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Tumors of the Immune System |  |
| Studies of the immunologic response to tumors conducted in the past 15 years have changed the definition of cancer, which is now perceived as a chronic immune disease. In response to genotoxic stress, carcinogens, or viruses, the targeted cells induce expression of stress proteins and tumor-associated antigens. These mark the abnormal cells for the immune system, which eliminates them through cell-mediated cytoxicity. In cancer patients, tumor cells escape from the immune attack by
relying on both immunoparalysis and tumor cell modification. The demonstration that even bulky, invasive tumors can undergo complete remission under appropriate stimulation (eg, IL2) has shown that it is indeed possible to treat cancer successfully by immune manipulation. |
| Cancer is the number one cause of death in dogs; 45% of dogs ≥10 yr old die of cancer. Lymphoma is one of the most prevalent tumors in dogs and cats. Boxers, Basset Hounds, and Rottweilers are predisposed to developing lymphomas. The normal immune response requires a burst of rapid proliferation of lymphocytes. On occasion however, this proliferation may be uncontrolled, and lymphoid neoplasms result. Because lymphocytes are present in all organs, tumor development can occur in any
organ. Lymphomas can be multicentric, mediastinal, gastrointestinal, renal, nervous, or leukemic. Less commonly, they occur in the eyes, skin, or nose. To determine the stage of the disease, CBC, serum chemistry profiles, abdominal ultrasound, abdominal radiographs, and bone marrow analyses are useful. Immunofluorescent staining can be performed in dogs and cats to characterize lymphomas. They may be either T cell or B cell in origin. |
| Most cases of canine lymphosarcoma, Marek’s disease, calf leukosis, and feline leukemia are of T-cell origin, as are thymomas. Thymomas, which are relatively uncommon in domestic animals, generally cause loss of condition and respiratory distress. They are commonly confirmed by radiography. In humans, thymomas may be associated with signs of myasthenia gravis. While this association has been reported in dogs, it is uncommon. Many T-cell lymphomas are associated with a simultaneous
immunosuppression manifest by a predisposition to recurrent infections. |
| Adult bovine and ovine leukosis, alimentary feline leukemia, and avian leukosis are usually of B-cell origin. Under some circumstances, neoplastic B cells may develop into plasma cells. Plasma-cell tumors are known as myelomas. Because neoplastic plasma cells can secrete immunoglobulins, they give rise to gammopathies. |
| Combination chemotherapy (eg, vincristine, l-asparaginase, cyclophosphamide, doxorubicin, prednisone) is recommended for lymphoma treatment. Side effects include vomiting, diarrhea, lack of appetite, and fever. Hair loss is not observed in dogs and cats. Lymphomas are rarely cured, but remission of up to 1 yr is common after combination chemotherapy. |
