Infection and inflammation during contact lens wear is often associated with microbial contamination of lenses. Several different types of microbes that colonize lenses can lead to infection and inflammation, but the most common cause of infection (microbial keratitis; MK) remains the Gram-negative bacterium Pseudomonas aeruginosa. P. aeruginosa has a battery of cell-associated and extracellular virulence factors it can use to initiate and maintain infection. Its ability to produce proteases, to either invade or kill corneal cells, and to coordinate expression of virulence factors via quorum-sensing have been shown to be important during MK. Another important factor that contributes to the destruction of the cornea during MK is excessive activation of the host defense system. P. aeruginosa can activate several pathways of the immune system during MK, and activation often involves receptors on the corneal epithelial cells called toll-like receptors (TLRs). These TLRs recognize e.g., lipopolysaccharide or flagella from P. aeruginosa and activate the epithelial cells to produce inflammatory mediators such as cytokines and chemokines. These cytokines or chemokines recruit white blood cells, predominantly polymorphonuclear leukocytes, to the infection in order that they can phagocytose and kill the P. aeruginosa. However, continued recruitment and presence of these polymorphonuclear neutrophils (PMNs) and other white blood cells in the corneal tissue leads to destruction of corneal cells and tissue components. This can ultimately lead to scarring and vision loss. Bacterial colonization of contact lens surfaces causes many adverse responses encountered during contact lens wear.1–8 The most devastating of these is microbial keratitis (MK) which, if not treated promptly or with the proper antibiotics, can lead to scarring and blindness. Other adverse responses associated with microbial colonization of contact lenses include contact lens-induced acute red eye (CLARE), contact lens-induced peripheral ulceration, infiltrative keratitis (IK), and asymptomatic infiltrative keratitis (AIK). For a detailed clinical description of these conditions the reader is referred to the article by Holden et al.9
Nonbacterial microbial colonization of contact lenses is associated with MK, especially colonization of lenses by Acanthamoeba10 or fungi.11,12 However, MK is most commonly caused by colonization of contact lenses with the bacterium Pseudomonas aeruginosa,13–15 a Gram-negative bacterium that is commonly found in many environments, including water. CLARE is associated with colonization of contact lenses with Gram-negative bacteria (in particular Haemophilus influenzae)3 or the Gram-positive bacterium Streptococcus pneumoniae.4 Contact Lens Induced Peripheral Ulceration is produced by Gram-positive colonization of contact lenses, with the bacterium Staphylococcus aureus being the most commonly associated bacterium.7 Causes of IK and AIK are many, but include colonization by large numbers of Gram-negative bacteria for IK and large numbers of Gram-positive bacteria for AIK (Table 1).
Pseudomonas aeruginosa Virulence Factors
As P. aeruginosa is a major causative agent of MK, CLARE, CPU and AIK, this review will focus on aspects of this bacterium that contribute to infection, and the host response to this bacterium in the cornea. A major part of the virulence of P. aeruginosa is attributed to its ability to produce several destructive proteins, and to induce an excessive host immune response in the ocular tissues.16 The P. aeruginosa virulence factors most associated with ocular damage are exoenzymes S (gene called exoS), and U (exoU),17 elastase (lasB),18 alkaline protease (aprA)19 and protease IV (prpL).20 Another important pathogenic mechanism is a cell-to-cell communication system called quorum sensing.21
P. aeruginosa strains can usually be distinguished by the presence of either genes to exoS or exoU, which encode the toxins exoenzyme S and exotoxin U respectively. Cells possessing the exoS gene tend to invade the interior of the cell, but do not secrete ExoU. Conversely, cells possessing exoU tend to remain outside the host cell but secrete ExoU directly into the cells interior.22 ExoS protein is both a GTPase-activating protein and an ADP-ribosyltransferase and both these activities lead to rearrangement of the cytoskeleton protein actin and ultimately cell death,23,24 whereas ExoU is an intracellular phospholipase and causes rapid cell death.25
Elastase (LasB) breaks down elastin, fibrin, and collagen, which are critical for the mechanical properties of connective tissue. It has also shown to degrade host immunological factors IgG, IgA, IFNγ, TNFα.26 Alkaline protease is known to degrade fibrin, complement molecules C1q, and C3, and in conjunction with LasB, cytokines IFNγ and TNFα.26 It was shown to enhance bacterial binding to corneal epithelium by exposure of lipase sensitive receptors,27 but subsequently determined not to be of major importance in virulence in the eye. Protease IV degrades important host immunological proteins such as complement and IgG.28 Protease IV also compromises the integrity of structural proteins such as elastin,29 therefore causing tissue damage and facilitating bacterial infection. Protease IV degrades the iron binding proteins lactoferrin and transferrin which enables P. aeruginosa to scavenge iron from the host.29 Protease IV has been shown to be a key virulence factor for P. aeruginosa in ocular infections.30
Controlled expression of several key P. aeruginosa virulence factors including the gene for elastase, lasB 31 and ExoS production32 are mediated via population density biosensor mechanisms, known as quorum-sensing. Quorum sensing activates genes only when a quorum, i.e., a certain density of bacteria, is present. Once the density of bacteria reaches a certain threshold, the bacteria respond as a population, switching on the production of genes, especially those associated with pathogenicity. This control of the expression of virulence or pathogenicity factors might help the bacterium be “invisible” to the host (i.e., eye) until such a time when the levels of the bacteria are sufficient to allow the bacteria to overcome host defense systems.
