This report describes a method to induce chronic experimental autoimmune dry eye in Lewis rats through immunization with an emulsion of rat lacrimal gland extract, ovalbumin, and complete Freund’s adjuvant, followed by the injection of lacrimal gland extract and ovalbumin into the forniceal subconjunctiva and lacrimal glands six weeks later.
Dry eye disease is a very common condition that causes morbidity and healthcare burden and decreases the quality of life. There is a need for a suitable dry eye animal model to test novel therapeutics to treat autoimmune dry eye conditions. This protocol describes a chronic autoimmune dry eye rat model. Lewis rats were immunized with an emulsion containing lacrimal gland extract, ovalbumin, and complete Freund's adjuvant. A second immunization with the same antigens in incomplete Freund's adjuvant was administered two weeks later. These immunizations were administered subcutaneously at the base of the tail. To boost the immune response at the ocular surface and lacrimal glands, lacrimal gland extract and ovalbumin were injected into the forniceal subconjunctiva and lacrimal glands 6 weeks after the first immunization. The rats developed dry eye features, including reduced tear production, decreased tear stability, and increased corneal damage. Immune profiling by flow cytometry showed a preponderance of CD3+ effector memory T cells in the eyeball.
Dry eye disease (DED) is a multifactorial disease of the tears and the ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability, which can lead to damage to the ocular surface. It is accompanied by increased osmolarity of the tear film and by inflammation of the ocular surface1. Symptoms associated with DED are burning, stinging, grittiness, foreign body sensation, tearing, ocular fatigue, and dryness2,3. The two main causes of DED are reduced tear production by the tear secretion gland and the excessive evaporation of the tear film4. In patients with autoimmune diseases, such as Sjogren syndrome, systemic lupus erythematosus, and rheumatoid arthritis immune damage to the meibomian glands reduces the expression of lipids essential for tear stability. Also, immune damage to the ocular surface decreases the production of mucins important for surface wettability. Together, these processes cumulatively cause chronic dry eye5,6,7.
Tear replacement and anti-inflammatory therapy are the mainstays of therapy. However, current anti-inflammatory therapies for DED (i.e., corticosteroids and cyclosporine) are broadly immunosuppressive, leading to serious adverse effects8,9,10. There is need for a suitable animal model to test novel immunomodulatory agents to treat autoimmune dry eye.
Mice with specific genetic defects11,12,13, mice lacking specific genes14,15, and transgenic mice overexpressing immunoregulatory genes have been used as models of autoimmune dry eye16,17. Antigen-induced autoimmune animal models have also been reported in mice18, rabbits19, and rats20,21. Here, we describe an antigen-induced model of chronic autoimmune dry eye. This model is a modification of two earlier models; one used lacrimal gland extracts, and the second used an autoantigen (i.e., klk1b22) from the lacrimal glands20,21.
The disease was induced by the subcutaneous immunization of 6 to 8 week-old female Lewis rats with ovalbumin, complete Freund's adjuvant, and an emulsion containing lacrimal gland extracts from Sprague-Dawley rats (Figure 1). A second immunization with the same antigen in incomplete Freund's adjuvant was administered two weeks later. To recruit antigen-specific immune cells to the lacrimal gland and ocular surface, mixture of lacrimal gland extract and ovalbumin (1 mg/mL) was injected into the forniceal subconjunctiva and lacrimal glands at the 6th-7th week (Figure 1). More than 85% of the rats developed characteristic features of dry eye 70 days after the first immunization. These features include reduced tear production (Figure 2), increased corneal fluorescein staining (Figure 3), and decreased tear stability (Figure 4). Immune profiling of the T cells in the eyeballs of normal rats by flow cytometry reveals a preponderance of CD3+ effector memory T cells (Figures 5 and 6). Rats with autoimmune DED show an increase in CD3+ effector memory T cells and a corresponding decreases in naïve and central memory T cells (Figure 6).
Animals were handled according to institutional guidelines and the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. The study protocol was approved by the Institutional Animal Care and Use Committee of SingHealth.
