Therapy of Pseudomonas aeruginosa eye infections

A Dual Role for Cysteinyl Leukotriene Receptors in the Pathogenesis of Corneal Infection

Cysteinyl leukotrienes (CysLTs) have been defined as central mediators of inflammation. Despite our extensive understanding of these bioactive lipid mediators in the pathogenesis of diseases such as asthma, allergic rhinitis, and even neurological disorders, information regarding the eye is markedly lacking. As a result, this study examined the expression profiles of two major CysLT receptors, CysLT1 and CysLT2, in the cornea using experimental mouse models of Pseudomonas aeruginosa-induced keratitis with contrasting outcomes: susceptible C57BL/6 (B6) and resistant BALB/c. Postinfection, disparate levels of CysLT receptors were accompanied by distinct expression profiles for select proinflammatory and anti-inflammatory cell surface markers detected on macrophages and polymorphonuclear neutrophils between the two strains. Further, inhibition of either CysLT receptor converted the disease response of both strains, where corneal perforation was prevented in B6 mice, and BALB/c mice fared significantly worse. In addition, receptor antagonist studies revealed changes in inflammatory cell infiltrate phenotypes and an influence on downstream CysLT receptor signaling pathways. Although the B6 mouse model highlights the established proinflammatory activities related to CysLT receptor activation, results generated from BALB/c mice indicate a protective mechanism that may be essential to disease resolution. Further, basal expression levels of CysLT1 and CysLT2 were significantly higher in uninfected corneas of both mouse strains as opposed to during infection, suggestive of a novel role in homeostatic maintenance within the eye. In light of these findings, therapeutic targeting of CysLT receptors extends beyond inhibition of proinflammatory activities and may impact inflammation resolution, as well as corneal surface homeostasis.

High-mobility group box 1: a novel target for treatment of Pseudomonas aeruginosa keratitis

High-mobility group box 1 (HMGB1), a prototypic alarmin, mediates the systemic inflammatory response syndrome. Treatment with vasoactive intestinal peptide, an anti-inflammatory neuropeptide, downregulates proinflammatory cytokines and promotes healing in a susceptible (cornea perforates) model of Pseudomonas aeruginosa keratitis, and also significantly downregulates HMGB1 expression. Therefore, we examined targeting HMGB1 for the treatment of P. aeruginosa keratitis to avoid delivery and other issues associated with vasoactive intestinal peptide. For this, HMGB1 was silenced using small interfering RNA, whereas controls were treated with a nonspecific scrambled sequence small interfering RNA. Less disease was seen postinfection in siHMGB1 compared with control mice and was documented by clinical score and photographs with a slit lamp. Real-time RT-PCR and ELISA confirmed HMGB1 knockdown. RT-PCR analysis also revealed reduced mRNA levels of IL-1β, MIP-2, TNF-α, TLR4, and receptor for advanced glycation end products, whereas mRNA levels of anti-inflammatory TLRs single Ig IL-1-related receptor and ST2 were increased significantly. HMGB1 knockdown also decreased IL-1β and MIP-2 proteins, reducing polymorphonuclear cell number in the infected cornea. mRNA and protein levels of CXCL12 and CXCR4, as well as mononuclear cells, were reduced significantly after HMGB1 knockdown. Ab neutralization of HMGB1, infection with a clinical isolate, and recombinant HMGB1 treatment of resistant mice supported the silencing studies. These data provide evidence that silencing HMGB1 promotes better resolution of P. aeruginosa keratitis by decreasing levels of proinflammatory mediators (decreasing polymorphonuclear cell infiltration), increasing anti-inflammatory TLRs, reducing CXCL12 (preventing HMGB1/CXCL12 heterodimer formation), and signaling through CXCR4, reducing monocyte/macrophage infiltration.

