Distinct patterns of virus-specific T cell-mediated cytolysis of transformed versus primary target cells

Ideas about the mechanism(s) by which cytotoxic T lymphocytes (CTL) lyse appropriate target cells are still controversial. We studied the action of established murine CTL clones as well as of freshly prepared primarily induced CTL against two types of target cells. Transformed tumour cell lines (MC57G and L929) and untransformed cells such as peritoneal exudate cells (PEC) or fibroblasts were examined as target cells by scanning and transmission electron microscopy and with the 51Cr-release assay. We found independent of which CTL used, that in transformed cells the cell membrane seemed to be the first target of CTL attack, whereas in untransformed cells the first noticeable events appeared to take place in the nucleus of the target cells; the membrane of attacked untransformed PEC or fibroblasts was found to be intact at a time when the cellular organelles already were disintegrated. The morphological observations were paralleled by differences in the kinetics of 51Cr-release; untransformed target cells released their label only after a 2 h long period, whereas transformed cells released 51Cr considerably earlier.