Targets in Ocular Allergy Alessandra Micera, Sergio Bonini, Alessandro Lambiase, Roberto Sgrulletta, Stefano Bonini

1.1

Introduction

Allergy or type I hypersensitivity is an im- munoglobulin E (IgE) driven reaction that can take place in different organs. The effector phas- es of IgE-associated immune responses occur in three temporal patterns: (1) an acute reaction, which develops within seconds or minutes after allergen exposure; (2) a late phase reaction, which develops within a few hours after allergen exposure; and (3) a chronic inflammatory re- sponse, which can persist for days, months or years [1]. Allergic inflammatory disorders may result in ocular surface allergy, and in most cas- es ocular allergy is associated with other sys- temic allergies such as rhinoconjunctivitis [1].

The immunopathogenic mechanisms in allergic disorders involve a combination of IgE-mediat- ed and T-cell-mediated responses. Type-2 T- helper lymphocytes are predominant in in- flamed allergic conjunctival tissues [2]. They are thought to play a prominent role in the de- velopment of allergic disorders by producing regulatory and inflammatory cytokines such as interleukin (IL)-3 (local differentiation of mast cells and eosinophils), IL-4 (development of mast cells, IgE synthesis and vascular cell adhe- sion molecules) and IL-13 and IL-5 (develop- ment of eosinophils, eosinophil chemotaxic, survival and degranulation) [1]. The IgE-medi- ated conjunctival allergic reaction can be repro- duced easily by specific conjunctival provoca- tion, which induces an early reaction followed by a predominant infiltration of eosinophils (EOSs) and mast cells (MCs), which are thought to play a pivotal role in disease pathogenesis [3].

EOSs and their proteins (ECP) are increased Alessandra Micera, Sergio Bonini, Alessandro Lambiase,

Roberto Sgrulletta, Stefano Bonini

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Allergic conjunctivitis is inflammation of the ocular surface with IgE, cells, cytokines and mediators

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Several chemokines/adhesion molecules, cytokines and neuropeptides are detectable in situ in allergic conjunctival tissues

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Growth factors, and mainly nerve growth factor (NGF) and transforming growth factor b1 (TGF-b1), are also found to be increased in the blood, tears and tissues of patients with allergic conjunctivitis

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A significant correlation is observed between plasma levels of NGF and the increased number of mast cells (MCs) and eosinophils (EOSs) in the tarsal and bulbar vernal keratoconjunctivitis (VKC) conjunctiva

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Fibrosis can be observed in the conjunctiva of VKC (giant papillae formations) as a consequent manifestation of allergy

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Overproduction and deposition of colla- gens and tissue remodelling in VKC may be due to imbalance of matrix metallopro- teinases and their physiological inhibitors

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Various growth factors (NGF, TGF-b1) and cytokines, produced by both inflammatory and stromal cells, have been proposed to cause tissue repair

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VKC and atopic keratoconjunctivitis (AKC) diseases can compromise the cornea, with ulcers and scarring ultimately leading to visual loss

Core Messages

and activated in tears and tissues of all allergic eye diseases even though the mechanisms of EOS recruitment to the ocular surface are not completely understood [4]. Tear fluid has also been found to contain a small quantity of hista- mine and tryptase [5], well-known products of MCs [1]. Interestingly, the conjunctival epitheli- um has also been considered to play a key role in ocular allergic disorders [6].

1.2

Chronic Allergic Eye Diseases

Chronic allergic ocular disease encompasses several disorders, such as seasonal atopic con- junctivitis, perennial atopic conjunctivitis, atopic keratoconjunctivitis (AKC) and vernal keratoconjunctivitis (VKC) [7]. Seasonal atopic

and in most cases conjunctivitis is only one manifestation of additional allergic reactions (rhinitis, hay fever or a hay fever like sympto- matology, and in severe cases conjunctivitis is associated with different forms of pulmonary affection). Atopic keratoconjunctivitis is a se- vere, bilateral, ocular allergic disease affecting adults. A familial history for atopy and an asso- ciation with systemic atopic dermatitis are com- mon. Symptoms commonly include itching, burning, and tearing. Signs include involvement of mainly upper conjunctiva in the form of a papillary conjunctivitis. The corneal epithelium reveals mild to moderate inflammatory changes that can result in scarring and neovasculariza- tion leading to blindness. Many patients may develop associated staphylococcal blepharitis.

