Borrelia burgdorferi-pulsed dendritic cells induce a protective immune response against tick-transmitted spirochetes

Immune Response to Borrelia: Lessons from Lyme Disease Spirochetes

The mammalian host responds to infection with Borrelia spirochetes through a highly orchestrated immune defense involving innate and adaptive effector functions aimed toward limiting pathogen burdens, minimizing tissue injury, and preventing subsequent reinfection. The evolutionary adaptation of Borrelia spirochetes to their reservoir mammalian hosts may allow for its persistence despite this immune defense. This review summarizes our current understanding of the host immune response to B. burgdorferi sensu lato, the most widely studied Borrelia spp. and etiologic agent of Lyme borreliosis. Pertinent literature will be reviewed with emphasis on in vitro, ex vivo and animal studies that influenced our understanding of both the earliest responses to B. burgdorferi as it enters the mammalian host and those that evolve as spirochetes disseminate and establish infection in multiple tissues. Our focus is on the immune response of inbred mice, the most commonly studied animal model of B. burgdorferi infection and surrogate for one of this pathogen's principle natural reservoir hosts, the white-footed deer mouse. Comparison will be made to the immune responses of humans with Lyme borreliosis. Our goal is to provide an understanding of the dynamics of the mammalian immune response during infection with B. burgdorferi and its relation to the outcomes in reservoir (mouse) and non-reservoir (human) hosts.

Borrelia miyamotoi Activates Human Dendritic Cells and Elicits T Cell Responses

The spirochete Borrelia miyamotoi has recently been shown to cause relapsing fever. Like the Lyme disease agent, Borrelia burgdorferi, B. miyamotoi is transmitted through the bite of infected ticks; however, little is known about the response of the immune system upon infection. Dendritic cells (DCs) play a central role in the early immune response against B. burgdorferi We investigated the response of DCs to two different strains of B. miyamotoi using in vitro and ex vivo models and compared this to the response elicited by B. burgdorferi. Our findings show that B. miyamotoi is phagocytosed by monocyte-derived DCs, causing upregulation of activation markers and production of proinflammatory cytokines in a similar manner to B. burgdorferi. Recognition of B. miyamotoi was demonstrated to be partially mediated by TLR2. DCs migrated out of human skin explants upon inoculation of the skin with B. miyamotoi. Finally, we showed that B. miyamotoi-stimulated DCs induced proliferation of naive CD4⁺ and CD8⁺ T cells to a larger extent than B. burgdorferi. In conclusion, we show in this study that DCs respond to and mount an immune response against B. miyamotoi that is similar to the response to B. burgdorferi and is able to induce T cell proliferation.

Modulation of dendritic cell by pathogen antigens: Where do we stand?

Dendritic cells (DCs) are essential players in the activation of T cells and in the development of adaptive immune response towards invading pathogens. Upon antigen (Ag) recognition of Pathogen Associated Molecular Patterns (PAMPs) by their receptors (PRRs), DCs are activated and acquire an inflammatory profile. DCs have the ability to direct the profile of helper T (Th) cells towards Th1, Th2, Th17, Th9 and regulatory (Treg) cells. Each subset of Th cells presents a unique gene expression signature and is endowed with the ability to conduct or suppress effector cells in inflammation. Pathogens target DCs during infection. Many studies demonstrated that antigens and molecules derived from pathogens have the ability to dampen DC maturation and activation, leading these cells to a permissive state or tolerogenic profile (tolDCs). Although tolDCs may represent a hindrance in infection control, they could be positively used to modulate inflammatory disorders, such as autoimmune diseases. In this review, we focus on discussing findings that use pathogen-antigen modulated DCs and tolDCs in prophylactics and therapeutics approaches for vaccination against infectious diseases or inflammatory disorders.

Ménage à trois: Borrelia, dendritic cells, and tick saliva interactions

Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis, is inoculated into the skin during an Ixodes tick bite where it is recognised and captured by dendritic cells (DCs). However, considering the propensity of Borrelia to disseminate, it would appear that DCs fall short in mounting a robust immune response against it. Many aspects of the DC-driven immune response to Borrelia have been examined. Recently, components of tick saliva have been identified that sabotage DC responses and aid Borrelia infection. In this review, we summarise what is currently known about the immune response of DCs to Borrelia and explore the mechanisms by which Borrelia manages to circumvent this immune response, with or without the help of tick salivary proteins.

Induction of potent protection against acute and latent herpes simplex virus infection in mice vaccinated with dendritic cells

BACKGROUND AIMS

Dendritic cells (DCs) are the most potent antigen presenting cells of the immune system and have been under intense study with regard to their use in immunotherapy against cancer and infectious disease agents. In the present study, DCs were employed to assess their value in protection against live virus challenge in an experimental model using lethal and latent herpes simplex virus (HSV) infection in Balb/c mice.

METHODS

DCs obtained ex vivo in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4 were loaded with HSV-1 proteins (DC/HSV-1 vaccine). Groups of mice were vaccinated twice, 7 days apart, via subcutaneous, intraperitoneal or intramuscular routes with DC/HSV-1 and with mock (DC without virus protein) and positive (alum adjuvanted HSV-1 proteins [HSV-1/ALH]) control vaccines. After measuring anti-HSV-1 antibody levels in blood samples, mice were given live HSV-1 intraperitoneally or via ear pinna to assess the protection level of the vaccines with respect to lethal or latent infection challenge.

RESULTS

Intramuscular, but not subcutaneous or intraperitoneal, administration of DC/HSV-1 vaccine provided complete protection against lethal challenge and establishment of latent infection as assessed by death and virus recovery from the trigeminal ganglia. It was also shown that the immunity was not associated with antibody production because DC/HSV-1 vaccine, as opposed to HSV-1/ALH vaccine, produced very little, if any, HSV-1-specific antibody.

CONCLUSIONS

Overall, our results may have some impact on the design of vaccines against genital HSV as well as chronic viral infections such as hepatitis B virus, hepatitis C virus and human immunodeficiency virus.

Therapeutic vaccines against HIV infection

Resistance to medication, adverse effects in the medium-to-long-term and cost all place important limitations on lifelong adherence to combined antiretroviral therapy (cART). In this context, new therapeutic alternatives to 'cART for life' in HIV-infected patients merit investigation. Some data suggest that strong T cell-mediated immunity to HIV can indeed limit virus replication and protect against CD4 depletion and disease progression. The combination of cART with immune therapy to restore and/or boost immune-specific responses to HIV has been proposed, the ultimate aim being to achieve a 'functional cure'. In this scenario, new, induced, HIV-specific immune responses would be able to control viral replication to undetectable levels, mimicking the situation of the minority of patients who control viral replication without treatment and do not progress to AIDS. Classical approaches such as whole inactivated virus or recombinant protein initially proved useful as therapeutic vaccines. Overall, however, the ability of these early vaccines to increase HIV-specific responses was very limited and study results were discouraging, as no consistent immunogenicity was demonstrated and there was no clear impact on viral load. Recent years have seen the development of new approaches based on more innovative vectors such as DNA, recombinant virus or dendritic cells. Most clinical trials of these new vectors have demonstrated their ability to induce HIV-specific immune responses, although they show very limited efficacy in terms of controlling viral replication. However, some preliminary results suggest that dendritic cell-based vaccines are the most promising candidates. To improve the effectiveness of these vaccines, a better understanding of the mechanisms of protection, virological control and immune deterioration is required; without this knowledge, an efficacious therapeutic vaccine will remain elusive.

