Significance of serum factors modifying cellular immune responses to growing tumours.

Lymphocytes from hepatoma bearing rats are cytotoxic for cells of the corresponding hepatoma and this reactivity can be specifically blocked by pretreating target cells with tumour bearer serum. This blocking activity is unlikely to be mediated by circulating tumour specific antibody, since none is detectable by cytotoxic assay in these sera. In contrast, serum from tumour immune rats is cytotoxic for plated hepatoma cells, but in the absence of complement these sera block lymphocyte cytotoxicity, findings which suggest that blocking by humoral antibody may not be a significant factor in modifying tumour growth. It has also been established that immune complexes prepared by adding hepatoma immune serum to solubilized tumour specific antigen can block target hepatoma cells from lymphocyte attack and this is probably the mechanism of blocking by tumour bearer serum. More significantly, lymph node cell cytotoxicity is specifically inhibited following incubation with tumour bearer serum. Similar lymphocyte inhibition is obtained by pretreating effector cells with solubilized hepatoma specific antigen. This suggests that the activity of tumour bearer serum may be effected by circulating tumour antigen or immune complexes and in support of this, there is evidence for these factors in the sera studied. These investigations indicate that inhibition of lymphocyte reactivity by tumour bearer serum is of a complex nature, but probably the most relevant with regard to immune responses is the tumour-antigen mediated inhibition of lymphocyte reactivity.