The Journal of Immunology, 2001, 166: 7404-7409.
Copyright © 2001 by The American Association of Immunologists
Gestational Attenuation of Lyme Arthritis Is Mediated by Progesterone and IL-41
Manuel H. Moro*,
Johannes Bjornsson
,
Eric V. Marietta*,
Erik K. Hofmeister*,
Jeffrey J. Germer*,
Elizabeth Bruinsma*,
Chella S. David* and
David H. Persing
,
Departments of
*
Immunology and
Laboratory Medicine and Pathology, Mayo Foundation, Rochester, MN 55905; and
Infectious Disease Research Institute and
Corixa Corporation, Seattle Life Sciences Center, Seattle, WA 98104
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Abstract
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Infection of different strains of laboratory mice with the agent of
Lyme disease, Borrelia burgdorferi, results in
arthritis, the severity of which has been correlated with the dominance
of Th1 cytokines. In this study, we demonstrate that changes in
B. burgdorferi-specific immunologic responses associated
with pregnancy can alter the outcome of Lyme arthritis in mice. Whereas
nonpregnant female C3H mice consistently developed severe Lyme
arthritis, pregnant mice had a marked reduction in arthritis severity
that was associated with a slight reduction in IFN-
and markedly
increased levels of IL-4 production by B.
burgdorferi-specific T cells. Similar reductions in arthritis
severity and patterns of cytokine production were observed in
nonpregnant, progesterone-implanted mice. Ab neutralization of IL-4 in
progesterone-implanted mice resulted in severe arthritis. Our results
are consistent with the known shift toward Th2 cytokine expression at
the maternal-fetal interface, and are the first to show a
pregnancy-related therapeutic effect in an infectious
model.
 |
Introduction
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Lyme
disease is an infectious multisystem inflammatory disorder that is
caused by the tick-borne spirochete, Borrelia burgdorferi.
This disease is initially characterized by the enlargement of skin
lesion around the site of the tick bite (erythema migrans); focal,
intermittent inflammation involving myocardium, neurologic tissue,
and/or joints may develop weeks to months after initial infection
(1). In a subset of untreated cases, chronic arthritis
ensues, which is associated with the persistence of small amounts of
B. burgdorferi nucleic acids in synovial tissues and joint
fluid (2).
In the murine model of Lyme disease, Th2 responses may protect against
the development of arthritis because treatment of arthritis-resistant
mice with anti-IL-4 increases arthritis severity, and treatment of
susceptible mice with rIL-4 or passive transfer of
CD4+ Th2 clones apparently reduces arthritis
activity (3, 4, 5, 6, 7). Mouse strain-dependent resistance to
arthritis is accompanied by a decrease in B.
burgdorferi-specific IFN-
production and serum IgG2a and IgG3
levels, along with an increase in IgG1 levels. Conversely, in
chronically infected animals, Th1 responses appear to promote
pathogenic inflammatory responses that may exacerbate arthritic
activity, because administration of anti-IL-12 or anti-IFN-
Abs reduces arthritis severity in susceptible animals (5, 6, 8). Th1-type cytokine production has also been reported in
synovial T cells from patients with Lyme arthritis
(9, 10, 11).
During pregnancy, a series of profound immunologic changes occurs,
including changes associated with altered tryptophan metabolism and
progesterone-mediated alterations in the balance of cytokine
elaboration (12, 13). One of the generally observed
effects on immune responses during pregnancy has been a bias toward
humoral responses, often at the expense of cell-mediated immunity and
associated inflammatory sequelae (14, 15, 16, 17, 18, 19). Pregnant mice
mount higher Ab levels to heterologous Ags compared with nonpregnant
mice and display concomitant reductions in delayed-type
hypersensitivity responses against paternal MHC and other nonself-Ags
(20, 21). On the basis of these and other observations, it
has been proposed that Th2 cytokines (IL-4, 5, 6, 10, and 13) produced
at murine fetal-maternal interface down-regulate Th1 responses
responsible for acute allograft rejection, thus along with other
mechanisms, promoting fetal survival (18, 22, 23).
