CD4 and CD8 double-negative immunophenotype of thymoma-associated lymphocytes in a dog

Abstract

Persistent small-cell lymphocytosis in dogs with a concurrent mediastinal mass has been associated with both thymoma and small-cell lymphoma. In thymomas, neoplastic thymic epithelial cells induce overproduction and release of polyclonal lymphocytes, whereas thymic lymphoma results in thymic effacement by a clonal expansion of neoplastic lymphocytes and subsequent leukemic phase of lymphoma. Flow cytometry has been used to differentiate these 2 entities by immunophenotyping mediastinal mass aspirates. It has been reported that cases with mediastinal masses in which ≥ 10% of the associated small-cell lymphocytes were double positive for CD4 and CD8 were thymomas, whereas masses associated with < 10% were suggestive of lymphoma. We report a unique case of thymoma-associated lymphocytosis lacking the classic CD4+CD8+ immunophenotype. Our findings suggest that there may be more diversity in the thymoma-associated lymphocyte immunophenotype than has been identified previously; immunophenotyping alone might not be sufficient to differentiate thymic small-cell lymphoma from thymoma-associated lymphocytosis. In dogs with mediastinal masses and peripheral lymphocytosis, employing a variety of testing modalities to avoid misdiagnosis is prudent. These modalities include cytologic and/or histologic evaluation, immunophenotyping, and clonality assessment.

Keywords: PCR for antigen receptor gene rearrangement; dogs; flow cytometry; immunophenotype; lymphocytosis; thymoma.

Figure 1.

Representative photomicrograph of the peripheral blood smear demonstrating moderate lymphocytosis and mild basophilia. Lymphocytes are small (nuclei smaller than the diameter of a neutrophil) with mature condensed chromatin and scant basophilic cytoplasm. A basophil is indicated by the arrow. Modified Wright stain. Bar = 25 µm.

Figure 2.

Mediastinal mass aspirate composed of a heterogeneous population of predominantly small lymphocytes with variably sized clusters of polygonal to slightly fusiform epithelial cells (arrows) and occasional scattered mast cells (asterisks). Modified Wright stain. Bar = 25 µm.

Figures 3, 4.

Flow cytometric immunophenotyping of peripheral blood (3) and thymic mass (4). Dot plots with subdivided T cells by detection of CD4 and CD8. The lower right quadrant contains CD4+ cells, lower left quadrant CD4−CD8− (double-negative) cells, upper left quadrant CD8+ cells, and the upper right quadrant CD4+CD8+ (double-positive) cells.