Cytotoxic T lymphocytes have been thought to lyse cellular targets in the past by a calcium-dependent pathway. This notion was recently supported by the identification and purification of a pore-forming protein (perforin) from the granules of these cell types. Here, we show that perforin is absent from a number of cell lines that nevertheless display vigorous cytolytic activity toward target cells. The cytotoxic activity of eight murine CTL lines is completely or partially retained in the absence of calcium. The calcium-independent lytic activity is associated with two subcellular fraction peaks isolated by Percoll gradient centrifugation, e.g., a heavy density band migrating with granule markers and a lighter band corresponding to free cytosolic material. These results suggest a complex picture of lymphocyte-mediated killing involving probably multiple mechanisms and mediators that may operate in concert or independently in the delivery of the lethal hit.

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