Inflammatory Response in the Cornea During Pseudomonas aeruginosa Keratitis
Pseudomonas aeruginosa infection of the cornea triggers an intense inflammatory response, which persists and can result in severe corneal damage, including perforation.33 Corneal infection with P. aeruginosa is characterized by the extensive recruitment of inflammatory cells with PMNs being the most predominant cell type seen in the cornea. PMNs are believed to be essential for the elimination of bacteria and to promote wound healing; however, persistence of these cells may contribute to corneal damage. A reduction in PMN recruitment is accompanied by a significant reduction in tissue damage during bacterial infection of the cornea in mice.34
The initial inflammatory cascade is most likely mediated by the innate immune system, i.e., the immune system not dependent on antibody production. The characteristics of the innate immune response are a broad-spectrum relatively nonspecific response, no memory or lasting protective immunity, a limited repertoire of recognition molecules and the fact that the responses are phylogenetically ancient. Innate immunity includes anatomic barriers such as the lids, lashes and an intact epithelial surface. A large number of diverse chemical compounds are involved in innate immunity. These include factors present all the time in tears such as lactoferrin (binds iron which is an essential nutrient for microbes), lysozyme (an enzyme which cleaves the cell wall peptidoglycan of bacteria), secretory phospholipase A2 (which cleaves membrane lipids of certain bacteria) and components of the complement cascade (which helps recruit white blood cells to sites of bacterial colonization and aids in killing of bacteria). Other factors are produced by resident ocular cells, such as epithelial cells, upon stimulation by bacteria. These factors include defensins (which help kill bacteria by boring holes in their membranes) and arachidonic acid metabolites (which help recruit white blood cells).
The adaptive antibody-associated immune system probably plays a smaller role in the initial inflammatory response during keratitis, but is likely to increase in importance as the infection develops. (It should be noted that in the case of Acanthamoeba keratitis, it is believed that the adaptive immune response is of major importance).35 During the course of the inflammatory response, migrating and resident white blood cells, including polymorphonuclear leukocytes, macrophages, dentritic cells, and T cells, take over as the major mediators of the inflammatory response. It seems that the activation of TLRs of dentritic cells is a major influence on the progression from an innate to adaptive immune response. For recent reviews of the role of the innate immune response during Pseudomonas aeruginosa keratitis see several publications arising from the Hazlett laboratory.16,36,37,38
Figure 1 outlines the stages involved in the production of the inflammatory response to Pseudomonas aeruginosa during MK. Epithelial cells respond to P. aeruginosa and other bacteria through several systems, including the use of TLRs. This host receptor–bacteria interaction has been described as occurring between pattern-recognition receptors (such as TLRs) on host cells and pathogen-associated molecular patterns (PAMPs) of microorganisms.39 The cornea or conjunctiva have been shown to possess TLR 2, TLR 4, TLR 5, and TLR 9. These receptors recognize distinct PAMPs; TLR 2 recognizes components from both Gram-positives (i.e., Staphylococcus aureus) and Gram-negatives (i.e., Pseudomonas aeruginosa), including ExoS of P. aeruginosa 40; TLR 4 recognizes lipopolysaccharide which is only produced by Gram-negatives and ExoS of P. aeruginosa 40; TLR 5 recognizes flagellin, the major protein component of flagella of many different bacteria; TLR 9 recognizes DNA and RNA from bacteria and viruses (see review by Yu and Hazlett36). Interestingly, it seems that the cornea may “hide” its TLRs, thus reducing the propensity for corneal inflammation. TLR 4 is only expressed at low levels until stimulated by inflammatory cytokines such as IL-1β41 and TLR 5 is only expressed in basal or wing cells of the corneal epithelium, not on the superficial cells42 and is therefore hidden until the cornea epithelial surface is disrupted. TLR 9 seems to be involved in P. aeruginosa keratitis, probably helping to stimulate bacterial killing in the cornea,43 and TLR 5 on corneal epithelial cells can recognize P. aeruginosa flagellin.42 Intracellular signaling cascades in corneal epithelial cells after PAMP recognition stimulate the production of cytokines and chemokines involved in the inflammatory response. TLRs can also mediate expression of proteins that are directly antimicrobial, e.g., the defensins. Several defensins can be produced by corneal epithelial cells directly as the result of TLR stimulation.41,44,45 Defensins can also be produced from the products of TLR activation, e.g., IL-1β.46