Evaluation of murine bone marrow-derived dendritic cells loaded with inactivated virus as a vaccine against Japanese encephalitis virus

Japanese encephalitis (JE) is a serious infectious disease in southern and eastern Asia. Design and development of safer and more efficacious vaccines against Japanese encephalitis virus (JEV) is a high-priority target in the world. Dendritic cells (DCs) are the most potent professional antigen-presenting cells (APCs) playing a central and unique role in the generation of primary T-cell responses, and are considered attractive "live adjuvants" for vaccination and immunotherapy against cancer and infectious diseases. In this study, mouse bone marrow-derived dendritic cells (bmDCs were generated and stimulated with inactivated JEV in vitro. BALB/c mice were immunized with stimulated bmDCs and then challenged with JEV wild-type strain. The neutralization antibody, interferon gamma (IFN-gamma), tumor necrosis factors alpha (TNF-alpha) or interleukin-6 (IL-6), and virus-specific CD8+ cytotoxic T-lymphocyte (CTL) levels, as well as survival rates, were analyzed and compared with inactivated vaccine and DCs control groups. The results demonstrated that intravenous (i.v.) injection of 2 x 10(5) JEV-pulsed bmDCs into each mouse produced notable levels of JEV-specific neutralizing antibodies and higher levels of CD8+ CTL, IFN-gamma and TNF-alpha compared with JEV-inactivated vaccine. Furthermore, stimulated bmDCs could elicit a highly protective efficacy (90%) against JEV challenge. It suggests that stimulated bmDCs can be considered as an attractive "live adjuvant" for vaccination against JEV infection.

Prostaglandin E2 is a major inhibitor of dendritic cell maturation and function in Ixodes scapularis saliva

Tick saliva is thought to contain a number of molecules that prevent host immune and inflammatory responses. In this study, the effects of Ixodes scapularis saliva on cytokine production by bone marrow-derived dendritic cells (DCs) from C57BL/6 mice stimulated by TLR-2, TLR-4, and TLR-9 ligands were studied. Saliva at remarkably diluted concentrations (<1/2000) promotes a dose-dependent inhibition of IL-12 and TNF-alpha production induced by all TLR ligands used. Using a combination of fractionation techniques (microcon filtration, molecular sieving, and reversed-phase chromatography), we unambiguously identified PGE(2) as the salivary inhibitor of IL-12 and TNF-alpha production by DCs. Moreover, we have found that I. scapularis saliva (dilution 1/200; approximately 10 nM PGE(2)) marginally inhibited LPS-induced CD40, but not CD80, CD86, or MHC class II expression. In addition, saliva significantly suppressed the ability of DCs to stimulate Ag-specific CD4(+) T cell proliferation and IL-2 production. Notably, the effect of saliva on DC maturation and function was reproduced by comparable concentrations of standard PGE(2). These findings indicate that PGE(2) accounts for most inhibition of DC function observed with saliva in vitro. The role of salivary PGE(2) in vector-host interaction and host immune modulation and inflammation in vivo is also discussed. This study is the first to identify molecularly a DC inhibitor from blood-sucking arthropods.

Mechanisms of dendritic cell-based vaccination against infection

Due to their unique capacity to initiate and regulate adaptive immune responses, dendritic cells (DC) represent the most potent antigen-presenting cells of the immune system. Immature DC reside in peripheral tissues, where they sample and process antigens and efficiently sense a large variety of signals from the surrounding environment. Toll-like receptors (TLR) expressed by DC play a critical role in the detection of invading pathogens as well as in triggering the subsequent immune responses. The differential expression of TLR by different DC subsets may correlate with the induction of different patterns of adaptive immune responses. The rapidly expanding and fundamental knowledge of DC biology furthers promising perspectives for the development of vaccination strategies in different fields. For example, the immunotherapeutic potential of antigen-pulsed DC for the treatment of cancer has been confirmed in a number of experimental tumour models. Furthermore, DC have been shown to serve as natural adjuvants in different models of infectious diseases, mediating protection against various types of pathogens. Using murine leishmaniasis as an example, we have demonstrated that DC, once properly conditioned ex vivo, mediate complete and durable protection against infection. Critical parameters determining the efficiency of DC-based vaccination against microbial pathogens include the origin of DC, the choice of antigen to be used for DC loading, the route of immunization and the state of DC maturation and activation. In the present review, we discuss the necessity to define the mechanisms responsible for the immunostimulatory capacity of DC in vivo, in order to exploit their full potential as vaccination tools.

Induction of Enhanced Immunity to Intestinal Nematodes Using IL-9-Producing Dendritic Cells

Dendritic cells can be considered natural adjuvants and are able to act as cellular vaccines to protect against disease. Adoptive transfer of Ag-pulsed bone marrow-derived dendritic cells (BMDCs) enhanced expulsion of the intestinal nematode, Trichinella spiralis, from the small intestine. IL 9 is a critical cytokine in protective immunity to intestinal nematode infection and is believed to enhance Th2 immune responses. Deriving dendritic cells from an IL-9 transgenic (IL-9t) mouse has enabled a detailed investigation of the importance of IL-9 during Ag presentation. Indeed, IL-9t dendritic cells significantly enhanced T cell proliferation and Th2 responses and, after adoptive transfer, enhanced parasite-specific IgG1 and intestinal mastocytosis in vivo, leading to accelerated expulsion of adult worms from the intestine. Overall, this paper demonstrates that dendritic cell vaccination can be used to successfully protect the host against intestinal nematode infection and suggests that IL-9 can act as a potent type 2 adjuvant during Ag presentation and the early stages of Th2 activation.

Innate immune responses in Lyme borreliosis: enhanced tumour necrosis factor-alpha and interleukin-12 in asymptomatic individuals in response to live spirochetes

Innate immunity is important for early defence against borrelia spirochetes and should play a role in the clinical outcome of the infection. In order to study early cytokine responses, in vitro differentiated dendritic cells (DCs) and whole blood cells from 21 patients with different clinical outcomes of Lyme neuroborreliosis were stimulated with live borrelia spirochetes. The borrelia-induced secretion of interleukin (IL)-4, IL-10, IL-12p70, interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha in DCs and IL-1beta, IL-6, IL-8, IL-10, IL-12p70, TNF-alpha, regulated upon activation normal T cell expressed and secreted (RANTES), monocyte chemoattractant protein (MCP)-1, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta and eotaxin in whole blood cells was measured by enzyme-linked immunospot (ELISPOT) and multiplex arrays, respectively. We found increased numbers of TNF-alpha-secreting DCs (P = 0.018) in asymptomatic seropositive individuals compared to patients with subacute neuroborreliosis and seronegative controls. Asymptomatic individuals were also found to have elevated levels of IL-12p70 (P = 0.031) in whole blood cell supernatants compared to seronegative controls. These results are in line with previous experiments using cells of the adaptive immune response, indicating that strong T helper type 1 (Th1) proinflammatory responses might be associated with a successful resolution of Lyme disease.

