Rapid antibody responses by low-dose, single-step, dendritic cell-targeted immunization

Effective production of snake antivenom by targeting epidermal dendritic cells via the 'low dose, low volume, multi-site' immunization

Snakebite envenomation (SBE) is a serious neglected tropical disease that affects about 3 million people every year and causes over 100,000 deaths annually, mostly in developing countries. WHO has pledged to cut in half the morbidity and mortality due to SBE by 2030. Animal plasma-derived antivenoms, produced mostly in horses and sheep, are the main treatment modality. However, for over a century, equine plasma antivenom production has faced many problems. These include: low neutralizing potency, failure of horses to develop a satisfactory immune response, and a long immunization period. These problems have led to antivenom shortages and higher costs resulting in otherwise avoidable morbidity and mortality in snake bite victims. Attempts have been made to improve the antivenom production process. For example, a number of adjuvants designed to improve the immune response have been tested. In 1997, an immunization protocol involving the use of multi-site, low-volume and venom doses was developed and is currently used in antivenom production. This protocol constituted a significant innovation that has resulted in highly potent antivenoms within much shortened immunization periods, with all the immunized horses responding and with much less venom immunogen used. It has resulted in an ample antivenom supply for use in Thailand and neighboring countries and has led to no reported deaths from snakebite in Thailand in the past few years. The effectiveness of this immunization protocol was the result of a strategy based on targeting dendritic cells which play a pivotal role in the immune response process. This communication summarizes the basis and results of this immunization strategy.

Straight to the point: targeted mRNA-delivery to immune cells for improved vaccine design

With the deepening of our understanding of adaptive immunity at the cellular and molecular level, targeting antigens directly to immune cells has proven to be a successful strategy to develop innovative and potent vaccines. Indeed, it offers the potential to increase vaccine potency and/or modulate immune response quality while reducing off-target effects. With mRNA-vaccines establishing themselves as a versatile technology for future applications, in the last years several approaches have been explored to target nanoparticles-enabled mRNA-delivery systems to immune cells, with a focus on dendritic cells. Dendritic cells (DCs) are the most potent antigen presenting cells and key mediators of B- and T-cell immunity, and therefore considered as an ideal target for cell-specific antigen delivery. Indeed, improved potency of DC-targeted vaccines has been proved in vitro and in vivo. This review discusses the potential specific targets for immune system-directed mRNA delivery, as well as the different targeting ligand classes and delivery systems used for this purpose.

Cell-targeted vaccines: implications for adaptive immunity

Recent advancements in immunology and chemistry have facilitated advancements in targeted vaccine technology. Targeting specific cell types, tissue locations, or receptors can allow for modulation of the adaptive immune response to vaccines. This review provides an overview of cellular targets of vaccines, suggests methods of targeting and downstream effects on immune responses, and summarizes general trends in the literature. Understanding the relationships between vaccine targets and subsequent adaptive immune responses is critical for effective vaccine design. This knowledge could facilitate design of more effective, disease-specialized vaccines.

An Engineered Monovalent Anti-TNF-α Antibody with pH-Sensitive Binding Abrogates Immunogenicity in Mice following a Single Intravenous Dose

Therapeutic Abs directed toward TNF-α display significant immunogenicity in humans, frequently leading to lower serum concentrations of the Ab that are associated with lower treatment efficacy. The enhanced incidence of immunogenicity observed with this class of therapeutics may be mediated by the expression of TNF-α as a homotrimer, both as a soluble serum protein and as a membrane-associated protein (mTNF-α) on the surface of dendritic cells. The TNF-α homotrimer enables the formation of polyvalent Ab-TNF-α immune complexes (ICs) that enhance binding to FcR and neonatal FcR. Polyvalent ICs and Ab bound to mTNF-α on the surface of dendritic cells can internalize, traffic to the lysosomes, and be processed for presentation by MHC molecules. To diminish immunogenicity caused by trafficking of ICs and mTNF-α to the lysosomes, we engineered a monovalent format of adalimumab with pH-sensitive binding to TNF-α. The engineered variant, termed AF-M2637, did not cross-link TNF-α trimers and consequently formed small, nonprecipitating ICs only. AF-M2637 bound TNF-α with high affinity at pH 7.4 (EC50 = 1.1 nM) and displayed a significantly faster dissociation rate than adalimumab at pH 6.0. No immune response to AF-M2637 was detected in mice following a single i.v. dose. In contrast, rapid immunization was detected following the injection of a single i.v. dose of adalimumab, monovalent adalimumab, or the bivalent form of the pH-sensitive variant. These data suggest that ICs and mTNF-α both contribute to the immunogenicity of adalimumab in mice and provide a general strategy for engineering less immunogenic therapeutic TNF-α Abs.

Pathological and protective roles of dendritic cells in Mycobacterium tuberculosis infection: Interaction between host immune responses and pathogen evasion

Dendritic cells (DCs) are principal defense components that play multifactorial roles in translating innate immune responses to adaptive immunity in Mycobacterium tuberculosis (Mtb) infections. The heterogeneous nature of DC subsets follows their altered functions by interacting with other immune cells, Mtb, and its products, enhancing host defense mechanisms or facilitating pathogen evasion. Thus, a better understanding of the immune responses initiated, promoted, and amplified or inhibited by DCs in Mtb infection is an essential step in developing anti-tuberculosis (TB) control measures, such as host-directed adjunctive therapy and anti-TB vaccines. This review summarizes the recent advances in salient DC subsets, including their phenotypic classification, cytokine profiles, functional alterations according to disease stages and environments, and consequent TB outcomes. A comprehensive overview of the role of DCs from various perspectives enables a deeper understanding of TB pathogenesis and could be useful in developing DC-based vaccines and immunotherapies.

Immunogenicity of the Xcl1-SARS-CoV-2 Spike Fusion DNA Vaccine for COVID-19

SARS-CoV-2 spike (S) variants that may evade antibody-mediated immunity are emerging. Evidence shows that vaccines with a stronger immune response are still effective against mutant strains. Here, we report a targeted type 1 conventional dendritic (cDC1) cell strategy for improved COVID-19 vaccine design. cDC1 cells specifically express X-C motif chemokine receptor 1 (Xcr1), the only receptor for chemokine Xcl1. We fused the S gene sequence with the Xcl1 gene to deliver the expressed S protein to cDC1 cells. Immunization with a plasmid encoding the S protein fused to Xcl1 showed stronger induction of antibody and antigen-specific T cell immune responses than immunization with the S plasmid alone in mice. The fusion gene-induced antibody also displayed more powerful SARS-CoV-2 wild-type virus and pseudovirus neutralizing activity. Xcl1 also increased long-lived antibody-secreting plasma cells in bone marrow. These preliminary results indicate that Xcl1 serves as a molecular adjuvant for the SARS-CoV-2 vaccine and that our Xcl1-S fusion DNA vaccine is a potential COVID-19 vaccine candidate for use in further translational studies.

