A robust neutralization test for Plasmodium falciparum malaria

Anti-malarial market and policy surveys in sub-Saharan Africa

At a recent meeting (Sept 18, 2009) in which reasons for the limited access to artemisinin-based combination therapy (ACT) in sub-Saharan Africa were discussed, policy and market surveys on anti-malarial drug availability and accessibility in Burundi and Sierra Leone were presented in a highly interactive brainstorming session among key stakeholders across private, public, and not-for-profit sectors. The surveys, the conduct of which directly involved the national malaria control programme managers of the two countries, provides the groundwork for evidence-based policy implementation. The results of the surveys could be extrapolated to other countries with similar socio-demographic and malaria profiles. The meeting resulted in recommendations on key actions to be taken at the global, national, and community level for better ACT accessibility. At the global level, both public and private sectors have actions to take to strengthen policies that lead to the replacement of loose blister packs with fixed-dose ACT products, develop strategies to ban inappropriate anti-malarials and regulate those bans, and facilitate technology and knowledge transfer to scale up production of fixed-dose ACT products, which should be readily available and affordable to those patients who are in the greatest need of these medicines. At the national level, policies that regulate the anti-malarial medicines market should be enacted and enforced. The public sector, including funding donors, should participate in ensuring that the private sector is engaged in the ACT implementation process. Research similar to the surveys discussed is important for other countries to develop and evaluate the right incentives at a local level. At the community level, community outreach and education about appropriate preventive and treatment measures must continue and be strengthened, with service delivery systems developed within both public and private sectors, among other measures, to decrease access to ineffective and inappropriate anti-malarial medicines. What was clear during the meeting is that continuing commitment, strengthened interaction and transparency among various stakeholders, with focus on communities, national governments, and evidence-based policy and action are the only way to sustainably address the control of malaria, a disease which continues to have a significant health and socio-economic impact worldwide, particularly in sub-Saharan Africa. Details on the methodology employed in carrying out the studies discussed at this meeting, as well as more detailed results, data analysis and discussion of the studies are soon to be published.

Anti-malarial market and policy surveys in sub-Saharan Africa

At a recent meeting (Sept 18, 2009) in which reasons for the limited access to artemisinin-based combination therapy (ACT) in sub-Saharan Africa were discussed, policy and market surveys on anti-malarial drug availability and accessibility in Burundi and Sierra Leone were presented in a highly interactive brainstorming session among key stakeholders across private, public, and not-for-profit sectors. The surveys, the conduct of which directly involved the national malaria control programme managers of the two countries, provides the groundwork for evidence-based policy implementation. The results of the surveys could be extrapolated to other countries with similar socio-demographic and malaria profiles. The meeting resulted in recommendations on key actions to be taken at the global, national, and community level for better ACT accessibility. At the global level, both public and private sectors have actions to take to strengthen policies that lead to the replacement of loose blister packs with fixed-dose ACT products, develop strategies to ban inappropriate anti-malarials and regulate those bans, and facilitate technology and knowledge transfer to scale up production of fixed-dose ACT products, which should be readily available and affordable to those patients who are in the greatest need of these medicines. At the national level, policies that regulate the anti-malarial medicines market should be enacted and enforced. The public sector, including funding donors, should participate in ensuring that the private sector is engaged in the ACT implementation process. Research similar to the surveys discussed is important for other countries to develop and evaluate the right incentives at a local level. At the community level, community outreach and education about appropriate preventive and treatment measures must continue and be strengthened, with service delivery systems developed within both public and private sectors, among other measures, to decrease access to ineffective and inappropriate anti-malarial medicines. What was clear during the meeting is that continuing commitment, strengthened interaction and transparency among various stakeholders, with focus on communities, national governments, and evidence-based policy and action are the only way to sustainably address the control of malaria, a disease which continues to have a significant health and socio-economic impact worldwide, particularly in sub-Saharan Africa. Details on the methodology employed in carrying out the studies discussed at this meeting, as well as more detailed results, data analysis and discussion of the studies are soon to be published.

CD11c(high )dendritic cells are essential for activation of CD4+ T cells and generation of specific antibodies following mucosal immunization

To generate vaccines that protect mucosal surfaces, a better understanding of the cells required in vivo for activation of the adaptive immune response following mucosal immunization is required. CD11c(high) conventional dendritic cells (cDCs) have been shown to be necessary for activation of naive CD8(+) T cells in vivo, but the role of cDCs in CD4(+) T cell activation is still unclear, especially at mucosal surfaces. The activation of naive Ag-specific CD4(+) T cells and the generation of Abs following mucosal administration of Ag with or without the potent mucosal adjuvant cholera toxin were therefore analyzed in mice depleted of CD11c(high) cDCs. Our results show that cDCs are absolutely required for activation of CD4(+) T cells after oral and nasal immunization. Ag-specific IgG titers in serum, as well as Ag-specific intestinal IgA, were completely abrogated after feeding mice OVA and cholera toxin. However, giving a very high dose of Ag, 30-fold more than required to detect T cell proliferation, to cDC-ablated mice resulted in proliferation of Ag-specific CD4(+) T cells. This proliferation was not inhibited by additional depletion of plasmacytoid DCs or in cDC-depleted mice whose B cells were MHC-II deficient. This study therefore demonstrates that cDCs are required for successful mucosal immunization, unless a very high dose of Ag is administered.

