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§
§
From the Department of Internal Medicine,*
Division of
Gastroenterology and Hepatology, University of Texas Medical Branch,
Galveston, Texas; the Department of Surgery,
Division of Urology, University of Texas Medical Branch, Galveston,
Texas; the Pulmonary Division,
Beth Israel
Deaconess Medical Center, Harvard Medical School, and the Ina Sue
Pelmutter Laboratory,§
Childrens Hospital,
Harvard Medical School, Boston, Massachusetts; and the Tulane
Environmental Astrobiology Center and Nephrology
Section,¶
Tulane University Medical Center and VA
Medical Center, New Orleans, Louisiana
| Abstract |
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| Introduction |
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Autoimmune and immune mechanisms have been implicated based on findings that at least a subset of IC patients present with allergy symptoms8 and that immunoglobulin and complement deposits are present in affected bladders.9 The immunological component is also supported by the findings of increased interleukin-6 levels in the urine of patients with IC10 and evidence of urothelial cell activation.11 Experimental animal models further support a possible role for an immunological component in cystitis.12 Moreover, experiments in intact animals have indicated that antigen sensitization and challenge induce cystitis.13,14 In this animal model, the inflammatory response is a complex cascade that involves disruption of the bladder permeability barrier.15
The trigger in IC is not entirely known, and pain often appears out of proportion to standard laboratory and pathological evaluation, thus continually raising questions as to the role of the sensory nerves in its pathogenesis.2 In fact, bladder biopsies from IC patients present increased density of neurokinin-1 (NK-1) receptors,18 nerves,19 and substance P-containing fibers.20 Furthermore, the finding that sensory C fiber desensitization decreases urinary bladder hyperreflexia further supports a role for sensory peptides in this disorder.16
Among the peptides released by C fibers, the tachykinins, substance P (SP) and NK-A, are known modulators of bladder inflammation.17 SP is spontaneously released within the bladder wall,21 activates NK receptors, and leads to increased vascular permeability28 and expression of adhesion molecules.29 The finding of increased NK-1 receptor density in experimental inflammation further supports a role for this receptor in cystitis.22 In fact, NK-1 receptor antagonists reduce detrusor hyperreflexia caused by chemical24 and bacterial27 cystitis, decrease cyclophosphamide-induced inflammation,25,26 and restore bladder endosomal fusion secondary to antigen-induced cystitis.23
Products of mast cell degranulation activate sensory C fibers to release SP7,17,30,31 and therefore induce inflammation. In this regard, recent experiments indicate that sensitized NK-1 receptor knockout (NK-1R-/-) mice do not mount an inflammatory response to antigen-complex stimulation. These results indicate a mandatory role for SP and its receptor in modulating the effects of mast cell degranulation.32 Additional findings using NK-1R-/- mice indicate that SP plays a major role in other forms of inflammation, such as pancreatitis (34), Clostrium difficile toxin A-induced colitis,33 and granuloma response of murine schistosomiasis.35 However, little is known of how SP regulates other proinflammatory substances within the bladder wall.
The availability of NK-1R-/- mice allows us to directly evaluate the importance of NK-1 receptors in cystitis and to examine the mechanisms by which SP participates in the early events of inflammation.
| Materials and Methods |
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NK-1R-/- (KO) and wild-type (WT) littermate control mice were generated by N. P. Gerard, and the colony at the University of Texas Medical Branch (UTMB) was genotyped as described previously.32 Female mice weighing 2030 g were used in this experiment, according to the approved animal protocol (UTMB, Animal Care & Use Committee protocol 98-05-033).
Sensitization Protocol
Groups of 32 mice (16 KO and 16 WT) were sensitized intraperitoneally (i.p.) with 1 µg of dinitrophenyl (DNP)4-human serum albumin (HSA) in 1 mg of alum on days 0, 7, 14, and 21. This protocol induces, in WT mice, sustained levels of immunoglobulin E (IgE) up to 56 days postsensitization.36 One week after the last sensitization, cystitis was induced.
