Comparative evaluation of the immune responses and protection engendered by LC16m8 and Dryvax smallpox vaccines in a mouse model

Live attenuated smallpox vaccine candidate (KVAC103) efficiently induces protective immune responses in mice

Smallpox, caused by the variola virus belonging to the genus Orthopoxvirus, is an acute contagious disease that killed 300 million people in the 20th century. Since it was declared to be eradicated and the national immunization program against it was stopped, the variola virus has become a prospective bio-weapon. It is necessary to develop a safe vaccine that protects people from terrorism using this biological weapon and that can be administered to immunocompromised people. Our previous study reported on the development of an attenuated smallpox vaccine (KVAC103). This study evaluated cellular and humoral immune responses to various doses, frequencies, and routes of administration of the KVAC103 strain, compared to CJ-50300 vaccine, and its protective ability against the wild-type vaccinia virus Western Reserve (VACV-WR) strain was evaluated. The binding and neutralizing-antibody titers increased in a concentration-dependent manner in the second inoculation, which increased the neutralizing-antibody titer compared to those after the single injection. In contrast, the T-cell immune response (interferon-gamma positive cells) increased after the second inoculation compared to that of CJ-50300 after the first inoculation. Neutralizing-antibody titers and antigen-specific IgG levels were comparable in all groups administered KVAC103 intramuscularly, subcutaneously, and intradermally. In a protective immunity test using the VACV-WR strain, all mice vaccinated with CJ-50300 or KVAC103 showed 100% survival. KVAC103 could be a potent smallpox vaccine that efficiently induces humoral and cellular immune responses to protect mice against the VACV-WR strain.

Monkeypox: epidemiology, pathogenesis, treatment and prevention

Monkeypox is a zoonotic disease that was once endemic in west and central Africa caused by monkeypox virus. However, cases recently have been confirmed in many nonendemic countries outside of Africa. WHO declared the ongoing monkeypox outbreak to be a public health emergency of international concern on July 23, 2022, in the context of the COVID-19 pandemic. The rapidly increasing number of confirmed cases could pose a threat to the international community. Here, we review the epidemiology of monkeypox, monkeypox virus reservoirs, novel transmission patterns, mutations and mechanisms of viral infection, clinical characteristics, laboratory diagnosis and treatment measures. In addition, strategies for the prevention, such as vaccination of smallpox vaccine, is also included. Current epidemiological data indicate that high frequency of human-to-human transmission could lead to further outbreaks, especially among men who have sex with men. The development of antiviral drugs and vaccines against monkeypox virus is urgently needed, despite some therapeutic effects of currently used drugs in the clinic. We provide useful information to improve the understanding of monkeypox virus and give guidance for the government and relative agency to prevent and control the further spread of monkeypox virus.

An increase in glycoprotein concentration on extracellular virions dramatically alters vaccinia virus infectivity and pathogenesis without impacting immunogenicity

The extracellular virion (EV) form of Orthopoxviruses is required for cell-to-cell spread and pathogenesis, and is the target of neutralizing antibodies in the protective immune response. EV have a double envelope that contains several unique proteins that are involved in its intracellular envelopment and/or subsequent infectivity. One of these, F13, is involved in both EV formation and infectivity. Here, we report that replacement of vaccinia virus F13L with the molluscum contagiosum virus homolog, MC021L, results in the production of EV particles with significantly increased levels of EV glycoproteins, which correlate with a small plaque phenotype. Using a novel fluorescence-activated virion sorting assay to isolate EV populations based on glycoprotein content we determine that EV containing either higher or lower levels of glycoproteins are less infectious, suggesting that there is an optimal concentration of glycoproteins in the outer envelope that is required for maximal infectivity of EV. This optimal glycoprotein concentration was required for lethality and induction of pathology in a cutaneous model of animal infection, but was not required for induction of a protective immune response. Therefore, our results demonstrate that there is a sensitive balance between glycoprotein incorporation, infectivity, and pathogenesis, and that manipulation of EV glycoprotein levels can produce vaccine vectors in which pathologic side effects are attenuated without a marked diminution in induction of protective immunity.

