A CD91-positive subset of CD11c+ blood dendritic cells: characterization of the APC that functions to enhance adaptive immune responses against CD91-targeted antigens

Sublingual Immunization with Chimeric C1q/CD40 Ligand/HIV Virus-like Particles Induces Strong Mucosal Immune Responses against HIV

Development of a vaccine that can elicit robust HIV specific antibody responses in the mucosal compartments is desired for effective prevention of HIV via sexual transmission. However, the current mucosal vaccines have either poor immunogenicity when administered orally or invite safety concerns when administered intranasally. Sublingual immunization has received more attention in recent years based on its efficiency in inducing systemic and mucosal immune responses in both mucosal and extra-mucosal tissues. To facilitate the transport of the immunogen across the sub-mucosal epithelial barrier, we found that CD91, the receptor of C1q, is prevalently expressed in the sublingual mucosal lining, and thus, a modified chimeric C1q surface conjugated CD40L/HIV VLP was generated. The ability of this chimeric C1q/CD40L/HIV VLP to bind, cross the epithelial layer, access and activate the sub-mucosal layer dendritic cells (DCs), and ultimately induce enhanced mucosal and systemic immune responses against HIV is evaluated in this study. We found that C1q/CD40L/HIV VLPs have enhanced binding, increased transport across the epithelial layer, and upregulate DC activation markers as compared to CD40L/HIV VLPs alone. Mice immunized with C1q/CD40L/HIV VLPs by sublingual administration showed higher levels of IgA salivary antibodies against both HIV Gag and Env than mice immunized with CD40L/HIV VLPs. Moreover, sublingual immunization with C1q/CD40L/HIV VLPs induced more Env- and Gag-specific IFN-γ producing T cells than the CD40L/HIV VLPs group. Interestingly, C1q/CD40L/HIV VLP immunization can also induce more mucosal homing T cells than that in CD40L/HIV VLP group. Our data suggest that incorporation of C1q to CD40L/HIV VLPs is a promising novel strategy and that the sublingual immunization can be a favorite immunization route for HIV mucosal vaccines.

Bivalent therapeutic vaccine against HPV16/18 genotypes consisting of a fusion protein between the extra domain A from human fibronectin and HPV16/18 E7 viral antigens

Background

In vivo targeting of human papillomavirus (HPV) derived antigens to dendritic cells might constitute an efficient immunotherapeutic strategy against cervical cancer. In previous works, we have shown that the extra domain A from murine fibronectin (mEDA) can be used to target antigens to toll-like receptor 4 (TLR4) expressing dendritic cells and induce strong antigen-specific immune responses. In the present study, we have produced a bivalent therapeutic vaccine candidate consisting of the human EDA (hEDA) fused to E7 proteins from HPV16 and HPV18 (hEDA-HPVE7-16/18) and evaluate its potential as a therapeutic vaccine against cervical cancer.

Materials and methods

Recombinant fusion proteins containing HPV E7 proteins from HPV16 and HPV18 virus subtypes fused to hEDA were produced and tested in vitro on their capacity to bind TLR4 and induce the production of tumor necrosis factor-α or interleukin (IL)-12 by human monocytes and dendritic cells. The immunogenicity and potential therapeutic activity of the vaccine in combination with cisplatin or with the TLR3 agonist molecules polyinosinic‐polycytidylic acid (Poly IC) or Poly ICLC was evaluated in mice bearing subcutaneous or genital orthotopic HPV16 TC-1 tumors.

Results

hEDA-HPVE7-16/18 prototype vaccine binds human TLR4 and stimulate TLR4-dependent signaling pathways and IL-12 production by human monocyte-derived dendritic cell. Vaccination with hEDA-HPVE7-16/18 induced strong HPVE7-specific Cytotoxic T lymphocyte (CTL) responses and eliminated established tumors in the TC-1-based tumor model. The antitumor efficacy was significantly improved by combining the fusion protein with cisplatin or with the TLR-3 ligand Poly IC and especially with the stabilized analog Poly ICLC. Moreover, hEDA-HPVE7-16/18+Poly ICLC induced full tumor regression in 100% of mice bearing orthotopic genital HPV tumors.

Conclusion

Our results suggest that this therapeutic vaccine formulation may be an effective treatment for cervical tumors that do not respond to current therapies.

The active contribution of OPCs to neuroinflammation is mediated by LRP1

Oligodendrocyte progenitor cells (OPCs) account for about 5% of total brain and spinal cord cells, giving rise to myelinating oligodendrocytes that provide electrical insulation to neurons of the CNS. OPCs have also recently been shown to regulate inflammatory responses and glial scar formation, suggesting functions that extend beyond myelination. Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifaceted phagocytic receptor that is highly expressed in several CNS cell types, including OPCs. Here, we have generated an oligodendroglia-specific knockout of LRP1, which presents with normal myelin development, but is associated with better outcomes in two animal models of demyelination (EAE and cuprizone). At a mechanistic level, LRP1 did not directly affect OPC differentiation into mature oligodendrocytes. Instead, animals lacking LRP1 in OPCs in the demyelinating CNS were characterized by a robust dampening of inflammation. In particular, LRP1-deficient OPCs presented with impaired antigen cross-presentation machinery, suggesting a failure to propagate the inflammatory response and thus promoting faster myelin repair and neuroprotection. Our study places OPCs as major regulators of neuroinflammation in an LRP1-dependent fashion.

Environmental cues received during development shape dendritic cell responses later in life

Environmental signals mediated via the aryl hydrocarbon receptor (AHR) shape the developing immune system and influence immune function. Developmental exposure to AHR binding chemicals causes persistent changes in CD4+ and CD8+ T cell responses later in life, including dampened clonal expansion and differentiation during influenza A virus (IAV) infection. Naïve T cells require activation by dendritic cells (DCs), and AHR ligands modulate the function of DCs from adult organisms. Yet, the consequences of developmental AHR activation by exogenous ligands on DCs later in life has not been examined. We report here that early life activation of AHR durably reduces the ability of DC to activate naïve IAV-specific CD8+ T cells; however, activation of naïve CD4+ T cells was not impaired. Also, DCs from developmentally exposed offspring migrated more poorly than DCs from control dams in both in vivo and ex vivo assessments of DC migration. Conditional knockout mice, which lack Ahr in CD11c lineage cells, suggest that dampened DC emigration is intrinsic to DCs. Yet, levels of chemokine receptor 7 (CCR7), a key regulator of DC trafficking, were generally unaffected. Gene expression analyses reveal changes in Lrp1, Itgam, and Fcgr1 expression, and point to alterations in genes that regulate DC migration and antigen processing and presentation as being among pathways disrupted by inappropriate AHR signaling during development. These studies establish that AHR activation during development causes long-lasting changes to DCs, and provide new information regarding how early life environmental cues shape immune function later in life.

