Safety and immunogenicity of a thermostable ID93 + GLA-SE tuberculosis vaccine candidate in healthy adults

Adjuvant-containing subunit vaccines represent a promising approach for protection against tuberculosis (TB), but current candidates require refrigerated storage. Here we present results from a randomized, double-blinded Phase 1 clinical trial (NCT03722472) evaluating the safety, tolerability, and immunogenicity of a thermostable lyophilized single-vial presentation of the ID93 + GLA-SE vaccine candidate compared to the non-thermostable two-vial vaccine presentation in healthy adults. Participants were monitored for primary, secondary, and exploratory endpoints following intramuscular administration of two vaccine doses 56 days apart. Primary endpoints included local and systemic reactogenicity and adverse events. Secondary endpoints included antigen-specific antibody (IgG) and cellular immune responses (cytokine-producing peripheral blood mononuclear cells and T cells). Both vaccine presentations are safe and well tolerated and elicit robust antigen-specific serum antibody and Th1-type cellular immune responses. Compared to the non-thermostable presentation, the thermostable vaccine formulation generates greater serum antibody responses (p < 0.05) and more antibody-secreting cells (p < 0.05). In this work, we show the thermostable ID93 + GLA-SE vaccine candidate is safe and immunogenic in healthy adults.

Structure and mechanism of monoclonal antibody binding to the junctional epitope of Plasmodium falciparum circumsporozoite protein

Lasting protection has long been a goal for malaria vaccines. The major surface antigen on Plasmodium falciparum sporozoites, the circumsporozoite protein (PfCSP), has been an attractive target for vaccine development and most protective antibodies studied to date interact with the central NANP repeat region of PfCSP. However, it remains unclear what structural and functional characteristics correlate with better protection by one antibody over another. Binding to the junctional region between the N-terminal domain and central NANP repeats has been proposed to result in superior protection: this region initiates with the only NPDP sequence followed immediately by NANP. Here, we isolated antibodies in Kymab mice immunized with full-length recombinant PfCSP and two protective antibodies were selected for further study with reactivity against the junctional region. X-ray and EM structures of two monoclonal antibodies, mAb667 and mAb668, shed light on their differential affinity and specificity for the junctional region. Importantly, these antibodies also bind to the NANP repeat region with equal or better affinity. A comparison with an NANP-only binding antibody (mAb317) revealed roughly similar but statistically distinct levels of protection against sporozoite challenge in mouse liver burden models, suggesting that junctional antibody protection might relate to the ability to also cross-react with the NANP repeat region. Our findings indicate that additional efforts are necessary to isolate a true junctional antibody with no or much reduced affinity to the NANP region to elucidate the role of the junctional epitope in protection.

The circumsporozoite protein (CSP) of Plasmodium falciparum malaria has been the foundation for the design of transmission blocking malaria vaccines. To date, the most promising CSP-based vaccine candidate is RTS,S, which consists of the central repeating NANP amino-acid sequence and the C-terminal domain of CSP fused to hepatitis B surface antigen that assembles into virus-like particles. Potential shortcomings of RTS,S includes the lack of other potential CSP epitopes such as the junctional epitope, which is located between the N-terminal domain of CSP and the start of the NANP repeat region. Here, we elicited antibodies against full-length CSP and screened for junctional epitope binding. We then used an array of biophysical techniques to elucidate the nature of the binding and tested the level of two protective antibodies in a mouse challenge model. Although the antibodies were able to bind both junctional and NANP epitopes, the in vivo data showed distinct levels of protection between themselves and also to an NANP-only binder. Our data suggest that their protection ability may be related to the strong cross-reactivity with NANP epitopes. Since all reported junctional antibodies to date have dual-specificity, we suggest that a true junctional binder with no or very low NANP affinity, if one can be found, is essential to evaluate the contribution of the junctional epitope to protection.