Construction of transgenic Plasmodium berghei as a model for evaluation of blood-stage vaccine candidate of Plasmodium falciparum chimeric protein 2.9

Background

The function of the 19 kDa C-terminal region of the merozoite surface protein 1 (MSP1-19) expressed by Plasmodium has been demonstrated to be conserved across distantly related Plasmodium species. The green fluorescent protein (GFP) is a reporter protein that has been widely used because it can be easily detected in living organisms by fluorescence microscopy and flow cytometry.

Methodology and Results

In this study, we used gene targeting to generate transgenic P. berghei (Pb) parasites (designated as PfMSP1-19Pb) that express the MSP1-19 of P. falciparum (Pf) and the GFP reporter protein simultaneously. The replacement of the PbMSP1-19 locus by PfMSP1-19 was verified by PCR and Southern analysis. The expression of the chimeric PbfMSP-1 and the GFP was verified by Western blot and fluorescence microscopy, respectively. Moreover, GFP-expressing transgenic parasites in blood stages can be readily differentiated from other blood cells using flow cytometry. A comparion of growth rates between wild-type and the PfMSP1-19Pb transgenic parasite indicated that the replacement of the MSP1-19 region and the expression of the GFP protein were not deleterious to the transgenic parasites. We used this transgenic mouse parasite as a murine model to evaluate the protective efficacy in vivo of specific IgG elicited by a PfCP-2.9 malaria vaccine that contains the PfMSP1-19. The BALB/c mice passively transferred with purified rabbit IgG to the PfCP-2.9 survived a lethal challenge of the PfMSP1-19Pb transgenic murine parasites, but not the wild-type P. berghei whereas the control mice passively transferred with purified IgG obtained from adjuvant only-immunized rabbits were vulnerable to both transgenic and wild-type infections.

Conclusions

We generated a transgenic P. berghei line that expresses PfMSP1-19 and the GFP reporter gene simultaneously. The availability of this parasite line provides a murine model to evaluate the protective efficacy in vivo of anti-MSP1-19 antibodies, including, potentially, those elicited by the PfCP-2.9 malaria vaccine in human volunteers.

Alveolar macrophages transport pathogens to lung draining lymph nodes

The first step in inducing pulmonary adaptive immunity to allergens and airborne pathogens is Ag acquisition and transport to the lung draining lymph nodes (dLN). Dendritic cells (DC) sample the airways, and active transfer of Ag to the lung dLN is considered an exclusive property of migratory DC. However, alveolar macrophages (AM) are the first phagocytes to contact inhaled particulate matter. Although having well-defined immunoregulatory capabilities, AM are generally considered as restricted to the alveoli. We show that murine AM constitutively migrate from lung to dLN and that following exposure to Streptococcus pneumoniae, AM rapidly transport bacteria to this site. Thus AM, and not DC, appear responsible for the earliest delivery of these bacteria to secondary lymphoid tissue. The identification of this novel transport pathway has important consequences for our understanding of lung immunity and suggests more widespread roles for macrophages in the transport of Ags to lymphoid organs than previously appreciated.

Concomitant inhalation of cigarette smoke and aerosolized protein activates airway dendritic cells and induces allergic airway inflammation in a TLR-independent way

Cigarette smoking is associated with the development of allergic asthma. In mice, exposure to cigarette smoke sensitizes the airways toward coinhaled OVA, leading to OVA-specific allergic inflammation. Pulmonary dendritic cells (DCs) are professional APCs involved in immunosurveillance and implicated in the induction of allergic responses in lung. We investigated the effects of smoking on some of the key features of pulmonary DC biology, including trafficking dynamics and cellular activation status in different lung compartments. We found that cigarette smoke inhalation greatly amplified DC-mediated transport of inhaled Ags to mediastinal lymph nodes, a finding supported by the up-regulation of CCR7 on airway DCs. Pulmonary plasmacytoid DCs, which have been involved in inhalational tolerance, were reduced in number after smoke exposure. In addition, combined exposure to cigarette smoke and OVA aerosol increased surface expression of MHC class II, CD86, and PDL2 on airway DCs, while ICOSL was strongly down-regulated. Although inhaled endotoxins, which are also present in cigarette smoke, have been shown to act as DC activators and Th2-skewing sensitizers, TLR4-deficient and MyD88 knockout mice did not show impaired eosinophilic airway inflammation after concomitant exposure to cigarette smoke and OVA. From these data, we conclude that cigarette smoke activates the pulmonary DC network in a pattern that favors allergic airway sensitization toward coinhaled inert protein. The TLR independency of this phenomenon suggests that alternative immunological adjuvants are present in cigarette smoke.

An anti-inflammatory role for plasmacytoid dendritic cells in allergic airway inflammation

It was previously shown that administration of recombinant human Fms-like tyrosine kinase receptor-3 ligand (Flt3L) before allergen challenge of sensitized mice suppresses the cardinal features of asthma through unclear mechanisms. Here, we show that Flt3L dramatically alters the balance of conventional to plasmacytoid dendritic cells (pDCs) in the lung favoring the accumulation of pDCs. Selective removal of pDCs abolished the antiinflammatory effect of Flt3L, suggesting a regulatory role for these cells in ongoing asthmatic inflammation. In support, we found that immature pDCs are recruited to the lungs of allergen-challenged mice irrespective of Flt3L treatment. Selective removal of pDCs during allergen challenge enhanced airway inflammation, whereas adoptive transfer of cultured pDCs before allergen challenge suppressed inflammation. Experiments in which TLR9 agonist CpG motifs were administered in vitro or in vivo demonstrated that pDCs were antiinflammatory irrespective of their maturation state. These effects were mediated through programmed death-1/programmed death ligand 1 interactions, but not through ICOS ligand, IDO, or IFN-alpha. These findings suggest a specialized immunoregulatory role for pDCs in airway inflammation. Enhancing the antiinflammatory properties of pDCs could be employed as a novel strategy in asthma treatment.