Viral glycoproteins are critical determinants of host cell tropism, immunity, and pathogenesis. Vaccinia virus was used for the most successful immunization program in history, and poxviruses continue to be used as vaccine vectors. Here, we report that vaccinia virus extracellular virion (EV) protein F13 plays an important, previously unappreciated, role in controlling glycoprotein incorporation, and that there is a direct relationship between glycoprotein concentrations and subsequent infectivity. Crucially, manipulation of the EV glycoprotein concentrations altered pathogenesis and lethality in an in vivo infection model, but did not markedly alter the induced immune response. These results have important implications that inform the design of safer and more efficacious poxvirus-based vaccine vectors by altering glycoprotein content.

Evidence for Residual Immunity to Smallpox After Vaccination and Implications for Re-emergence

INTRODUCTION

Smallpox has been eradicated but advances in synthetic biology have increased the risk of its re-emergence. Residual immunity in individuals who were previously vaccinated may mitigate the impact of an outbreak, but there is a high degree of uncertainty about the duration and degree of residual immunity. Both cell-mediated and humoral immunity are thought to be important but the exact mechanisms of protection are unclear. Guidelines usually suggest vaccine-induced immunity wanes to zero after 3-10 years post vaccination, whereas other estimates show long term immunity over decades.

MATERIALS AND METHODS

A systematic review of the literature was conducted to quantify the duration and extent of residual immunity to smallpox after vaccination.

RESULTS

Twenty-nine papers related to quantifying residual immunity to smallpox after vaccination were identified: neutralizing antibody levels were used as immune correlates of protection in 11/16 retrospective cross-sectional studies, 2/3 epidemiological studies, 6/7 prospective vaccine trials and 0/3 modeling studies. Duration of protection of >20 years was consistently shown in the 16 retrospective cross-sectional studies, while the lowest estimated duration of protection was 11.7 years among the modeling studies. Childhood vaccination conferred longer duration of protection than vaccination in adulthood, and multiple vaccinations did not appear to improve immunity.

CONCLUSIONS

Most studies suggest a longer duration of residual immunity (at least 20 years) than assumed in smallpox guidelines. Estimates from modeling studies were less but still greater than the 3-10 years suggested by the WHO Committee on International Quarantine or US CDC guidelines. These recommendations were probably based on observations and studies conducted while smallpox was endemic. The cut-off values for pre-existing antibody levels of >1:20 and >1:32 reported during the period of endemic smallpox circulation may not be relevant to the contemporary population, but have been used as a threshold for identifying people with residual immunity in post-eradication era studies. Of the total antibodies produced in response to smallpox vaccination, neutralizing antibodies have shown to contribute significantly to immunological memory. Although the mechanism of immunological memory and boosting is unclear, revaccination is likely to result in a more robust response. There is a need to improve the evidence base for estimates on residual immunity to better inform planning and preparedness for re-emergent smallpox.

Challenges and Achievements in Prevention and Treatment of Smallpox

Declaration of smallpox eradication by the WHO in 1980 led to discontinuation of the worldwide vaccination campaign. The increasing percentage of unvaccinated individuals, the existence of its causative infectious agent variola virus (VARV), and the recent synthetic achievements increase the threat of intentional or accidental release and reemergence of smallpox. Control of smallpox would require an emergency vaccination campaign, as no other protective measure has been approved to achieve eradication and ensure worldwide protection. Experimental data in surrogate animal models support the assumption, based on anecdotal, uncontrolled historical data, that vaccination up to 4 days postexposure confers effective protection. The long incubation period, and the uncertainty of the exposure status in the surrounding population, call for the development and evaluation of safe and effective methods enabling extension of the therapeutic window, and to reduce the disease manifestations and vaccine adverse reactions. To achieve these goals, we need to evaluate the efficacy of novel and already licensed vaccines as a sole treatment, or in conjunction with immune modulators and antiviral drugs. In this review, we address the available data, recent achievements, and open questions.

