Development of organised conjunctival leucocyte aggregates after corneal transplantation in rats

Mucosa-Associated Lymphoid Tissue and Tertiary Lymphoid Structures of the Eye and Ear in Laboratory Animals

Mucosa-associated lymphoid tissue (MALT) of special senses is poorly described and can be confused with nonspecific mononuclear cell infiltrates and tertiary lymphoid structures (TLS). In the eye, MALT consists mostly of conjunctiva-associated lymphoid tissue (CALT) and lacrimal drainage-associated lymphoid tissue (LDALT). In humans, CALT and LDALT are important components of the normal eye-associated lymphoid tissue (EALT), but EALT is less frequently described in ocular tissues of animals. The EALT are acquired postnatally in preferential mucosal sites, expand with antigenic exposure, form well-developed lymphoid follicles, and are reported to senesce. Lymphoid follicles that are induced concurrently with chronic inflammation are more appropriately considered TLS but must be differentiated from inflammation in MALT. Less understood is the etiology for formation of lymphoid tissue aggregates in the ciliary body, limbus, or choroid of healthy eyes in animals and humans. In the healthy eustachian tube and middle ear of animals and humans, MALT may be present but is infrequently described. Concurrent with otitis media, lymphoid follicles in the eustachian tube are probably expanded MALT, but lymphoid follicles in the middle ear may be TLS. The purpose of this comparative review is to familiarize toxicologic pathologists with MALT in the special senses and to provide considerations for differentiating and reporting eye and ear MALT from immune or inflammatory cell infiltrates or inflammation in nonclinical studies, and the circumstances for reporting TLS in compartments of the eye and ear.

Comparative outcomes of penetrating and component endothelial cell corneal allografts in outbred sheep

Lamellar (component cell) corneal transplantation is replacing penetrating keratoplasty for some corneal disorders in humans, but the relative risks of immunological graft rejection for the two procedures remain uncertain. A model of component endothelial cell keratoplasty (endokeratoplasty) was developed in outbred sheep. Clinical and histological graft outcomes after endokeratoplasty were then compared with contemporaneous penetrating corneal allografts. No topical or systemic immunosuppression was administered to any recipient sheep. Endothelial cell allografts (n = 10) took significantly longer to achieve perfect transparency following surgery than did penetrating corneal grafts (n = 7) (day 10 vs. day 4; p = 0.003; two-tailed Mann-Whitney U test). The median day to rejection of penetrating grafts was postoperative day 18; for endothelial cell grafts, it was day 48 (p = 0.04; two-tailed Mann-Whitney U test). The clinical courses of the two procedures were therefore quite different. Penetrating grafts gained clarity quickly but exhibited rapid graft neovascularization. Clinical rejection was preceded by inflammation in the anterior segment. Endothelial cell grafts exhibited a fluctuating, more indolent course of opacification, although all did eventually fail. Histological analysis confirmed immunological rejection in all failed grafts, but with different patterns of leukocytic infiltration in endokeratoplasties compared with penetrating keratoplasties. Inflammatory cells in endothelial cell grafts were generally fewer in number and were more often found in the posterior stroma. We conclude that, in the absence of immunosuppression, all endothelial cell allografts do undergo immunological rejection, albeit at a slower rate than penetrating grafts.

Conjunctiva-associated lymphoid tissue (CALT) reactions to antiglaucoma prostaglandins with or without BAK-preservative in rabbit acute toxicity study

Conjunctiva-associated lymphoid tissue (CALT) is closely associated with ocular surface immunity. This study investigated the effects of antiglaucoma prostaglandin analogs with or without benzalkonium chloride (BAK) preservative on organized CALT using an acute toxic model. A total of 48 albino rabbits were used and seven groups of treatments were constituted. Solutions (50 µl) of PBS, 0.02%BAK, (0.02%BAK+)latanoprost, (0.015%BAK+)travoprost, (0.005%BAK+)bimatoprost, (BAK-free)travoprost preserved with the SofZia® system or (BAK-free)tafluprost were instilled 15 times at 5-min intervals in both eyes. CALT changes were analyzed using in vivo confocal microscopy (IVCM), immunohistology in cryosections for detecting MUC-5AC+ mucocytes and CD45+ hematopoietic cells. Antiglaucoma eye drops stimulated inflammatory cell infiltration in the CALT, and seemed to be primarily related to the concentration of their BAK content. The CALT reaction after instillation of BAK-containing eye drops was characterized by inflammatory cell infiltration in the dome and intrafollicular layers and by cell circulation inside the lymph vessels. CD45 was strongly expressed in the CALT after instillation of all BAK-containing solutions at 4 h and decreased at 24 h. The number of MUC-5AC+ mucocytes around the CALT structure decreased dramatically after instillation of BAK-containing solutions. This study showed for the first time the in vivo aspect of rabbit CALT after toxic stimuli, confirming the concentration-dependent toxic effects of BAK. IVCM-CALT analysis could be a pertinent tool in the future for understanding the immunotoxicologic challenges in the ocular surface and would provide useful criteria for evaluating newly developed eye drops.

