Among dendritic cells, plasmacytoid dendritic cells (PDC) represent a functionally distinct lineage. Regarding innate immunity, PDC secrete large amounts of type I IFN upon viral exposure or stimulation by microbial products such as unmethylated CpG-motif containing oligo-DNA due to their selective expression of TLR7 and TLR9. We asked whether they could acquire cytotoxic functions during the early phases of infection or after activation with TLR7 or TLR9 agonists. In the present study, we describe a human PDC cell line called GEN2.2, derived from leukemic PDC, that shares most of the phenotypic and functional features of normal PDC. We show that after contact with the influenza virus, GEN2.2, as well as normal PDC, acquires TRAIL and killer activity against TRAIL-sensitive target cells. Moreover, we show that activation of GEN2.2 cells by CpG-motif containing oligo-DNA or R848 also induces TRAIL and endows them with the ability to kill melanoma cells. Therefore, PDC may represent a major component of innate immunity that could participate to the clearance of infected cells and tumor cells. This phenomenon could be relevant for the efficacy of TLR7 or TLR9 agonists in the therapy of infectious disease and cancer.

To assess the sensitivity of primary nonHodgkin lymphoma cells to rituximabmediated cytotoxicity, we compared the potency of several rituximab-mediated killing mechanisms on fresh lymphoma cells. All lymphoma cells tested were equally sensitive to antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-mediated phagocytosis of tumor cells, and rituximab-induced apoptosis. However, they were differentially lysed by complement-dependent cytotoxicity (CDC).We found that taking into account both CD20 and complement regulatory protein expression on tumor cells could predict CDC sensitivity in vitro. Importantly, the sensitivity of lymphoma cells to CDC was consistent with the reported different clinical response rates of lymphomas: rituximab induced high CDC killing of follicular lymphoma cells, whereas mantle cell lymphoma and diffuse large cell lymphoma cells were moderately sensible to CDC, and small lymphocytic lymphoma cells were almost all resistant. We propose that CDC is a determinant mechanism of rituximab-induced killing in vivo. Poor sensitivity to CDC in vitro might predict a poor clinical response, whereas high sensitivity to CDC would only indi-

On recognition of influenza virus (Flu) by TLR7, plasmacytoid dendritic cells (pDCs) produce type I IFN in significant amounts. Synthetic TLR7 ligands induce the maturation of pDCs, as evidenced by the expression of costimulatory molecules and the production of proinflammatory cytokines; however, they induce only lowlevel production of IFN-␣. To dissect the TLR7 signaling in pDCs and how these different profiles are induced, we studied the effects of 2 TLR7 ligands (Flu and CL097) on the activation of blood-isolated pDCs and the human GEN2.2 pDC cell line. Type I IFN production by pDCs correlates with differential interferon regulatory factor 7 (IRF7) translocation into the nucleus induced by the 2 TLR7 ligands. Surprisingly, with both activators we nevertheless observed the rapid expression of the IFN-inducible genes mxa, cxcl10, and trail within 4 hours of stimulation. This expression, controlled by STAT1 phosphorylation, was independent of type I IFN. STAT1 activation was found to be strictly dependent on the PI3K-p38MAPK pathway, showing a new signaling pathway leading to rapid expression of IFNinducible genes after TLR7 triggering. Thus, pDCs, through this unusual TLR7 signaling, have the capacity to promptly respond to viral infection during the early phases of the innate immune response. IntroductionToll-like receptors (TLRs) contain a leucine-rich repeat ectodomain that enables the recognition of pathogen-associated molecular patterns. 1 Among the 10 TLRs identified in humans, TLR7 is the least studied. This TLR binds single-stranded viral RNA, localizes to endosomal compartments, and is particularly expressed by plasmacytoid dendritic cells (pDCs). 2 Ligand binding to TLR7 activates human pDCs that respond by either producing proinflammatory cytokines or substantial levels of type I IFN and activating specific T cells. [3][4][5] In addition, we have recently shown that Fluand TLR7 agonist-activated pDCs express TNF-related apoptosisinducing ligand (TRAIL). This renders them capable of direct cytotoxic activity toward infected and tumor cells. 6 This has been observed in vivo: Stary et al 7 describe the presence of TRAILexpressing and IFN-␣-producing pDCs in tumors after topical use of the TLR7 ligand imiquimod in basal cell carcinoma treatment. Taken together, these data suggest that pDCs represent key target cells in the striking antitumor effects observed with synthetic TLR7 agonists, such as imidazoquinolines, in the treatment of cutaneous virus-induced neoplasia (eg, condyloma-HPV), 8 and other skin tumors, such as basal cell carcinoma. 9 The signaling pathways triggered by TLR7 activation have been partially described. After ligand recognition, endosomal TLR7 interacts with the key adaptor molecule MyD88 (myeloid differentiation primary-response gene 88) that recruits a signal complex comprising IRAK1 (interleukin-1 receptor-associated kinase 1), IRAK4, and TRAF6 (tumor necrosis factor receptor-associated factor 6). TLR7 triggering leads to the activation of NF-B (nuclear factor-B) and MAPKs (mito...