Dendritic cell entrapment within the pregnant uterus inhibits immune surveillance of the maternal/fetal interface in mice

Embryo implantation induces formation of the decidua, a stromal cell-derived structure that encases the fetus and placenta. Using the mouse as a model organism, we have found that this tissue reaction prevents DCs stationed at the maternal/fetal interface from migrating to the lymphatic vessels of the uterus and thus reaching the draining lymph nodes. Strikingly, decidual DCs remained immobile even after being stimulated with LPS and exhibiting responsiveness to CCL21, the chemokine that drives DC entry into lymphatic vessels. An analysis of maternal T cell reactivity toward a surrogate fetal/placental antigen furthermore revealed that regional T cell responses toward the fetus and placenta were driven by passive antigen transport and thus the tolerogenic mode of antigen presentation that predominates when there is negligible input from tissue-resident DCs. Indeed, the lack of involvement of tissue-resident DCs in the T cell response to the fetal allograft starkly contrasts with their prominent role in organ transplant rejection. Our results suggest that DC entrapment within the decidua minimizes immunogenic T cell exposure to fetal/placental antigens and raise the possibility that impaired development or function of the human decidua, which unlike that of the mouse contains lymphatic vessels, might lead to pathological T cell activation during pregnancy.

Mononuclear phagocyte-derived IL-10 suppresses the innate IL-12/IFN-gamma axis in lung-challenged aged mice

Previously, we reported that IL-10-producing mononuclear phagocytes increase in lungs of aged mice, causing impaired innate cytokine expression. Since dendritic cells (DCs) contribute to innate NK cell and adaptive T cell immunity, we tested the hypothesis that age-related IL-10 might influence DC function with effects on NK and T cell activation. The results showed that DC recruitment to sites of lung inflammation was normal in aged mice (>20 mo). However, IFN-gamma-producing NK cells in LPS-challenged lungs were decreased in aged as compared with young mice, which was associated with increased IL-10(+)CD11b(+)Gr-1(low)CD11c(-) cells consistent with mononuclear phagocytes. In vivo or in vitro blockade of IL-10 signaling restored IFN-gamma-producing NK cells. This restoration was reversed by IL-12 neutralization, indicating that IL-10 suppressed sources of IL-12 in aged mice. To probe DC function in adaptive immunity, we transferred young naive OVA-specific TCR transgenic T cells to old mice. Following challenge with OVA plus LPS, Ag presentation in the context of MHC-I and MHC-II occurred with similar kinetics and intensity in draining lymph nodes of young and old recipients as measured by proliferation. Despite this, aged hosts displayed impaired induction of IFN-gamma(+)CD4(+), but not IFN-gamma(+)CD8(+), effector T cells. Blockade of IL-10 signaling reversed age-associated defects. These studies indicate that the innate IL-12/IFN-gamma axis is not intrinsically defective in lungs of aged mice, but is rather suppressed by enhanced production of mononuclear phagocyte-derived IL-10. Our data identify a novel mechanism of age-associated immune deficiency.

Blood monocyte subsets differentially give rise to CD103+ and CD103- pulmonary dendritic cell populations

There are two major myeloid pulmonary dendritic cell (DC) populations: CD103+ DCs and CD11bhigh DCs. In this study, we investigated in detail the origins of both myeloid DC pools using multiple experimental approaches. We show that, in resting lung, Ly-6ChighCCR2high monocytes repopulated CD103+ DCs using a CCR2-dependent mechanism, and these DCs preferentially retained residual CCR2 in the lung, whereas, conversely, Ly-6ClowCCR2low monocytes repopulated CD11bhigh DCs. CX3CR1 was required to generate normal numbers of pulmonary CD11bhigh DCs, possibly because Ly-6Clow monocytes in the circulation, which normally express high levels of CX3CR1, failed to express bcl-2 and may have diminished survival in the circulation in the absence of CX3CR1. Overall, these data demonstrate that the two circulating subsets of monocytes give rise to distinct tissue DC populations.

Leukotriene B4 receptor 1 expression on dendritic cells is required for the development of Th2 responses and allergen-induced airway hyperresponsiveness

Dendritic cells (DC) are important APCs that control allergen-induced airway responses by interacting directly with T cells. Leukotriene B(4) (LTB(4)), interacting with its high-affinity receptor, LTB(4) receptor 1 (BLT1), is known to attract and activate leukocytes during inflammation. We have previously shown that BLT1 expression on Ag-primed T cells is required for the development of airway hyperresponsiveness (AHR; Miyahara et al. 2005. Am. J. Respir. Crit. Care Med. 172: 161-167). However, the role for the LTB(4)-BLT1 pathway in DC function in allergen-induced airway responses has not been defined. Bone marrow-derived DCs (BMDC) were generated. Naive BALB/c mice received OVA-pulsed BLT1-deficient (BLT1(-/-)) BMDCs or wild-type BMDCs intratracheally and were then challenged with OVA for 3 days. Airway responses were monitored 48 h after the last allergen challenge. BLT1(-/-) BMDCs showed normal maturation judged from surface expression of CD markers. Compared with recipients of wild-type BMDCs, mice that received BLT1(-/-) BMDCs developed significantly lower AHR to inhaled methacholine, lower goblet cell metaplasia, and eosinophilic infiltration in the airways and decreased levels of Th2 type cytokines in the bronchoalveolar lavage fluid. Migration of BLT1(-/-) BMDCs into peribronchial lymph nodes was significantly impaired compared with BLT1(+/+) BMDCs after intratracheal instillation. These data suggest that BLT1 expression on DCs is required for migration of DCs to regional lymph nodes as well as in the development of AHR and airway inflammation.

