Cancer immunology for the clinician

Cancer immunotherapy is coming of age. It has become abundantly clear that immunotherapy-which has been described as treating the body's immune system so the immune system can treat the cancer-can be routinely effective, and may indeed cure advanced cancers. Accordingly, it is important to understand the basic, clinically relevant principles of cancer immunology to better prepare for an increasingly exciting future. The host immune system is the only active enemy faced by a malignant cell population as it develops. So it is helpful to think of the battle between the cancer cell population and the developing cancer as a Darwinian crucible in which only the malignant cells most fit to thrive in the face of active immune system attack are able to survive in the reluctant host. All successful cancers thus have overcome the defenses mounted by host immune systems by actively thwarting the evolution of anticancer immunity. A malignant cell population that has "solved" the riddle of the host immune system need not employ all of these mechanisms in order to survive in a particular host. Hence, it may be that the dominant mechanism or mechanisms of immune evasion in fact represent potential Achilles' heels that can be therapeutically attacked to restore immune control of a cancer. To better understand where opportunities exist for immunotherapy, it is important to first consider how developing cancers overcome host immunity: by overwhelming, hiding from, subverting, shielding from, defending against, and outlasting the host immune response. Clearly, more than one of these mechanisms may be present in any particular patient, but it is likely that many cancer types employ dominant immune defense mechanisms. There can be no doubt that mobilizing the immune system to attack a cancer, remember the enemy, and continually target emerging clones represents an extremely promising path to cancer prevention and cure.