Soluble tumour-specific antigen and its relationship to tumour growth.

Hooded rats bearing a syngeneic methylcholanthrene-induced tumour were evaluated for extent of in vivo host immunity and this was correlated by in vitro techniques with the levels of circulating tumour antigen and specific antibody. Early tumour growth was associated with detectable immunity, as measured by the capacity of the animal to reject a second direct challenge of the same tumour at a remote site. Radioimmunoassay for circulating tumour antigen and indirect membrane immunofluorescence for antitumour antibody did not detect either component at this stage. Animals with advanced tumours lost immunity as detected by direct tumour challenge, and this closely coincided with the appearance of rising levels of circulating soluble tumour antigen. Although the host possessed the immunologic ability to react against its own neoplasm, this ability was insufficient to produce tumour rejection. Active immunotherapy initiated at the time of, or up to 10 days after, intramuscular challenge with tumour, increased tumour immunity sufficiently for tumour growth to be prevented. Successful immunization was associated with the early appearance (16 days) of measurable levels of antitumour antibody and absence of circulating antigen. It is concluded that soluble tumour antigen present in the local microenvironment of the tumour in the early stages of tumour growth interferes with the ability of immune cells to cause tumour rejection. As the tumour progressively grows, sufficient soluble antigen is produced and released systemically to suppress the effector arm of the host's tumour immune response at distant sites. The levels of circulating soluble tumour antigen attained may be of critical importance in the suppression of rejection responses that prevent metastasis.