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Lymphoid Leukemias, Myeloid Neoplasia, and Myelodysplastic Syndrome
Angela R. Kozicki, DVM, DACVIM (Oncology)

Bluepearl Veterinary Partners, Southfield, MI, USA

Lymphoid Leukemias and flow cytometry in which cell suspensions are stained
with specific fluorochromes, analyzed by a focused laser
Etiology/Pathophysiology beam and sorted by size and DNA content (ploidy of
cells). Immunophenotyping can identify the presence of
Lymphoid leukemias are differentiated as either chronic different blood cell lineages but cannot predict clonality
lymphocytic leukemia (CLL) or acute lymphoblastic leu­ (whether a population arises from a single cell).
kemia (ALL). CLL is a clonal, neoplastic proliferation of It is important to know normal percentages of lym­
small, mature‐looking lymphocytes that manifests as a phocyte subsets found in the peripheral blood of dogs
persistent, often severe lymphocytosis. Three subsets and cats to be able to distinguish a benign or reactive
have been recognized based on immunophenotyping. process from a malignant one. In general, peripheral
The most common is granular T‐CLL which consists of lymphocytes of dogs consist of ~80% T cells (CD3+ or
proliferations of cytotoxic (CD8+) T cells. Despite con­ CD5+) with 45% being helper T cells (CD4+), 25% being
ventional thinking that CLL is always a primary bone cytotoxic T cells (CD8+) and the remainder being dou­
marrow disease, granular T‐CLL is considered a primary ble‐negative T cells (CD4‐, CD8‐). Approximately 15%
splenic disease which can be verified by expression of a are B lymphocytes (CD21+) and the remaining lympho­
marker for splenic red pulp (CD11d). The second is B‐ cytes are natural killer (NK) cells. Cats are very similar
CLL (CD21+, CD79a+) and the third is nongranular/ but have slightly fewer T cells (~75%) and a greater per­
atypical T‐CLL (CD3+) in which no consistent pattern of centage of B cells (~25%) in the peripheral blood.
concurrent CD4 or CD8 expression is found. ALL is The etiology of lymphoid leukemias is largely
defined by proliferation of immature lymphoblasts in the unknown. In people, there is no known association with
bone marrow or peripheral blood. The immunopheno­ radiation, chemicals or alkylating agents and no viral
type of ALL is typically B cell (CD21+, CD3‐, CD4‐, cause. Genetic factors are suspected in both humans and
CD8‐) with an acute, immature lineage based on expres­ dogs but immunophenotype of the neoplastic cells in
sion of CD34, a stem cell marker. these species is different in that most human lymphoid
Classification of lymphocytes involves assessment of leukemias are B cell in origin. Therefore, it is unwise to
cell surface antigen (marker) expression that pertains to try to make direct correlations between species since dif­
both lineage and function. This classification is usually ferent causative factors may be involved. In cats, retrovi­
performed by immunophenotyping. Immunophenotyping ruses such as feline leukemia virus (FeLV) play a role
consists of the application of antibodies specific for dif­ since ~60–80% of cats with ALL are FeLV+. There is no
ferentiation antigens of lymphocytes and other immune known association between FeLV and CLL in cats and no
cells to identify lineages of these cells present in neoplas­ evidence of a retroviral cause in dogs.
tic or reactive diseases. Immunophenotyping can be per­
formed on cytology smears, blood smears, fluids (blood
or bone marrow), formalin‐fixed paraffin sections or Epidemiology
snap‐frozen tissue sections. Immunophenotyping tests In people, CLL is an indolent disease of older adults (>60
include immunohistochemistry (on tissue samples), years of age) and accounts for 25–30% of all adult leuke­
immunocytochemistry (on cytologic or fluid samples), mias among white populations in North America and

Clinical Small Animal Internal Medicine Volume II, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical