Broadening the horizons for yellow fever: new uses for an old vaccine

Re-thinking yellow fever vaccines: fighting old foes with new generation vaccines

Despite the existence of a highly efficient yellow fever vaccine, yellow fever reemergence throughout Africa and the Americas has put 900 million people in 47 countries at risk of contracting the disease. Although the vaccine has been key to controlling yellow fever epidemics, its live-attenuated nature comes with a range of contraindications that prompts advising against its administration to pregnant and lactating women, immunocompromised individuals, and those with hypersensitivity to chicken egg proteins. Additionally, large outbreaks have highlighted problems with insufficient vaccine supply, whereby manufacturers rely on slow traditional manufacturing processes that prevent them from ramping up production. These limitations have contributed to an inadequate control of yellow fever and have favored the pursuit of novel yellow fever vaccine candidates that aim to circumvent the licensed vaccine's restrictions. Here, we review the live-attenuated vaccine's limitations and explore the epitome of a yellow fever vaccine, whilst scrutinizing next-generation vaccine candidates.

Molecular and immunological characterization of a DNA-launched yellow fever virus 17D infectious clone

Yellow fever virus (YFV)-17D is an empirically developed, highly effective live-attenuated vaccine that has been administered to human beings for almost a century. YFV-17D has stood as a paradigm for a successful viral vaccine, and has been exploited as a potential virus vector for the development of recombinant vaccines against other diseases. In this study, a DNA-launched YFV-17D construct (pBeloBAC-FLYF) was explored as a new modality to the standard vaccine to combine the commendable features of both DNA vaccine and live-attenuated viral vaccine. The DNA-launched YFV-17D construct was characterized extensively both in cell culture and in mice. High titres of YFV-17D were generated upon transfection of the DNA into cells, whereas a mutant with deletion in the capsid-coding region (pBeloBAC-YF/ΔC) was restricted to a single round of infection, with no release of progeny virus. Homologous prime-boost immunization of AAD mice with both pBeloBAC-FLYF and pBeloBAC-YF/ΔC elicited specific dose-dependent cellular immune response against YFV-17D. Vaccination of A129 mice with pBeloBAC-FLYF resulted in the induction of YFV-specific neutralizing antibodies in all vaccinated subjects. These promising results underlined the potential of the DNA-launched YFV both as an alternative to standard YFV-17D vaccination and as a vaccine platform for the development of DNA-based recombinant YFV vaccines.

Yellow fever 17D-vectored vaccines expressing Lassa virus GP1 and GP2 glycoproteins provide protection against fatal disease in guinea pigs

Yellow Fever (YF) and Lassa Fever (LF) are two prevalent hemorrhagic fevers co-circulating in West Africa and responsible for thousands of deaths annually. The YF vaccine 17D has been used as a vector for the Lassa virus glycoprotein precursor (LASV-GPC) or their subunits, GP1 (attachment glycoprotein) and GP2 (fusion glycoprotein). Cloning shorter inserts, LASV-GP1 and -GP2, between YF17D E and NS1 genes enhanced genetic stability of recombinant viruses, YF17D/LASV-GP1 and -GP2, in comparison with YF17D/LASV-GPC recombinant. The recombinant viruses were replication competent and properly processed YF proteins and LASV GP antigens in infected cells. YF17D/LASV-GP1 and -GP2 induced specific CD8+ T cell responses in mice and protected strain 13 guinea pigs against fatal LF. Unlike immunization with live attenuated reassortant vaccine ML29, immunization with YF17D/LASV-GP1 and -GP2 did not provide sterilizing immunity. This study demonstrates the feasibility of YF17D-based vaccine to control LF in West Africa.

The yellow fever virus vaccine induces a broad and polyfunctional human memory CD8+ T cell response