CCR2 mediates conventional dendritic cell recruitment and the formation of bronchovascular mononuclear cell infiltrates in the lungs of mice infected with Cryptococcus neoformans

Pulmonary clearance of the encapsulated yeast Cryptococcus neoformans requires the development of T1-type immunity. CCR2-deficient mice infected with C. neoformans develop a non-protective T2 immune response and persistent infection. The mechanisms responsible for this aberrant response are unknown. The objective of this study was to define the number, phenotype, and microanatomic location of dendritic cells (DC) residing within the lung of CCR2+/+ or CCR2-/- mice throughout a time course following infection with C. neoformans. Results demonstrate the CCR2-mediated recruitment of conventional DC expressing modest amounts of costimulatory molecules. DC recruitment was preceded by the up-regulation in the lung of the CCR2 ligands CCL2 and CCL7. Colocalization of numerous DC and CD4+ T cells within bronchovascular infiltrates coincided with increased expression of IL-12 and IFN-gamma. By contrast, in the absence of CCR2, DC recruitment was markedly impaired, bronchovascular infiltrates were diminished, and mice developed features of T2 responses, including bronchovascular collagen deposition and IL-4 production. Our results demonstrate that CCR2 is required for the recruitment of large numbers of conventional DC to bronchovascular infiltrates in mice mounting a T1 immune response against a fungal pathogen. These findings shed new insight into the mechanism(s) by which DC recruitment alters T cell polarization in response to an infectious challenge within the lung.

TNF receptor-associated factor 1 expressed in resident lung cells is required for the development of allergic lung inflammation

TNF is a major therapeutic target in a range of chronic inflammatory disorders, including asthma. TNFR-associated factor (TRAF)1 is an intracellular adaptor molecule important for signaling by TNFR. In this study, we investigated the role of TRAF1 in an adoptive transfer model of allergic lung inflammation. Mice deficient in TRAF1 (TRAF1(-/-)) and wild-type (WT) control animals were adoptively transferred with WT OVA-immune CD4(+) T cells, exposed to an aerosol of LPS-free OVA, and analyzed for the development of allergic lung inflammation. In contrast to WT mice, TRAF1(-/-) recipients failed to display goblet cell hyperplasia, eosinophilic inflammation, and airway hyperresponsiveness in this model of asthma. Neither T cell recruitment nor expression of the proinflammatory cytokines IL-4, IL-5, IL-13, or TNF occurred in the lungs of TRAF1(-/-) mice. Although purified myeloid TRAF1(-/-) dendritic cells (DCs) exhibited normal Ag-presenting function and transmigratory capacity in vitro and were able to induce OVA-specific immune responses in the lung draining lymph nodes (LNs) following adoptive transfer in vivo, CD11c(+)CD11b(+) DCs from airways of TRAF1(-/-) recipients were not activated, and purified draining LN cells did not proliferate in vitro. Moreover, transfer of WT or TRAF1(-/-) DCs failed to restore T cell recruitment and DC activation in the airways of TRAF1(-/-) mice, suggesting that the expression of TRAF1 in resident lung cells is required for the development of asthma. Finally, we demonstrate that T cell-transfused TRAF1(-/-) recipient mice demonstrated impaired up-regulation of ICAM-1 expression on lung cells in response to OVA exposure.

The kidney-renal lymph node-system contributes to cross-tolerance against innocuous circulating antigen

Soluble Ags devoid of inflammatory stimuli, derived for example from self-serum or food proteins, induce T cell tolerance, predominantly in the spleen. In this study, we describe an additional role of the kidney-renal LN (rLN) system in tolerogenic presentation of circulating soluble Ags. Protein below albumin molecular mass constitutively passed the kidney glomerular filter and was concentrated in the tubular compartment. Enriched filterable Ag was endocytosed by kidney dendritic cells (kDCs). Simultaneously, it was transported cell independently within 2 min to DCs resident in rLNs. These DC phenotypically differed from kDCs carrying filterable Ag, and used a distinct mechanism, mannose receptor-mediated endocytosis, to internalize Ag. They activated specific CD8+ T cells, which subsequently proliferated without producing effector cytokines or developing cytotoxic activity, showed a curtailed lifespan and signs of apoptosis. Such T cell tolerization was independent of steady-state migratory kDC, because it occurred also when nephrectomy was performed soon after Ag injection. These findings demonstrate that the kidney dispatches concentrated blood-borne Ags to the rLNs, where they are captured by resident DCs, resulting in CD8+ T cell cross-tolerance. This mechanism may contribute to avoiding immunity against innocuous circulating protein Ags below albumin size.

