The observation that most cellular responses occurred within 1 h after exposure to Ebola virions suggested that virion binding/ Figure 5 ) confirmed that at 1 h cRNA amounts were below that of genomic RNA, whereas after 6 h cRNA was greatly increased. LPS served as a positive control for the responsiveness of the macrophages used in the virion exposure studies. A comparison of the effects of Ebola virion exposure versus LPS treatment revealed that the majority of genes responded similarly (Supplemental Table 1 ), further substantiating the hypothesis that the observed responses with Ebola virions were triggered by binding and/or entry. To further investigate which changes in expression levels in macrophages occur due to virion binding/entry, macrophage responses following exposure with infectious virions were directly compared to exposure to Ebola virus-like particles (VLPs) lacking viral genetic material and containing only the ZEBOV matrix protein VP40 and the ZEBOV spike glycoprotein GP 1,2 (VLP VP40-GP ) (see also [19] ). GP 1,2 is the only known surface-exposed structural component of Ebola virions and contains the cell-surface receptor-binding domain [30, 31] . We therefore hypothesized that if virion binding is responsible for the observed macrophage responses, then these effects would be mediated by GP 1,2 . To evaluate the role of GP 1,2 , VLPs consisting of VP40, but lacking GP 1,2 (VLP VP40 ), were tested in parallel with VLP VP40-GP . VLP VP40 particles were shown previously to be morphologically similar to the GP 1,2 -containing VLPs [32, 33] . LPS was again used as a control for macrophage responsiveness and also to directly compare the LPS-induced responses to those of Ebola virion and VLP exposures. Additionally, cells were exposed to latex particles, which were used in approximate equal quantity to VLPs. Ebola VLP preparations were tested by negative-stain electron microscopy for authentic appearance and presence (VLP VP40-GP ) or absence (VLP VP40 ) of GP 1,2 (data not shown). Particles resembled those previously described [32, 33] . In addition, Ebola virion, VLP, and latex preparations were tested for endotoxin-contamination before incubation with macrophages from donors D4, D5, and D6. The endotoxin concentrations of all samples used in this study were not above those of the tissue culture media (,0.5 U/ml). To determine whether the quantitative differences in gene expression levels between cells of donors D1-D3 observed in the first experiment were due to the MOI used, a higher MOI of Ebola virions (<100) was used for this experiment. Cells were exposed to VLPs and latex particles at a concentration of 100 particles per cell. LPS was used at the same concentration as in the first experiment (10 ng/ml), which allowed for a direct comparison of the responsiveness of macrophages used in both experiments. The results revealed that while a higher MOI did increase the overall intensity of responses compared to the first experiments (compare to Figure 4) , it did not eliminate or noticeably reduce the degree of variation of gene expression levels between donors ( Figure 6C &  6D ). This suggests that genetic variation among identical cell types of different donors may have an influence on the effect of ZEBOV infection and therefore may influence chances of survival.