Protective vaccine-induced CD4(+) T cell-independent B cell responses against rabies infection

T cell independent antibody responses with class switch and memory using peptides anchored on liposomes

Vaccines generally require T lymphocytes for B-cell activation and immunoglobulin class switching in response to peptide or protein antigens. In the absence of T cells, limited IgG class switch takes place, germinal centers are short-lived, and the B cells lack memory. Here, immunization of mice with liposomes containing 15mer peptides and monophosphoryl lipid A (MPLA) as adjuvant, induced T-cell independent (TI) IgG class switch within three days, as well as germinal center formation. The antibody responses were long-lived, strictly dependent on Toll-like receptor 4 (TLR4) signaling, partly dependent on Bruton's tyrosine kinase (BTK) signal transmission, and independent of signaling through T-cell receptors, MHC class II and inflammasome. The antibody response showed characteristics of both TI type 1 and TI type 2. All IgG subclasses could be boosted months after primary immunization, and the biological function of the secreted antibodies was demonstrated in murine models of allergic anaphylaxis and of bacterial infection. Moreover, antibody responses after immunization with peptide- and MPLA-loaded liposomes could be triggered in neonatal mice and in mice receiving immune-suppressants. This study demonstrates T-cell independent endogenous B-cell memory and recall responses in vivo using a peptide antigen. The stimulation of these antibody responses required a correct and dense assembly and administration of peptide and adjuvant on the surface of liposomes. In the future, TI vaccines may prove beneficial in pathological conditions in which T-cell immunity is compromised through disease or medicines or when rapid, antibody-mediated immune protection is needed.

Development and Evaluation of an Enzyme-Linked Immunosorbent Assay Targeting Rabies-Specific IgM and IgG in Human Sera

Immunity from rabies depends on rabies virus neutralizing antibodies (RVNA) induced after immunization; however, the influence of antibody isotype switching has not been extensively investigated. This has become particularly relevant with changes in World Health Organization (WHO) recommended rabies vaccine regimens that may influence RVNA isotype kinetics, potentially affecting the peak, and longevity, of RVNA immunoglobulin (IgG) levels. We developed rapid and reliable assays for quantifying the anti-rabies IgM/IgG class switch in human serum based on an indirect ELISA technique. The immune response was tracked in ten individuals naïve to the rabies vaccine by quantifying serum titers weekly, from day seven to day 42 post-immunization, using a serum neutralization assay and the ELISA IgM/IgG assays. The average RVNA IU/mL levels were at D0 ≤ 0.1, D7 0.24, D14 8.36, D21 12.84, D28 25.74 and D42 28.68. Levels of specific IgM antibodies to rabies glycoprotein (EU/mL) were higher, on average, at D7, 1.37, and from D14, 5.49, to D21, 6.59. In contrast, average IgG antibodies (EU/mL) predominated from D28, 10.03, to D42, 14.45. We conclude that levels of anti-rabies IgM/IgG at D28 characterize the isotype class switch. These assays, combined with serum neutralization assays, distinguished the RVNA levels in terms of the IgM/IgG responses and are expected to add to the diagnostic repertoire, provide additional information in establishing rabies vaccine regimens, both post- and pre-exposure prophylaxis, and contribute to research efforts.

Immunogenicity of replication-deficient vesicular stomatitis virus based rabies vaccine in mice

BACKGROUND

Rabies is a viral disease that causes severe neurological manifestations both in humans and various mammals. Although inactivated and/or attenuated vaccines have been developed and widely used around the world, there are still concerns with regard to their safety, efficacy, and costs.

OBJECTIVE

As demand has grown for a new rabies vaccine, we have developed a new vesicular stomatitis viruses (VSVs) based rabies vaccine that replaces glycoproteins with rabies virus (RABV) glycoprotein (GP), or so-called VSV/RABV-GP.

METHODS

VSV/RABV-GP production was measured by sandwich ELISA. The generation of VSV/RABV-GP was evaluated with GP-specific antibodies and reduced transduction with GP-specific neutralizing antibodies. Virus entry was quantified by measuring the luciferase levels at 18-h post-transduction. BALB/c mice (three groups of six mice each) were intraperitoneally immunized with PBS, RABA, or VSV/RABV-GP at 0 and 14 days. At 28 days post-immunization serology was performed. Statistical significance was calculated using the Holm-Sidak multiple Student's t test.

RESULTS

Mice immunized with VSV/RABV-GP produced IgM and IgG antibodies, whereas IgM titers were significantly higher in mice immunized with VSV/RABV-GP compared to inactivated RABV. The secretion profiles of IgG1 and IgG2a production suggested that VSV/RAVB-GP induces the T helper cell type-2 immune bias. In addition, the average (±SD; n = 3) serum neutralization titers of the inactivated RABV and VSV/RABV-GP groups were 241 ± 40 and 103 ± 54 IU/mL, respectively.

CONCLUSION

Our results confirm that VSV/RABV-GP could be a new potential vaccination platform for RABV.