A monoclonal antibody-based immunoassay to measure the antibody response against the repeat region of the circumsporozoite protein of Plasmodium falciparum

Background

The malaria vaccine candidate RTS,S/AS01 (GSK Vaccines) induces high IgG concentration against the circumsporozoite protein (CSP) of Plasmodium falciparum. In human vaccine recipients circulating anti-CSP antibody concentrations are associated with protection against infection but appear not to be the correlate of protection. However, in a humanized mouse model of malaria infection prophylactic administration of a human monoclonal antibody (MAL1C), derived from a RTS,S/AS01-immunized volunteer, directed against the CSP repeat region, conveyed full protection in a dose-dependent manner suggesting that antibodies alone are able to prevent P. falciparum infection when present in sufficiently high concentrations. A competition ELISA was developed to measure the presence of MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients and study their possible contribution to protection against infection.

Results

MAL1C-like antibodies present in polyclonal vaccine-induced sera were evaluated for their ability to compete with biotinylated monoclonal antibody MAL1C for binding sites on the capture antigen consisting of the recombinant protein encompassing 32 NANP repeats of CSP (R32LR). Serum samples were taken at different time points from participants in two RTS,S/AS01 vaccine studies (NCT01366534 and NCT01857869). Vaccine-induced protection status of the study participants was determined based on the outcome of experimental challenge with infected mosquito bites after vaccination. Optimal conditions were established to reliably detect MAL1C-like antibodies in polyclonal sera. Polyclonal anti-CSP antibodies and MAL1C-like antibody content were measured in 276 serum samples from RTS,S/AS01 vaccine recipients using the standard ELISA and MAL-1C competition ELISA, respectively. A strong correlation was observed between the results from these assays. However, no correlation was found between the results of either assay and protection against infection.

Conclusions

The competition ELISA to measure MAL1C-like antibodies in polyclonal sera from RTS,S/AS01 vaccine recipients was robust and reliable but did not reveal the elusive correlate of protection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1596-8) contains supplementary material, which is available to authorized users.

The biological function of antibodies induced by the RTS,S/AS01 malaria vaccine candidate is determined by their fine specificity

Background

Recent vaccine studies have shown that the magnitude of an antibody response is often insufficient to explain efficacy, suggesting that characteristics regarding the quality of the antibody response, such as its fine specificity and functional activity, may play a major role in protection. Previous studies of the lead malaria vaccine candidate, RTS,S, have shown that circumsporozoite protein (CSP)-specific antibodies and CD4⁺ T cell responses are associated with protection, however the role of fine specificity and biological function of CSP-specific antibodies remains to be elucidated. Here, the relationship between fine specificity, opsonization-dependent phagocytic activity and protection in RTS,S-induced antibodies is explored.

Methods

A new method for measuring the phagocytic activity mediated by CSP-specific antibodies in THP-1 cells is presented and applied to samples from a recently completed phase 2 RTS,S/AS01 clinical trial. The fine specificity of the antibody response was assessed using ELISA against three antigen constructs of CSP: the central repeat region, the C-terminal domain and the full-length protein. A multi-parameter analysis of phagocytic activity and fine-specificity data was carried out to identify potential correlates of protection in RTS,S.

Results

Results from the newly developed assay revealed that serum samples from RTS,S recipients displayed a wide range of robust and repeatable phagocytic activity. Phagocytic activity was correlated with full-length CSP and C-terminal specific antibody titres, but not to repeat region antibody titres, suggesting that phagocytic activity is primarily driven by C-terminal antibodies. Although no significant difference in overall phagocytic activity was observed with respect to protection, phagocytic activity expressed as ‘opsonization index’, a relative measure that normalizes phagocytic activity with CS antibody titres, was found to be significantly lower in protected subjects than non-protected subjects.

Conclusions

Opsonization index was identified as a surrogate marker of protection induced by the RTS,S/AS01 vaccine and determined how antibody fine specificity is linked to opsonization activity. These findings suggest that the role of opsonization in protection in the RTS,S vaccine may be more complex than previously thought, and demonstrate how integrating multiple immune measures can provide insight into underlying mechanisms of immunity and protection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1348-9) contains supplementary material, which is available to authorized users.