Vasoactive Intestinal Peptide Balances Pro- and Anti-Inflammatory Cytokines in thePseudomonas aeruginosa-Infected Cornea and Protects against Corneal Perforation

Corneal infection with Pseudomonas aeruginosa perforates the cornea in susceptible C57BL/6 (B6), but not resistant BALB/c, mice. To determine whether vasoactive intestinal peptide (VIP) played a role in development of the resistant response, protein expression levels were tested by immunocytochemistry and enzyme immunoassay in BALB/c and B6 corneas. Both mouse strains showed constitutive expression of corneal VIP protein and nerve fiber distribution. However, disparate expression patterns were detected in the cornea after infection. VIP protein was elevated significantly in BALB/c over B6 mice at 5 and 7 days postinfection. Therefore, B6 mice were injected with rVIP and subsequently demonstrated decreased corneal opacity and resistance to corneal perforation compared with PBS controls. rVIP- vs PBS-treated B6 mice also demonstrated down-regulation of corneal mRNA and/or protein levels for proinflammatory cytokines/chemokines: IFN-gamma, IL-1beta, MIP-2, and TNF-alpha, whereas anti-inflammatory mediators, IL-10 and TGF-beta1, were up-regulated. Treatment with rVIP decreased NO levels and polymorphonuclear neutrophil (PMN) number. To further define the role of VIP, peritoneal macrophages (Mphi) and PMN from BALB/c and B6 mice were stimulated with LPS and treated with rVIP. Treatment of LPS-stimulated Mphi from both mouse strains resulted in decreased IL-1beta and MIP-2 protein levels; PMN responded similarly. Both cell types also displayed a strain-dependent differential response to rVIP, whereby B6 Mphi/PMN responded only to a higher concentration of VIP compared with cells from BALB/c mice. These data provide evidence that neuroimmune regulation of the cytokine network and host inflammatory cells functions to promote resistance against P. aeruginosa corneal infection.

Role of cytokines and chemokines in pseudomonal keratitis

Pseudomonal keratitis usually progresses rapidly, often resulting in corneal perforation and blindness. Remarkable events in pseudomonal keratitis include massive polymorphonuclear leukocyte infiltration in the cornea and various degrees of tissue destruction. With regard to initiation of these inflammatory events, various inflammatory cytokines and chemokines appear to be key substances and have been the subject of several studies. Inflammatory cytokines and chemokines believed to be important in pseudomonal keratitis include interleukin (IL)-1 beta, IL-6, macrophage inflammatory protein (MIP)-2 (homologous to human IL-8), macrophage inhibitory factor (MIF), IL-12, IL-18, interferon (IFN)-gamma, and tumor necrosis factor (TNF)-alpha. In this article, current concepts related to the role of inflammatory cytokines and chemokines in pseudomonal keratitis are reviewed.

Corneal response to Pseudomonas aeruginosa infection

Pseudomonas aeruginosa (P. aeruginosa) is a common organism associated with bacterial keratitis, especially in those who use extended wear contact lenses. Recent advances in our understanding of host innate and adaptive immune responses to experimental infection have been made using a variety of animal models, including inbred murine models that are classed as resistant (cornea heals) vs. susceptible (cornea perforates). Evidence has been provided that sustained IL-12-driven IFN-gamma production in dominant Th1 responder strains such as C57BL/6 (B6) contributes to corneal destruction and perforation, while IL-18-driven production of IFN-gamma in the absence of IL-12 is associated with bacterial killing and less corneal destruction in dominant Th2 responder strains such as BALB/c. The critical role of IL-1 and chemotactic cytokines such as MIP-2 in PMN recruitment and the critical role of this cell in the innate immune response to bacterial infection is reviewed. Regulation of PMN persistence is also discussed and evidence provided that persistence of PMN in B6 cornea is regulated by CD4+ T cells, while macrophages regulate PMN number in the cornea of BALB/c mice. The studies provide a better understanding of the inflammatory mechanisms that are operative in the cornea after P. aeruginosa challenge and are consistent with long-term goals of providing targets for alternative or adjunctive treatment for this disease. Future studies will be aimed at better defining the role of Toll receptors, neuropeptides (as unconventional modulators of the immune response) and exploitation of disease control by new techniques, such as RNA silencing.