Intranasal delivery of a cDC1 targeted influenza vaccine with poly(I:C) enhances T cell responses and protects against influenza infection

Targeting antigens to dendritic cells represent a promising method for enhancing immune responses against specific antigens. However, many studies have focused on systemic delivery (intravenous or intraperitoneally) of targeted antigen, approaches that are not easily transferable to humans. Here we evaluate the efficacy of an influenza vaccine targeting Xcr1⁺ cDC1 administered by intranasal immunization. Intranasal delivery of antigen fused to the chemokine Xcl1, the ligand of Xcr1, resulted in specific uptake by lung CD103⁺ cDC1. Interestingly, intranasal immunization with influenza A/PR/8/34 haemagglutinin (HA) fused to Xcl1, formulated with poly(I:C), resulted in enhanced induction of antigen-specific IFNγ⁺ CD4⁺ and IFNγ⁺ CD8⁺ T cell responses in lung compared non-targeted anti-NIP-HA (αNIP-HA). Induction of antibody responses was, however, similar in Xcl1-HA and αNIP-HA immunized mice, but significantly higher than in mice immunized with monomeric HA. Both Xcl1-HA and αNIP-HA vaccines induced full protection when mice were challenged with a lethal dose of influenza PR8 virus, reflecting the strong induction of HA-specific antibodies. Our results demonstrate that i.n. delivery of Xcl1-HA is a promising vaccine strategy for enhancing T cell responses in addition to inducing strong antibody responses.

Targeting Haemagglutinin Antigen of Avian Influenza Virus to Chicken Immune Cell Receptors Dec205 and CD11c Induces Differential Immune-Potentiating Responses

Improving the immunogenicity and protective efficacy of vaccines is critical to reducing disease impacts. One strategy used to enhance the immunogenicity of vaccines is the selective delivery of protective antigens to the antigen presenting cells (APCs). In this study, we have developed a targeted antigen delivery vaccine (TADV) system by recombinantly fusing the ectodomain of hemagglutinin (HA) antigen of H9N2 influenza A virus to single chain fragment variable (scFv) antibodies specific for the receptors expressed on chicken APCs; Dec205 and CD11c. Vaccination of chickens with TADV containing recombinant H9HA Foldon-Dec205 scFv or H9HA Foldon-CD11c scFv proteins elicited faster (as early as day 6 post primary vaccination) and higher anti-H9HA IgM and IgY, haemagglutination inhibition, and virus neutralisation antibodies compared to the untargeted H9HA protein. Comparatively, CD11c scFv conjugated H9HA protein showed higher immunogenic potency compared to Dec205 scFv conjugated H9HA protein. The higher immune potentiating ability of CD11c scFv was also reflected in ex-vivo chicken splenocyte stimulation assay, whereby H9HA Foldon-CD11c scFv induced higher levels of cytokines (IFNγ, IL6, IL1β, and IL4) compared to H9HA Foldon-Dec205 scFv. Overall, the results conclude that TADV could be a better alternative to the currently available inactivated virus vaccines.

Autotransporter-Mediated Display of Complement Receptor Ligands by Gram-Negative Bacteria Increases Antibody Responses and Limits Disease Severity

The targeting of immunogens/vaccines to specific immune cells is a promising approach for amplifying immune responses in the absence of exogenous adjuvants. However, the targeting approaches reported thus far require novel, labor-intensive reagents for each vaccine and have primarily been shown as proof-of-concept with isolated proteins and/or inactivated bacteria. We have engineered a plasmid-based, complement receptor-targeting platform that is readily applicable to live forms of multiple gram-negative bacteria, including, but not limited to, Escherichia coli, Klebsiella pneumoniae, and Francisella tularensis. Using F. tularensis as a model, we find that targeted bacteria show increased binding and uptake by macrophages, which coincides with increased p38 and p65 phosphorylation. Mice vaccinated with targeted bacteria produce higher titers of specific antibody that recognizes a greater diversity of bacterial antigens. Following challenge with homologous or heterologous isolates, these mice exhibited less weight loss and/or accelerated weight recovery as compared to counterparts vaccinated with non-targeted immunogens. Collectively, these findings provide proof-of-concept for plasmid-based, complement receptor-targeting of live gram-negative bacteria.

Contribution of Dendritic Cells in Protective Immunity against Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is a major cause of severe respiratory disease in infants and the elderly. The socioeconomic burden of RSV infection is substantial because it leads to serious respiratory problems, subsequent hospitalization, and mortality. Despite its clinical significance, a safe and effective vaccine is not yet available to prevent RSV infection. Upon RSV infection, lung dendritic cells (DCs) detecting pathogens migrate to the lymph nodes and activate the adaptive immune response. Therefore, RSV has evolved various immunomodulatory strategies to inhibit DC function. Due to the capacity of RSV to modulate defense mechanisms in hosts, RSV infection results in inappropriate activation of immune responses resulting in immunopathology and frequent reinfection throughout life. This review discusses how DCs recognize invading RSV and induce adaptive immune responses, as well as the regulatory mechanisms mediated by RSV to disrupt DC functions and ultimately avoid host defenses.

Targeting Conventional Dendritic Cells to Fine-Tune Antibody Responses

Dendritic cells (DCs) facilitate cross talk between the innate and adaptive immune system. They sense and phagocytose invading pathogens, and are not only capable of activating naïve T cells, but can also determine the polarization of T cell responses into different effector subtypes. Polarized T cells in turn have a crucial role in antibody class switching and affinity maturation, and consequently the quality of the resulting humoral immunity. Targeting vaccines to DCs thus provides a great deal of opportunities for influencing the humoral immune responses, by fine-tuning the T cell response as well as regulating antigen availability for B cells. In this review we aim to outline how different DC targeted vaccination strategies can be utilized to induce a desired humoral immune response. A range of factors, including route of vaccine administration, use of adjuvants, choice of DC subset and surface receptor to target have been reported to influence the resulting immune response and will be reviewed herein. Finally, we will discuss opportunities for designing improved vaccines and challenges with translating this knowledge into clinical or veterinary medicine.

Endocytosis Deficient Murine Xcl1-Fusion Vaccine Enhances Protective Antibody Responses in Mice

Targeting antigen to surface receptors on dendritic cells (DCs) can improve antibody response against subunit vaccines. We have previously observed that human XCL1-fusion vaccines target murine Xcr1⁺ DCs without actively inducing endocytosis of the antigen, resulting in enhanced antibody responses in mice. However, the use of foreign chemokines for targeting is undesirable when translating this observation to human or veterinary medicine due to potential cross-reactive responses against the endogenous chemokine. Here we have identified a mutant version of murine Xcl1, labeled Xcl1(Δ1) owing to removal of a conserved valine in position 1 of the mature chemokine, that retains specific binding to Xcr1⁺ DCs without inducing endocytosis of the receptor. DNA immunization with Xcl1(Δ1) conjugated to influenza hemagglutinin (HA) induced improved antibody responses, with higher end point titers of IgG compared to WT Xcl1-HA. The Xcl1(Δ1) fusion vaccine also resulted in an increased number of HA reactive germinal center B cells with higher avidity toward the antigen, and serum transfer experiments show that Xcl1(Δ1)-HA induced antibody responses provided better protection against influenza infection as compared to WT Xcl1-HA. In summary, our observations indicate that targeting antigen to Xcr1⁺ DCs in an endocytosis deficient manner enhances antibody responses. This effect was obtained by introducing a single mutation to Xcl1, suggesting our strategy may easily be translated to human or veterinary vaccine settings.

