CD45+ and CD45− lymphocyte populations identified by flow cytometry from dogs with lymphoma exhibit similar morphology and the same clonal (B cell or T cell) lineage

•CD45+ and CD45− cells were identified by flow cytometry in canine lymphoma.•CD45+ and CD45− cells were sorted using high speed cell sorting.•Morphology was assessed by cytology and clonal origin by PARR.•In 10/11 dogs, CD45+ and CD45− morphology was similar.•In 12/12 dogs, PARR of paired CD45+ and CD45− samples produced amplicons of similar size.

Flow cytometric analysis of canine lymphoma sometimes demonstrates a mixed population of CD45+ and CD45− lymphocytes. Recently, indolent forms of canine lymphoma have been described which are associated with the loss of CD45 expression, warranting further investigation of the role of CD45 in canine lymphoma. The purpose of this study was to compare morphology and assess clonal origin between CD45+ and CD45− lymphocyte populations identified by flow cytometry in confirmed cases of canine B- and T-cell lymphoma. Our hypothesis was that the CD45− population of lymphocytes represented a phenotypic variant of the CD45+ population. Fifteen client-owned dogs with lymphoma and distinct CD45+ and CD45− lymphocyte populations identified by flow cytometry were identified for a blinded, prospective assessment of morphology and clonal origin (B cell or T cell) between populations of sorted CD45+ and CD45− cells. Lymphocytes were isolated from 11 dogs for paired cytologic evaluation. In 10/11 dogs, the CD45+ and CD45− samples were similar (95% C.I., 0.301-1.00). DNA was harvested from sorted populations of CD45+ and CD45− cells from 12/15 dogs and PARR analysis produced amplicons of identical size from both populations, indicating that 100% (12/12) were of the same lineage, B cell or T cell (95% C.I., 0.757-1.00). Collectively, our data suggests that the CD45− population identified in dogs with lymphoma represents a phenotypic variant of the CD45+ population.