Comparing macrophages and dendritic leukocytes as antigen-presenting cells for humoral responses in vivo by antigen targeting

Fused Mycobacterium tuberculosis multi-stage immunogens with an Fc-delivery system as a promising approach for the development of a tuberculosis vaccine

Tuberculosis (TB) remains a major health problem worldwide. Currently, the Bacilli Calmette-Guérin (BCG) is the only available licensed TB vaccine, which has low efficacy in protection against adult pulmonary TB. Therefore, the development of a safe and effective vaccine against TB needs global attention. In the present study, a novel multi-stage subunit vaccine candidate from culture filtrate protein-10 (CFP-10) and heat shock protein X (HspX) of Mycobacterium tuberculosis fused to the Fc domain of mouse IgG2a as a selective delivery system for antigen-presenting cells (APCs) was produced and its immunogenicity assessed. The optimized gene constructs were introduced into pPICZαA expression vectors, and the resultant plasmids (pPICZαA-CFP-10:Hspx:Fcγ2a and pPICZαA-CFP-10:Hspx:His) were transferred into Pichia pastoris by electroporation. The identification of both purified recombinant fusion proteins was evaluated by SDS-PAGE and immunoblotting. Then the immunogenicity of the recombinant proteins with and without BCG was evaluated in BALB/c mice by assessing the level of IFN-γ, IL-12, IL-4, IL-17 and TGF-β cytokines. Both multi-stage vaccines (CFP-10:HspX:Fcγ2a and CFP-10:HspX:His) induced Th1-type cellular responses by producing high level of IFN-γ (272 pg/mL, p<0.001) and IL-12 (191 pg/mL, p<0.001). However, the Fc-tagged recombinant protein induced more effective Th1-type cellular responses with a low level of IL-4 (10 pg/mL) compared to the CFP-10:HspX:His group. The production of IFN-γ to CFP-10:HspX:Fcγ2a was markedly consistent and showed an increasing trend for IL-12 compared with the BCG or CFP-10:HspX:His primed and boosted groups. Findings revealed that CFP-10:Hspx:Fcγ2a fusion protein can elicit strong Th1 antigen-specific immune responses in favor of protective immunity in mice and could provide new insight for introducing an effective multi-stage subunit vaccine against TB.

Targeting antigens to CD180 rapidly induces antigen-specific IgG, affinity maturation, and immunological memory

Targeting antigen to B cells and dendritic cells via direct conjugation to anti-CD180 antibody promotes robust antigen-specific antibody responses in the absence of adjuvant.

Antigen (Ag) targeting is an efficient way to induce immune responses. Ag is usually coupled to an antibody (Ab) specific for a receptor expressed on dendritic cells (DCs), and then the Ag–anti-receptor is inoculated with an adjuvant. Here we report that targeting Ag to a receptor expressed on both B cells and DCs, the TLR orphan receptor CD180, in the absence of adjuvant rapidly induced IgG responses that were stronger than those induced by Ag in alum. Ag conjugated to anti-CD180 (Ag-αCD180) induced affinity maturation and Ab responses that were partially T cell independent, as Ag-specific IgGs were generated in CD40- and T cell–deficient mice. After preimmunization with Ag-αCD180 and boosting with soluble Ag, both WT and CD40 knockout (KO) mice rapidly produced Ag-specific IgG-forming cells, demonstrating that Ag–anti-CD180 induces immunological memory. The potent adjuvant effect of Ag-αCD180 required Ag to be coupled to anti-CD180 and the responsive B cells to express both CD180 and an Ag-specific B cell receptor. Surprisingly, CD180 Ag targeting also induced IgG Abs in BAFF-R KO mice lacking mature B cells and in mice deficient in interferon signaling. Targeting Ag to CD180 may be useful for therapeutic vaccination and for vaccinating the immune compromised.

