“It also suggests that if the TRegs are only suppressing a subset of effector T cells, there’s something else preventing most effector T cells from, well, effecting.”

There is evidence that some tumors themselves secrete TGF-beta and Il-10. Potentially, T cells infiltrating and trying to attack the tumor may lose their effector function that way.

Maybe the tumor’s microenvironment creates an iron curtain for immune cell infiltration?