Immunophenotypic Characterization of Canine Nodal T-Zone Lymphoma.

T-zone lymphoma (TZL) is an indolent nodal T-cell lymphoma most commonly observed in submandibular lymph nodes in dogs. The diagnosis is based on its distinct morphology and expression of CD3. TZL has been reported to have a low Ki67 index and to lack expression of CD45. The latter feature has been used to diagnose this type of lymphoma via fine needle aspirate and flow cytometry without confirmation of the characteristic tissue architecture. The goal of this study was to characterize the immunophenotype of canine nodal TZL in greater detail. Twenty-seven TZLs were selected based on their characteristic morphology. A tissue microarray was generated, and immunohistochemical expression of CD3, CD5, CD20, CD21, CD25, CD45, Bcl-6, and Ki67 was evaluated. Neoplastic T cells in all cases were positive for CD3, CD5, and CD25, and negative for CD20, CD21, and Bcl-6. Positive labelling for CD45 was detected in 2 of the 27 cases with the remaining 25 cases being negative. All cases had a low Ki67 index with an average index of 19.56%. For the CD45-positive TZLs, clonality of the T-cell antigen receptor gamma gene was confirmed in only one of these cases. The observed immunophenotype of canine TZL is similar to previous publications with the exception that 2 cases expressed CD45. Expression of CD45 in TZLs in this study emphasizes the importance of interpreting immunophenotypic findings in conjunction with histopathology to reach an accurate diagnosis and not to use lack of expression of a particular antigen as the sole diagnostic criterion.