Formation of giant cells in the corneal endothelium during its regeneration after destruction by freezing

Immunology and Pathology in Ocular Drug Development

Clear vision is dependent on features that protect the anatomical integrity of the eye (cornea and sclera) and those that contribute to internal ocular homeostasis by conferring hemangiogenic (avascular tissues and antiangiogenic factors), lymphangiogenic (lack of draining lymphatics), and immunologic (tight junctions that form blood-ocular barriers, immunosuppressive cells, and modulators) privileges. The later examples are necessary components that enable the eye to maintain an immunosuppressive environment that responds to foreign invaders in a deviated manner, minimizing destructive inflammation that would impair vision. These conditions allowed for the observations made by Medawar, in 1948, of delayed rejection of allogenic tissue grafts in the anterior chamber of mouse eye and permit the sequestration of foreign invaders (eg, Toxoplasma gondii) within the retina of healthy individuals. Yet successful development of intraocular drugs (biologics and delivery devices) has been stymied by adverse ocular pathology, much of which is driven by immune pathways. The eye can be intolerant of foreign protein irrespective of delivery route, and endogenous ocular cells have remarkable plasticity when recruited to preserve visual function. This article provides a review of current understanding of ocular immunology and the potential role of immune mechanisms in pathology observed with intraocular drug delivery.

Corneal Dystrophy in Dutch Belted Rabbits as a Possible Model of Thiel-Behnke Subtype of Epithelial-Stromal TGFβ-Induced Corneal Dystrophy

The International Committee for Classification of Corneal Dystrophies (IC3D) categorized corneal dystrophies in humans using anatomic, genotypic, and clinicopathologic phenotypic features. Relative to the IC3D classification, a review of the veterinary literature confirmed that corneal dystrophy is imprecisely applied to any corneal opacity and to multiple poorly characterized histologic abnormalities of the cornea in animals. True corneal dystrophy occurs in mice with targeted mutations and spontaneously in pet dogs and cats and in Dutch belted (DB) rabbits, but these instances lack complete phenotyping or genotyping. Corneal dystrophy in DB rabbits can be an important confounding finding in ocular toxicology studies but has only been described once. Therefore, the ophthalmology and pathology of corneal dystrophy in 13 DB rabbits were characterized to determine whether the findings were consistent with or a possible model of any corneal dystrophy subtypes in humans. Slit lamp and optical coherence tomography (OCT) imaging were used to characterize corneal dystrophy over 4 months in young DB rabbits. The hyperechoic OCT changes correlated with light microscopic findings in the anterior stroma, consisting of highly disordered collagen fibers and enlarged keratocytes. Histochemical stains did not reveal abnormal deposits. Small clusters of 8 to 16 nm diameter curly fibers identified by transmission electron microscopy were consistent with Thiel-Behnke (TBCD) subtype of epithelial-stromal transforming growth factor β-induced dystrophies. Sporadic corneal dystrophy in DB rabbits appears to be a potential animal model of TBCD, but genotypic characterization will be required to confirm this categorization.

Toxicity of Intracameral Injection of Fourth-Generation Fluoroquinolones on the Corneal Endothelium

PURPOSE

The aim of this study was to compare the cellular susceptibility patterns and morphologic changes in the corneal endothelium associated with the use of fourth-generation fluoroquinolones.

METHOD

Endothelial susceptibility was assessed through intracameral injection of besifloxacin, gatifloxacin, and moxifloxacin. Human umbilical vein endothelial cells (HUVECs) were used as the standard cellular lineage to assess the quantitative toxicity of each antibiotic solution. Qualitative changes in the morphologic character of the corneal structure and the endothelial layer were generated using a combination of ex vivo and in vivo assays. Experimental assays were conducted in triplicate, and the results were statistically analyzed.

