Chronic lymphocytic leukemia or small cell lymphoma in a cat

 
CLL

This cat had a persistent lymphocytosis (increasing progressively over several months from 50,500/uL to 150,000/uL). The lymphocytes were small cells, with clumped chromatin compatible with a chronic lymphocytic leukemia (CLL). This lymphocytosis is too marked to attribute to an epinephrine response (usually < 25,000/uL) and was compatible with a leukemia. This was phenotyped with flow cytometry as a helper/regulatory T cell, i.e. the tumor cells were positive for CD5 and CD4 (the most common form of CLL in a cat). The cat was concurrently anemic and had mild rouleaux formation, which was due to antigenic stimulation (a polyclonal gammopathy was evident on electrophoresis). The cat concurrently was diagnosed with intestinal low grade small cell lymphoma, that was a T cell on immunohistochemical staining (positive for CD3). It is possible that the cat had a leukemia associated with the intestinal lymphoma, however it is quite rare for cats to have an overt leukemia with this type of lymphoma. It is also possible that they were two different disorders. If the intestinal tumor cells were also helper/regulatory T cells, this would argue for a leukemia associated with lymphoma, however CD4 immunostaining cannot not be performed on formalin-fixed sections. Regardless, the peripheral lymphocytosis and intestinal lymphoma responded to chemotherapy.

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