The virulence of Salmonella is linked to its invasive capacity and suppression of adaptive immunity. This does not explain, however, the rapid dissemination of the pathogen after breaching the gut. Here we showed that early in infection, S. Typhimurium suppressed degranulation of local mast cells (MCs), resulting in limited neutrophil recruitment and restricted outflow of vascular contents into infection sites, thus facilitating bacterial spread. MC suppression was mediated by the Salmonella phosphatase (SptP), which shares structural homology with Yersinia YopH. SptP functioned by dephosphorylating the vesicle fusion protein N-ethylmalemide-sensitive factor (NSF) and by blocking phosphorylation of Syk. Without SptP, orally challenged S. Typhimurium failed to suppress MC degranulation and exhibited limited colonization of the mesenteric lymph nodes. Administration of SptP to sites of Escherichia coli infection markedly enhanced its virulence. Thus, SptP-mediated inactivation of local MCs is a powerful mechanism utilized by S. Typhimurium to impede early innate immunity.
A growing body of evidence indicates that the kidneys contribute substantially to immune defense against pathogens in the urinary tract. In this issue, Paragas et al. report that α–intercalated cells (A-ICs) within the nephron collecting duct sense infecting Gram-negative bacteria, resulting in simultaneously secretion of the iron chelating protein lipocalin 2 (LCN2) and protons, which acidify the urine. A-IC–specific LCN2 and proton secretion markedly reduced the ability of infecting uropathogenic E. coli (UPEC) to grow and sustain infection. The capacity of A-ICs to sense and actively promote clearance of infecting bacteria in the lower urinary tract represents a novel function for these specialized kidney cells, which are best known for their role in modulating acid-base homeostasis.
The lower urinary tract’s virtually inevitable exposure to external microbial pathogens warrants efficient tissue-specialized defenses to maintain sterility. The observation that the bladder can become chronically infected in combination with clinical observations that antibody responses following bladder infections are not detectable, suggest defects in the formation of adaptive immunity and immunological memory. We have identified a broadly immunosuppressive transcriptional program specific to the bladder, but not the kidney, during infection of the urinary tract that is dependent on tissue-resident mast cells (MCs). This involves localized production of interleukin-10 and results in suppressed humoral and cell mediated responses and bacterial persistence. Therefore, in addition to the previously described role of MCs orchestrating the early innate immunity during bladder infection, they subsequently play a tissue-specific immunosuppressive role. These findings may explain the prevalent recurrence of bladder infections and suggest the bladder as a site exhibiting an intrinsic degree of MC-maintained immune privilege.
Allergic asthma is characterized by airway hyperresponsiveness, inflammation, and a cellular infiltrate dominated by eosinophils. Numerous epidemiological studies have related the exacerbation of allergic asthma with an increase in ambient inhalable particulate matter from air pollutants. This is because inhalable particles efficiently deliver airborne allergens deep into the airways, where they can aggravate allergic asthma symptoms. However, the cellular mechanisms by which inhalable particulate allergens (pAgs) potentiate asthmatic symptoms remain unknown, in part because most in vivo and in vitro studies exploring the pathogenesis of allergic asthma use soluble allergens (sAgs). Using a mouse model of allergic asthma, we found that, compared with their sAg counterparts, pAgs triggered markedly heightened airway hyperresponsiveness and pulmonary eosinophilia in allergen-sensitized mice. Mast cells (MCs) were implicated in this divergent response, as the differences in airway inflammatory responses provoked by the physical nature of the allergens were attenuated in MC-deficient mice. The pAgs were found to mediate MC-dependent responses by enhancing retention of pAg/IgE/FcεRI complexes within lipid raft–enriched, CD63+ endocytic compartments, which prolonged IgE/FcεRI-initiated signaling and resulted in heightened cytokine responses. These results reveal how the physical attributes of allergens can co-opt MC endocytic circuitry and signaling responses to aggravate pathological responses of allergic asthma in mice.