Development of an animal model of progressive vaccinia in nu/nu mice and the use of bioluminescence imaging for assessment of the efficacy of monoclonal antibodies against vaccinial B5 and L1 proteins

Bioluminescence imaging (BLI) was used to follow dissemination of recombinant vaccinia virus (VACV) expressing luciferase (IHD-J-Luc) in BALB/c nu/nu mice treated post-challenge with monoclonal antibodies (MAbs) against L1 and B5 VACV proteins in a model of Progressive Vaccinia (PV). Areas Under the flux Curve (AUC) were calculated for viral loads in multiple organs in individual mice. Following scarification with 10⁵ pfu, IHD-J-Luc VACV undergoes fast replication at the injection site and disseminates rapidly to the inguinal lymph nodes followed by spleen, liver, and axillary lymph nodes within 2-3 days and before primary lesions are visible at the site of scarification. Extension of survival in nude mice treated with a combination of anti-B5 and anti-L1 MAbs 24 h post challenge correlated with a significant reduction in viral load at the site of scarification and delayed systemic dissemination. Nude mice reconstituted with 10⁴ T cells prior to challenge with IHD-J-Luc, and treated with MAbs post-challenge, survived infection, cleared the virus from all organs and scarification site, and developed anti-VACV IgG and VACV-specific polyfunctional CD8⁺ T cells that co-expressed the degranulation marker CD107a, and IFNγ and TNFα cytokines. All T cell reconstituted mice survived intranasal re-challenge with IHD-J-Luc (10⁴ pfu) two months after the primary infection. Thus, using BLI to monitor VACV replication in a PV model, we showed that anti-VACV MAbs administered post challenge extended survival of nude mice and protected T cell reconstituted nude mice from lethality by reducing replication at the site of scarification and systemic dissemination of VACV.

Recent advances in the study of live attenuated cell-cultured smallpox vaccine LC16m8

LC16m8 is a live, attenuated, cell-cultured smallpox vaccine that was developed and licensed in Japan in the 1970s, but was not used in the campaign to eradicate smallpox. In the early 2000s, the potential threat of bioterrorism led to reconsideration of the need for a smallpox vaccine. Subsequently, LC16m8 production was restarted in Japan in 2002, requiring re-evaluation of its safety and efficacy. Approximately 50,000 children in the 1970s and about 3500 healthy adults in the 2000s were vaccinated with LC16m8 in Japan, and 153 adults have been vaccinated with LC16m8 or Dryvax in phase I/II clinical trials in the USA. These studies confirmed the safety and efficacy of LC16m8, while several studies in animal models have shown that LC16m8 protects the host against viral challenge. The World Health Organization Strategic Advisory Group of Experts on Immunization recommended LC16m8, together with ACAM2000, as a stockpile vaccine in 2013. In addition, LC16m8 is expected to be a viable alternative to first-generation smallpox vaccines to prevent human monkeypox.

Genomic Analysis, Phenotype, and Virulence of the Historical Brazilian Smallpox Vaccine Strain IOC: Implications for the Origins and Evolutionary Relationships of Vaccinia Virus