Pathogenic mechanisms of P. aeruginosa keratitis: a review of the role of T cells, Langerhans cells, PMN, and cytokines

The aim of this article is to review our current understanding of the role of cytokines, chemokines, T cells, Langerhans cells, and neutrophils (PMN) and their interactions in vivo in the host response to Pseudomonas aeruginosa ocular challenge. The cellular/cytokine network in vivo has begun to be unraveled, and the data discussed provide substantive evidence for a regulatory role of CD4(+) T cells (Th1 type) contributing directly to persistence of PMN in the cornea of susceptible C57BL/6 (cornea perforates) versus resistant BALB/c (cornea heals) mice. Additionally, in the susceptible mouse model, CD4(+) T cells interact with Langerhans cells and B7/CD28 ligation appears critical for antigen presentation and the susceptibility response. Various cytokines and chemokines (e.g., MIP-1alpha, IL-1beta, MIP-2, IL-12, and IFN-gamma) and their pattern of sustained upregulation after infection in susceptible versus resistant mice also will be discussed in light of an in vivo cytokine network. T-cell-mediated pathogenic mechanisms are of importance in development of the susceptible response to P. aeruginosa ocular infection. In the absence of T-cell infiltration into the cornea, PMN do not persist in the stroma, and cytokines and chemokines are better balanced, resulting in decreased stromal destruction and the resistance response.

Balance of pro- and anti-inflammatory cytokines correlates with outcome of acute experimental Pseudomonas aeruginosa keratitis

The purpose of this study was to elucidate the expression of pro- and anti-inflammatory cytokines in mouse corneas infected with Pseudomonas aeruginosa. Three bacterial strains (invasive, cytotoxic, or CLARE [contact lens-induced acute red eye]) which have recently been shown to produce distinct patterns of corneal disease in the mouse were used. The left mouse (BALB/c) corneas were scarified and infected with 2 x 10(6) CFU of one of the three P. aeruginosa strains, while right eyes served as controls. Animals were examined at 1, 4, 8, 16, and 24 h with a slit lamp biomicroscope to grade the severity of infection. Following examination, eyes were collected and processed for histopathology, multiprobe RNase protection assay for cytokine mRNA, enzyme-linked immunosorbent assay to quantitate cytokine proteins, and myeloperoxidase activity to quantitate polymorphonuclear leukocytes. The kinetics of appearance and magnitude of expression of key cytokines varied significantly in the three different phenotypes of P. aeruginosa infection. The predominant cytokines expressed in response to all three phenotypes were interleukin-1 beta (IL-1 beta), IL-1Ra, and IL-6. In response to the invasive strain, which induced severe corneal inflammation, significantly lower ratios of IL-1Ra to IL-1 beta were present at all time points, whereas corneas challenged with the CLARE strain, which induced very mild inflammation, showed a high ratio of IL-1Ra to IL-1 beta. The outcome of infection in bacterial keratitis correlated with the relative induction of these pro- and anti-inflammatory cytokines, and exogenous administration of recombinant rIL-1Ra (rIL-1Ra) was able to reduce the disease severity significantly. These findings point to the therapeutic potential of rIL-1Ra protein in possible treatment strategies for bacterial keratitis.

Prolonged elevation of IL-1 in Pseudomonas aeruginosa ocular infection regulates macrophage-inflammatory protein-2 production, polymorphonuclear neutrophil persistence, and corneal perforation

The kinetics of IL-1 (alpha and beta) production after Pseudomonas aeruginosa corneal infection was examined in susceptible (cornea perforates) C57BL/6J (B6) and resistant (cornea heals) BALB/cByJ (BALB/c) mice. IL-1alpha and -1beta (mRNA and protein) were elevated in both mouse strains, and levels peaked at 1 day postinfection (p.i. ). Significantly greater amounts of IL-1 protein were detected in B6 vs BALB/c mice at 1 and 3 days p.i. At 5 days p.i., IL-1alpha and -1beta (mRNA and protein) remained elevated in B6, but began to decline in BALB/c mice. To test the significance of elevated IL-1 in B6 mice, a polyclonal neutralizing Ab against IL-1beta was used to treat infected B6 mice. A combination of subconjunctival and i.p. administration of IL-1beta polyclonal Ab significantly reduced corneal disease. The reduction in disease severity in infected B6 mice was accompanied by a reduction in corneal polymorphonuclear neutrophil number, bacterial load, and macrophage inflammatory protein-2 mRNA and protein levels. These data provide evidence that IL-1 is an important contributor to P. aeruginosa corneal infection. At least one mechanism by which prolonged and/or elevated IL-1 expression contributes to irreversible corneal tissue destruction appears to be by increasing macrophage inflammatory protein-2 production, resulting in a prolonged stimulation of polymorphonuclear neutrophil influx into cornea. In contrast, a timely down-regulation of IL-1 appears consistent with an inflammatory response that is sufficient to clear the bacterial infection with less corneal damage.

