The specificity of a lymphocyte-mediated in vitro cytotoxic reaction was examined in a series of patients with primary intracranial tumors. Specific tumor-directed LMC was observed in 85% (35/41) of patients with gliomas and in 96% (24/25) of patients with nonglial tumors (meningiomas and acoustic neurilomas). The tumor specificity of these reactions was affirmed by testing the lymphocytes of each patient and the controls against nine to 12 different target cells. These target cells, each from an early passage culture, included both neoplastic and normal cells from the patient himself, as well as allogeneic tumor cells of types both related and unrelated to the patient's tumor. The specificity was further affirmed by showing that the LMC could be abolished by prior absorption of the lymphocytes on appropriate cell monolayers.

The direct assays, in concert with the absorption studies, showed that, in glioma patients, the LMC was directed against two different determinants. One was found on cells from all glial tumors, regardless of their degree of anaplasia, and was called the common glioma antigen (CGA). The other was expressed on anaplastic gliomas, melanomas, and fetal glial cells, but not on well-differentiated gliomas, normal adult glial cells, fetal fibroblasts, or other tumors. This distribution of the second antigen was believed due to the common origin of melanocytes and glial cells from the ectodermal cells of the embryonic neural tube, and it was thus called the glioembryonic antigen (GEA).

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