Smallpox was declared eradicated in 1980 after an intensive vaccination program using different strains of vaccinia virus (VACV; Poxviridae). VACV strain IOC (VACV-IOC) was the seed strain of the smallpox vaccine manufactured by the major vaccine producer in Brazil during the smallpox eradication program. However, little is known about the biological and immunological features as well as the phylogenetic relationships of this first-generation vaccine. In this work, we present a comprehensive characterization of two clones of VACV-IOC. Both clones had low virulence in infected mice and induced a protective immune response against a lethal infection comparable to the response of the licensed vaccine ACAM2000 and the parental strain VACV-IOC. Full-genome sequencing revealed the presence of several fragmented virulence genes that probably are nonfunctional, e.g., F1L, B13R, C10L, K3L, and C3L. Most notably, phylogenetic inference supported by the structural analysis of the genome ends provides evidence of a novel, independent cluster in VACV phylogeny formed by VACV-IOC, the Brazilian field strains Cantagalo (CTGV) and Serro 2 viruses, and horsepox virus, a VACV-like virus supposedly related to an ancestor of the VACV lineage. Our data strongly support the hypothesis that CTGV-like viruses represent feral VACV that evolved in parallel with VACV-IOC after splitting from a most recent common ancestor, probably an ancient smallpox vaccine strain related to horsepox virus. Our data, together with an interesting historical investigation, revisit the origins of VACV and propose new evolutionary relationships between ancient and extant VACV strains, mainly horsepox virus, VACV-IOC/CTGV-like viruses, and Dryvax strain.

IMPORTANCE

First-generation vaccines used to eradicate smallpox had rates of adverse effects that are not acceptable by current health care standards. Moreover, these vaccines are genetically heterogeneous and consist of a pool of quasispecies of VACV. Therefore, the search for new-generation smallpox vaccines that combine low pathogenicity, immune protection, and genetic homogeneity is extremely important. In addition, the phylogenetic relationships and origins of VACV strains are quite nebulous. We show the characterization of two clones of VACV-IOC, a unique smallpox vaccine strain that contributed to smallpox eradication in Brazil. The immunogenicity and reduced virulence make the IOC clones good options for alternative second-generation smallpox vaccines. More importantly, this study reveals the phylogenetic relationship between VACV-IOC, feral VACV established in nature, and the ancestor-like horsepox virus. Our data expand the discussion on the origins and evolutionary connections of VACV lineages.

VaxCelerate II: rapid development of a self-assembling vaccine for Lassa fever

Development of effective vaccines against emerging infectious diseases (EID) can take as much or more than a decade to progress from pathogen isolation/identification to clinical approval. As a result, conventional approaches fail to produce field-ready vaccines before the EID has spread extensively. Lassa is a prototypical emerging infectious disease endemic to West Africa for which no successful vaccine is available. We established the VaxCelerate Consortium to address the need for more rapid vaccine development by creating a platform capable of generating and pre-clinically testing a new vaccine against specific pathogen targets in less than 120 d A self-assembling vaccine is at the core of the approach. It consists of a fusion protein composed of the immunostimulatory Mycobacterium tuberculosis heat shock protein 70 (MtbHSP70) and the biotin binding protein, avidin. Mixing the resulting protein (MAV) with biotinylated pathogen-specific immunogenic peptides yields a self-assembled vaccine (SAV). To meet the time constraint imposed on this project, we used a distributed R&D model involving experts in the fields of protein engineering and production, bioinformatics, peptide synthesis/design and GMP/GLP manufacturing and testing standards. SAV immunogenicity was first tested using H1N1 influenza specific peptides and the entire VaxCelerate process was then tested in a mock live-fire exercise targeting Lassa fever virus. We demonstrated that the Lassa fever vaccine induced significantly increased class II peptide specific interferon-γ CD4(+) T cell responses in HLA-DR3 transgenic mice compared to peptide or MAV alone controls. We thereby demonstrated that our SAV in combination with a distributed development model may facilitate accelerated regulatory review by using an identical design for each vaccine and by applying safety and efficacy assessment tools that are more relevant to human vaccine responses than current animal models.

Bioluminescent imaging of vaccinia virus infection in immunocompetent and immunodeficient rats as a model for human smallpox