Mucosal immune responses in the trachea after chronic infection with Mycoplasma gallisepticum in unvaccinated and vaccinated mature chickens

Tracheitis associated with the chronic respiratory disease in chickens caused by Mycoplasma gallisepticum is marked by infiltration of leukocytes into the mucosa. Although cytokines/chemokines are known to play a key role in the recruitment, differentiation, and proliferation of leukocytes, those that are produced and secreted into the trachea during the chronic stages of infection with M. gallisepticum have not been described previously. In this study, the levels of transcription in the trachea of genes encoding a panel of 13 cytokines/chemokines were quantified after experimental infection with the M. gallisepticum wild-type strain Ap3AS in unvaccinated chickens and chickens vaccinated 40-, 48- or 57-weeks previously with the novel attenuated strain ts-304. These transcriptional levels in unvaccinated/infected and vaccinated/infected chickens were compared with those of unvaccinated/uninfected and vaccinated/uninfected chickens. Pathological changes and subsets of leukocytes infiltrating the tracheal mucosa were concurrently assessed by histopathological examination and indirect immunofluorescent staining. After infection, unvaccinated birds had a significant increase in tracheal mucosal thickness and in transcription of genes for cytokines/chemokines, including those for IFN-γ, IL-17, RANTES (CCLi4), and CXCL-14, and significant downregulation of IL-2 gene transcription. B cells, CD3⁺ or CD4⁺ cells and macrophages (KUL01⁺ ) accumulated in the mucosa but CD8⁺ cells were not detected. In vaccinated birds, the levels of transcription of the genes for IL-6, IL-2, RANTES and CXCL-14 were significantly lower after infection than in the unvaccinated/infected and/or unvaccinated/uninfected birds, while the transcription of the IFN-γ gene was significantly upregulated, and there were aggregations of B cells in the tracheal mucosa. These observations indicated that M. gallisepticum may have suppressed Th2 responses by upregulating secretion of IFN-γ and IL-17 by CD4⁺ cells and induced immune dysregulation characterized by depletion of CD8⁺ cells and downregulation of IL-2 in the tracheas of unvaccinated birds. The ts-304 vaccine appeared to induce long-term protection against this immune dysregulation. TAKE AWAY: The ts-304 vaccine-induced long-term protection against immune dysregulation caused by M. gallisepticum Detection of B cells and plasma cells in the tracheal mucosa suggested that long-term protection is mediated by mucosal B cell memory Infection of unvaccinated birds with M. gallisepticum resulted in CD8⁺ cell depletion and downregulation of IL-2 in the tracheal mucosa, suggestive of immune dysregulation Infection of unvaccinated birds with M. gallisepticum resulted in upregulation of IFN-γ and infiltration of CD4⁺ cells and antigen presenting cells (B and KUL01⁺ cells) into the tracheal mucosa, suggesting enhanced antigen processing and presentation during chronic infection Th2 responses to infection with M. gallisepticum may be dampened by CD4⁺ cells through upregulation of IFN-γ and IL-17 during chronic infection.

Quantitative characterization of the T cell receptor repertoires of human immunized by rabies virus vaccine

Cellular immunity is crucial for an efficient host immune response against rabies virus (RABV) infection. But the T cell receptor (TCR) repertoire in human after RABV vaccine immunization remained unclear. In this study, we conducted high-throughput sequencing of TCR β chain complementarity determining region 3(CDR3) repertoires in 4 healthy volunteers before and after immunization with RABV vaccine. Our data showed that RABV vaccination changed the TCR diversity and the usage of V/J gene segments, as well as V-J pairing. The high-frequency clonotypes that altered after vaccination were identified. These results may provide us with new insights into T cell receptor condition after RABV vaccination.

Incorporating B cell activating factor (BAFF) into the membrane of rabies virus (RABV) particles improves the speed and magnitude of vaccine-induced antibody responses

B cell activating factor (BAFF) is a member of the tumor necrosis factor (TNF) superfamily of cytokines that links innate with adaptive immunity. BAFF signals through receptors on B cells, making it an attractive molecule to potentiate vaccine-induced B cell responses. We hypothesized that a rabies virus (RABV)-based vaccine displaying both antigen and BAFF on the surface of the same virus particle would target antigen-specific B cells for activation and improve RABV-specific antibody responses. To test this hypothesis, we constructed a recombinant RABV-based vector expressing virus membrane-anchored murine BAFF (RABV-ED51-mBAFF). BAFF was incorporated into the RABV particle and determined to be biologically functional, as demonstrated by increased B cell survival of primary murine B cells treated ex-vivo with RABV-ED51-mBAFF. B cell survival was inhibited by pre-treating RABV-ED51-mBAFF with an antibody that blocks BAFF functions. RABV-ED51-mBAFF also activated primary murine B cells ex-vivo more effectively than RABV as shown by significant upregulation of CD69, CD40, and MHCII on the surface of infected B cells. In-vivo, RABV-ED51-mBAFF induced significantly faster and higher virus neutralizing antibody (VNA) titers than RABV while not adversely affecting the longevity of the vaccine-induced antibody response. Since BAFF was incorporated into the virus particle and genome replication was not required for BAFF expression in-vivo, we hypothesized that RABV-ED51-mBAFF would be effective as an inactivated vaccine. Mice immunized with 250 ng/mouse of β-propriolactone-inactivated RABV-ED51-mBAFF showed faster and higher anti-RABV VNA titers compared to mice immunized with inactivated RABV. Together, this model stands as a potential foundation for exploring other virus membrane-anchored molecular adjuvants to make safer, more effective inactivated RABV-based vaccines.