The recruitment of leukocytes from the blood vessels to the site of infection or inflammation is a highly orchestrated process, involving a cascade of chemoattractant signals and adhesion molecules. Chemokines are a superfamily of structurally related proteins whose principal function seems to be regulation of leukocyte recruitment.47 The family is divided into four groups (CXC, CC, C, and CX3C) based on the position of the first two cysteine amino acids.48–51 Based on in vitro experimentation and corroborative in vivo studies, members of the CXC chemokines that contain a tripeptide motif Glu-Leu-Arg (ELR+),52 such as interleukin (IL)-8,53 have been shown to promote the migration of PMNs, but not of mononuclear cells, whereas those that do not contain the motif (ELR−) are potent chemoattractants for mononuclear cells. By contrast, members of the CC chemokine subfamily, such as MIP-1α (CCL5) and the monocyte chemoattractant protein-1 (MCP-1; CCL2) usually only promote the migration of monocytes and T lymphocytes, but not of PMNs.54,55
Apart from the chemokines directly associated with recruitment of PMNs, several other cytokines or chemokines have been shown to play major roles in the inflammatory response associated with Pseudomonas aeruginosa keratitis, and these include IL-1β, IL-6, IL-10, IL-18 and interferon (IFN)γ. IL-1β may be the master mediator of the inflammatory response; after its appearance other mediators are generated. Certainly, IL-1β appears very early in the infection in animal models56,57 and human corneal epithelial cells exposed to P. aeruginosa in tissue culture.58 Blocking IL-1β reduces corneal inflammation during keratitis.56,59–61 IL-6, on the other hand, may be involved in down-regulating corneal inflammation and the resolution of disease.62 Mice that have been constructed without the IL-6 gene (IL-6 gene knockouts) show more severe disease63 and if IL-6 is given to mice by injection during P. aeruginosa infection, the animals show better disease progression,64 whereas IL-1β and IL-6 are initially produced by the corneal epithelial cells, and IL-10 is predominantly produced by a subset of infiltrating white blood cells65 and as such mediates its effects later in the infection and inflammation. IL-10 may be involved in preventing excessive angiogenesis during corneal infection,65 thereby controlling vision loss during infection. IL-18 is involved in the regulation of IFNγ production, and it seems that IL-18 has a role in preventing growth of P. aeruginosa during infection through IFNγ.66 IL-18 is produced mainly by resident macrophages or dentritic cells of the cornea and stimulates T, NK, and NKT cells to produce IFNγ. IL-18 driven IFNγ production, in the absence of IL-12 is associated with increased bacterial killing and less corneal destruction.37
T-cells are specialized white blood cells that play a central role in cell-mediated immunity. CD4+ T-cells (also known as helper T cells) are involved in activating and directing other immune cells, and are particularly important in the acquired immune system. They are essential in determining B cell antibody class switching, in the activation and growth of cytotoxic T cells, and in promoting bactericidal activity of phagocytes such as macrophages. In corneal infection, CD4+ T cells seem to promote corneal perforation and susceptibility during infection with P. aeruginosa.67 There are two distinct types of helper T cells, Th1, and Th2. Th1 cells produce for example IFN-γ, whereas Th2 cells produce IL-4, IL-5 and IL-13, among numerous other cytokines. Cytokines produced by Th1 cells maximize the killing efficacy of the macrophages and the proliferation of cytotoxic CD8+ T cells. The cytokines produced by Th2 cells stimulate B cells into proliferation, to undergo antibody class switching, and to increase antibody production. It has become clear that, in mouse strains, the Th1 response is associated with susceptibility of the cornea to perforation following infection with P. aeruginosa, whereas the Th2 response is associated with resistance to perforation.68 The Th1 response seems to assist in the continued recruitment of PMNs to the cornea, and it is these PMNs that mediate perforation.
