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

To describe clinical, in vivo confocal microscopic, histopathologic, polymerase chain reaction assay for antigen receptor rearrangement (PARR) and immunohistochemical features of a horse with suspected malignant transformation of corneal B lymphocytes in immune-mediated keratitis (IMMK) and to also report surgical management and adjunctive treatment including topical and radiation therapies.

A 15-year-old Oldenburg gelding was examined for presumptive progressive stromal IMMK of the right eye which became refractory to medical management. Corneal in vivo confocal microscopy revealed a marked infiltration of neovascularization, leukocytes, and dendritic cells. Superficial keratectomy was performed. Histopathologic and immunohistopathologic abnormalities of the cornea were restricted to the anterior and mid-stroma and included sheets of medium-size atypical lymphocytes which stained positively for CD79a and CD20 and also demonstrated clonal rearrangements of immunoglobulin heavy chain 2 (IGH2) and 3 (IGH3) confirming B-cell lymphoma. Complete physical examination, complete blood count, serum chemistry, thoracic radiographs, and thoracic ultrasound were performed. No evidence of systemic metastasis was found. Adjunctive local therapy included topical dexamethasone, mitomycin C, and radiation therapy using electronic surface brachytherapy. Follow-up at 16 months revealed a lack of corneal infiltrate and progressive clearing of corneal fibrosis and superficial pigment.

This case represents a unique presentation of primary corneal lymphoma in the horse. Tumor development was associated with chronic IMMK and chronic topical anti-inflammatory therapy. Surgical excision with adjunctive therapy using mitomycin C and electronic surface brachytherapy was well tolerated and considered curative. Future immunohistochemistry and PARR will be important in characterizing the IMMK syndrome and its variations.