Calcein release assays were performed as described previously [36] . Briefly, target cells were labeled with Calcein-AM (1% solution in medium) for 30 min at 37uC. After intensive washing the cells were either incubated with 10 mg of exosomes for 4 h or loaded with CLL-specific, HLA-A2-restricted peptides (PU1.423: aa-sequence VLFYLGQYI, mdm2.53: YLAPENGYL and ETV5.45: ELFQDL-SQL) at 20 mg/ml for 1 h or left untreated. The cytotoxicity assays were performed over a time period of 6 h, using an effector-targetratio of 40:1, and the calcein released into the supernatant was quantified with excitation and emission wavelengths of 490 nm and 530 nm, respectively, in a plate-reader (Perkin Elmer, Waltham). All experiments were performed at least three times. The Ebola Virus Glycoprotein and HIV-1 Vpu Employ Different Strategies to Counteract the Antiviral Factor Tetherin The antiviral protein tetherin/BST2/CD317/HM1.24 restricts cellular egress of human immunodeficiency virus (HIV) and of particles mimicking the Ebola virus (EBOV), a hemorrhagic fever virus. The HIV-1 viral protein U (Vpu) and the EBOV-glycoprotein (EBOV-GP) both inhibit tetherin. Here, we compared tetherin counteraction by EBOV-GP and Vpu. We found that EBOV-GP but not Vpu counteracted tetherin from different primate species, indicating that EBOV-GP and Vpu target tetherin differentially. Tetherin interacted with the GP2 subunit of EBOV-GP, which might encode the determinants for tetherin counteraction. Vpu reduced cell surface expression of tetherin while EBOV-GP did not, suggesting that both proteins employ different mechanisms to counteract tetherin. Finally, Marburg virus (MARV)–GP also inhibited tetherin and downregulated tetherin in a cell type–dependent fashion, indicating that tetherin antagonism depends on the cellular source of tetherin. Collectively, our results indicate that EBOV-GP counteracts tetherin by a novel mechanism and that tetherin inhibition is conserved between EBOV-GP and MARV-GP.  Detection of a Fourth Orbivirus Non-Structural Protein The genus Orbivirus includes both insect and tick-borne viruses. The orbivirus genome, composed of 10 segments of dsRNA, encodes 7 structural proteins (VP1–VP7) and 3 non-structural proteins (NS1–NS3). An open reading frame (ORF) that spans almost the entire length of genome segment-9 (Seg-9) encodes VP6 (the viral helicase). However, bioinformatic analysis recently identified an overlapping ORF (ORFX) in Seg-9. We show that ORFX encodes a new non-structural protein, identified here as NS4. Western blotting and confocal fluorescence microscopy, using antibodies raised against recombinant NS4 from Bluetongue virus (BTV, which is insect-borne), or Great Island virus (GIV, which is tick-borne), demonstrate that these proteins are synthesised in BTV or GIV infected mammalian cells, respectively. BTV NS4 is also expressed in Culicoides insect cells. NS4 forms aggregates throughout the cytoplasm as well as in the nucleus, consistent with identification of nuclear localisation signals within the NS4 sequence. Bioinformatic analyses indicate that NS4 contains coiled-coils, is related to proteins that bind nucleic acids, or are associated with membranes and shows similarities to nucleolar protein UTP20 (a processome subunit). Recombinant NS4 of GIV protects dsRNA from degradation by endoribonucleases of the RNAse III family, indicating that it interacts with dsRNA. However, BTV NS4, which is only half the putative size of the GIV NS4, did not protect dsRNA from RNAse III cleavage. NS4 of both GIV and BTV protect DNA from degradation by DNAse. NS4 was found to associate with lipid droplets in cells infected with BTV or GIV or transfected with a plasmid expressing NS4. The genus Orbivirus currently includes twenty two distinct virus species, with genomes composed of 10 segments of linear double stranded RNA (dsRNA), that are vectored by Culicoides midges, ticks, phlebotomine flies, anopheline or culicine mosquitoes. The three economically most important orbiviruses: Bluetongue virus (BTV) (the Orbivirus 'type-species') African horse sickness virus (AHSV) and Epizootic hemorrhagic disease virus (EHDV) are all transmitted by Culicoides biting-midges [1] . Several tick-borne orbiviruses can infect humans, including members of the Changuinola virus, Corriparta virus, Lebombo virus, Orungo virus and Great island virus (GIV) species.