The development of the most successful
cancer immunotherapies in solid
tumors,
immune-checkpoint blockade, has focused on factors regulating T-cell activation. Until recently, the field has maintained a predominately T-cell centric view of
immunotherapy, leaving aside the impact of innate immunity and especially myeloid cells. Dendritic cells (DC) are dominant partners of T cells, necessary for initiation of adaptive immune responses. Emerging evidence supports a broader role for DCs in
tumors including the maintenance and support of effector functions during T-cell responses. This relationship is evidenced by the association of activated DCs with
immune-checkpoint blockade responses and transcriptional analysis of responding
tumors demonstrating the presence of type I IFN transcripts and DC relevant
chemokines. T-cell-inflamed
tumors preferentially respond to
immunotherapies compared with non-T-cell-inflamed
tumors and this model suggests a potentially modifiable spectrum of
tumor microenvironmental immunity. Although host and commensal factors may limit the T-cell-inflamed phenotype,
tumor cell intrinsic factors are gaining prominence as therapeutic targets. For example,
tumor WNT/β-
catenin signaling inhibits production of
chemokine gradients and blocking DC recruitment to
tumors. Conversely, mechanisms of innate immune
nucleic acid sensing, normally operative during pathogen response, may enhance DC accumulation and make
tumors more susceptible to
cancer immunotherapy. Elucidating mechanisms whereby DCs infiltrate and become activated within
tumors may provide new opportunities for therapeutic intervention. Conceptually, this would facilitate conversion of non-T-cell-inflamed to T-cell-inflamed states or overcome secondary resistance mechanisms in T-cell-inflamed
tumors, expanding the proportion of patients who benefit from
cancer immunotherapy.