Poor Antigen Processing of Poxvirus Particles Limits CD4+ T Cell Recognition and Impacts Immunogenicity of the Inactivated Vaccine

CD4⁺ T cells play critical roles in defending against poxviruses, both by potentiating cellular and humoral responses and by directly killing infected cells. Despite this central role, the basis for pox-specific CD4⁺ T cell activation, specifically the origin of the poxvirus-derived peptides (epitopes) that activate CD4⁺ T cells, remains poorly understood. In addition, because the current licensed poxvirus vaccines can cause serious adverse events and even death, elucidating the requirements for MHC class II (MHC-II) processing and presentation of poxviral Ags could be of great use. To address these questions, we explored the CD4⁺ T cell immunogenicity of ectromelia, the causative agent of mousepox. Having identified a large panel of novel epitopes via a screen of algorithm-selected synthetic peptides, we observed that immunization of mice with inactivated poxvirus primes a virtually undetectable CD4⁺ T cell response, even when adjuvanted, and is unable to provide protection against disease after a secondary challenge. We postulated that an important contributor to this outcome is the poor processability of whole virions for MHC-II-restricted presentation. In line with this hypothesis, we observed that whole poxvirions are very inefficiently converted into MHC-II-binding peptides by the APC as compared with subviral material. Thus, stability of the virion structure is a critical consideration in the rational design of a safe alternative to the existing live smallpox vaccine.

Combination therapy of rabies-infected mice with inhibitors of pro-inflammatory host response, antiviral compounds and human rabies immunoglobulin

Recent studies demonstrated that inhibitors of pro-inflammatory molecular cascades triggered by rabies infection in the central nervous system (CNS) can enhance survival in mouse model and that certain antiviral compounds interfere with rabies virus replication in vitro. In this study different combinations of therapeutics were tested to evaluate their effect on survival in rabies-infected mice, as well as on viral load in the CNS. C57Bl/6 mice were infected with Silver-haired bat rabies virus (SHBRV)-18 at virus dose approaching LD₅₀ and LD₁₀₀. In one experimental group daily treatments were initiated 4 h before-, in other groups 48 or 96 h after challenge. In the first experiment therapeutic combination contained inhibitors of tumour necrosis factor-α (infliximab), caspase-1 (Ac-YVAD-cmk), and a multikinase inhibitor (sorafenib). In the treated groups there was a notable but not significant increase of survival compared to the virus infected, non-treated mice. The addition of human rabies immunoglobulins (HRIG) to the combination in the second experiment almost completely prevented mortality in the pre-exposure treatment group along with a significant reduction of viral titres in the CNS. Post-exposure treatments also greatly improved survival rates. As part of the combination with immunomodulatory compounds, HRIG had a higher impact on survival than alone. In the third experiment the combination was further supplemented with type-I interferons, ribavirin and favipiravir (T-705). As a blood-brain barrier opener, mannitol was also administered. This treatment was unable to prevent lethal consequences of SHBRV-18 infection; furthermore, it caused toxicity in treated mice, presumably due to interaction among the components. In all experiments, viral loads in the CNS were similar in mice that succumbed to rabies regardless of treatment. According to the findings, inhibitors of detrimental host response to rabies combined with antibodies can be considered among the possible therapeutic and post-exposure options in human rabies cases.

Recombinant rabies virus with the glycoprotein fused with a DC-binding peptide is an efficacious rabies vaccine

Our previous studies demonstrated that recruiting and/or activating dendritic cells (DCs) enhanced the immunogenicity of recombinant rabies viruses (rRABV). In this study, rRABV LBNSE with a small DC-binding peptide (designated as rLBNSE-DCBp) or a negative control peptide (designated as rLBNSE-DCCp) fused to the glycoprotein (G) was constructed and rescued. As expected, significantly more activated DCs were detected in rLBNSE-DCBp-immunized mice than those immunized with rLBNSE or rLBNSE-DCCp. Subsequently, significantly more generation of TFH and GC B cells were observed in rLBNSE-DCBp immunized mice than those in rLBNSE or rLBNSE-DCCp-immunized mice. In addition, significantly higher levels of virus neutralizing antibodies (VNAs) were observed in mice immunized with rLBNSE-DCBp than those immunized with rLBNSE or rLBNSE-DCCp, resulting in a better protection of rLBNSE-DCBp immunized mice against the lethal challenge. Taken together, our results suggest that rRABV with G fused with DCBp is a promising rabies vaccine candidate.

Rabies - epidemiology, pathogenesis, public health concerns and advances in diagnosis and control: a comprehensive review

Rabies is a zoonotic, fatal and progressive neurological infection caused by rabies virus of the genus Lyssavirus and family Rhabdoviridae. It affects all warm-blooded animals and the disease is prevalent throughout the world and endemic in many countries except in Islands like Australia and Antarctica. Over 60,000 peoples die every year due to rabies, while approximately 15 million people receive rabies post-exposure prophylaxis (PEP) annually. Bite of rabid animals and saliva of infected host are mainly responsible for transmission and wildlife like raccoons, skunks, bats and foxes are main reservoirs for rabies. The incubation period is highly variable from 2 weeks to 6 years (avg. 2-3 months). Though severe neurologic signs and fatal outcome, neuropathological lesions are relatively mild. Rabies virus exploits various mechanisms to evade the host immune responses. Being a major zoonosis, precise and rapid diagnosis is important for early treatment and effective prevention and control measures. Traditional rapid Seller's staining and histopathological methods are still in use for diagnosis of rabies. Direct immunofluoroscent test (dFAT) is gold standard test and most commonly recommended for diagnosis of rabies in fresh brain tissues of dogs by both OIE and WHO. Mouse inoculation test (MIT) and polymerase chain reaction (PCR) are superior and used for routine diagnosis. Vaccination with live attenuated or inactivated viruses, DNA and recombinant vaccines can be done in endemic areas. This review describes in detail about epidemiology, transmission, pathogenesis, advances in diagnosis, vaccination and therapeutic approaches along with appropriate prevention and control strategies.