Macrophage inflammatory protein-2 is a mediator of polymorphonuclear neutrophil influx in ocular bacterial infection

Polymorphonuclear neutrophils (PMN) in Pseudomonas aeruginosa-infected cornea are required to clear bacteria from affected tissue, yet their persistence may contribute to irreversible tissue destruction. This study examined the role of C-X-C chemokines in PMN infiltration into P. aeruginosa-infected cornea and the contribution of these mediators to disease pathology. After P. aeruginosa challenge, corneal PMN number and macrophage inflammatory protein-2 (MIP-2) and KC levels were compared in mice that are susceptible (cornea perforates) or resistant (cornea heals) to P. aeruginosa infection. While corneal PMN myeloperoxidase activity (indicator of PMN number) was similar in both groups of mice at 1 and 3 days postinfection, by 5-7 days postinfection corneas of susceptible mice contained a significantly greater number of inflammatory cells. Corneal MIP-2, but not KC, levels correlated with persistence of PMN in the cornea of susceptible mice. To test the biological relevance of these data, resistant mice were treated systemically with rMIP-2. This treatment resulted in increased corneal PMN number and significantly exacerbated corneal disease. Conversely, administration of neutralizing MIP-2 pAb to susceptible mice reduced both PMN infiltration and corneal destruction. Collectively, these findings support an important role for MIP-2 in recruitment of PMN to P. aeruginosa-infected cornea. These data also strongly suggest that a timely down-regulation of the host inflammatory response is critical for resolution of infection.

Effect of galardin on collagen degradation by Pseudomonas aeruginosa

The authors examined the effect of a synthetic peptidyl hydroxamate inhibitor of matrix metalloproteinase, Galardin, on collagen degradation by Pseudomonas aeruginosa (P. aeruginosa) in the presence or absence of keratocytes. Type I collagen gels, with or without suspended keratocytes, were incubated under medium containing sterile P. aeruginosa culture broth and/or Galardin for 24 hr. Degradation of collagen fibrils during culture was measured by the release of hydroxyproline. The conditioned media were also subjected to gelatin zymography and Western blotting to analyse the activation, by P. aeruginosa factor(s), of matrix metalloproteinases (MMPs) released by keratocytes. The effects of protease inhibitors, aprotinin, leupeptin and pepstatin, on collagen degradation by P. aeruginosa were also examined. P. aeruginosa broth by itself induced collegen gel degradation. When keratocytes were present, P. aeruginosa broth increased the amount of degraded collagen even further. Galardin significantly reduced the amounts of collagen degraded by P. aeruginosa culture broth, whether keratocytes were present or absent in the gel. However, the protease inhibitors had no inhibitory effects on collagen degradation. Gelatin zymography and Western blotting revealed that inactive proMMP-1, -2 and -3, released by keratocytes, were converted to active forms in the presence of P. aeruginosa broth. Galardin decreased the amounts of active MMPs and increased those of inactive proMMPs, suggesting that Galardin inhibited the activation of proMMPs by P. aeruginosa. The present results suggest that Galardin inhibits the keratocyte-mediated collagen degradation by P. aeruginosa culture broth, resulting from preventing the conversion of proMMPs to active MMPs.

