Suspected malignant transformation of B lymphocytes in the equine cornea from immune-mediated keratitis.

This study discusses a rare case of B-cell lymphoma in a horse’s eye. The horse, which had been suffering from immune-mediated keratitis (an inflammation of the cornea), underwent a range of diagnostic and treatment procedures, from confocal microscopy to physical examination to histopathology. Researchers confirmed the presence of lymphoma and, after ruling out systemic metastasis, administered a combination of surgical and local therapies. Sixteen months after the interventions, the horse’s condition showed significant improvement.

Background and Case Overview

• This case study involves a 15-year-old Oldenburg gelding (a type of horse) diagnosed with immune-mediated keratitis (IMMK) – which is an inflammation of the cornea – in its right eye.
• The condition was progressive, meaning it was worsening over time, and had become resistant to medical treatment.
• Via in vivo confocal microscopy, a type of advanced imaging technique, the researchers observed that the cornea had severe encroaching of new blood vessels, and was heavily populated by leukocytes (white blood cells) and dendritic cells.

Diagnostic Procedures and Findings

• The horse underwent a surgical procedure known as superficial keratectomy – the removal of the outer layer of the cornea.
• Upon examining the tissue thus removed, the researchers found that the cornea displayed histopathologic and immunohistopathologic abnormalities, specifically in the front and middle layers of the cornea’s tissue (anterior and mid-stroma).
• The abnormalities were characterised by clusters of unusually large lymphocytes that tested positive for CD79a and CD20 – markers of B lymphocytes, a type of white blood cell.
• These lymphocytes also demonstrated clonal rearrangements of immunoglobulin heavy chain 2 (IGH2) and 3 (IGH3), which confirmed the diagnosis of B-cell lymphoma, a type of cancer.

Treatment and Outcomes

• After confirming that the cancer had not spread systemically, the treatment team administered a series of topical drugs, including dexamethasone (a steroid) and mitomycin C (a chemotherapy drug) to the eye.
• They also employed radiation therapy using electronic surface brachytherapy – a type of radiation therapy that uses an electronic source.
• 16 months following treatment, the horse’s cornea no longer showed signs of infiltration and demonstrated progressive reduction of corneal fibrosis (growth of excess tissue) and superficial pigment.

Conclusions and Future Directions

• This single case presented a unique instance of primary corneal lymphoma in a horse.
• The researchers suggest that the chronic IMMK and the long-term anti-inflammatory therapy may have triggered this unique malignancy.
• They also conclude that the combination of surgical resection and adjunctive therapy with mitomycin C and electronic surface brachytherapy proved to be successful and well-tolerated.
• The researchers expect that future applications of immunohistochemistry and tests such as PARR will help better understand IMMK syndrome and its variants.