Low-density lipoprotein receptor-related protein 1 attenuates house dust mite-induced eosinophilic airway inflammation by suppressing dendritic cell-mediated adaptive immune responses

BACKGROUND

Low-density lipoprotein receptor-related protein 1 (LRP-1) is a scavenger receptor that regulates adaptive immunity and inflammation. LRP-1 is not known to modulate the pathogenesis of allergic asthma.

OBJECTIVE

We sought to assess whether LRP-1 expression by dendritic cells (DCs) modulates adaptive immune responses in patients with house dust mite (HDM)-induced airways disease.

METHODS

LRP-1 expression on peripheral blood DCs was quantified by using flow cytometry. The role of LRP-1 in modulating HDM-induced airways disease was assessed in mice with deletion of LRP-1 in CD11c⁺ cells (Lrp1^fl/fl; CD11c-Cre) and by adoptive transfer of HDM-pulsed CD11b⁺ DCs from Lrp1^fl/fl; CD11c-Cre mice to wild-type (WT) mice.

RESULTS

Human peripheral blood myeloid DC subsets from patients with eosinophilic asthma have lower LRP-1 expression than cells from healthy nonasthmatic subjects. Similarly, LRP-1 expression by CD11b⁺ lung DCs was significantly reduced in HDM-challenged WT mice. HDM-challenged Lrp1^fl/fl; CD11c-Cre mice have a phenotype of increased eosinophilic airway inflammation, allergic sensitization, T_H2 cytokine production, and mucous cell metaplasia. The adoptive transfer of HDM-pulsed LRP-1-deficient CD11b⁺ DCs into WT mice generated a similar phenotype of enhanced eosinophilic inflammation and allergic sensitization. Furthermore, CD11b⁺ DCs in the lungs of Lrp1^fl/fl; CD11c-Cre mice have an increased ability to take up HDM antigen, whereas bone marrow-derived DCs display enhanced antigen presentation capabilities.

CONCLUSION

This identifies a novel role for LRP-1 as a negative regulator of DC-mediated adaptive immune responses in the setting of HDM-induced eosinophilic airway inflammation. Furthermore, the reduced LRP-1 expression by circulating myeloid DCs in patients with eosinophilic asthma suggests a possible role for LRP-1 in modulating type 2-high asthma.

Surrogate in vitro activation of innate immunity synergizes with interleukin-7 to unleash rapid antigen-driven outgrowth of CD4+ and CD8+ human peripheral blood T-cells naturally recognizing MUC1, HER2/neu and other tumor-associated antigens

Effective adoptive immunotherapy has proved elusive for many types of human cancer, often due to difficulties achieving robust expansion of natural tumor-specific T-cells from peripheral blood. We hypothesized that antigen-driven T-cell expansion might best be triggered in vitro by acute activation of innate immunity to mimic a life-threatening infection. Unfractionated peripheral blood mononuclear cells (PBMC) were subjected to a two-step culture, first synchronizing their exposure to exogenous antigens with aggressive surrogate activation of innate immunity, followed by γ-chain cytokine-modulated T-cell hyperexpansion. Step 1 exposure to GM-CSF plus paired Toll-like receptor agonists (resiquimod and LPS), stimulated abundant IL-12 and IL-23 secretion, as well as upregulated co-stimulatory molecules and CD11c expression within the myeloid (CD33+) subpopulation. Added synthetic long peptides (>20aa) derived from widely expressed oncoproteins (MUC1, HER2/neu and CMVpp65), were reliably presented to CD4+ T-cells and cross-presented to CD8+ T-cells. Both presentation and cross-presentation demonstrated proteasomal and Sec61 dependence that could bypass the endoplasmic reticulum. Step 2 exposure to exogenous IL-7 or IL-7+IL-2 produced selective and sustained expansion of both CD4+ and CD8+ peptide-specific T-cells with a predominant interferon-γ-producing T1-type, as well as the antigen-specific ability to lyse tumor targets. Other γ-chain cytokines and/or combinations were initially proliferogenic, but followed by a contractile phase not observed with IL-7 or IL-7+IL-2. Regulatory T-cells were minimally propagated under these culture conditions. This mechanistically rational culture sequence, effective even for unvaccinated donors, enables rapid preparation of T-cells recognizing tumor-associated antigens expressed by the majority of human cancers, including pancreatic cancers, breast cancers and glioblastomas.

Human CD1c(+) DCs are critical cellular mediators of immune responses induced by immunogenic cell death

Chemotherapeutics, including the platinum compounds oxaliplatin (OXP) and cisplatin (CDDP), are standard care of treatment for cancer. Although chemotherapy has long been considered immunosuppressive, evidence now suggests that certain cytotoxic agents can efficiently stimulate antitumor responses, through the induction of a form of apoptosis, called immunogenic cell death (ICD). ICD is characterized by exposure of calreticulin and heat shock proteins (HSPs), secretion of ATP and release of high-mobility group box 1 (HMGB1). Proper activation of the immune system relies on the integration of these signals by dendritic cells (DCs). Studies on the crucial role of DCs, in the context of ICD, have been performed using mouse models or human in vitro-generated monocyte-derived DCs (moDCs), which do not fully recapitulate the in vivo situation.

Here, we explore the effect of platinum-induced ICD on phenotype and function of human blood circulating DCs. Tumor cells were treated with OXP or CDDP and induction of ICD was investigated. We show that both platinum drugs triggered translocation of calreticulin and HSP70, as well as the release of ATP and HMGB1. Platinum treatment increased phagocytosis of tumor fragments by human blood DCs and enhanced phenotypic maturation of blood myeloid and plasmacytoid DCs. Moreover, upon interaction with platinum-treated tumor cells, CD1c⁺ DCs efficiently stimulated allogeneic proliferation of T lymphocytes. Together, our observations indicate that platinum-treated tumor cells may exert an active stimulatory effect on human blood DCs. In particular, these data suggest that CD1c⁺ DCs are critical mediators of immune responses induced by ICD.

Immunological Characterization of Whole Tumour Lysate-Loaded Dendritic Cells for Cancer Immunotherapy

INTRODUCTION

Dendritic cells play a key role as initiators of T-cell responses, and even if tumour antigen-loaded dendritic cells can induce anti-tumour responses, their efficacy has been questioned, suggesting a need to enhance immunization strategies.