Early cytokine and chemokine gene expression during Pseudomonas aeruginosa corneal infection in mice

Using a multiprobe RNase protection assay, we examined cytokine and chemokine mRNAs that were expressed after corneal infection with Pseudomonas aeruginosa in mice. Cytokines that were upregulated included interleukin-1alpha (IL-1alpha) and -1beta, IL-1 receptor antagonist, IL-6, IL-11, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, macrophage colony-stimulating factor, stem cell factor, lymphotoxin beta, transforming growth factor beta1, and tumor necrosis factor alpha. Chemokine transcripts that were upregulated included Eotaxin; gamma-interferon-inducible protein 10; monocyte chemoattractant protein 1; macrophage inflammatory proteins 1alpha, 1beta, and 2; and RANTES. Peak expression of these cytokines and chemokines was observed between 1 and 3 days after infection. These responses returned to or approached baseline preinfection levels by 7 days after ocular challenge. Identification of the various cytokines and chemokines upregulated during corneal infection provides important information relevant to unraveling the pathogenesis induced by this bacterium and provides hope that specific molecules can be targeted for therapy.

Bacterial keratitis in the critically ill

BACKGROUND

In the 4 year period (1988-91 there were nine cases of bacterial keratitis in five critically ill patients on an intensive care unit ('unit A'), all except one due to Pseudomonas aeruginosa. Many of these patients had serious ocular complications requiring surgery and all surviving patients were left with significant visual deficits. One further case of keratitis due to P aeruginosa occurred on unit A in April 1993. The problem of keratitis in ventilated patients is not unique to this unit as a further four cases in three patients from additional units in this area have been treated.

METHODS

Predisposing factors in unit A were established through subsequent investigations. It was found, in particular, that all the ocular infections were preceded by colonisation of the respiratory tract with the pathogenic organism. Recommendations concerning eye care and tracheal suctioning were adopted by unit A in 1991.

RESULTS

In the subsequent 4 years (1991-5), the frequency of isolation of pseudomonas from the respiratory tract per patient treated in unit A remained relatively high at 3.8% (153/4032). However, the conjunctival pseudomonas isolation rate has decreased significantly (p < 0.001) from 0.8% (19/2430) to 0.05% (2/4032).

CONCLUSIONS

Ventilated patients may be at risk from inoculation of pathogens into the eyes. The principal risk factor for bacterial keratitis in this series was corneal exposure secondary to conjunctival chemosis or lid damage. The adoption of simple preventative measures on unit A had a significant impact on the incidence of eye infections due to pseudomonas, despite the high proportion of patients whose respiratory tracts were colonised with the same organism. There is a need for additional research into the most effective method of eye care for ventilated patients in order to reduce the frequency of this avoidable condition.

Inhibition of bacterial adherence to host tissue does not markedly affect disease in the murine model of Pseudomonas aeruginosa corneal infection

The prevention of bacterial infections by the inhibition of binding to host tissues is an oft-touted approach, but few studies with appropriate models of infection have tested its feasibility. Pseudomonas aeruginosa causes severe corneal infections in mice after inoculations with low doses, and infection is thought to depend upon an initial adherence of the bacteria to corneal cells. In vitro, adherence to corneal cells is mediated to a large degree by the complete-outer-core oligosaccharide of the bacterial lipopolysaccharide (LPS). However, bacteria adhering to tissues in vivo are difficult to differentiate from nonadherent bacteria. Since a direct correlate of P. aeruginosa adherence to corneal epithelial cells is the degree to which these cells internalize P. aeruginosa, the level of adherence in vivo can be approximated by measuring P. aeruginosa ingestion by cells by using gentamicin exclusion assays. To determine the degree to which inhibition of the corneal cell adherence affects the course of infection and disease in the murine model, we evaluated the ability of LPS-outer-core oligosaccharide to inhibit bacterial association and entry into corneal cells and to modulate the development of disease. Mice were anesthetized, and their corneas were scratched and inoculated with virulent P. aeruginosa 6294 or PAO1, along with either 50 microg of oligosaccharide derived from LPS from P. aeruginosa PAC557 (complete outer core but no O side chains) or oligosaccharide derived from LPS of P. aeruginosa PAC1RalgC::tet (incomplete-core oligosaccharide). After 4 h, there were no differences between groups in the counts of infecting and internalized bacteria. At 24 h, the complete-core oligosaccharide decreased the levels of bacteria per eye by 70 to 99.7% compared with the levels achieved by including the incomplete-core oligosaccharide in the infectious inoculum. Epithelial cell ingestion of bacteria was comparably affected. However, the effect on disease was modest and only evident at lower challenge doses that elicited mild disease in controls and when the bacterial association and ingestion were inhibited by >99%. Overall, it appears that in the murine model of P. aeruginosa corneal infection at challenge doses of bacteria 10-fold or greater than the minimal amount needed to cause disease, the absolute level of inhibition of bacterial adherence is insufficient to reduce the bacterial counts below that which elicits disease.