During infection, signals from the periphery are known to reach draining lymph nodes (DLNs), but how these molecules, such as inflammatory cytokines, traverse the significant distances involved without dilution or degradation remains unclear. We show that peripheral mast cells, upon activation, release stable submicrometer heparin-based particles containing tumor necrosis factor and other proteins. These complexes enter lymphatic vessels and rapidly traffic to the DLNs. This physiological drug delivery system facilitates communication between peripheral sites of inflammation and remote secondary lymphoid tissues.
Mast cells have been associated with obliterative bronchiolitis (OB) in human pulmonary allografts, although their role in the development of OB remains unknown.
In this study, we evaluated the role of mast cells in pulmonary allograft rejection using an orthotopic rat pulmonary allograft model that utilizes chronic aspiration of gastric fluid to reliably obtain OB. Pulmonary allograft recipients (n = 35) received chronic aspiration of gastric fluid with (n = 10) and without (n = 16) treatment with a mast cell membrane stabilizer, cromolyn sodium, or chronic aspiration with normal saline (n = 9) as a control.
The acute graft injury associated with long ischemic time in the model (6 hours total ischemic time; typical acute graft injury rate ~30%) was apparently blocked by cromolyn, because peri-operative mortality associated with the acute graft injury was not observed in any of the animals receiving cromolyn (p = 0.045). Further, the rats receiving cromolyn developed significantly fewer OB lesions than those treated with gastric fluid alone (p < 0.001), with a mean reduction of 46% of the airways affected.
These findings provide impetus for further studies aimed at elucidating the effects of cromolyn and the role of mast cells in pulmonary allotransplantation.
mast cells; aspiration; pulmonary allograft; gastric fluid; obliterative bronchiolitis
Mast cells (MCs), granulated tissue-resident cells of hematopoietic lineage, constitute a major sensory arm of the innate immune system. This review discusses the evidence supporting the dual role of MCs, both as sentinels for invading pathogens, as well as regulatory cells throughout the course of acute inflammation, from its initiation to resolution. This versatility is dependent on the MC’s ability to detect pathogens and danger signals and release a unique panel of mediators to promote pathogen-specific clearance mechanisms, such as through cellular recruitment or vascular permeability. It is increasingly understood that MCs also contribute to the regulated contraction of immune activation that occurs within tissues as inflammation resolves. This overarching regulatory control over innate immune processes has made MCs successful targets to purposefully enhance or, alternatively, suppress MC responses in multiple therapeutic contexts.
Development of nasal immunization for human use is hindered by the lack of acceptable adjuvants. Although CT is an effective adjuvant, its toxicity will likely prevent its use in nasal vaccines. This study compared non-toxin adjuvants to CT for their ability to induce protective antibody responses with nasal immunization. C3H/HeN and C57BL/6 mice were immunized with rPA formulated with the following adjuvants: CT, IL-1α, LPS, CpG, Pam3CSK4, 3M-019, resiquimod/R848 or c48/80. Serum and nasal wash cytokine concentrations were monitored 6 hours post-vaccination as biomarkers for acute activation of the innate immune system. Not all of the adjuvants induced significant changes in innate serum or nasal wash cytokines, but when changes were observed, the cytokine signatures were unique for each adjuvant. All adjuvants except Pam3CSK4 induced significantly increased anti-rPA serum IgG titers in both strains of mice, while only IL-1α, c48/80 and CpG enhanced mucosal anti-rPA IgA. Pam3CSK4 was the only adjuvant unable to enhance the induction of serum LeTx-neutralizing antibodies in C3H/HeN mice while c48/80 was the only adjuvant to induce increased serum LeTx-neutralizing antibodies in C57BL/6 mice. Only CT enhanced total serum IgE in C3H/HeN mice while IL-1α enhanced total serum IgE in C57BL/6 mice. The adjuvant influenced antigen-specific serum IgG subclass and T cell cytokine profiles, but these responses did not correlate with the induction of LeTx-neutralizing activity. Our results demonstrate the induction of diverse innate and adaptive immune responses by non-toxin nasal vaccine adjuvants that lead to protective humoral immunity comparable to CT and that these responses may be influenced by the host strain.
