Clonality testing for rearrangements in the complementarity-determining region 3 of the immunoglobulin heavy chain of B lymphocytes (B cell receptor) and the T cell receptor of T lymphocytes helps distinguish between clonal and non-clonal expansions of lymphocytes. There are rare reports of clonally rearranged T and B cell receptors in dogs with acute myeloid leukemia (AML). Our objective was to determine the frequency of clonally rearranged T and B cell receptors in dogs with AML. Archived slides from historical cases of AML (from January 2010 to June 2013) and slides or liquid specimens [blood, bone marrow (BM), body cavity fluid, or tissue aspirates] from cases of AML diagnosed between June 2013 and February 2017 were used in the study. A diagnosis of AML was made on the basis of more than 20% immature neoplastic cells (“blasts”) in blood, BM, or extramedullary tissues, displaying features of myeloid differentiation. Myeloid differentiation was based on a combination of morphologic criteria, positive flow cytometric labeling for surface antigens typical of myeloid origin (e.g., CD11b, CD11c, CD14 with a general lack of expression of T or B cell markers), or positive cytochemical staining reactions for myeloid-associated enzymes (e.g., alkaline phosphatase, chloroacetate esterase). There were 63 cases of AML diagnosed during this period; however, slides or liquid specimens with sufficient DNA for testing were only obtained from 25 dogs. Affected dogs represented various breeds and were a median of 8 years old, with more male (64%) than female (36%) dogs. Common clinical signs were peripheral or internal lymphadenopathy (10/25 dogs, 40%) and hepatomegaly or splenomegaly (10/25 dogs combined, 40%). Typical hematologic findings were bi- or pancytopenia (23/25 dogs, 92%), with circulating blasts (21/25, 84%). Solitary clonal (4 B cell, 6 T cell) and biclonal (6 B and T cell) rearrangements in B or T cell receptors were found in 16 dogs (64%). Our results indicate that dogs with AML can have a high frequency of clonally rearranged T or B cell receptors, including biclonality, and clonality testing should not be used as a tool to distinguish between acute leukemia of myeloid or lymphoid origin.

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