Immunization with DNA Plasmids Coding for Crimean-Congo Hemorrhagic Fever Virus Capsid and Envelope Proteins and/or Virus-Like Particles Induces Protection and Survival in Challenged Mice

Abstract

Crimean-Congo hemorrhagic fever virus (CCHFV) is a bunyavirus causing severe hemorrhagic fever disease in humans, with high mortality rates. The requirement of a high-containment laboratory and the lack of an animal model hampered the study of the immune response and protection of vaccine candidates. Using the recently developed interferon alpha receptor knockout (IFNAR−/−) mouse model, which replicates human disease, we investigated the immunogenicity and protection of two novel CCHFV vaccine candidates: a DNA vaccine encoding a ubiquitin-linked version of CCHFV Gc, Gn, and N and one using transcriptionally competent virus-like particles (tc-VLPs). In contrast to most studies that focus on neutralizing antibodies, we measured both humoral and cellular immune responses. We demonstrated a clear and 100% efficient preventive immunity against lethal CCHFV challenge with the DNA vaccine. Interestingly, there was no correlation with the neutralizing antibody titers alone, which were higher in the tc-VLP-vaccinated mice. However, the animals with a lower neutralizing titer, but a dominant cell-mediated Th1 response and a balanced Th2 response, resisted the CCHFV challenge. Moreover, we found that in challenged mice with a Th1 response (immunized by DNA/DNA and boosted by tc-VLPs), the immune response changed to Th2 at day 9 postchallenge. In addition, we were able to identify new linear B-cell epitope regions that are highly conserved between CCHFV strains. Altogether, our results suggest that a predominantly Th1-type immune response provides the most efficient protective immunity against CCHFV challenge. However, we cannot exclude the importance of the neutralizing antibodies as the surviving immunized mice exhibited substantial amounts of them.IMPORTANCE Crimean-Congo hemorrhagic fever virus (CCHFV) is responsible for hemorrhagic diseases in humans, with a high mortality rate. There is no FDA-approved vaccine, and there are still gaps in our knowledge of the immune responses to infection. The recently developed mouse models mimic human CCHF disease and are useful to study the immunogenicity and the protection by vaccine candidates. Our study shows that mice vaccinated with a specific DNA vaccine were fully protected. Importantly, we show that neutralizing antibodies are not sufficient for protection against CCHFV challenge but that an extra Th1-specific cellular response is required. Moreover, we describe the identification of five conserved B-cell epitopes, of which only one was previously known, that could be of great importance for the development of diagnostics tools and the improvement of vaccine candidates.

Development of vaccines against Crimean-Congo haemorrhagic fever virus

“…In mouse efficacy studies using A129 mice, despite the induction of strong neutralising antibody titres, the tc-VLPs only protected 40% of animals [91]. Results with these subunit vaccine approaches therefore demonstrate that neutralisation activity is unlikely alone to be an accurate measure of protection against CCHFV infection.…”

“…Additionally, the use of convalescent plasma as a post-exposure prophylactic has not been particularly effective [63]. Recent vaccine studies have also indicated that the induction of neutralisation antibodies does not correspond to vaccine efficacy [91], [92]. Thus cell-mediated immunity looks likely to play a major role in developing effective protection against CCHFV.…”

“…Studies with this CCHFV DNA vaccine were limited to immunogenicity assessment by serological assay, as a suitable model of disease was not available at the time. Recently, a DNA vaccine coding for CCHFV Gc, Gn and NP demonstrated induction of both humoral and cell-mediated immune responses which elicited complete protection from lethal disease in a mouse model [91].…”

“…For example data has identified neutralising epitopes in both Gn and Gc [78], [91]. Interestingly, one of these reports [78] studied several strains which segregated into three of the four previously described M segment lineage groups [120].…”

Crimean-Congo Hemorrhagic Fever

“…Recently, a DNA vaccine against the M genome segment of CCHFV has been developed, but reports from experimental animal models indicate that it is not highly immunogenic [98] . Despite insufficient neutralizing antibody titers in mice immunized with DNA plasmids developed against virus capsid and envelope proteins, Th1 response was reported to be important in protection [99] . A vaccine developed in Turkey against the Turkey-Kelkit-06 CCHFV strain resulted in measurable levels of neutralizing antibody in the serum of infected mice after a second vaccination [100] .…”