Due to the increasing concern of using smallpox virus as biological weapons for terrorist attack, there is renewed interest in studying the pathogenesis of human smallpox and development of new therapies. Animal models are highly demanded for efficacy and safety examination of new vaccines and therapeutic drugs. Here, we demonstrated that both wild type and immunodeficient rats infected with an engineered vaccinia virus carrying Firefly luciferase reporter gene (rTV-Fluc) could recapitulate infectious and clinical features of human smallpox. Vaccinia viral infection in wild type Sprague-Dawley (SD) rats displayed a diffusible pattern in various organs, including liver, head and limbs. The intensity of bioluminescence generated from rTV-Fluc correlated well with viral loads in tissues. Moreover, neutralizing antibodies had a protective effect against virus reinfection. The recombination activating gene 2 (Rag2) knockout rats generated by transcription activator-like effector nucleases (TALENs) technology were further used to examine the infectivity of the rTV-Fluc in immunodeficient populations. Here we demonstrated that Rag2-/- rats were more susceptible to rTV-Fluc than SD rats with a slower virus clearance rate. Therefore, the rTV-Fluc/SD rats and rTV-Fluc/Rag2-/- rats are suitable visualization models, which recapitulate wild type or immunodeficient populations respectively, for testing human smallpox vaccine and antiviral drugs.

Vaccinia virus strain LC16m8 defective in the B5R gene keeps strong protection comparable to its parental strain Lister in immunodeficient mice

BACKGROUND

Attenuated vaccinia virus strain, LC16m8, defective in the B5R envelope protein gene, is used as a stockpile smallpox vaccine strain in Japan against bioterrorism: the defect in the B5R gene mainly contributes to its highly attenuated properties.

METHODS

The protective activity of LC16m8 vaccine against challenge with a lethal dose of vaccinia Western Reserve strain was assessed in wild-type and immunodeficient mice lacking CD4, MHC class I, MHC class II or MHC class I and II antigens.

RESULTS

The immunization with LC16m8 induced strong protective activity comparable to that of its parent strain, Lister (Elstree) strain, in wild-type mice from 2 days to 1 year after vaccination, as well as in immunodeficient mice at 2 or 3 weeks after vaccination. These results implicated that the defect in the B5R gene hardly affected the potential activity of LC16m8 to induce innate, cell-mediated and humoral immunity, and that LC16m8 could be effective in immunodeficient patients.

CONCLUSION

LC16m8 with truncated B5 protein has an activity to induce immunity, such as innate immunity and subsequent cell-mediated and humoral immunity almost completely comparable to the activity of its parental strain Lister.

The evolution of poxvirus vaccines

After Edward Jenner established human vaccination over 200 years ago, attenuated poxviruses became key players to contain the deadliest virus of its own family: Variola virus (VARV), the causative agent of smallpox. Cowpox virus (CPXV) and horsepox virus (HSPV) were extensively used to this end, passaged in cattle and humans until the appearance of vaccinia virus (VACV), which was used in the final campaigns aimed to eradicate the disease, an endeavor that was accomplished by the World Health Organization (WHO) in 1980. Ever since, naturally evolved strains used for vaccination were introduced into research laboratories where VACV and other poxviruses with improved safety profiles were generated. Recombinant DNA technology along with the DNA genome features of this virus family allowed the generation of vaccines against heterologous diseases, and the specific insertion and deletion of poxvirus genes generated an even broader spectrum of modified viruses with new properties that increase their immunogenicity and safety profile as vaccine vectors. In this review, we highlight the evolution of poxvirus vaccines, from first generation to the current status, pointing out how different vaccines have emerged and approaches that are being followed up in the development of more rational vaccines against a wide range of diseases.

Effect of the deletion of genes encoding proteins of the extracellular virion form of vaccinia virus on vaccine immunogenicity and protective effectiveness in the mouse model

