Introduction
Leptin is a peptide hormone that plays an important role in the regulation of body weight by inhibiting food intake and stimulating energy expenditure. Moreover, leptin exhibits a variety of other effects, including regulation of endocrine function, reproduction and immunity. Consistently, leptin-deficient (
ob/
ob) mice and leptin-receptor deficient (
db/
db) mice are not only obese, but also exhibit important hormonal imbalances, infertility, abnormalities in thermoregulation, and evidence of immune and haematopoietic defects [
1‐
4].
The role of leptin in the modulation of immune response and inflammation has recently become increasingly evident. The increase in leptin production that occurs during infection and inflammation strongly suggests that leptin is a part of the cytokine cascade, which orchestrates the innate immune response and host defence mechanisms [
5]. However, both proinflammatory and anti-inflammatory effects have been described for leptin, depending on the experimental model investigated [
5]. Leptin plays an important role in inflammatory processes involving T cells, and has been reported to promote T-helper (Th)1 polarization of the cellular immune response [
6‐
9]. Several studies have implicated leptin in the pathogenesis of autoimmune inflammatory conditions, such as experimental autoimmune encephalomyelitis and type 1 diabetes [
10‐
13].
Furthermore, in leptin-deficient
ob/
ob mice we recently demonstrated reduced severity of antigen-induced arthritis (AIA), which is a model of immune-mediated joint inflammation [
14]. Leptin appeared to contribute to the mechanisms of joint inflammation in AIA by regulating both humoral and cell-mediated immune responses. Essentially identical results were obtained in
db/
db mice, which lack expression of the functional leptin receptor, long isoform (OB-Rb). However, joint inflammation in AIA depends on the adaptive immune response, which is known to be impaired in
ob/
ob and
db/
db mice. We conducted the present study to investigate the effect of leptin and leptin receptor deficiency on inflammatory events in the joint, independent of their effects on T-cell and B-cell responses. Therefore, we explored the effect of leptin deficiency in zymosan-induced arthritis (ZIA), a model of proliferative arthritis, which is restricted to the joint injected with zymosan A and is not dependent on the adaptive immune response. Indeed, zymosan A, which is a ligand for toll-like receptor 2 as well as an activator of the alternate complement pathway, triggers local activation of the innate immune system, causing inflammation in the injected joint [
15,
16].
We followed the development of ZIA in ob/ob C57BL/6 mice and in their control +/? (i.e. +/+ or ob/+) lean littermates. In addition, in order to evaluate the role of OB-Rb in ZIA, we also used db/db and control db/+ C57BL/KS mice. The results of the experiments show that, in contrast to AIA, ZIA is not impaired in ob/ob and db/db mice. Furthermore, resolution of acute inflammation during ZIA appears to be delayed in the absence of leptin.
Materials and methods
Animals
Eight-week-old male, leptin deficient C57BL/6 ob/ob mice and their control +/+ or ob/+ (i.e. +/?) littermates, as well as OB-Rb leptin receptor deficient C57BL/KS db/db mice and their control db/+ littermates, were purchased from Elevage Janvier (Le Genest-St-Isle, France). Animals were housed under conventional conditions in the animal facility of the Geneva University School of Medicine. Water and standard laboratory chow were provided ad libitum. The experimental protocol received the approval from the Animal Ethics Committee of the Geneva University School of Medicine and of the Geneva Veterinarian Office.
Induction of arthritis
Arthritis was induced by injection of 180 μg zymosan A via a small skin cut along the suprapatellar ligament directly into the knee joint cavity, as described previously [
17]. Zymosan A (30 mg) from
Saccharomyces cerevisiae (Sigma-Aldrich, Buchs, Switzerland) was suspended in 1 ml endotoxin-free saline (Laboratory Dr G Bichsel AG, Interlaken, Switzerland) by boiling and sonification. Mice were injected with 6 μl of this suspension into the right knee joint, under inhalation anaesthesia with 5% isofluran (Forene
®; Abbott AG, Baar, Switzerland). The left knee joint was simultaneously injected with an equal amount (6 μl) of saline and served as the control.
Study design
Ob/ob and +/? mice were killed at different time points after injection of zymosan A (i.e. on days 1 and 3 for evaluation of the acute phase of arthritis, and on days 14 or 21 for evaluation of the chronic phase). The knee joints were either processed for histology (days 3, 14 and 21) or used for RNA extraction (days 1 and 14). One experiment was performed using db/db and control db/+ mice to evaluate the effect of OB-Rb deficiency on ZIA. In this experiment, mice were killed on day 14 after injection of zymosan A and the knee joints were processed for histology. All animals were killed by exsanguination (cardiac puncture) followed by cervical dislocation, under intraperitoneal anaesthesia with 0.01 ml/g saline solution containing 12 mg/ml ketasol (Dr E Graub AG, Bern, Switzerland) and 0.16% rompun (Bayer, Provet AG, Lyssach, Switzerland).