Induction of Cystitis
Sensitized WT and KO mice were anesthetized (ketamine, 40 mg/kg, and xylazine, 2.5 mg/kg i.p.), then transurethrally catheterized (24 Ga, 3/4 in; Angiocath, Becton Dickinson, Sandy, UT), and the urine was drained by applying slight digital pressure to the lower abdomen. The urinary bladders were instilled with either 150 µl of saline or DNP4-ovalbumin (-OVA; 1 µg/ml) infused at a slow rate to avoid trauma and vesicoureteral reflux.37 To ensure consistent contact of substances with the bladder, infusion was repeated twice within a 30-minute interval. Twenty-four hours after instillation, mice were sacrificed with pentobarbital (20 mg/kg i.p.), and bladders were removed rapidly and fixed in buffered formalin for histology (n = 8 per group).
Alterations at Histological Level
The urinary bladder was evaluated for inflammatory cell infiltrates, mast cell numbers, and the presence of interstitial edema. A semiquantitative score with defined criteria of inflammation severity was used to evaluate cystitis.38 A cross-section of bladder wall was fixed in formalin, dehydrated in graded alcohol and xylene, embedded in paraffin, and cut serially into four 5-µm sections (8 µm apart) to be stained with hematoxylin and eosin (H&E) and Giemsa. Histology slides were scanned with a CoolSNAP camera (RS Photometrics, Tucson, AZ) mounted on an Olympus microscope. Image analysis was performed with a MetaMorph Imaging System (Universal Imaging Corp., West Chester, PA). The severity of lesions in the urinary bladder was graded as follows: 1+, mild (infiltration of a low number of neutrophils in the lamina propria and little or no interstitial edema); 2+, moderate (infiltration of moderate numbers of neutrophils in the lamina propria and moderate interstitial edema); 3+, severe (diffuse infiltration of moderate to large numbers of neutrophils in the lamina propria and severe interstitial edema).
Identification of mast cells and quantification of their degree of degranulation was performed in Giemsa-stained sections because the presence of granules is the identifying characteristic for mast cells. Degrees of degranulation are presented as the percentage of mast cells per cross-section that exhibited degranulation.
Statistical Analysis
The statistical analysis of data was performed using Wilcoxons rank sum test. Results are expressed as mean ± SEM. The n values reported refer to the number of animals used for each experiment. In all cases, a value of P < 0.05 was considered indicative of significant difference.39
Reagents
DNP was conjugated to OVA and HSA (Sigma Chemical Co., St. Louis, MO) as previously described.40 Alum adjuvant was purchased from Intergen (Purchase, NY).
| Results |
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We first examined whether the bladder of sensitized mice
developed inflammation secondary to antigen challenge. Bladders
isolated from sensitized WT mice that were challenged with saline did
not present any sign of inflammation or edema (Figure 1a)
. WT sensitized mice challenged with
antigen developed an inflammatory response characterized by
vasodilation, edema, intense polymorphonuclear neutrophil (PMN)
infiltration in the mucosa and submucosal layers (Figure 1b)
, and
activation of resident mast cells (Table 1)
. This inflammatory response, which
was observed 24 hours after antigen stimulation, had predominant
characteristics of acute inflammation based on the strong vascular
component, predominance of PMNs, and near absence of
macrophages/monocytes (Figure 1b)
. In contrast, histological
quantification of the bladder sections indicated that
NK-1R-/- mice had significant attenuation of
congestion and edema of the mucosa, as well as reduced PMN infiltration
in response to antigen challenge (Figure 1c
; Table 1
). Histologically,
antigen-challenged bladders of these mice appeared to be no different
than those from sensitized WT mice exposed to saline (Figure 1a)
.
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Morphological analysis was used to investigate the participation
of the mast cell in cystitis. Urinary bladders isolated from sensitized
NK-1R-/- mice that were exposed either to
saline or antigen presented fourfold more mast cells than counterpart
WT control mice. Increased numbers of mast cells were found in the
subepithelial layers (Figure 1d)
and within the detrusor smooth muscle
around blood vessels (Figure 1e)
and between muscle fibers (Figure 1f)
.