MATHERIALS & METHODS

We focused on the characterization of bone marrow-derived dendritic cells pulsed with whole tumour lysate (TAA-DC), as a source of known and unknown antigens, in a mouse model of breast cancer (MMTV-Ras). Dendritic cells were evaluated for antigen uptake and for the expression of MHC class I/II and costimulatory molecules and markers associated with maturation.

RESULTS

Results showed that antigen-loaded dendritic cells are characterized by a phenotypically semi-mature/mature profile and by the upregulation of genes involved in antigen presentation and T-cell priming. Activated dendritic cells stimulated T-cell proliferation and induced the production of high concentrations of IL-12p70 and IFN-γ but only low levels of IL-10, indicating their ability to elicit a TH1-immune response. Furthermore, administration of Antigen loaded-Dendritic Cells in MMTV-Ras mice evoked a strong anti-tumour response in vivo as demonstrated by a general activation of immunocompetent cells and the release of TH1 cytokines.

CONCLUSION

Data herein could be useful in the design of antitumoral DC-based therapies, showing a specific activation of immune system against breast cancer.

A fusion protein between streptavidin and the endogenous TLR4 ligand EDA targets biotinylated antigens to dendritic cells and induces T cell responses in vivo

The development of tools for efficient targeting of antigens to antigen presenting cells is of great importance for vaccine development. We have previously shown that fusion proteins containing antigens fused to the extra domain A from fibronectin (EDA), an endogenous TLR4 ligand, which targets antigens to TLR4-expressing dendritic cells (DC), are highly immunogenic. To facilitate the procedure of joining EDA to any antigen of choice, we have prepared the fusion protein EDAvidin by linking EDA to the N terminus of streptavidin, allowing its conjugation with biotinylated antigens. We found that EDAvidin, as streptavidin, forms tetramers and binds biotin or biotinylated proteins with a Kd ~ 2.6 × 10(-14) mol/L. EDAvidin favours the uptake of biotinylated green fluorescent protein by DC. Moreover, EDAvidin retains the proinflammatory properties of EDA, inducing NF- κβ by TLR4-expressing cells, as well as the production of TNF- α by the human monocyte cell line THP1 and IL-12 by DC. More importantly, immunization of mice with EDAvidin conjugated with the biotinylated nonstructural NS3 protein from hepatitis C virus induces a strong anti-NS3 T cell immune response. These results open a new way to use the EDA-based delivery tool to target any antigen of choice to DC for vaccination against infectious diseases and cancer.

Heat-shock proteins as dendritic cell-targeting vaccines--getting warmer

Heat-shock proteins (hsp) provide a natural link between innate and adaptive immune responses by combining the ideal properties of antigen carriage (chaperoning), targeting and activation of antigen-presenting cells (APC), including dendritic cells (DC). Targeting is achieved through binding of hsp to distinct cell surface receptors and is followed by antigen internalization, processing and presentation. An improved understanding of the interaction of hsp with DC has driven the development of numerous hsp-containing vaccines, designed to deliver antigens directly to DC. Studies in mice have shown that for cancers, such vaccines generate impressive immune responses and protection from tumour challenge. However, translation to human use, as for many experimental immunotherapies, has been slow partly because of the need to perform trials in patients with advanced cancers, where demonstration of efficacy is challenging. Recently, the properties of hsp have been used for development of prophylactic vaccines against infectious diseases including tuberculosis and meningitis. These hsp-based vaccines, in the form of pathogen-derived hsp-antigen complexes, or recombinant hsp combined with selected antigens in vitro, offer an innovative approach against challenging diseases where broad antigen coverage is critical.

[Macroglobulin signaling system]

This review will focus on the systematization of knowledge about structure of macroglobulin signaling system, which includes macroglobulin family proteins (alpha-2-macroglobulin, alpha-2-glycoprotein, pregnancy associated plasma protein A), their receptors (LRP, grp78), ligands (proteinases, cytokines, hormones, lipids, et al.) transforming and transcriptional factors for regulation of macroglobulins synthesis. After reviewing the functions of macroglobulin signaling system, and mechanisms of their realization, we discuss the complex and significant role of this system in different physiological and pathological processes.

Eradication of large tumors expressing human papillomavirus E7 protein by therapeutic vaccination with E7 fused to the extra domain a from fibronectin

Cervical carcinoma is one of the most common cancers in women worldwide. It is well established that chronic infection of the genital tract by various mucosatropic human papillomavirus (HPV) types causes cervical cancer. Cellular immunity to E7 protein from HPV (HPVE7) has been associated with clinical and cytologic resolution of HPV-induced lesions. Thus, we decided to test if targeting of HPVE7 to dendritic cells using a fusion protein containing the extra domain A (EDA) from fibronectin, a natural ligand for TLR4, and HPVE7 (EDA-HPVE7) might be an efficient vaccine for the treatment of cervical carcinoma. We found that EDA-HPVE7 fusion protein was efficiently captured by bone marrow derived dendritic cells in vitro and induced their maturation, with the upregulation of maturation markers and the production of IL-12. Immunization of mice with EDA-HPVE7 fusion protein induced antitumor CD8(+) T cell responses in the absence of additional adjuvants. Repeated intratumoral administration of EDA-HPVE7 in saline was able to cure established TC-1 tumors of 5-7 mm in diameter. More importantly, intravenous injection with EDA-HPVE7 in combination with the TLR ligand polyinosinic-polycytidylic acid (pIC), or with low doses of cyclophosphamide and the TLR9 ligand CpG-B complexed in cationic lipids, were able to eradicate large established TC-1 tumors (1.2 cm in diameter). Thus, therapeutic vaccination with EDA-HPVE7 fusion protein may be effective in the treatment of human cervical carcinoma.