Beta2 integrins control the severity of murine Lyme carditis

Infection of C57BL/6 (B6) mice with the Lyme disease spirochete Borrelia burgdorferi can result in development of arthritis and carditis. B. burgdorferi induces expression of beta2/CD18 integrins, adhesion molecules that mediate the firm adhesion of leukocytes to the endothelium necessary for cellular extravasation during inflammation. The important role of beta2/CD18 integrins during extravasation suggests that these molecules play a role in the development of Lyme arthritis and carditis. The dependency of these inflammatory processes on the beta2 integrins was investigated in CD18 hypomorph mice, which express low levels of CD18. The results indicate that CD18 deficiency did not abrogate development of Lyme arthritis or carditis. Moreover, it resulted in increased severity of Lyme carditis. B. burgdorferi-infected CD18 hypomorph mice showed an increased macrophage infiltration of the heart, while they produced lower levels of borreliacidal anti-B. burgdorferi antibodies compared to wild-type mice. In accordance with these results, we demonstrate that dendritic cells from CD18 hypomorph mice secrete higher levels of monocyte/macrophage chemoattractant protein 1 (MCP-1/CCL2) in response to B. burgdorferi. Similarly, we show by real-time PCR that B. burgdorferi-infected hearts from CD18 hypomorph mice express increased levels of MCP-1 RNA compared to wild-type mice. Overall, our results indicate that beta2 integrin deficiency does not abrogate B. burgdorferi-induced inflammation; rather, it results in increased recruitment of macrophages into the B. burgdorferi-infected heart, likely due to the increased expression of MCP-1 in this tissue. Thus, beta2 integrins may play a regulatory role in B. burgdorferi-induced inflammation beyond mediating adhesion of leukocytes to the endothelium.

A live and inactivated Chlamydia trachomatis mouse pneumonitis strain induces the maturation of dendritic cells that are phenotypically and immunologically distinct

The intracellular bacterial pathogen Chlamydia trachomatis is a major cause of sexually transmitted disease worldwide. While protective immunity does appear to develop following natural chlamydial infection in humans, early vaccine trials using heat-killed C. trachomatis resulted in limited and transient protection with possible enhanced disease during follow-up. Thus, immunity following natural infection with live chlamydia may differ from immune responses induced by immunization with inactivated chlamydia. To study this differing immunology, we used murine bone marrow-derived dendritic cells (DC) to examine DC maturation and immune effector function induced by live and UV-irradiated C. trachomatis elementary bodies (live EBs and UV-EB, respectively). DC exposed to live EBs acquired a mature DC morphology; expressed high levels of major histocompatibility complex (MHC) class II, CD80, CD86, CD40, and ICAM-1; produced elevated amounts of interleukin-12 and tumor necrosis factor alpha; and were efficiently recognized by Chlamydia-specific CD4+ T cells. In contrast, UV-EB-pulsed DC expressed low levels of CD40 and CD86 but displayed high levels of MHC class II, ICAM-1, and CD80; secreted low levels of proinflammatory cytokines; and exhibited reduced recognition by Chlamydia-specific CD4+ T cells. Adoptive transfer of live EB-pulsed DC was more effective than that of UV-EB-pulsed DC at protecting mice against challenge with live C. trachomatis. The expression of DC maturation markers and immune protection induced by UV-EB could be significantly enhanced by costimulation of DC ex vivo with UV-EB and oligodeoxynucleotides containing cytosine phosphate guanosine; however, the level of protection was significantly less than that achieved by using DC pulsed ex vivo with viable EBs. Thus, exposure of DC to live EBs results in a mature DC phenotype which is able to promote protective immunity, while exposure to UV-EB generates a semimature DC phenotype with less protective potential. This result may explain in part the differences in protective immunity induced by natural infection and immunization with whole inactivated organisms and is relevant to rational chlamydia vaccine design strategies.

Dendritic cells transduced with an adenovirus vector encoding interleukin-12 are a potent vaccine for invasive pulmonary aspergillosis

Invasive pulmonary aspergillosis (IPA) is a common and devastating pneumonia. We developed a novel antiinfective vaccine that couples the potent Ag-presenting capacity of dendritic cells (DCs) with paracrine delivery of interleukin-12 (IL-12) to local immune response sites. Our results showed that DCs engulfed Aspergillus conidia through coiling phagocytosis. Transfection of DCs with adenovirus encoding the cDNA of IL-12 did not affect their morphology and capacity to engulf conidia. The transduced DCs secreted IL-12, which was biologically active, to induce the production of gamma interferon (IFN-gamma) from spleen cells. Adoptive transfer of DCs pulsed with heat-inactivated Aspergillus fumigatus (HAF) to naive mice induced the Ag-specific production of IFN-gamma; the transduced HAF-pulsed DCs augmented this immune response further. Animals receiving HAF-pulsed DCs had lower fungal burdens, a more than three-fold higher survival rate at day 3. This protection was associated with a pronounced enhancement in the Aspergillus-specific IFN-gamma response. IL-12-engineered DCs augmented this protection strikingly as judged by a higher survival, and almost no Aspergillus could be detected in the lung of mice that had received IL-12-transduced HAF-pulsed DCs. These results suggest that antigen-pulsed DCs and IL-12 gene therapy could be used as adjunct therapy for aspergillosis.

Dendritic Cell Biology

Dendritic cells (DCs) are a special type of leukocytes able to alert the immune system to the presence of infections. They play a central role in the initiation of both innate and adaptive immune responses. This particular DC feature is regulated by the activation of specific receptors at the cell surface called Toll-like receptors (TLRs) that bind a number of microbial products collectively referred to as microbial-associated molecular patterns (MAMP). TLRs initiate a cascade of events, which together define the process of DC maturation. This phenomenon allows DCs to progressively acquire varying specific functions. DC maturation depends on the nature of the perturbation and permits unique and efficient immune responses for each pathogen. In this review the discussion is focused on DCs in the context of interactions with pathogens and DC-specific functions are highlighted.

Antigen delivery by dendritic cells

Dendritic cells (DC) link the innate and adaptive arms of the immune system and thus orchestrate the immune response to pathogens. A novel immune intervention strategy to control infectious diseases is based on the use of the potent immunostimulatory properties of DC for vaccination and immunotherapy. Recent advances in our understanding of DC biology and the molecular mechanisms by which DC instruct the development of an appropriate immune response to microorganisms provide means for DC-based approaches to manipulate the immune system. In experimental systems, DC vaccination has been documented to mediate protection against a wide spectrum of infectious diseases caused by viral, bacterial, parasitic and fungal pathogens. The protocols for the generation, stimulation and antigen loading of DC are being optimized, and methods for DC targeting in situ are likely to become available soon, thus paving the way for clinical applications of DC-based vaccines.