DNA vaccine that targets hemagglutinin to MHC class II molecules rapidly induces antibody-mediated protection against influenza

New influenza A viruses with pandemic potential periodically emerge due to viral genomic reassortment. In the face of pandemic threats, production of conventional egg-based vaccines is time consuming and of limited capacity. We have developed in this study a novel DNA vaccine in which viral hemagglutinin (HA) is bivalently targeted to MHC class II (MHC II) molecules on APCs. Following DNA vaccination, transfected cells secreted vaccine proteins that bound MHC II on APCs and initiated adaptive immune responses. A single DNA immunization induced within 8 d protective levels of strain-specific Abs and also cross-reactive T cells. During the Mexican flu pandemic, a targeted DNA vaccine (HA from A/California/07/2009) was generated within 3 wk after the HA sequences were published online. These results suggest that MHC II-targeted DNA vaccines could play a role in situations of pandemic threats. The vaccine principle should be extendable to other infectious diseases.

Ablation of tumor progression locus 2 promotes a type 2 Th cell response in Ovalbumin-immunized mice

The protein kinase encoded by the Tpl2 proto-oncogene regulates ERK activation and cytokine gene expression in macrophages in response to LPS and TNF-alpha. In this study we show that OVA-immunized Tpl2(-/-) mice express high levels of IgE and develop more severe bronchoalveolar eosinophilic inflammation than Tpl2(+/+) controls, when challenged with OVA intranasally. Bronchoalveolar exudates and supernatants of OVA-stimulated splenocytes from immunized Tpl2(-/-) mice express elevated levels of IL-4 and IL-5, suggesting that Tpl2 ablation promotes the Th2 polarization of the T cell response. Anti-CD3 stimulation of CD4(+) T cells of wild-type and Tpl2 knockout mice revealed that Tpl2 ablation gives rise to a cell autonomous T cell defect that is primarily responsible for the Th2 polarization of the T cell response to Ag. This observation was further supported by experiments addressing the expression of Th1 and Th2 cytokines in OVA-stimulated mixed cultures of CD4(+) T cells from Tpl2(+/+)/OT2 or Tpl2(-/-)/OT2 mice and dendritic cells from Tpl2(+/+) or Tpl2(-/-) mice. Further studies revealed that Th1 cells express significantly higher levels of Tpl2 than Th2 cells. As a result, Tpl2(-/-) Th1 cells exhibit a stronger defect in ERK activation by anti-CD3 than Th2 cells and express low levels of T-bet. Given that the development of Th1 and Th2 cells depends on positive feedback signals from the T cells, themselves, the functional defect of the Tpl2(-/-) Th1 cells provides a mechanistic explanation for the T cell autonomous Th2 polarization in Tpl2(-/-) mice.

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.

Rapid monoclonal antibody generation via dendritic cell targeting in vivo

Dendritic cells (DC) are the professional antigen-presenting cells of the immune system. Previous studies have demonstrated that targeting foreign antigens to DC leads to enhanced antigen (Ag)-specific responses in vivo. However, the utility of this strategy for the generation of MAbs has not been investigated. To address this question we immunized mice with IgG-peptide conjugates prepared with the hamster anti-murine CD11c MAb N418. Synthetic peptides corresponding to two different exposed regions of DC-specific ICAM-3 grabbing nonintegrin (DC-SIGN), a human C-type lectin, were conjugated to N418 using thiol-based chemistry. The N418 MAb served as the targeting molecule and synthetic peptides as the Ag (MAb-Ag). A rapid and peptide specific serum IgG response was produced by Day 7 when the synthetic peptides were linked to the N418 MAb, compared to peptide co-delivered with the N418 without linkage. Spleen cells from N418-peptide immunized mice were fused on Day 10, and three IgG1/k monoclonal antibodies (MAbs) were selected to one of the peptide epitopes (MID-peptide). One of the MAbs, Novik 2, bound to two forms of recombinant DC-SIGN protein in enzyme-linked immunosorbent assay (ELISA), and was specifically inhibited by the MID-peptide in solution. Two of these MAbs show specific binding to DC-SIGN expressed by cultured human primary DC. We conclude that in vivo DC targeting enhances the immunogenicity of synthetic peptides and is an effective method for the rapid generation of MAbs to predetermined epitopes.

