Immunotherapy of a murine B-cell lymphoma by antibodies to idiotypic determinants of its IgM, resulted in surviving tumour-free mice. Several of these mice, however, did develop tumours a long time after the initial inoculation of the tumour cells, or upon rechallenge of the survivors with B-lymphoma cells. The presence of tumour cells during this long latent period may have been due to the development in the host of an anti-idiotype immune response. These late-developing tumours were detected by a radioimmunoassay and characterized by immunofluorescent staining and sodium dodecyl sulphate/polyacrylamide gel electrophoresis. Cells of some late-developing tumours lost the idiotype recognized by the antibodies used for the immunotherapy. Several of these tumours expressed IgM on the cell surface, while others did not, because of the absence of light chains. They were identical in the structure of their rearranged mu chain genes proving their derivation from the original inoculation. Cell lines obtained from the late-developing tumours differed in their tumorigenicity. The appearance of idiotype-negative tumour cells as a result of anti-idiotype immunotherapy has a great impact in our efforts to cure lymphoma by this modality.

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