In summary, microbial keratitis during contact lens wear is most often caused by P. aeruginosa. This bacterium can produce a variety of virulence factors, including toxins and proteases that help it to initiate and maintain the infection. Aspects of the innate immune system help control the bacterial infection and during infection, the host mounts an inflammatory response to try to overcome the bacteria. This inflammatory response is mediated, at least initially, through the innate immune system via pathogen-associated recognition receptors on corneal epithelial cells. Subsequent to this recognition of infection, epithelial cells and resident lymphocytes stimulate the recruitment of predominately PMNs. It is the recruitment of large numbers of PMNs that helps contain the infection, but may ultimately lead to corneal destruction and vision loss.
Institute for Eye Research
Rupert Myers Building
University of New South Wales
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1. Poggio EC, Glynn RJ, Schein OD, Seddon JM, Shannon MJ, Scardino VA, Kenyon KR. The incidence of ulcerative keratitis among users of daily-wear and extended-wear soft contact lenses. N Engl J Med 1989;321:779–83.
2. Holden BA, La Hood D, Grant T, Newton-Howes J, Baleriola-Lucas C, Willcox MD, Sweeney DF. Gram-negative bacteria can induce contact lens related acute red eye (CLARE) responses. CLAO J 1996;22:47–52.
3. Sankaridurg PR, Willcox MD, Sharma S, Gopinathan U, Janakiraman D, Hickson S, Vuppala N, Sweeney DF, Rao GN, Holden BA. Haemophilus influenzae adherent to contact lenses associated with production of acute ocular inflammation. J Clin Microbiol 1996;34:2426–31.
4. Sankaridurg PR, Sharma S, Willcox M, Sweeney DF, Naduvilath TJ, Holden BA, Rao GN. Colonization of hydrogel lenses with Streptococcus pneumoniae: risk of development of corneal infiltrates. Cornea 1999;18:289–95.
5. Sankaridurg PR, Sharma S, Willcox M, Naduvilath TJ, Sweeney DF, Holden BA, Rao GN. Bacterial colonization of disposable soft contact lenses is greater during corneal infiltrative events than during asymptomatic extended lens wear. J Clin Microbiol 2000;38:4420–4.
6. Corrigan KM, Harmis NY, Willcox MD. Association of acinetobacter species with contact lens-induced adverse responses. Cornea 2001;20:463–6.
7. Jalbert I, Willcox MD, Sweeney DF. Isolation of Staphylococcus aureus from a contact lens at the time of a contact lens-induced peripheral ulcer: case report. Cornea 2000;19:116–20.
8. Keay L, Harmis N, Corrigan K, Sweeney D, Willcox M. Infiltrative keratitis associated with extended wear of hydrogel lenses and Abiotrophia defectiva. Cornea 2000;19:864–9.
9. Holden BA, Sankaridurg PR, Jalbert I. Adverse events and infections: which ones and how many? In: Sweeney DF, ed. Silicone Hydrogels: The Rebirth of Continuous Wear Contact Lenses. Oxford, UK: Butterworth-Heinemann; 2000:150–213.
10. Schaumberg DA, Snow KK, Dana MR. The epidemic of Acanthamoeba keratitis: where do we stand? Cornea 1998;17:3–10.
11. Srinivasan M. Fungal keratitis. Curr Opin Ophthalmol 2004;15:321–7.
12. Khor WB, Aung T, Saw SM, Wong TY, Tambyah PA, Tan AL, Beuerman R, Lim L, Chan WK, Heng WJ, Lim J, Loh RS, Lee SB, Tan DT. An outbreak of Fusarium keratitis associated with contact lens wear in Singapore. JAMA 2006;295:2867–73.
13. Schein OD, Ormerod LD, Barraquer E, Alfonso E, Egan KM, Paton BG, Kenyon KR. Microbiology of contact lens-related keratitis. Cornea 1989;8:281–5.