The development and use of vaccines and their ability to prevent infection/disease is a shining example of the benefit of biomedical research. Modern vaccines often utilize subunit immunogens that exhibit minimal immunogenicity and require the use of adjuvants to maximize the induction of protective immune responses. We recently described a novel class of vaccine adjuvants, mast cell (MC) activators, that exhibit safe and effective vaccine adjuvant activity when administered by intranasal or intradermal routes. A compound library containing 580 functionalized benzopyrans, a structural motif found in a diverse array of natural and designed bioactive compounds, was screened using a MC degranulation assay to identify novel MC activating compounds for future evaluation as novel vaccine adjuvants. This approach identified 12 novel MC degranulating compounds. Therefore, MC degranulation can be used to reliably detect novel compounds for evaluation as adjuvants for use in mucosal vaccine strategies.
Caveolin-1, the hallmark protein of caveolae, is highly expressed within the lung in the epithelium, endothelium, and in immune cells. In addition to its classical roles in cholesterol metabolism and endocytosis, caveolin-1 has also been shown to be important in inflammatory signaling pathways. In particular, caveolin-1 is known to associate with the nitric oxide synthase enzymes, downregulating their activity. Endotoxins, which are are composed mainly of lipopolysaccharide (LPS), are found ubiquitously in the environment and can lead to the development of airway inflammation and increased airway hyperresponsiveness (AHR).
We compared the acute responses of wild-type and caveolin-1 deficient mice after LPS aerosol, a well-accepted mode of endotoxin exposure, to investigate the role of caveolin-1 in the development of environmental lung injury.
Although the caveolin-1 deficient mice had greater lung inflammatory indices compared to wild-type mice, they exhibited reduced AHR following LPS exposure. The uncoupling of inflammation and AHR led us to investigate the role of caveolin-1 in the production of nitric oxide, which is known to act as a bronchodilator. The absence of caveolin-1 resulted in increased nitrite levels in the lavage fluid in both sham and LPS treated mice. Additionally, inducible nitric oxide synthase expression was increased in the lung tissue of caveolin-1 deficient mice following LPS exposure and administration of the potent and specific inhibitor 1400W increased AHR to levels comparable to wild-type mice.
We attribute the relative airway hyporesponsiveness in the caveolin-1 deficient mice after LPS exposure to the specific role of caveolin-1 in mediating nitric oxide production.
Caveolin-1; Lipopolysaccharide; Airway hyperresponsiveness; Nitric oxide; Lung injury; Cytokines
Granules of mast cells (MCs) enhance adaptive immunity when, on activation, they are released as stable particles. Here we show that submicrometre particles modelled after MC granules augment immunity when used as adjuvants in vaccines. The synthetic particles, which consist of a carbohydrate backbone with encapsulated inflammatory mediators such as tumour necrosis factor, replicate attributes of MCs in vivo including the targeting of draining lymph nodes and the timed release of the encapsulated mediators. When used as an adjuvant during vaccination of mice with haemagglutinin from the influenza virus, the particles enhanced adaptive immune responses and increased survival of mice on lethal challenge. Furthermore, differential loading of the particles with the cytokine IL-12 directed the character of the response towards Th1 lymphocytes. The synthetic MC adjuvants replicate and enhance the functions of MCs during vaccination, and can be extended to polarize the resulting immunity.
