Plasmodium falciparum-infected erythrocytes modulate the maturation of dendritic cells

Danger signals: SOS to the immune system

The activation of dendritic cells, necessary for the initiation of primary and secondary immune responses, can be induced by endogenous danger signals - released by tissues undergoing stress, damage or abnormal death - and also by exogenous danger signals elaborated by pathogens. Some endogenous danger signals that recently have been discovered are heat-shock proteins, nucleotides, reactive oxygen intermediates, extracellular-matrix breakdown products, neuromediators and cytokines like the IFNs. We propose that allergy may be initiated by the direct damage of dendritic or other cells by toxic chemicals and allergenic proteases, and suggest that the triggering of danger signal receptors by exogenous pathogen-derived molecules may be more to the advantage of the pathogen than to the host.

Modulation of host responses to blood-stage malaria by interleukin-12: from therapy to adjuvant activity

This review focuses on the role of interleukin (IL)-12, a proinflammatory cytokine with pleiotropic effects as a potent immunoregulatory molecule and hematopoietic growth factor, in infection with Plasmodium parasites, the causative agents of malaria. IL-12 has been demonstrated to have profound effects on the immune response to blood-stage malaria, to induce protection, and to alleviate malarial anemia. In combination with an anti-malarial drug, IL-12 is effective in an established malaria infection. This cytokine also has potent immune effects as a malaria vaccine adjuvant. However, IL-12 can also mediate pathology during blood-stage malaria.

The malaria-infected red blood cell: structural and functional changes

The asexual stage of malaria parasites of the genus Plasmodium invade red blood cells of various species including humans. After parasite invasion, red blood cells progressively acquire a new set of properties and are converted into more typical, although still simpler, eukaryotic cells by the appearance of new structures in the red blood cell cytoplasm, and new proteins at the red blood cell membrane skeleton. The red blood cell undergoes striking morphological alterations and its rheological properties are considerably altered, manifesting as red blood cells with increased membrane rigidity, reduced deformability and increased adhesiveness for a number of other cells including the vascular endothelium. Elucidation of the structural changes in the red blood cell induced by parasite invasion and maturation and an understanding of the accompanying functional alterations have the ability to considerably extend our knowledge of structure-function relationships in the normal red blood cell. Furthermore, interference with these interactions may lead to previously unsuspected means of reducing parasite virulence and may lead to the development of novel antimalarial therapeutics.

Salmonella enterica serovar typhimurium-induced maturation of bone marrow-derived dendritic cells

Murine bone marrow-derived dendritic cells (DC) can phagocytose and process Salmonella enterica serovar Typhimurium for peptide presentation on major histocompatibility complex class I (MHC-I) and MHC-II molecules. To investigate if a serovar Typhimurium encounter with DC induces maturation and downregulates their ability to present antigens from subsequently encountered bacteria, DC were pulsed with serovar Typhimurium 24 h prior to coincubating with Escherichia coli expressing the model antigen Crl-OVA. Quantitating presentation of OVA epitopes contained within Crl-OVA showed that Salmonella-pulsed DC had a reduced capacity to process Crl-OVA-expressing E. coli for OVA(257-264)/K(b) and OVA(265-277)/I-A(b) presentation. In addition, time course studies of DC pulsed with Crl-OVA-expressing serovar Typhimurium showed that OVA(257-264)/K(b) complexes could stimulate CD8OVA T-hybridoma cells for <24 h following a bacterial pulse, while OVA(265-277)/I-A(b) complexes could stimulate OT4H T-hybridoma cells for >24 but <48 h. The phoP-phoQ virulence locus of serovar Typhimurium also influenced the ability of DC to process Crl-OVA-expressing serovar Typhimurium for OVA(265-277)/I-A(b) presentation but not for OVA(257-264)/K(b) presentation. Furthermore, pulsing of DC with serovar Typhimurium followed by incubation for 24 or 48 h altered surface expression of MHC-I, MHC-II, CD40, CD54, CD80, and CD86, generating a DC population with a uniform, high expression level of these molecules. Finally, neither the serovar Typhimurium phoP-phoQ locus nor lipopolysaccharides (LPS) containing lipid A modifications purified from phoP mutant strains had a different effect on DC maturation from that of wild-type serovar Typhimurium or purified wild-type LPS. Thus, these data show that Salmonella or Salmonella LPS induces maturation of DC and that this process is not altered by the Salmonella phoP virulence locus. However, phoP did influence OVA(265-277)/I-A(b) presentation by DC infected with Crl-OVA-expressing serovar Typhimurium when quantitated after 2 h of bacterial infection.

