Failure at the Effector Phase: Immune Barriers at the Level of the Melanoma Tumor Microenvironment

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Abstract
The clinical investigation of numerous therapeutic cancer vaccine strategies has resulted in relative disappointment. Whereas a minority of patients have indeed experienced clinical benefit, the majority of patients show disease progression even in cases in which induction of functional tumor antigen–specific T-cell responses as measured in the blood is easily detected. This observation has led to interrogation of the tumor microenvironment for potential mechanisms of tumor resistance to the effector phase of the antitumor T-cell response. Poor chemokine-mediated trafficking of effector cells and the action of negative regulatory pathways that inhibit T-cell function have been identified as key limiting factors. Important negative regulatory pathways include T-cell anergy from insufficient B7 costimulation, extrinsic suppression by regulatory T-cell populations, direct inhibition through inhibitory ligands such as PD-L1, and metabolic dysregulation such as through the activity of indoleamine 2,3-dioxygenase. Recognition of these evasion mechanisms has pointed toward new therapeutic approaches for cancer immunotherapy.