RESULTS

At 1 hour of exposure, all HUVECs exposed to antibiotics showed viability above 85%, after 3 hours of exposure to besifloxacin, gatifloxacin, and moxifloxacin, the percentages of viable cells were 68.3 ± 4.0 (P < 0.001), 90.7 ± 4.2 (P < 0.05), and 93.3 ± 1.5 (P > 0.05), respectively. All fluoroquinolones tested showed toxicity to HUVECs, resulting in significant (P < 0.001) loss of cellular viability after 24 hours of drug exposure. Giant endothelial cells were observed in animals treated with the 3 fluoroquinolones in contrast to the absence of these abnormal cells in the untreated group. Early cellular detachment was seen in the endothelial layer after exposure to gatifloxacin and moxifloxacin.

CONCLUSIONS

We concluded that injection of fourth-generation fluoroquinolones in the aqueous humor did not adversely affect the corneal endothelium. However, these results suggested that prophylactic intracameral injection of besifloxacin, gatifloxacin, or moxifloxacin, if needed, should be administered as a last therapeutic resource in clinical practice, with careful and constant monitoring of corneal endothelium.

A Surgical Cryoprobe for Targeted Transcorneal Freezing and Endothelial Cell Removal

Purpose

To examine the effects of transcorneal freezing using a new cryoprobe designed for corneal endothelial surgery.

Methods

A freezing console employing nitrous oxide as a cryogen was used to cool a series of different cryoprobe tip designs made of silver for high thermal conductivity. In vitro studies were conducted on 426 porcine corneas, followed by preliminary in vivo investigations on three rabbit corneas.

Results

The corneal epithelium was destroyed by transcorneal freezing, as expected; however, the epithelial basement membrane remained intact. Reproducible endothelial damage was optimally achieved using a 3.4 mm diameter cryoprobe with a concave tip profile. Stromal edema was seen in the pre-Descemet's area 24 hrs postfreeze injury, but this had been resolved by 10 days postfreeze. A normal collagen fibril structure was seen 1 month postfreeze, concurrent with endothelial cell repopulation.

Conclusions

Transcorneal freezing induces transient posterior stromal edema and some residual deep stromal haze but leaves the epithelial basement membrane intact, which is likely to be important for corneal re-epithelialization. Localized destruction of the endothelial monolayer was achieved in a consistent manner with a 3.4 mm diameter/concave profile cryoprobe and represents a potentially useful approach to remove dysfunctional corneal endothelial cells from corneas with endothelial dysfunction.

Transforming growth factor-beta suppresses proliferation of rabbit corneal endothelial cells in vitro

Corneal endothelial cells in vivo appear to be inhibited in G1 phase of the cell cycle. Studies were carried out to determine whether cultured rabbit corneal endothelium expresses transforming growth factor-beta (TGF-beta) receptor types I, II, and III, suggesting they would be sensitive to a TGF-beta-induced signal. In addition, we explored if TGF-beta might mediate this G1 phase inhibition by implementing flow cytometry and 5-bromo-2'-deoxyuridine (BrdU) immunofluorescence. Reverse transcription-polymerase chain reaction (RT-PCR) products of the expected size were obtained for all three TGF-beta receptor types. Flow cytometry revealed a dose-dependent suppression in the percentage of S phase cells in cultures treated with TGF-beta1 or TGF-beta2. The lowest percentage of S phase cells was found for 10 ng/ml TGF-beta1 and 0.1 ng/ml TGF-beta2. BrdU, an S phase marker, was immunolocalized, and semiquantitative analysis of stained cells showed a maximum suppression of S phase entry at 18 h for 10 ng/ml of TGF-beta11 and 24 h for 10 ng/ml of TGF-beta2. In rabbit, the corneal endothelium expresses TGF-beta receptor types I, II, and III, permitting a TGF-beta signal to be transduced. Flow cytometry reveals a dose-dependent response to both TGF-beta1 and TGF-beta2, and the cells are more sensitive to TGF-beta2. At optimal TGF-beta concentrations, the percentage of S phase cells is comparable to that of a non-proliferating culture, suggesting TGF-beta prevents the cells from proceeding through the G1/S phase transition. This suppression was also seen with BrdU labeling. Together, these results indicate that TGF-beta could be one of the pathways that leads to G1 phase arrest in corneal endothelial cells.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