Isotopic quantification of joint swelling
Joint swelling was quantified at various time points after injection of zymosan A by measuring uptake of circulating
99mTc-pertechnetate in the knee joint, as previously described [
14,
17]. Animals were injected subcutaneously in the neck region with 10 μCi of
99mTc-pertechnetate in 0.2 ml saline. After 30 min mice were sedated by inhalation anaesthesia with 5% isofluran, and accumulation of the isotope due to increased blood flow and oedema in the knee was determined in duplicate by external gamma counting. The ratio between
99mTc-pertechnetate uptake in the inflamed and that in the contralateral knee joint was calculated. A ratio greater than 1.1 was taken to indicate joint swelling.
Histological studies
Knee joints were fixed in 10% formalin for a minimum of 24 hours and decalcified using 'd-calcifier' (Lerner Laboratories, Pittsburg, PA, USA), containing 14% HCl, over 6–7 hours. They were embedded in paraffin and serial sections of 4 μm were cut for histological analysis. Sections were stained with either haematoxylin–eosin or toluidine blue. Histological assessment was performed in a blinded manner, using an established scoring system for synovial hyperplasia (from 0 = no hyperplasia to 3 = most severe hyperplasia) and inflammatory cell infiltration in the synovium (from 0 = no inflammation to 3 = most severe inflammation). Cartilage damage was determined by toluidine blue staining (from 0 = no change and fully stained cartilage to 3 = total loss of toluidine blue staining and erosions in cartilage). These three scores were added together to obtain a total histological score, as previously described [
18]. Only processes taking place inside the joint cavity were taken into account.
Measurement of IL-6, serum amyloid A and corticosterone levels
Blood samples (100 μl) were taken at baseline and different time points after injection of zymosan A from the tail vein and at the end of experiment by cardiac punction. Serum levels of IL-6 were measured using a commercial DuoSet ELISA Development System (R&D Systems, Abington, UK). Detection limit for this test is 39 pg/ml. Serum levels of serum amyloid A (SAA) were determined using a direct enzyme-linked immunosorbent assay, as previously described [
19]. The detection limit for this test is 13 μg/ml. Serum corticosterone levels were determined by using a radioimmunoassay (Diagnostic Systems Laboratories, Webster, TX, USA), as previously described [
20].
RNase protection assay
On days 1 and 14 after injection of zymosan A, knee joints from five ob/ob and five +/? mice were used for RNA isolation. Total RNA was prepared using the TRIzol reagent (Gibco – Life Technologies AG, Basel, Switzerland) according to the manufacturer's instructions. Expression levels of IL-12, IL-10, IL-1α, IL-1β, IL-1 receptor antagonist, IL-18, IL-6, interferon-γ, macrophage migration inhibitory factor (MIF), L32 ribosomal protein and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA were analyzed by RiboQuant™ RNase protection assay, using the mCK-2b multiprobe template set from BD Biosciences (Heidelberg, Germany). Briefly, riboprobes were 32P-labelled and hybridized overnight in solution with 10 μg total RNA. The hybridized RNA was digested with RNases A and T1, and the remaining RNase-protected probes were purified, resolved on denaturing polyacrylamide gels, and imaged by autoradiography according to the RiboQuant protocol. The protected bands representing cytokine mRNA expression were quantified by phosphor-imaging using a Cyclone Storage Phosphor System (PerkinElmer Life Sciences, Zaventem, Belgium) and normalized for GAPDH expression. Data represent mean ± SEM (n = 5) of values obtained for all detectable cytokines.
Statistical analysis
All data were analyzed using one-way analysis of variance. Data are expressed as means ± SEM. P < 0.05 was considered statistically significant.
Discussion
The results of the present study indicate that resolution of joint swelling in ZIA was delayed in leptin deficient mice. Accordingly, the acute phase response, as assessed by circulating levels of IL-6 and SAA, remained elevated for a longer period of time in ob/ob mice than in control, lean littermates. Furthermore, at late time points histological features of arthritis tended to be more severe in ob/ob mice. Similar results were obtained in db/db mice, suggesting that the observed changes in the course of ZIA were mediated by a lack of interaction of leptin with OB-Rb.
In contrast to these findings we previously observed a milder form of AIA in
ob/
ob and
db/
db mice as compared with their controls, with decreased synovial levels of IL-1β and tumour necrosis factor (TNF)-α, and a switch toward production of Th2 cytokines [
14]. These contrasting observations in AIA and ZIA further suggest a greater sensitivity to agents stimulating the innate immune responses in leptin or leptin signalling deficient animals, as opposed to attenuated inflammation in models involving T-cell responses, in particular Th1-mediated diseases [
4]. Indeed, both T and B lymphocytes participate in the mechanisms that lead to articular inflammation in AIA, whereas ZIA exclusively involves the innate immune response.