Allergic airways disease develops after an increase in allergen capture and processing in the airway mucosa

Airway mucosal dendritic cells (AMDC) and other airway APCs continuously sample inhaled Ags and regulate the nature of any resulting T cell-mediated immune response. Although immunity develops to harmful pathogens, tolerance arises to nonpathogenic Ags in healthy individuals. This homeostasis is thought to be disrupted in allergic respiratory disorders such as allergic asthma, such that a potentially damaging Th2-biased, CD4(+) T cell-mediated inflammatory response develops against intrinsically nonpathogenic allergens. Using a mouse model of experimental allergic airways disease (EAAD), we have investigated the functional changes occurring in AMDC and other airway APC populations during disease onset. Onset of EAAD was characterized by early and transient activation of airway CD4(+) T cells coinciding with up-regulation of CD40 expression exclusively on CD11b(-) AMDC. Concurrent enhanced allergen uptake and processing occurred within all airway APC populations, including B cells, macrophages, and both CD11b(+) and CD11b(-) AMDC subsets. Immune serum transfer into naive animals recapitulated the enhanced allergen uptake observed in airway APC populations and mediated activation of naive allergen-specific, airway CD4(+) T cells following inhaled allergen challenge. These data suggest that the onset of EAAD is initiated by enhanced allergen capture and processing by a number of airway APC populations and that allergen-specific Igs play a role in the conversion of normally quiescent AMDC subsets into those capable of inducing airway CD4(+) T cell activation.

Controls for lung dendritic cell maturation and migration during respiratory viral infection

Dendritic cells are ideally suited to orchestrate the innate and adaptive immune responses to infection, but we know little about how these cells respond to infection with common respiratory viruses. Paramyxoviral infections are the most frequent cause of serious respiratory illness in childhood and are associated with an increased risk of asthma. We therefore used a high-fidelity mouse model of paramyxoviral respiratory infection triggered by Sendai virus to examine the response of conventional and plasmacytoid dendritic cells (cDCs and pDCs, respectively) in the lung. We found that pDCs are scarce at baseline but become the predominant population of lung dendritic cells during infection. This recruitment allows for a source of IFN-alpha locally at the site of infection. In contrast, cDCs rapidly differentiate into myeloid cDCs and begin to migrate from the lung to draining lymph nodes within 2 h after viral inoculation. These events cause the number of lung cDCs to decrease rapidly and remain decreased at the site of viral infection. Maturation and migration of lung cDCs depends on Ccl5 and Ccr5 signals because these events are significantly impaired in Ccl5(-/-) and Ccr5(-/-) mice. cDCs failure to migrate to draining lymph nodes in Ccl5(-/-) or Ccr5(-/-) mice is associated with impaired up-regulation of CCR7 that would normally direct this process. Our results indicate that pDCs and cDCs respond distinctly to respiratory paramyxoviral infection with patterns of movement that should serve to coordinate the innate and adaptive immune responses, respectively.

Lung dendritic cells rapidly mediate anthrax spore entry through the pulmonary route

Inhalational anthrax is a life-threatening infectious disease of considerable concern, especially because anthrax is an emerging bioterrorism agent. The exact mechanisms leading to a severe clinical form through the inhalational route are still unclear, particularly how immobile spores are captured in the alveoli and transported to the lymph nodes in the early steps of infection. We investigated the roles of alveolar macrophages and lung dendritic cells (LDC) in spore migration. We demonstrate that alveolar macrophages are the first cells to phagocytose alveolar spores, and do so within 10 min. However, interstitial LDCs capture spores present in the alveoli within 30 min without crossing the epithelial barrier suggesting a specific mechanism for rapid alveolus sampling by transepithelial extension. We show that interstitial LDCs constitute the cell population that transports spores into the thoracic lymph nodes from within 30 min to 72 h after intranasal infection. Our results demonstrate that LDCs are central to spore transport immediately after infection. The rapid kinetics of pathogen transport may contribute to the clinical features of inhalational anthrax.

Scavenger Receptors SR-AI/II and MARCO limit pulmonary dendritic cell migration and allergic airway inflammation

The class A scavenger receptors (SR-A) MARCO and SR-AI/II are expressed on lung macrophages (MPhis) and dendritic cells (DCs) and function in innate defenses against inhaled pathogens and particles. Increased expression of SR-As in the lungs of mice in an OVA-asthma model suggested an additional role in modulating responses to an inhaled allergen. After OVA sensitization and aerosol challenge, SR-AI/II and MARCO-deficient mice exhibited greater eosinophilic airway inflammation and airway hyperresponsiveness compared with wild-type mice. A role for simple SR-A-mediated Ag clearance ("scavenging") by lung MPhis was excluded by the observation of a comparable uptake of fluorescent OVA by wild-type and SR-A-deficient lung MPhis and DCs. In contrast, airway instillation of fluorescent Ag revealed a significantly higher traffic of labeled DCs to thoracic lymph nodes in SR-A-deficient mice than in controls. The increased migration of SR-A-deficient DCs was accompanied by the enhanced proliferation in thoracic lymph nodes of adoptively transferred OVA-specific T cells after airway OVA challenge. The data identify a novel role for SR-As expressed on lung DCs in the down-regulation of specific immune responses to aeroallergens by the reduction of DC migration from the site of Ag uptake to the draining lymph nodes.

The importance of human FcgammaRI in mediating protection to malaria

The success of passive immunization suggests that antibody-based therapies will be effective at controlling malaria. We describe the development of fully human antibodies specific for Plasmodium falciparum by antibody repertoire cloning from phage display libraries generated from immune Gambian adults. Although these novel reagents bind with strong affinity to malaria parasites, it remains unclear if in vitro assays are predictive of functional immunity in humans, due to the lack of suitable animal models permissive for P. falciparum. A potentially useful solution described herein allows the antimalarial efficacy of human antibodies to be determined using rodent malaria parasites transgenic for P. falciparum antigens in mice also transgenic for human Fc-receptors. These human IgG1s cured animals of an otherwise lethal malaria infection, and protection was crucially dependent on human FcγRI. This important finding documents the capacity of FcγRI to mediate potent antimalaria immunity and supports the development of FcγRI-directed therapy for human malaria.