Heterologous HIV-based lentiviral/adenoviral vectors immunizations result in enhanced HIV-specific immunity

Viral vectors are considered as one of the major means for the induction of strong immune responses against recombinant antigens by genetic immunization. Among these, lentiviral vectors are particularly attractive vehicles, as they can infect a wide variety of cells and can transduce replicating as well as non-replicating cells. We have engineered VRX1023, an HIV-1-based lentiviral vector (LV) vaccine candidate, to deliver HIV-1 Gag, Pol and Rev antigens under control of the native LTR promoter. While VRX1023 has been shown to elicit strong cell-mediated and humoral immunity as a stand-alone vaccine, we report here its combination in a heterologous prime-boost approach. Its combination with an adenovirus serotype 5 (Ad5)-based vector in the mouse model increased the frequency and polyfunctionality of HIV-specific CD4+ and CD8+ T cells. Homologous prime-boost regimens induced high levels of anti-vector neutralizing antibodies in Ad5-immunized mice, whereas the VSV-G-pseudotyped VRX1023 LV elicited low levels of anti-lentiviral vector neutralization. In addition, the heterologous prime-boost strategy resulted in a 5-fold reduction in Ad5-specific vector neutralization as compared to Ad5 homologous immunization. In conclusion, this study demonstrates that LV and Ad5 vector candidates can be combined in a heterologous immunization regimen, yielding dramatically improved immunogenicity while overcoming anti-vector immunity. These findings may have implications for the development of HIV vaccine regimens in populations with elevated Ad5 seroprevalence or when repeated vector administrations are required.

Lentiviral vectors for HIV disease prevention and treatment

HIV/AIDS has posed major challenges to the scientific community, both in terms of treatment and prevention. Current drug regimens, while efficacious, are expensive, inaccessible to major parts of the world, induce major side effects, and cannot prevent escape mutants due to lack of compliance and drug fatigue. In the vaccine field, recent setbacks related to the interruption and cancellation of major advanced clinical trials using adenoviral vectors have highlighted the need for new and innovative strategies. Unique features of HIV-based lentiviral vectors (LVs) and the current progress in the LV-based platform development make them an attractive alternative for the further LV-based HIV vaccine development. In preclinical studies, they have demonstrated a high degree of immunogenicity, while overcoming pitfalls faced by other viral vectors. These findings, combined with recent progress in large scale LV production/purification, make this strategy worth considering for further vaccine development.

Novel adenovirus vaccine vectors based on the enteric-tropic serotype 41

Replication-defective adenovirus vectors, primarily developed from serotype 5 (Ad5) viruses, have been widely used for gene transfer and vaccination approaches. Vectors based on other serotypes of adenovirus could be used in conjunction with, or in place of, Ad5 vectors. In this study, Ad41, an enteric adenovirus usually described as 'non-cultivable' or 'fastidious,' has been successfully cloned, rescued and propagated on 293-ORF6 cells. The complementation capabilities of this cell line allow generation of Ad41 vectors at titers comparable to those obtained for Ad5 vectors. Mice immunized with an Ad41 vector containing an HIV envelope (Env) gene mounted anti-Env cellular and humoral immune responses. Ad41-Env vectors appear to be particularly attractive when used in heterologous prime-boost regimens, where they induce significantly higher cellular immune responses to HIV-Env than Ad5-based regimens. Ad41-based constructs are attractive vaccine vectors alone or in combination with Ad5 adenovectors, since each vector type can provide circumvention of pre-existing immunity to the other.