Leptin was previously reported to play an important role in T-cell-mediated immune responses. Evidence of defective cell-mediated immunity and lymphoid atrophy, analogous to those observed in chronic under-nutrition in humans, are detected in
ob/
ob and
db/
db mice [
23‐
25]. Leptin stimulates the proliferation of CD4
+ T cells and promotes Th1 responses [
6]. Congenital leptin deficiency in humans is associated with a decreased number of circulating CD4
+ T cells, impaired T-cell proliferation and cytokine release, all of which could be reversed by the administration of recombinant leptin [
26]. In addition, the OB-Rb receptor is also expressed on B cells and may participate in the development of humoral responses [
14]. Consistent with these findings, leptin deficient mice are protected from inflammation mediated by T and B cells in different disease models, including AIA, experimental autoimmune encephalomyelitis, type 1 diabetes and experimental colitis [
11,
12,
14,
27].
Our results in ZIA suggest that chronic leptin deficiency interferes with adequate control of the inflammatory reaction. Protective effects of leptin were previously observed in studies of other experimental models conducted to explore innate immune responses.
Ob/ob mice are significantly more susceptible to lipopolysaccharide (LPS)-induced death, and this feature can partly be reversed by administration of leptin [
28]. OB receptor deficient
fa/fa rats also exhibit enhanced LPS-induced hepatotoxicity [
29]. Similarly,
ob/
ob and
db/
db mice are more likely to succumb after administration of TNF-α. The protective role of leptin against TNF-α induced toxicity was further supported by the deleterious effect of neutralizing anti-leptin antibodies administered to TNF-α injected mice [
30]. The mechanisms underlying these protective effects of leptin are still unclear. Although thymic and circulating lymphocytes are reduced, a fourfold increase in the number of circulating monocytes was observed in leptin deficient mice, suggesting enhanced responses to monocyte activators [
7]. Furthermore, an imbalance between proinflammatory and anti-inflammatory monokines has been observed in
ob/
ob mice injected with LPS, with plasma levels of the anti-inflammatory cytokines IL-10 and IL-1 receptor antagonist being lower in leptin deficient than in normal mice [
28]. However, LPS or TNF-α mediated systemic inflammation is a complex syndrome, and susceptibility to these systemic stimuli might also be influenced by the effects of leptin on nervous, endocrine, or other responses, independent of the production of inflammatory mediators.
Intravenous injection of
Staphylococcus aureus results in a severe form of septic arthritis in mice, which is associated with decreased circulating levels of leptin. In this model, treatment with leptin significantly decreased the severity of septic arthritis without interfering with staphylococcal load in the joints [
31]. The levels of IL-6 were significantly lower in mice with septic arthritis after administration of leptin [
31]. Consistent with these findings, our results in ZIA indicate that IL-6 levels were higher in
ob/
ob mice than in controls. IL-6 plays an important role in turning acute inflammation into a chronic synovitis, as demonstrated by limited duration of ZIA in IL-6 deficient mice [
32]. In addition, IL-6 has also been shown to play a major role in other models of arthritis [
33,
34]. Thus, control of IL-6 production may be one of the mechanisms by which leptin is involved in the control of the inflammatory response during ZIA.
It is noteworthy that anaesthesia, skin cut and intra-articular injection with saline slightly enhanced serum levels of IL-6 and SAA in ob/ob and +/? mice, although to a lesser extent than in animals infected with zymosan A. Interestingly, this small, zymosan A independent inflammatory response was also greater in ob/ob mice than in lean controls, further supporting the presence of an inappropriate control of inflammatory responses in leptin deficiency.
Corticosterone secretion is known to be elevated in all forms of leptin deficiency and in leptin insensitivity [
21,
22]. However, despite the presence of elevated levels of glucocorticoids,
ob/
ob mice still exhibited a more pronounced acute phase response and longer lasting arthritis than did controls. Thus, it is conceivable that leptin deficiency could result in an even more severe form of arthritis in the absence of hypercorticosteronaemia.
The mRNA levels of different cytokines were determined in arthritic and control joints at two time points. Consistent with a previous report [
17], the levels of IL-1α, IL-1β and IL-6 mRNA were increased during ZIA. In addition, we also detected elevated levels of IL-1 receptor antagonist and MIF, which to the best of our knowledge have not previously been reported in the joint during ZIA. MIF is a broad-spectrum proinflammatory cytokine that is implicated both in animal models of immune-mediated arthritis and in human rheumatoid arthritis [
35]. The levels of mRNAs encoding these different cytokines, including IL-6, were not different between
ob/
ob mice and their lean controls. However, we cannot exclude variations in post-transcriptional regulation, which might still result in different levels of active proteins at the site of inflammation.