In vivo characterization of host and bacterial protease expression during Pseudomonas aeruginosa corneal infections in naive and immunized mice

PURPOSE

The corneal destruction resulting from P. aeruginosa keratitis is thought to be due, in part, to the actions of host and bacterial proteases on corneal tissue. The intent of the current study was to compare host and bacterial protease expression in the ocular tissues of mice that have been shown to be susceptible to corneal infection with mice that have been protected from such infection as a result of prior immunization.

METHODS

Supernatants of infected corneal homogenates from naive mice or from mice immunized as a result of prior corneal infection or that had been immunized against bacterial protease were analyzed by gelatin zymography for the presence of host gelatinases and bacterial proteases at various times after infection. In addition, studies were carried out to examine the effect of immunization with bacterial protease on numbers of ocular bacteria.

RESULTS

Mice that were protected against the tissue destruction during infection were found to have reduced proteolytic activity in corneal tissues when compared with susceptible mice. More specifically, the resistant mice displayed lower levels of bacterial alkaline protease, latent MMP-2, and both the active and latent forms of MMP-9. In addition, immunization against either bacterial alkaline protease or elastase failed to reduce bacterial numbers in ocular tissues from 6 to 72 hours after infection.

CONCLUSIONS

These results suggest that increased proteolytic activity in ocular tissues during P. aeruginosa infection may contribute to the irreversible corneal damage observed during the infection.

Pseudomonas aeruginosa keratitis in leukopenic rabbits

To study the role of the host inflammatory response in Pseudomonas aeruginosa keratitis, rabbits were made leukopenic with intravenous injections of cyclophosphamide and dexamethasone. Twenty-four hr later, keratitis was initiated in all rabbits with an intrastromal injection of 1,000 log phase P. aeruginosa strain 27853. Slit lamp examination of eyes showed that leukopenic rabbits had significantly less (P < 0.0001) ocular pathology at 16, 22, and 27 hr postinfection. The number of viable bacteria recovered from corneas of leukopenic rabbits was the same as the number recovered from nonleukopenic rabbits (P = 0.95). These results suggest that the host inflammatory response significantly contributes to the overall ocular pathology associated with P. aeruginosa keratitis, but does not influence the survival of the infecting organism in the cornea at the height of the infection.

Pathogenesis and therapy of Pseudomonas aeruginosa keratitis

Considerable progress has been made recently in understanding the pathogenesis of pseudomonas keratitis including its adhesion, invasion and the role of the glycocalyx. Adhesion to epithelial cells has been shown in vitro to depend on pili but their relationship to the glycocalyx biofilm has yet to be explored. The actions of its toxins and proteases have been established by studying the effects of deletion mutants. However, in animal models pseudomonads have been unable to cause an invasive infection in the presence of an intact corneal epithelium and have required associated trauma. Why early necrosis of the cornea can occur in an immunocompetent individual is still not clear but may reflect the delayed access of polymorphonuclear cells (PMNs) in an avascular tissue. Whether the corneal necrosis that later occurs is then due to the PMNs or pseudomonas toxins is not established. Topical fortified gentamicin therapy remains the treatment of choice for early infection. The use of medical and surgical adjunctive therapy is often needed to cope with the complications of advanced disease and has been reviewed.