ELQ-331 as a prototype for extremely durable chemoprotection against malaria

Background

The potential benefits of long-acting injectable chemoprotection (LAI-C) against malaria have been recently recognized, prompting a call for suitable candidate drugs to help meet this need. On the basis of its known pharmacodynamic and pharmacokinetic profiles after oral dosing, ELQ-331, a prodrug of the parasite mitochondrial electron transport inhibitor ELQ-300, was selected for study of pharmacokinetics and efficacy as LAI-C in mice.

Methods

Four trials were conducted in which mice were injected with a single intramuscular dose of ELQ-331 or other ELQ-300 prodrugs in sesame oil with 1.2% benzyl alcohol; the ELQ-300 content of the doses ranged from 2.5 to 30 mg/kg. Initial blood stage challenges with Plasmodium yoelii were used to establish the model, but the definitive study measure of efficacy was outcome after sporozoite challenge with a luciferase-expressing P. yoelii, assessed by whole-body live animal imaging. Snapshot determinations of plasma ELQ-300 concentration ([ELQ-300]) were made after all prodrug injections; after the highest dose of ELQ-331 (equivalent to 30 mg/kg ELQ-300), both [ELQ-331] and [ELQ-300] were measured at a series of timepoints from 6 h to 5½ months after injection.

Results

A single intramuscular injection of ELQ-331 outperformed four other ELQ-300 prodrugs and, at a dose equivalent to 30 mg/kg ELQ-300, protected mice against challenge with P. yoelii sporozoites for at least 4½ months. Pharmacokinetic evaluation revealed rapid and essentially complete conversion of ELQ-331 to ELQ-300, a rapidly achieved (< 6 h) and sustained (4–5 months) effective plasma ELQ-300 concentration, maximum ELQ-300 concentrations far below the estimated threshold for toxicity, and a distinctive ELQ-300 concentration versus time profile. Pharmacokinetic modeling indicates a high-capacity, slow-exchange tissue compartment which serves to accumulate and then slowly redistribute ELQ-300 into blood, and this property facilitates an extremely long period during which ELQ-300 concentration is sustained above a minimum fully-protective threshold (60–80 nM).

Conclusions

Extrapolation of these results to humans predicts that ELQ-331 should be capable of meeting and far-exceeding currently published duration-of-effect goals for anti-malarial LAI-C. Furthermore, the distinctive pharmacokinetic profile of ELQ-300 after treatment with ELQ-331 may facilitate durable protection and enable protection for far longer than 3 months. These findings suggest that ELQ-331 warrants consideration as a leading prototype for LAI-C.

Radical cure of experimental babesiosis in immunodeficient mice using a combination of an endochin-like quinolone and atovaquone

Human babesiosis is a tick-borne multisystem disease caused by Babesia species of the apicomplexan phylum. Most clinical cases and fatalities of babesiosis are caused by Babesia microti Current treatment for human babesiosis consists of two drug combinations, atovaquone + azithromycin or quinine + clindamycin. These treatments are associated with adverse side effects and a significant rate of drug failure. Here, we provide evidence for radical cure of experimental babesiosis in immunodeficient mice using a combination of an endochin-like quinolone (ELQ) prodrug and atovaquone. In vivo efficacy studies in mice using ELQ-271, ELQ-316, and the ELQ-316 prodrug, ELQ-334, demonstrated excellent growth inhibitory activity against the parasite, with potency equal to that of orally administered atovaquone at 10 mg/kg. Analysis of recrudescent parasites after ELQ or atovaquone monotherapy identified genetic substitutions in the Qi or Qo sites, respectively, of the cytochrome bc1 complex. Impressively, a combination of ELQ-334 and atovaquone, at doses as low as 5.0 mg/kg each, resulted in complete clearance of the parasite with no recrudescence up to 122 d after discontinuation of therapy. These results will set the stage for future clinical evaluation of ELQ and atovaquone combination therapy for treatment of human babesiosis.