However, this Th2 bias during pregnancy may have a deleterious effect
on the outcome of certain infectious processes. In mice and in some
instances in humans, pregnancy may increase susceptibility to certain
intracellular pathogens such as Leishmania major (mice
only), Listeria monocytogenes, Mycobacterium
leprae, Mycobacterium tuberculosis, and
Toxoplasma gondii (mice only), in which protective immunity
is associated with Th1 responses (15, 16, 24, 25, 26).
In this study, we show that during pregnancy in a murine model, the
severity of pathogenic inflammatory responses associated with Lyme
arthritis is significantly attenuated. The pregnancy-associated
reduction in disease severity was associated with modestly reduced
production of IFN-
and significantly higher relative production
levels of the Th2 cytokine IL-4 compared with nonpregnant infected
controls. This gestational therapeutic effect was reproduced in
nonpregnant mice by progesterone treatment.
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Materials and Methods
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Mice
Inbred 4- to 5-wk-old female C3H.HeJ mice and 9-wk-old male
BALB/c x SJL were purchased from The Jackson Laboratory (Bar
Harbor, ME) and housed in the Mayo Clinic animal facility.
Spirochetes
B. burgdorferi N40 strain, originally obtained from
S. Barthold, was used in all the experiments. Frozen aliquots of
low passage N40 were thawed and grown to log phase in 10 ml modified
Barbour Stoenner Kelly II (BSK
II)3 medium in plastic
screw-top tubes at 32°C before each experiment. Spirochetes were
visualized to assess viability and counted by dark-field microscopy
before inoculation into mice.
Experimental infection in mice
Depending on the experiment, female mice were individually mated
with stud BALB/c x SJL males. Evidence of vaginal plugs was
considered day 0 of pregnancy. Control females were not mated. Mice
were infected by syringe inoculation behind the left shoulder
intradermally. A total of 104 spirochetes per
animal in 100 µl of BSK II was used. At 19, 30, or 45 days (depending
on the experiment), the animals were sacrificed, and regional draining
lymph nodes and spleen were harvested for cytokine analysis. At every
point, the infection status was assessed by culturing ear tissue in 10
ml BSK II medium at 32°C for 3 wk. The presence of viable spirochetes
was assessed by dark-field microscopy at the end of the culture
period.
Bb-specific IgG isotype ELISA
Ninety-six-well polystyrene plates were coated with whole cell
lysates (1 µg/well) and kept at -20°C until further use. Plates
were thawed and then blocked by the addition of 1% BSA/PBS Tween 20
solution for 1 h at 37°C. The cells were then washed extensively
(PBS/Tween 20). Duplicate samples of murine serum (90 µl/well 1:150)
were applied to the plates and incubated for 1 h at 37°C. Goat
anti-mouse (IgG1 and IgG2a), each diluted 1/4000 and linked to HRP
(Southern Biotechnology Associates, Birmingham, AL), were added to each
well, then incubated for 1 h at 37°C and washed. Then 100 µl
of ABTS peroxidase (Kirkegaard & Perry Laboratories, Gaithersburg, MD)
substrate was added and monitored at 405 nm. The reaction was
stopped with SDS buffer.