Targeting Vaccine-Induced Extrafollicular Pathway of B Cell Differentiation Improves Rabies Postexposure Prophylaxis

Vaccine-induced B cells differentiate along two pathways. The follicular pathway gives rise to germinal centers (GCs) that can take weeks to fully develop. The extrafollicular pathway gives rise to short-lived plasma cells (PCs) that can rapidly secrete protective antibodies within days of vaccination. Rabies virus (RABV) postexposure prophylaxis (PEP) requires rapid vaccine-induced humoral immunity for protection. Therefore, we hypothesized that targeting extrafollicular B cell responses for activation would improve the speed and magnitude of RABV PEP. To test this hypothesis, we constructed, recovered, and characterized a recombinant RABV-based vaccine expressing murine B cell activating factor (BAFF) (rRABV-mBAFF). BAFF is an ideal molecule to improve early pathways of B cell activation, as it links innate and adaptive immunity, promoting potent B cell responses. Indeed, rRABV-mBAFF induced a faster, higher antibody response in mice and enhanced survivorship in PEP settings compared to rRABV. Interestingly, rRABV-mBAFF and rRABV induced equivalent numbers of GC B cells, suggesting that rRABV-mBAFF augmented the extrafollicular B cell pathway. To confirm that rRABV-mBAFF modulated the extrafollicular pathway, we used a signaling lymphocytic activation molecule (SLAM)-associated protein (SAP)-deficient mouse model. In response to antigen, SAP-deficient mice form extrafollicular B cell responses but do not generate GCs. rRABV-mBAFF induced similar anti-RABV antibody responses in SAP-deficient and wild-type mice, demonstrating that BAFF modulated immunity through the extrafollicular and not the GC B cell pathway. Collectively, strategies that manipulate pathways of B cell activation may facilitate the development of a single-dose RABV vaccine that replaces current complicated and costly RABV PEP.IMPORTANCE Effective RABV PEP is currently resource- and cost-prohibitive in regions of the world where RABV is most prevalent. In order to diminish the requirements for rabies immunoglobulin (RIG) and multiple vaccinations for effective prevention of clinical rabies, a more rapidly protective vaccine is needed. This work presents a successful approach to rapidly generate antibody-secreting PCs in response to vaccination by targeting the extrafollicular B cell pathway. We demonstrate that the improved early antibody responses induced by rRABV-mBAFF confer improved protection against RABV in a PEP model. Significantly, activation of the early extrafollicular B cell pathway, such as that demonstrated here, could improve the efficacy of vaccines targeting other pathogens against which rapid protection would decrease morbidity and mortality.

Overexpression of Interleukin-7 Extends the Humoral Immune Response Induced by Rabies Vaccination

Rabies continues to present a public health threat in most countries of the world. The most efficient way to prevent and control rabies is to implement vaccination programs for domestic animals. However, traditional inactivated vaccines used in animals are costly and have relatively low efficiency, which impedes their extensive use in developing countries. There is, therefore, an urgent need to develop single-dose and long-lasting rabies vaccines. However, little information is available regarding the mechanisms underlying immunological memory, which can broaden humoral responses following rabies vaccination. In this study, a recombinant rabies virus (RABV) that expressed murine interleukin-7 (IL-7), referred to here as rLBNSE-IL-7, was constructed, and its effectiveness was evaluated in a mouse model. rLBNSE-IL-7 induced higher rates of T follicular helper (Tfh) cells and germinal center (GC) B cells from draining lymph nodes (LNs) than the parent virus rLBNSE. Interestingly, rLBNSE-IL-7 improved the percentages of long-lived memory B cells (Bmem) in the draining LNs and plasma cells (PCs) in the bone marrow (BM) for up to 360 days postimmunization (dpi). As a result of the presence of the long-lived PCs, it also generated prolonged virus-neutralizing antibodies (VNAs), resulting in better protection against a lethal challenge than that seen with rLBNSE. Moreover, consistent with the increased numbers of Bmem and PCs after a boost with rLBNSE, rLBNSE-IL-7-immunized mice promptly produced a more potent secondary anti-RABV neutralizing antibody response than rLBNSE-immunized mice. Overall, our data suggest that overexpressing IL-7 improved the induction of long-lasting primary and secondary antibody responses post-RABV immunization.IMPORTANCE Extending humoral immune responses using adjuvants is an important method to develop long-lasting and efficient vaccines against rabies. However, little information is currently available regarding prolonged immunological memory post-RABV vaccination. In this study, a novel rabies vaccine that expressed murine IL-7 was developed. This vaccine enhanced the numbers of Tfh cells and the GC responses, resulting in upregulated quantities of Bmem and PCs. Moreover, we found that the long-lived PCs that were elicited by the IL-7-expressing recombinant virus (rLBNSE-IL-7) were able to sustain VNA levels much longer than those elicited by the parent rLBNSE virus. Upon reexposure to the pathogen, the longevous Bmem, which maintained higher numbers for up to 360 dpi with rLBNSE-IL-7 compared to rLBNSE, could differentiate into antibody-secreting cells, resulting in rapid and potent secondary production of VNAs. These results suggest that the expression of IL-7 is beneficial for induction of potent and long-lasting humoral immune responses.

