22 Breed-specific Canine Lymphoproliferative Diseases

Abstract

The lymphatic system consists of T cells, B cells, NK cells, and other less common subsets of small mononuclear cells that arise from pluripotent hematopoietic stem cells. These cells differentiate primarily in the bone marrow or in the thymus into long-lived cells that are responsible for adaptive immune responses. During a normal immune response to a pathogenic challenge, these cells undergo frequent rounds of cell division to expand their numbers. T cells and B cells, in particular, follow this process of clonal expansion with a carefully orchestrated apoptotic program that contracts their populations to physiological levels while allowing survival of memory cells.

Considering the high rate of proliferation that is required for self-renewal and to support immune responses, as well as the absolute requirement for extensive apoptosis that restores the balance of lymphocyte populations, it is not surprising that cells of the lymphoid system are highly susceptible to neoplastic transformation. Lymphoproliferative diseases (LPDs) encompass a constellation of tumors that originate from lymphocytes. LPDs that manifest as solid tumors are called lymphomas, whereas LPDs that arise from the bone marrow or spleen and (generally) present with malignant cells in the peripheral circulation are called leukemias (Vernau and Moore 1999; Valli et al. 2002). A group of tumors called non-Hodgkin’s lymphoma (NHL) comprises the most common LPDs in people. As a group, NHL is the fifth most common cancer in the U.S. with a lifetime risk of ~1 in 50 and an overall 5-year survival rate of approximately 55% (Jemal et al....