Antigen-presenting cells. Experimental and clinical studies of dendritic cells

We present a summary of our experimental and clinical studies of dendritic cells (DCs) in five different areas. First, human DCs undergo chemotaxis and transendothelial migration in response to distinct chemokines, depending on their maturation state; inducible chemokines may recruit immature DCs to sites of inflammation and infection, whereas constitutive chemokines may regulate subsequent migration to lymphoid tissues. Second, M-tropic but not T-tropic strains of HIV-1 induce and modulate DC chemotaxis in culture; conceivably this may facilitate viral infection and contribute to pathogenesis in vivo. Third, malaria-infected erythrocytes modulate the maturation and inhibit the function of DCs; adhesion of infected erythrocytes to DCs may impart a selective advantage to the parasite by modulating host immunity. Fourth, a phase I clinical trial of DC-based immunotherapy of cancer has been initiated in which autologous DCs are administered directly into metastatic skin lesions; immune responses to a recall antigen, with which the cells were pulsed, were enhanced in several patients. Finally we describe the growth and characterization of a stably immature, maturation-resistant stage of DCs generated from mouse bone marrow that can induce transplantation tolerance in vivo, and outline studies of their capacity to induce responses of transgenic T cells in vivo.

Malaria research: host-parasite interactions and new developments in chemotherapy, immunology and vaccinology

Malaria remains the major parasitic disease, with 300-500 million new infections each year. This survey covers recent advances in the field of parasite-host interactions, focusing on Plasmodium falciparum, the most virulent of the human parasites. Rapid progress in genomic research is creating a basis for the development of new drugs and vaccines. Identification of drug-resistance mutations facilitates evaluation of improved drug policies, and attempts are being made to develop new compounds that inhibit metabolic pathways that are specific to the parasite. Cytoadherence of parasitized erythrocytes to microvascular endothelium is responsible for the sequestration of parasites, causing pathology and severe disease. Newly identified molecular fine structures that mediate cytoadherence may provide new targets for specific therapies. Humoral and cell-mediated immunity induced by the parasite may be protective, but may also be harmful by generating imbalance in cytokine responses. Efforts are made to determine the pathways that give rise to protection, with vaccination being the principal goal for achieving malaria control. Different vaccine constructs are being evaluated in preclinical and clinical trials, including modified viral vectors, synthetic peptides, DNA and new adjuvants.

Classification of adhesive domains in the Plasmodium falciparum erythrocyte membrane protein 1 family

The Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) family of cytoadherent proteins has a central role in disease from malaria infection. This highly diverse gene family is involved in binding interactions between infected erythrocytes and host cells and is expressed in a clonally variant pattern at the erythrocyte surface. We describe by sequence analysis the structure and domain organization of 20 PfEMP1 from the GenBank database. Four domains comprise the majority of PfEMP1 extracellular sequence: the N-terminal segment (NTS) located at the amino terminus of all PfEMP1, the C2, the Cysteine-rich Interdomain Region (CIDR) and the Duffy Binding-like (DBL) domains. Previous work has shown that CIDR and DBL domains can possess adhesive properties. CIDR domains grouped as three distinct sequence classes (alpha, beta, and gamma) and DBL domains as five sequence classes (alpha, beta, gamma, delta, and epsilon). Consensus motifs are described for the different DBL and CIDR types. Whereas the number of DBL and CIDR domains vary between PfEMP1, PfEMP1 domain architecture is not random in that certain tandem domain associations--such as DBLalphaCIDRalpha, DBLdeltaCIDRbeta, and DBLbetaC2--are preferentially observed. This conservation may have functional significance for PfEMP1 folding, transport, or binding activity. Parasite binding phenotype appears to be a determinant of infected erythrocyte tissue tropism that contributes to parasite survival, transmission, and disease outcome. The sequence classification of DBL and CIDR types may have predictive value for identifying PfEMP1 domains with a particular binding property. This information might be used to develop interventions targeting parasite binding variants that cause disease.