Cytokine analysis
Regional lymph nodes and spleens were processed for IL-4, IL-10,
IL-13, and IFN-
analysis from pregnant/progesterone mice and from
control, nonpregnant mice. Lymph nodes and spleens were individually
processed into single cell suspensions in RPMI medium supplemented with
10% FCS, 100 U penicillin/ml, and 100 mg streptomycin/ml. Lymph node
cells and splenocytes were washed with RPMI, resuspended in RPMI with
10% FCS and supplements at 1 x 107
cells/ml, and aliquoted into 96-well (total volume 200 µl) tissue
culture plates. Lymph node cells and splenocytes were stimulated with
B. burgdorferi sonicate at a final concentration of 25
µg/ml or with an equivalent volume of PBS. Supernatants were
harvested at 72 h, and IL-4, IL-10, IL-13, and IFN-
were
measured by a sandwich ELISA, as specified by the manufacturer
(PharMingen, San Diego, CA). Concentrations for the previously
mentioned cytokines were based on standard curves obtained from serial
dilutions of recombinant IL-4, IL-10, IL-13, and IFN-
.
Histopathology
The joints of hind limbs (tibiotarsal joints) were fixed in
neutral-buffered Formalin (pH 7.2), demineralized, and then processed
and stained with hematoxylin-eosin by routine histologic techniques.
The tibiotarsal joints were blindly scored for arthritis severity on a
scale of 0 (no inflammation), 1 (mild, usually focal inflammation of
synovium within the tibiotarsal complex, 2 (multiple sites of moderate
inflammation without necrosis), and 3 (diffusely and severely inflamed
lesions with areas of necrosis). The severity of synovial hyperplasia
correlated with the intensity of inflammation. The histopathology of
B. burgdorferi arthritis in the mouse has been thoroughly
described (27, 28). Differences in mean values of
arthritis severity in the different experiments were analyzed by
Students two-tailed test.
Progesterone implants
Mice were implanted s.c. with a 21-day time release progesterone
pellet (Innovative Research of America, Sarasota, FL). Briefly, while
under methoxyflurane anesthesia (Medical Developments,
Springvale, Australia), the hair on the back of the neck was clipped, a
small (0.3-cm) incision was made, and a progesterone pellet (25
mg/pellet) was inserted. Control animals received placebo pellets
containing all the components of the progesterone pellet, except the
active product itself. The implants delivered 1.2 mg/day of
progesterone. This dose was based on the minimum amount of progesterone
required to maintain pregnancy (29). The incision was
closed with silk, and the animals were maintained in individual
cages.
Anti-IL-4 neutralization
Five milligrams of 11B.11 mAb (National Cancer Institute
Biological Resources Branch, Frederick, MD) were administered i.p. on
the day of infection, and were repeated weekly for 2 wk.
Quantitative B. burgdorferi detection by competitive
PCR
B. burgdorferi DNA was purified from
preweighed mouse ear tissue using a modified QIAamp tissue kit protocol
(Qiagen, Valencia, CA) (30). A 256-bp region within the
B. burgdorferi flagellin gene was used as a genomic target
for amplification and quantitation. This mixture also contained 200
copies/µl of a 356-bp competitive internal control consisting
of a 303-bp product from Staphylococcus aureus plasmid
pUB112, tailed with the appropriate B.
burgdorferi flagellin primer sequences. The PCR products generated
during the amplification process were detected using a
modification of the PCR ELISA detection kit (Roche Molecular
Biochemicals; Indianapolis, IN) (30).
