When long term cultures of mouse lymphoma cells, known to possess the isoantigenic phenotype determined by the H-2d allele, are incubated with anti H-2d isoantibody and guinea pig complement, slightly more than 99 per cent of cells are killed under optimal conditions. Growth in mass culture and colony formation by single cells after incubation with isoantibody and complement are employed to assess the cytotoxic effect. The cytotoxic action of isoantibody is complement-dependent, for viability of cells exposed to antibody alone is unaltered. When excess isoantibody and optimum concentrations of complement are used, killing begins as soon as these reagents are mixed with the cells, and no further killing occurs after 5 to 15 minutes at 37°C. About 80 per cent of cells are killed with an isoantiserum containing antibody to two isoantigenic components of the H-2d complex. That the cytotoxic action is mediated through the H-2 isoantigen is shown by (a) isoantiserum containing only anti H-2d antibody produces maximal cell killing, and (b) isoantiserum from which anti H-2d antibody has been removed by absorption loses all cytotoxic activity.

Variant cells resistant to the cytotoxic action of anti H-2d isoantibody were isolated from lymphoma cell populations surviving multiple exposures to isoantibody and complement. These variants can be distinguished morphologically from the isoantibody-sensitive parent cell line. Although variants are resistant to anti H-2d isoantibody, these cells possess H-2d isoantigen but in a lower concentration than found in cells of the parent line. The basis for resistance to cytotoxic isoantibody is discussed.

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