Fully competent dendritic cells as inducers of T cell anergy in autoimmunity

Mature immunologically competent dendritic cells are the most efficient antigen-presenting cells that powerfully activate T cells and initiate and sustain immune responses. Indeed, dendritic cells are able to efficiently capture antigens, express high levels of costimulatory molecules, and produce the combination of cytokines required to create a powerful immune response. They are also considered to be important in initiating autoimmune disease by efficiently presenting autoantigens to self-reactive T cells that, in this case, will mount a pathogenic autoimmune reaction. Triggering T cells is not a simple on-off procedure, as T cell receptor responds to minor changes in ligand with gradations of T cell activation and effector functions. These "misfit" peptides have been called Altered Peptide Ligands, and have been shown to have important biological significance. Here, we show that fully capable dendritic cells may present, upon natural antigen processing, a self-epitope with Altered Peptide Ligands features that can unexpectedly induce anergy in a human autoreactive T cell clone. These results indicate that presentation of a self-epitope by immunologically competent dendritic cells does not always mean "danger" and show a mechanism involved in the fine balance between activation and tolerance induction in humans.

Human dendritic cells discriminate between viable and killed Toxoplasma gondii tachyzoites: dendritic cell activation after infection with viable parasites results in CD28 and CD40 ligand signaling that controls IL-12-dependent and -independent T cell production of IFN-gamma

We studied how the interaction between human dendritic cells (DC) and Toxoplasma gondii influences the generation of cell-mediated immunity against the parasite. We demonstrate that viable, but not killed, tachyzoites of T. gondii altered the phenotype of immature DC. DC infected with viable parasites up-regulated the expression of CD40, CD80, CD86, and HLA-DR and down-regulated expression of CD115. These changes are indicative of DC activation induced by T. gondii. Viable and killed tachyzoites had contrasting effects on cytokine production. DC infected with viable T. gondii rather than DC that phagocytosed killed parasites induced secretion of high amounts of IFN-gamma by T cells from T. gondii-seronegative donors. IFN-gamma production in response to DC infected with viable parasites required CD28 and CD40 ligand (CD40L) signaling. In addition, this IFN-gamma response was dependent in part on IL-12 secretion. Production of IL-12 p70 occurred after interaction between T cells and DC infected with viable T. gondii, but not after incubation of T cells with DC plus killed tachyzoites. IL-12 synthesis was inhibited by blockade of CD40L signaling. IL-12-independent IFN-gamma production required CD80/CD86-CD28 interaction and, to a lesser extent, CD40-CD40L signaling. Taken together, T. gondii-induced activation of human DC is associated with T cell production of IFN-gamma through CD40-CD40L-dependent release of IL-12 and through CD80/CD86-CD28 and CD40-CD40L signaling that mediate IFN-gamma secretion even in the absence of bioactive IL-12.

The immunology of malaria infection

As global malaria mortality increases the urgency for vaccine development, analysis of immune responses in naturally exposed populations is providing clues to the nature of protective immunity. Recently, sophisticated immune evasion strategies adopted by the parasite have been analysed at the molecular level. More immunogenic vaccination strategies have been identified, providing renewed optimism that effective malaria control through vaccination should be feasible.

Immunobiology of dendritic cells

Dendritic cells (DCs) are antigen-presenting cells with a unique ability to induce primary immune responses. DCs capture and transfer information from the outside world to the cells of the adaptive immune system. DCs are not only critical for the induction of primary immune responses, but may also be important for the induction of immunological tolerance, as well as for the regulation of the type of T cell-mediated immune response. Although our understanding of DC biology is still in its infancy, we are now beginning to use DC-based immunotherapy protocols to elicit immunity against cancer and infectious diseases.

Plasmodium chabaudi-infected erythrocytes adhere to CD36 and bind to microvascular endothelial cells in an organ-specific way

Adherence of erythrocytes infected with Plasmodium falciparum to microvascular endothelial cells (sequestration) is considered to play an important role in parasite virulence and pathogenesis. However, the real importance of sequestration for infection and disease has never been fully assessed. The absence of an appropriate in vivo model for sequestration has been a major barrier. We have examined the rodent malaria parasite Plasmodium chabaudi chabaudi AS in mice as a potential model. Erythrocytes infected with this parasite adhere in vitro to purified CD36, a critical endothelium receptor for binding P. falciparum-infected erythrocytes. P. c. chabaudi-infected erythrocytes adhere in vitro to endothelial cells in a gamma interferon-dependent manner, suggesting the involvement of additional adhesion molecules in the binding process, as is also the case with P. falciparum-infected cells. Furthermore, plasma or sera from infected and hyperimmune mice, respectively, have the ability to block binding of infected erythrocytes to endothelial cells. In vivo, erythrocytes containing mature P. c. chabaudi parasites are sequestered from the peripheral circulation. Sequestration is organ specific, occurring primarily in the liver, although intimate contact between infected erythrocytes and endothelial cells is also observed in the spleen and brain. The results are discussed in the context of the use of this model to study (i) the relationship between endothelial cell activation and the level of sequestration and (ii) the primary function of sequestration in malaria infection.