The assessment of endothelial integrity by scanning electron microscopy and fluorescein diacetate staining following treatment with cryoprotective additives

As part of the development of methods of corneal cryostorage for transplantation, a toxicity study was carried out on the rabbit corneal endothelium using four cryoprotective additives (CPA's) 1) dimethyl sulphoxide (Me2SO), 2) propane-1,2-diol (PG), 3) glycerol (GLY), 4) polyvinylpyrrolidone (PVP). A fifth group, based upon a CPA combination of Me2SO and PVP, was used to characterize both the assays, and the response of the endothelial layer to osmotic stress. The effect upon the cell membrane was assessed using scanning electron microscopy (SEM) and fluorescein diacetate with ethidium bromide staining (FDA/EB). Two sampling points were used, one immediately after treatment and the other following an incubation period. Calculations were performed to predict the maximum relative volume of cells during CPA exchange. Immediately following serial addition and removal of 2 or 3 mol/L (M) PG or GLY, the cells exhibited adverse morphological changes shown with SEM, and the proportion of intact cells judged by FDA/EB staining was significantly reduced when CPA equilibration was performed at 37 degrees C rather than at 20 degrees C. A 3M Me2SO concentration gave less morphological change than 3M PG or GLY, but even after treatment with 4M Me2SO more than 95% cells were judged intact by FDA/EB staining. PVP at 40% w/v showed minimal damage with both assays, and the fifth experimental group suggested that PVP may protect from injury during hypotonic stress. With all groups, the integrity of the cell layer recovered during incubation, so that for each sample the percentage of intact cells was high. However, although confluency was often restored following incubation, total cell density was usually reduced. The results indicate that serial addition and removal of 3M Me2SO is tolerated by the cornea, whereas PG or GLY cannot be used at 2 or 3M without inducing osmotic damage. There was low toxicity to PVP, and it was an effective osmotic buffer.

Cytochrome oxidase activity of postsurgical bullous keratopathy endothelium

Proper corneal hydration is normally maintained by a pump-leak mechanism located on the lateral cell membranes of the endothelium (1,2). When endothelial cells become sufficiently dysfunctional in response to trauma, the cornea becomes irreversibly edematous and opaque. Previous studies in our laboratory have shown that mitochondrial cytochrome oxidase (CO) activity, an enzyme important in respiratory activities, is correlated with endothelial cell functional activity in corneas with Fuchs' endothelial dystrophy. In this study we investigated cytochrome oxidase activity in corneas with postsurgical bullous keratopathy. Corneas with aphakic and pseudophakic bullous keratopathy (ABK, PBK) were incubated in diaminobenzidine-cytochrome C media. Results showed that the staining pattern of ABK and PBK corneas was uniformly low compared to keratoconus corneas, and in contrast to previous studies on Fuchs' dystrophy corneas which demonstrated a regional staining pattern. This suggests that CO staining patterns correlate with functional activity and may be a useful technique to detect dysfunctional cells in various disease categories.

Long term observations on an anterior chamber Ridley intraocular lens

In 1965 the patient, aged 6, sustained a perforating eye injury which was repaired and a traumatic cataract was aspirated within five weeks. Five years later a Ridley Mk 2 A/C intraocular lens was inserted. Several episodes of blunt trauma occurred over a three year period following this procedure. This paper reports the clinical, corneal pachometric and specular microscopic findings of both traumatized and normal fellow eyes 18 years after the initial incident. The corneal endothelial mosaic of the traumatized right eye was very irregular in the vicinity of the initial site of perforation. These marked variations in cell size and shape were less apparent at peripheral corneal areas. The estimated cell loss to the traumatized eye was in the region of 74% although in spite of this corneal function was maintained. The effects of trauma on the corneal endothelium are discussed and a brief review of the literature presented.