Ocular complications include blepharoconjunc- tivitis, cataract, ocular herpes simplex and kera- toconus. Vernal keratoconjunctivitis is essen- tially a chronic recurrent seasonal childhood disease (worldwide spread, especially in coun- tries around the Mediterranean basin), which in approximately half the children’s population is associated with other allergic manifestations [8]. Unlike severe AKC, VKC tends to resolve spontaneously after several years. Genetic stud- ies have not been performed that confirm a re- lationship of VKC with a particular genotype.

The disease is mostly seasonal, lasting from the

beginning of spring (marked exacerbation of clinical symptoms) until autumn/winter (milder symptoms), even though for children the disease remains variably active the year around. Nevertheless, perennial cases that are persistent throughout the year are not rare, es- pecially in patients living in warm subtropical or desert climates. Its predominance during the high pollen season highly strengthens the wide- ly accepted hypothesis that VKC is an immuno- logically mediated hypersensitivity reaction to environmental antigens. The characteristic fea- tures of the two clinical forms of VKC, the giant papillae on the upper tarsal conjunctiva (tarsal VKC) or the gelatinous limbal infiltrates (limbal VKC), leave no doubt as to the diagnosis of ver- nal disease. All VKC forms are characterized by intense itching, tearing, mucous secretions and a severe photophobia that often forces children to live virtually in the dark. The commonly not- ed foreign body sensation is caused by conjunc- tival surface irregularity and copious mucous secretions, while the presence of pain is indica- tive of a compromised cornea, which can be present in the form of a superficial punctate keratitis, epithelial macroerosions or ulcers and plaque. According to the other allergic conjunc- tivitis, pronounced infiltration of lymphocytes, mainly of the Th2 subtype, EOSs and MCs has been reported for VKC [4, 6, 9]. In particular, Th2-derived IL-4 and IL-13, essential to IgE an- tibody production in tissue eosinophilia and asthma airway remodelling [1], have been re- ported in tears of both VKC and AKC as well as in patients with SAC, VKC and AKC, respective- ly [10].

1.3

Effector Cells and Cytokine Release

Allergic conjunctivitis is due to direct exposure

of the ocular mucosal surfaces to environmental

allergens. The pathogenesis of allergic conjunc-

tival disorders is multifactorial and not fully

understood. Ragweed represents the primary

responsible allergen (75 % of cases of rhino-

conjunctivitis). Conjunctival symptoms include

itching, tearing, and perhaps burning. Clinical

examination may reveal various forms of im-

munologic reaction, all of these presenting as various forms of “red eye”. Photophobia and blurred vision may also occur. Clinical signs in- clude milky or pale pink conjunctivae with vas- cular congestion, which may progress to con- junctival swelling (chemosis). The immunology hypersensitivity reactions include mast cell de- granulation, eosinophils, and other inflamma- tory cells and modulators. The hallmark and ef- fector cells of allergic phenomena are mast cells and eosinophils.

Mast cells (MCs) are bone-marrow-derived tissue cells containing prominent cytoplasmic granules. They have a clear and pivotal role in allergy, but also take part in wound healing and native immunity [11]. In type I hypersensitivity reactions, MCs elicit the early stages of the aller- gic inflammatory response. In fact the cross- linking of IgE antibodies bound to high-affinity IgE receptors by the antigen and the release of various preformed (histamine) and newly syn- thesized mediators causes immediate MC de- granulation. In addition, the later release of chemiotactic factors, cytokines and chemo- kines, from activated MCs also induces the on- set of the late-phase reaction, characterized by tissue infiltration and activation of various in- flammatory cells and notably the eosinophils.

The cytokines/growth factors detected in situ in human MCs include interleukin (IL)-3, IL-4, IL-5, IL-6, IL-8, IL-10, IL-13, IL-16, tumor necro- sis factor-a (TNF-a), vascular endothelial growth factor (VEGF), granulocyte/monocyte- colony forming factor (GM-CSF), stem cell factor (SCF), nerve growth factor (NGF), fibrob- last growth factor (b-FGF) and MIP-1a [12]. In particular, by releasing IL-5, GM-CSF, TNF-a, and tryptase, MCs promote eosinophil recruit- ment, activation and survival into the tissue [12].

Eosinophils (EOSs) are blood granulocytes produced in the bone marrow under the influ- ence of IL-3, IL-5 and GM-CSF that infiltrate the tissues in various pathological situations, such as parasitic infections, allergic inflammation and neoplastic diseases [13]. EOSs possess cyto- plasmic granules that, among non-EOS-specific mediators, contain characteristically preformed basic proteins, namely EOS peroxidase (EPO), EOS-derived neurotoxin (EDN), major basic

protein (MBP), and EOS cationic protein (ECP).

