Identification of a polyI:C-inducible membrane protein that participates in dendritic cell–mediated natural killer cell activation

Abstract

In myeloid dendritic cells (mDCs), TLR3 is expressed in the endosomal membrane and interacts with the adaptor toll/interleukin 1 receptor homology domain–containing adaptor molecule 1 (TICAM-1; TRIF). TICAM-1 signals culminate in interferon (IFN) regulatory factor (IRF) 3 activation. Co-culture of mDC pretreated with the TLR3 ligand polyI:C and natural killer (NK) cells resulted in NK cell activation. This activation was triggered by cell-to-cell contact but not cytokines. Using expression profiling and gain/loss-of-function analyses of mDC genes, we tried to identify a TICAM-1–inducing membrane protein that participates in mDC-mediated NK activation. Of the nine candidates screened, one contained a tetraspanin-like sequence and satisfied the screening criteria. The protein, referred to as IRF-3–dependent NK-activating molecule (INAM), functioned in both the mDC and NK cell to facilitate NK activation. In the mDC, TICAM-1, IFN promoter stimulator 1, and IRF-3, but not IRF-7, were required for mDC-mediated NK activation. INAM was minimally expressed on NK cells, was up-regulated in response to polyI:C, and contributed to mDC–NK reciprocal activation via its cytoplasmic tail, which was crucial for the activation signal in NK cells. Adoptive transfer of INAM-expressing mDCs into mice implanted with NK-sensitive tumors caused NK-mediated tumor regression. We identify a new pathway for mDC–NK contact-mediated NK activation that is governed by a TLR signal-derived membrane molecule.