Evidence for mitosis in the adult corneal endothelium

Intracellular structures probably representative of mitotic figures were seen by specular microscopy in the endothelium of a corneal graft following a rejection reaction. Serial measurements over an eight month period initially showed grossly enlarged cells and apparent mitotic figures. Subsequently, clusters of cells smaller than any cells previously seen were observed. Measurements of endothelial cell area over this period demonstrated a highly significant (P less than 0.0001) decrease in cell area, or increase in cell density, with time. These observations indicate that at least under some circumstances mitosis occurs in the endothelium of the adult human cornea.

Initial healing of the posterior corneal surface following perforating trauma in guinea pig: a scanning electron microscope study

Using a scanning electron microscope, we have studied the healing of the posterior corneal surface after a small perforating trauma. Both corneas of adult guinea pigs were perforated with a needle and studied at various time intervals during recovery. Thirty minutes after the perforation the wound was sealed with a fibrous plug. Leukocytes adhered to the wounded area from the first day of healing on, but usually most of them disappeared after the third day. Fibroblast-like cells were present during the entire healing process from the first day on. Endothelial cells at the margin of the wound lost their hexagonal shape and began to slide over the plug. In 7 days the wound was completely covered by irregularly arranged endothelium. The endothelial cells did not reach their regular arrangement during the 14-day follow-up period.

Quantitative analysis of endothelial mosaic pattern changes in anterior keratoconus

The corneal endothelia of 21 eyes with anterior keratoconus and 15 eyes of age-matched controls were investigated with a specular microscope. Of the 21 eyes, 15 had definite keratoconus while six apparently normal fellow eyes were designated as latent. The corneal endothelial photographs were subjected to a computerized digitizer analysis of the area and shape of individual cells. The endothelium in keratoconus showed a significant increase in the extent of polymegethism (the coefficient of variation in cell area; 0.36 +/- 0.07) as compared with controls (0.24 +/- 0.03), with the mean cell area staying within normal limits. The relative frequency of hexagonal cells in keratoconus (50.5 +/- 5.7%) was significantly lower than that of controls (70.6 +/- 5.5%). Additionally, an increase of various cell shapes was noted, indicating that there was also a significant increase in cellular pleomorphism. In the latent group, alterations in cellular structure could not be detected by analysis of cell area when compared to controls. However, when these eyes were subjected to cell shape analysis and compared to fellow keratoconic eyes, both eyes showed similar pleomorphic characteristics. These results suggest that in the fellow eye in unilateral keratoconus, even when there are no ocular signs and symptoms, diagnosis can be made early with specular microscopy and computerized cell shape analysis.

The localization of actin in dividing corneal endothelial cells demonstrated with nitrobenzoxadiazole phallacidin

The distribution of actin in dividing endothelial cells of the rat cornea was studied by fluorescence microscopy by means of the nitrobenzoxadiazole conjugated derivative of the actin-binding toxin phallacidin (NBD-Ph). In normal noninjured tissue, fluorescence is limited to an area at or near the plasma membrane. Twenty-four hours after a corneal freeze injury, stress fibers are detected but only in those cells that are migrating into the wound area. By 48 h post-injury, cells in various stages of mitosis can be identified. During metaphase, anaphase, and telophase, diffuse cytoplasmic staining is observed, although the spindle region remains free of fluorescence. At various sites along the plasma membrane, fluorescence appears stronger compared to other regions. During the latter two stages of proliferation, NBD-Ph positive material can be seen within cell processes. In addition, a band of this material is observed within the region that corresponds to the cleavage furrow. As the daughter cells separate, actin can be detected within the cytoplasmic bridge. The results indicate that NBD-Ph can be used to study the distribution of actin in cells that were proliferating in vivo, and these patterns appear similar to those obtained with immunological methods on cultured cells.