Present status of antileishmanial vaccines

The term leishmaniasis refers collectively to various clinical syndromes that are caused by obligate intracellular protozoa of the genus Leishmania. Approximately 350 million people in 8 countries are estimated to be threatened by the disease. The World Health Organization estimated that there are 12 million cases of all forms of leishmaniasis worldwide, with over 500,000 new cases of visceral disease occurring each year. Most of the drugs commonly used to treat different forms of leishmaniasis are toxic and have unacceptable side effects. Moreover, cases of drug resistant leishmaniasis are on the rise. Due to non-existence of effective vaccine to date, improved immunoprophylactic approaches still remain desirable to combat leishmaniasis. Antileishmanial vaccines developed around the globe are discussed.

Toxoplasma gondii antigen-pulsed-dendritic cell-derived exosomes induce a protective immune response against T. gondii infection

It was previously demonstrated that immunizing mice with spleen dendritic cells (DCs) that had been pulsed ex vivo with Toxoplasma gondii antigens triggers a systemic Th1-biased specific immune response and induces protection against infection. T. gondii can cause severe sequelae in the fetuses of mothers who acquire the infection during pregnancy, as well as life-threatening neuropathy in immunocompromised patients, in particular those with AIDS. Here, we investigate the efficacy of a novel cell-free vaccine composed of DC exosomes, which are secreted antigen-presenting vesicles that express functional major histocompatibility complex class I and II and T-cell-costimulatory molecules. They have already been shown to induce potent antitumor immune responses. We investigated the potential of DC2.4 cell line-derived exosomes to induce protective immunity against toxoplasmosis. Our data show that most adoptively transferred T. gondii-pulsed DC-derived exosomes were transferred to the spleen, elicited a strong systemic Th1-modulated Toxoplasma-specific immune response in vivo, and conferred good protection against infection. These findings support the possibility that DC-derived exosomes can be used for T. gondii immunoprophylaxis and for immunoprophylaxis against many other pathogens.

In vitro reconstructed mucosa-integrating Langerhans' cells

All three-dimensional in vitro mucosal models constructed, thus far, have only been reconstituted by epithelial cells. We have developed a reconstructed oral and vaginal epithelium that integrates Langerhans' cells (LC), the dendritic cells (DC) of malpighian epithelia. The epithelium was composed of gingival or vaginal keratinocytes seeded on a de-epidermized dermis (DED) and grown in submerged culture for 2 weeks. LC precursors, obtained after differentiation of cord blood-derived CD34+ hematopoietic progenitor cells (CD34+HPC) by granulocyte macrophage-colony stimulating factor (GM-CSF), tumor necrosis factor-alpha (TNF-alpha), transforming growth factor-beta (TGF-beta) and Flt3-ligand (Flt3-L), were introduced after 6-8 days of culture into the reconstituted epithelium. The in vitro reconstituted mucosal epithelium formed a multilayered, well-differentiated epithelial structure, confirmed by the immunohistochemical expression of cytokeratins 4, 6, 10, 13, 14, 16 and involucrin. LC were identified in the basal and suprabasal epithelial layers by CD1a antigen, S100 protein and Langerin/CD207 expression, and by transmission electron microscopy. Type IV collagen was expressed at the chorio-epithelial junction, and most ultrastructural features of this junction were visualized by electron microscopy. This in vitro reconstructed gingiva or vagina integrating LC represents interesting models very similar to native tissues. Because LC play an important role in the mucosal immune system, our models could be useful for conducting studies on interactions with pathogenic agents (viruses, bacteria etc.), as well as in pharmacological, toxicological and clinical research.

Dendritic cell vaccination and viral infection--animal models

Dendritic cells (DCs) play a pivotal role in the initiation and maintenance of immune responses against viruses and other microbial pathogens. Adoptively transferred, in vitro manipulated DCs presenting antigen derived from different viruses have been shown to elicit cytotoxic T cell (CTL) and T helper (Th) cell responses and to induce protective antiviral immunity. Furthermore, DC-based adoptive immunotherapies have the potential to specifically (re)activate antiviral immunity in chronic viral diseases such as HIV or hepatitis virus infections. Cellular dendritic cell vaccines, however, are not suitable for large-scale prophylactic immunization. Strategies for vaccine development should therefore aim at the specific delivery of microbial antigens to DCs in situ. Furthermore, appropriate mobilization and activation of DCs by the vaccine is important for the generation of optimal antimicrobial immune responses. Here, we discuss recent data on induction of antiviral immunity with various DC-vaccination approaches and outline future directions for the development of specific antigen targeting to DCs.

Dendritic cell-based immunotherapy

Dendritic cell (DC)-based vaccinations represent a promising approach for the immunotherapy of cancer and infectious diseases as DCs play an essential role in initiating cellular immune responses. A number of clinical trials using ex vivo-generated DCs have been performed so far and only minor toxicity has been reported. Both the induction of antigen-specific T cells and clinical responses have been observed in vaccinated cancer patients. Nevertheless, DC-based immunotherapy is still in its infancy and there are many issues to be addressed such as antigen loading procedures, DC source and maturational state, migration properties, route, frequency, and dosage of DC vaccination. The increasing knowledge of DC biology should be used to improve the efficacy of this new therapy.

The immune response is initiated by dendritic cells via interaction with microorganisms and interleukin-2 production

The immune system of vertebrate animals is characterized by the capacity to respond to disturbances. This function requires 2 different approaches. First, the immune system responds in a few hours to infectious agents (innate immunity) by recognizing molecular patterns typical of microorganisms (but absent in self-tissues). Second, it mounts a late response that differentiates among different microbes, giving rise to memory (adaptive immunity). In this context, dendritic cells (DCs) play a central role, becoming efficient stimulators of both innate and adaptive responses after microbial activation. Recent data generated by global transcriptional profiling of DCs after bacterial encounter are discussed, as are the unique DC functional plasticity and the central role of DC-derived interleukin-2 in priming early and late immune responses.

Dendritic cells, new tools for vaccination

Our rapidly expanding knowledge of the biology of the dendritic cell (DC), a major antigen-presenting cell connecting innate and adaptive immunity, suggests new possibilities for the development of vaccines and therapeutic strategies against pathogens, through the manipulation of their function in vivo, or the injection of the DC itself, once properly instructed ex vivo.

Dendritic cell (DC)-based protection against an intracellular pathogen is dependent upon DC-derived IL-12 and can be induced by molecularly defined antigens

Upon loading with microbial Ag and adoptive transfer, dendritic cells (DC) are able to induce immunity to infections. This offers encouragement for the development of DC-based vaccination strategies. However, the mechanisms underlying the adjuvant effect of DC are not fully understood, and there is a need to identify Ag with which to arm DC. In the present study, we analyzed the role of DC-derived IL-12 in the induction of resistance to Leishmania major, and we evaluated the protective efficacy of DC loaded with individual Leishmania Ag. Using Ag-pulsed Langerhans cells (LC) from IL-12-deficient or wild-type mice for immunization of susceptible animals, we showed that the inability to release IL-12 completely abrogated the capacity of LC to mediate protection against leishmaniasis. This suggests that the availability of donor LC-derived IL-12 is a requirement for the development of protective immunity. In addition, we tested the protective effect of LC loaded with Leishmania homolog of receptor for activated C kinase, gp63, promastigote surface Ag, kinetoplastid membrane protein-11, or Leishmania homolog of eukaryotic ribosomal elongation and initiation factor 4a. The results show that mice vaccinated with LC that had been pulsed with selected molecularly defined parasite proteins are capable of controlling infection with L. major. Moreover, the protective potential of DC pulsed with a given Leishmania Ag correlated with the level of their IL-12 expression. Analysis of the cytokine profile of mice after DC-based vaccination revealed that protection was associated with a shift toward a Th1-type response. Together, these findings emphasize the critical role of IL-12 produced by the sensitizing DC and suggest that the development of a DC-based subunit vaccine is feasible.