Dendritic cell targeted Ccl3- and Xcl1-fusion DNA vaccines differ in induced immune responses and optimal delivery site

Fusing antigens to chemokines to target antigen presenting cells (APC) is a promising method for enhancing immunogenicity of DNA vaccines. However, it is unclear how different chemokines compare in terms of immune potentiating effects. Here we compare Ccl3- and Xcl1-fusion vaccines containing hemagglutinin (HA) from influenza A delivered by intramuscular (i.m.) or intradermal (i.d.) DNA vaccination. Xcl1 fusion vaccines target cDC1s, and enhance proliferation of CD4⁺ and CD8⁺ T cells in vitro. In contrast, Ccl3 target both cDC1 and cDC2, but only enhance CD4⁺ T cell proliferation in combination with cDC2. When Ccl3- or Xcl1-HA fusion vaccines were administered by i.m. DNA immunization, both vaccines induced Th1-polarized immune responses with antibodies of the IgG2a/IgG2b subclass and IFNγ-secreting T cells. After i.d. DNA vaccination, however, only Xcl1-HA maintained a Th1 polarized response and induced even higher numbers of IFNγ-secreting T cells. Consequently, Xcl1-HA induced superior protection against influenza infection compared to Ccl3-HA after i.d. immunization. Interestingly, i.m. immunization with Ccl3-HA induced the strongest overall in vivo cytotoxicity, despite not inducing OT-I proliferation in vitro. In summary, our results highlight important differences between Ccl3- and Xcl1- targeted DNA vaccines suggesting that chemokine fusion vaccines can be tailor-made for different diseases.

CD11c-specific bio-nanocapsule enhances vaccine immunogenicity by targeting immune cells

BACKGROUND

Various nanocarriers have been used to deliver subunit vaccines specifically to dendritic cells (DCs) for the improvement of immunogenicity. However, due to their insufficient DC priming ability, these vaccines could not elicit effective innate immunity. We have recently developed a DC-targeting bio-nanocapsule (BNC) by displaying anti-CD11c IgGs via protein A-derived IgG Fc-binding Z domain on the hepatitis B virus envelope L protein particles (α-DC-ZZ-BNC).

RESULTS

After the chemical modification with antigens (Ags), the α-DC-ZZ-BNC-Ag complex could deliver Ags to DCs efficiently, leading to effective DC maturation and efficient endosomal escape of Ags, followed by Ag-specific T cell responses and IgG productions. Moreover, the α-DC-ZZ-BNC modified with Japanese encephalitis virus (JEV) envelope-derived D3 Ags could confer protection against 50-fold lethal dose of JEV injection on mice.

CONCLUSION

The α-DC-ZZ-BNC-Ag platform was shown to induce humoral and cellular immunities effectively without any adjuvant.

Leaf-Encapsulated Vaccines: Agroinfiltration and Transient Expression of the Antigen Staphylococcal Endotoxin B in Radish Leaves

Transgene introgression is a major concern associated with transgenic plant-based vaccines. Agroinfiltration can be used to selectively transform nonreproductive organs and avoid introgression. Here, we introduce a new vaccine modality in which Staphylococcal enterotoxin B (SEB) genes are agroinfiltrated into radishes (Raphanw sativus L.), resulting in transient expression and accumulation of SEB in planta. This approach can simultaneously express multiple antigens in a single leaf. Furthermore, the potential of high-throughput vaccine production was demonstrated by simultaneously agroinfiltrating multiple radish leaves using a multichannel pipette. The expression of SEB was detectable in two leaf cell types (epidermal and guard cells) in agroinfiltrated leaves. ICR mice intranasally immunized with homogenized leaves agroinfiltrated with SEB elicited detectable antibody to SEB and displayed protection against SEB-induced interferon-gamma (IFN-γ) production. The concept of encapsulating antigens in leaves rather than purifying them for immunization may facilitate rapid vaccine production during an epidemic disease.

Antibody Response after Vaccination with Antigen-Pulsed Dendritic Cells

Dendritic cells (DCs) are the most potent antigen-presenting cells of the immune system capable of initiating immune responses to antigens. It is also well documented that cancer patients often experience anergy against tumor antigens. In this study we selected the best protocol for inducing the production of antibodies against the HER2 oncoprotein using DCs to overcome anergy. Murine DCs were pulsed in vitro, using different protocols, with recombinant HER2 fused to a human Fc (in order to improve DC antigen uptake) and were used to vaccinate mice. The obtained results indicate that antigen-pulsed DCs can induce an antibody response and that adding CpG after antigen pulsing greatly increases anti-HER2 antibody production.

Dendritic cell targeted vaccines: Recent progresses and challenges

Dendritic cells (DCs) are known to be a set of morphology, structure and function of heterogeneous professional antigen presenting cells (APCs), as well as the strongest functional antigen presenting cells, which can absorb, process and present antigens. As the key regulators of innate and adaptive immune responses, DCs are at the center of the immune system and capable of interacting with both B cells and T cells, thereby manipulating the humoral and cellular immune responses. DCs provide an essential link between the innate and adaptive immunity, and the strong immune activation function of DCs and their properties of natural adjuvants, make them a valuable target for antigen delivery. Targeting antigens to DC-specific endocytic receptors in combination with the relevant antibodies or ligands along with immunostimulatory adjuvants has been recently recognized as a promising strategy for designing an effective vaccine that elicits a strong and durable T cell response against intracellular pathogens and cancer. This opinion article provides a brief summary of the rationales, superiorities and challenges of existing DC-targeting approaches.

A Universal Influenza Vaccine Can Lead to Disease Exacerbation or Viral Control Depending on Delivery Strategies

The development of influenza A virus (IAV) vaccines, which elicits cross-strain immunity against seasonal and pandemic viruses is a major public health goal. As pigs are susceptible to human, avian, and swine-adapted IAV, they would be key targets of so called universal IAV vaccines, for reducing both the zoonotic risk and the economic burden in the swine industry. They also are relevant preclinical models. However, vaccination with conserved IAV antigens (AGs) in pigs was reported to elicit disease exacerbation. In this study, we assessed whether delivery strategies, i.e., dendritic cell (DC) targeting by the intradermal (ID) or intramuscular (IM) routes, impact on the outcome of the vaccination with three conserved IAV AGs (M2e, NP, and HA2) in pigs. The AGs were addressed to CD11c by non-covalent binding to biotinylated anti-CD11c monoclonal antibody. The CD11c-targeted AGs given by the ID route exacerbated disease. Conversely, CD11c-targeted NP injected by the IM route promoted T cell response compared to non-targeted NP. Furthermore, the conserved IAV AGs injected by the IM route, independently of DC targeting, induced both a reduction of viral shedding and a broader IgG response as compared to the ID route. Our findings highlight in a relevant animal species that the route of vaccine delivery impacts on the protection induced by conserved IAV AGs and on vaccine adverse effects. Finally, our results indicate that HA2 stands as the most promising conserved IAV AG for universal vaccine development.