14. Cheng KH, Leung SL, Hoekman HW, Beekhuis WH, Mulder PG, Geerards AJ, Kijlstra A. Incidence of contact-lens-associated microbial keratitis and its related morbidity. Lancet 1999;354:181–5.
15. Verhelst D, Koppen C, van Looveren J, Meheus A, Tassignon MS and the Belgian Keratitis Study Group. Clinical, epidemiological and cost aspects of contact lens related infectious keratitis in Belgium: results of a seven-year retrospective study. Bull Soc Belge Ophtalmol 2005;7–15.
16. Hazlett LD. Pathogenic mechanisms of P. aeruginosa
keratitis: a review of the role of T cells, Langerhans cells, PMN, and cytokines. DNA Cell Biol 2002;21:383–90.
17. Fleiszig SM, Wiener-Kronish JP, Miyazaki H, Vallas V, Mostov KE, Kanada D, Sawa T, Yen TS, Frank DW. Pseudomonas aeruginosa
-mediated cytotoxicity and invasion correlate with distinct genotypes at the loci encoding exoenzyme S. Infect Immun 1997;65:579–86.
18. Kessler E, Blumberg S. Specific inhibitors of Pseudomonas aeruginosa
elastase as potential drugs for the treatment of Pseudomonas keratitis. Antibiot Chemother 1987;39:102–12.
19. Twining SS, Kirschner SE, Mahnke LA, Frank DW. Effect of Pseudomonas aeruginosa
elastase, alkaline protease, and exotoxin A on corneal proteinases and proteins. Invest Ophthalmol Vis Sci 1993;34:2699–712.
20. O’Callaghan RJ, Engel LS, Hobden JA, Callegan MC, Green LC, Hill JM. Pseudomonas keratitis. The role of an uncharacterized exoprotein, protease IV, in corneal virulence. Invest Ophthalmol Vis Sci 1996;37:534–43.
21. Zhu H, Bandara R, Conibear TC, Thuruthyil SJ, Rice SA, Kjelleberg S, Givskov M, Willcox MD. Pseudomonas aeruginosa
with lasI quorum-sensing deficiency during corneal infection. Invest Ophthalmol Vis Sci 2004;45:1897–903.
22. Lee EJ, Cowell BA, Evans DJ, Fleiszig SM. Contribution of ExsA-regulated factors to corneal infection by cytotoxic and invasive Pseudomonas aeruginosa
in a murine scarification model. Invest Ophthalmol Vis Sci 2003;44:3892–8.
23. Maresso AW, Baldwin MR, Barbieri JT. Ezrin/radixin/moesin proteins are high affinity targets for ADP-ribosylation by Pseudomonas aeruginosa
ExoS. J Biol Chem 2004;279:38402–8.
24. Sun J, Barbieri JT. ExoS Rho GTPase-activating protein activity stimulates reorganization of the actin cytoskeleton through Rho GTPase guanine nucleotide disassociation inhibitor. J Biol Chem 2004;279:42936–44.
25. Sato H, Frank DW. ExoU is a potent intracellular phospholipase. Mol Microbiol 2004;53:1279–90.
26. Caballero AR, Moreau JM, Engel LS, Marquart ME, Hill JM, O’Callaghan RJ. Pseudomonas aeruginosa
protease IV enzyme assays and comparison to other Pseudomonas proteases. Anal Biochem 2001;290:330–7.
27. Gupta SK, Masinick SA, Hobden JA, Berk RS, Hazlett LD. Bacterial proteases and adherence of Pseudomonas aeruginosa
to mouse cornea. Exp Eye Res 1996;62:641–50.
28. Engel LS, Hill JM, Caballero AR, Green LC, O’Callaghan RJ. Protease IV, a unique extracellular protease and virulence factor from Pseudomonas aeruginosa
. J Biol Chem 1998;273:16792–7.
29. Wilderman PJ, Vasil AI, Johnson Z, Wilson MJ, Cunliffe HE, Lamont IL, Vasil ML. Characterization of an endoprotease (PrpL) encoded by a PvdS-regulated gene in Pseudomonas aeruginosa
. Infect Immun 2001;69:5385–94.
30. Caballero A, Thibodeaux B, Marquart M, Traidej M, O’Callaghan R. Pseudomonas keratitis: protease IV gene conservation, distribution, and production relative to virulence and other Pseudomonas proteases. Invest Ophthalmol Vis Sci 2004;45:522–30.