Dendritic cells as a tool to combat infectious diseases

Dendritic cells (DCs) form a network of potent antigen-presenting cells that initiate and amplify immune responses. The detection and capture of microorganisms by DCs trigger stimulus-specific maturation programs that enable DCs to convey pathogen-associated signals to the adaptive branch of the immune system. The appropriate activation of DCs is critical for their ability to direct the development of either a Th1 or a Th2 response, thereby determining the outcome of microbial infections. Advances in the understanding of DC interactions with microbes provide new concepts for immune interventions. In different models of infectious disease, it has been demonstrated that DCs can serve as vaccine carriers, mediating protection against various types of pathogens. The studies of the requirements of ex vivo manipulations of DCs may lead to the design of vaccines that induce protective immunity to infections by appropriate targeting of DCs in vivo.

Effective induction of acquired resistance to Listeria monocytogenes by immunizing mice with in vivo-infected dendritic cells

Splenic dendritic cells (DCs) obtained from mice at 48 h after Listeria monocytogenes infection exhibited up-regulation of CD80 and produced higher titers of gamma interferon (IFN-gamma) and interleukin-12 (IL-12) than did DCs obtained from uninfected mice. Mice immunized with DCs obtained from mice that had been infected with L. monocytogenes 48 h before acquired host resistance to lethal infection with L. monocytogenes at 4 and 8 weeks. Immunization with DCs from heat-killed L. monocytogenes failed to induce resistance. Acquired antilisterial resistance is specific, since the immunized mice could not be protected from Salmonella enterica serovar Typhimurium infection. Infected DCs stimulated proliferation of naive CD4(+) and CD8(+) cells in vitro, suggesting that in vivo-infected DCs activate CD8(+) T cells, which are critical in acquired antilisterial resistance, as well as CD4(+) T cells. When wild-type mice were immunized with DCs from IFN-gamma-deficient mice, they were protected against a lethal L. monocytogenes challenge. In contrast, when mice were immunized with DCs from anti-IL-12 p40 monoclonal antibody-injected mice, they failed to gain acquired antilisterial resistance. These results suggest that DC-derived IL-12, but not IFN-gamma, may play a critical role in induction of acquired antilisterial resistance. Our present results suggest that splenic DCs obtained from mice infected with L. monocytogenes in vivo may be an effective immunogen with which to induce antigen-specific immunity.

How dendritic cells and microbes interact to elicit or subvert protective immune responses

B and T lymphocytes recognize antigens with high specificity, but neither initiate immune responses, nor decide their types. These functions rest upon dendritic cells (DCs), which can determine and maintain Th1/Th2 polarization. Immune responses are thus dependent on the DC subset, the receptors that recognize each pathogen and the microenvironment. Microbes employ an array of mechanisms to evade and disrupt DC functions; some even hijack DCs for transport around the body. Our progress in the understanding of DC physiology will hopefully help us create the necessary vaccines to counteract the infectious agents that still plague mankind.

A role for peripheral blood fibrocytes in Lyme disease?

It is proposed that peripheral blood fibrocytes will be a new and important player in the pathogenesis of Lyme disease. Peripheral blood fibrocytes are a circulating leukocyte subpopulation that: (a) express collagen; (b) are an abundant source of cytokines, chemoattractants and growth factors; and (c) are able to recruit and activate naive T-cells and memory T-cells. We predict that peripheral blood fibrocytes will represent a new and important antigen-presenting cell which will play an important role in directing the immune response from the pathogenic Th1 to the protective Th2 response cell in Borrelia infections.

Fluorine-18 labeled mouse bone marrow-derived dendritic cells can be detected in vivo by high resolution projection imaging

Immunization with ex vivo generated dendritic cells has become a focus for many clinical applications. The optimal site of injection and the migration pattern of these cells remain to be elucidated. We therefore developed a novel method for labeling mouse bone marrow-derived dendritic cells (BMDC) with the positron emitting radioisotope F-18 using N-succinimidyl-4-[F-18]fluorobenzoate, which covalently binds to the lysine residues of cell surface proteins. When we determined the stability of F-18 labeled BMDC, we found that at 4 h only 44+/-10% of the initial cell-bound activity was retained at 37 degrees C, whereas considerably more (91+/-3%) was retained at 4 degrees C. Labeled cells did not exhibit any significant alteration in cell viability or phenotype as determined by trypan blue exclusion and FACS analysis 24 h after radiolabeling. Furthermore, F-18-labeled BMDC stimulated allogeneic T cells in a mixed leukocyte reaction as potently as did sham-treated BMDC and migrated towards secondary lymphoid tissue chemokine (SLC) in a chemotaxis assay in vitro with the same efficiency as sham-treated BMDC. Migration of F-18-labeled BMDC was studied after footpad injection by (1) ex vivo counting of dissected tissues using a gamma counter and (2) in vivo by imaging mice with PiPET, a 2-mm resolution positron projection imager. After 4 h, the ratio between measured activity in draining vs. contralateral (D/C) lymph nodes (LN) was 166+/-96 (n=7) in the case of live cell injections, whereas if we injected heat-killed F-18-labeled BMDC or F-18-labeled macrophages the D/C ratios were 17+/-2 (n=2) and 14+/-4 (n=2), respectively. Injection of cell-free activity in the form of F-18-labeled 4-fluorobenzoic acid resulted in a D/C ratio of 7+/-2 (n=3), suggesting that the activity measured in the draining lymph node was associated with migrated F-18-labeled BMDC. When F-18-labeled live cells were injected into the footpad, 0.18+/-0.04% (n=7) of footpad activity was found in the draining LN within 4 h, whereas none was found in the contralateral LN. Quantitative assessment of cell migration by PET projection imaging of mice confirmed the ex-vivo counting results. These studies indicate that PET imaging offers a new approach for in vivo studies of dendritic cell biodistribution and migration.

Dendritic cells pulsed with intact Streptococcus pneumoniae elicit both protein- and polysaccharide-specific immunoglobulin isotype responses in vivo through distinct mechanisms

Immature bone marrow-derived myeloid dendritic cells (BMDCs) are induced to undergo phenotypic maturation and secretion of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-12, and IL-10 when pulsed in vitro with intact Streptococcus pneumoniae. After transfer to naive mice, pulsed BMDCs induce immunoglobulin (Ig) isotype responses specific for both protein and polysaccharide pneumococcal antigens, having in common the requirement for viable BMDCs, T cells, and B7-dependent costimulation in the recipient mice. Whereas primary Ig isotype responses to bacterial proteins uniformly require BMDC expression of major histocompatibility complex class II, CD40, and B7, and the secretion of IL-6, but not IL-12, similar requirements for antipolysaccharide Ig responses were only observed for the IgG1 isotype.

