Immune response to a syngeneic rat tumour: evolution of serum cytotoxicity and blockade

The development of serum factors by Wistar rats during the growth of a syngeneic squamous cell carcinoma has been investigated to clarify the nature of the local lymphocyte anergy reported previously in this system. Sera from tumour bearing animals were tested for cytotoxicity against tumour cells by in vitro microassay, and their ability to inhibit cell mediated cytotoxicity was also studied. Serum cytotoxicity was first detected after 2 weeks of tumour growth, reaching a peak at 4 weeks and then declining. Inhibitory activity was found only in the sera of animals with advanced tumours. Anti-tumour antibody either in the sera or bound to tumour cells was not detected by immunofluorescence techniques. No evidence of general immunological debilitation was found, the animals showing normal immune responses to sheep erythrocytes and killed Brucella abortus organisms throughout tumour growth. Serum inhibitory factors may be responsible for the decline in anti-tumour immunoreactivity and the local lymphocyte anergy observed in tumour bearing hosts.