Inhibition of cytochrome bc1 as a strategy for single-dose, multi-stage antimalarial therapy

Single-dose therapies for malaria have been proposed as a way to reduce the cost and increase the effectiveness of antimalarial treatment. However, no compound to date has shown single-dose activity against both the blood-stage Plasmodium parasites that cause disease and the liver-stage parasites that initiate malaria infection. Here, we describe a subset of cytochrome bc1 (cyt bc1) inhibitors, including the novel 4(1H)-quinolone ELQ-400, with single-dose activity against liver, blood, and transmission-stage parasites in mouse models of malaria. Although cyt bc1 inhibitors are generally classified as slow-onset antimalarials, we found that a single dose of ELQ-400 rapidly induced stasis in blood-stage parasites, which was associated with a rapid reduction in parasitemia in vivo. ELQ-400 also exhibited a low propensity for drug resistance and was active against atovaquone-resistant P. falciparum strains with point mutations in cyt bc1. Ultimately, ELQ-400 shows that cyt bc1 inhibitors can function as single-dose, blood-stage antimalarials and is the first compound to provide combined treatment, prophylaxis, and transmission blocking activity for malaria after a single oral administration. This remarkable multi-stage efficacy suggests that metabolic therapies, including cyt bc1 inhibitors, may be valuable additions to the collection of single-dose antimalarials in current development.

Mefloquine and psychotomimetics share neurotransmitter receptor and transporter interactions in vitro

RATIONALE

Mefloquine is used for the prevention and treatment of chloroquine-resistant malaria, but its use is associated with nightmares, hallucinations, and exacerbation of symptoms of post-traumatic stress disorder. We hypothesized that potential mechanisms of action for the adverse psychotropic effects of mefloquine resemble those of other known psychotomimetics.

OBJECTIVES

Using in vitro radioligand binding and functional assays, we examined the interaction of (+)- and (-)-mefloquine enantiomers, the non-psychotomimetic anti-malarial agent, chloroquine, and several hallucinogens and psychostimulants with recombinant human neurotransmitter receptors and transporters.

RESULTS

Hallucinogens and mefloquine bound stereoselectively and with relatively high affinity (K i = 0.71-341 nM) to serotonin (5-HT) 2A but not 5-HT1A or 5-HT2C receptors. Mefloquine but not chloroquine was a partial 5-HT2A agonist and a full 5-HT2C agonist, stimulating inositol phosphate accumulation, with similar potency and efficacy as the hallucinogen dimethyltryptamine (DMT). 5-HT receptor antagonists blocked mefloquine's effects. Mefloquine had low or no affinity for dopamine D1, D2, D3, and D4.4 receptors, or dopamine and norepinephrine transporters. However, mefloquine was a very low potency antagonist at the D3 receptor and mefloquine but not chloroquine or hallucinogens blocked [(3)H]5-HT uptake by the 5-HT transporter.

CONCLUSIONS

Mefloquine, but not chloroquine, shares an in vitro receptor interaction profile with some hallucinogens and this neurochemistry may be relevant to the adverse neuropsychiatric effects associated with mefloquine use by a small percentage of patients. Additionally, evaluating interactions with this panel of receptors and transporters may be useful for characterizing effects of other psychotropic drugs and for avoiding psychotomimetic effects for new pharmacotherapies, including antimalarial quinolines.

Eluding anaphylaxis allows peptide-specific prevention of the relapsing stage of experimental autoimmune encephalomyelitis

We have used a peptide derived from Acanthamoeba castellanii (ACA) to treat the relapsing phase of EAE that develops in SJL mice following immunization with the PLP 139-151 peptide. The native sequence of the ACA 81-95 peptide that shares key residues with the PLP 139-151 peptide is weakly encephalitogenic in SJL mice but is not recognized by antiserum from SJL mice immunized with PLP 139-151. A single amino acid change to the ACA 81-95 peptide sequence significantly enhanced its encephalitogenicity. When administered to SJL mice as a nonlinear peptide octamer, the modified ACA peptide prevented relapsing episodes of EAE in SJL mice previously immunized with the PLP 139-151 encephalitogenic peptide.