Protection against pulmonary infection with Pseudomonas aeruginosa following immunization with P. aeruginosa-pulsed dendritic cells

To develop a Pseudomonas aeruginosa vaccine that allows the host immune system to select the antigens, we hypothesized that dendritic cells (DC) pulsed with P. aeruginosa would induce protective immunity against pulmonary infections with P. aeruginosa. Incubation of murine bone marrow-derived DC with P. aeruginosa in vitro led to uptake of P. aeruginosa and activation of the DC. Spleen-derived CD4(+) cells from mice immunized with P. aeruginosa-pulsed DC showed increased proliferation, demonstrating that DC pulsed with P. aeruginosa were capable of eliciting a P. aeruginosa-specific immune response. To evaluate if P. aeruginosa-pulsed DC can induce protective immunity against P. aeruginosa pulmonary infection, DC incubated with P. aeruginosa in vitro were administered systemically to syngeneic mice, and the mice were then challenged by intrapulmonary infection with P. aeruginosa (5 x 10(4) CFU/mouse) 13 days later. Unimmunized control mice and mice who had previously received naive DC or DC stimulated with lipopolysaccharide or Escherichia coli died within 72 h. In contrast, 45% of mice receiving P. aeruginosa-pulsed DC demonstrated prolonged survival (>14 days). Finally, DC-pulsed with heat-inactivated P. aeruginosa protected CD8(-/-) but not CD4(-/-) mice, demonstrating that CD4(+) T cells were required for the DC pulsed with P. aeruginosa to induce protective immunity.

Dendritic cell activation and cytokine production induced by group B Neisseria meningitidis: interleukin-12 production depends on lipopolysaccharide expression in intact bacteria

Interactions between dendritic cells (DCs) and microbial pathogens are fundamental to the generation of innate and adaptive immune responses. Upon stimulation with bacteria or bacterial components such as lipopolysaccharide (LPS), immature DCs undergo a maturation process that involves expression of costimulatory molecules, HLA molecules, and cytokines and chemokines, thus providing critical signals for lymphocyte development and differentiation. In this study, we investigated the response of in vitro-generated human DCs to a serogroup B strain of Neisseria meningitidis compared to an isogenic mutant lpxA strain totally deficient in LPS and purified LPS from the same strain. We show that the parent strain, lpxA mutant, and meningococcal LPS all induce DC maturation as measured by increased surface expression of costimulatory molecules and HLA class I and II molecules. Both the parent and lpxA strains induced production of tumor necrosis factor alpha (TNF-alpha), interleukin-1alpha (IL-1alpha), and IL-6 in DCs, although the parent was the more potent stimulus. In contrast, high-level IL-12 production was only seen with the parent strain. Compared to intact bacteria, purified LPS was a very poor inducer of IL-1alpha, IL-6, and TNF-alpha production and induced no detectable IL-12. Addition of exogenous LPS to the lpxA strain only partially restored cytokine production and did not restore IL-12 production. These data show that non-LPS components of N. meningitidis induce DC maturation, but that LPS in the context of the intact bacterium is required for high-level cytokine production, especially that of IL-12. These findings may be useful in assessing components of N. meningitidis as potential vaccine candidates.

Immunohistochemical analysis of Lyme disease in the skin of naive and infection-immune rabbits following challenge

In this study, skin histopathology from naive and infection-derived immune rabbits was compared following intradermal challenge using Borrelia burgdorferi B31 strain. The presence or absence of spirochetes in relationship to host cellular immune responses was determined from the time of intradermal inoculation to the time of erythema migrans (EM) development (approximately 7 days in naive rabbits) and through development of challenge immunity (approximately 5 months in naive rabbits). Skin biopsies were obtained and analyzed for the presence of spirochetes, B cells, T cells, polymorphonuclear cells (PMNs), and macrophages by immunohistochemical techniques. In infected naive animals, morphologically identifiable spirochetes were detected at 2 h and up to 3 weeks postinfection. At 12 and 24 h postinfection there was a marked PMN response that decreased by 36 to 48 h; by 72 h the PMNs were replaced by a few infiltrating macrophages. At the time of EM development and 14 days postinfection, the PMNs and macrophages were replaced by a lymphocytic infiltrate. There was a greater number of spirochetes at 14 days, a time when EM had resolved, than at 7 days postinfection. By 3 weeks postinfection there were few organisms and lymphocytes detectable. In contrast to infected naive rabbits, intact spirochetes were never visualized in skin biopsies from infection-immune rabbits; only spirochetal antigen was detected at 2, 12, and 24 h in the presence of a numerous PMN infiltrate. By 36 h postchallenge, spirochetal antigen could not be detected and the PMN response was replaced by a few infiltrating macrophages. By 72 h postchallenge, PMNs and macrophages were absent from the skin; B and T cells were never detected at any time point in skin from infection-immune rabbits. The destruction of spirochetes in immune animals in the presence of PMNs and in the absence of a lymphocytic infiltrate suggests that infection-derived immunity is antibody mediated.

Vaccination with Bordetella pertussis-pulsed autologous or heterologous dendritic cells induces a mucosal antibody response in vivo and protects against infection

This study demonstrates for the first time that vaccination with either autologous or heterologous dendritic cells (DC) pulsed with specific antigen induces protective immune responses against noninvasive bacteria, namely Bordetella pertussis. The DC-mediated protection is associated with strong B. pertussis-specific immunoglobulin G (IgG) and IgA responses in the lung.

Major histocompatibility complex class II-independent generation of neutralizing antibodies against T-cell-dependent Borrelia burgdorferi antigens presented by dendritic cells: regulation by NK and gammadelta T cells

We previously showed that adoptive transfer of Borrelia burgdorferi-pulsed dendritic cells (DCs) into syngeneic mice protects animals from challenge with tick-transmitted spirochetes. Here, we demonstrate that the protective immune response is antibody (Ab) dependent and does not require the presence of major histocompatibility complex (MHC) class II molecules on DCs. Mice sensitized with B. burgdorferi-pulsed MHC class II-deficient (MHC class II(-/-)) DCs mounted a humoral response against protective antigens, including B. burgdorferi outer surface protein A (OspA) and OspC. B-cell help for the generation of neutralizing anti-OspC immunoglobulin G Abs could be provided by gammadelta T cells. In contrast, anti-OspA Ab production required the presence of alphabeta T cells, although this pathway could be independent of MHC class II molecules on antigen-presenting cells. Moreover, depletion of NK cells prior to transfer of antigen-pulsed MHC class II(-/-) DCs resulted in significant increases in the levels of neutralizing Abs induced by DCs. Altogether, these data suggest that the initial interactions between DCs and innate immune cells, such as gammadelta and NK cells, can influence the generation of a protective humoral response against B. burgdorferi antigens.