Gene transfer in humans using a conditionally replicating lentiviral vector

We report findings from a clinical evaluation of lentiviral vectors in a phase I open-label nonrandomized clinical trial for HIV. This trial evaluated the safety of a conditionally replicating HIV-1-derived vector expressing an antisense gene against the HIV envelope. Five subjects with chronic HIV infection who had failed to respond to at least two antiviral regimens were enrolled. A single i.v. infusion of gene-modified autologous CD4 T cells was well tolerated in all patients. Viral loads were stable, and one subject exhibited a sustained decrease in viral load. CD4 counts remained steady or increased in four subjects, and sustained gene transfer was observed. Self-limiting mobilization of the vector was observed in four of five patients. There is no evidence for insertional mutagenesis after 21-36 months of observation. Immune function improved in four subjects. Lentiviral vectors appear promising for gene transfer to humans.

Enhanced mucosal immunoglobulin A response of intranasal adenoviral vector human immunodeficiency virus vaccine and localization in the central nervous system

Replication-defective adenovirus (ADV) vectors represent a promising potential platform for the development of a vaccine for AIDS. Although this vector is typically administered intramuscularly, it would be desirable to induce mucosal immunity by delivery through alternative routes. In this study, the immune response and biodistribution of ADV vectors delivered by different routes were evaluated. ADV vectors expressing human immunodeficiency virus type 1 (HIV-1) Gag, Pol, and Env were delivered intramuscularly or intranasally into mice. Intranasal immunization induced greater HIV-specific immunoglobulin A (IgA) responses in mucosal secretions and sera than in animals with intramuscular injection, which showed stronger systemic cellular and IgG responses. Administration of the vaccine through an intranasal route failed to overcome prior ADV immunity. Animals exposed to ADV prior to vaccination displayed substantially reduced cellular and humoral immune responses to HIV antigens in both groups, though the reduction was greater in animals immunized intranasally. This inhibition was partially overcome by priming with a DNA expression vector expressing HIV-1 Gag, Pol, and Env before boosting with the viral vector. Biodistribution of recombinant adenovirus (rADV) vectors administered intranasally revealed infection of the central nervous system, specifically in the olfactory bulb, possibly via retrograde transport by olfactory neurons in the nasal epithelium, which may limit the utility of this route of delivery of ADV vector-based vaccines.

The transmembrane protein of HIV-1 primary isolates modulates cell surface expression of their envelope glycoproteins

We have recently shown that the level of cell surface expression of envelope glycoproteins derived from various human immunodeficiency virus type 1 (HIV-1) primary isolates (PI) was lower than those of envelope glycoproteins derived from T-cell laboratory-adapted (TCLA) HIV-1 (D. Brand et al., 2000, Virology 271, 350-362). We investigated this phenomenon by comparing the cell surface expression of chimeric envelope glycoproteins constructed by swapping the gp120 surface and gp41 transmembrane glycoproteins of the TCLA HIV-1MN and the PI HIV-1(133), HIV-1G365, or HIV-1EFRA. We found that each chimeric envelope construct had a cell surface-specific pattern of expression similar to that of the parental envelope glycoproteins corresponding to the gp41. Thus, the difference in cell surface expression observed between TCLA viruses and various PI is probably due to a signal located in gp41. Identification of this signal may be important for the design of PI envelope-derived immunogens and may increase our understanding of the mechanisms by which HIV-1 escapes from the immune system.

Immunogenicity of recombinant envelope glycoproteins derived from T-cell line-adapted isolates or primary HIV isolates: a comparative study using multivalent vaccine approaches

We investigated immunogenic properties of native envelope glycoproteins derived from HIV-1 (subtype B). Our main objective was to assess whether the design of multivalent vaccines affects generation of neutralizing antibodies against primary viruses. Recombinant Semliki Forest virus (SFV) particles producing various HIV-1 envelope glycoproteins were used as vaccine vectors. The following multivalent vaccination approaches were compared: 1) immunization with a mixture of recombinant SFV expressing envelope glycoproteins derived from three HIV-1 primary isolates and two T-cell laboratory-adapted (TCLA) viruses; 2) immunization with a mixture of recombinant SFV expressing only the envelope glycoproteins derived from three HIV-1 primary isolates; 3) sequential immunizations with the recombinant SFV expressing the envelope glycoproteins derived from three HIV-1 primary isolates and two TCLA viruses, respectively. Two monovalent vaccine approaches using SFV expressing envelope glycoproteins derived from a single primary isolate or TCLA virus were also included in the study. The multivalent vaccination strategies based on SFV vaccine vectors did not induce more neutralizing antibodies than the previously tested TCLA envelope immunogens, which gave disappointing results against primary isolates.