Rabies Control and Treatment: From Prophylaxis to Strategies with Curative Potential

Rabies is an acute, fatal, neurological disease that affects almost all kinds of mammals. Vaccination (using an inactivated rabies vaccine), combined with administration of rabies immune globulin, is the only approved, effective method for post-exposure prophylaxis against rabies in humans. In the search for novel rabies control and treatment strategies, live-attenuated viruses have recently emerged as a practical and promising approach for immunizing and controlling rabies. Unlike the conventional, inactivated rabies vaccine, live-attenuated viruses are genetically modified viruses that are able to replicate in an inoculated recipient without causing adverse effects, while still eliciting robust and effective immune responses against rabies virus infection. A number of viruses with an intrinsic capacity that could be used as putative candidates for live-attenuated rabies vaccine have been intensively evaluated for therapeutic purposes. Additional novel strategies, such as a monoclonal antibody-based approach, nucleic acid-based vaccines, or small interfering RNAs (siRNAs) interfering with virus replication, could further add to the arena of strategies to combat rabies. In this review, we highlight current advances in rabies therapy and discuss the role that they might have in the future of rabies treatment. Given the pronounced and complex impact of rabies on a patient, a combination of these novel modalities has the potential to achieve maximal anti-rabies efficacy, or may even have promising curative effects in the future. However, several hurdles regarding clinical safety considerations and public awareness should be overcome before these approaches can ultimately become clinically relevant therapies.

Current Status and Development of Vaccines and Other Biologics for Human Rabies Prevention

Rabies is a neglected viral zoonosis with the highest case fatality of any infectious disease. Pasteur's historical accomplishments during the late 19(th) century began the process of human vaccine development, continuing to evolve into the 21(st) century. Over the past 35 years, great improvements occurred in the production of potent tissue culture vaccines and the gradual removal from the market of unsafe nerve tissue products. Timely and appropriate administration of modern biologics virtually assures survivorship, even after severe exposures. Nevertheless, in the developing world, if not provided for free nationally, the cost of a single course of human prophylaxis exceeds the average monthly wage of the common worker. Beyond traditional approaches, recombinant, sub-unit and other novel methods are underway to improve the availability of safe, effective and more affordable rabies biologics.

Interplay between rabies virus and the mammalian immune system

Rabies is a disease caused following infection of the brain by the rabies virus (RABV). The principle mechanism of transmission is through a bite wound. The virus infects peripheral nerves and moves to the central nervous system (CNS). There appears to be little involvement of other organ systems and little detectable immune stimulation prior to infection of the CNS. This failure of the mammalian immune system to respond to rabies virus infection leads, in the overwhelming majority of cases, to death of the host. To some extent, this failure is likely due to the exclusive replication of RABV in neurons and the limited ability to generate, sufficiently rapidly, an anti-viral antibody response in situ. This is reflected in the ability of post-exposure vaccination, when given early after infection, to prevent disease. The lack of immune stimulation during RABV infection preceding neural invasion is the Achilles heel of the immune response. Whilst many viruses infect the brain, causing encephalitis and neuronal deficit, none are as consistently fatal to the host as RABV. This is in part due to prior replication of many viruses in peripheral, non-neural tissue by other viruses that allows timely activation of the immune response before the host is overwhelmed. Our current understanding of the correlates of protection for rabies suggests that it is the action of neutralising antibodies that prevent infection and control spread of RABV. Furthermore, it tells us that the induction of immunity can protect and understanding how and why this happens is critical to controlling infection. However, the paradigm of antibody development suggests that antigen presentation overwhelmingly occurs in lymphoid tissue (germinal and non-germinal centres) and these are external to the CNS. In addition, the blood-brain-barrier may provide a block to the delivery of immune effectors (antibodies/plasma B-cells) entering where they are needed. Alternatively, there may be insufficient antigen exposure after natural infection to mount an effective response or the virus actively suppresses immune function. To improve our ability to treat this fatal infection it is imperative to understand how immunity to RABV develops and functions so that parameters of protection are better defined.

A Bivalent, Chimeric Rabies Virus Expressing Simian Immunodeficiency Virus Envelope Induces Multifunctional Antibody Responses

We previously showed that a matrix (M) gene-deleted rabies virus (RABV)-based vaccine (RABV-ΔM) is highly immunogenic and induces potent B cell responses in the context of RABV infection. We speculated that RABV-ΔM expressing HIV proteins would also induce potent B cell responses against HIV antigens. As a prerequisite to future studies in nonhuman primates, we completed immunogenicity studies in mice to confirm the ability of RABV-ΔM to induce polyfunctional B cell responses in the context of HIV. To that end, the envelope protein from the mac239 strain of SIV (SIVmac239Env) was cloned into RABV-ΔM, resulting in RABV-ΔM-Env. Infectious virus was recovered following standard methods and propagated on baby hamster kidney cells stably expressing RABV M [>10(7) focus forming units (ffu)/ml]. Western blot analysis of cell lysates or of purified virions confirmed Env expression on the surface of infected cells and within virus particles, respectively. Positive neutralization activity against a neutralization-sensitive SIV strain and to a lesser extent against a neutralization-resistant SIV strain was detected in mice after a single intramuscular inoculation with RABV-ΔM-Env. The quality, but not quantity, of the antibody response was enhanced via boosting with recombinant gp130 or RABV-ΔM-Env as measured by an increase in antibody avidity and a skewing toward a Th1-type antibody response. We also show that an intradermal inoculation induces higher antibodies than an intramuscular or intranasal inoculation. An intradermal inoculation of RABV-ΔM-Env followed by a boost inoculation with recombinant gp130 produced anti-SIV antibodies with neutralizing and nonneutralizing antibody (nNAb) effector functions. Together, RABV-ΔM-Env induces B cells to secrete antibodies against SIV with the potential to clear both "free" and cell-associated virus. Strategies capable of eliciting both NAbs as well as nNAbs might help to improve the efficacy of HIV-1 vaccines.