Recently, it has been reported that EOSs main- tain their ability to release ECP even after re- peated stimulation, implying that mature EOSs may not require a significant ECP resynthesis [14]. In addition, their secretory granules con- tain several preformed cytokines such as GM- CSF, IL-2, IL-6, IL-4, IL-5, transforming growth factor b (TGF-b), NGF and TNF-a. The presence of EOSs in inflammation and allergy has been recognized as the critical element inducing the inflammatory process and causing tissue dam- age, even though more recently EOS presence has also been associated with tissue repair. An- other recent observation is that an MC-EOS cross-talk exists that may amplify the local al- lergic inflammatory response [12]. MCs en- hance EOS survival and functional activity by TNF-a and tryptase and EOSs cause histamine, GM-CSF and PGD

release from MC by MBP and SCF. Moreover, other mediators such as GM-CSF, IL-3, g-IFN and NGF also take part in this cellular cross-talk [12]. Early studies have shown a role for different growth factors in the growth and survival of both MCs and EOSs [12].

1.4

Chemokines and Adhesion Molecules

Chemokines and adhesion molecules have been

found to strengthen associated with the selec-

tive infiltrate entity observed in these ocular

disorders, controlling the specific cell recruit-

ment in the conjunctiva [15, 16]. Chemokines

have been shown specifically to attract and acti-

vate EOSs [17], acting through chemokine

receptor-3 (CCR3) expressed on EOSs [17]. In-

creased eotaxin-1 and eotaxin-2 (a recently dis-

covered chemokine that is functionally very

similar to eotaxin-1) preferentially recruit cir-

culating EOSs, attracting them to the epithelium

and subepithelial stroma and inducing EOS de-

granulation with release of epithelium-damag-

ing proteins [17]. A recent study showed that

cultured conjunctival FBs produce eotaxin-1 in

response to IL-4, IL-13 and TNF-a, strongly sup-

porting the hypothesis of a real contribution of

FBs in allergic status [18]. These EOS-derived

compounds have been shown to be highly con- centrated in tears of all allergic patients, like- wise representing a diagnostic marker, mainly localized in corneal ulcers and highly toxic to corneal epithelial cells. In normal conjunctiva, the adhesion molecule ICAM-1 is downregulat- ed and confined to the vascular endothelium.

Otherwise, ICAM-1/-3 expression is highly in- creased in active VKC biopsies and not confined to the lower levels, but expressed by vascular, epithelial, stromal and inflammatory cells [19].

ICAM-1 is necessary to monocyte, lymphocyte and neutrophil adhesion to activated endotheli- um. Several proinflammatory cytokines con- tribute ICAM-1 induction [19].

1.5

Neuropeptide and Growth Factor Involvement

Allergic inflammation might also follow the re- lease of neuropeptides, mainly substance P, as observed in VKC [20], which cause characteris- tic features of allergic inflammation, including vasodilatation, increased vascular permeability and a contribution to further release of hista- mine from MCs. Specifically, tryptase- and his- tamine-releasing factors might greatly con- tribute to SP release and to amplifying the chronic allergic reaction, triggering nerves to release neuropeptides by binding to protease- activated receptors.

4 Chapter 1 Targets in Ocular Allergy

Fig. 1.1. A possible cross-talk between mast cells (MCs), eosinophils (EOS) and fibroblasts (FBs) during the early and late phase of allergic reactions. MCs and EOS effects exerted on FBs seem to be related to the release of either fibrogenic or fibrolytic mediators in-

fluencing tissue repair/fibrosis. Furthermore, FBs in- fluenced MCs and EOS by increasing their survival and functional activity and contributing to the local inflammation

Growth factors, and mainly NGF and TGF-b1 [21], are also found increased in the blood, tears and tissues of patients with allergic conjunctivi- tis. NGF involvement in allergic disorders was postulated after the discovery of an increased NGF plasma level in patients with VKC, which correlated with increased total IgE levels and ECP [22]. A significant correlation was also ob- served between plasma levels of NGF and the increased number of MCs and EOSs in the tarsal and bulbar VKC conjunctiva [22]. The discovery of the increased NGF plasma levels in VKC prompted the investigation of the presence of NGF in other allergic rhinoconjunctivitis [23].