Morphological studies on the regeneration of rabbit corneal endothelium under the influence of corticosteroids

In adult albino rabbits, the influence of corticosteroids on the regeneration of the corneal endothelium after cryocoagulation was studied using light-, scanning- and transmission electron microscopy. The follow-up period varied from 10 h to 10 days. In the regenerating endothelium, both mitotic and amitotic cells division were seen. In the dexamethasone-treated eyes, the regeneration process seemed to be stimulated rather than inhibited. In five of six eyes, areas of hyperproliferation were found, consisting of numerous elongated cells that formed several cell layers. Most of these cells revealed an extensively developed endoplasmic reticulum and Golgi apparatus as well as numerous mitochondria. There were signs of secretory activity. The mechanism for the development of these multilayered hyperproliferation zones is unknown.

In situ demonstration of actin in normal and injured ocular tissues using 7-nitrobenz-2-oxa-1,3-diazole phallacidin

The fluorescent derivative of the actin-binding toxin phallacidin, 7-nitrobenz-2-oxa-1,3 diazole phallacidin, has been used to cytologically demonstrate the presence of actin in lens epithelium, corneal endothelium, and retinal pigment epithelium. In these noninjured tissues, no stress fibers are observed and fluorescence is confined mainly to an area at or near the cell membrane, although some diffuse cytoplasmic staining can also be seen. However, following injury to either the lens epithelium or corneal endothelium of rats and frogs, stress fibers are detected, but only in those cells that migrate into the wound area. Cells on the periphery of each tissue do not partake in would repair and thus maintain their normal appearance. After the tissue has regenerated, stress fibers disappear, and those cells involved in the injury response return to their normal morphology. When rabbit corneal endothelium is placed in tissue culture, stress fibers are observed as the cells migrate away from the initial explant. Upon reaching confluency, these cells spread out and each is surrounded by thick actin-containing bands. Furthermore, they exhibit some stress cables within their cytoplasm. This is in contrast to their appearance in vivo where stress fibers are absent and fluorescence is limited to a region near the cell membrane.

Thymidine incorporation by human corneal endothelium during organ culture

Nine corneas from 5 human adult donors were obtained 11 to 31 h post mortem. In 4 corneas the endothelium was wounded by freezing and in 1 cornea by mechanical means. Care was taken to minimize endothelial damage in the remaining 4 corneas. The corneas were incubated at 31 degrees C for 6 days in a tissue culture medium containing 3H thymidine. Autoradiographs were made of the flat preparations of the endothelium. All corneas contained radioactive endothelial cell nuclei, with the highest concentration of labelled nuclei being in the wound areas. The greatest number of labelled cells was found in the cornea from the youngest donor, 19 years of age, but thymidine uptake also occurred in the oldest cornea, 89-years-old, which additionally had signs of endothelial dystrophy.

Effects of artificial intraocular pressure elevation on the corneal endothelium in the vervet monkey (Cercopithecus ethiops)

Both eyes of anesthetized vervet monkeys were perfused with mock aqueous humor for 3-7 hours. By adjusting the height of a reservoir connected to the anterior chamber of each eye the intraocular pressure in one eye was maintained at 33-44 mmHg and in the other eye it was a few mmHg above the spontaneous level (12-15 mmHg). Morphologically the control eyes appeared normal, whereas pronounced changes were observed in the high pressure eyes. Thus the corneal endothelium showed an uneven surface towards the anterior chamber with vacuolization, bledding and disruption of the cytoplasm. Pycnosis, excaryocytosis and even loss of whole endothelial cells were observed as well. The morphological changes were most pronounced in the peripheral part of the cornea and furthermore differed among neighbouring cells. During the healing process one could observe mitosis, amitosis and cell surface increase.