Discovery, synthesis, and optimization of antimalarial 4(1H)-quinolone-3-diarylethers

The historical antimalarial compound endochin served as a structural lead for optimization. Endochin-like quinolones (ELQ) were prepared by a novel chemical route and assessed for in vitro activity against multidrug resistant strains of Plasmodium falciparum and against malaria infections in mice. Here we describe the pathway to discovery of a potent class of orally active antimalarial 4(1H)-quinolone-3-diarylethers. The initial prototype, ELQ-233, exhibited low nanomolar IC₅₀ values against all tested strains including clinical isolates harboring resistance to atovaquone. ELQ-271 represented the next critical step in the iterative optimization process, as it was stable to metabolism and highly effective in vivo. Continued analoging revealed that the substitution pattern on the benzenoid ring of the quinolone core significantly influenced reactivity with the host enzyme. This finding led to the rational design of highly selective ELQs with outstanding oral efficacy against murine malaria that is superior to established antimalarials chloroquine and atovaquone.

Targeting T cells responsive to the priming epitope prevent the relapsing phase of experimental autoimmune encephalomyelitis

Upon recovery from the initial episode of experimental autoimmune encephalomyelitis (EAE), virtually all SJL mice develop relapsing/remitting episodes of disease. These relapses may occur due to the reactivation of memory T cells initially stimulated as part of the disease-inducing protocol or naïve T-cell populations stimulated by distinct encephalitogens derived from the inflammatory disease process (epitope spread). We have used encephalitogen-specific non-linear peptide octamers to modify the course of relapsing EAE (rEAE) in SJL mice immunized with an oliogodendrocyte-specific protein peptide (OSP 55-71). Our studies show that the peptide-octamers, which target the T cells stimulated by the priming encephalitogen, but not other candidate encephalitogens, prevent rEAE.

Sontochin as a guide to the development of drugs against chloroquine-resistant malaria

Sontochin was the original chloroquine replacement drug, arising from research by Hans Andersag 2 years after chloroquine (known as "resochin" at the time) had been shelved due to the mistaken perception that it was too toxic for human use. We were surprised to find that sontochin, i.e., 3-methyl-chloroquine, retains significant activity against chloroquine-resistant strains of Plasmodium falciparum in vitro. We prepared derivatives of sontochin, "pharmachins," with alkyl or aryl substituents at the 3 position and with alterations to the 4-position side chain to enhance activity against drug-resistant strains. Modified with an aryl substituent in the 3 position of the 7-chloro-quinoline ring, Pharmachin 203 (PH-203) exhibits low-nanomolar 50% inhibitory concentrations (IC(50)s) against drug-sensitive and multidrug-resistant strains and in vivo efficacy against patent infections of Plasmodium yoelii in mice that is superior to chloroquine. Our findings suggest that novel 3-position aryl pharmachin derivatives have the potential for use in treating drug resistant malaria.

Costimulation of Th17 cells: Adding fuel or putting out the fire in the inflamed gut?

Inflammatory bowel disease, typified by Crohn's disease and ulcerative colitis, is a common disorder characterized by recurrent and serious inflammation of the gastrointestinal tract. It is well documented that T cells play a pivotal role in the development of inflammatory bowel disease. Th17 cells are a unique T cell subpopulation implicated in inflammatory bowel disease and many other autoimmune/inflammatory diseases. However, the regulatory mechanism of Th17 activation and proliferation has not been defined completely. Recent studies have shown that the ligation of several costimulatory receptor-ligand pairs contributes to the activation, differentiation, and proliferation of T lymphocytes including the Th17 subset. In this review, we will discuss the emerging evidence on the role of Th17 cells in inflammatory bowel disease pathogenesis as well as the effect of costimulatory molecules on Th17 development and consider if the need for such costimulation of T lymphocytes provides a target for the development of novel therapeutic strategy.

