The natural killer (NK) cell activity of splenocytes and recycling capacity of NK cells were observed by combining the 51Cr-release cytotoxicity assay and single cell cytotoxicity assay against YAC-1. The ICR mice were infected intranasally with Naegleria fowleri, that is a pathogenic free-living amoeba. The mice infected with 1 x 10(5) trophozoites showed mortality rate of 76.7% and mean survival time of 12.9 days. The cytotoxic activity of NK cells in infected mice was significantly higher than that of non-infected mice during the period between 12 hours and day 3 after infection, and highest on day 1. The target-binding capacity of NK cells in infected mice was not different from that of non-infected ones. Maximal killing potential and maximal recycling capacity were remarkably increased in infected mice as compared with the control. The results obtained in this observation indicated that elevated NK cell activity in mice infected with N. fowleri was not due to target-binding capacity of NK cells but due to the increased activity of NK cells and increased recycling capacity of individual NK cells.
