Canine Cerebral Intravascular Lymphoma: Neuropathological and Immunohistochemical Findings.

Intravascular lymphoma (IVL) is a rare angiotropic large-cell lymphoma in which neoplastic lymphocytes proliferate within the lumina of small blood vessels in the absence of a primary extravascular mass or leukemia. This study included 10 cases of canine IVL restricted to the CNS. Dogs had an average age of 8 years and neurological signs mainly referred to brain involvement such as depression, seizures, and ambulatory deficits. Gross examination at necropsy showed focal extensive or multiple hemorrhagic areas mainly distributed in the telencephalon and diencephalon. Histopathologically, numerous veins and capillaries were filled with neoplastic lymphoid cells, accompanied by edema, hemorrhage, and thrombosis. Immunohistochemistry (IHC) for CD3, CD20, and PAX5 was performed to phenotype the neoplastic lymphocytes. IHC for CD44 and CD29 were used to investigate the pathogenetic mechanism leading to the intravascular aggregation of the neoplastic lymphocytes. The same IHC panel was applied to 8 cases of primary and metastatic canine CNS lymphoma in order to compare IVL immunoreactivity. Three IVLs were typified as T-cell, 3 as B-cell, and 4 as non-T non-B. Neoplastic lymphocytes showed marked expression of CD44 in all IVL cases, and CD29-immunolabeled cells were observed in 4 IVLs. CD44 immunoreactivity was consistent with the findings reported in human IVL, suggesting a predisposition to the formation of lymphocyte aggregates. CD29 was inconsistently immunonegative in canine IVL, confirming only partially the pathogenetic mechanism suggested for the human counterpart.