Prolonged maturation and enhanced transduction of dendritic cells migrated from human skin explants after in situ delivery of CD40-targeted adenoviral vectors

Therapeutic tumor vaccination with viral vectors or naked DNA, carrying the genetic code for tumor-associated Ags, critically depends on the in vivo transduction of dendritic cells (DC). Transfection of predominantly nonprofessional APC and only small numbers of DC may hamper proper T cell activation. Aim of this study was, therefore, the targeted, selective, and enhanced in situ transduction of DC. A human skin explant model was used to explore targeted transduction of cutaneous DC after intradermal injection of a bispecific Ab conjugate to link adenoviral (Ad) vectors directly to CD40 on the DC surface. A significantly enhanced transduction efficiency and selectivity, and an increased activation state of migrating DC were thus achieved. Moreover, DC transduced by CD40-targeted Ad maintained their Ag-specific CTL-stimulatory ability for up to 1 wk after the start of migration, in contrast to DC transduced by untargeted Ad, which had lost this capacity by that time. Because DC targeting in vivo might obviate the need for the in vitro culture of autologous DC for adoptive transfer, CD40-targeted Ad vectors constitute a promising new vaccine modality for tumor immunotherapy.

'Troy-bodies': antibodies as vector proteins for T cell epitopes

A major objective in vaccine development is the design of reagents that give a strong, specific T cell response. Targeting of antigens to antigen presenting cells (APC) results in enhanced antigen presentation and T cell activation. In this paper, we describe a novel targeting reagent denoted 'Troy-bodies', namely recombinant antibodies with APC-specificity and with T cell epitopes integrated in their C regions. We have made such antibodies with V regions specific for either IgD or MHC class II, and five different T cell epitopes have been tested. All epitopes could be introduced into loops of C domains without disrupting immunoglobulin (Ig) folding. Four have been tested in T cell activation studies, and all could be released and presented by APC. Furthermore, whether IgD- or MHC-specific, the molecules tested enhanced T cell stimulation compared to non-specific control antibodies in vitro as well as in vivo. Using this technology, specific reagents can be designed that target selected antigenic peptides to an APC of choice. Troy-bodies may therefore be useful for manipulation of immune responses, and in particular for vaccination purposes.

"Troy-bodies": recombinant antibodies that target T cell epitopes to antigen presenting cells

Targeting of antigens to antigen presenting cells (APC) results in enhanced antigen presentation and T cell activation. In this paper, we describe a novel targeting reagent denoted "Troy-bodies", namely recombinant antibodies with APC-specific V regions and C regions with integrated T cell epitopes. We have made such antibodies with V regions specific for either IgD or MHC class II, and four different T cell epitopes have been tested. All four epitopes could be introduced into loops of C domains without disrupting Ig folding, and they could be released and presented by APC. Furthermore, whether IgD- or MHC-specific, the molecules enhanced T cell stimulation compared to non-specific control antibodies in vitro as well as in vivo. Using this technology, specific reagents can be designed that target selected antigenic peptides to an APC of choice. Troy-bodies may therefore be useful for manipulation of immune responses, and in particular for vaccination purposes.

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

We have compared the kinetics of antibody responses in conventional and dendritic cell-targeted immunization by using a model antigen in mice. Targeting was achieved by linking the reporter antigen (polyclonal goat anti-hamster antibody) to N418, a hamster mAb that binds to the CD11c molecule on the surface of murine dendritic cells. Intradermal injection of submicrogram quantities of goat anti-hamster antibody complexed to mAb N418 elicited goat antibody-specific serum IgG in mice. Antigen-specific IgG titers were detectable by day 5, with titers that ranged from 1:1000 to 1:100,000 by day 7. In contrast, when the goat antigen was injected alone or in the presence of a hamster antibody control to form nontargeted complexes, goat-specific serum IgG was undetectable at day 7. Additional control experiments showed that the interaction between the model antigen and mAb N418 is required for amplification of the serum antibody response. These studies demonstrate that a single-step, facilitated-delivery of small amounts of protein antigen to dendritic cells in vivo can give very rapid and high antibody responses. The approach may be particularly useful for vaccination immediately before or just after exposure to a pathogen and may enhance the utility of subunit antigens as immunogens.