1. Preparation of Lacrimal Gland Extract
NOTE: Rats were anesthetized with the intraperitoneal injection of ketamine (75 mg/kg) and xylazine (10 mg/kg). Proper anesthetization was confirmed by toe pinching and tail pinching. Ophthalmic gel was applied to the rat eyes to prevent dryness after each procedure. The anesthetized rats were placed under far infrared lights to keep the animals warm until they fully recovered. During the procedures and recovery time, animals were closely monitored by researchers. All the materials and surgery tools were sterile before use. At the end of the experiment, rats were euthanized by the intraperitoneal injection of pentobarbital (80 mg/kg). Complete euthanasia was verified by lack of cardiac pulse and no blink reflex by touching the eyeball. Rats were housed in standard conditions: room temperature, 21-23 °C; relative humidity, 30-70%; light-dark cycle, alternating 12 h (7 AM to 7 PM).
2. Preparation of Emulsion and Pertussis Toxin
3. Immunization of the Lewis Rats
4. Injection of the Antigen Mixture into the Forniceal Subconjunctiva and Lacrimal Gland to Recruit Antigen-specific Immune Cells and Cause Local Inflammation
5. Assessment of Dry Eye Features
NOTE: For steps 5.1-5.3, the anesthetized rats should be held gently by a gloved hand in an upright position on a flat surface to avoid movement.
Figure 1 illustrates the experiment design. On both day 48 and day 70, dry eye clinical features are assessed in the immunized rats. The tear volume is represented by the length of the wet part of the phenol red thread. Figure 2 shows representative images of phenol red threads from control and DED rats. The length of the phenol red threads in the DED group is shorter than the control group, indicating less tear volume.
Fluorescein binds to damaged corneal epithelium. Thus, corneal damage is measured by corneal fluorescein staining. Fluorescein spots on the corneal surface of DED rats were graded from 0 to 2 and compared to control rats. Rats with DED have more fluorescein staining than control rats (Figure 3), suggesting corneal damage.
The corneal smoothness in DED and control rats was assessed by the ring illuminator. If the corneal surface is smooth, with high tear stability, the image of the illuminator ring on the ocular surface is round and perfect. Distortion of the image indicates reduced corneal smoothness and an unstable tear film. The distortion degree of the ring was graded from 0 to 2. A higher ring distortion level was noted in the DED group (Figure 4), indicating less tear stability.
Rats are defined as having dry eye when at least two clinical features of dry eye are abnormal. Among the 24 immunized rats, 21 rats developed DED on day 48. The results were consistent when evaluated on day 70.
The flow cytometry analysis shows that the predominant T-cell subset in normal rat eyeball tissues are effector memory T cells (Figure 5). In the eyeballs of DED rats, ~70% of the CD3+ T cells are effector memory T cells, while in control rats, this number is ~50%. Eyeballs of DED rats have significantly higher effector memory T cells than those of control rats (Figure 6).
Figure 1: Schematic of the Experimental Design. LG: lacrimal gland; DED: dry eye disease; CFA: complete Freund's adjuvant; IFA: incomplete Freund's adjuvant. Please click here to view a larger version of this figure.
Figure 2: Phenol Red Thread Measures the Tear Volume. Phenol red thread is placed at the proximal corner of both rat eyes for 1 min and is then removed. Representative images of the phenol red thread, together with a ruler, from both control and DED groups are shown. ImageJ was used to measure the length of the wet part of the phenol red threads. Scale bar = 1 mm. Please click here to view a larger version of this figure.
Figure 3: Representative Images of the Corneal Epithelial Damage Measured by Fluorescein Staining. Each rat cornea was stained with 0.2% fluorescein for 1 min and flushed with at least 1 mL of saline. Images were taken under an eye imaging microscope with cobalt blue light. The first column shows representative images of control corneas. The second column contains representative images of corneal staining from rats with DED features. The green fluorescent spots indicate corneal epithelial damage. All images were produced on the same color scale. The quantification of fluorescein staining was performed according to the area and density of the green spots. Scale bar = 1 mm. Please click here to view a larger version of this figure.
Figure 4: Representative Cornea Images Showing the Reflection of the Ring Illuminator. Rat corneal/tear smoothness was measured by a ring illuminator. The distortion degree of the ring in the captured images is a measure of relative tear stability. The left column shows the representative images in control animals, and the right column shows representative images after the induction of dry eye. Scale bar = 1 mm. Please click here to view a larger version of this figure.