Human defensins activate monocyte-derived dendritic cells, promote the production of proinflammatory cytokines, and up-regulate the surface expression of CD91

Defensins are endogenous defense peptides with well defined antimicrobial activity against a broad spectrum of pathogens including bacteria, fungi, viruses, and parasites.Several lines of evidence suggest that defensins might also contribute to the regulation of host innate and adaptive immunity, but their immunomodulatory functions are still poorly understood. Herein, we studied the impact of human defensins on multiple functions of DCs, which are a central player in all immune responses, bridging innate and adaptive immunity. We challenged DCs differentiated in vitro from human moDCs with HNP-1 alpha-defensin or HBD-1. HNP-1 and HBD-1 were chemotactic for moDCs. Both defensins promoted the activation and maturation of moDCs, as assessed by up-regulation of surface expression of the costimulatory molecules CD80, CD86, and CD40, the maturation marker CD83, and HLA-DR. HNP-1 and HBD-1 also enhanced the production of the proinflammatory cytokines TNF-alpha, IL-6, and IL-12p70 but did not affect the production of the regulatory cytokine IL-10. According to these stimulatory effects, HNP-1 and HBD-1 increased the allostimulatory activity of moDCs significantly. Finally, HNP-1 and HBD-1 promoted the up-regulation of CD91 on the DC surface. CD91 is a scavenger receptor involved in the recognition of multiple ligands including defensins, thus suggesting that defensins may amplify their own effects through the activation of an autocrine loop. Taken together, our observations may provide new insight into the immunomodulatory properties of human defensins and may aid the exploration of new therapeutic strategies to potentiate antimicrobial and antitumor immunity.

Alarmins link neutrophils and dendritic cells

Neutrophils are the first major population of leukocyte to infiltrate infected or injured tissues and are crucial for initiating host innate defense and adaptive immunity. Although the contribution of neutrophils to innate immune defense is mediated predominantly by phagocytosis and killing of microorganisms, neutrophils also participate in the induction of adaptive immune responses. At sites of infection and/or injury, neutrophils release numerous mediators upon degranulation or death, among these are alarmins which have a characteristic dual capacity to mobilize and activate antigen-presenting cells. We describe here how alarmins released by neutrophil degranulation and/or death can link neutrophils to dendritic cells by promoting their recruitment and activation, resulting in the augmentation of innate and adaptive immune responses.

Antigen delivery by alpha(2)-macroglobulin enhances the cytotoxic T lymphocyte response

alpha(2)M* targets antigens to APCs for rapid internalization, processing, and presentation. When used as an antigen-delivery vehicle, alpha(2)M* amplifies MHC class II presentation, as demonstrated by increased antibody titers. Recent evidence, however, suggests that alpha(2)M* encapsulation may also enhance antigen-specific CTL immunity. In this study, we demonstrate that alpha(2)M*-delivered antigen (OVA) enhances the production of specific in vitro and in vivo CTL responses. Murine splenocytes expressing a transgenic TCR specific for CTL peptide OVA(257-264) (SIINFEKL) demonstrated up to 25-fold greater IFN-gamma and IL-2 secretion when treated in vitro with alpha(2)M*-OVA compared with soluble OVA. The frequency of IFN-gamma-producing cells was increased approximately 15-fold, as measured by ELISPOT. Expansion of the OVA-specific CD8+ T cell population, as assayed by tetramer binding and [3H]thymidine incorporation, and OVA-specific cell-mediated cytotoxicity, as determined by a flow cytometric assay, were also enhanced significantly by alpha(2)M*-OVA. Furthermore, significant CTL responses were observed at antigen doses tenfold lower than those required with OVA alone. Finally, we also observed enhanced humoral and CTL responses by naïve mice following intradermal immunization with alpha(2)M*-OVA. These alpha(2)M*-OVA-immunized mice demonstrated increased protection against a s.c.-implanted, OVA-expressing tumor, as demonstrated by delayed tumor growth and prolonged animal survival. The observation that alpha(2)M*-mediated antigen delivery elicits specific CTL responses suggests the cross-presentation of antigen onto MHC class I. These results support alpha(2)M* as an effective antigen-delivery system that may be particularly useful for vaccines based on weakly immunogenic subunits or requiring dose sparing.

Heat Shock Proteins and Scavenger Receptors: Role in Adaptive Immune Responses

Tumor-derived heat shock proteins have shown promise as anti-cancer vaccines in clinical trials. Heat shock proteins (HSPs) can generate potent anti-tumor immunity and elicit antigen-specific CD8+ T cell responses in murine studies. Antigen presenting cells (APC), such as macrophages and dendritic cells (DCs), can elicit antigen-specific CD8+ T cell responses mediated by HSPs. CD91 was the first identified endocytic scavenger receptor for HSPs on APC that can facilitate the process of cross-presentation. Other scavenger receptors may also play a similar role in this process. The present review critically evaluates the identified HSP endocytic receptors on APCs that may generate adaptive immune responses. A better understanding of this interaction between HSPs and APCs may further unravel mechanisms of immunoadjuvant function of HSPs.

Modulation of the immunogenicity of the Trypanosoma congolense cysteine protease, congopain, through complexation with alpha(2)-macroglobulin

The protozoan parasite Trypanosoma congolense is the main causative agent of livestock trypanosomosis. Congopain, the major lysosomal cysteine proteinase of T. congolense, contributes to disease pathogenesis, and antibody-mediated inhibition of this enzyme may contribute to mechanisms of trypanotolerance. The potential of different adjuvants to facilitate the production of antibodies that would inhibit congopain activity was evaluated in the present study. Rabbits were immunised with the recombinant catalytic domain of congopain (C2), either without adjuvant, with Freund’s adjuvant or complexed with bovine or rabbit α₂-macroglobulin (α₂M). The antibodies were assessed for inhibition of congopain activity. Rabbits immunised with C2 alone produced barely detectable anti-C2 antibody levels and these antibodies had no effect on recombinant C2 or native congopain activity. Rabbits immunised with C2 and Freund’s adjuvant produced the highest levels of anti-C2 antibodies. These antibodies either inhibited C2 and native congopain activity to a small degree, or enhanced their activity, depending on time of production after initial immunisation. Rabbits receiving C2-α₂M complexes produced moderate levels of anti-C2 antibodies and these antibodies consistently showed the best inhibition of C2 and native congopain activity of all the antibodies, with maximum inhibition of 65%. Results of this study suggest that antibodies inhibiting congopain activity could be raised in livestock with a congopain catalytic domain-α₂M complex. This approach improves the effectiveness of the antigen as an anti-disease vaccine candidate for African trypanosomosis.