Differences in IgG subclass do not effect immune complex-enhanced T cell activation despite differential binding to antigen presenting cells

Ag presentation to CD4 T cells is a critical event in the generation of protective immunity. IgG, in the form of IgG-pathogen (Ag) complexes, is capable of mediating FcgammaR-dependent Ag presentation, and thereby enhanced T cell activation. Therefore, it is important to understand the ability of the individual human IgG subclasses to function in enhanced T cell activation. We hypothesized that increased delivery of Ag to monocyte FcgammaR by high affinity human IgG subclasses, IgG1 and IgG3, would lead to increased Ag presentation, as compared to low affinity IgG subclasses, IgG2 and IgG4. To create immune complexes, we linked biotinylated IgG subclasses to biotinylated Ag via an avidin bridge, and examined T cell responses to them. Although IgG2- and IgG4-Ag complexes bound to monocytes at significantly lower levels than those made with IgG1 and IgG3, we observed no significant difference in the ability of the four human IgG subclasses to mediate enhanced T cell activation. Studies suggest the explanation for this dichotomy lies within the first 24 h of Ag processing, and that processing efficiency may vary with IgG subclass. They also suggest the existence of a highly efficient, and selective processing pathway, which is dependent on IgG subclass, and can compensate for low level production and FcgammaR binding of IgG2- and IgG4-Ag complexes.

Increasing Immunogenicity of Antigens Fused to Ig-Binding Proteins by Cell Surface Targeting

Fusion of antigenic proteins to Ig-binding proteins such as protein A from Staphylococcus aureus and its derived ZZ fragment is known to increase immunogenicity of the fused Ag in vivo. To shed light on the origin of this effect, we used snake toxins as Ags and observed that 1) fusion of toxins to ZZ enhanced their presentation to a toxin-specific T cell hybridoma (T1B2), using A20 B lymphoma cells, splenocytes, or peritoneal exudate cells as APCs; 2) this enhancement further increased when the number of fused Ig-binding domains varied from two with ZZ to five with protein A; and 3) the phenomenon vanished when the fusion protein was preincubated with an excess of free ZZ or when P388D1 monocytes cells were used as APCs. Therefore, ZZ-fused toxins are likely to be targeted to surface Igs of APCs by their ZZ moiety. Furthermore, ZZ-α and toxin α stimulated similar profiles of toxin-specific T cells in BALB/c mice, suggesting a comparable processing and presentation in vivo for both toxin forms. To improve the targeting efficiency, ZZ-α was noncovalently complexed to various Igs directed to different cell surface components of APCs. The resulting complexes were up to 103-fold more potent than the free toxin at stimulating T1B2. Also, they elicited both a T cell and an Ab response in BALB/c mice, without the need of any adjuvant. This simple approach may find practical applications by increasing the immunogenicity of recombinant proteins without the use of adjuvant.

CD34+ Hematopoietic Progenitors From Human Cord Blood Differentiate Along Two Independent Dendritic Cell Pathways in Response to Granulocyte-Macrophage Colony-Stimulating Factor Plus Tumor Necrosis Factor α: II. Functional Analysis

In response to granulocyte-macrophage colony-stimulating factor plus tumor necrosis factor α, cord blood CD34+ hematopoietic progenitor cells differentiate along two unrelated dendritic cell (DC) pathways: (1) the Langerhans cells (LCs), which are characterized by the expression of CD1a, Birbeck granules, the Lag antigen, and E cadherin; and (2) CD14+ cell-derived DCs, characterized by the expression of CD1a, CD9, CD68, CD2, and factor XIIIa (Caux et al, J Exp Med 184:695, 1996). The present study investigates the functions of each population. Although the two populations are equally potent in stimulating naive CD45RA cord blood T cells through apparently identical mechanisms, each also displays specific activities. In particular CD14-derived DCs show a potent and long-lasting (from day 8 to day 13) antigen uptake activity (fluorescein isothiocyanate dextran or peroxidase) that is about 10-fold higher than that of CD1a+ cells, which is restricted to the immature stage (day 6). The antigen capture is exclusively mediated by receptors for mannose polymers. The high efficiency of antigen capture of CD14-derived cells is coregulated with the expression of nonspecific esterase activity, a tracer of lysosomial compartment. In contrast, the CD1a+ population never expresses nonspecific esterase activity. The most striking difference is the unique capacity of CD14-derived DCs to induce naive B cells to differentiate into IgM-secreting cells, in response to CD40 triggering and interleukin-2. Thus, although the two populations can allow T-cell priming, initiation of humoral responses might be preferentially regulated by the CD14-derived DCs. Altogether, those results show that different pathways of DC development might exist in vivo: (1) the LC type, which might be mainly involved in cellular immune responses, and (2) the CD14-derived DC related to dermal DCs or circulating blood DCs, which could be involved in humoral immune responses.

