Flow cytometry expression pattern of CD44 and CD18 markers on feline leukocytes

The paucity of specific feline antibodies for flow cytometry (FC) is an ongoing challenge. Flow cytometrists must extrapolate information from relatively few markers. We evaluated the expression pattern of the panleukocyte markers CD18 and CD44 on leukocyte (white blood cell, WBC) subclasses in the peripheral blood (PB) of 14 healthy cats. The degree of expression of CD18 and CD44 was calculated as the ratio between the median fluorescence intensity (MFI) value of antibody-stained cells and autofluorescence. All samples were acquired with the same cytometer with constant photomultiplier setting and compensation matrices. Both molecules were expressed at higher levels on monocytes, intermediate levels on polymorphonuclear cells (PMNs), and lower levels on lymphocytes. CD18-MFI discriminated well among the 3 populations, whereas CD44-MFI mostly overlapped between monocytes and PMNs. However, CD44-MFI had a lower intra-population variability. Evaluation of CD18 and CD44, together with morphologic parameters, was useful for discriminating among WBC subclasses in healthy cats. This information may be helpful for future studies given that an increase in CD18-MFI may indicate reactive changes, whereas fluctuations in CD44-MFI may suggest neoplasia.