Human umbilical cord blood NK T cells kill tumors by multiple cytotoxic mechanisms.

Natural killer (NK) T cells are restricted by CD1d and play an important role in the rejection of malignant tumors, but how kill these tumors is unclear. To investigate this, we cultured Valpha24+CD4+ NK T cells in human umbilical cord blood, which was enriched by immunomagnetic beads. In short-term (4 h) cytotoxicity assays, the NK T cells could kill only those targets expressing CD1d. In longer cytotoxicity assays (20 h), however, the NK T cells were able to kill all the tumors, regardless of CD1d expression. When each of the perforin, Fas-FasL, and TNF-alpha cytotoxic mechanisms were blocked, it was apparent that perforin killing dominated in both the short- and long-term assays. In the short-term assay, perforin killing required that the targets expressed CD1d, but killing was more efficient if Fas was present because then the Fas-FasL mechanism was also used. Thus, cells that lacked Fas and CD1d and were not killed in the 4-h assay, were instead lysed in 20-h assay through a combination of perforin and TNF-alpha killing. NK T cells can kill tumor targets by perforin, Fas-FasL, and TNF-alpha mechanisms. TNF-alpha killing requires longer contact between effectors and targets, suggesting that TNF-alpha acts by enhancing perforin killing.