The exonuclease activity of Lassa virus nucleoprotein is involved in 2 APC - mediated NK cell responses

23 24 Lassa virus is an Old World Arenavirus which causes for Lassa hemorrhagic fever in humans 25 mostly in West Africa. Lassa fever is an important public health problem and a safe and effective 26 vaccine is urgently needed. The infection causes immunosuppression, probably due to the 27 absence of activation of antigen-presenting cells (dendritic cells and macrophages), and in low 28 type I IFN production and deficient NK cell function. However, a recombinant Lassa virus 29 carrying D389A and G392A substitutions in the nucleoprotein that abolish the exonuclease 30 activity and IFN activation loses its inhibitory activity and induces strong type I IFN production 31 by dendritic cells and macrophages. We show here that during infection by this mutant Lassa 32 virus, antigen-presenting cells trigger efficient human NK cell responses in vitro including 33 production of IFN-γ and cytotoxicity. NK cell activation involves close contact with both 34 antigen-presenting cells and soluble factors. We report that infected dendritic cells and 35 macrophages express the NKG2D ligands MHC class I-related chains A and B and they may 36 produce IL-12, IL-15 and IL-18, all involved in NK cell functions. NK cell degranulation is 37 significantly increased in cocultures suggesting that NK cells seem to kill infected dendritic cells 38 and macrophages. This work confirms the inhibitory function of Lassa virus nucleoprotein. 39 Importantly, we demonstrate for the first time that Lassa virus nucleoprotein is involved in the 40 inhibition of antigen presenting cell-mediated NK cell responses. 41 42 on A uust 0, 2017 by gest http/jvi.asm .rg/ D ow nladed fom Importance of the study 43 44 The pathogenesis and immune responses induced by Lassa virus are poorly known. Recently, an 45 exonuclease domain contained in the viral nucleoprotein has been shown to be able to inhibit type 46 I IFN response by avoiding the recognition of viral RNA by cell sensors. Here, we studied the 47 responses of NK cells to dendritic cells and macrophages infected with a recombinant Lassa virus 48 in which the exonuclease functions have been abolished and demonstrated that NK cells are 49 strongly activated and presented effective functions. These results show that the strategy 50 developed by Lassa virus to evade innate immunity is also effective on NK cells, explaining the 51 weak NK cell activation observed with the wild-type virus. By providing a better understanding 52 of the interactions between Lassa virus and the host immune system, these results are important 53 for the field of arenavirus biology and may be useful for a vaccine approach against Lassa fever. 54 on A uust 0, 2017 by gest http/jvi.asm .rg/ D ow nladed fom