31. Pearson JP, Pesci EC, Iglewski BH. Roles of Pseudomonas aeruginosa
las and rhl quorum-sensing systems in control of elastase and rhamnolipid biosynthesis genes. J Bacteriol 1997;179:5756–67.
32. Hogardt M, Roeder M, Schreff AM, Eberl L, Heesemann J. Expression of Pseudomonas aeruginosa
exoS is controlled by quorum sensing and RpoS. Microbiology 2004;150:843–51.
33. Laibson PR. Pseudomonas aeruginosa
. In: Fraunfelder FT, Hampton Roy F, Meyer M, eds. Current Ocular Therapy 3. Philadelphia, PA: Saunders; 1990:35–7.
34. Hazlett LD, Zucker M, Berk RS. Distribution and kinetics of the inflammatory cell response to ocular challenge with Pseudomonas aeruginosa
in susceptible versus resistant mice. Ophthalmic Res 1992;24:32–9.
35. Niederkorn JY. The role of the innate and adaptive immune responses in Acanthamoeba keratitis. Arch Immunol Ther Exp (Warsz) 2002;50:53–9.
36. Yu FS, Hazlett LD. Toll-like receptors and the eye. Invest Ophthalmol Vis Sci 2006;47:1255–63.
37. Hazlett LD. Corneal response to Pseudomonas aeruginosa
infection. Prog Retin Eye Res 2004;23:1–30.
38. Hazlett LD. Role of innate and adaptive immunity in the pathogenesis of keratitis. Ocul Immunol Inflamm 2005;13:133–8.
39. Janeway CA Jr, Medzhitov R. Innate immune recognition. Annu Rev Immunol 2002;20:197–216.
40. Epelman S, Stack D, Bell C, Wong E, Neely GG, Krutzik S, Miyake K, Kubes P, Zbytnuik LD, Ma LL, Xie X, Woods DE, Mody CH. Different domains of Pseudomonas aeruginosa
exoenzyme S activate distinct TLRs. J Immunol 2004;173:2031–40.
41. Ueta M, Nochi T, Jang MH, Park EJ, Igarashi O, Hino A, Kawasaki S, Shikina T, Hiroi T, Kinoshita S, Kiyono H. Intracellularly expressed TLR2s and TLR4s contribution to an immunosilent environment at the ocular mucosal epithelium. J Immunol 2004;173:3337–47.
42. Zhang J, Xu K, Ambati B, Yu FS. Toll-like receptor 5-mediated corneal epithelial inflammatory responses to Pseudomonas aeruginosa
flagellin. Invest Ophthalmol Vis Sci 2003;44:4247–54.
43. Huang X, Barrett RP, McClellan SA, Hazlett LD. Silencing toll-like receptor-9 in Pseudomonas aeruginosa
keratitis. Invest Ophthalmol Vis Sci 2005;46:4209–16.
44. Kumar A, Zhang J, Yu FS. Toll-like receptor 2-mediated expression of beta-defensin-2 in human corneal epithelial cells. Microbes Infect 2006;8:380–9.
45. Rodriguez-Martinez S, Cancino-Diaz ME, Cancino-Diaz JC. Expression of CRAMP via PGN-TLR-2 and of alpha-defensin-3 via CpG-ODN-TLR-9 in corneal fibroblasts. Br J Ophthalmol 2006;90:378–82.
46. McDermott AM, Redfern RL, Zhang B, Pei Y, Huang L, Proske RJ. Defensin expression by the cornea: multiple signalling pathways mediate IL-1beta stimulation of hBD-2 expression by human corneal epithelial cells. Invest Ophthalmol Vis Sci 2003;44:1859–65.
47. Springer TA. Traffic signals for lymphocyte recirculation and leukocyte emigration: the multistep paradigm. Cell 1994;76:301–14.
48. Baggiolini M, Dewald B, Moser B. Interleukin-8 and related chemotactic cytokines—CXC and CC chemokines. Adv Immunol 1994;55:97–179.
49. Bazan JF, Bacon KB, Hardiman G, Wang W, Soo K, Rossi D, Greaves DR, Zlotnik A, Schall TJ. A new class of membrane-bound chemokine with a CX3C motif. Nature 1997;385:640–4.
50. Baggiolini M. Chemokines and leukocyte traffic. Nature 1998;392:565–8.
51. Pan Y, Lloyd C, Zhou H, Dolich S, Deeds J, Gonzalo JA, Vath J, Gosselin M, Ma J, Dussault B, Woolf E, Alperin G, Culpepper J, Gutierrez-Ramos JC, Gearing D. Neurotactin, a membrane-anchored chemokine upregulated in brain inflammation. Nature 1997;387:611–7.
