Lassa virus is a rat-borne virus endemic to West Africa. Characterized by an acute hemorrhagic illness known commonly as Lassa fever, Lassa virus has become a serious health concern in the African region. Approximately 100,000 to 300,000 infections occur each year which results in around 5,000 death annually. However, despite the virus’s extraordinary pervasiveness, no vaccine has been developed in the 50 years since its discovery.
While there are measures besides vaccine distribution that public health officials can employ to prevent infections, Lassa virus remains prevalent for several reasons. One of the most significant issues is the population’s high contact with rats. The natal multimammate rat, or African rat, acts as a reservoir host for the virus, allowing it to replicate before it spreads among humans. Because controlling the rat population is so difficult, officials have instead attempted to enact public education campaigns which encourage personal hygiene to diminish human-to-human transmission as well as promote rodent precaution. However, this has not prevented recent large outbreaks of the disease, with some mortality rates of up to 40% (1). With an estimated 180 million people living at risk of the disease (2), efforts to procure a vaccine after such outbreaks have become more urgent. This study, conducted by the Pasteur Institute and the University of Lyon, provides exciting progress in the development of a Lassa virus vaccine.
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| Nigeria Centre for Disease Control "How to prevent Lassa Fever." January 29, 2019, https://ncdc.gov.ng/news/206/lassa-fever-public-health-advisory. Accessed December 8, 2019 |
Generally speaking, vaccination is the manipulation of a patient’s immune system to combat against a specific pathogen. To accomplish this, the vaccine must contain substances derived from the pathogen called antigens. White blood cells uptake these antigens to form antibodies that can bind to the pathogen at molecules on its surface called epitopes. This binding signals a response from the immune system to attack the pathogen. Preliminarily generating antibodies against the pathogen by vaccination thereby ensures that a patient’s immune system can effectively defend against it.
Attempts to produce a vaccine against Lassa virus have been largely hindered by the high genetic diversity of the virus strains (3). As is the case for many viruses, high mutation rates result in the development of epitopes on the virus surface to which antibodies from the immune system can’t bind. To address this issue, researchers in this study tested three antigens for the virus: Lassa virus glycoproteins (GPC), nucleoproteins (NP), and Z proteins (4). The researchers also tested two virus platforms which enabled the expression of multiple viral antigens: Schwarz measles (MeV) and MOPEVAC. These platforms were both live-attenuated viruses, which are living but weakened versions of the virus that allow researchers to express virus antigens for immune targeting. With various Lassa antigens to choose from and two viral platforms to express them on, the researchers began building and testing the effectiveness of potential vaccines.
Vectors are organisms that do not cause disease but can infect host cells and cause an immune response. Five MeV vectors and one MOPEVAC vector expressing different viral antigens were generated for the study (Figure 1A). These were successfully grown in a cell line (Figure 1B) and all desired antigens were found to be present in the vectors (Figure 1C). All vectors induced an immune response from infected cells, as measured by the production of defensive proteins called INF’s (Figure 1D). The vectors also triggered the production of activation molecules in antigen-presenting cells (APCs) (CD8+ and CD4+) (Figure 1E). These measures confirmed to the researchers that the viral vectors were expressing the correct antigens and inducing an immune response from cells prior to their introduction into live hosts.
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| Figure 1. Characterization of Viral Vectors for Lassa Virus Vaccine. (A) A depiction of vector DNA for MeV-based vaccines. (B) Growth kinetics of vaccines in cells, as measured by viral concentration through viral titer. (C) Antigen expression in supernatants and cell extracts (cells) of infected Vero NK cells analyzed by Western blot. (D) Induction of IFN expression in human dendritic cells (left graph) and macrophages (right graph) 1 day after infection by the MeV-based constructs. (E) Cell surface expression of coactivation molecules in primary human monocyte-derived dendritic cells (left graph) and macrophages (right graph) at day 2 after infection. |
After the successful development of vaccine candidates for Lassa virus, the vaccines were injected into three groups of four cynomolgus monkeys, otherwise known as macaques, by a single shot to observe immunization against Lassa. Immunization was assessed by measuring the presence of Lassa-specific IgM and IgG antibodies, with their development occurring in most of the vectors (Figure 2B). They also monitored the presence of Lassa-specific antigens in immune cells (CD8+ and CD4+), observing the best immune response from the MeV-NP vector, which consisted of the MeV viral platform expressing the NP Lassa antigen (Figure 2C). After vaccine injection, the biological parameters of the animals (body temperature, weight, and respiratory rate) and saw no adverse effects, indicating that these vaccines did not cause Lassa disease in the subjects.