Primary human immunodeficiency virus type 1-specific CD8+ T-cell responses induced by myeloid dendritic cells

Induction of an antigenically broad and vigorous primary T-cell immune response by myeloid dendritic cells (DC) in blood and tissues could be important for an effective prophylactic or therapeutic vaccine to human immunodeficiency virus type 1 (HIV-1). Here we show that a primary CD8(+) T-cell response can be induced by HIV-1 peptide-loaded DC derived from blood monocytes of HIV-1-negative adults and neonates (moDC) and by Langerhans cells (LC) and interstitial, dermal-intestinal DC (idDC) derived from CD34(+) stem cells of neonatal cord blood. Optimal priming of single-cell gamma interferon (IFN-gamma) production by CD8(+) T cells required CD4(+) T cells and was broadly directed to multiple regions of Gag, Env, and Nef that corresponded to known and predicted major histocompatibility complex class I epitopes. Polyfunctional CD8(+) T-cell responses, defined as single-cell production of more than one cytokine (IFN-gamma, interleukin 2, or tumor necrosis factor alpha), chemokine (macrophage inhibitory factor 1beta), or cytotoxic degranulation marker CD107a, were primed by moDC, LC, and idDC to HIV-1 Gag and reverse transcriptase epitopes, as well as to Epstein-Barr virus and influenza A virus epitopes. Thus, three major types of blood and tissue myeloid DC targeted by HIV-1, i.e., moDC, LC, and idDC, can prime multispecific, polyfunctional CD8(+) T-cell responses to HIV-1 and other viral antigens.

Low-density lipoprotein receptor-related protein 1 is an essential receptor for myelin phagocytosis

Multiple sclerosis (MS) is an autoimmune disease in which myelin is progressively degraded. Because degraded myelin may both initiate and accelerate disease progression, clearing degraded myelin from extracellular spaces may be critical. In this study, we prepared myelin vesicles (MV) from rat brains as a model of degraded myelin. Murine embryonic fibroblasts (MEFs) rapidly internalized MVs, which accumulated in lysosomes only when these cells expressed low-density lipoprotein receptor-related protein (LRP1). Receptor-associated protein (RAP), which binds LRP1 and inhibits interaction with other ligands, blocked MV uptake by LRP1-expressing MEFs. As a complementary approach, we prepared primary cultures of rat astrocytes, microglia and oligodendrocytes. All three cell types expressed LRP1 and mediated MV uptake, which was inhibited by RAP. LRP1 gene-silencing in oligodendrocytes also blocked MV uptake. Myelin basic protein (MBP), which was expressed as a recombinant protein, bound directly to LRP1. MBP-specific antibody inhibited MV uptake by oligodendrocytes. In experimental autoimmune encephalomyelitis in mice, LRP1 protein expression was substantially increased in the cerebellum and spinal cord. LRP1 colocalized with multiple CNS cell types. These studies establish LRP1 as a major receptor for phagocytosis of degraded myelin, which may function alone or in concert with co-receptors previously implicated in myelin phagocytosis.

Alpha 2-macroglobulin binds CpG oligodeoxynucleotides and enhances their immunostimulatory properties by a receptor-dependent mechanism

CpG oligodeoxynucleotides (ODN) stimulate the immune system and are under evaluation as treatments and vaccine adjuvants for infectious diseases, cancer, and immune system disorders. Although they have shown promising results in numerous clinical trials, the ultimate use of CpG ODN-based therapeutics may hinge on improved pharmacokinetics and reduced systemic side-effects. CpG ODN efficacy and potency might be enhanced greatly by packaging them into particles that protect them from degradation and specifically target them for uptake by immune-competent cells. The plasma proteinase inhibitor alpha 2-macroglobulin (alpha 2M) binds numerous biologically active macromolecules, including cytokines, chemokines, and growth factors, and can modulate their activity. Molecules bound to alpha 2M are protected from interactions with neighboring macromolecules and are targeted for receptor-mediated uptake by immune-competent cells. Here, we report that activated alpha 2M (alpha 2M*) binds CpG ODN and enhances their immunostimulatory properties significantly. Murine macrophages treated with alpha 2M*-ODN complexes respond more rapidly and produce a greater cytokine response than induced by free CpG ODN. Using human PBMC, alpha 2M*-ODN complexes exhibit fourfold enhanced potency and 15-fold greater efficacy for stimulating production of inflammatory cytokines. alpha 2M* targets delivery of CpG ODN specifically to immune-competent cells, which endocytose the complexes sixfold more rapidly than free CpG ODN. CpG ODN bound to alpha 2M* are also protected from degradation by nucleases. This novel targeting technology may improve CpG ODN-based therapeutics by increasing efficacy at reduced doses, thus reducing side-effects and cost.

Heat shock proteins and autoimmunity in patients with carotid atherosclerosis

Studies aimed at elucidating the pathogenetic mechanisms underlying the initiation and progression of human atherosclerosis have emphasized the central role of inflammatory and immune cells. Atherosclerotic plaques are infiltrated by activated macrophages, T and B lymphocytes, plasma cells, and mast cells, releasing inflammatory molecules, which amplify the severity of the disease. Endothelial cells subjected to various stress conditions express increased amounts of heat shock proteins (HSPs), some of the most successfully conserved proteins throughout evolution. Many experimental observations reviewed in this article draw attention to several HSPs targeted by a specific cellular and humoral immune response in patients with atherosclerotic disease. The review also reports preliminary data obtained by our group on the possible role of HSP90 as a candidate autoantigen in carotid atherosclerosis. Our study deals with the presence of specific antibodies and T cells directed against HSP90 in patients with carotid atherosclerotic plaques. In 60% of these subjects' sera but in none of the sera from healthy controls immunoblotting (IB) detected the presence of specific antibodies. Moreover, 20% of peripheral blood mononuclear cells (PBMC) samples from patients but none from healthy subjects proliferated in response to human purified HSP90. In vitro experiments showed an upregulation of HSP90 expression in endothelial cells exposed to oxidative stress by treatment with H(2)O(2) and greater release of soluble HSP90 in culture supernatants from H(2)O(2)-treated cells than from untreated cells.