Identification of the glycoprotein 41(TM) cytoplasmic tail domains of human immunodeficiency virus type 1 that interact with Pr55Gag particles

We investigated the protein/protein interactions that occur during human immunodeficiency virus (HIV-1) budding. We evaluated the binding to Pr55Gag particles of peptides mapping to the cytoplasmic tail of gp41TM and of host-cell proteins, in a cell-free, in vitro assay. Host-cell proteins and irrelevant viral envelope peptides did not bind. Peptides corresponding to a large central domain of the gp41TM cytoplasmic tail (93 residues) bound to Pr55Gag particles. This demonstrates that a Gag/Env interaction is responsible for the specific incorporation of the Env glycoprotein into nascent HIV-1 virions, and defines more accurately the gp41TM domain involved in this interaction.

Antigenic properties of recombinant envelope glycoproteins derived from T-cell-line-adapted isolates or primary human immunodeficiency virus isolates and their relationship to immunogenicity

The native envelope glycoproteins of primary HIV-1 virions have weaker antigenicity than do T-cell laboratory-adapted (TCLA) viruses. These antigenic properties require further evaluation if recombinant envelope glycoproteins are produced as part of a vaccine strategy. In this study, we compared the antigenicity of recombinant envelope glycoproteins derived from three primary isolates (PI) (HIV-1(BX08), HIV-1(CHA), and HIV-1(133)) and two TCLA viruses (HIV-1(HXB2) and HIV-1(MN)) produced using the Semliki Forest virus (SFV) system. This analysis was performed by radioimmunoprecipitation assays and flow cytometry. The results suggest that the SFV produces envelope glycoproteins with features in common with the envelopes found in naturally occurring virions. In particular, the PI envelopes had weak heterogeneous antigenic properties. However, the cytometric analysis also showed that there was less envelope glycoprotein on the cell surface for the PI envelopes than for those of TCLA viruses, suggesting differences in their intracellular trafficking. The immunogenic properties of the various envelope glycoproteins were evaluated in mice using recombinant SFV particles as vaccine vectors. The PI envelopes were less immunogenic than the TCLA envelopes, probably due to both their low antigenicity and cell surface expression level. Thus, it may be difficult to design an effective vaccine based on native recombinant PI envelopes.

Comparative analysis of humoral immune responses to HIV type 1 envelope glycoproteins in mice immunized with a DNA vaccine, recombinant Semliki Forest virus RNA, or recombinant Semliki Forest virus particles

The Semliki Forest virus (SFV) system seems to be a useful new approach for generating effective immune responses against HIV-1 in animal models. We evaluated this system by comparing the humoral immune responses raised in mice immunized against the HIV-1 envelope with the SFV system, a DNA vaccine, and a recombinant Env glycoprotein. gp160 ELISA antibody titers (204,800) were highest in the sera from mice immunized with recombinant Semliki Forest virus particles. These sera contained antibodies to the CD4-binding site and recognized linear epitopes on gp120 and gp41 that were also recognized by a pool of sera from HIV1-infected individuals. This demonstrates that the HIV-1 envelope produced in vivo by the SFV system does not fold aberrantly. A low level of neutralizing antibodies against the HIV-1LAI strain was also detected in the serum of one mouse immunized with recombinant SFV particles, suggesting that booster injections should be given to achieve a more effective immune response. SFV recombinant particles induced the strongest humoral responses to the HIV-1 envelope of all the potential HIV env vaccines tested.