The fact that NGF levels correlated with total IgE antibody titre strengthens support for the hypothesis of a link between NGF and an aller- gic response in the visual system [24]. Addition- al evidence of an involvement of NGF and NGF receptors in allergic ocular disorders has come from a newly described ocular allergic condi- tion, named inflamed juvenile conjunctival ne- vus (IJCN [25]). In the conjunctiva of IJCN pa- tients increased EOSs and MCs have been found to synthesize and release NGF as well as express its specific receptor [25]. The increase in the lo- cal NGF expression has been correlated to an in- cremented number of MCs and EOSs [25]. TGF-

patients, mainly as a source of EOSs, without significant differences between the tarsal and the limbal forms [26]. Increased levels of TGF-

sues and tears indicate a local production of these cytokines, mainly of EOS derivation [26].

Moreover, these increased levels of TGF-b1, IL-1 and IL-6 might be related to collagen hyperpro- duction [26]. Taken together, all these data indi- cate that growth factors are synthesized, re- leased and utilized by MCs, EOSs and FBs, and that NGF and TGF-b1 might be viewed as an ad- ditional marker of ocular allergic conditions as a result of their relevant association with aller- gic ocular disorders (Fig. 1.1).

1.6

Tissue Remodelling

and the Contribution of Fibroblasts

Tissue remodelling and formation of giant

papillae are some of the consequences of chron-

ic allergic disorders, including the eye [1, 21]. Re-

modelling involves both production and depo-

sition of extracellular matrix (ECM) compo-

nents, as well as degradation and clearance of

newly synthesized products [21].Any inflamma-

tory reaction can induce tissue damage and a

resulting healing process, which is a very com-

plex event involving interactions of both in-

flammatory and structural cells [1]. Three main

overlapping phases have been identified in tis-

sue response to injury, which can continue for

months to years: inflammation, granulation tis-

sue formation – including fibroblast (FB) prolif-

eration, migration, differentiation and remodel-

ling. This process can be physiological and well

balanced or can result in an exaggerated patho-

logical process, which includes remodelling

and/or fibrosis [21]. FBs represent the main tar-

get and effector cells of these processes due to

their ability to migrate to the injured area, pro-

liferate, produce ECM, differentiate into myofi-

broblasts (myoFBs) and finally to contract the

wound [21]. Previously viewed as simple struc-

tural cells providing the cellular and ECM scaf-

fold to tissues, FBs are now recognized as regu-

latory cells exerting active roles in tissue

homeostasis, by producing cytokines and sever-

al growth factors with autocrine/paracrine ac-

tivities [12]. Several in vitro studies suggest that

myoFBs may play a role in fibrosis, remodelling,

and repair processes associated with IgE-medi-

ated hypersensitivity, at least in other organs

where chronic allergic conditions might devel-

op [21]. MyoFBs are specialized cells thought

to rise locally from FBs and displaying the

contractile protein a-smooth muscle actin

(a-SMA). MyoFBs represent the main transient

cellular phenotype responsible for wound con-

traction and their deletion, via apoptosis, or as

recently hypothesized via their phenotypical re-

version to FBs, is necessary to terminate suc-

cessfully the repair process.Although somewhat

controversial, prolonged survival of myoFBs

might contribute to the abnormal ECM deposi- tion and fibrosis.

Fibrosis can be observed in the conjunctiva of VKC, an interesting example of a chronic inflammatory response result in tissue remod- elling (Fig. 1.2). In VKC, fibrosis can be a conse- quent and sometimes a concomitant mani- festation of allergy [27], and only recently have the role of FBs and myoFBs in VKC fibrosis been taken into consideration. Structural changes in the conjunctiva, such as giant papillae forma- tion, epithelial in growth, and subepithelial fi- brosis associated with ECM deposition, are present and may contribute to the chronicity of the disease. No data have been reported on the presence of myoFBs in VKC. Normal cultured conjunctival FBs are induced to express a-SMA protein upon growth factor stimulation, and

VKC derived FBs directly express this protein (Micera et al., unpublished data). FBs from VKC biopsies express in vitro a-SMA protein and ap- pear to have a cytokine profile typical of activat- ed myoFBs (Micera et al., unpublished data).

1.7

Metabolism of Extracellular Matrix

The analysis of ECM in VKC demonstrated a reduction in proteoglycans and a substantial increase in total collagen with an altered ratio between collagen type I and III, due to a consis- tent increase in collagen type III [27]. Moreover, a high amount of procollagens type I and III in the tears during active tarsal VKC has also been reported. This increased deposition of collagen

Fig. 1.2. Repeated/prolonged exposure of the ocular surface to allergens can lead to a perpetuation of local inflammation and damage (mediators from both mast cells and eosinophils), resulting in a repair process due to migration/proliferation/active metab-

olism of fibroblasts/myofibroblasts. Remodelling of the entire structure might sometimes induce func- tion decline. In the frame is shown a picture from a VKC biopsy stained for haematoxylin and eosin

types I, III and IV in the giant papillae might be viewed as a result of increased expression of cytokines and growth factors from EOS, MCs or other inflammatory cells which are known to stimulate resident FBs to overproduce ECM.

