher concentrations of saliva resulted
in more profound reductions (Fig.
3). The reduction in CD18
expression on saliva-treated PMN was apparent in the presence
or absence of TNF-

activation and was statistically different
from CD18 expression by PMN not exposed to saliva (for 1:10
dilution of saliva,
P = 0.0004 without TNF-

and
P = 0.08 in
the presence of TNF-

;
n = 4) (Table
1). Expression of the PMN
surface marker CD15 (siayl Lewis X antigen) was not appreciably
changed by saliva treatment or by TNF-

stimulation.
We examined PMN surface marker expression after treatment with
reagents similar to components of tick saliva. No significant
differences from untreated PMN were detected for PMN expression
of the integrin molecules Cd11a, CD11b, CD11c, and CD18 or of
surface protein CD15 after 1 h of incubation in the presence
of the reagent apyrase (10 U/ml), prostaglandin E
2 (PGE
2) (20
µg/ml), or calcitonin gene-related peptide (1
x 10
8 M) at concentrations reported to be effective for activity (
22,
23,
26,
27). As expected, TNF-

produced a statistically significant
increase in expression of all integrins tested (
P = 0.04), regardless
of treatment, but not of CD15 (
n = 3) (data not shown).
Saliva treatment impairs spirochete killing in suspension.
Saliva-treated PMN were dramatically less adherent (Fig. 1) and thus were not suitable for direct microscopic observation of spirochete killing by vital staining. To measure killing, we quantified the recovery of spirochetes in suspension using a well-characterized [3H]adenine regrowth assay (10). PMN were preincubated in suspension with saliva at a 1:10 dilution for 1 h before addition of spirochetes at a B. burgdorferi/PMN ratio of 5:1 or 2:1. In three experiments in which PMN killed unopsonized spirochetes, a modest increase in the level of viability was noted in saliva-treated PMN (Fig. 4); the reduction was significant at a B. burgdorferi/PMN ratio of 2:1 (P = 0.04). While this suspension assay does not require adherence of the PMN to a surface, the increased survival of spirochetes in the presence of treated PMN is in keeping with the observed decrease in attachment of spirochetes to PMN.
Tick saliva does not affect PMN orientation or chemotaxis.
In preparations in which PMN have access to only one surface
and adhesive properties are important, the absence of integrins
greatly reduces locomotion (
13). We have found that compressed
PMN in very thin slide preparations exhibit normal random locomotion
and chemotaxis even in the presence of antibodies that disable
their integrins. Normal locomotion was also noted in compressed
PMN from a patient whose PMN lack ß2-integrins (
13,
14). In the present study, as expected, control PMN were well
attached to the glass slides and locomoted flat on the slide
before and after stimulation. In contrast, saliva-treated PMN
were less adherent and often rounded up (data not shown). Saliva-treated
cells resembled cells treated with EDTA or anti-CD18 (ß2-integrin)
antibodies, conditions that disable integrins (
13,
14). To determine
which aspects of chemotaxis were affected by saliva, we examined
saliva-treated PMN in compressed slide preparations, in which
integrins were not required for a PMN response to a chemotactic
target. Compressed PMN showed no effect of tick saliva at a
dilution of 1:10 and responded appropriately to the chemotactic
gradient created by laser irradiation of erythrocytes (Fig.
5), indicating that orientation and directed locomotion were
intact.
In the presence of saliva, both in pretreatment and throughout
the interaction, we saw the disaggregation of platelets described
previously (
24). As noted above for surface marker integrin
expression as assessed by FACS, apyrase (1 to 10 U/ml) and PGE
2 (2 to 20 µg/ml), reagents similar to salivary components,
had no effect on random locomotion or chemotaxis in thin or
thick preparations (data not shown). PMN responded normally
to the chemotactic stimulus even in the presence of 200 µg
of PGE
2 per ml.

DISCUSSION
We found that PMN are less adherent, bind fewer spirochetes,
and have a reduced cellular footprint after saliva treatment.
The mechanism for reduced spirochete attachment after saliva
treatment is unclear and may include the reduced surface area
or the absence or masking of a specific receptor. FACS analysis
of saliva-treated PMN revealed a dose-dependent decrease in
expression of CD18, the common monomer of the three ß2-integrins,
evident in the presence or absence of stimulation with TNF-

.
Moreover, killing of spirochetes was reduced in the presence
of saliva, in keeping with evidence that integrins mediate binding
of unopsonized spirochetes to leukocytes (
3,
4). Although we
did not distinguish between decreased appearance and increased
removal of CD18 from the cell surface, the behavior of saliva-treated
PMN during chemotaxis suggests that there is aggregation of
integrin molecules that are swept to the rear of moving PMN.
In compressed slide preparations, in which adhesion was less
important for locomotion (
13), tick saliva had no effect on
the random locomotion of PMN or on their orientation or chemotaxis.
After a tick bite, a local cement plug or absorbent reservoir forms, which contains high concentrations of saliva that are replenished by intermittent bursts of salivation during feeding (1). Generally, attachment of ticks for 48 h is necessary for efficient transmission of B. burgdorferi to a host (25), which provides a critical window of time for saliva to exert its modulatory effects on the host immune response.
Besides their apparent contribution to the adhesion of PMN to B. burgdorferi and killing of spirochetes as demonstrated here, integrins are known to be important in adherence and locomotion and other aspects of phagocyte function; the absence of these molecules results in severe complications of infectious diseases and wound healing (8). For example, masking of CD18 blocked cytokine stimulation of H2O2 production by PMN (19) and delayed apoptosis (5). The downregulation of PMN integrins by saliva compromises the abilities of the responding PMN and provides spirochetes with critical time to establish themselves in situ before they disseminate to cause systemic illness.

ACKNOWLEDGMENTS
This work was supported in part by grants from the National
Institutes of Health (grants AI 43558, AR 10493, AR 48513, and
AR 07107), from the Eshe Fund, from the Mathers Foundation,
and initially from the Centers for Disease Control and Prevention.
D.L. was a fellow of the Arthritis Foundation.
We are grateful for ticks supplied by John Anderson and Durland Fish and for the technical assistance of Rita Palmarozza and Deborah Beck.

FOOTNOTES
* Corresponding author. Mailing address: Department of Internal Medicine, Yale University School of Medicine, 333 Cedar Street/TAC S413, New Haven, CT 06520-8031. Phone: (203) 785-7039. Fax: (203) 785-7053. E-mail:
ruth.montgomery{at}yale.edu.

Editor: D. L. Burns

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Infection and Immunity, May 2004, p. 2989-2994, Vol. 72, No. 5
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.5.2989-2994.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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