There is a current biodefense interest in protection against Anthrax. Here we developed a new generation of stable and effective anthrax vaccine. We studied the immune response elicited by rPA delivered intranasally with a novel mucosal adjuvant, a mast cell activator Compound 48/80. The vaccine formulation was prepared in a powder form by spray-freeze-drying (SFD) under optimized conditions to produce particles with a target size of D50=25μm, suitable for delivery to the rabbit nasal cavity. Physicochemical properties of the powder vaccines were characterized to assess their delivery and storage potential. Structural stability of rPA was confirmed by CD and ATR-FTIR, while functional stability of rPA and C48/80 was monitored by cell-based assays. Animal study was performed using a unitdose powder device for direct nasal application. Results showed that C48/80 provided effective mucosal adjuvant activity in rabbits. Freshly prepared SFD powder vaccine formulations or powders stored for over two years at room temperature elicited significantly elevated serum PA-specific and lethal toxin neutralization antibody titers that were comparable to that induced by IM immunization with rPA. Nasal delivery of this vaccine formulation may be a viable alternative to the currently licensed vaccine, or an attractive vaccine platform for other mucosally transmitted diseases.
Mycoplasma pneumoniae (Mp) frequently colonizes the airways of patients with chronic asthma and likely contributes to asthma exacerbations. We previously reported that mice lacking surfactant protein A (SP-A) have increased airway hyperresponsiveness (AHR) during M pneumoniae infection versus wild-type mice mediated by TNF-α. Mast cells (MCs) have been implicated in AHR in asthma models and produce and respond to TNF-α.
Determine the contribution of MC/TNF interactions to AHR in airways lacking functional SP-A during Mp infection. Methods: Bronchoalveolar lavage fluid was collected from healthy and asthmatic subjects to examine TNF-α levels and M pneumoniae positivity. To determine how SP-A interactions with MCs regulate airway homeostasis, we generated mice lacking both SP-A and MCs (SP-A−/−KitW-sh/W-sh) and infected them with M pneumoniae.
Our findings indicate that high TNF-α levels correlate with M pneumoniae positivity in human asthmatic patients and that human SP-A inhibits M pneumoniae–stimulated transcription and release of TNF-α by MCs, implicating a protective role for SP-A. MC numbers increase in M pneumoniae–infected lungs, and airway reactivity is dramatically attenuated when MCs are absent. Using SP-A−/−KitW-sh/W-sh mice engrafted with TNF-α−/− or TNF receptor (TNF-R)−/− MCs, we found that TNF-α activation of MCs through the TNF-R, but not MC-derived TNF-α, leads to augmented AHR during M pneumoniae infection when SP-A is absent. Additionally, M pneumoniae– infected SP-A−/−KitW-sh/W-sh mice engrafted with TNF-α−/− or TNF-R−/− MCs have decreased mucus production compared with that seen in mice engrafted with wild-type MCs, whereas burden was unaffected.
Our data highlight a previously unappreciated but vital role for MCs as secondary responders to TNF-α during the host response to pathogen infection.
Mast cells; TNF; Mycoplasma species; airway hyperres-ponsiveness; mucus
Vesicoureteric reflux (VUR) is a common congenital defect of the urinary tract that is usually discovered after a child develops a urinary tract infection. It is associated with reflux nephropathy, a renal lesion characterized by the presence of chronic tubulointersitial inflammation and fibrosis. Most patients are diagnosed with reflux nephropathy after one or more febrile urinary tract infections, suggesting a potential role for infection in its development. We have recently shown that the C3H mouse has a 100% incidence of VUR. Here, we evaluate the roles of VUR and uropathogenic Escherichia coli infection in the development of reflux nephropathy in the C3H mouse. We find that VUR in combination with sustained kidney infection is crucial to the development of reflux nephropathy, whereas sterile reflux alone fails to induce reflux nephropathy. A single bout of kidney infection without reflux fails to induce reflux nephropathy. The host immune response to infection was examined in two refluxing C3H substrains, HeN and HeJ. HeJ mice, which have a defect in innate immunity and bacterial clearance, demonstrate more significant renal inflammation and reflux nephropathy compared with HeN mice. These studies demonstrate the crucial synergy between VUR, sustained kidney infection and the host immune response in the development of reflux nephropathy in a mouse model of VUR.