Antibodies to both infectious forms of vaccinia virus, the mature virion (MV) and the enveloped virion (EV), as well as cell-mediated immune response appear to be important for protection against smallpox. EV virus particles, although more labile and less numerous than MV, are important for dissemination and spread of virus in infected hosts and thus important in virus pathogenesis. The importance of the EV A33 and B5 proteins for vaccine induced immunity and protection in a murine intranasal challenge model was evaluated by deletion of both the A33R and B5R genes in a vaccine-derived strain of vaccinia virus. Deletion of either A33R or B5R resulted in viruses with a small plaque phenotype and reduced virus yields, as reported previously, whereas deletion of both EV protein-encoding genes resulted in a virus that formed small infection foci that were detectable and quantifiable only by immunostaining and an even more dramatic decrease in total virus yield in cell culture. Deletion of B5R, either as a single gene knockout or in the double EV gene knockout virus, resulted in a loss of EV neutralizing activity, but all EV gene knockout viruses still induced a robust neutralizing activity against the vaccinia MV form of the virus. The effect of elimination of A33 and/or B5 on the protection afforded by vaccination was evaluated by intranasal challenge with a lethal dose of either vaccinia virus WR or IHD-J, a strain of vaccinia virus that produces relatively higher amounts of EV virus. The results from multiple experiments, using a range of vaccination doses and virus challenge doses, and using mortality, morbidity, and virus dissemination as endpoints, indicate that the absence of A33 and B5 have little effect on the ability of a vaccinia vaccine virus to provide protection against a lethal intranasal challenge in a mouse model.

Evaluation of a non-viral vaccine in smallpox-vaccinated individuals and immunized HLA-transgenic mice

The current poxvirus vaccine is associated with rare, but serious adverse events. Therefore, we investigated a non-replicating approach to vaccine design. Peptides encoding potential HLA-binding motifs were derived from the orthopoxvirus genes, D8L, A27L, and C12L (the IL-18-binding protein [vIL18BP105]), all of which are preserved among poxviruses that infect humans, and which may be a target of host immunity. The peptides were tested with poxvirus-vaccinated human PBMC and serum for eliciting memory responses, as well as with splenocytes and serum from peptide-immunized, human HLA-DR04 transgenic (HLA tg) mice. vIL18BP105 induced 5-fold proliferation of vaccinated-donor PBMC over non-vaccinated (P<0.001), including IL-2-producing CD8+ cells. Serum IgG recognizing vIL18BP105 was detected (P<0.002 vs non-vaccinated) by ELISA. Viral peptides were conjugated to the HLA-targeting mAb, L243, for immunization of HLA tg mice. Splenocytes from vIL18BP105-L243-immunized mice proliferated upon exposure to vIL18BP105 (P<0.001). Proliferating splenocytes were interferon-γ-producing CD4(+)CD45RA(neg). vIL18BP105-L243-immunized mice generated IgG more rapidly than free-peptide-immunized mice. Peptide-specific antibody was also detected when different L243-peptide conjugates were combined. vIL18BP, by eliciting human memory responses, is a viable antigen for inclusion in a virus-free vaccine. The immunogenicity of peptides was boosted by conjugation to L243, whether administered alone or combined.

Vaccinia viruses: vaccines against smallpox and vectors against infectious diseases and tumors

Less than 200 years after its introduction, widespread use of vaccinia virus (VACV) as a smallpox vaccine has eradicated variola virus. Along with the remarkable success of the vaccination program, frequent and sometimes severe adverse reactions to VACV were encountered. After eradication, VACV has been reserved for select populations who might be at significant risk for orthopoxvirus infections. Events over the past decade have renewed concerns over the potential use of variola virus as a biological weapon. Accordingly, interest in VACV and attenuated derivatives has increased, both as vaccines against smallpox and as vectors for other vaccines. This article will focus on new developments in the field of orthopoxvirus immunization and will highlight recent advances in the use of vaccinia viruses as vectors for infectious diseases and malignancies.

Safety and immunogenicity of LC16m8, an attenuated smallpox vaccine in vaccinia-naive adults

INTRODUCTION

LC16m8 is an attenuated cell culture-adapted Lister vaccinia smallpox vaccine missing the B5R protein and licensed for use in Japan.

METHODS

We conducted a phase I/II clinical trial that compared the safety and immunogenicity of LC16m8 with Dryvax in vaccinia-naive participants. Adverse events were assessed, as were electrocardiography and laboratory testing for cardiotoxicity and viral culturing of the vaccination sites. Neutralization titers to vaccinia, monkeypox, and variola major were assessed and cell-mediated immune responses were measured by interferon (IFN)-γ enzyme-linked immunosorbent spot and lymphoproliferation assays.