Expanding the tools for identifying mononuclear phagocyte subsets in swine: Reagents to porcine CD11c and XCR1

Pig is a domestic species of major importance in the agro-economy and in biomedical research. Mononuclear phagocytes (MNP) are organized in subsets with specialized roles in the orchestration of the immune response and new tools are awaited to improve MNP subset identification in the pig. We cloned pig CD11c cDNA and generated a monoclonal antibody to pig CD11c which showed a pattern of expression by blood and skin MNP subsets similar to humans. We also developed a porcine XCL1-mCherry dimer which specifically reacted with the XCR1-expressing dendritic cell subset of the type 1 lineage in blood and skin. These original reagents will allow the efficient identification of pig MNP subsets to study their role in physiological and pathological processes and also to target these cells in novel intervention and vaccine strategies for veterinary applications and preclinical evaluations.

Efficient Qualitative and Quantitative Determination of Antigen-induced Immune Responses

To determine the effectiveness of immunization strategies used in therapeutic antibody or vaccine development, it is critical to assess the quality of immunization-induced polyclonal antibody responses. Here, we developed a workflow that uses sensitive methods to quantitatively and qualitatively assess immune responses against foreign antigens with regard to antibody binding affinity and epitope diversity. The application of such detailed assessments throughout an immunization campaign can significantly reduce the resources required to generate highly specific antibodies. Our workflow consists of the following two steps: 1) the use of surface plasmon resonance to quantify antigen-specific antibodies and evaluate their apparent binding affinities, and 2) the recovery of serum IgGs using an automated small scale purification system, followed by the determination of their epitope diversity using hydrogen deuterium exchange coupled with mass spectrometry. We showed that these methods were sensitive enough to detect antigen-specific IgGs in the nanogram/μl range and that they provided information for differentiating the antibody responses of the various immunized animals that could not be obtained by conventional methods. We also showed that this workflow can guide the selection of an animal that produces high affinity antibodies with a desired epitope coverage profile, resulting in the generation of potential therapeutic monoclonal antibody clones with desirable functional profiles. We postulate that this workflow will be an important tool in the development of effective vaccines to combat the highly sophisticated evasion mechanisms of pathogens.

Direct Delivery of Antigens to Dendritic Cells via Antibodies Specific for Endocytic Receptors as a Promising Strategy for Future Therapies

Dendritic cells (DCs) are the most potent professional antigen presenting cells and are therefore indispensable for the control of immunity. The technique of antibody mediated antigen targeting to DC subsets has been the basis of intense research for more than a decade. Many murine studies have utilized this approach of antigen delivery to various kinds of endocytic receptors of DCs both in vitro and in vivo. Today, it is widely accepted that different DC subsets are important for the induction of select immune responses. Nevertheless, many questions still remain to be answered, such as the actual influence of the targeted receptor on the initiation of the immune response to the delivered antigen. Further efforts to better understand the induction of antigen-specific immune responses will support the transfer of this knowledge into novel treatment strategies for human diseases. In this review, we will discuss the state-of-the-art aspects of the basic principles of antibody mediated antigen targeting approaches. A table will also provide a broad overview of the latest studies using antigen targeting including addressed DC subset, targeted receptors, outcome, and applied coupling techniques.

Enhanced Humoral Responses Induced by Targeting of Antigen to Murine Dendritic Cells

Dendritic cells (DCs) are superior in their ability to induce and control adaptive immune responses. These qualities have motivated the hypothesis that targeted delivery of antigen to DCs in vivo may be an effective way of enhancing immunization. Recent results show that antigen targeted to certain DC surface molecules may indeed induce robust immune responses. Targeting of antigen to DCs can be accomplished by the means of monoclonal antibodies. This study compared the humoral responses induced in mice by in vivo targeting of DCs using monoclonal antibodies specific for CD11c, CD36, CD205, Clec6A, Clec7A, Clec9A, Siglec-H and PDC-TREM. The results demonstrate that antigen delivery to different targets on DCs in vivo gives rise to humoral responses that differ in strength. Targeting of antigen to CD11c, CD36, CD205, Clec6A, Clec7A and PDC-TREM induced significantly stronger antibody responses compared to non-targeted isotype-matched controls. Targeting of Clec9A and Siglec-H did not lead to efficient antibody responses, which may be due to unfavourable properties of the targeting antibody, in which case, other antibodies with the same specificity might elicit a different outcome. Anti-CD11c was additionally used for elucidating the impact of the route of vaccination, and the results showed only minor differences between the antibody responses induced after immunization either s.c., i.v. or i.p. Altogether, these data show that targeting of different surface molecules on DCs result in very different antibody responses and that, even in the absence of adjuvants, strong humoral responses was induced.

ESAT-6 Targeting to DEC205+ Antigen Presenting Cells Induces Specific-T Cell Responses against ESAT-6 and Reduces Pulmonary Infection with Virulent Mycobacterium tuberculosis

Airways infection with Mycobacterium tuberculosis (Mtb) is contained mostly by T cell responses, however, Mtb has developed evasion mechanisms which affect antigen presenting cell (APC) maturation/recruitment delaying the onset of Ag-specific T cell responses. Hypothetically, bypassing the natural infection routes by delivering antigens directly to APCs may overcome the pathogen's naturally evolved evasion mechanisms, thus facilitating the induction of protective immune responses. We generated a murine monoclonal fusion antibody (α-DEC-ESAT) to deliver Early Secretory Antigen Target (ESAT)-6 directly to DEC205+ APCs and to assess its in vivo effects on protection associated responses (IFN-γ production, in vivo CTL killing, and pulmonary mycobacterial load). Treatment with α-DEC-ESAT alone induced ESAT-6-specific IFN-γ producing CD4+ T cells and prime-boost immunization prior to Mtb infection resulted in early influx (d14 post-infection) and increased IFN-γ+ production by specific T cells in the lungs, compared to scarce IFN-γ production in control mice. In vivo CTL killing was quantified in relevant tissues upon transferring target cells loaded with mycobacterial antigens. During infection, α-DEC-ESAT-treated mice showed increased target cell killing in the lungs, where histology revealed cellular infiltrate and considerably reduced bacterial burden. Targeting the mycobacterial antigen ESAT-6 to DEC205+ APCs before infection expands specific T cell clones responsible for early T cell responses (IFN-γ production and CTL activity) and substantially reduces lung bacterial burden. Delivering mycobacterial antigens directly to APCs provides a unique approach to study in vivo the role of APCs and specific T cell responses to assess their potential anti-mycobacterial functions.

