Using chemical tests in which individual tumor cells can be tagged with a fluorescent dye that allows them to glow when they are not bound to T cells, Florida researchers measured the immune response in mice to various foreign proteins, with and without injections of MDSCs. They found an 80 percent suppression of the immune response in the presence of MDSCs, confirming that the suppressor cells were inactivating the T cells.
The Florida team then turned to Schneck, who in 1993 developed several novel proteins to test how various antigens, such as those on cancer cells, specifically latch on to T cells.
Researchers then began experiments to determine if the MDSC T-cell interference was simply genetic or had some biochemical explanation, testing a half-dozen major reactions known to occur during infection to see if any set path was particularly active during interference.
In tissue tests from tumor-filled mice bred to lack a biochemical reaction, the scientists found that one specific pathway, the reactive-oxygen species, or ROS pathway, stood out, because when inactivated, T-cell tolerance did not develop. Researchers were surprised when subsequent tests showed that ROS actually modified the T cells, altering their structure so they could no longer bind to tumor-cell antigens.
When a known byproduct of ROS, the chemical peroxynitriate, was neutralized, T-cell tolerance failed to develop in test tube studies, pinning down peroxynitrate as the culprit prohibiting immune cell binding to and marking of foreign tumor cells.