Blocking factors against leucocyte-dependent melanoma antibody in the sera of melanoma patients

Summary.-Previous studies, using plasmapheresis to remove blocking factors of cell-mediated cytotoxity to melanoma cells from the circulation of melanoma patients, suggested that leucocyte-dependent antibody to melanoma cells may also be blocked by factors in their sera. The present study confirms these findings, by showing that most patients with disseminated melanoma had melanoma LDA activity in the IgG fraction when this was separated from their sera. This also applied to a high percentage of patients with primary melanoma. Evidence that the blocking factors may be immune complexes was shown by experiments in which LDA activity to melanoma cells was revealed after acidification of melanoma sera to dissociate immune complexes, followed by ultrafiltration through membranes retaining mole-cules of size greater than 100,000 daltons. Blocking of LDA activity in the retentate recurred when the retentate was recombined with the filtrate. Further studies indicated that the blocking activity showed affinity for the target cell and not the effector cell. Preliminary analysis of the specificity of the blocking suggests that this was similar to that of melanoma antisera. These results appear to show that blocking of LDA activity to melanoma cells is common in melanoma patients and that the assay system may provide a quantitative method for their analysis that may yield information of biological importance in the management of melanoma patients. THE presence of blocking factors which inhibit cell-mediated cytotoxicity (CMC) against tumour cells appears to provide one of the more important mechanisms for escape of tumours from the immune cytotoxic response of the host. Serum factors having this activity have been demonstrated in both animal (Hellstr6