Lassa virus causes a hemorrhagic fever endemic in West Africa. The pathogenesis and the immune responses associated with the disease are poorly understood, and no vaccine is available. We followed virological, pathological, and immunological markers associated with fatal and nonfatal Lassa virus infection of cynomolgus monkeys. The clinical picture was characterized by fever, weight loss, depression, and acute respiratory syndrome. Transient thrombocytopenia and lymphopenia, lymphadenopathy, splenomegaly, infiltration of mononuclear cells, and alterations of the liver, lungs, and endothelia were observed. Survivors exhibited fewer lesions and a lower viral load than nonsurvivors. Although all animals developed strong humoral responses, antibodies appeared more rapidly in survivors and were directed against GP 1 , GP 2 , and NP. Type I interferons were detected early after infection in survivors but only during the terminal stages in fatalities. The mRNAs for CXCL10 (IP-10) and CXCL11 (I-TAC) were abundant in peripheral blood mononuclear cells and lymph nodes from infected animals, but plasma interleukin-6 was detected only in fatalities. In survivors, high activated-monocyte counts were followed by a rise in the total number of circulating monocytes. Activated T lymphocytes circulated in survivors, whereas T-cell activation was low and delayed in fatalities. In vitro stimulation with inactivated Lassa virus induced activation of T lymphocytes from all infected monkeys, but only lymphocytes from survivors proliferated. Thus, early and strong immune responses and control of viral replication were associated with recovery, whereas fatal infection was characterized by major alterations of the blood formula and, in organs, weak immune responses and uncontrolled viral replication.

Lassa fever is a major threat in Western Africa. The large number of people living at risk for this disease calls for the development of a vaccine against Lassa virus (LASV). We generated live-attenuated LASV vaccines based on measles virus and Mopeia virus platforms and expressing different LASV antigens, with the aim to develop a vaccine able to protect after a single shot. We compared the efficacy of these vaccines against LASV in cynomolgus monkeys. The vaccines were well tolerated and protected the animals from LASV infection and disease after a single immunization but with varying efficacy. Analysis of the immune responses showed that complete protection was associated with robust secondary T cell and antibody responses against LASV. Transcriptomic and proteomic analyses showed an early activation of innate immunity and T cell priming after immunization with the most effective vaccines, with changes detectable as early as 2 days after immunization. The most efficacious vaccine candidate, a measles vector simultaneously expressing LASV glycoprotein and nucleoprotein, has been selected for further clinical evaluation.

Despite several clinical trials implemented, no antiviral drug could demonstrate efficacy against Ebola virus. In non-human primates, early initiation of polymerase inhibitors favipiravir and remdesivir improves survival, but whether they could be effective in patients is unknown. Here we analyze the impact of antiviral therapy by using a mathematical model that integrates virological and immunological data of 44 cynomolgus macaques, left untreated or treated with favipiravir. We estimate that favipiravir has a ~50% efficacy in blocking viral production, which results in reducing virus growth and cytokine storm while IFNα reduces cell susceptibility to infection. Simulating the effect of delayed initiations of treatment, our model predicts survival rates of 60% for favipiravir and 100% for remdesivir when treatment is initiated within 3 and 4 days post infection, respectively. These results improve the understanding of Ebola immuno-pathogenesis and can help optimize antiviral evaluation in future outbreaks.