CD34+ Hematopoietic Progenitors From Human Cord Blood Differentiate Along Two Independent Dendritic Cell Pathways in Response to Granulocyte-Macrophage Colony-Stimulating Factor Plus Tumor Necrosis Factor α: II. Functional Analysis

In response to granulocyte-macrophage colony-stimulating factor plus tumor necrosis factor α, cord blood CD34+ hematopoietic progenitor cells differentiate along two unrelated dendritic cell (DC) pathways: (1) the Langerhans cells (LCs), which are characterized by the expression of CD1a, Birbeck granules, the Lag antigen, and E cadherin; and (2) CD14+ cell-derived DCs, characterized by the expression of CD1a, CD9, CD68, CD2, and factor XIIIa (Caux et al, J Exp Med 184:695, 1996). The present study investigates the functions of each population. Although the two populations are equally potent in stimulating naive CD45RA cord blood T cells through apparently identical mechanisms, each also displays specific activities. In particular CD14-derived DCs show a potent and long-lasting (from day 8 to day 13) antigen uptake activity (fluorescein isothiocyanate dextran or peroxidase) that is about 10-fold higher than that of CD1a+ cells, which is restricted to the immature stage (day 6). The antigen capture is exclusively mediated by receptors for mannose polymers. The high efficiency of antigen capture of CD14-derived cells is coregulated with the expression of nonspecific esterase activity, a tracer of lysosomial compartment. In contrast, the CD1a+ population never expresses nonspecific esterase activity. The most striking difference is the unique capacity of CD14-derived DCs to induce naive B cells to differentiate into IgM-secreting cells, in response to CD40 triggering and interleukin-2. Thus, although the two populations can allow T-cell priming, initiation of humoral responses might be preferentially regulated by the CD14-derived DCs. Altogether, those results show that different pathways of DC development might exist in vivo: (1) the LC type, which might be mainly involved in cellular immune responses, and (2) the CD14-derived DC related to dermal DCs or circulating blood DCs, which could be involved in humoral immune responses.

Granulocyte-macrophage colony-stimulating factor and the immune system

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a multifunctional cytokine currently used for the reversal of neutropenia associated with cytotoxic chemotherapy, bone marrow and haemopoietic stem cell transplantation. GM-CSF also modulates the function of differentiated white blood cells. In the context of local inflammatory responses, GM-CSF stimulates macrophages for antimicrobial and antitumor effects. GM-CSF further enhances healing and repair by its actions on fibroblasts and epidermal cells. GM-CSF is the pivotal mediator of the maturation and function of dendritic cells, the most important cell type for the induction of primary T cell immune responses. GM-CSF may enhance antibody dependent cellular cytotoxicity (ADCC) in several cell types, and the generation and cytotoxicity of natural killer (NK) cells. On this basis, GM-CSF may be useful for inducing or augmenting antibody responses to antimicrobial vaccines, to enhance killing of intracellular microorganisms, to accelerate epidermal and mucosal wound healing, and to stimulate protective immunity against tumors.

