Preferential expression of IgG1 antibodies specific for L2C leukaemia IgM idiotypic determinants in tumour-protected strain 2 guinea-pigs.

Immunization of strain 2 guinea-pigs with 10 syngeneic Ia L2C leukaemia cells in adjuvant leads to L2C tumour protection. After subsequent challenges with L2C tumour cells, the sera of twelve out of seventy protected guinea-pigs had detectable L2C reactivity as determined by a [I]-protein A binding assay. The antibodies bound equally well to Ia and Ia L2C tumour cells, but did not bind to L1 and L10 guinea-pig hepatocarcinoma cells or normal guinea-pig B and T lymphocytes. The binding was blocked appreciably by F(ab') reagents specific for the L2C IgM idiotype but not by those specific for Ia or B.1 alloantigens or β microglobulin. These results lead to provisional identification of anti-idiotype among the syngeneic antibody population. After ion-exchange chromatography, the L2C reactivity in eleven of the twelve immune sera analysed was exclusively in the IgG1 fraction. The syngeneic anti-idiotypic antibodies precipitated only IgM molecules from the NP-40 extracts of L2C tumour cells and were dissociated from the L2C leukaemia cells more readily than the xenogeneic anti-idiotypic antibodies at pH 6.5 and 6.0. These results suggest that the L2C IgM idiotype may function as a tumour-associated antigen or is near the antigenic complex recognized by the low affinity L2C antibodies. The preferential expression of IgG1 antibodies suggests that humoral immunity effects a minimum level of protection because this isotype, in the guinea-pig, has a restricted capacity to mediate tumour rejection by secondary immune mechanisms.