Inflammatory skin disease in K5.hTGF-beta1 transgenic mice is not dependent on the IL-23/Th17 inflammatory pathway

In the presence of IL-6, transforming growth factor (TGF)-beta1 induces differentiation of T helper (Th) 17 cells in mice. Interleukin (IL)-23, a heterodimeric cytokine composed of IL-23p19 and IL-12/23p40 subunits, stimulates the growth and expansion of Th17 cells, and has been implicated in psoriasis pathogenesis. To study the associations between TGF-beta1, the IL-23/Th17 inflammatory pathway, and psoriasis, we investigated inflammatory skin disease in transgenic mice that constitutively overexpress human TGF-beta1 in basal keratinocytes (K5.hTGF-beta1 transgenic mice); these mice had previously been reported as having a psoriasis-like disease. K5.hTGF-beta1 transgenic mice had high levels of TGF-beta1 mRNA and protein in both skin and serum. Levels of cytokines involved in IL-23/Th17-mediated inflammation were not elevated in lesional skin compared with those in non-lesional and wild-type skin. It is noteworthy that IL-4 and IgE were markedly elevated in inflamed skin and serum, respectively, of transgenic mice. Monoclonal antibodies (mAbs) specifically directed against IL-23p19 or IL-12/23p40 had no clinical effect on established inflammatory skin disease in K5.hTGF-beta1 transgenic mice, whereas the same mAbs were able to block the development of murine experimental autoimmune encephalomyelitis, an IL-23/Th17-mediated disease. In summary, the IL-23/Th17 inflammatory pathway is not responsible for the maintenance of inflammatory skin disease in K5.hTGF-beta1 transgenic mice.

A drug-selected Plasmodium falciparum lacking the need for conventional electron transport

Mitochondrial electron transport is essential for survival in Plasmodium falciparum, making the cytochrome (cyt) bc(1) complex an attractive target for antimalarial drug development. Here we report that P. falciparum cultivated in the presence of a novel cyt bc(1) inhibitor underwent a fundamental transformation in biochemistry to a phenotype lacking a requirement for electron transport through the cyt bc(1) complex. Growth of the drug-selected parasite clone (SB1-A6) is robust in the presence of diverse cyt bc(1) inhibitors, although electron transport is fully inhibited by these same agents. This transformation defies expected molecular-based concepts of drug resistance, has important implications for the study of cyt bc(1) as an antimalarial drug target, and may offer a glimpse into the evolutionary future of Plasmodium.

Depressive symptoms in patients with chronic hepatitis C are correlated with elevated plasma levels of interleukin-1beta and tumor necrosis factor-alpha

Studies suggest that cytokines have a role in the biology of depression. In this study, we evaluated depression and cytokine levels in patients with and without chronic hepatitis C (HCV) to better assess how chronic infection alters cytokines levels and may contribute to depressive symptomotology. Twenty-three adults with (n=16) and without (n=7) HCV were recruited through the Portland VA Medical Center. Research participants were excluded for current substance abuse, psychotic disorder, liver cirrhosis, or interferon (IFN) therapy. Participants completed the Beck Depression Inventory-II (BDI-II) and a blood draw to evaluate plasma cytokine levels [i.e., interleukin (IL)-1beta, IL-10 and tumor necrosis factor (TNF)-alpha]. t-Tests were performed to compare cytokine levels in patients with or without HCV. HCV patients showed higher TNF-alpha values compared to patients without HCV (group means=7.94 vs. 3.41pg/mL, respectively, p=0.047). There were no significant differences between the groups for the other cytokines assessed. In patients with HCV, TNF-alpha and IL-1beta levels (but not IL-10) were correlated with BDI-II scores [r=0.594, p=0.020 and r=0.489, p=0.055 (trend), respectively]. Taken together, these results show an association between severity of depressive symptoms and expression of pro-inflammatory cytokines in patients with HCV. Future studies should investigate how inflammatory mediators play a role in the expression of specific depressive symptoms in patients with chronic infection. Patients with HCV represent an interesting model to examine this relationship.

