Nodal T-zone lymphoma and T-zone hyperplasia in dogs

Abstract

T-zone lymphoma (TZL) is an indolent, nodal lymphoma that has been clinically characterized in detail in dogs, and T-zone hyperplasia (TZH) is a hyperplastic change in lymph nodes associated with antigen processing. In some cases, histopathological features of TZL and TZH are similar, and are difficult to differentiate by morphology alone. Since there have been few publications characterizing their immunohistochemical profiles, histological, immunohistochemical, and clonality examinations were performed using formalin-fixed paraffin-embedded samples of canine lymph nodes with TZL (14 cases) and canine lymph nodes with TZH associated with nonlymphocytic tumors (10 cases). Immunohistochemically, small- to medium-sized lymphocytes of TZL were immunopositive for CD3, CD5, and HLA-DR, and negative for CD45, FOXP3, and granzyme B (GRB) in all cases. Among these 14 cases, 11 were immunopositive for CD8 and 1 was CD20 positive. Paracortical lymphocytes in TZH were diffusely immunopositive for CD3, CD5, and CD45, with scattered immunopositivity for CD8, HLA-DR, FOXP3, and GRB, and negative for CD20 in all cases. A clonal TCR gene rearrangement was detected in 13/14 TZL and none of the TZH cases. The present study revealed that TZL is a clonal proliferation of monomorphic CD8+CD45-GRB- T cells, while TZH consists of an immunophenotypically heterogenous population of CD45+ T cells that are variably positive for CD8 and FOXP3. These results suggest that canine TZL is a clonal proliferation of naïve or premature cytotoxic T cells. Regarding TZH, variable immunopositivity for cytotoxic and regulatory T-cell antigens may reflect immune responses to a variety of regional neoplastic lesions.

Extreme lymphocytosis in a dog with T-zone lymphoma

“…Histologically, TZL is characterized as a diffuse proliferation of small to medium lymphocytes in the paracortical area ( Kojima et al , 2022 ). Usually, cytology is the screening tool for lymphomas, as it is less invasive and has lower costs ( Lima, 2020 ).…”

“…In the case of TZL, the diagnosis can rarely be obtained on cytology, as the presence of a high concentration of small to medium cells with clear and elongated cytoplasm, can also indicate a reactive process in the paracortex region, which appears as a primary response preceding plasma cell proliferation, in cases of chronic dermatitis or in response to certain antigens, such as in cases of leishmaniasis or viral infections ( Comazzi and Riondato, 2021 ). However, a TZL diagnosis can be suspected by the presence of a high concentration of small to medium cells with clear and elongated cytoplasm, resembling a “hand mirror” shape ( Mizutani et al , 2016 ; Kojima et al , 2022 ). In addition to morphological analysis, samples collected by fine needle aspiration are amenable to flow cytometry immunophenotyping.…”