Thymoma-associated lymphocytosis in a dog.

A 9-year-old female spayed English Springer Spaniel was evaluated for a cranial mediastinal mass and lymphocytosis. Flow cytometric immunophenotyping of peripheral blood lymphocytes revealed 97% as CD3 positive, confirming a T-cell lineage. Additionally, T-cell subset assessment showed 53.2% to be double-negative T-lymphocytes, expressing neither CD4 nor CD8 surface markers. The number of double-negative lymphocytes in circulation coincided with the number of T-cell receptor (TCR) γδ-expressing T-cells in circulation. Molecular T-cell clonality analysis of TCR Gamma (TCRG) gene rearrangement showed a polyclonal expansion of T-lymphocytes. Histopathology confirmed the mass to be a thymoma, supporting the diagnosis of thymoma-associated T-cell lymphocytosis. Resolution of the lymphocytosis after removal of the thymoma provided further evidence for this diagnosis. To the authors' knowledge, this case is only the second report of thymoma-associated peripheral lymphocytosis in the veterinary literature, and is the first to report a confirmed thymoma-associated peripheral γδ T-cell lymphocytosis in a dog.