Flow cytometric features of lymphoid subsets in healthy and diseased cats

Introduction: Flow cytometry (FC) is widely used in humans and dogs to diagnose and characterize hematopoietic neoplasms. Conversely, its use in feline patients is still limited, leading to a lack of standardized protocols and subjective data interpretation. Methods: Herein, we describe FC features of circulating lymphoid subsets in a total of 20 cats: 9 healthy cats, 6 diseased cats without hematopoietic neoplasm, and 5 cats with probable chronic lymphocytic leukemia (CLL), using a panel of 10 antibodies and a multicolor approach, in terms of both cell size (nFSC) and degree of antigen expression (MFI). Results: Three main subsets were identified in healthy cats and diseased cats without hematopoietic neoplasm (namely, CD5 + CD45R-, CD21 + CD45R + and CD5 + CD45R+). CD4 + CD8- cells outnumbered CD4- CD8 + cells. Low percentages of CD4 + CD8 + and CD134 + cells were also present. MHCII had higher fluorescence intensity in B- than in T-cells. CD9 was not expressed on leukocytes surface, but on small events possibly referable to platelet clumps. In diseased cats without hematopoietic neoplasm, each T-cell subset was larger in size than in healthy cats. Finally, in cats with probable CLL the leading phenotype was CD5 + CD45R-CD4 + CD8-CD134 + MHCII+ and cell size overlapped with the one of the other diseased cats. Discussion: Our results are expected to lay the ground for a more standardized approach to feline samples for FC, and a more objective data interpretation, ultimately leading to improved diagnostic accuracy. Further studies are needed to assess the biological, diagnostic and prognostic value of specific FC patterns in feline medicine.