A filarial nematode-secreted product signals dendritic cells to acquire a phenotype that drives development of Th2 cells

Although exogeneous "danger" signals such as LPS can activate APC to produce a Th1 response, the nature of events initiating a Th2 response is controversial. We now show that pathogen-derived products have the capacity to induce bone marrow-derived dendritic cell cultures to acquire a phenotype that promotes the differentiation of naive CD4+ T cells toward either a Th1 or Th2 phenotype. Thus, LPS-matured dendritic cells (DC1) promote a Th1 response (increased generation of IFN-gamma and reduced production of IL-4) by Ag-stimulated CD4+ T cells from the DO.11.10 transgenic mouse expressing a TCR specific for an OVA peptide (OVA323-339). In contrast, a phosphorylcholine-containing glycoprotein, ES-62, secreted by the filarial nematode, Acanthocheilonema viteae, which generates a Th2 Ab response in vivo, is found to induce the maturation of dendritic cells (DC2) with the capacity to induce Th2 responses (increased IL-4 and decreased IFN-gamma). In addition, we show that the switch to either Th1 or Th2 responses is not effected by differential regulation through CD80 or CD86 and that a Th2 response is achieved in the presence of IL-12.

'Anergic' T cells modulate the T-cell activating capacity of antigen-presenting cells

Nowadays there is compelling evidence for immunoregulation by T cells. Recently, we showed that so-called 'anergic' T cells are not functionally inert but can act as regulatory cells by actively suppressing other T cell responses. We now show that 'anergic' T cells mediate this suppressive effect via modulation of the T-cell activating capacity of the antigen-presenting cell (APC). Upon removal of the 'anergic' T cells, the suppressive APC phenotype persisted, indicating that 'anergic' T cells conditioned the APC to become a mediator of T cell suppression. The inhibitory signal delivered by 'anergic' T cells depended on the presence of the cognate ligand for the 'anergic' T cell, and appeared to be dominant since previously activated APC were rendered inhibitory as well. These findings imply that APC upon cross-talk with T cells can adopt distinct functional phenotypes ranging from T-cell stimulatory to T-cell suppressive. The contribution of 'anergic' T cells to the functional tuning of APC offers an explanation for the maintenance of 'anergic' T cells in the repertoire, and for their role in immunoregulation.

Haptoglobin 1-1 is associated with susceptibility to severe Plasmodium falciparum malaria

The haptoglobin (Hp) phenotypes were determined by polyacrylamide-gel electrophoresis in plasma samples obtained in 1997 from 113 Plasmodium falciparum malaria patients (aged 1-12 years) with strictly defined cerebral malaria, severe malarial anaemia, or uncomplicated malaria and 42 age-matched healthy controls from the same area (coastal Ghana). Hp1-1 was significantly more prevalent among the patients (43%) than among healthy controls (7.1%), whereas Hp2-1 and Hp2-2 were underrepresented among the patients (11% and 2%, respectively) compared to the control donors (33% and 14%, respectively). No significant difference in frequency of Hp0 was observed between patients and controls. Among the malaria patients, the Hp1-1 phenotype was significantly more prevalent among patients with the complications of cerebral malaria and severe anaemia compared to patients with uncomplicated disease, whereas the reverse was seen with respect to Hp2-1 and Hp2-2. Our data suggest that the Hp1-1 phenotype is associated with susceptibility to P. falciparum malaria in general, and to the development of severe disease in particular.

