Progress toward the development of Lassa vaccines

KEYWORDS:

• Mammarenavirus
• arenavirus
• arenaviridae
• Lassa virus
• Junín virus
• Pichindé virus

Mammarenaviruses (also known simply as arenaviruses) originate primarily in rodents and are divided into Old World (OW) and New World (NW) groups based on their geographical, serological, and phylogenetic (sequence) differences. Several members from both groups can cause severe and lethal viral hemorrhagic fevers (VHFs) in humans, such as Lassa fever (LF) caused by Lassa virus (LASV) and Argentine hemorrhagic fever caused by Junín virus (JUNV). LASV infection is endemic in West Africa that can cause up to 300,000 cases and 5,000 deaths annually, which ranks second only to Dengue fever in terms of VHF global health burden. While the overall fatality rate of LF is estimated to be only about 1%, it can be much higher in hospitalized patients (~18%) and during some outbreaks (25%) (reviewed by Murphy and Ly [1]).

Multiple vaccine platforms have been explored for use to develop vaccines against LASV that show various degrees of success and failure over the years. Some of them include killed vaccine, subunit, and viral-like particle (VLP) vaccines, as well as strategy to present a stabilized LASV glycoprotein trimer as an antigen (immunogen) on protein nanoparticles, all of which have been shown to provide some levels of protection to various experimental animals against lethal LASV challenge (reviewed by Murphy and Ly [2]). Data gathered from these studies suggest that some of the LASV vaccine candidates can induce a relatively low level of protective neutralizing antibodies (NAbs). This finding correlates well with evidence from recovered Lassa fever patients and experimentally infected animals, which suggests that cell-mediated immunity (CMI) plays a more prominent role in protection than NAbs. As such, some recent efforts have focused on the development of vaccines that can stimulate CMI to protect animals against LASV challenge. These efforts include testing of live-attenuated LASV vaccines that have been shown to provide complete protection against LASV challenge in animals. Other efforts to express LASV antigens via the use of replication-competent viral vectors, including vesicular stomatitis virus (rVSV) and measles virus (rMV), have also demonstrated good protective efficacy in animal models, and some of these vaccines have entered early human clinical trials [2].

Those vaccines that have entered clinical trials include the rVSV-based vaccines expressing the LASV glycoprotein antigen (GPC) [i.e. the rVSVdeltaG-LASV-GPC that was developed by the International AIDS Vaccine Initiative (IAVI) and the Public Health Agency of Canada (PHAC) and another rVSV-based LASV GPC vaccine developed by Emergent BioSolutions, Inc.] that were modeled after the rVSV-EBOV-GP (Ervebo EBOV vaccine) vaccine that was successfully developed and approved by the FDA in 2019 to prevent Ebola virus (EBOV) infections [3]. A recombinant measles virus (rMV) expressing the LASV GPC and nucleoprotein (NP) that was developed by Themis Bioscience and a DNA vaccine encoding the LASV GPC gene (INO-4500) developed by the Inovio Pharmaceuticals were also being evaluated mainly for their safety profiles in healthy adults (18–50 or 55 years old) in phase 1 clinical trials in endemic countries (Liberia, Ghana) and nonendemic countries (U.S.A., Belgium).

Altogether, there are five registered phase 1 trials and one phase 2 trial of Lassa vaccine testing that are currently at various stages of advancement [4]. The rVSVdelta-LASV-GPC vaccine was shown to induce protective immune responses, such as T cell and neutralizing antibody responses against LASV challenge in non-human primates (NHPs) and guinea pigs prior to its entering phase 1 trial. Similarly, the other viral vectored vaccine (rMV-LASV-GP-NP vaccine) has been shown to induce efficient protection against both homologous and heterologous LASV challenges in cynomolgus macaques prior to entering clinical trial. Results from a phase 1 clinical trial of the rMV-LASV showed an acceptable safety and tolerability profile, and immunogenicity seemed to be unaffected by preexisting immunity against the vector [5]. The INO-4500 DNA vaccine has also been shown to protect NHPs and guinea pigs against LASV challenge infection, and some data from its clinical trial showed its ability to induce strong cellular immunity, including an increase in LASV GP-peptide reactive CD4 and CD8 T-cell responses in human volunteers.

Besides those vaccines that have entered clinical trials, other experimental vaccines that include but are not necessarily limited to other recombinant virus-based platforms, such as the chimpanzee adenovirus-vectored vaccine (ChAdOx1-Lassa-GPC), the live attenuated yellow fever virus strain 17D (YF17D)-based Lassa vaccine, and those that use vaccinia virus, rabies virus, alpha virus replicon as viral vaccine platforms are at various stages of development [2]. It was demonstrated that a single vaccinated dose of the ChAdOx1 vector-based vaccine expressing the Josiah LASV GPC, for example, could induce robust T-cell and antibody responses in mice and protected guinea pigs against morbidity and mortality following lethal challenge of the vaccinated animal with a guinea pig-adapted Josiah LASV strain. A prime-and-boost vaccination of this vaccine has also been shown to significantly enhance LASV antigen-specific antibody titers and clear LASV from the tissues of the virus-challenged animals [6].