Dendritic cell physiology and function in the eye

The eye and the brain are immunologically privileged sites, a property previously attributed to the lack of a lymphatic circulation. However, recent tracking studies confirm that these organs have good communication through classical site-specific lymph nodes, as well as direct connection through the blood circulation with the spleen. In addition, like all tissues, they contain resident myeloid cell populations that play important roles in tissue homeostasis and the response to foreign antigens. Most of the macrophage and dendritic cell (DC) populations in the eye are restricted to the supporting connective tissues, including the cornea, while the neural tissue (the retina) contains almost no DCs, occasional macrophages (perivascularly distributed), and a specialized myeloid cell type, the microglial cell. Resident microglial cells are normally programmed for immunological tolerance. The privileged status of the eye, however, is relative, as it is susceptible to immune-mediated inflammatory disease, both infectious and autoimmune. Intraocular inflammation (uveitis and uveoretinitis) and corneal graft rejection constitute two of the more common inflammatory conditions affecting the eye leading to considerable morbidity (blindness). As corneal graft rejection occurs almost exclusively by indirect allorecognition, host DCs play a major role in this process and are likely to be modified in their behavior by the ocular microenvironment. Ocular surface disease, including allergy and atopy, also comprise a significant group of immune-mediated eye disorders in which DCs participate, while infectious disease such as herpes simplex keratitis is thought to be initiated via corneal DCs. Intriguingly, some more common conditions previously thought to be degenerative (e.g. age-related macular degeneration) may have an autoimmune component in which ocular DCs and macrophages are critically involved. Recently, the possibility of harnessing the tolerizing potential of DCs has been applied to experimental models of autoimmune uveoretinitis with good effect. This approach has considerable potential for use in translational clinical therapy to prevent sight-threatening disease caused by ocular inflammation.

Nasal-associated lymphoid tissue is not an absolute requirement for the induction of rat tear IgA antibody responses

PURPOSE/AIM OF STUDY: The purpose of this work was to determine whether rat nasal-associated lymphoid tissue is required for the induction of tear IgA responses.

MATERIALS AND METHODS

Particulate antigen in the form of DNP-BSA encapsulated in cationic microparticles was applied topically to the eyes (ocular topically) of rats that had the nasolacrimal ducts temporarily plugged with chromic gut suture material. Eye washes and serum were monitored for development of antigen specific IgA and IgG, respectively. To track the particulate uptake, fluorescent latex beads were applied topically to the eyes of plugged and unplugged animals. The nasal-associated lymphoid tissue and the draining lymph nodes were then examined for the presence of the fluorescent beads.

RESULTS

It was found that the chromic gut suture was effective in blocking the passage of antigen into the nasopharyngeal cavity for at least 24 hr. Tear antigen-specific IgA levels found in the eyes of plugged animals were not significantly lower from those of unplugged animals. Serum IgG antibody levels were also similar between the two groups. In animals with plugged nasolacrimal ducts, fluorescent beads were found predominately in the superficial cervical lymph nodes, which have been shown to drain the surface of the eye.

CONCLUSIONS

These results indicate that particulate antigen can be taken up by the conjunctiva and transported to the draining lymph nodes, showing that antigen does not need to access nasal-associated lymphoid tissue to induce tear IgA antibody responses.