Induction of antitumor immunity against cervical cancer by protein HPV-16 E7 in fusion with ricin B chain in tumor-bearing mice

OBJECTIVE

In immunotherapy of HPV-16-associated cervical cancers, the E7 protein is considered as a prime candidate. However, it is a poor inducer of a cytotoxic T-cell response when used as a singular antigen in protein vaccination. Therefore, to design effective cancer vaccines, the best tumor antigens should be combined with the most effective immunogens or drug delivery tools to achieve positive clinical results. In this study, we fused HPV-16 E7 with the lectin subunit of ricin toxin (RTB) from castor plant as a vaccine adjuvant/carrier.

MATERIALS AND METHODS

After reaching the soluble form of the recombinant protein, we designed 2 preventive and inhibition tumor models for investigation of the prevention and rejection of TC-1 cell growth in female C57BL/6 mice, respectively. In each model, mice were immunized with the recombinant protein of E7-RTB or E7 without any adjuvant.

RESULTS

We demonstrated that prophylactic immunization of E7-RTB protected mice against challenge from TC-1 cells. Also in the therapeutic model, E7-RTB could inhibit TC-1 tumor growth in the lung. The results were significant compared with the immunization of E7 singularly.

CONCLUSIONS

We concluded that immunization with E7-RTB protein without any adjuvant could generate antitumor effects in mice challenged with TC-1 cells. This research verifies the clinical applications and the future prospects for development of HPV-16 E7 therapeutic vaccines fused to immunoadjuvants.

The antigen presenting cells instruct plasma cell differentiation

The professional antigen presenting cells (APCs), including many subsets of dendritic cells and macrophages, not only mediate prompt but non-specific response against microbes, but also bridge the antigen-specific adaptive immune response through antigen presentation. In the latter, typically activated B cells acquire cognate signals from T helper cells in the germinal center of lymphoid follicles to differentiate into plasma cells (PCs), which generate protective antibodies. Recent advances have revealed that many APC subsets provide not only “signal 1” (the antigen), but also “signal 2” to directly instruct the differentiation process of PCs in a T-cell-independent manner. Herein, the different signals provided by these APC subsets to direct B cell proliferation, survival, class switching, and terminal differentiation are discussed. We furthermore propose that the next generation of vaccines for boosting antibody response could be designed by targeting APCs.

Prevention and Inhibition of TC-1 Cell Growth in Tumor Bearing Mice by HPV16 E7 Protein in Fusion with Shiga Toxin B-Subunit from shigella dysenteriae

OBJECTIVE

For immunotherapy of human papillomavirus (HPV) -16-associated cervical cancers the E7 protein is considered a prime candidate. However it is a poor inducer of cytotoxic T-cell response, when being used as a singular antigen in protein vaccination. Hence, in this study we focused on the utilization of a vaccine delivery system for prevention or treatment of cervical cancer.

MATERIALS AND METHODS

In this experimental study, we designed and evaluated a novel fusion protein comprising HPV16 E7 antigen fused to Shiga toxin B-subunit (STxB) as both an antigen vector and an adjuvant. Then we designed two preventive and therapeutic tumor models to investigate the prevention and inhibition of TC-1 cell growth in female C57BL/6 mice, respectively. In each model, mice were immunized with the recombinant protein of E7-STxB or E7 without any adjuvant.

RESULTS

We demonstrated that prophylactic immunization of E7-STxB protected mice against TC-1 cells. Also in the therapeutic model, E7-STxB inhibited TC-1 tumor growth inlungs. The results were significant when compared with the immunization of E7 singularly.

CONCLUSION

We concluded that immunization with the E7-STxB protein without any adjuvant could generate anti-tumor effect in mice challenged with TC-1 cells.This research verifies the clinical applications and the future prospects of developing HPV16 E7 therapeutic vaccines fused to immunoadjuvants.

Bone marrow dendritic cell-mediated regulation of TLR and B cell receptor signaling in B cells

Dendritic cells (DCs) play an essential role in regulation of immune responses. In the periphery, Ag presentation by DCs is critical for adaptive responses; for this reason, DCs are often targets of adjuvants that enhance vaccine responses. Activated mature DCs enhance B cell activation and differentiation by providing cytokines like BAFF and a proliferation-inducing ligand. However, the role of immature DCs in B cell tolerance is not well studied. Recently, mouse immature bone marrow-derived DCs (iBMDCs) have been shown to suppress anti-IgM-induced B cell activation. In this study, we tested the ability of mouse DCs to modulate B cell functions during TLR activation. We found that iBMDCs potently suppressed proliferation and differentiation of various B cell subsets on TLR stimulation. However, iBMDCs did not affect CD40-mediated B cell activation. Optimal suppression of B cell activation by iBMDCs required cell contact via the CD22 receptor on B cells. The B cell suppression was a property of iBMDCs or DCs resident in the bone marrow (BM), but not mature BM-derived DCs or DCs resident in the spleen. Presence of iBMDCs also enhanced the Ag-induced apoptotic response of BM B cells, suggesting that the suppressive effects of iBMDCs may have a role in B cell tolerance.

Antibody-antigen-adjuvant conjugates enable co-delivery of antigen and adjuvant to dendritic cells in cis but only have partial targeting specificity

Antibody-antigen conjugates, which promote antigen-presentation by dendritic cells (DC) by means of targeted delivery of antigen to particular DC subsets, represent a powerful vaccination approach. To ensure immunity rather than tolerance induction the co-administration of a suitable adjuvant is paramount. However, co-administration of unlinked adjuvant cannot ensure that all cells targeted by the antibody conjugates are appropriately activated. Furthermore, antigen-presenting cells (APC) that do not present the desired antigen are equally strongly activated and could prime undesired responses against self-antigens. We, therefore, were interested in exploring targeted co-delivery of antigen and adjuvant in cis in form of antibody-antigen-adjuvant conjugates for the induction of anti-tumour immunity. In this study, we report on the assembly and characterization of conjugates consisting of DEC205-specific antibody, the model antigen ovalbumin (OVA) and CpG oligodeoxynucleotides (ODN). We show that such conjugates are more potent at inducing cytotoxic T lymphocyte (CTL) responses than control conjugates mixed with soluble CpG. However, our study also reveals that the nucleic acid moiety of such antibody-antigen-adjuvant conjugates alters their binding and uptake and allows delivery of the antigen and the adjuvant to cells partially independently of DEC205. Nevertheless, antibody-antigen-adjuvant conjugates are superior to antibody-free antigen-adjuvant conjugates in priming CTL responses and efficiently induce anti-tumour immunity in the murine B16 pseudo-metastasis model. A better understanding of the role of the antibody moiety is required to inform future conjugate vaccination strategies for efficient induction of anti-tumour responses.

Targeting Dendritic Cells in vivo for Cancer Therapy

Monoclonal antibodies that recognize cell surface molecules have been used deliver antigenic cargo to dendritic cells (DC) for induction of immune responses. The encouraging anti-tumor immunity elicited using this immunization strategy suggests its suitability for clinical trials. This review discusses the complex network of DC, the functional specialization of DC subsets, the immunological outcomes of targeting different DC subsets and their cell surface receptors, and the requirements for the induction of effective anti-tumor CD4 and CD8 T cell responses that can recognize tumor-specific antigens. Finally, we review preclinical experiments and the progress toward targeting human DC in vivo.