We report that infection of draining lymph nodes (DLNs) by Salmonella typhimurium results in the specific downregulation of the homeostatic chemokines CCL21 and CXCL13, which are essential for normal DLN organization and function. Our data reveal that the mechanism of this suppression is dependent on S. typhimurium LPS (sLPS). The decrease in CCL21 expression involves interaction between sLPS and CCL21-producing cells within DLNs, triggering a distinct Toll-like receptor 4 (TLR4)-mediated host signaling response. In this response, suppressor of cytokine signaling-3 (Socs3) is upregulated, which negatively regulates mothers against decapentaplegic homolog-3 (Smad3)-initiated production of CCL21. Disruption of lymph node architecture and cellular trafficking enhances S. typhimurium virulence and could represent a mechanism of immune suppression used by pathogens that primarily target lymphoid tissue.
Rationale: Previously, we demonstrated a candidate region for susceptibility to airspace enlargement on mouse chromosome 5. However, the specific candidate genes within this region accounting for emphysema-like changes remain unrecognized. c-Kit is a receptor tyrosine kinase within this candidate gene region that has previously been recognized to contribute to the survival, proliferation, and differentiation of hematopoietic stem cells. Increases in the percentage of cells expressing c-Kit have previously been associated with protection against injury-induced emphysema.
Objectives: Determine whether genetic variants of c-Kit are associated with spontaneous airspace enlargement.
Methods: Perform single-nucleotide polymorphism association studies in the mouse strains at the extremes of airspace enlargement phenotype for variants in c-Kit tyrosine kinase. Characterize mice bearing functional variants of c-Kit compared with wild-type controls for the development of spontaneous airspace enlargement. Epithelial cell proliferation was measured in culture.
Measurements and Main Results: Upstream regulatory single-nucleotide polymorphisms in the divergent mouse strains were associated with the lung compliance difference observed between the extreme strains. c-Kit mutant mice (KitW-sh/W-sh), when compared with genetic controls, developed altered lung histology, increased total lung capacity, increased residual volume, and increased lung compliance that persist into adulthood. c-Kit inhibition with imatinib attenuated in vitro proliferation of cells expressing epithelial cell adhesion molecule.
Conclusions: Our findings indicate that c-Kit sustains and/or maintains normal alveolar architecture in the lungs of mice. In vitro data suggest that c-Kit can regulate epithelial cell clonal expansion. The precise mechanisms that c-Kit contributes to the development of airspace enlargement and increased lung compliance remain unclear and warrants further investigation.
genetic; tyrosine kinase; SASH; chronic obstructive pulmonary disease; aging
Candida glabrata is an emerging human fungal pathogen that is frequently drug tolerant, resulting in difficulties in treatment and a higher mortality in immunocompromised patients. The calcium-activated protein phosphatase calcineurin plays critical roles in controlling drug tolerance, hyphal growth, and virulence in diverse fungal pathogens via distinct mechanisms involving survival in serum or growth at host temperature (37° and higher). Here, we comprehensively studied the calcineurin signaling cascade in C. glabrata and found novel and uncharacterized functions of calcineurin and its downstream target Crz1 in governing thermotolerance, intracellular architecture, and pathogenesis in murine ocular, urinary tract, and systemic infections. This represents a second independent origin of a role for calcineurin in thermotolerant growth of a major human fungal pathogen, distinct from that which arose independently in Cryptococcus neoformans. Calcineurin also promotes survival of C. glabrata in serum via mechanisms distinct from C. albicans and thereby enables establishment of tissue colonization in a murine systemic infection model. To understand calcineurin signaling in detail, we performed global transcript profiling analysis and identified calcineurin- and Crz1-dependent genes in C. glabrata involved in cell wall biosynthesis, heat shock responses, and calcineurin function. Regulators of calcineurin (RCN) are a novel family of calcineurin modifiers, and two members of this family were identified in C. glabrata: Rcn1 and Rcn2. Our studies demonstrate that Rcn2 expression is controlled by calcineurin and Crz1 to function as a feedback inhibitor of calcineurin in a circuit required for calcium tolerance in C. glabrata. In contrast, the calcineurin regulator Rcn1 activates calcineurin signaling. Interestingly, neither Rcn1 nor Rcn2 is required for virulence in a murine systemic infection model. Taken together, our findings show that calcineurin signaling plays critical roles in thermotolerance and virulence, and that Rcn1 and Rcn2 have opposing functions in controlling calcineurin signaling in C. glabrata.