Chimeric rabies virus-like particles containing membrane-anchored GM-CSF enhances the immune response against rabies virus

Rabies remains an important public health threat in most developing countries. To develop a more effective and safe vaccine against rabies, we have constructed a chimeric rabies virus-like particle (VLP), which containing glycoprotein (G) and matrix protein (M) of rabies virus (RABV) Evelyn-Rokitnicki-Abelseth (ERA) strain, and membrane-anchored granulocyte-macrophage colony-stimulating factor (GM-CSF), and it was named of EVLP-G. The immunogenicity and protective efficacy of EVLP-G against RABV were evaluated by intramuscular administration in a mouse model. The EVLP-G was successfully produced in insect cells by coinfection with three recombinant baculoviruses expressing G, M, and GM-CSF, respectively. The membrane-anchored GM-CSF possesses a strong adjuvant activity. More B cells and dendritic cells (DCs) were recruited and/or activated in inguinal lymph nodes in mice immunized with EVLP-G. EVLP-G was found to induce a significantly increased RABV-specific virus-neutralizing antibody and elicit a larger and broader antibody subclass responses compared with the standard rabies VLP (sRVLP, consisting of G and M). The EVLP-G also elicited significantly more IFN-γ- or IL-4-secreting CD4+ and CD8+ T cells than the sRVLP. Moreover, the immune responses induced by EVLP-G protect all vaccinated mice from lethal challenge with RABV. These results suggest that EVLP-G has the potential to be developed as a novel vaccine candidate for the prevention and control of animal rabies.

Lymph node but not intradermal injection site macrophages are critical for germinal center formation and antibody responses to rabies vaccination

Replication-deficient rabies virus (RABV)-based vaccines induce rapid and potent antibody responses via T cell-independent and T cell-dependent mechanisms. To further investigate early events in vaccine-induced antibody responses against RABV infections, we studied the role of macrophages as mediators of RABV-based vaccine immunogenicity. In this report, we show that a recombinant matrix gene-deleted RABV-based vaccine (rRABV-ΔM) infects and activates primary murine macrophages in vitro. Immunization of mice with live RABV-based vaccines results in accumulation of macrophages at the site of immunization, which suggests that macrophages in tissues support the development of effective anti-RABV B cell responses. However, we show that draining lymph node macrophages, but not macrophages at the site of immunization, are essential for the generation of germinal center B cells, follicular T helper cells, and RABV-specific antibodies. Our findings have implications for the design of new RABV-based vaccines for which early immunological events are important for the protection against RABV in postexposure settings.

IMPORTANCE

More than two-thirds of the world's population live in regions where rabies is endemic. Postexposure prophylaxis is the primary means of treating humans. Identifying immunological principles that guide the development of rapid and potent antibody responses against rabies infections will greatly increase our ability to produce more-effective rabies vaccines. Here we report that macrophages in the draining lymph node, but not in the tissue at the site of immunization are important for vaccine-induced antibody responses to rabies. Information gleaned from this study may help guide the development of a single-dose vaccine against rabies infections.

Lymph node but not intradermal injection site macrophages are critical for germinal center formation and antibody responses to rabies vaccination

Replication-deficient rabies virus (RABV)-based vaccines induce rapid and potent antibody responses via T cell-independent and T cell-dependent mechanisms. To further investigate early events in vaccine-induced antibody responses against RABV infections, we studied the role of macrophages as mediators of RABV-based vaccine immunogenicity. In this report, we show that a recombinant matrix gene-deleted RABV-based vaccine (rRABV-ΔM) infects and activates primary murine macrophages in vitro. Immunization of mice with live RABV-based vaccines results in accumulation of macrophages at the site of immunization, which suggests that macrophages in tissues support the development of effective anti-RABV B cell responses. However, we show that draining lymph node macrophages, but not macrophages at the site of immunization, are essential for the generation of germinal center B cells, follicular T helper cells, and RABV-specific antibodies. Our findings have implications for the design of new RABV-based vaccines for which early immunological events are important for the protection against RABV in postexposure settings.

IMPORTANCE

More than two-thirds of the world's population live in regions where rabies is endemic. Postexposure prophylaxis is the primary means of treating humans. Identifying immunological principles that guide the development of rapid and potent antibody responses against rabies infections will greatly increase our ability to produce more-effective rabies vaccines. Here we report that macrophages in the draining lymph node, but not in the tissue at the site of immunization are important for vaccine-induced antibody responses to rabies. Information gleaned from this study may help guide the development of a single-dose vaccine against rabies infections.

Rabies vaccines: where do we stand, where are we heading?

Rabies being the most lethal zoonotic, vaccine-preventable viral disease with worldwide distribution of reservoir wild animals presents unique challenges for its diagnosis, management and control. Although vaccines available are highly effective, which had played the key role in controlling rabies in North America, western Europe and in a number of Asian and Latin American countries, the requirement of multiple doses along with boosters, associated cost to reduce the incidence in wild animals and prophylactic human vaccination has remained a major impediment towards achieving the same goals in poorer parts of the world such as sub-Saharan Africa and southeast Asia. Current efforts to contain rabies worldwide are directed towards the development of more safe, cheaper and efficacious vaccines along with anti-rabies antibodies for post-exposure prophylaxis. The work presented here provides an overview of the advances made towards controlling the human rabies, particularly in last 10 years, and future perspective.