Dendritic cells pulsed with unfractionated helminthic proteins to generate antiparasitic cytotoxic T lymphocyte

Dendritic cells (DC) are sentinels of immunity. We determined their role in the induction of immunity against alveolar echinococcosis, caused by the larval stage of the cestode Echinococcus multilocularis. Furthermore, we evaluated if unfractionated protein from E. multilocularis (Em-Ag) can be used as loading agent for DC (comparable to unfractionated tumour proteins) in order to generate antiparasitic cytotoxic T lymphocyte (CTL). Interestingly, immature DC did not mature in the presence of 1 microg/ml Em-Ag as analysed by FACS and mixed leucocyte reactions. Yet, their capacity to take up dextran was markedly reduced. Further maturation of immature Em-Ag pulsed DC could be induced by proinflammatory cytokines. These mature DC were slightly better inducers of T cell proliferation when compared with unpulsed mature DC. Importantly, by repetetive stimulation of autologous CD8+ lymphocytes with the Em-Ag pulsed mature DC, we were able to generate specifically proliferating CTL lines. Thus, immunotherapy with ex vivo generated Em-Ag pulsed DC might be of benefit for patients inheriting this incurable disease.

Prominent T cell response to a selectively in vivo expressed Borrelia burgdorferi outer surface protein (pG) in patients with Lyme disease

Diagnosis of Lyme disease by analysis of T cell immune responses in vitro is curtailed by poor correlation between test results and status of infection. This is probably due to the inherent nonspecific activation potential of the causative agent, the spirochete Borrelia burgdorferi, for bystander lymphocytes, in particular via their outer surface lipoproteins. We have now applied a novel protocol to determine specific T cell responses in Lyme disease patients and exclude unrelated cellular responses in vitro. Non-lipidated spirochetal antigens (OspA, OspC and P39) including those selectively expressed in the mammalian host (pG and BapA) were used for antigenic stimulation and autologous dendritic cells served as antigen-presenting cells. The majority of patients with well-defined early and late manifestations of Lyme disease exhibited specific T cell proliferative responses to one or more of the indicated antigens, however at distinct levels. Most notably, among the five antigens tested, pG was specifically recognized by the majority of T cell populations (>70%) - mainly Th1 cells - from patients but not control individuals. These data indicate a causal relationship between B. burgdorferi infection and T cell reactivity to pG, thus making this protein a promising additional diagnostic marker for Lyme disease.

Fate of Mycobacterium tuberculosis within murine dendritic cells

The interaction of microbes with dendritic cells (DCs) is likely to have an enormous impact on the initiation of the immune response against a pathogen. In this study, we compared the interaction of Mycobacterium tuberculosis with murine bone marrow-derived DCs and macrophages (M phi) in vitro. M. tuberculosis grew equally well within nonactivated DCs and M phi. Activation of DCs and M phi with gamma interferon and lipopolysaccharide inhibited the growth of the intracellular bacteria in a nitric oxide synthase-dependent fashion. However, while this activation enabled M phi to kill the intracellular bacteria, the M. tuberculosis bacilli within activated DCs were not killed. Thus, DCs could restrict the growth of the intracellular mycobacteria but were less efficient than M phi at eliminating the infection. These results may have implications for priming immune responses to M. tuberculosis. In addition, they suggest that DCs may serve as a reservoir for M. tuberculosis in tissues, including the lymph nodes and lungs.

Molecular events of bacterial-induced maturation of dendritic cells

In order to protect the body from infectious microorganisms, mammals have developed powerful lines of defense, consisting in innate and adaptive immune responses. The innate response is phylogenetically more ancient and, for a long time, it has been considered to be broadly directed to microorganisms. However, the discovery of a new class of receptors, involved in recognition of patterns characteristic of groups of microorganisms (the toll-like receptor family) has re-evaluated the role of the innate immune system as a discriminating system. Indeed, there is increasing evidence that the induction of different types of effector adaptive responses are directed by the innate immune system after recognition of particular groups of pathogens. The central role of Dendritic cells (DC) in the induction of adaptive immune responses towards infectious agents has been extensively described, but, recently, a new role of DC as a link between the non-antigen- and the antigen-specific responses has been proposed. DC have, indeed, the capacity to recruit and activate cells of the innate immune system upon inflammation. Thus. understanding the interaction of bacteria with DC, and the early molecular events resulting from this interaction may shed some light on the mechanisms of initiation of the immune response to infectious agents and on aspects of invasiveness, pathogenicity, and the persistence of certain bacteria.

Lipoproteins from Borrelia burgdorferi applied in liposomes and presented by dendritic cells induce CD8(+) T-lymphocytes in vitro

Borrelia burgdorferi (Bb) is the tick-borne etiologic agent of Lyme borreliosis, which has many aspects of autoimmune diseases. Bb is unable to recycle synthesized membrane lipids and lipoproteins. Consequently, a large amount of liposome-like vesicle (Bb-blebs) is shed from the outer bacterial membrane. The influence of Bb-blebs on the cellular immune response is not yet known. As a Bb-blebs model, we established standardized Bb-liposomes, produced from freshly extracted lipids and lipoproteins of live Bb. Bb-liposomes were incorporated via nonendocytotic mechanisms by different human cell types, namely dendritic cells (DC), lymphocytes, and fibroblasts, as visualized by immunofluorescence and transmission electron microscopy. Bb-liposomes were localized in the cytosol and in the nucleus of the cells. With this in mind, we generated in vitro Bb-specific T-cells from nonadherant peripheral blood mononuclear cells by use of Bb-liposomes loaded autologous DC. More than 95% of those T-cells were CD8(+) and they killed autologous Bb-liposome-loaded T-cell blasts. These results suggest that Bb-blebs may be responsible for the autoimmune-like appearance of Lyme disease.

Analysis of mouse dendritic cell migration in vivo upon subcutaneous and intravenous injection

Dendritic cells (DC) have an increasingly important role in vaccination therapy; therefore, this study sought to determine the migratory capacity and immunogenic function of murine bone-marrow (BM)-derived DC following subcutaneous (s.c.) and intravenous (i.v.) injection in vivo. DC were enriched from BM cultures using metrizamide. Following centrifugation, the low-buoyant density cells, referred to throughout as DC, were CD11c(high), Iab(high), B7-1(high) and B7-2(high) and potently activated alloreactive T cells in mixed lymphocyte reactions (MLR). In contrast, the high-density cells expressed low levels of the above markers, comprised mostly of granulocytes based on GR1 expression, and were poor stimulators in MLR. Following s.c. injection of fluorescently labelled cells into syngeneic recipient mice, DC but not granulocytes migrated to the T-dependent areas of draining lymph nodes (LN). DC numbers in LN were quantified by flow-cytometric analysis, on 1, 2, 3, 5 and 7 days following DC transfer. Peak numbers of around 90 DC per draining LN were found at 2 days. There was very little migration of DC to non-draining LN, thymus or spleen at any of the time-points studied. In contrast, following i.v. injection, DC accumulated mainly in the spleen, liver and lungs of recipient mice but were largely absent from peripheral LN and thymus. The ability of DC to induce T-cell-mediated immune responses was examined using trinitrobenzenesulphate (TNBS)-derivatized DC (TNBS-DC) to sensitize for contact hypersensitivity responses (CHS) in naive syngeneic recipients. Following s.c. injection, as few as 105 TNBS-DC, but not TNBS-granulocytes, sensitized for CHS responses. However, the same number of TNBS-DC failed to induce CHS following i.v. injection. In summary, this study provides new and quantitative data on the organ specific migration of murine BM-derived DC following s.c. and i.v. injection. The demonstration that the route of DC administration determines the potency of CHS induction, strongly suggests that the route of immunization should be considered in the design of vaccine protocols using DC.