phosphatase; calcium; calmodulin; Crz1; Rcn1; Rcn2; thermotolerance; cell wall integrity; ER stress; drug tolerance; pH homeostasis; urinary tract infection; ocular infection; virulence
We previously reported that the immunogenicity of Hcβtre, a botulinum neurotoxin A (BoNT/A) immunogen, was enhanced by fusion to an epithelial cell binding domain, Ad2F, when nasally delivered to mice with cholera toxin (CT). This study was performed to determine if Ad2F would enhance the nasal immunogenicity of Hcβtre in rabbits, an animal model with a nasal cavity anatomy similar to humans. Since CT is not safe for human use, we also tested the adjuvant activity of compound 48/80 (C48/80), a mast cell activating compound previously determined to safely exhibit nasal adjuvant activity in mice.
New Zealand White or Dutch Belted rabbits were nasally immunized with Hcβtre or Hcβtre-Ad2F alone or combined with CT or C48/80, and serum samples were tested for the presence of Hcβtre-specific binding (ELISA) or BoNT/A neutralizing antibodies.
Hcβtre-Ad2F nasally administered with CT induced serum anti-Hcβtre IgG ELISA and BoNT/A neutralizing antibody titers greater than those induced by Hcβtre + CT. C48/80 provided significant nasal adjuvant activity and induced BoNT/A-neutralizing antibodies similar to those induced by CT.
Ad2F enhanced the nasal immunogenicity of Hcβtre, and the mast cell activator C48/80 was an effective adjuvant for nasal immunization in rabbits, an animal model with a nasal cavity anatomy similar to that in humans.
Mast cells have primarily been associated with mediating the pathological secondary responses to allergens in sensitized hosts. In view of the recent evidence for a mast cell role in modulating primary immune responses to pathogens, the likelihood for a role of mast cells in influencing primary immune response to allergens has grown. New evidence suggests that mast cells drive the development of Th2 responses to allergens, particularly when allergen exposure occurs concomitantly with exposure to pathogen products present in the environment. These new roles for mast cells in allergy and infection suggest additional drug targets to prevent development of allergic disease and allergic exacerbations of established disease.
Mast cells (MCs) are best known for eliciting harmful reactions, mostly after primary immunity has been established. Here, we report that during E. coli infection, the primary humoral response in MC-deficient mice is significantly diminished, and was found to be less protective in a urinary tract infection (UTI) model compared to the response from MC-sufficient counterparts. MCs were found to recruit large numbers of dendritic cells (DCs) into the infected tissue site, which eventually migrated into draining lymph nodes (DLNs) over a prolonged time-course. This pattern of trafficking was facilitated by MC generated TNF, which increased the expression of E-selectin on local blood vessels. Antibody blockade of E-selectin inhibited DC recruitment into the site of infection and DLNs, and consequently impaired the primary humoral immune response. Thus, during infection, resident MCs contribute to the primary protective adaptive response through recruitment of DCs from the circulation into infected sites.
Inflammatory bowel disease (IBD) is hypothesized to result from stimulation of immune responses against resident intestinal bacteria within a genetically susceptible host. Mast cells may play a critical role in IBD pathogenesis, since they are typically located just beneath the intestinal mucosal barrier and can be activated by bacterial antigens.
