An immunodominant MHC class II-restricted tumor antigen is conformation dependent and binds to the endoplasmic reticulum chaperone, calreticulin

There is accumulating evidence that CD4(+) T cell responses are important i n antitumor immunity. Accordingly, we generated CD4(+) T cells against the murine CT26 colon cancer. Three of three independent CT26-specific CD4(+) h ybridomas were found to recognize the high m.w. precursor of the env gene p roduct gp90. The CD4(+) response was completely tumor specific in that the same glycoprotein expressed by other tumors was not recognized by the CT26- specific hybridomas. The recognition of gp90 by the hybridomas was strictly dependent on the conformation of gp90. Different procedures that disrupted the conformation of the glycoprotein, such as disulfide bond reduction and thermal denaturation, completely abrogated recognition of gp90 by all thre e hybridomas. In CT26 cells, but not in other tumor cells tested, a large p roportion of gp90 was retained in the endoplasmic reticulum, mostly bound t o the endoplasmic reticulum. chaperone, calreticulin. Although calreticulin was not essential for the stimulation of the gp90-specific hybridomas, mos t of the antigenic form of gp90 was bound to it. The antigenicity of gp90 c orrelated well with calreticulin binding, reflecting the fact that specific ity of binding of calreticulin to its substrate required posttranslational modifications that were also necessary for the generation of this tumor-spe cific CD4(+) epitope.