Metastatic thymic lymphosarcoma in a calf.

Metastatic thymic lymphosarcoma was diagnosed in a 16-month-old mixed-breed heifer with a history of progressive weight loss. Physical examination revealed cachexia, pale mucous membranes, large peripheral lymph nodes, and a 15 X 40-cm mass in the ventral portion of the neck, extending cranially from the thoracic inlet. Neoplastic lymphocytes were identified in aspirates of pleural effusion and bone marrow. Histologic examination of necropsy specimens substantiated metastatic dispersal of the tumor into lymphoid tissue, liver, intestine, heart, and kidney. This case differs from other reported cases of thymic lymphosarcoma because of the involvement of organs other than the thymus and lymph nodes. Analytical flow cytometry was performed on mononuclear leukocytes obtained from blood by use of density gradient centrifugation. The majority of cells (65%) appeared to be of an immature, poorly differentiated phenotype, on the basis of the small numbers of cells stained with monoclonal antibodies specific for the following cell surface markers: 8% BoCD2 (IL-A26; sheep erythrocyte receptor on T-lymphocytes); less than 10% B-lymphocytes as determined by expression of MHC class II proteins and surface immunoglobulin; 12% monocytes (IL-A24); and 5% null cells (IL-A29). Although the leukemic cell population did not express traditional surface markers for T-lymphocytes, we hypothesize that the leukemic cell population represents an early stage of T-cell maturation that has failed to differentiate and express characteristic cell surface antigens.