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Results
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Pregnant mice have mild Lyme arthritis
In three separate experiments, female, 6-wk-old C3H.HeJ mice were
mated with BALB/c x SJL stud males and checked daily for evidence
of vaginal plugs (day 0 of pregnancy). Within 24 h of plug
development, 104 spirochetes (strain N40) were
inoculated intradermally at a consistent suprascapular location into
the newly pregnant females (n = 37), and unmated
controls (n = 25). Animals were followed up to day 19
of pregnancy and then euthanized. Tibiotarsal joints were collected and
evaluated blindly for histopathologic determination of arthritis
severity. Arthritis severity scores indicated a dramatic contrast
between pregnant mice (mean score = 0.9; SD = 0.9) and
control mice (mean score = 2; SD = 0.9) (Fig. 1
). In most cases, pregnant infected mice
showed mild infiltration of inflammatory cells in the tibiotarsal
joint, whereas nonpregnant infected mice harbored moderate to severe
infiltration (Fig. 2
). Regional lymph
nodes and spleens were collected and assayed individually for Th1
(IFN-
)- and Th2 (IL-4, IL-10, and IL-13)-specific cytokines. A
moderate reduction of IFN-
and a concomitant increase in IL-4 were
observed in pregnant infected mice, whereas control nonpregnant mice
demonstrated a predominant Th1-specific cytokine pattern (slightly
higher IFN-
production, little IL-4 production) (Fig. 3
, A and B). We
then examined serum levels of B. burgdorferi-specific IgG1
and IgG2a Abs to ascertain the relative influence of Th1 vs Th2
cytokines in vivo (31); a net increase in the ratio of the
IgG2a subclass relative to IgG1 is associated with Th1-dominant immune
responses. Consistent with the cytokine measurements, we observed a
significant reduction of IgG2a levels in pregnant mice compared with
nonpregnant controls (Fig. 4
). No
significant differences were observed for IgG1. All animals were proven
to be infected by recovery of B. burgdorferi from a site
distant from the site of inoculation. However, quantitative B.
burgdorferi PCR performed on ear punch biopsy tissues indicated no
significant differences in spirochetal tissue burden between pregnant
and nonpregnant mice that might account for the differences in
inflammatory activity (32) (data not shown).

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FIGURE 2. A, Normal tibiotarsal joint. Junction of tibia and
tarsus. B, Detail of arrowed area in A.
Uninflamed joint space, synovial lining, and adjacent soft tissues.
C, Mild (1+) acute inflammation limited to
synovium. D, Detail of arrowed area in C.
Edema and polymorphonuclear infiltrate in synovial tissues.
E, Severe (3+) acute inflammation. Heavy
polymorphonuclear infiltrates in synovium with mucoid inflammatory
exudate in joint space. F, Detail of arrowed area in
E.
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FIGURE 3. Ag-specific cytokine production by regional lymphoid organs and spleen
in B. burgdorferi-infected pregnant
(n = 37) and unmated control (n
= 25) C3H mice at day 19 of pregnancy (day 18 of infection).
a, IFN- ; b, IL-4.
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FIGURE 4. IgG isotypes in B. burgdorferi-infected pregnant
(n = 37) and unmated control (n
= 25) C3H mice. Serum samples were collected on day 18 of
infection.
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Timing of the anti-inflammatory effects of pregnancy
To determine whether arthritis activity would rebound during the
postpartum period, mice were mated and infected, as described initially
in the previous experiments, but pups were removed immediately at time
of delivery to allow progesterone hormone to approach pregestational
levels (1920 days after inoculation of B. burgdorferi).
One cohort was followed up to day 30 after inoculation (10 days after
removal of pups), and another cohort was followed up to day 45
postinoculation (25 days after removal of pups). Results for both
cohorts indicated a sustained reduction in arthritis severity (30-day
cohort, n = 15, mean arthritic score = 0.9; 45-day
cohort, n = 15, mean arthritic score = 0.6). To
determine whether the timing of infection relative to the onset of
pregnancy determined the therapeutic effect of pregnancy, we infected,
in two separate experiments, 6-wk-old C3H.HeJ females and mated them 7
days later. Animals that did not show vaginal plugs in the following 2
days were excluded from the experiment; thus, all animals were pregnant
by day 89 after inoculation with B. burgdorferi. Mice
(n = 16) were followed up to day 19 of pregnancy (day
30 after inoculation); this time point was selected to coincide with
the usual peak of arthritic activity. Again, pregnant infected mice
showed a significant reduction in arthritis severity (mean arthritic
score = 1; SD = 0.6) compared with nonpregnant controls
(n = 12, mean arthritic score = 2.1; SD
= 0.5).