Boosting antibody responses by targeting antigens to dendritic cells

Delivering antigens directly to dendritic cells (DCs) in situ, by injecting antigens coupled to antibodies specific for DC surface molecules, is a promising strategy for enhancing vaccine efficacy. Enhanced cytotoxic T cell responses are obtained if an adjuvant is co-administered to activate the DC. Such DC targeting is also effective at enhancing humoral immunity, via the generation of T follicular helper cells. Depending on the DC surface molecule targeted, antibody production can be enhanced even in the absence of adjuvants. In the case of Clec9A as the DC surface target, enhanced antibody production is a consequence of the DC-restricted expression of the target molecule. Few other cells absorb the antigen-antibody construct, therefore, it persists in the bloodstream, allowing sustained antigen presentation, even by non-activated DCs.

Ag-bearing liposomes engrafted with peptides that interact with CD11c/CD18 induce potent Ag-specific and antitumor immunity

Dendritic cells (DCs) play key role in eliciting antigen (Ag)-specific immune responses, and crucial to this is the uptake of Ag via surface receptors including the heterodimeric integrin CD11c/CD18. Here we report that CD11c/CD18-interacting peptides can be used as targeting moieties to deliver liposomal Ag to antigen presenting cells (APCs) and elicit Ag-specific and antitumor immunity. Two peptides of sequence related to human ICAM-4 and previously reported to bind CD11c/CD18, and a 12-mer cyclic peptide previously identified by phage display to bind CD11c/CD18, were produced synthetically, and tested for their ability to target liposomal Ag. The three peptides were designed to contain a shorter spacer to reduce steric hindrance, and a His-tag to enable engraftment onto liposomes incorporated with chelator lipid. Our results show that the three peptides, denoted as p17, p18 and p30, promote strong binding of liposomes to CD11c(+) and CD11b(+) cells in vitro and in vivo. Vaccination of mice with Ag-bearing liposomes engrafted with the peptides, particularly p18 and p30, induced Ag-specific T cell priming and antibody production. Importantly, the vaccination of C57BL/6 mice with syngeneic B16-OVA-derived plasma membrane vesicles (PMVs) engrafted with p18 and p30 peptide showed dramatic antitumor responses, inhibiting tumor growth/metastasis in both the lung and subcutaneous tumor models, with a high proportion of the mice apparently being "cured" of their tumors. The engraftment of p18 and p30 peptides onto liposomes and PMVs, thus provides an effective means to target Ags to DCs in vivo, for the development of effective cancer vaccines and immunotherapies.

Biochemical, pathological and oncological relevance of Gb3Cer receptor

Glycosphingolipids are amphipathic molecules composed of hydrophilic oligosaccharide chain and a hydrophobic ceramide part, located primarily in the membrane microdomains of animal cells. Their oligosaccharide chains make them excellent candidates for the cell surface recognition molecules. Natural glycosphingolipid, globotriaosylceramide (Gal α1-4, Gal β1-4, Glc β1-1, ceramide), is also called CD77 and its expression was previously associated with proliferating centroblasts undergoing somatic hypermutation, but it has been demonstrate that globotriaosylceramide is not a reliable marker to discriminate human centroblasts from centrocytes. Globotriaosylceramide constitutes rare P k blood group antigen on erythrocytes, and it is also known as Burkitt's lymphoma antigen. On endothelial cells, globotriaosylceramide plays as the receptor for bacterial toxins of the Shiga family, also called verotoxins. Precise biological function and significance of globotriaosylceramide expression on endothelial cells remains to be the subject of many studies and it is believed globotriaosylceramide represents an example of a glycolipid antigen able to transduce a signal leading to apoptosis. In past decade, cancer researches put a great afford in determining new therapeutic agents such as bacterial toxins against tumor malignancies. Reports have demonstrated that verotoxin-1 induces apoptosis in solid tumor cell lines expressing globotriaosylceramide such as astrocytoma, renal cell carcinoma, colon cancer and breast cancer due to verotoxin-1 high specificity and apoptosis-inducing properties, and therefore, it is suggested to be an anticancer agent. Verotoxins have been investigated weather they could reduce treatment side-effects and toxicity to normal tissues and become a new oncological tool in cancer labeling.

Human mannose receptor (CD206) in immune response: novel insights into vaccination strategies using a humanized mouse model

Evaluation of: He LZ, Crocker A, Lee J et al. Antigenic targeting of the human mannose receptor induces tumor immunity. J. Immunol. 178(10), 6259-6267 (2007). Induction of adaptive immune response requires, in most cases, internalization of the antigen by professional antigen-presenting cells. Various endocytic receptors may mediate such interaction. However, little is known regarding the exact participation of such receptors in the context of immune response in vivo. This paper evaluates a recent work that utilizes one such molecule, the human CD206, transgenically incorporated into mouse, in order to establish its role in immune response. Such a study benefits from the fact that the function of the human form of the receptor is elaborated and thus would help to define new targets for future vaccine strategies.

Syk-coupled C-type lectin receptors that mediate cellular activation via single tyrosine based activation motifs

Different dendritic cell (DC) subsets have distinct specialized functions contributed in part by their differential expression of pattern recognition receptors (PRRs). C-type lectin receptors (CLRs) are a group of PRRs expressed by DCs and other myeloid cells that can recognize endogenous ligands as well as a wide range of exogenous structures present on pathogens. Dual roles in homeostasis and immunity have been demonstrated for some members of this receptor family. Largely due to their endocytic ability and subset specific expression, DC-expressed CLRs have been the focus of significant antigen-targeting studies. A number of CLRs function on the basis of signaling via association with immunoreceptor tyrosine-based activation motif (ITAM)-containing adapter proteins. Others contain ITAM-related motifs or immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic tails. Here we review CLRs that induce intracellular signaling via a single tyrosine-based ITAM-like motif and highlight their relevance in terms of DC function.

Ligation of CD11c during vaccination promotes germinal centre induction and robust humoral responses without adjuvant

In this study, we investigated the mouse dendritic cell (DC) receptor, complement receptor 4 (CR4; CD11c/CD18), as an immunotarget for triggering humoral immunity. Comparison of antibody titres generated against a panel of 13 anti-antigen-presenting cell receptor monoclonal antibodies, with or without conjugated ovalbumin (OVA), revealed uniquely rapid and robust responses following CR4 targeting, with antibody titres approaching 1 : 100 000 7 days after a single dose of antigen. Furthermore, using just 100 ng OVA conjugated to anti-CD11c Fab', we generated anti-OVA titres greater than those produced by a 100-fold higher dose of OVA in complete Freund's adjuvant at day 28. These anti-OVA antibody titres were sustained and could be boosted further with targeted OVA on day 21. Investigations to explain this vaccine potency showed that, in addition to targeting splenic DC, anti-CDl1c antibodies delivered a powerful adjuvant effect and could boost humoral immunity against OVA even when the OVA was targeted to other molecules on DC, such as major histocompatibility complex class II, CD11a and CD11b. However, interestingly, this adjuvant effect was lost if OVA was targeted to other cells such as B cells via CD21 or CD19. The adjuvant effect was mediated through a marked enhancement of both germinal centre and extrafollicular plasma cell formation in responding spleens. These results demonstrate that anti-CD11c monoclonal antibody can both target antigen and act as a powerful adjuvant for rapid and sustained antibody responses. They also point to an interesting role for CR4 on DC in triggering B cells during humoral immunity.