Kininogens Coordinate Adaptive Immunity through the Proteolytic Release of Bradykinin, an Endogenous Danger Signal Driving Dendritic Cell Maturation

Strategically positioned in peripheral tissues, immune sentinel cells sense microbes and/or their shed products through different types of pattern-recognition receptors. Upon secretion, pre-formed pro-inflammatory mediators activate the microvasculature, inducing endothelium/neutrophil adherence and impairing endothelium barrier function. As plasma proteins enter into peripheral tissues, short-lived proinflammatory peptides are rapidly generated by limited proteolysis of complement components and the kininogens (i.e. kinin-precursor proteins). While much emphasis has been placed on the studies of the vascular functions of kinins, their innate effector roles remain virtually unknown. A few years ago, we reported that exogenous bradykinin (BK) potently induces dendritic cell (DC) maturation, driving IL-12-dependent Th1 responses through the activation of G-protein-coupled BK B(2) receptors (B(2)R). The premise that immature DC might sense kinin-releasing pathogens through B(2)R was demonstrated in the subcutaneous mouse model of Trypanosoma cruzi infection. Analysis of the dynamics of parasite-evoked inflammation revealed that activation of TLR2/neutrophils drives the influx of plasma proteins, including kininogens, into peripheral tissues. Once associated to cell surfaces and/or extracellular matrices, the surface-bound kininogens are cleaved by T. cruzi cysteine proteases. Acting as short-lived 'danger' signals, kinins activate DC via B(2)R, converting them into Th1 inducers. Fine tuned control of the extravascular levels of these natural peptide adjuvants is exerted by kinin-degrading metallopeptidases, e.g. Angiotensin converting enzyme (ACE/CD143). In summary, the studies in the subcutaneous model of T. cruzi infection revealed that the peripheral levels of BK, a DC maturation signal, are controlled by TLR2/neutrophils and ACE, respectively characterized as positive and negative modulators of innate/adaptive immunity.

VWF protects FVIII from endocytosis by dendritic cells and subsequent presentation to immune effectors

Von Willebrand factor (VWF) is a chaperone molecule for procoagulant factor VIII (FVIII). Its role in the reduction of the immunogenicity of therapeutic FVIII in patients with hemophilia A has been evoked but lacks clear cellular and molecular rationale. Here, we demonstrate that VWF protects FVIII from being endocytosed by human dendritic cells (DCs) and subsequently presented to FVIII-specific T cells. The immunoprotective effect of VWF requires a physical interaction with FVIII because the endocytosis of FVIII was significantly restored on hindering the formation of the VWF-FVIII complex. Interestingly, VWF had no direct inhibitory effect either on the ability of DCs to present antigenic peptides or on the activation potency of CD4+ T cells. We thus propose that VWF may reduce the immunogenicity of FVIII by preventing, upstream from the activation of immune effectors, the entry of FVIII in professional antigen-presenting cells.

Oral immunogenicity of the plant proteinase bromelain

Bromelain is a natural mixture of proteolytic enzymes derived from pineapple stem that has been shown to have anti-inflammatory activity when administered orally. Although most proteins given orally without adjuvant (e.g., food) result in tolerance, we previously reported that long-term oral exposure to bromelain stimulated the development of high serum anti-bromelain antibody titers. The purpose of these studies was to further investigate the mechanisms responsible for the immunogenicity of oral bromelain. Results showed that repeated exposure was required for development of anti-bromelain antibodies, with strong antibody responses in all mice that received at least 12 doses of bromelain either orally or intragastrically over 3-6 weeks. Proteolytic activity was required for strong oral immunogenicity in the absence of conventional adjuvant, with strong serum antibody responses generated against proteolytically active bromelain and trypsin, but not against ovalbumin, lysozyme, or inactivated bromelain. Significantly higher anti-bromelain antibody titers were seen in IL-10-deficient versus wild-type mice, suggesting that simultaneous treatments that decrease IL-10 activity may further enhance systemic antibody responses following oral exposure. The antibodies generated did not affect the proteolytic activity of bromelain. The data demonstrate that proteolytically active antigens such as bromelain can stimulate both systemic and mucosal immune responses following repeated oral exposure. Further studies of the mechanisms involved in generation of immune responses following oral exposure to proteolytically active antigens can lead to a better understanding of mechanisms of oral tolerance and to the development of novel adjuvants for oral vaccines.

CD163 positive subsets of blood dendritic cells: the scavenging macrophage receptors CD163 and CD91 are coexpressed on human dendritic cells and monocytes

CD163 and CD91 are scavenging receptors with highly increased expression during the differentiation of monocytes into the anti-inflammatory macrophage phenotype. In addition, CD91 is expressed in monocyte-derived dendritic cells (MoDCs), where the receptor is suggested to be important for internalization of CD91-targeted antigens to be presented on the dendritic cell surface for T-cell stimulation. Despite their overlap in functionality, the expression of CD91 and CD163 has never been compared and the expression of CD163 in the monocyte-dendritic cell lineage is not yet characterized. CD163 expression in dendritic cells (DCs) was investigated using multicolor flow cytometry in peripheral blood from 31 healthy donors and 15 HIV-1 patients in addition to umbilical cord blood from 5 newborn infants. Total RNA was isolated from MACS purified DCs and CD163 mRNA was determined with real-time reverse transcriptase polymerase chain reaction. The effect of glucocorticoid and phorbol ester stimulation on monocyte and dendritic cell CD163 and CD91 expression was investigated in cell culture of mononuclear cells using multicolor flow cytometry. We identified two CD163+ subsets in human blood with dendritic cell characteristics, CD163lo and CD163hi, together constituting a substantial fraction of DCs. Both subsets were characterized as [lin]- CD4+ ILT3+ HLA-DR+ CD11c+ by flow cytometry, and CD163 mRNA was readily detectable in MACS purified human DCs. CD163 on DCs was upregulated by glucocorticoid, and treatment by phorbol ester significantly decreased surface expression. Overall, the expression of CD163 on DCs was significantly increased in HIV-1 patients (19.3% [95% CI: 14.7-26.3%]) compared to healthy patients (10.5% [95% CI: 8.0-12.5]) p < 0.001. The CD163lo subset was CD16+, whereas the CD163hi subset was CD16-. Both subsets were CD91+, thereby constituting a subfraction of the recently described CD91+ CD11c+ dendritic cell subset. Coexpression of CD163 and CD91 was also demonstrated on human monocytes, which upon glucocorticoid treatment exhibited an increase in both CD163 and CD91 expression. We have now shown that CD163 and CD91 are coexpressed and coregulated on human monocytes. In addition, two subsets of CD163+ DCs constituting a fraction of the recently described CD91+ CD11c+ dendritic cell subset have been identified. The CD163 expression pattern suggests that if antigens are targeted to CD163 they may induce an immunostimulatory response like that of CD91-targeted antigens.

Heat Shock Protein–Based Cancer Vaccines

The ability to duplicate the remarkable success of infectious disease vaccines in cancer, with durably robust and highly specific antitumor immune responses, has been long held as one of the keys in developing true "magic bullet" cancer therapies. This article attempts to explain why cancer vaccines have failed (so far), delineate the increasingly complex barriers that prevent the eliciting of effective antitumor immunity and examines the ability of heat shock protein-based vaccines to overcome these barriers. This article is not a definitive compendium of the huge body of relevant literature but rather focuses on the major concepts underlying active specific immunotherapy in general and heat shock protein vaccines in particular.