Current status of rabies and prospects for elimination

Rabies is one of the most deadly infectious diseases, with a case-fatality rate approaching 100%. The disease is established on all continents apart from Antarctica; most cases are reported in Africa and Asia, with thousands of deaths recorded annually. However, the estimated annual figure of almost 60,000 human rabies fatalities is probably an underestimate. Almost all cases of human rabies result from bites from infected dogs. Therefore, the most cost-effective approach to elimination of the global burden of human rabies is to control canine rabies rather than expansion of the availability of human prophylaxis. Mass vaccination campaigns with parenteral vaccines, and advances in oral vaccines for wildlife, have allowed the elimination of rabies in terrestrial carnivores in several countries worldwide. The subsequent reduction in cases of human rabies in such regions advocates the multidisciplinary One Health approach to rabies control through the mass vaccination of dogs and control of canine populations.

Ginsenoside Re as an adjuvant to enhance the immune response to the inactivated rabies virus vaccine in mice

The inactivated rabies virus vaccine (RV) is a relatively expensive vaccine, prone to failure in some cases. Ginsenoside Re (Re) is a saponin isolated from Panax ginseng, and has an adjuvant property. Here the adjuvant effect of Re to improve the immune response to the RV is evaluated in mice. ICR mice were immunized with saline, 2.50mg/kg Re, 20μl RV, 100μl RV, or 20μl of RV adjuvanted with Re (1.25, 2.50 or 5.00mg/kg). Different time points after boosting, we measured serum antibodies in blood samples and separated splenocytes to detect lymphocyte proliferation and the production of IL-4, IL-10, IL-12, and IFN-γ in vitro. We also compared immunizations containing 20μl RV and 20μl RV adjuvanted with Re (5.00mg/kg) for the expression of CD4(+) and CD8(+) T-cell subsets at different time points. Results indicated that co-administration of Re significantly enhanced serum antibody titers, increased the CD4(+):CD8(+) ratio, and enhanced both proliferation responses and IL-4, IL-10, IL-12 and IFN-γ secretions. Both Th1 and Th2 immune responses were activated. The supplementation of the Re (5.00mg/kg) to 20μl of RV significantly amplified serum antibody responses and Th1/Th2 responses inducing similar protection as did 100μl of RV. This suggests that Re could be used to reduce the dose, and therefore the cost, of the RV to achieve the same effective protection. Re merits further studies for use with vaccines of mixed Th1/Th2 immune responses.

Safety and serological response to a matrix gene-deleted rabies virus-based vaccine vector in dogs

Dogs account for the majority of human exposures and deaths due to rabies virus (RABV) worldwide. In this report, we show that a replication-deficient RABV-based vaccine in which the matrix gene is deleted (RABV-ΔM) is safe and induces rapid and potent VNA titers after a single inoculation in dogs. Average VNA titers peaked at 3.02 or 5.11 international units (IU/ml) by 14 days post-immunization with a single dose of 10(6) or 10(7) focus forming units (ffu), respectively, of RABV-ΔM. By day 70 post immunization, all dogs immunized with either dose of vaccine showed VNA titers >0.5IU/ml, the level indicative of a satisfactory immunization. Importantly, no systemic or local reactions were noted in any dog immunized with RABV-ΔM. The elimination of dog rabies through mass vaccination is hindered by limited resources, requirement for repeat vaccinations often for the life of a dog, and in some parts of the world, inferior vaccine quality. Our preliminary safety and immunogenicity data in dogs suggest that RABV-ΔM might complement currently used inactivated RABV-based vaccines in vaccination campaigns by helping to obtain 100% response in vaccinated dogs, thereby increasing overall vaccination coverage.

ICAM-1-based rabies virus vaccine shows increased infection and activation of primary murine B cells in vitro and enhanced antibody titers in-vivo

We have previously shown that live-attenuated rabies virus (RABV)-based vaccines infect and directly activate murine and human primary B cells in-vitro, which we propose can be exploited to help develop a single-dose RABV-based vaccine. Here we report on a novel approach to utilize the binding of Intracellular Adhesion Molecule-1 (ICAM-1) to its binding partner, Lymphocyte Function-associated Antigen-1 (LFA-1), on B cells to enhance B cell activation and RABV-specific antibody responses. We used a reverse genetics approach to clone, recover, and characterize a live-attenuated recombinant RABV-based vaccine expressing the murine Icam1 gene (rRABV-mICAM-1). We show that the murine ICAM-1 gene product is incorporated into virus particles, potentially exposing ICAM-1 to extracellular binding partners. While rRABV-mICAM-1 showed 10-100-fold decrease in viral titers on baby hamster kidney cells compared to the parental virus (rRABV), rRABV-mICAM-1 infected and activated primary murine B cells in-vitro more efficiently than rRABV, as indicated by significant upregulation of CD69, CD40, and MHCII on the surface of infected B cells. ICAM-1 expression on the virus surface was responsible for enhanced B cell infection since pre-treating rRABV-mICAM-1 with a neutralizing anti-ICAM-1 antibody reduced B cell infection to levels observed with rRABV alone. Furthermore, 100-fold less rRABV-mICAM-1 was needed to induce antibody titers in immunized mice equivalent to antibody titers observed in rRABV-immunized mice. Of note, only 10³ focus forming units (ffu)/mouse of rRABV-mICAM-1 was needed to induce significant anti-RABV antibody titers as early as five days post-immunization. As both speed and potency of antibody responses are important in controlling human RABV infection in a post-exposure setting, these data show that expression of Icam1 from the RABV genome, which is then incorporated into the virus particle, is a promising strategy for the development of a single-dose RABV vaccine that requires only a minimum of virus.