Thus, in vitro VKC-derived FBs spontaneously produce collagen types I and III (Micera et al., unpublished data). The Th2-type IL-4 and -13 seem to control collagens I and III on normal FBs, whereas Th1-type derived IFN-g may have an inhibitory effect on FBs, according to other tissues [1]. Overproduction and deposition of collagens and tissue remodelling may be due to an imbalance between matrix metalloproteinas- es (MMPs) and their physiological inhibitors, tissue inhibitors of MMPs (TIMPs). Thus, MMP- 1 and MMP-9 were found altered in the tear in VKC [28]. MMPs are a family of zinc- and calci- um-dependent enzymes involved in many phys- iological and pathologic processes, including progression of tumours, metastasis, inflamma- tory diseases, and wound healing, by several cytokines. Some of the 17 known members of the human MMP family, collagenase I (MMP-1), gelatinase A and B (MMP-2 and -9), stromelysin (MMP-3), and matrilysin (MMP-7), are those most involved in anterior segment disease and wound healing. These enzymes are produced and secreted by a variety of cells, including ep- ithelial cells, inflammatory cells, and conjuncti- val FBs. MMP-1 can cleave the triple helix of in- terstitial collagen types I, II, and III. Stromelysin and matrilysin are involved in degrading a vari- ety of ECM components such as proteoglycans, fibronectin, and laminin. Gelatinase A and B are involved in cleaving collagen types IV (the main component of basal membrane), V, VII, and X;

fibronectin; laminin; elastin; and collagen degradation products; and TIMP-1 is the natural inhibitor of both collagenase I and gelatinase.

Various growth factors and cytokines, pro- duced by both inflammatory and stromal cells, have been proposed to cause tissue repair. How- ever, the main pro-fibrogenic factor remains TGF-b [21] , which is responsible for the stimu- lation of ECM formation, the inhibition of its degradation and the chemoattraction of FBs.

There are three different isoforms of TGF-b, namely TGF-b1 (mainly profibrogenic), TGF-b2 (mainly immunoregulatory) and TGF-b3. TGF-

b1 is the main isoform involved in the fibrotic

process [21]. TGF-b1 is not the only growth fac- tor found increased in ocular allergic disorders showing tissue remodelling [21]. NGF recently has been postulated to be another growth factor important in the stromal-epithelial interaction during the wound-healing processes occurring in different organs [29]. The healing effect of topical long-term treatment of NGF of human corneal neurotrophic ulcers has indicated that this factor might influence FBs, myoFBs and ep- ithelial cells during the repair process occurring in the eye [29], showing the potential of NGF as a pharmacological tool for disorders character- ized by impaired healing.

Finally, VKC and AKC diseases can compro- mise the cornea severely, with ulcers and scar- ring ultimately leading to visual loss. In partic- ular,VKC affected boys and AKC patients might develop keratoconus [30]. Whether these chil- dren develop keratoconus as a result of a sus- ceptible genetic background or as a secondary manifestation remains an open question.

Summary for the Clinician

∑ Ocular allergic manifestations range from

mild to severe diseases

∑ Itching is a cardinal symptom: no itching,

no allergy!

∑ Transient or persistent corneal damage rep-

resents signs of the severity of the disease and is the expression of severe inflamma- tion

∑ In the mildest form a common treatment

may be suggested (multiple action drugs)

∑ Topical steroids or topically available new

immunosuppressive agents such as cy- closporine represent alternative treatment for severe diseases

1.8

Conclusions

The topical therapy for chronic allergic ocular

disorders implies the use of steroids to quieten

the disease and of MC stabilizers or leukotriene

inhibitors, mainly in association with steroid

treatment. MCs and EOSs have been shown to

participate in the development of allergic in-

flammatory disorders and tissue repair/fibrosis by modulating the activities of immunocompe- tent cells and of FBs and ECM homeostasis. MC and EOS effects exerted on FBs seem to be relat- ed to the production of different mediators and in turn FBs can promote local inflammation by producing factors responsible for recruitment, adhesion or cell activation. This would imply that novel immunotherapeutic tools directed to different targets, including antagonists of some cytokines or growth factors, should be studied to develop new strategies for controlling espe- cially MC activation, EO recruitment, and Th2 activity.

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