Targeted delivery of tumor antigens to activated dendritic cells via CD11c molecules induces potent antitumor immunity in mice

PURPOSE

CD11c is an antigen receptor predominantly expressed on dendritic cells (DC), to which antigen targeting has been shown to induce robust antigen-specific immune responses. To facilitate targeted delivery of tumor antigens to DCs, we generated fusion proteins consisting of the extracellular domain of human HER or its rat homologue neu, fused to the single-chain fragment variable specific for CD11c (scFv(CD11c)-HER2/neu).

EXPERIMENTAL DESIGN

Induction of cellular and humoral immune responses and antitumoral activity of the fusion proteins admixed with DC-activating CpG oligonucleotides (scFv(CD11c)-HER2/neu(CpG)) were tested in transplantable HER2/neu-expressing murine tumor models and in transgenic BALB-neuT mice developing spontaneous neu-driven mammary carcinomas.

RESULTS

Vaccination of BALB/c mice with scFv(CD11c)-HER2(CpG) protected mice from subsequent challenge with HER2-positive, but not HER2-negative, murine breast tumor cells, accompanied by induction of strong HER2-specific T-cell and antibody responses. In a therapeutic setting, injection of scFv(CD11c)-HER2(CpG) caused rejection of established HER2-positive tumors. Importantly, antitumoral activity of such a fusion protein vaccine could be reproduced in immunotolerant BALB-neuT mice, where scFv(CD11c)-neu(CpG) vaccination significantly protected against a subsequent challenge with neu-expressing murine breast tumor cells and markedly delayed the onset of spontaneous mammary carcinomas.

CONCLUSIONS

CD11c-targeted protein vaccines for in vivo delivery of tumor antigens to DCs induce potent immune responses and antitumoral activities and provide a rationale for further development of this approach for cancer immunotherapy.

Enhancing immune responses by targeting antigen to DC

mAb that recognise various cell surface receptors have been used to deliver antigen to DC and thereby elicit immune responses. The encouraging data obtained in mouse models suggests that this immunisation strategy is efficient and could lead to clinical trials. We discuss a number of issues pertinent to this vaccination approach. These include which molecules are the best targets for delivering antigen to DC, which DC subtypes should be targeted, the types of immune responses to be generated and whether additional adjuvants are required. Finally, we discuss some progress towards targeting antigen to human DC.

CD11c provides an effective immunotarget for the generation of both CD4 and CD8 T cell responses

The magnitude and quality of T cell responses generated when Ag is targeted to receptors on DC is influenced by both the specific receptor targeted and its distribution among DC subsets. Here we examine the targeting of the model Ag OVA to potential DC targets, including CD11c, CD205, MHC class II, CD40, TLR2 and FcgammaRII/III, using a panel of (Fab' x OVA) conjugates. In vitro studies identified CD11c, CD205 and MHC class II as superior and comparably effective immunotargets for the delivery of OVA to APC for presentation to T cells. In vivo studies, however, showed a marked advantage of targeting Ag to CD11c for both CD4 (OT-II) and CD8 (OT-I) responses, with robust stimulation after a single, low dose (equivalent to 0.5 microg OVA); in contrast, (anti-CD205 x OVA) and (anti-MHC class II x OVA) resulted in markedly less proliferation of both OT-I and OT-II cells. Biodistribution and immunohistochemical studies suggest that the exceptional ability of CD11c to capture Ag in lymphoid tissues may, at least partially, explain its ability to promote T cell responses. These results suggest that targeting antigen via CD11c offers a previously unappreciated strategy for vaccine development which, unlike most targets, delivers robust responses of both CD4 and CD8 T cells.

CD11c: not merely a murine DC marker, but also a useful vaccination target

The induction of robust CD4(+) and CD8(+) T cell responses is a central aim in antiviral and anticancer vaccination. To this end, dendritic cells (DC) need to capture the vaccine, process and present it on MHC class I and II molecules. The mechanisms by which DC acquire antigen predetermine the quantity and quality of the ensuing T cell activation. In this issue of the European Journal of Immunology, it is demonstrated that targeting antigen towards CD11c, an integrin expressed preferentially by murine DC, facilitates efficient CD4(+) and CD8(+) T cell activation. The underlying mechanisms involve efficient antigen delivery into the marginal zone and T/DC zone of the spleen. This commentary discusses these findings in the context of current knowledge on antigen presentation.

The dendritic cell subtype-restricted C-type lectin Clec9A is a target for vaccine enhancement

A novel dendritic cell (DC)-restricted molecule, Clec9A, was identified by gene expression profiling of mouse DC subtypes. Based on sequence similarity, a human ortholog was identified. Clec9A encodes a type II membrane protein with a single extracellular C-type lectin domain. Both the mouse Clec9A and human CLEC9A were cloned and expressed, and monoclonal antibodies (mAbs) against each were generated. Surface staining revealed that Clec9A was selective for mouse DCs and was restricted to the CD8(+) conventional DC and plasmacytoid DC subtypes. A subset of human blood DCs also expressed CLEC9A. A single injection of mice with a mAb against Clec9A, which targets antigens (Ags) to the DCs, produced a striking enhancement of antibody responses in the absence of added adjuvants or danger signals, even in mice lacking Toll-like receptor signaling pathways. Such targeting also enhanced CD4 and CD8 T-cell responses. Thus, Clec9A serves as a new marker to distinguish subtypes of both mouse and human DCs. Furthermore, targeting Ags to DCs with antibodies to Clec9A is a promising strategy to enhance the efficiency of vaccines, even in the absence of adjuvants.

Making friends in out-of-the-way places: how cells of the immune system get together and how they conduct their business as revealed by intravital imaging

A central characteristic of the immune system is the constantly changing location of most of its constituent cells. Lymphoid and myeloid cells circulate in the blood, and subsets of these cells enter, move, and interact within, then leave organized lymphoid tissues. When inflammation is present, various hematopoietic cells also exit the vasculature and migrate within non-lymphoid tissues, where they carry out effector functions that support host defense or result in autoimmune pathology. Effective innate and adaptive immune responses involve not only the action of these individual cells but also productive communication among them, often requiring direct membrane contact between rare antigen-specific or antigen-bearing cells. Here, we describe our ongoing studies using two-photon intravital microscopy to probe the in situ behavior of the cells of the immune system and their interactions with non-hematopoietic stromal elements. We emphasize the importance of non-random cell migration within lymphoid tissues and detail newly established mechanisms of traffic control that operate at multiple organizational scales to facilitate critical cell contacts. We also describe how the methods we have developed for imaging within lymphoid sites are being applied to other tissues and organs, revealing dynamic details of host-pathogen interactions previously inaccessible to direct observation.

