Large Granular Lymphosarcoma/Leukemia in Dogs and Cats

Large granular lymphosarcaoma / leukemia is a neoplastic disease of lymphocytes that contain intracytoplasmic azurophilic granules. Large granular lymphocytes (LGL) can be found in the blood of healthy domestic species and usually constitute 10% or less of the total circulating lymphocyte pool.2,7 Large granular lymphocytes have lymphocyte morphology, possessing a round or bean-shaped nucleus, and slightly basophilic cytoplasm. The major morphologic difference between LGLs and typical lymphocytes is that the former contain varying numbers (3 or more) of small to large azurophilic granules in their cytoplasm. Granular size usually is inversely proportional to the number of cytoplasmic granules.5 In cats, the granules may be large with internal vesicles.6 The exact origin of LGLs is unknown, however, studies have shown that at least two lines (cytotoxic T lymphocytes and natural killer cells) produce large granular lymphocytes. Studies show that most large granular lymphocytes in dogs are of T cell origin, with the minority of natural killer (NK) cell origin.2,9 Most cases of LGL leukemia originate outside of the bone marrow and thus are technically lymphosarcoma. Infiltration of the bone marrow subsequently can occur, and patients will present with a leukemic blood picture.2