In vivo release of glycoprotein I from the Ha subline of TA3 murine tumor into ascites fluid and serum.

Previous studies have demonstrated that a unique glycoprotein can be cleaved by trypsin from the plasma membrane of the Ha, but not the St, subline of the TA(3) murine mammary adenocarcinoma. Using an automated quantitative method for measurement of trypsincleaved fragments (glycoprotein fraction I) by inhibition of Vicia graminea lectin hemagglutination, we find evidence that glycoprotein fraction I-like molecules appear in the ascites fluid and serum of the Ha-bearing, but not the St-bearing, syngeneic mice. These molecules were shown by gel filtration to be larger than the trypsincleaved glycoprotein fraction I but have a carbohydrate composition very similar to glycoprotein fraction I. It is likely that these ascites and serum glycoproteins have been released in vivo from the membranes of the viable Ha tumor cells. In view of the ability of the Ha, but not the St, cells to grow in allogeneic recipients, it is possible that these circulating membrane-derived molecules may be playing a blocking role in the immune response to the tumor.