Carbohydrate-mediated targeting of antigen to dendritic cells leads to enhanced presentation of antigen to T cells

The unique therapeutic value of dendritic cells (DCs) for the treatment of allergy, autoimmunity and transplant rejection is predicated upon our ability to selectively deliver antigens, drugs or nucleic acids to DCs in vivo. Here we describe a method for delivering whole protein antigens to DCs based on carbohydrate-mediated targeting of DC-expressed lectins. A series of synthetic carbohydrates was chemically-coupled to a model antigen, ovalbumin (OVA), and each conjugate was evaluated for its ability to increase the efficiency of antigen presentation by murine DCs to OVA-specific T cells (CD4(+) and CD8(+)). In vitro data are presented that demonstrate that carbohydrate modification of OVA leads to a 50-fold enhancement of presentation of antigenic peptide to CD4(+) T cells. A tenfold enhancement is observed for CD8(+) T cells; this indicates that the targeted lectin(s) can mediate cross-presentation of antigens on MHC class I. Our data indicate that the observed enhancements in antigen presentation are unique to OVA that is conjugated to complex oligosaccharides, such as a high-mannose nonasaccharide, but not to monosaccharides. Taken together, our data suggest that a DC targeting strategy that is based upon carbohydrate-lectin interactions is a promising approach for enhancing antigen presentation via class I and class II molecules.

Antibody-targeted vaccines

The specificity and high affinity binding of antibodies provides these molecules with ideal properties for delivering a payload to target cells. This concept has been commercialized for cancer therapies using toxin- or radionucleotide-conjugated antibodies that are designed to selectively deliver cytotoxic molecules to cancer cells. Exploiting the same effective characteristics of antibodies, antibody-targeted vaccines (ATV) are designed to deliver disease-specific antigens to professional antigen-presenting cells (APCs), thus enabling the host's immune system to recognize and eliminate malignant or infected cells through adaptive immunity. The concept of ATVs has been in development for many years, and recently has entered clinical trials. Early studies with ATVs focused on the ability to induce humoral immunity in the absence of adjuvants. More recently, ATVs targeted to C-type lectin receptors have been exploited for induction of potent helper and cytolytic T-cell responses. To maximize their stimulatory capacity, the ATVs are being evaluated with a variety of adjuvants or other immunostimulatory agents. In the absence of co-administered immunostimulatory signals, APC-targeting can induce antigen-specific tolerance and, thus, may also be exploited in developing specific treatments for autoimmune and allergic diseases, or for preventing transplant rejection. The successful clinical application of this new class of antibody-based products will clearly depend on using appropriate combinations with other strategies that influence the immune system.

Human monoclonal antibodies against anthrax lethal factor and protective antigen act independently to protect against Bacillus anthracis infection and enhance endogenous immunity to anthrax

The unpredictable nature of bioterrorism and the absence of real-time detection systems have highlighted the need for an efficient postexposure therapy for Bacillus anthracis infection. One approach is passive immunization through the administration of antibodies that mitigate the biological action of anthrax toxin. We isolated and characterized two protective fully human monoclonal antibodies with specificity for protective antigen (PA) and lethal factor (LF). These antibodies, designated IQNPA (anti-PA) and IQNLF (anti-LF), were developed as hybridomas from individuals immunized with licensed anthrax vaccine. The effective concentration of IQNPA that neutralized 50% of the toxin in anthrax toxin neutralization assays was 0.3 nM, while 0.1 nM IQNLF neutralized the same amount of toxin. When combined, the antibodies had additive neutralization efficacy. IQNPA binds to domain IV of PA containing the host cell receptor binding site, while IQNLF recognizes domain I containing the PA binding region in LF. A single 180-mug dose of either antibody given to A/J mice 2.5 h before challenge conferred 100% protection against a lethal intraperitoneal spore challenge with 24 50% lethal doses [LD50s] of B. anthracis Sterne and against rechallenge on day 20 with a more aggressive challenge dose of 41 LD50s. Mice treated with either antibody and infected with B. anthracis Sterne developed detectable murine anti-PA and anti-LF immunoglobulin G antibody responses by day 17 that were dependent on which antibody the mice had received. Based on these results, IQNPA and IQNLF act independently during prophylactic anthrax treatment and do not interfere with the establishment of endogenous immunity.

Antigenic targeting of the human mannose receptor induces tumor immunity

Pattern recognition receptors are preferentially expressed on APCs allowing selective uptake of pathogens for the initiation of antimicrobial immunity. In particular, C-type lectin receptors, including the mannose receptor (MR), facilitate APC-mediated adsorptive endocytosis of microbial glyconjugates. We have investigated the potential of antigenic targeting to the MR as a means to induce Ag-specific humoral and cellular immunity. hMR transgenic (hMR Tg) mice were generated to allow specific targeting with the anti-hMR Ab, B11. We show that hMR targeting induced both humoral and cellular antigenic specific immunity. Immunization of hMR Tg mice with B11 mAbs induced potent humoral responses independent of adjuvant. Injection of hMR Tg mice with mouse anti-hMR Ab clone 19.2 elicited anti-Id-specific humoral immunity while non-Tg mice were unresponsive. B11-OVA fusion proteins (B11-OVA) were efficiently presented to OVA-specific CD4 and CD8 T cells in MR Tg, but not in non-Tg, mice. Effector differentiation of responding T cells in MR Tg mice was significantly enhanced with concomitant immunization with the TLR agonist, CpG. Administration of both CpG and B11-OVA to hMR Tg mice induced OVA-specific tumor immunity while WT mice remained unprotected. These studies support the clinical development of immunotherapeutic approaches in cancer using pattern recognition receptor targeting systems for the selective delivery of tumor Ags to APCs.

A mechanistic study of immune system activation by fusion of antigens with the ligand-binding domain of CTLA4

Fusion proteins consisting of the ligand-binding domain of CTLA4 covalently attached to an antigen (Ag) are potent immunogens. This fusion strategy effectively induces Ag-specific immunity both when introduced as a DNA-based vaccine and as a recombinant protein. CTLA4 is a ligand for B7 molecules expressed on the surface of antigen-presenting cells (APCs), and this interaction is critical for the fusion protein to stimulate Ag-specific immunity. We show that interaction of the fusion protein with either B7-1 or B7-2 is sufficient to stimulate immune activity, and that T cells are essential for the development of IgG responses. In addition, we demonstrate that human dendritic cells (DCs) pulsed with CTLA4-Ag fusion proteins can efficiently present Ag to T cells and induce an Ag-specific immune response in vitro. These studies provide further mechanistic understanding of the process by which CTLA4-Ag fusion proteins stimulate the immune system, and represent an efficient means of generating Ag-specific T cells for immunotherapy.