Malaria rivals HIV and tuberculosis as the world's most deadly infection killing a child every 30 seconds. Antibodies and their receptors (Fc-receptors) have been shown to be vital for the development of protective immunity, and as such they act as correlates of protection in studies aimed at defining the best antigens to incorporate into current vaccines. Understanding antibody types and Fc-receptors that optimally induce immunity is therefore vital to developing the best vaccines. Surrogate markers of antibody efficacy currently rely on in vitro assays that are laborious and difficult to reproduce. It remains unclear if such in vitro assays are predictive of functional immunity in humans due to the lack of suitable animal models permissive for Plasmodium falciparum. Here, we create a transgenic in vivo mouse model that has significant advantage over the use of new world primates, the only other model for human malaria. We demonstrate that this model defines an Fc-dependent mechanism of parasite destruction that cannot be assessed in current in vitro assays. The model provides both a test for therapeutic antibody efficacy prior to clinical trials in humans and an important tool in malaria vaccine development.

Chemokine receptor CCR2 but not CCR5 or CCR6 mediates the increase in pulmonary dendritic cells during allergic airway inflammation

Increased numbers of pulmonary dendritic cells (DCs) are recruited to the lungs during allergic airway inflammation and contribute to the maintenance of the inflammatory immune response. The chemokine receptors that directly control DC accumulation into the lungs are largely unknown. To explore this issue, we generated mixed bone marrow chimeric mice containing both wild-type and knockout cells for a given chemokine receptor. After induction of allergic airway inflammation, we specifically tracked and compared chemokine receptor knockout vs wild-type DC populations through various lung compartments. Using this approach, we show that CCR2, but not CCR5 or CCR6, directly controls the accumulation of DCs into allergic lungs. Furthermore, the size of inflammatory monocyte populations in peripheral blood was strikingly CCR2 dependent, suggesting that CCR2 primarily mediates the release of monocytic DC precursors into the bloodstream.

Distinct responses of lung and spleen dendritic cells to the TLR9 agonist CpG oligodeoxynucleotide

Dendritic cells (DCs) sense various components of invading pathogens via pattern recognition receptors such as TLRs. CpG oligodeoxynucleotides (ODNs), which mimic bacterial DNA, inhibit allergic airways disease and promote responses in the spleen to bacterial components. Because many TLR agonists are currently being tested for potential therapeutic effects, it is important to characterize the expression and function of TLRs in different tissues. We show that both myeloid and plasmacytoid DCs in the spleen express TLR9, the receptor for CpG ODNs, but lung DCs show no detectable expression in either subset. TLR4 expression in contrast was detected on both lung and spleen DCs. LPS was superior to CpG ODN in increasing the allostimulatory potential of lung DCs and their expression of CD40. However, both agonists efficiently stimulated spleen DCs. CpG ODNs administered to mice efficiently inhibited Th2 cytokine production both in the lung draining lymph node and in the spleen. Surprisingly, inhibition of Th2 cytokine production was evident despite high levels of expression of GATA-3 and additional transcription factors that regulate Th2 responses. Although in the spleen CpG ODNs induced IL-6, a key cytokine induced via TLR9-MyD88 signaling, no IL-6 was detectable in lung LN cells. These studies show for the first time that lung DCs lack TLR9 expression, but, despite this deficiency, CpG ODNs induce potent inhibitory effects on Th2 cytokine production in the lung without inducing expression of the proinflammatory cytokine, IL-6, which has been linked to chronic diseases in the lung and the gut.

Mast cell-associated TNF promotes dendritic cell migration

Mast cells represent a potential source of TNF, a mediator which can enhance dendritic cell (DC) migration. Although the importance of mast cell-associated TNF in regulating DC migration in vivo is not clear, mast cells and mast cell-derived TNF can contribute to the expression of certain models of contact hypersensitivity (CHS). We found that CHS to FITC was significantly impaired in mast cell-deficient Kit(W-sh/W-sh) or TNF(-/)(-) mice. The reduced expression of CHS in Kit(W-sh/W-sh) mice was fully repaired by local transfer of wild-type bone marrow-derived cultured mast cells (BMCMCs), but was only partially repaired by transfer of TNF(-/)(-) BMCMCs. Thus, mast cells, and mast cell-derived TNF, were required for optimal expression of CHS to FITC. We found that the migration of FITC-bearing skin DCs into draining lymph nodes (LNs) 24 h after epicutaneous administration of FITC in naive mice was significantly reduced in mast cell-deficient or TNF(-/)(-) mice, but levels of DC migration in these mutant mice increased to greater than wild-type levels by 48 h after FITC sensitization. Mast cell-deficient or TNF(-/)(-) mice also exhibited significantly reduced migration of airway DCs to local LNs at 24 h after intranasal challenge with FITC-OVA. Migration of FITC-bearing DCs to LNs draining the skin or airways 24 h after sensitization was repaired in Kit(W-sh/W-sh) mice which had been engrafted with wild-type but not TNF(-/)(-) BMCMCs. Our findings indicate that mast cell-associated TNF can contribute significantly to the initial stages of FITC-induced migration of cutaneous or airway DCs.