This study investigated effects of mast cells on inflammation and associated neoplasia in IBD-susceptible interleukin (IL)-10-deficient mice with and without mast cells. IL-10-deficient mast cells produced more pro-inflammatory cytokines in vitro both constitutively and when triggered, compared with wild type mast cells. However despite this enhanced in vitro response, mast cell-sufficient Il10−/− mice actually had decreased cecal expression of tumor necrosis factor (TNF) and interferon (IFN)-γ mRNA, suggesting that mast cells regulate inflammation in vivo. Mast cell deficiency predisposed Il10−/− mice to the development of spontaneous colitis and resulted in increased intestinal permeability in vivo that preceded the development of colon inflammation. However, mast cell deficiency did not affect the severity of IBD triggered by non-steroidal anti-inflammatory agents (NSAID) exposure or helicobacter infection that also affect intestinal permeability.
Mast cells thus appear to have a primarily protective role within the colonic microenvironment by enhancing the efficacy of the mucosal barrier. In addition, although mast cells were previously implicated in progression of sporadic colon cancers, mast cells did not affect the incidence or severity of colonic neoplasia in this inflammation-associated model.
We evaluated the safety and efficacy of the mast cell activator compound 48/80 (C48/80) when used as an adjuvant delivered intradermally (ID) with recombinant anthrax protective antigen (rPA) in comparison with two well-known adjuvants. Mice were vaccinated in the ear pinnae with rPA or rPA + C48/80, CpG oligodeoxynucleotides (CpG), or cholera toxin (CT). All adjuvants induced similar increases in serum anti-rPA IgG and lethal toxin neutralizing antibodies. C48/80 induced a balanced cytokine production (Th1/Th2/Th17) by antigen-restimulated splenocytes, minimal injection site inflammation, and no antigen-specific IgE. Histological analysis demonstrated that vaccination with C48/80 reduced the number of resident mast cells and induced an injection-site neutrophil influx within 24 hours. Our data demonstrate that C48/80 is a safe and effective adjuvant, when used by the intradermal route, to induce protective antibody and balanced Th1/Th2/Th17 responses.
Compound 48/80; adjuvant safety; intradermal vaccination
The urinary tract is one of the most intractable mucosal surfaces for pathogens to colonize. In addition to the natural barriers at this site, potential pathogens have to contend with the vigorous local innate immune system. Several Toll-Like Receptors (TLRs) have been identified on epithelial cells of the bladder and the kidneys which mediate a variety of powerful immune responses. A common finding among successful uropathogens is their intrinsic ability to suppress TLR-mediated responses. As antibiotic therapy becomes increasingly ineffective, employing boosters of the innate immune system in the urinary tract may become a viable option.
Caveolin proteins have been implicated in a wide range of cellular functions including lipid raft mediated endocytosis and regulation of cell signaling cascades. Recent discoveries have shown that these proteins are involved not only in regulating these homeostatic cellular functions, but also in the host response to a wide range of different infections. Both caveolin-1 and 2 have been shown to play important roles in pathogen uptake. While caveolin-1 is the most well studied member of this family, a growing body of evidence has now recognized the role of caveolin-2 in these host pathogen interactions and novel host defense mechanisms.
caveolin; lipid rafts; microbial pathogenesis; pneumonia; pseudomonas
The remarkable resistance of the urinary tract to infection has been attributed to its physical properties and the innate immune responses triggered by pattern recognition receptors lining the tract. We report a distinct TLR4 mediated mechanism in bladder epithelial cells (BECs) that abrogates bacterial invasion, a necessary step for successful infection. Compared to controls, uropathogenic type 1 fimbriated Escherichia coli and Klebsiella pneumoniae invaded BECs of TLR4 mutant mice in 10-fold or greater numbers. TLR4 mediated suppression of bacterial invasion was linked to increased intracellular cAMP levels which negatively impacted Rac-1 mediated mobilization of the cytoskeleton. Artificially increasing intracellular cAMP levels in BECs of TLR4 mutant mice restored resistance to type 1 fimbriated bacterial invasion. This finding reveals a novel function for TLR4 and another facet of bladder innate defense.