IFITMs are broad antiviral factors that block incoming virions in endosomal vesicles, protecting target cells from infection. In the case of HIV-1, we and others reported the existence of an additional antiviral mechanism through which IFITMs lead to the production of virions of reduced infectivity. However, whether this second mechanism of inhibition is unique to HIV or extends to other viruses is currently unknown. To address this question, we have analyzed the susceptibility of a broad spectrum of viruses to the negative imprinting of the virion particles infectivity by IFITMs. The results we have gathered indicate that this second antiviral property of IFITMs extends well beyond HIV and we were able to identify viruses susceptible to the three IFITMs altogether (HIV-1, SIV, MLV, MPMV, VSV, MeV, EBOV, WNV), as well as viruses that displayed a member-specific susceptibility (EBV, DUGV), or were resistant to all IFITMs (HCV, RVFV, MOPV, AAV). The swapping of genetic elements between resistant and susceptible viruses allowed us to point to specificities in the viral mode of assembly, rather than glycoproteins as dominant factors of susceptibility. However, we also show that, contrarily to X4-, R5-tropic HIV-1 envelopes confer resistance against IFITM3, suggesting that viral receptors add an additional layer of complexity in the IFITMs-HIV interplay. Lastly, we show that the overall antiviral effects ascribed to IFITMs during spreading infections, are the result of a bimodal inhibition in which IFITMs act both by protecting target cells from incoming viruses and in driving the production of virions of reduced infectivity. Overall, our study reports for the first time that the negative imprinting of the virion particles infectivity is a conserved antiviral property of IFITMs and establishes IFITMs as a paradigm of restriction factor capable of interfering with two distinct phases of a virus life cycle.

Lassa virus (LASV; Arenaviridae) is responsible for severe hemorrhagic fevers in Africa. LASV nucleoprotein (NP) plays important roles in regulating viral transcription and replication and in inhibiting type I interferon (IFN) production. The NP C-terminal domain contains a 3-to-5 exonuclease activity involved in suppressing IFN induction. We have established a murine polymerase (Pol) I reverse genetics system for LASV, showing that residues D389 and G392 of NP were critical for LASV viability, while the D389A/G392A and D389T/392A double mutants were severely altered in the ability to suppress IFN in macrophages and dendritic cells. Assessing their attenuation in vivo may open new perspectives in vaccinology.Lassa fever is a viral hemorrhagic fever caused by an Old World arenavirus, Lassa virus (LASV; family Arenaviridae) (45) transmitted by infected Mastomys natalensis, a peridomestic rodent (35). It is a major public health concern in regions of endemicity in West Africa and a threat for importation and misuse as a bioterrorism agent in industrial countries (9). The severity of the disease varies from asymptomatic infection to fatal hemorrhagic fever (19,33,34). Whether infection leads to death seems to depend on host immune responses, although the mechanisms involved remain to be clarified. LASV tropism for antigen-presenting cells (APC), such as dendritic cells (DC) and macrophages (MP), in the early stages of infection probably plays a key role in the defective cellular responses observed for severe cases (5,28,29,51). DC and MP massively release LASV but are not activated and do not produce cytokines, except for a modest type I interferon (IFN) production (5, 7).LASV is enveloped and has two single-stranded RNA genome segments (L and S) of ambisense polarity (9). The L segment codes for the small zinc-finger protein Z and for the RNA-dependent RNA polymerase (Pol) L. The S segment codes for the nucleoprotein (NP) and the precursor to the glycoproteins maturated by subtilase SKI-1/S1P (24) into GP1 and GP2. In each segment, the open reading frames (ORFs) are separated by an intergenic region forming a hairpin structure acting as a transcription termination signal for the mRNA synthesis (25,40,41,47). The two RNA segments contribute to virulence. A reassortant exchanging the L segment of LASV with that of the attenuated Mopeia arenavirus (MOPV) is attenuated and protects guinea pigs against a LASV challenge (26, 27). In the S segment, NP plays a role not only in regulating transcription and replication (41) but also in inhibiting type I IFN production by interfering with IFN regulatory factor 3 (IRF-3) activation (32). Reverse genetics systems have been developed for several arenaviruses: lymphocytic choriomeningitis virus (LCMV) (10,13,14,46), Junin virus (1, 13), Pichinde virus (22), and very recently, Lassa virus (2). Singlecycle LCMV (44)-or Junin-based chimeras (3) have been developed as well. Here, we established a reverse genetics system for LASV, enabling us to determine the role of specific amino ac...

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The events leading to death in severe cases of Lassa fever (LF) are unknown. Fatality seems to be linked to high viremia and immunosuppression, and cellular immunity, rather than neutralizing antibodies, appears to be essential for survival. We previously compared Lassa virus (LV) with its genetically close but nonpathogenic homolog Mopeia virus (MV), which was used to model nonfatal LF. We showed that strong and early activation of antigen-presenting cells (

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