Differential CD91 dependence for calreticulin and Pseudomonas exotoxin-A endocytosis

Calreticulin (CRT) interaction with cell-surface receptors is integral to its function in escorting associated peptides into the antigen-presenting cell (APC) antigen presentation pathway. Additionally, extracellular CRT is proposed to be required for lung APC interaction with collectins. In both cases, CD91 has been proposed to act as the APC cell-surface receptor requisite for mediating these processes. However, the evidence for a CRT interaction with CD91 is indirect, predicated on partial competition of cellular binding by gp96, of which CD91 has been proposed as the unique endocytic receptor, and by the CD91 ligand alpha2-macroglobulin. Here, we directly investigate the function of CD91 in binding and trafficking CRT. We find that the ability of CRT to interact with APC does not correlate with cellular CD91 expression or function. Additionally, in the first genetic test of CD91 function regarding CRT, CD91 expression neither conferred CRT association nor did CD91-deficient (CD91-/-) and CD91-expressing cells differ in their ability to traffic CRT. Finally, cellular CRT trafficking did not parallel that of Pseudomonas exotoxin-A, an obligate CD91 ligand, by the criteria of CD91 dependence, cell-type specificity and endocytic itinerary. These data identify that CRT trafficking is not, as previously hypothesized, CD91 dependent and indicate usage of alternative cellular trafficking pathways.

Parasite cysteine proteinase interactions with α2-macroglobulin or kininogens: differential pathways modulating inflammation and innate immunity in infection by pathogenic trypanosomatids

Plasma extravasation is a common endothelium response to tissue injury provoked by pathogens. Herein I will review studies showing that host proteinase inhibitors (e.g., alpha2-macroglobulin (alpha2M) or kininogens) interact with protozoan cysteine proteinases (CPs) in extravascular infection sites, linking inflammation to innate immunity by different mechanisms. Using human monocytes as antigen presenting cells, we first demonstrated that alpha2M entrapment of cruzipain, a Trypanosoma cruzi CP, reduced the activation threshold of cruzipain-specific CD4 T cells due to facilitated uptake of alpha2M-cruzipain complexes by the multiscavenger receptor (CD91). More recently, studies of the mechanisms underlying inflammation elicited by T. cruzi revealed that kininogens, once bound to glycosaminoglycans, are not able to efficiently inactivate cruzipain via their inhibitory cystatin-like domains. Instead, we found that cruzipain readily processes surface-bound kininogens, liberating bioactive kinins. Acting as paracrine hormones, kinins vigorously activate host cells through bradykinin (BK) receptors, thus stimulating endocytic uptake of the pathogen. Rather than unilaterally enhancing parasite infectivity, the liberated kinins activate innate immunity by potently stimulating dendritic cell maturation via the BK B2 receptor. The discovery of chagasin, a novel family of endogenous inhibitors expressed by trypanosomatids, is likely another regulatory player involved in the dynamics of the inflammatory response.

Immunodominance and Immunodomination: Critical Factors in Developing Effective CD8+ T‐Cell–Based Cancer Vaccines

The focusing of cellular immunity toward one, or just a few, antigenic determinant, even during immune responses to complex microorganisms or antigens, is known as immunodominance. Although described in many systems, the mechanisms of determinant immunodominance are only just beginning to be appreciated, especially in relation to the interplay between T cells of differing specificities and the interactions between T cells and the antigen-presenting cells (APCs). The outcome of these cellular interactions can lead to a form of immune suppression of one specificity by another-described as "immunodomination". The specific and detailed mechanisms involved in this process are now partly defined. A full understanding of all the factors that control immunodominance and influence immunodomination will help us to develop better viral and cancer vaccines.

Binding of activated alpha2-macroglobulin to its cell surface receptor GRP78 in 1-LN prostate cancer cells regulates PAK-2-dependent activation of LIMK

Two characteristics of highly malignant cells are their increased motility and secretion of proteinases allowing these cells to penetrate surrounding basement membranes and metastasize. Activation of 21-kDa activated kinases (PAKs) is an important mechanism for increasing cell motility. Recently, we reported that binding of receptor-recognized forms of the proteinase inhibitor alpha2-macroglobulin (alpha2M*) to GRP78 on the cell surface of 1-LN human prostate cancer cells induces mitogenic signaling and cellular proliferation. In the current study, we have examined the ability of alpha2M* to activate PAK-1 and PAK-2. Exposure of 1-LN cells to alpha2M* caused a 2- to 3-fold increase in phosphorylated PAK-2 and a similar increase in its kinase activity toward myelin basic protein. By contrast, the phosphorylation of PAK-1 was only negligibly affected. Silencing the expression of the GRP78 gene, using either of two different mRNA sequences, greatly attenuated the appearance of phosphorylated PAK-2 in alpha2M*-stimulated cells. Treatment of 1-LN cells with alpha2M* caused translocation of PAK-2 in association with NCK to the cell surface as evidenced by the co-immunoprecipitation of PAK-2 and NCK in the GRP78 immunoprecipitate from plasma membranes. alpha2M*-induced activation of PAK-2 was inhibited by prior incubation of the cells with specific inhibitors of tyrosine kinases and phosphatidylinositol 3-kinase. PAK-2 activation was accompanied by significant increases in the levels of phosphorylated LIMK and phosphorylated cofilin. Silencing the expression of the PAK-2 gene greatly attenuated the phosphorylation of LIMK. In conclusion, we show for the first time the activation of PAK-2 in 1-LN prostate cancer cells by a proteinase inhibitor, alpha2-macroglobulin. These studies suggest a mechanism by which alpha2M* enhances the metastatic potential of these cells.

Expression of the common heat-shock protein receptor CD91 is increased on monocytes of exposed yet HIV-1-seronegative subjects

The significantly higher surface expression of the surface heat-shock protein receptor CD91 on monocytes of human immunodeficiency virus type-1 (HIV-1)-infected, long-term nonprogressors suggests that HIV-1 antigen uptake and cross-presentation mediated by CD91 may contribute to host anti-HIV-1 defenses and play a role in protection against HIV-1 infection. To investigate this further, we performed phenotypic analysis to compare CD91 surface expression on CD14(+) monocytes derived from a cohort of HIV-1-exposed seronegative (ESN) subjects, their seropositive (SP) partners, and healthy HIV-1-unexposed seronegative (USN) subjects. The median fluorescent intensity (MFI) of CD91 on CD14(+) monocytes was significantly higher in ESN compared with SP (P = 0.028) or USN (P = 0.007), as well as in SP compared with USN subjects (P = 0.018). CD91 MFI was not normalized in SP subjects on highly active antiretroviral therapy (HAART) despite sustainable, undetectable plasma viraemia. Data in three SP subjects experiencing viral rebounds following interruption of HAART showed low CD91 MFI comparable with levels in USN subjects. There was a significant positive correlation between CD91 MFI and CD8(+) T cell counts in HAART-naïve SP subjects (r = 0.7, P = 0.015). Increased surface expression of CD91 on CD14(+) monocytes is associated with the apparent HIV-1 resistance that is observed in ESN subjects.