Mechanisms of corneal allograft rejection and regional immunosuppression

Corneal transplantation has not matched the improvements in outcome seen with other clinical transplantation procedures. The therapeutic strategies, which have improved the outcomes of solid vascularised organs are not applicable to corneal transplantation. Corneal transplantation is different with respect to relevant transplantation biology and the clinical context in which it is practiced. New approaches need to be developed which provide regional rather than systemic immunosuppression. The accessibility of the cornea makes it particularly suitable for topical medication and for gene therapy approaches. Engineered antibodies, small enough to pass through the cornea, and directed at key molecules in the allograft response have been developed. Gene therapy had been developed using viral vectors to transfect the corneal endothelium with the genes for immunosuppressive lymphokines. Both approaches show promise.

The immunobiology of corneal transplantation

Corneal allotransplantation is highly successful in the short term, but much less successful in the longer term. Many corneal grafts in recipients with corneal neovascularization or the sequelae of ocular inflammation undergo irreversible rejection, despite topical immunosuppression with glucocorticosteroids. Sensitization to cornea-derived alloantigen proceeds by both direct and indirect routes, but the anatomic location of sensitization remains unclear. Multiple and redundant mechanisms operate in the effector phase of corneal graft rejection, which is largely cell-mediated rather than antibody-mediated. Human leukocyte antigen matching may improve outcomes in high-risk patients but systemic immunosuppression is frequently ineffective and is seldom used.

The impact of corneal allograft rejection on the long-term outcome of corneal transplantation

PURPOSE

To examine the influence of corneal allograft rejection on the survival of penetrating corneal transplantation, to review the status of conventional therapies to improve graft survival, and to consider prospects for alternative approaches to reduce the impact of rejection.

DESIGN

Perspective, including prospective, observational cohort study.

METHODS

An examination of the literature on human corneal graft rejection and data from the Australian Corneal Graft Registry, reviewed in the context of clinical experience.

RESULTS

Corneal graft outcome is not improving with era. The sequelae of inflammation, whether occurring before corneal transplantation or subsequently, exert a profound influence by predisposing the graft to rejection. Of the developments that have been instrumental in reducing rejection in vascularized organ transplantation, living-related donation is not an option for corneal transplantation. However, HLA matching may be beneficial and requires reassessment. The evidence base to support the use of systemic immunosuppressive agents in corneal transplantation is thin, and topical glucocorticosteroids remain the drugs of choice to prevent or reverse rejection episodes. Experimental approaches to local allospecific immunosuppression, including the use of antibody-based reagents and gene therapy, are being developed but may be difficult to translate from the laboratory bench to the clinic.

CONCLUSIONS

Corneal allograft rejection remains a major cause of graft failure. High-level evidence to vindicate the use of a particular approach or treatment to prevent or treat corneal graft rejection is lacking. In the absence of extensive data from randomized, controlled clinical trials, corneal graft registers and extrapolation from experimental models provide some clinically useful information.

Cellular inflammatory response induced by sensory denervation of the conjunctiva in monkeys

The inflammatory response induced by sensory denervation of the cornea, neuroparalytic (neurotrophic) keratitis, has been widely reported in the literature. Clinical evidence has shown that the conjunctiva also responds to sensory denervation, but little is known of the cytology of the conjunctival tissue response to denervation. The purpose of this study therefore was to investigate the cytological aspects of tissue response induced in the conjunctiva of monkeys by sensory denervation. Intracranial ophthalmic neurotomy was carried out in three monkeys, maxillary neurotomy in four, combined ophthalmic and maxillary neurotomy in two, and infraorbital nerve transaction in one monkey. These various operations were performed for other experimental purposes, but the conjunctival tissues from the animals were suitable and available to study the cytology of the inflammatory response induced in the tissue following the sensory denervation. The cytological changes were studied using light microscopy. Complete or severe ophthalmic nerve transection induced significant inflammatory responses, which were largely confined to the tarsal region of the conjunctiva. The responses included a substantial increase in the infiltration of the epithelium by polymorphonuclear leucocytes and severe disruption of the epithelium. The number of macrophages in the conjunctiva also increased. The response induced by maxillary denervation, however, was not as pronounced as that induced by sensory denervation. The restriction of the conjunctival tissue response to the tarsal region was considered to be due to the friction between the tarsal conjunctiva (and the hard tarsal plate) and the cornea during blinking. This study shows that sensory denervation of the conjunctiva elicits an inflammatory response characterized by substantial infiltration of the epithelium by neutrophil and macrophage and disorganization of the conjunctival tissue.