Assessment of feline peripheral blood lymphocyte subpopulations and CD18 expression pattern by flow cytometry

Flow cytometry-based immunophenotyping plays a critical role in the diagnosis, classification, staging, and monitoring of hematopoietic malignancies. However, its application in feline medicine remains limited primarily because of the scarcity of species-specific antibodies. Existing studies in cats predominantly focus on analyzing lymphocyte subpopulations in lymph nodes, with fewer investigations conducted on bone marrow and peripheral blood. This study aimed to assess the utility of peripheral blood samples from healthy cats for quantifying lymphocyte subpopulations by flow cytometry to provide basic data useful for future studies on animals with diseases. Using specific antibody panels, we successfully quantified lymphocyte subpopulations in 15 healthy cats. Total leukocytes (CD18+) accounted for a median of 89.7% of peripheral blood cells. Among lymphocytes, CD5+ T cells were the predominant subset, followed by CD21+ B cells. Among T cells, CD4+ helper cells outnumbered CD8+ cytotoxic cells. Notably, CD18 expression exhibited a biphasic pattern: B lymphocytes showed lower fluorescence intensity compared with T lymphocytes. In some cases, three distinct fluorescence peaks suggested further heterogeneity within the T-cell population. To the best of our knowledge, this is the first study to clearly identify a biphasic expression pattern of CD18 in the peripheral blood lymphocytes of cats. These findings reveal the complexity of immune cell marker expression in the peripheral blood of felines and support the broader application of flow cytometry in feline immunological and diagnostic research. The establishment of these baseline immunophenotypic profiles in healthy cats is crucial for improving the diagnosis and monitoring of feline hematopoietic diseases, ultimately contributing to better clinical management and therapeutic strategies.