The live yellow fever vaccine (YF-17D) offers a unique opportunity to study memory CD8(+) T cell differentiation in humans following an acute viral infection. We have performed a comprehensive analysis of the virus-specific CD8(+) T cell response using overlapping peptides spanning the entire viral genome. Our results showed that the YF-17D vaccine induces a broad CD8(+) T cell response targeting several epitopes within each viral protein. We identified a dominant HLA-A2-restricted epitope in the NS4B protein and used tetramers specific for this epitope to track the CD8(+) T cell response over a 2 year period. This longitudinal analysis showed the following. 1) Memory CD8(+) T cells appear to pass through an effector phase and then gradually down-regulate expression of activation markers and effector molecules. 2) This effector phase was characterized by down-regulation of CD127, Bcl-2, CCR7, and CD45RA and was followed by a substantial contraction resulting in a pool of memory T cells that re-expressed CD127, Bcl-2, and CD45RA. 3) These memory cells were polyfunctional in terms of degranulation and production of the cytokines IFN-gamma, TNF-alpha, IL-2, and MIP-1beta. 4) The YF-17D-specific memory CD8(+) T cells had a phenotype (CCR7(-)CD45RA(+)) that is typically associated with terminally differentiated cells with limited proliferative capacity (T(EMRA)). However, these cells exhibited robust proliferative potential showing that expression of CD45RA may not always associate with terminal differentiation and, in fact, may be an indicator of highly functional memory CD8(+) T cells generated after acute viral infections.

Construction and characterization of recombinant flaviviruses bearing insertions between E and NS1 genes

BACKGROUND

The yellow fever virus, a member of the genus Flavivirus, is an arthropod-borne pathogen causing severe disease in humans. The attenuated yellow fever 17D virus strain has been used for human vaccination for 70 years and has several characteristics that are desirable for the development of new, live attenuated vaccines. We described here a methodology to construct a viable, and immunogenic recombinant yellow fever 17D virus expressing a green fluorescent protein variant (EGFP). This approach took into account the presence of functional motifs and amino acid sequence conservation flanking the E and NS1 intergenic region to duplicate and fuse them to the exogenous gene and thereby allow the correct processing of the viral polyprotein precursor.

RESULTS

YF 17D EGFP recombinant virus was grew in Vero cells and reached a peak titer of approximately 6.45 +/- 0.4 log10 PFU/mL at 96 hours post-infection. Immunoprecipitation and confocal laser scanning microscopy demonstrated the expression of the EGFP, which was retained in the endoplasmic reticulum and not secreted from infected cells. The association with the ER compartment did not interfere with YF assembly, since the recombinant virus was fully competent to replicate and exit the cell. This virus was genetically stable up to the tenth serial passage in Vero cells. The recombinant virus was capable to elicit a neutralizing antibody response to YF and antibodies to EGFP as evidenced by an ELISA test. The applicability of this cloning strategy to clone gene foreign sequences in other flavivirus genomes was demonstrated by the construction of a chimeric recombinant YF 17D/DEN4 virus.

CONCLUSION

This system is likely to be useful for a broader live attenuated YF 17D virus-based vaccine development for human diseases. Moreover, insertion of foreign genes into the flavivirus genome may also allow in vivo studies on flavivirus cell and tissue tropism as well as cellular processes related to flavivirus infection.

New developments in flavivirus vaccines with special attention to yellow fever

PURPOSE OF REVIEW

Here we review recent epidemiological trends in flavivirus diseases, findings related to existing vaccines, and new directions in flavivirus vaccine research. We emphasize the need for stepped-up efforts to stop further spread and intensification of these infections worldwide.

RECENT FINDINGS

Although the incidence and geographic distribution of flavivirus diseases have increased in recent years, human vaccines are available only for yellow fever, Japanese encephalitis, tick-borne encephalitis and Kyasanur forest disease. Factors contributing to resurgence include insufficient supplies of available vaccines, incomplete vaccination coverage and relaxation in vector control. Research has been underway for 60 years to develop effective vaccines against dengue, and recent progress is encouraging. The development of vaccines against West Nile, virus recently introduced to North America, has been initiated. In addition, there is considerable interest in improving existing vaccines with respect to increasing safety (e.g. eliminating the newly recognized syndrome of yellow fever vaccine-associated viscerotropic adverse disease), and to reducing the cost and number of doses required for effective immunization.

SUMMARY

Traditional approaches to flavivirus vaccines are still employed, while recent advancements in biotechnology produced new approaches to vaccine design, such as recombinant live virus, subunit and DNA vaccines. Live chimeric vaccines against dengue, Japanese encephalitis and West Nile based on yellow fever 17D virus (ChimeriVax) are in phase I/II trials, with encouraging results. Other chimeric dengue, tick-borne encephalitis and West Nile virus candidates were developed based on attenuated dengue backbones. To further reduce the impact of flavivirus diseases, vaccination policies and vector control programs in affected countries require revision.