A major lung CD103 (alphaE)-beta7 integrin-positive epithelial dendritic cell population expressing Langerin and tight junction proteins

Dendritic cells (DC) mediate airway Ag presentation and play key roles in asthma and infections. Although DC subsets are known to perform different functions, their occurrence in mouse lungs has not been clearly defined. In this study, three major lung DC populations have been found. Two of them are the myeloid and plasmacytoid DC (PDC) well-characterized in other lymphoid organs. The third and largest DC population is the integrin alpha(E) (CD103) beta(7)-positive and I-A(high)CD11c(high)-DC population. This population was found to reside in the lung mucosa and the vascular wall, express a wide variety of adhesion and costimulation molecules, endocytose avidly, present Ag efficiently, and produce IL-12. Integrin alpha(E)beta(7)(+) DC (alphaE-DC) were distinct from intraepithelial lymphocytes and distinguishable from CD11b(high) myeloid and mPDCA-1(+)B220(+)Gr-1(+) PDC populations in surface marker phenotype, cellular functions, and tissue localization. Importantly, this epithelial DC population expressed high levels of the Langerhans cell marker Langerin and the tight junction proteins Claudin-1, Claudin-7, and ZO-2. In mice with induced airway hyperresponsiveness and eosinophilia, alphaE-DC numbers were increased in lungs, and their costimulation and adhesion molecules were up-regulated. These studies show that alphaE-DC is a major and distinct lung DC population and a prime candidate APC with the requisite surface proteins for migrating across the airway epithelia for Ag and pathogen capture, transport, and presentation. They exhibit an activated phenotype in allergen-induced lung inflammation and may play significant roles in asthma pathogenesis.

Antibody- and Fc-receptor-based therapeutics for malaria

Abs (antibodies) are complex glycoproteins that play a crucial role in protective immunity to malaria, but their effectiveness in mediating resistance can be enhanced by genetically engineered modifications that improve on nature. These Abs also aid investigation of immune mechanisms operating to control the disease and are valuable tools in developing neutralization assays for vaccine design. This review explores how this might be achieved.

Opinion: antibody-based therapies for malaria

Antibodies are multifunctional glycoproteins that are found in blood and tissue fluids, and can protect against malaria by binding and neutralizing malaria parasites and preparing them for destruction by immune cells. Important technical advances mean that it is now possible to synthesize antibodies against important Plasmodium antigens that could be used for therapeutic purposes. These reagents could be designed to act like a drug and kill parasites directly, or could be used in vaccine strategies to protect individuals from infection. In this article, we discuss the possible therapeutic uses of antibodies in the treatment and prevention of malaria.

Immunogenicity and protective efficacy of Escherichia coli expressed Plasmodium falciparum merozoite surface protein-1(42) using human compatible adjuvants

The C-terminal 42-kDa fragment of the merozoite surface protein-1 of Plasmodium falciparum (PfMSP-1(42)) was expressed as a recombinant protein in Escherichia coli and purified to near homogeneity. We tested the immunogenicity of recombinant PfMSP-1(42) in three clinically acceptable adjuvants (Montanide ISA 720, alum and MF59) in mice and in rabbits. High antibody responses were obtained with two adjuvant formulations with IgGl being the predominant immunoglobulin isotype. Significant T-cell proliferation responses were also observed. Competitive enzyme linked immunosorbant assay (ELISA) showed the presence of both invasion and processing inhibitory antibodies in sera obtained from the immunized rabbits. Passive immunizations of mice with anti-PfMSP-1(42) IgG purified from the rabbit-sera were found to be protective against a parasite challenge with P. berghei/P. falciparum chimeric line (Pb-PfM19) that expresses Plasmodium falciparum MSP-1(19). These findings may be useful for the development of a malaria vaccine based on Plasmodium falciparum MSP-1(42).

Flt3-ligand, IL-4, GM-CSF, and adherence-mediated isolation of murine lung dendritic cells: assessment of isolation technique on phenotype and function

Lung dendritic cells (DCs) are difficult to study due to their limited quantities and the complexities required for isolation. Although many procedures have been used to overcome this challenge, the effects of isolation techniques on lung DCs have not been reported. The current study shows that freshly isolated DCs (CD11c+) have limited ability to induce proliferation in allogeneic T cells, and are immature as indicated by low cell surface expression of costimulatory molecules compared with liver or splenic DCs. DCs isolated after overnight culture or from mice treated with Flt3L are phenotypically mature and potent stimulators of allogeneic T cells. DCs could not be propagated from lung mononuclear cells in response to IL-4 and GM-CSF. Contrary to data reported for nonpulmonary DCs, expression of CCR6 was decreased on mature lung DCs, and only a subset of mature DCs expressed higher levels of CCR7. Absence of CD8alpha expression indicates that freshly isolated DCs are myeloid-type, whereas mature DCs induced by overnight culture are both "lymphoid" (CD8alpha+) and "myeloid" (CD8alpha-). DCs from mice genetically deficient in CD8alpha expression were strong simulators of allogeneic T cells which was consistent with data showing that CD8alpha- DCs from CD8alpha-sufficient mice are better APCs compared with CD8alpha+ DCs from the same mice. These data show that freshly isolated lung DCs are phenotypically and functionally distinct, and that the isolation technique alters the biology of these cells. Therefore, lung DC phenotype and function must be interpreted relative to the technique used for isolation.

Inhibition of experimental asthma by indoleamine 2,3-dioxygenase

Epidemiological evidence points to the inverse relationship between microbial exposure and the prevalence of allergic asthma and autoimmune diseases in Westernized countries. The molecular basis for this observation has not yet been completely delineated. Here we report that the administration of certain toll-like receptor (TLR) ligands, via the activation of innate immunity, induces high levels of indoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme of tryptophan catabolism in various organs. TLR9 ligand-induced pulmonary IDO activity inhibits Th2-driven experimental asthma. IDO activity expressed by resident lung cells rather than by pulmonary DCs suppressed lung inflammation and airway hyperreactivity. Our results provide a mechanistic insight into the various formulations of the hygiene hypothesis and underscore the notion that activation of innate immunity can inhibit adaptive Th cell responses.

