Natural killer cells: frequency, phenotype and function in healthy cats.

Natural killer (NK) cells play a central role in innate immunity and have been shown to influence adaptive immune responses as well. This study aimed to provide a general NK cell quantification and phenotyping in several compartments of healthy cats and assess their functional properties. The results indicated that NK numbers, both absolute and relative, and phenotype mostly correspond with those found in bovine, ovine, human and murine immunology. However, there were also distinct differences, especially with regard to the expression of the integrin CD11b and the selectin CD62L (between 10 and 30% of feline NK cells stain positive for these markers) and the relative frequencies in lymph nodes (6.7%), which stand central in NK cell development. Caution should be taken when extrapolating findings on NK cell properties over species, notwithstanding the generally accepted evolutionary conservation of NK cells and their subtypes. It was also shown that K562 cells, the 'golden' target cell line for NK functionality tests did not work for feline cells. The feline kidney cell line CRFK proved to be very responsive to NK- and NKT-mediated lysis and therefore, represents an ideal alternative target. This study is a good reference for NK cell numbers, both absolute and relative, phenotype and function in several anatomical compartments of healthy cats and for cat-specific cytotoxic assays involving both NK and NKT cells.