RESULTS

Local and systemic reactions after vaccination with LC16m8 were similar to those reported after Dryvax. No clinically significant abnormalities consistent with cardiac toxicity were seen for either vaccine. Both vaccines achieved antivaccinia, antivariola, and antimonkeypox neutralizing antibody titers >1:40, although the mean plaque reduction neutralization titer of LC16m8 at day 30 after vaccination was significantly lower than Dryvax for anti-NYCBH vaccinia (P < .01), antimonkeypox (P < .001), and antivariola (P < .001). LC16m8 produced robust cellular immune responses that trended higher than Dryvax for lymphoproliferation (P = .06), but lower for IFN-γ ELISPOT (P = .02).

CONCLUSIONS

LC16m8 generates neutralizing antibody titers to multiple poxviruses, including vaccinia, monkeypox, and variola major, and broad T-cell responses, indicating that LC16m8 may have efficacy in protecting individuals from smallpox. Clinical Trials Registration. NCT00103584.

Serological responses in humans to the smallpox vaccine LC16m8

In response to potential bioterrorism with smallpox, members of the Japanese Self-Defense Forces were vaccinated with vaccinia virus (VACV) strain LC16m8, an attenuated smallpox vaccine derived from VACV strain Lister. The serological response induced by LC16m8 to four virion-surface proteins and the intracellular mature virus (IMV) and extracellular enveloped virus (EEV) was investigated. LC16m8 induced antibody response against the IMV protein A27 and the EEV protein A56. LC16m8 also induced IMV-neutralizing antibodies, but unlike the VACV strain Lister, did not induce either EEV-neutralizing antibody or antibody to EEV protein B5, except after revaccination. Given that B5 is the only target for EEV-neutralizing antibody and that neutralization of both IMV and EEV give optimal protection against orthopoxvirus challenge, these data suggest that immunity induced by LC16m8 might be less potent than that deriving from strain Lister. This potential disadvantage should be balanced against the advantage of the greater safety of LC16m8.

Smallpox vaccine safety is dependent on T cells and not B cells

The licensed smallpox vaccine, ACAM2000, is a cell culture derivative of Dryvax. Both ACAM2000 and Dryvax are administered by skin scarification and can cause progressive vaccinia, with skin lesions that disseminate to distal sites. We have investigated the immunologic basis of the containment of vaccinia in the skin with the goal to identify safer vaccines for smallpox. Macaques were depleted systemically of T or B cells and vaccinated with either Dryvax or an attenuated vaccinia vaccine, LC16m8. B cell depletion did not affect the size of skin lesions induced by either vaccine. However, while depletion of both CD4(+) and CD8(+) T cells had no adverse effects on LC16m8-vaccinated animals, it caused progressive vaccinia in macaques immunized with Dryvax. As both Dryvax and LC16m8 vaccines protect healthy macaques from a lethal monkeypox intravenous challenge, our data identify LC16m8 as a safer and effective alternative to ACAM2000 and Dryvax vaccines for immunocompromised individuals.

Deletion of major nonessential genomic regions in the vaccinia virus Lister strain enhances attenuation without altering vaccine efficacy in mice

The vaccinia virus (VACV) Lister strain was one of the vaccine strains that enabled smallpox eradication. Although the strain is most often harmless, there have been numerous incidents of mild to life-threatening accidents with this strain and others. In an attempt to further attenuate the Lister strain, we investigated the role of 5 genomic regions known to be deleted in the modified VACV Ankara (MVA) genome in virulence in immunodeficient mice, immunogenicity in immunocompetent mice, and vaccine efficacy in a cowpox virus challenge model. Lister mutants were constructed so as to delete each of the 5 regions or various combinations of these regions. All of the mutants replicated efficiently in tissue culture except region I mutants, which multiplied more poorly in human cells than the parental strain. Mutants with single deletions were not attenuated or only moderately so in athymic nude mice. Mutants with multiple deletions were more highly attenuated than those with single deletions. Deleting regions II, III, and V together resulted in total attenuation for nude mice and partial attenuation for SCID mice. In immunocompetent mice, the Lister deletion mutants induced VACV specific humoral responses equivalent to those of the parental strain but in some cases lower cell-mediated immune responses. All of the highly attenuated mutants protected mice from a severe cowpox virus challenge at low vaccine doses. The data suggest that several of the Lister mutants combining multiple deletions could be used in smallpox vaccination or as live virus vectors at doses equivalent to those used for the traditional vaccine while displaying increased safety.