Mechanism and biological significance of CD4-mediated cytotoxicity

It is now well established that CD4+ T cells can express cytotoxic activity. This type of cell-mediated cytotoxicity is associated with the Th1-, but not with the Th2-phenotype. While the activation of CD4+ CTL is MHC class II-restricted, the effector phase, i.e. the target cell killing is unrestricted and antigen non-specific. In analogy to CD8+ CTL, CD4-mediated target cell death is by DNA fragmentation. However, the molecular mechanism of killing differs from CD8-mediated lysis. Thus, CD4+ CTL preferentially lyse their targets via Fas-Fas ligand interaction, whereas the major cytotoxic effect of CD8+ CTL is by granule exocytosis, i.e. perforin and granzymes. Although CD8+ CTL can also express the FasL, their lytic activity through interaction with Fas is of less importance. Likewise, some CD4+ CTL may also kill by perforin/granzymes activity, but this pathway is of minor significance. The aims of CD8- or CD4-mediated lysis are also different. Thus, the major task of CD8+ CTL which recognize and kill their targets in the context of MHC class I molecules, is the lysis of virally infected cells and battling against tumor cells. CD4+ CTL, on the other hand, have an immunomodulatory role. Thus, they preferentially eliminate activated MHC class II-positive cells, i.e. APC, be they monocytes/macrophages, B cells or T cells. They may lyse these cells in order to prevent an overreaction of the ongoing immune response or in order to remove potentially hazardous cells upon completion of the immune response. The Fas-FasL pathway is particularly suitable for this task as myeloid or lymphoid cells express Fas only if activated, while FasL is preferentially expressed on activated CD4+ Th1 cells. Moreover, activated T cells eliminate themselves by the Fas-mediated pathway. Whether this happens by fratricide only, or also by suicide or both is open. Moreover, CD4+ CTL are particularly suitable for killing tumor cells as well, as they are efficient effectors in bystander lysis in contrast to CD8+ CTL. On the other hand, the non-specific killing via Fas-FasL interaction, which is an important reason for the bystander lysis, may have unwanted effects in that cells which should not be eliminated could be killed. Such reactions affecting various organs and cells, e.g. the liver, thyroid or islet cells of the pancreas could be an explanation for certain autoimmune diseases.

Intestinal immunization of mice with antigen conjugated to anti-MHC class II antibodies

We have explored a new technique for immunization of the intestinal tract of mice, using protein antigens bound to antibodies with specificity for murine MHC class II molecules (MHC-II). Either of two protein antigens, hen avidin (AV) or hen egg lysozyme (HEL) were covalently conjugated to anti-MHC-II antibodies and the purified conjugates were given orally (p.o.) or by direct intraduodenal (i.d.) injection into the intestinal lumen of mice. A secondary immunization p.o. with the same conjugate or with the non-conjugated antigen in the presence of cholera toxin (CTX) resulted in production of both intestinal secretory IgA and serum IgA antibody by those mice. In addition, serum IgG antibodies were produced. Conjugates with appropriate MHC-II specificity targeted the antigen because they induced more IgA and IgG antibody than conjugates with irrelevant antibody specificity or antigen alone, and because they induced antibody in mice that were genetic low responders to antigen. The results indicate the feasibility of oral subunit type vaccines with antibody targeting technology.

Dendritic leucocytes pulsed with monoclonal antibody-hapten conjugates elicit vigorous primary humoral responses in vivo

Purified dendritic leucocytes (DL) were pulsed briefly in vitro with haptenated monoclonal antibodies (MoAb) to MHC class II and immediately injected i.v. into syngeneic recipients. Strong anti-hapten humoral responses were observed even though only a few picomoles of specific MoAb-hapten conjugates were injected with the DL. In contrast, DL pulsed with control conjugates, i.e. haptenated non-binding MoAbs, gave only weak responses. DL thus, can take up, process and present protein antigens even after brief exposure in vitro, and their immunogenicity is enhanced by pulsing with antigen conjugated to specific MoAbs. Furthermore, in the presence of fetal calf serum (FCS), but not normal rat serum, the control MoAb W6/32 (against human MHC class I) bound to DL. The vigorous primary humoral response achieved following this pulsing indicates that it is the binding and the corresponding increased uptake that enhances the immunogenicity of the DL.