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| Figure 2. Immunization of Cynomolgus Monkeys with Vaccine Candidates. (B) Detection of MeV-specific IgM and IgG in plasma according to the time after immunization. (C) Quantification of CD8+ (left graphs) and CD4+ (right graphs) cells specific for LASV GPC (top graphs), NP (middle graphs), and Z (bottom graphs) peptides. Percentage of cells producing antibodies in response to vectors among total cells is presented according to the time after immunization. |
The last step in assessing the potential vaccine candidates was infecting the immunized macaques with unaltered Lassa virus and observing their immune resistance. Three unimmunized macaques infected with the virus served as controls. These subjects presented severe symptoms, like high fevers, and were euthanized two weeks after virus introduction (Figure 3A, B). Blood tests were used to assess tissue damage and inflammation in both immunized and unimmunized macaques. Unimmunized subjects demonstrated increased disease indicators across all four parameters assessed while the immunized subjects demonstrated low disease indicators for the parameters (Figure 3C). This assessment found the MeV-NP vector to be the best vaccine candidate, as its blood tests for all disease parameters were remarkably lower and consistent for each subject when compared to the other two vectors tested. Further monitoring of viral replication and immune response in the subjects also revealed the MeV-NP vector to have performed the best as a vaccine candidate.
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| Figure 3. Clinical Monitoring of Immunized Macaques After Lassa Virus Introduction. (A) A clinical score of individual animals was assigned with respect to time in weeks after Lassa infection. (B) Monitoring of body temperature during Lassa infection. Individual data are presented for each animal. (C) Analysis of four biological disease parameters during Lassa infection as measured by blood concentration: alanine aminotransferase (ALT), aspartate aminotransferase (AST), lactate dehydrogenase (LDH), and C-reactive protein (CRP) (Top to Bottom). |
This study offers extraordinarily promising results for the development of a robust Lassa virus vaccine. Given that nearly all the vaccine candidates exhibited strong immunity when compared to the control, the vector platforms tested appear to be well suited for vaccine development, especially MeV-NP. Immunization by a single shot is also a significant advancement, as it greatly improves potential for mass vaccination. To verify the findings of this study and improve upon it, future research should refine the successful vaccine candidates and test them in other primate comparable to humans.
After decades of Lassa virus devastating West Africa and infecting millions of people, the introduction of a Lassa vaccine could mean saving hundreds of thousands of lives. However, while the immunized macaques demonstrated incredible immunity to the virus, there is a long and strenuous process of testing potential vaccines before distribution among human populations. Still, with motivation from the scientific community as well as help from groups like the World Health Organization, a vaccine for Lassa virus will hopefully be saving humans, rather than macaques, very soon.
1. Roberts, L. (2018). Nigeria hit by unprecedented Lassa fever outbreak. Science, 359, 1201–1202, doi: 10.1126/science.359.6381.1201
2. Fichet-Calvet, E. and Rogers D.J. (2009). Risk maps of Lassa fever in West Africa. PLoS Negl Trop Dis., 3(3): e388, doi: 10.1371/journal.pntd.0000388
3. Lukashevich I.S. and Pushko P. (2016). Vaccine platforms to control Lassa fever. Expert Rev. Vaccines, 15(9):1135-1150, doi: 10.1080/14760584.2016.1184575
4. Fisher-Hoch, S.P., Hutwagner, L., Brown, B., and McCormick, J.B. (2000). Effective vaccine for Lassa fever. Journal of Virology, 74, 6777–6783, doi: 10.1128/JVI.74.15.6777-6783.2000
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