Detection of circulating tumor antigens in mice carrying a highly metastatic pulmonary squamous--cell carcinoma.

Recently we reported that the highly metastatic MSC-10 (mouse squamous carcinoma) is incapable of inducing transplantation immunity. Studies reported here were undertaken to determine whether or not the tumor is devoid of tumor-associated antigen. We found that sera from MSC-10 tumor-bearing mice contain soluble protein antigens which react with rabbit antisera made against the MSC-10 tumor, as demonstrated by immuno-diffusion. Such proteins were not detected in the sera of normal mice or mice bearing fibrosarcomas. A close temporal relationship was demonstrated between the appearance of circulating antigens and the occurrence of lung metastases. Protein components serologically indistinguishable from the circulating antigens were isolated from tumor cells. The molecular weight of these proteins is between 30,000 and 100,000 daltons. Studies with antisera to mouse leukemia virus showed that hte MSC-10 tumor antigens are not viral proteins. The lack of immunogenicity of this tumor for syngeneic hosts as well as its high metastatic activity may be due to the early appearance of soluble antigens in the circulation.