Reinvestigating the role of IgM in rabies virus postexposure vaccination

B cells secreting IgG antibodies, but not IgM, are thought to be solely responsible for vaccine-induced protection against rabies virus (RABV) infections in postexposure settings. In this report, we reinvestigated the potential for IgM to mediate protection in a mouse model of RABV vaccination. Immunocompetent mice immunized with an experimental live replication-deficient RABV-based vaccine produced virus neutralizing antibodies (VNAs) within 3 days of vaccination. However, mice unable to produce soluble IgM (sIgM(-/-)) did not produce VNAs until 7 days postimmunization. Furthermore, sIgM(-/-) mice were not protected against RABV infection when challenged 3 days postimmunization, while all wild-type mice survived challenge. Consistent with the lack of protection against pathogenic RABV challenge, approximately 50- to 100-fold higher viral loads of challenge virus were detected in the muscle, spinal cord, and brain of immunized sIgM(-/-) mice compared to control mice. In addition, IgG antibody titers in vaccinated wild-type and sIgM(-/-) mice were similar at all time points postimmunization, suggesting that protection against RABV challenge is due to the direct effects of IgM and not the influence of IgM on the development of effective IgG antibody titers. In all, early vaccine-induced IgM can limit dissemination of pathogenic RABV to the central nervous system and mediate protection against pathogenic RABV challenge. Considering the importance for the rapid induction of VNAs to protect against RABV infections in postexposure prophylaxis settings, these findings may help guide the development of a single-dose human rabies vaccine.

B cell infection and activation by rabies virus-based vaccines

Replication-deficient rabies viruses (RABV) are promising rabies postexposure vaccines due to their prompt and potent stimulation of protective virus neutralizing antibody titers, which are produced in mice by both T-dependent and T-independent mechanisms. To promote such early and robust B cell stimulation, we hypothesized that live RABV-based vaccines directly infect B cells, thereby activating a large pool of antigen-presenting cells (APCs) capable of providing early priming and costimulation to CD4(+) T cells. In this report, we show that live RABV-based vaccine vectors efficiently infect naive primary murine and human B cells ex vivo. Infection of B cells resulted in the significant upregulation of early markers of B cell activation and antigen presentation, including CD69, major histocompatibility complex class II (MHC-II), and CD40 in murine B cells or HLA-DR and CD40 in human B cells compared to mock-infected cells or cells treated with an inactivated RABV-based vaccine. Furthermore, primary B cells infected with a live RABV expressing ovalbumin were able to prime and stimulate naive CD4(+) OT-II T cells to proliferate and to secrete interleukin-2 (IL-2), demonstrating a functional consequence of B cell infection and activation by live RABV-based vaccine vectors. We propose that this direct B cell stimulation by live RABV-based vaccines is a potential mechanism underlying their induction of early protective T cell-dependent B cell responses, and that designing live RABV-based vaccines to infect and activate B cells represents a promising strategy to develop a single-dose postexposure rabies vaccine where the generation of early protective antibody titers is critical.

Investigating the role for IL-21 in rabies virus vaccine-induced immunity

Over two-thirds of the world's population lives in regions where rabies is endemic, resulting in over 15 million people receiving multi-dose post-exposure prophylaxis (PEP) and over 55,000 deaths per year globally. A major goal in rabies virus (RABV) research is to develop a single-dose PEP that would simplify vaccination protocols, reduce costs associated with RABV prevention, and save lives. Protection against RABV infections requires virus neutralizing antibodies; however, factors influencing the development of protective RABV-specific B cell responses remain to be elucidated. Here we used a mouse model of IL-21 receptor-deficiency (IL-21R-/-) to characterize the role for IL-21 in RABV vaccine-induced immunity. IL-21R-/- mice immunized with a low dose of a live recombinant RABV-based vaccine (rRABV) produced only low levels of primary or secondary anti-RABV antibody response while wild-type mice developed potent anti-RABV antibodies. Furthermore, IL-21R-/- mice immunized with low-dose rRABV were only minimally protected against pathogenic RABV challenge, while all wild-type mice survived challenge, indicating that IL-21R signaling is required for antibody production in response to low-dose RABV-based vaccination. IL-21R-/- mice immunized with a higher dose of vaccine produced suboptimal anti-RABV primary antibody responses, but showed potent secondary antibodies and protection similar to wild-type mice upon challenge with pathogenic RABV, indicating that IL-21 is dispensable for secondary antibody responses to live RABV-based vaccines when a primary response develops. Furthermore, we show that IL-21 is dispensable for the generation of Tfh cells and memory B cells in the draining lymph nodes of immunized mice but is required for the detection of optimal GC B cells or plasma cells in the lymph node or bone marrow, respectively, in a vaccine dose-dependent manner. Collectively, our preliminary data show that IL-21 is critical for the development of optimal vaccine-induced primary but not secondary antibody responses against RABV infections.