Th1/Th2 balance in infection

Cytokines produced by T helper (Th) cells are of critical importance for the outcome of many infectious diseases. Producing the "right" set of cytokines in response to an infectious agent can be a matter of life or death. The Th1/Th2 dichotomy, although an oversimplification has proven useful in the analysis of immune responses to infections. In some infectious diseases, most notably leishmaniasis or infections with gastrointestinal helminths, one Th subset is indispensable for clearing the infection, whereas the opposite Th subset is detrimental. More frequently, both Th1 and Th2 responses are required at different time points to effectively eradicate an infectious agent. The granuloma responses to either Mycobacterium tuberculosis or Schistosoma mansoni provide illustrative examples and are discussed in this review. There is accumulating evidence for frequent coexpression of Th1 and Th2 cytokines during the in vivo immune response to infections. The mechanisms by which infectious agents modulate Th1/Th2 phenotype development are summarized here. Finally, we review here the current evidence for cytokine imbalances induced by infections as pathogenic or protective factors in autoimmunity and allergy.

CD47 engagement inhibits cytokine production and maturation of human dendritic cells

Upon encounter with bacterial products, immature dendritic cells (iDCs) release proinflammatory cytokines and develop into highly stimulatory mature DCs. In the present study, we show that human monocyte-derived DCs functionally express the CD47 Ag, a thrombospondin receptor. Intact or F(ab')2 of CD47 mAb suppress bacteria-induced production of IL-12, TNF-alpha, GM-CSF, and IL-6 by iDCs. 4N1K, a peptide derived from the CD47-binding site of thrombospondin, also inhibits cytokine release. The inhibition of IL-12 and TNF-alpha is IL-10-independent inasmuch as IL-10 production is down-modulated by CD47 mAb and blocking IL-10 mAb fails to restore cytokine levels. CD47 ligation counteracts the phenotypic and functional maturation of iDCs in that it prevents the up-regulation of costimulatory molecules, the loss of endocytic activity, and the acquisition of an increased capacity to stimulate T cell proliferation and IFN-gamma production. Interestingly, regardless of CD47 mAb treatment during DC maturation, mature DC restimulated by soluble CD40 ligand and IFN-gamma, to mimic DC/T interaction, produce less IL-12 and more IL-18 than iDCs. Finally, CD47 ligation on iDCs does not impair their capacity to phagocytose apoptotic cells. We conclude that following exposure to microorganisms, CD47 ligation may limit the intensity and duration of the inflammatory response by preventing inflammatory cytokine production by iDCs and favoring their maintenance in an immature state.

The ves multigene family of B. bovis encodes components of rapid antigenic variation at the infected erythrocyte surface

B. bovis, an intraerythrocytic protozoal parasite, establishes chronic infections in cattle in part through rapid variation of the polymorphic, heterodimeric VESA1 protein on the infected erythrocyte surface and sequestration of mature parasites. We describe the characterization of the ves1 alpha gene encoding the VESA1a subunit, thus providing a description of a gene whose product is involved in rapid antigenic variation in a babesial parasite. This three-exon gene, a member of a multigene family (ves), encodes a polypeptide with no cleavable signal sequence, a single predicted transmembrane segment, and a cysteine/lysine-rich domain. Variation appears to involve creation and modification or loss of a novel, transcribed copy of the gene.

Vaccinia Virus Inhibits the Maturation of Human Dendritic Cells: A Novel Mechanism of Immune Evasion

Vaccinia virus employs multiple mechanisms to evade the immune system, yet is highly immunogenic. We studied the interaction between vaccinia and human dendritic cells (DCs), potent APCs. DCs develop from precursor cells in two stages: an immature stage in which Ag uptake and processing occur, and a mature stage in which there is up-regulation of costimulatory and HLA molecules and efficient T cell activation. Vaccinia virus undergoes an abortive replication in both stages of DCs and induces apoptotic cell death. Furthermore, maturation of immature DCs and consequently T cell activation are inhibited. Obstruction of DC maturation may constitute a novel mechanism by which vaccinia attempts to evade the immune response.

Genes involved in phenotypic and antigenic variation in African trypanosomes and malaria

Large polymorphic gene families that are involved in clonal phenotypic variation have been identified in both African trypanosomes and malaria parasites. Many of these gene families are necessary for host adaptation, allowing the parasite to infect different species of host or types of host cells. In many cases, switching between these functionally variable proteins also results in antigenic variation.

Antigenic variation in Plasmodium falciparum: mechanisms and consequences

In the past year, the major advances in malaria antigenic variation have been concerned with the transcription and switching of variant antigen genes, and the functional expression of regions of the major variant antigen. Also, new variant gene families have been discovered as a result of the Malaria Genome Project.