Activation of dendritic antigen-presenting cells expressing common heat shock protein receptor CD91 during induction of psoriasis

BACKGROUND

Psoriasis is a common and chronic relapsing inflammatory skin disorder. Although a role for T cells in mediating the induction and maintenance of psoriatic lesions is well established, mechanisms responsible for activation of T cells by antigen-presenting cells (APCs) during disease relapse are poorly understood.

OBJECTIVES

(i) To determine whether expression of the common heat shock protein (HSP) receptor CD91 correlated with development of psoriasis in a mouse model of psoriasis, (ii) to characterize the lesional cells on which CD91 was expressed, and (iii) to investigate whether CD91+ cells in psoriasis showed signs of activation.

METHODS

Two systems were used in order to study the above-mentioned objectives: (i) skin biopsies taken directly from patients with psoriasis (either psoriatic plaques or symptomless prepsoriatic skin) or from healthy donors, respectively, or (ii) (human) skin biopsies collected during development of psoriasis using a novel xenograft mouse model of psoriasis. The skin samples were then either processed for analysis by light microscopy, or labelled with fluorochrome-conjugated antibodies and analysed by confocal laser scanning microscopy.

RESULTS

We observed a markedly increased number of CD91+ cells which paralleled development of new psoriatic lesions in the psoriasis mouse model and in established psoriatic plaques compared with symptomless prepsoriatic or healthy skin. Morphology as well as cell-specific markers showed that CD91 was predominantly expressed by dermal dendritic APCs characterized by activation of nuclear factor-kappaB signalling and the presence of tumour necrosis factor-alpha, an important proinflammatory cytokine in the immunopathogenesis of psoriasis. In addition, HSP70, a ligand for CD91, was increased in keratinocytes in close vicinity to CD91-bearing APCs in psoriatic lesions.

CONCLUSIONS

These findings indicate massive presence of CD91+ dendritic cells juxtaposed to lesional keratinocytes expressing HSP70, and suggest a novel pathophysiological pathway and therapeutic target for this chronic inflammatory skin disease.

Mycobacterium avium ssp. paratuberculosis recombinant heat shock protein 70 interaction with different bovine antigen-presenting cells

Abstract Heat shock proteins (Hsp) can deliver antigen into the major histocompatibility complex class I presentation pathway of antigen-presenting cells (APC), a process called cross priming, thus stimulating antigen-specific CD8+ T-cell reactions. Hsp were shown to elicit proinflammatory responses in APC. Both processes require interaction of Hsp with APC via specific receptors. This study describes the interaction of recombinant Hsp70 (rHsp70) of Mycobacterium avium subspecies paratuberculosis with bovine peripheral blood mononuclear cells that was restricted to CD14+ cells. Characterized monocyte-derived macrophages, monocyte-derived dendritic cells (DC) and BoMac, an immortalized bovine macrophage cell line, were used to investigate the interaction of rHsp70 with different bovine APC. Saturation of immature DC with high concentrations of rHsp70 is demonstrated, and it was found that interaction of rHsp70 with DC was related to the maturation stage of the DC. Involvement of CD91 as a cellular receptor for rHsp70 was demonstrated; however, competition studies with immature DC demonstrated that other receptors exist on bovine APC. These data suggest that rHsp70-based vaccines may be useful for the successful immunization of cattle.

Highly efficient antigen targeting to M-DC8+ dendritic cells via FcgammaRIII/CD16-specific antibody conjugates

Conjugates of peptide antigens with antibodies specifically recognizing surface molecules on dendritic cells (DC) represent an attractive approach to target antigens to antigen-presenting cells (APC) for the induction of specific T cell responses. The present study evaluates the potential of M-DC8(+) DC, a sub-population of professional APC in the blood, for an antibody-based vaccination strategy. We prepared, by chemical cross-linking, conjugates of peptide model antigens with antibodies directed against different cell surface molecules of DC. Antigen-peptide conjugates using an anti-CD16 (FcgammaRIII) antibody were most potent in inducing in vitro activation of a specific CD4(+) T cell response. They were at least 300 times more efficient than two other antibody-antigen conjugates and approximately 500 times more efficient than unconjugated antigen peptides. Our data demonstrate that specific antigen targeting via CD16 on M-DC8(+) DC is a promising vaccination approach for the efficient induction of specific CD4(+) T cell responses ex vivo, and perhaps in vivo.

SalmonellaEscape from Antigen Presentation Can Be Overcome by Targeting Bacteria to Fcγ Receptors on Dendritic Cells

Dendritic cells (DCs) are professional APCs with the unique ability to activate naive T cells, which is required for initiation of the adaptive immune response against pathogens. Therefore, interfering with DC function would be advantageous for pathogen survival and dissemination. In this study we provide evidence suggesting that Salmonella enterica serovar typhimurium, the causative agent of typhoid disease in the mouse, interferes with DC function. Our results indicate that by avoiding lysosomal degradation, S. typhimurium impairs the ability of DCs to present bacterial Ags on MHC class I and II molecules to T cells. This process could correspond to a novel mechanism developed by this pathogen to evade adaptive immunity. In contrast, when S. typhimurium is targeted to FcgammaRs on DCs by coating bacteria with Salmonella-specific IgG, bacterial Ags are efficiently processed and presented on MHC class I and class II molecules. This enhanced Ag presentation leads to a robust activation of bacteria-specific T cells. Laser confocal microscopy experiments show that virulent S. typhimurium is rerouted to the lysosomal degradation pathway of DCs when internalized through FcgammaR. These observations are supported by electron microscopy studies demonstrating that internalized S. typhimurium shows degradation signs only when coated with IgG and captured by FcgammaRs on DCs. Therefore, our data support a potential role for bacteria-specific IgG on the augmentation of Ag processing and presentation by DCs to T cells during the immune response against intracellular bacteria.