Figure 5: Dot Plots Derived from Flow Cytometry Analysis. T cells isolated from eyeball tissues were stained with a panel of antibodies. In the CD45+CD3+7AAD– population, CD3+7-AAD– T cells were gated. Among the CD3+7-AAD– T cells, the naïve, central memory, and effector memory T cell populations were determined. Please click here to view a larger version of this figure.
Figure 6: T-cell Subpopulation Profile in the Eyeballs. CD3+CD45RC+ naïve T cells, CD3+CD45RC–CD44+CD62L– effector memory T (TEM) cells, and CD3+CD45RC–CD44+CD62L+ central memory T (TCM) cells are presented as the percentage of CD3+ T cells. Results are from 3 control rats and 6 DED rats. Similar results were obtained from the analysis of T cells from isolated lacrimal glands (data not shown). The unpaired Student's t-test was used for statistical comparison. The error bars represent the SD. * p <0.05, ** p <0.01. Please click here to view a larger version of this figure.
A critical step of this protocol is ensuring the homogeneity of the emulsion. In well-prepared emulsions, the antigens are completely coated with oil, ensuring the slow release of the injected antigen and continuous immune stimulation. Another critical feature of this protocol is the use of Lewis rats. Lewis rats are more sensitive to the development of autoimmune disease than other strains27.
This protocol has been modified from two previously published protocols, which either used lacrimal gland extract only or recombinant Klk1b2220,21. In the current protocol, ovalbumin plus lacrimal gland extract are used as the antigen, and antigen-specific immune cells are attracted to the ocular surface and the lacrimal gland, inducing local tissue damage. Dry eye develops slowly, reaching ~85% by day 48 after the initial immunization. Antigenic challenge to the eye and lacrimal gland on day 48 exacerbates dry eye and ensures its chronicity up to day 70.
Compared to the recombinant Klk1b22 in the Klk-induced DED model, the lacrimal gland extract and ovalbumin used in the current model are cheaper and easier to obtain. The lacrimal gland extract also contains other proteins, apart from Klk, that may induce autoimmunity, so this extract is theoretically more potent than the Klk method at inducing DED. We have also tried immunizing rats with lacrimal gland extract only; although these immunized rats developed DED, there was no significant increase of effector memory T cells in eyeball tissues compared to controls.
The limitation of this technique is that it takes 70 days to achieve the model. Effector memory T cells are the main T-cell subsets in the normal rat eye. In this model, autoimmune DED results in an increase in CD3+ effector memory T cells in the eyeball. Drugs that preferentially suppress effector memory T cells, such as selective inhibitors of the Kv1.3 potassium channel, may therefore have a therapeutic benefit on autoimmune DED28.
The authors have nothing to disclose.
The authors would like to thank Ms. Tin Min Qi and Dr. Veluchamy Amutha Barathi and her team for their help with handling the animals. This work was supported by NHIC-I2D-1409007, SingHealth Foundation SHF/FG586P/2014, and NMRC/CSA/045/2012.
Reagents | |||
Protease inhibitor cocktail | Sigma-Aldrich | P2714-1BTL | |
Pierce BCA Protein Assay Kit | Thermal Scientific | 23227 | |
Mycobacterium tuberculosis H37Ra | Becton, Dickinson and company | 231141 | |
complete Freund's adjuvant | Becton, Dickinson and company | 231131 | |
ovalbumin | Sigma-Aldrich | A5503-10G | |
incomplete Freund's adjuvant | Sigma-Aldrich | F5506-6X10ML | |
pertussis toxin | Sigma-Aldrich | P7208-50UG | |
fluorescein sodium solution | Bausch & Lomb U.K Limited | NA | |
Name | Company | Catalog Number | Comments |
Equipment | |||
Sonicator | Sonics | Vibra-Cell | |
phenol red thread | Tianjin Jingming New Technological development Co. LTD. | NA | |
Stereo microscope with ring light illuminator and camera | Carl Zeiss | NA | |
Micro IV microscope | Phoenix Research Labs | NA |