Evaluation of Lassa virus vaccine immunogenicity in a CBA/J-ML29 mouse model

Abstract

Lassa virus (LASV) is widely spread in West Africa and can cause fatal Lassa fever. In addition to the significant public health problem in endemic regions there have been numerous imported cases to non-endemic countries. Due to limitations in treatment options and difficulties posed by reservoir control vaccination remains the most logical method of disease control; however we have no vaccine approved for human use. LASV is a pathogen that requires the highest level of biocontainment for study and as such vaccine development is both difficult and costly. ML29 is a reassortant virus containing the replication machinery of the nonpathogenic Mopeia virus and major immunogens of LASV. It displays an attenuated phenotype both in vitro and in vivo as compared to wild type LASV and therefore ML29 offers a safer immunogenic surrogate of LASV for vaccine research outside of BSL-4 facilities. We have established a small animal model for the evaluation of immunogenicity of LASV vaccine candidates based on unique phenotypic characteristics of ML29 in CBA/J mice. A single intraperitoneal immunization with the reassortant virus ML29 into CBA/J mice is non-pathogenic and sufficient to protect animals against a lethal homologous intracerebral challenge. Immunized mice display negligible levels of ML29-specific antibody titers, but LASV antigen-specific cell mediated immune (CMI) responses are detectable early and peak around day 8-10 after immunization. ML29 immune splenocytes display high numbers of IFN-? producing cells by ELISPOT and robust numbers of IFN-?+ and TNF-?+ CD4 and CD8 T lymphocytes by flow cytometry. In vivo CTL experiments show a correlation between Ag-specific cytotoxicity and the timing of protection induced by a single immunization with ML29. Furthermore, splenocyte transfers using donor cells from ML29 immunized mice display a similar kinetics of protection. Finally, mice that received splenocytes from ML29 immunized mice depleted of CD8+ cells all succumbed to a lethal challenge, further demonstrating the critical role of CD8 T cells in protection. This model has proven a useful immunological tool for the preliminary evaluation of immunogenicity and efficacy for other heterologous vaccine candidates against LASV outside of BSL-4 containment facilities necessitated by LASV.