A common cross-species function for the double epidermal growth factor-like modules of the highly divergent plasmodium surface proteins MSP-1 and MSP-8

An understanding of structural and functional constraints on the C-terminal double epidermal growth factor (EGF)-like modules of merozoite surface protein (MSP)-1 and related proteins is of importance to the development of these molecules as malaria vaccines and drug targets. Using allelic replacement, we show that Plasmodium falciparum parasites can invade erythrocytes and grow efficiently in the absence of an MSP-1 protein with authentic MSP-1 EGF domains. In this mutant parasite line, the MSP-1 EGFs were replaced by the corresponding double EGF module from P. berghei MSP-8, the sequence of which shares only low identity with its MSP-1 counterpart. Hence, the C-terminal EGF domains of at least some Plasmodium surface proteins appear to perform the same function in asexual blood-stage development. Mapping the surface location of the few residues that are common to these functionally complementary EGF modules revealed the presence of a highly conserved pocket of potential functional significance. In contrast to MSP-8, an even more divergent double EGF module, that from the sexual stage protein PbS25, was not capable of complementing MSP-1 EGF function. More surprisingly, two chimeric double EGF modules comprising hybrids of the EGF domains from P. falciparum and P. chabaudi MSP-1 were also not capable of replacing the P. falciparum MSP-1 EGF module. Together, these data suggest that although the MSP-1 EGFs can accommodate extensive sequence diversity, there appear to be constraints that may restrict the simple accumulation of point mutations in the face of immune pressure in the field.

Novel antimalarial antibodies highlight the importance of the antibody Fc region in mediating protection

Parasite drug resistance and difficulties in developing effective vaccines have precipitated the search for alternative therapies for malaria. The success of passive immunization suggests that immunoglobulin (Ig)-based therapies are effective. To further explore the mechanism(s) by which antibody mediates its protective effect, we generated human chimeric IgG1 and IgA1 and a single-chain diabody specific for the C-terminal 19-kDa region of Plasmodium yoelii merozoite surface protein 1 (MSP119), a major target of protective immune responses. These novel human reagents triggered in vitro phagocytosis of merozoites but, unlike their parental mouse IgG2b, failed to protect against parasite challenge in vivo. Therefore, the Fc region appears critical for mediating protection in vivo, at least for this MSP119 epitope. Such antibodies may serve as prototype therapeutic agents, and as useful tools in the development of in vitro neutralization assays with Plasmodium parasites.

A new rodent model to assess blood stage immunity to the Plasmodium falciparum antigen merozoite surface protein 119 reveals a protective role for invasion inhibitory antibodies

Antibodies capable of inhibiting the invasion of Plasmodium merozoites into erythrocytes are present in individuals that are clinically immune to the malaria parasite. Those targeting the 19-kD COOH-terminal domain of the major merozoite surface protein (MSP)-119 are a major component of this inhibitory activity. However, it has been difficult to assess the overall relevance of such antibodies to antiparasite immunity. Here we use an allelic replacement approach to generate a rodent malaria parasite (Plasmodium berghei) that expresses a human malaria (Plasmodium falciparum) form of MSP-119. We show that mice made semi-immune to this parasite line generate high levels of merozoite inhibitory antibodies that are specific for P. falciparum MSP-119. Importantly, protection from homologous blood stage challenge in these mice correlated with levels of P. falciparum MSP-119-specific inhibitory antibodies, but not with titres of total MSP-119-specific immunoglobulins. We conclude that merozoite inhibitory antibodies generated in response to infection can play a significant role in suppressing parasitemia in vivo. This study provides a strong impetus for the development of blood stage vaccines designed to generate invasion inhibitory antibodies and offers a new animal model to trial P. falciparum MSP-119 vaccines.

Antiviral immunity and the role of dendritic cells

Using our experimental model we demonstrate the need for Th1 immune responses for recovery from influenza virus infection. Inoculation of IL-4 concurrent with infection significantly delays virus clearance and converts the immune response to a Th2 response. Immunization using live virus in the presence of IL-4 leads to generation of Th2 memory cells that fail to facilitate recovery upon subsequent virus infection. Inactivated virus expands Th2 cells, leading to responses similar to those observed following IL-4 infusion. Immunization using cultured dendritic cells incubated with live or inactivated virus mimics the results observed with direct virus injection. We demonstrate that in contrast to live virus-infected dendritic cells, inactivated virus fails to elicit Th1 immunity. This failure correlates with the inactivated viruses' inability to induce dendritic cell maturation. Thus, our data suggest that the polarity of the immune response is dictated by the nature of the antigen, and the trigger for influenza virus-induced DC maturation leading to Th1 immunity is dependent on virus replication.

Progress and challenges for malaria vaccines

Malaria causes much physical and economic hardship in tropical regions, particularly in communities where medical care is rudimentary. Should a vaccine be developed, it is the residents of these areas that stand to benefit the most. But the vaccine, which has been promised to be 'just round the corner' for many years, remains elusive. It is important to ask why this is so, when effective vaccines exist for many other infectious diseases. What are the reasons for the slow rate of progress, and what has been learned from the first clinical trials of candidate malaria vaccines? What are the remaining challenges, and what strategies can be pursued to address them?