Another promising vaccine candidate is the live attenuated mammarenavirus ML29. The ML29 reassortant viral vaccine contains the genomic segments of LASV and a related but nonpathogenic arenavirus called Mopea virus MOPV. Carrion and colleagues first tested this ML29 vaccine in inbred strain 13 guinea pigs and showed that animals vaccinated with a single subcutaneous (s.c.) dose of ML29 30 days before challenge with 10³ PFU of LASV provided complete protection against LASV infection [7]. The ML29 vaccine had also been shown to completely protect common marmosets against LASV infection (at 10³ PFU) when a single s.c. dose of 10³ PFU of the ML29 vaccine was used [8]. All the vaccinated marmosets survived the observational period of 35 days after LASV challenge and showed evidence of specific cell-mediated T cell responses as evidenced by increased populations of CD14+ and CD3+ T cells, as well as recruitment of CD3+ T cells and over-expression of HLA-DR, P, and Q in the target tissues. Additionally, injection of a low s.c. dose of ML29 was enough to induce specific cell-mediated T cell responses (as determined by IFNγ and TNF-α ELISPOT) but induced weak antibody (IgG) responses as determined by IgG ELISA.

In addition to ML29, another live-attenuated rLASV-GPC/CD has also been developed using codon deoptimization technique [9]. Codon deoptimization (CD) is a technique that reduces gene expression by substituting the wild-type codons with less-preferred codons across the entire coding sequence of the target gene. Immunization with rLASV-GPC/CD (via a single s.c. injection) of strain 13 inbred or outbred Hartley guinea pigs resulted in 100% survival and no clinical signs of disease. The anti-LASV IgG plasma titers were detected, indicating that those animals were properly vaccinated, despite showing no evidence of viral RNA in the blood or tissues of the vaccinated animals. There were no anti-LASV NAbs detected in any of the immunized animals nor were there any significant histopathological findings or NP antigens detected in the examined organs and tissues, and thus, indicating a complete attenuation of the vaccine. When the vaccinated animals were exposed to a lethal challenge dose of LASV (10⁵ PFU, Josiah strain), they were completely protected. Additionally, the levels of anti-LASV IgG antibody titers were not significantly boosted after challenge, and anti-LASV NAb titers were not detected, implying the insignificant role of NAbs in protection. This finding mirrors other studies, including a recent report that lipid nanoparticle encapsulated, modified mRNA vaccines (mRNA-LNP) that encode the wild-type LASV Josiah GPC or its prefusion stabilized conformation can protection of guinea pigs against LASV challenge despite the lack of NAbs [10].

Recognizing the challenges of vaccine development against LASV, the World Health Organization (WHO) has placed LF on their Blueprint list of priority diseases for vaccine and therapeutic development. Specifically, the 2017 WHO Target Product Profile (TPP) for LF vaccines emphasizes a high priority for the development of prophylactic vaccines, and optimal candidates should meet WHO-acceptable safety/reactogenicity, and should be single-dose and greater than or equal to 70% efficacy in preventing infection or disease caused by the LASV lineages I-IV, and should be long lasting (greater than or equal to 5 years). Published data have argued for cellular immunity as more effective in clearing LASV infection as compared to humoral immunity, and so, GP and NP antigens (which could induce robust CD4 and CD8 T cell responses) should be chosen for vaccine formulations using several emerging concepts and technologies for vaccine development [11] that include but are not necessarily limited to new viral vector designs [12], such as arenavirus vaccine vectors that are based on recombinant lymphocytic choriomeningitis virus (rLCMV) and Pichindé virus (rPICV) [13].

Results of a phase 1/2 clinical trial to test the immunogenicity and safety profiles of these two arenavirus vectors that were used to express the non-oncogenic version of the human papilloma virus 16 (HPV16) antigens E7 and E6 in patients with recurrent or metastatic HPV16+ cancers showed that these viral vectors could induce significantly heightened levels of proinflammatory cytokines/chemokines and antigen-specific CD8⁺ T cell responses, which were further enhanced by alternating injections of the two vectors expressing the tumor antigens (reviewed by Lauterbach and colleagues [14]). Similarly, recent studies demonstrated that alternating immunizations of mice or non-human primates with rPICV- and rLCMV-vectored vaccines expressing protein antigens of hepatitis B virus (HBV) [15] or SIV [16] (as a surrogate model of HIV-1) could result in a heightened magnitude of T cell responses against the respective vaccine antigens, highlighting the safety and potential utility of these new arenaviral vectors for use toward Lassa vaccine development.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or material discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or mending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Acknowledgments

The author would like to thank the anonymous reviewers for their helpful comments and critiques.

Data availability statement

No primary data are included in this article.

Additional information

Funding

The work in the author’s laboratory was supported in part by NIH NIAID grant R01AI131586.