Smallpox vaccines: targets of protective immunity

The eradication of smallpox, one of the great triumphs of medicine, was accomplished through the prophylactic administration of live vaccinia virus, a comparatively benign relative of variola virus, the causative agent of smallpox. Nevertheless, recent fears that variola virus may be used as a biological weapon together with the present susceptibility of unimmunized populations have spurred the development of new-generation vaccines that are safer than the original and can be produced by modern methods. Predicting the efficacy of such vaccines in the absence of human smallpox, however, depends on understanding the correlates of protection. This review outlines the biology of poxviruses with particular relevance to vaccine development, describes protein targets of humoral and cellular immunity, compares animal models of orthopoxvirus disease with human smallpox, and considers the status of second- and third-generation smallpox vaccines.

Third-generation smallpox vaccines: challenges in the absence of clinical smallpox

Smallpox, a disease caused by variola virus, is estimated to have killed hundreds of millions to billions of people before it was certified as eradicated in 1980. However, there has been renewed interest in smallpox vaccine development due in part to zoonotic poxvirus infections and the possibility of a re-emergence of smallpox, as well as the fact that first-generation smallpox vaccines are associated with relatively rare, but severe, adverse reactions in some vaccinees. An understanding of the immune mechanisms of vaccine protection and the use of suitable animal models for vaccine efficacy assessment are paramount to the development of safer and effective smallpox vaccines. This article focuses on studies aimed at understanding the immune responses elicited by vaccinia virus and the various animal models that can be used to evaluate smallpox vaccine efficacy. Harnessing this information is necessary to assess the effectiveness and potential usefulness of new-generation smallpox vaccines.

Complement-bound human antibodies to vaccinia virus B5 antigen protect mice from virus challenge

Evaluation of: Benhnia MR, McCausland MM, Laudenslager J et al. Heavily isotype dependent protective activities of human antibodies against the vaccinia virus extracellular virion antigen B5. J. Virol. 83, 12355–12367 (2009).

The mechanism of protection afforded by vaccinia virus (VACV) – the smallpox vaccine – is a key issue for the development of modern vaccines and countermeasures. Antibodies to VACV antigens of the extracellular virion (EV) form play a central role in protection against poxvirus diseases in animal models, and contribute to the protection of immunized humans against poxviruses. B5, a viral EV protein, is conserved among different orthopoxviruses and antibodies to B5 that protect mice against VACV challenge. Antibodies to B5 are primarily responsible for neutralization of vaccinia EVs, yet the mechanism of EV neutralization by antibodies to B5 is not fully understood. The paper under evaluation demonstrates that most of the neutralization in vitro and protection in vivo in a mouse model, by monoclonal human anti-B5 IgGs, is heavily dependent on the ability of the IgGs to bind complement (C3 and C1q). Similarly, IgGs capable of complement binding control complement-dependent cytotoxicity of VACV-infected cells. Human polyclonal antibodies induced by the smallpox vaccine were similarly dependent on complement for EV neutralization and the complement-dependent destruction of infected cells. These findings not only contribute to a better understanding of the mechanism of protection by antibodies, but might also help in the development and evaluation of newly-developed therapeutic and prophylactic antibody-based products against virulent orthopoxviruses, and for the prevention or treatment of smallpox vaccine-related post-vaccinal adverse effects.