Dendritic Cell (DC)-Based Anti-Infective Strategies: DCs Engineered to Secrete IL-12 Are a Potent Vaccine in a Murine Model of an Intracellular Infection

Infections with intracellular pathogens such as Leishmania donovani and Mycobacterium tuberculosis pose serious health problems worldwide. Effective vaccines for these pathogens are not available. Furthermore, despite optimal therapy, disease progression is often seen with several intracellular infections. For these reasons, we initiated studies to develop novel anti-infective vaccine and treatment strategies that couple the potent Ag-presenting capacity of dendritic cells (DC) with paracrine delivery of potent anti-infective cytokines such as IL-12 to local immune response sites. We tested this strategy in a murine model of visceral leishmaniasis. Adoptive transfer of DCs pulsed ex vivo with soluble L. donovani Ags (SLDA) to naive mice induced the Ag-specific production of IFN-γ, and increased the percentage of activation markers on spleen lymphocytes. SLDA-pulsed DCs engineered by retroviral gene transfer techniques to secrete high levels of biologically active murine IL-12 augmented this immune response further. In several different vaccination and immunotherapy protocols, compared with sham-treated mice, animals receiving SLDA-pulsed DCs either before or following infection had 1–3 log lower parasite burdens, and this protection was associated with a pronounced enhancement in the parasite-specific IFN-γ response. The augmentation of this protection by IL-12-engineered DCs was striking. First, live parasites were not detected in the liver of mice vaccinated with IL-12-transduced, SLDA-pulsed DCs. Second, this parasitological response was associated with a nearly normal liver histology. In contrast, parasites and granulomas were found in mice vaccinated with SLDA-pulsed, nontransduced DCs. Collectively, these studies provide the rationale for the development of potent DC-based immunotherapies.

The role of dendritic cells in the induction and regulation of immunity to microbial infection

Dendritic cells (DCs) play a critical role in the initiation and regulation of immune responses. Recent advances have begun to uncover the nature and diversity of DC-pathogen interactions and the modulation of DC function by microbial stimuli. Antigen pulsed DCs have also been shown in several infection models to induce high levels of protective immunity and to display immunotherapeutic potential. The study of the function of DCs in the response to infection is thus an exciting and rapidly expanding field with important implications for both fundamental and clinical immunology.

Mucosal immunity and tolerance: relevance to vaccine development

The mucosal immune system of mammals consists of an integrated network of lymphoid cells which work in concert with innate host factors to promote host defense. Major mucosal effector immune mechanisms include secretory antibodies, largely of immunoglobulin A (IgA) isotype, cytotoxic T cells, as well as cytokines, chemokines and their receptors. Immunologic unresponsiveness (tolerance) is a key feature of the mucosal immune system, and deliberate vaccination or natural immunization by a mucosal route can effectively induce immune suppression. The diverse compartments located in the aerodigestive and genitourinary tracts and exocrine glands communicate via preferential homing of lymphocytes and antigen-presenting cells. Mucosal administration of antigens may result in the concomitant expression of secretory immunoglobulin A (S-IgA) antibody responses in various mucosal tissues and secretions, and under certain conditions, in the suppression of immune responses. Thus, developing formulations based on efficient delivery of selected antigens/tolerogens, cytokines and adjuvants may impact on the design of future vaccines and of specific immunotherapeutic approaches against diseases associated with untoward immune responses, such as autoimmune disorders, allergic reactions, and tissue-damaging inflammatory reactions triggered by persistent microorganisms.

Vaccination as a modality to prevent Lyme disease. A status report

Recombinant outer surface protein A (OspA) of Borrelia burgdorferi is a highly protective immunogen for prevention of Lyme disease in experimental animals. Humoral immunity is sufficient for protection. The principal mechanism of action is prevention of transmission of the spirochete from tick to host. A recombinant OspA vaccine has been licensed for use in dogs. The recent licensure of an OspA vaccine for humans resulted from a critical analysis of recently completed efficacy studies.

Murine dendritic cells pulsed in vitro with Toxoplasma gondii antigens induce protective immunity in vivo

The activation of a predominant T-helper-cell subset plays a critical role in disease resolution. In the case of Toxoplasma gondii, the available evidence indicates that CD4(+) protective cells belong to the Th1 subset. The aim of this study was to determine whether T. gondii antigens (in T. gondii sonicate [TSo]) presented by splenic dendritic cells (DC) were able to induce a specific immune response in vivo and to protect CBA/J mice orally challenged with T. gondii cysts. CBA/J mice immunized with TSo-pulsed DC exhibited significantly fewer cysts in their brains after oral infection with T. gondii 76K than control mice did. Protected mice developed a strong humoral response in vivo, with especially high levels of anti-TSo immunoglobulin G2a antibodies in serum. T. gondii antigens such as SAG1 (surface protein), SAG2 (surface protein), MIC1 (microneme protein), ROP2 through ROP4 (rhoptry proteins), and MIC2 (microneme protein) were recognized predominantly. Furthermore, DC loaded with TSo, which synthesized high levels of interleukin-12 (IL-12), triggered a strong cellular response in vivo, as assessed by the proliferation of lymph node cells in response to TSo restimulation in vitro. Cellular proliferation was associated with gamma interferon and IL-2 production. Taken together, these results indicate that immunization of CBA/J mice with TSo-pulsed DC can induce both humoral and Th1-like cellular immune responses and affords partial resistance against the establishment of chronic toxoplasmosis.

Vaccination against chlamydial genital tract infection after immunization with dendritic cells pulsed ex vivo with nonviable Chlamydiae

Chlamydia trachomatis, an obligate intracellular bacterial pathogen of mucosal surfaces, is a major cause of preventable blindness and sexually transmitted diseases for which vaccines are badly needed. Despite considerable effort, antichlamydial vaccines have proven to be elusive using conventional immunization strategies. We report the use of murine bone marrow-derived dendritic cells (DC) pulsed ex vivo with killed chlamydiae as a novel approach to vaccination against chlamydial infection. Our results show that DC efficiently phagocytose chlamydiae, secrete IL-12 p40, and present chlamydial antigen(s) to infection sensitized CD4(+) T cells. Mice immunized intravenously with chlamydial-pulsed DC produce protective immunity against chlamydial infection of the female genital tract equal to that obtained after infection with live organisms. Immunized mice shed approximately 3 logs fewer infectious chlamydiae and are protected from genital tract inflammatory and obstructive disease. Protective immunity is correlated with a chlamydial-specific Th1-biased response that closely mimics the immune response produced after chlamydial infection. Thus, ex vivo antigen-pulsed DC represent a powerful tool for the study of protective immunity to chlamydial mucosal infection and for the identification of chlamydial protective antigens through reconstitution experiments. Moreover, these findings might impact the design of vaccine strategies against other medically important sexually transmitted diseases for which vaccines are sought but which have proven difficult to develop.