52. Baggiolini M, Dewald B, Moser B. Human chemokines: an update. Annu Rev Immunol 1997;15:675–705.
53. Spanaus KS, Nadal D, Pfister HW, Seebach J, Widmer U, Frei K, Gloor S, Fontana A. C-X-C and C-C chemokines are expressed in the cerebrospinal fluid in bacterial meningitis and mediate chemotactic activity on peripheral blood-derived polymorphonuclear and mononuclear cells in vitro. J Immunol 1997;158:1956–64.
54. Tessier PA, Naccache PH, Clark-Lewis I, Gladue RP, Neote KS, McColl SR. Chemokine networks in vivo: involvement of C-X-C and C-C chemokines in neutrophil extravasation in vivo in response to TNF-alpha. J Immunol 1997;159:3595–602.
55. Colvin BL, Thomson AW. Chemokines, their receptors, and transplant outcome. Transplantation 2002;74:149–55.
56. Rudner XL, Kernacki KA, Barrett RP, Hazlett LD. Prolonged elevation of IL-1 in Pseudomonas aeruginosa
ocular infection regulates macrophage-inflammatory protein-2 production, polymorphonuclear neutrophil persistence, and corneal perforation. J Immunol 2000;164:6576–82.
57. Thakur A, Xue M, Stapleton F, Lloyd AR, Wakefield D, Willcox MD. Balance of pro- and anti-inflammatory cytokines correlates with outcome of acute experimental Pseudomonas aeruginosa
keratitis. Infect Immun 2002;70:2187–97.
58. Xue ML, Zhu H, Willcox M, Wakefield D, Lloyd A, Thakur A. The role of IL-1beta in the regulation of IL-8 and IL-6 in human corneal epithelial cells during Pseudomonas aeruginosa
colonization. Curr Eye Res 2001;23:406–14.
59. Xue ML, Wakefield D, Willcox MD, Lloyd AR, Di Girolamo N, Cole N, Thakur A. Regulation of MMPs and TIMPs by IL-1beta during corneal ulceration and infection. Invest Ophthalmol Vis Sci 2003;44:2020–5.
60. Thakur A, Barrett RP, McClellan S, Hazlett LD. Regulation of Pseudomonas aeruginosa
corneal infection in IL-1 beta converting enzyme (ICE, caspase-1) deficient mice. Curr Eye Res 2004;29:225–33.
61. Thakur A, Barrett RP, Hobden JA, Hazlett LD. Caspase-1 inhibitor reduces severity of Pseudomonas aeruginosa
keratitis in mice. Invest Ophthalmol Vis Sci 2004;45:3177–84.
62. Cole N, Bao S, Willcox M, Husband AJ. Expression of interleukin-6 in the cornea in response to infection with different strains of Pseudomonas aeruginosa
. Infect Immun 1999;67:2497–502.
63. Cole N, Bao S, Stapleton F, Thakur A, Husband AJ, Beagley KW, Willcox MD. Pseudomonas aeruginosa
keratitis in IL-6-deficient mice. Int Arch Allergy Immunol 2003;130:165–72.
64. Cole N, Krockenberger M, Bao S, Beagley KW, Husband AJ, Willcox M. Effects of exogenous interleukin-6 during Pseudomonas aeruginosa
corneal infection. Infect Immun 2001;69:4116–9.
65. Cole N, Krockenberger M, Stapleton F, Khan S, Hume E, Husband AJ, Willcox M. Experimental Pseudomonas aeruginosa
keratitis in interleukin-10 gene knockout mice. Infect Immun 2003;71:1328–36.
66. Huang X, Hazlett LD. Analysis of Pseudomonas aeruginosa
corneal infection using an oligonucleotide microarray. Invest Ophthalmol Vis Sci 2003;44:3409–16.
67. Kwon B, Hazlett LD. Association of CD4+
T cell-dependent keratitis with genetic susceptibility to Pseudomonas aeruginosa
ocular infection. J Immunol 1997;159:6283–90.
68. Hazlett LD, McClellan S, Kwon B, Barrett R. Increased severity of Pseudomonas aeruginosa
corneal infection in strains of mice designated as Th1 versus Th2 responsive. Invest Ophthalmol Vis Sci 2000;41:805–10.