It has been known for almost twenty years that tumor cells express and present antigens that can elicit an immune response. So why doesn’t our immune system destroy them? It had been suggested that cancers somehow induce tolerance or become less immunogenic as they grow, but neither of these ideas has withstood experimental scrutiny. The fact that a foreign tumor can be implanted into an animal but is not rejected by the host’s immune system suggests that the tumors are probably inducing some of the host’s cells to produce the tolerance.

Stromal cells—normal cells in the connective tissue that typically surrounds a tumor—are well positioned to undercut the immune response. A specific type of stromal cell has been implicated in immune suppression in other contexts; these cells are characterized by their expression of fibroblast activation protein-α (FAP).