Progesterone mediates resistance to Lyme arthritis in pregnant mice
To determine whether the protective effects of pregnancy were due
to high levels of progesterone attained during pregnancy, we implanted
21-day progesterone time-released pellets s.c. into nonpregnant
female mice (release rate of 1.2 mg/day). Mice were inoculated with
104 spirochetes 24 h after receiving the
implant and followed up to day 19. Tibiotarsal joints, lymph nodes, and
spleen were collected and processed as described. Arthritic scores
indicated a significantly lower level of arthritis for mice with
progesterone implants (n = 13, mean arthritic
score = 0.7; control mice n = 10, mean arthritic
score = 2.1) (Fig. 5
). Cytokine
profiles and IgG subclass determinations were equivalent to those
obtained in pregnant mice (Fig. 6
, A and B, and Fig. 7
). Finally, to establish whether the
observed differences in cytokine secretion in pregnant mice influenced
the severity of arthritis, we treated progesterone-implanted mice with
anti-IL-4 mAbs once per week for 3 wk (5 mg/wk). There was a
significant increase in the severity of arthritis in the
anti-IL-4-treated mice, closely matching the scores in nonpregnant,
infected mice (Fig. 8
).

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FIGURE 6. Ag-specific cytokine production by regional lymphoid organs and spleen
in B. burgdorferi-infected progesterone-treated and
control C3H mice at day 19 after progesterone pellet implantation (day
18 of infection). a, IFN- ; b,
IL-4.
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FIGURE 7. IgG isotypes in B. burgdorferi-infected
progesterone-treated and control C3H mice. Results were derived from
cohorts of animals at day 19 of progesterone implant, which
corresponded to day 18 of infection. Progesterone-implanted mice,
n = 13; control mice, n =
10.
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Discussion
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Pregnancy, in general, is associated with a general decline in
production of Th1- associated cytokines and an increase in
Th2-associated cytokines (33). Administration of Th1
cell-associated cytokines (IFN-
or IL-2) or TNF-
is associated
with immunologic rejection of fetal tissues, and can result in abortion
(34, 35). Pregnancy also increases susceptibility to many
diseases caused by helminths, protozoa, and bacteria, although this
heightened susceptibility is most evident for infections with
intracellular protozoa and bacteria (24, 25, 26). NK cells,
Th1 T lymphocyte subsets, cytotoxic CD8+ T cell
activities, and the production of IFN-
, IL-2, and TNF-
are all
decreased during pregnancy and all contribute to the protective immune
response against these pathogens (36). IL-4, IL-5, and
IL-10, which down-regulate and antagonize the effects of
IFN-
, IL-2, and TNF-
, are produced locally at the maternal-fetal
interface during pregnancy. This may explain the decreased production
of IFN-
observed in pregnant mice infected with L. major
and T. gondii compared with nonpregnant mice (16, 37). Administration of IL-2 or IFN-
to pregnant mice can
significantly increase their resistance to T. gondii
infection, but also increases the risk of abortion
(38).
Ours is the first conclusive study showing decreased tissue pathology
associated with an infectious process during pregnancy, and is
consistent with what is known about the pathogenesis of Lyme arthritis
in inbred mouse models. We found little evidence of a stochastic effect
leading to Th2-dominant responses during pregnancy, but rather an
attenuation of the Th1 response and a corresponding decrease in
inflammatory cytokine production.
Ag-stimulated lymphocytes from pregnant mice secreted higher IL-4 and
lower levels of IFN-
than nonpregnant controls. This differential
cytokine secretion was noted in vivo, with the marked decrease in the
IgG2a subclass of B. burgdorferi Abs in pregnant mice
compared with nonpregnant mice. We also examined regional (popliteal)
lymph nodes in footpad-inoculated pregnant mice. The arthritis results
as well as IFN-
and IL-4 values were similar to animals infected by
the suprascapular route (data not shown).
