The mechanism of T cell mediated cytotoxicity III. Changes in target cell susceptibility during the cell cycle

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Abstract
Sedimentation velocity techniques have been used to fractionate cells according to cell cycle phase. Mastocytoma P815 shows higher susceptibility to specific anti-H2d cytotoxic T cells during the early phases of the cycle. The changes in H2d antigen density as measured by susceptibility to limiting dilutions of anti-H2d and complement vary independently from the changes in susceptibility to T cells. Further there is no change in P815 susceptibility to antibody-dependent cytotoxicity by K cells during the cell cycle. A methylcholanthrene induced tumour (Meth A) is less susceptible to cytotoxic T cells than is P815. This is shown not to be due to differences in H2d antigen density. Fractionation of this cell line indicated that suscep­tibility to cytotoxic T cells did not change significantly during the cell cycle. It is suggested that differences in susceptibility between different cell lines are found mainly in the early phases of the cell cycle. Although the causes of the changes in susceptibility remain unexplained, these obser­vations point to an important contribution by the target cell, apart from the possession of the relevant antigen, to the events leading to lysis by T cells.