Humoral response against murine lymphoma cells xenogenized by drug treatment in vivo

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Abstract
Treatment of murine lymphomas with triazene derivatives may lead to the appearance of novel drugmediated tumor antigens, a phenomenon known as chemical xenogenization. Such antigens, which are capable of eliciting specific transplantation resistance in histocompatible mice, have been previously detected by in vivo and in vitro cell‐mediated immune responses. In the present report we address the question of the humoral antibody response to a chemically xenogenized lymphoma. Histocompatible mice were given several injections of live cells of the xenogenized tumor. Ten days after each immunization, pooled sera from different animals were analyzed for Ab content by means of flow microfluorometry analysis and CELISA assay. The results reveal that antibodies of both IgG and IgM classes capable of binding the xenogenized tumor can already be detected after one single sensitization. However, the Ab titer gradually increases through subsequent immunizations, reaching a peak level after 3–4 injections at a time when most of the humoral response is made up of antibodies of IgG class. The specificity of the anti‐xenogenized tumor hyperimmune sera was subsequently investigated by its reaction with the parental, nonxenogenized line and with normal tissue cells of the same or allogeneic haplotypes. The data obtained point out that cross‐reactivity with the parental line could be completely removed by absorption of the hyperimmune sera on parental cells, which removed most of the IgM antibodies. Moreover, the presence of an excess of anti‐parental antibodies on the xenogenized tumor cells does not prevent the subsequent binding of the hyperimmune absorbed serum, thus indicating that the novel determinant(s) recognized on xenogenized cells are not spatially related to those shared with the original parental tumor. In addition, the hyperimmune absorbed serum does not crossreact with normal hemopoietic or lymphoid cells of the same (H‐r, or allogeneic H‐t> and H‐r haplotypes. Furthermore, no alien histocompatibility antigens of H‐t> or H‐r haplotypes could be detected on the xenogenized tumor cell surface. Taken together, these data provide evidence that chemical xenogenization of a murine lymphoma leads to the appearance of novel determinant(s) detectable by specific antibodies.