Tumor-Infiltrating γδ T Cells Suppress T and Dendritic Cell Function via Mechanisms Controlled by a Unique Toll-like Receptor Signaling Pathway

γδ T cells are important contributors to innate immunity against cancer, but their regulatory role in controlling immune responses remains largely unknown. Here we report that a dominant γδ1 T cell population among lymphocytes infiltrating breast tumors possessed a potent ability to suppress naive and effector T cell responses and to block the maturation and function of dendritic cells. Adoptive cotransfer experiments demonstrated their in vivo suppressive activity. However, their immunosuppressive activity could be reversed by human Toll-like receptor (TLR) 8 ligands both in vitro and in vivo. siRNA-mediated knockdown experiments revealed that MyD88, TRAF6, IKKα IKKβ, and p38α molecules in γδ1 cells were required for these cells to respond to TLR8 ligands, whereas TAK1, JNK, and ERK molecules did not appear to be involved in functional regulation. These results provide new insights into the regulatory mechanisms of tumor-specific γδ T cells and identify a unique TLR8 signaling pathway linking to their functional regulation.