Tumor-specific T cells can be detected in the blood and the tumors of many melanoma patients, and yet these cells are unable to kill the tumor. What causes the impotence of these T cells is a mystery. Equally mysterious is why vaccination against tumor-specific proteins sometimes causes tumor regression without expanding large numbers of vaccine-specific killer T cells.
Pierre Coulie's group studied anti-tumor T cells in patients vaccinated with a tumor antigen called MAGE-3. In one patient whose tumor regressed after vaccination, the authors found significantly more T cells specific for non-vaccine tumor proteins than were detected before vaccination. Vaccine-specific T cells, on the other hand, became detectable but did not expand to large numbers. Thus, reinvigoration of existing tumor-specific T cells after vaccination did not require large numbers of vaccine-specific T cells.
Although it is not known how these tumor-specific cells get activated, Coulie thinks that the few T cells stimulated by the vaccine may change the local, suppressive environment of the tumor such that other T cells can snap out of their stupor and attack the tumor.