Interestingly, clinical observations in humans suggest that the
severity of rheumatoid arthritis is ameliorated during pregnancy,
whereas systemic lupus erythematosus, in which the principal pathology
is associated with autoantibody production, may become exacerbated
during gestation (39, 40). The delicate cytokine balance
between the host response to infection and maintenance of pregnancy may
also work against the fetus in some cases; host responses to certain
pathogens, even if the site of infection is distant from the
fetal-placental interface, may overwhelm fetal tolerance mechanisms and
impair successful pregnancy (12).
Experimental infection of inbred mice with B. burgdorferi
results in arthritis, the severity of which appears to be genetically
determined by pathogen/host interactions that control the spirochete
burden and/or the host inflammatory response (27, 28).
B. burgdorferi-infected C3H mice characteristically mount a
Th1-dominant response in the postacute phase, as evidenced by high
levels of IFN-
and low or undetectable levels of IL-4 in lymphocytes
restimulated by B. burgdorferi Ags (3, 6).
Consistent with this, B. burgdorferi-specific IgG2a serum
levels exhibited a marked increase (6, 41). The
arthritis-modulating effects of IL-4 have been shown by depletion of
IL-4 with mAb and by experiments in which this cytokine has been
administered exogenously during the course of B. burgdorferi
infection of mice (3, 5, 6). The precise mechanism by
which this cytokine is associated with joint inflammation has not been
elucidated, but the known inhibitory effects of IL-4 on Th1-specific
inflammatory cytokines could lead to a decrease in joint inflammation
independent of direct effects on pathogen burden (42).
Indeed, we did not detect significant differences in spirochetal tissue
burden between pregnant and nonpregnant controls. In one study in which
T cell cytokine secretion was monitored at different time intervals
after initial inoculation during the development of murine Lyme
arthritis, the evolution of a Th2-type immune response developed after
an initial Th1-dominant response in a disease-resistant mouse
(43), suggesting modulation of proinflammatory effects
over time.
We used progesterone treatment of C3H mice to test the hypothesis that
this hormone was responsible for the therapeutic effect of pregnancy
(44, 45). In comparing pregnant mice with nonpregnant,
progesterone-treated mice, our results showed similar degrees of
reduced arthritis severity, differential cytokine secretion, and
predominance of the IgG2a Ab subclass. We then observed a significant
increase in arthritic activity when progesterone-treated animals were
treated with anti-IL-4 Abs, indicating that the
anti-inflammatory effects of progesterone were mediated by IL-4 in
Borrelia-infected mice.
In conclusion, our study demonstrates that pregnancy alters the
equilibrium of cytokine elaboration toward reduction of a pathogenic
inflammatory response to an infectious challenge in pregnant mice. This
down-regulation of Th1 responses, most likely via progesterone-mediated
up-regulation of Th2 cytokine production, provides a plausible
explanation for the significant reduction of Lyme arthritis in pregnant
mice. Further clarification in the dynamics of the immune response in
pregnant mice may in this, and in other infectious models, be useful
for understanding the basis of the profound immunologic changes
associated with pregnancy.
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Acknowledgments
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We thank T. Mossman, Yasir Skeiky, and Antonio
Campos-Neto for helpful suggestions and discussions, and J.
Hanson, L. Cummins, T. Trejo, and O.
Zegarra-Moro for technical assistance. We also thank G. Reynolds from
the National Cancer Institute for providing the 11B.11 mAb.
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Footnotes
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1 This work was supported by the National Institutes of Health Grants AI30548, AR41497, and AI41103. 
2 Address correspondence and reprint requests to Dr. David H. Persing, Infectious Disease, Research Institute, Seattle Life Sciences Center, Suite 600, 1124 Columbia Street, Seattle, WA 98104. E-mail address: persing{at}corixa.com 
3 Abbreviation used in this paper: BSK II, Barbour Stoenner Kelly II. 
Received for publication January 19, 2001.
Accepted for publication April 12, 2001.
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