These results are summarized in Table 1
. In addition, antigen challenge
of sensitized WT and NK-1R-/- mice induced
activation of resident mast cells as indicated by intense degranulation
(Figure 1, gi
; Table 1
). Despite the increased number of mast
cells in the bladders of NK-1R-/- mice and the
intense degranulation observed, there were no evident signs of
edema or PMN infiltrate in response to antigen challenge (Figure 1, ac)
.
| Discussion |
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In regard to mast cell numbers, urinary bladders isolated from
NK-1R-/- mice had fourfold more mast cells than
those from WT control mice (Table 1)
. In this context, we reported that
products of mast cells stimulate sensory neurons and release
SP.17
In a positive feedback loop, an important aspect of
SP action is its capacity to stimulate mast cells through activation of
NK receptors31
and to enhance mast cell degranulation
secondary to other stimuli.30
The capacity of these
substances to stimulate each others release reciprocally suggests a
nerve-mast cell axis31
that may perpetuate cystitis,
regardless of the initial stimulus. Our previous findings of a close
physical contact between mast cells and sensory nerves containing SP in
the urinary bladder further support this theory.42
However, despite the increased numbers of mast cells, no other signs of
inflammation were observed in the bladder of
NK-1R-/- mice. This would indicate a mandatory
participation of NK-1 receptors on the chain of events linking mast
cell degranulation and inflammation. Therefore, we suggest that
increased mast cell numbers without a concomitant participation of
sensory peptides is not enough to induce bladder inflammation.
Another possible explanation for absence of inflammation in response to antigen stimulation is that NK-1R-/- mice are unable to mount an immune response. In this regard, it has been shown that splenocytes and granuloma cells in NK-1R-/- mice present an appreciable impairment in IgG2a and IgE expression.35 However, NK-1R-/- mice also failed to respond with an inflammatory response when passively sensitized with an immunoglobulin,32 indicating that the NK-1 receptor plays a major role downstream of immune-complex activation.
SP and its receptor are widely distributed and implicated in the regulation of a wide range of inflammatory pathways, such as activation of vascular endothelial cells, vascular permeability, plasma extravasation,28 and expression of adhesion molecules.29 The development and use of NK-1 receptor KO mice has demonstrated the central mechanistic role of this receptor in other forms of inflammation.32-35 In the bladder, although the involvement of SP in cystitis has been suggested previously,4,7,17,18,20 our results provide direct evidence for the importance of NK-1 receptors as an upstream regulator of an inflammatory cascade.
The results obtained with NK-1R-/- mice indicate that this receptor subtype is essential for induction of inflammation. Other pathways activated by SP appear to have minor contributions. These effects fall into two major categories. The first involves the effects of SP on NK-2 and NK-3 receptors.24 Second, SP may also induce release of other mediators, such as bradykinin, which activates its own receptor.21,30
Our data suggest that the NK-1 receptor is by far the most quantitatively important proximate mediator of this type of inflammatory response. The specific role of NK-2 and NK-3 receptors in inflammation needs to be reassessed. In addition, understanding receptor-independent pathways may be critical to refining any therapeutic strategy based on NK-1 receptor inhibition.
Our studies were performed 24 hours after antigen stimulation and, therefore, represent the early events of cystitis. However, they strongly indicate a mandatory role for SP and NK-1 receptors, mediating inflammation secondary to mast cell activation. Because both mast cells7 and NK-1 receptors18 are increased in bladder biopsies of IC patients, it is tempting to propose a role for sensory peptides in the pathogenesis of this disorder. Further studies, using a specific nonpeptide antagonist of NK-1 receptors, are necessary to investigate whether blockade of this receptor will reduce the inflammation and symptoms of IC.
| Acknowledgements |
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| Footnotes |
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Supported by National Institutes of Health grants DK 5582801 (to R. S.), DK51392 (to T. G. H.), and HL41587 (to N. P. G.).
Accepted for publication November 2, 1999.
| References |
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, and the LFA1/ICAM-1 interaction in substance P-induced granulocyte infiltration. J Leuk Biol 1997, 61:445-451[Abstract]
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