Design, synthesis, and evaluation of 10-N-substituted acridones as novel chemosensitizers in Plasmodium falciparum

A series of novel 10-N-substituted acridones, bearing alkyl side chains with tertiary amine groups at the terminal position, were designed, synthesized, and evaluated for the ability to enhance the potency of quinoline drugs against multidrug-resistant (MDR) Plasmodium falciparum malaria parasites. A number of acridone derivatives, with side chains bridged three or more carbon atoms apart between the ring nitrogen and terminal nitrogen, demonstrated chloroquine (CQ)-chemosensitizing activity against the MDR strain of P. falciparum (Dd2). Isobologram analysis revealed that selected candidates demonstrated significant synergy with CQ in the CQ-resistant (CQR) parasite Dd2 but only additive (or indifferent) interaction in the CQ-sensitive (CQS) D6. These acridone derivatives also enhanced the sensitivity of other quinoline antimalarials, such as desethylchloroquine (DCQ) and quinine (QN), in Dd2. The patterns of chemosensitizing effects of selected acridones on CQ and QN were similar to those of verapamil against various parasite lines with mutations encoding amino acid 76 of the P. falciparum CQ resistance transporter (PfCRT). Unlike other known chemosensitizers with recognized psychotropic effects (e.g., desipramine, imipramine, and chlorpheniramine), these novel acridone derivatives exhibited no demonstrable effect on the uptake or binding of important biogenic amine neurotransmitters. The combined results indicate that 10-N-substituted acridones present novel pharmacophores for the development of chemosensitizers against P. falciparum.

After interleukin-12p40, are interleukin-23 and interleukin-17 the next therapeutic targets for inflammatory bowel disease?

Inflammatory bowel disease (IBD), typified by Crohn's disease and ulcerative colitis, is a common disorder characterized by recurrent and serious inflammation of the gastrointestinal tract. Recent immunologic advances have established that T cells and inflammatory cytokines play a pivotal role in the gastrointestinal inflammation of IBD. However, many cytokines not only elicit inflammation but also protect host against microbial invasion. Hence, suppression of these dual-purpose cytokines often exposes the patients to significant risk of infection. Recent research on Interleukin (IL)-23, IL-17, and IL-17 producing T cells has become the vanguard of further understanding the contribution of cytokines to autoimmune and inflammatory diseases. IL-23 is a newly discovered member of the IL-12-related cytokine family, and is primarily involved in the differentiation of pathogenic T cells characterized by their production of IL-17. IL-17 is a potent inflammatory mediator implicated in a number of autoimmune diseases. The discovery of this IL-23/IL-17-mediated inflammatory axis is having a profound impact on the elucidation of T cell-mediated pathogenesis as well as development of novel therapeutic targets. In this review, we discuss the current literature and present our recent studies on the role of IL-23 and IL-17 in the pathogenesis of IBD. Controlling the expression/production of IL-23 and IL-17 is an approach that would allow the development of a novel treatment strategy with more anti-inflammatory efficacy and potentially with less suppressive effects on host defenses.

Selective killing of the human malaria parasite Plasmodium falciparum by a benzylthiazolium dye

Malaria is an infectious disease caused by protozoan parasites of the genus Plasmodium. The most virulent form of the disease is caused by Plasmodium falciparum which infects hundreds of millions of people and is responsible for the deaths of 1-2 million individuals each year. An essential part of the parasitic process is the remodeling of the red blood cell membrane and its protein constituents to permit a higher flux of nutrients and waste products into or away from the intracellular parasite. Much of this increased permeability is due to a single type of broad specificity channel variously called the new permeation pathway (NPP), the nutrient channel, and the Plasmodial surface anion channel (PSAC). This channel is permeable to a range of low molecular weight solutes both charged and uncharged, with a strong preference for anions. Drugs such as furosemide that are known to block anion-selective channels inhibit PSAC. In this study, we have investigated a dye known as benzothiocarboxypurine, BCP, which had been studied as a possible diagnostic aid given its selective uptake by P. falciparum infected red cells. We found that the dye enters parasitized red cells via the furosemide-inhibitable PSAC, forms a brightly fluorescent complex with parasite nucleic acids, and is selectively toxic to infected cells. Our study describes an antimalarial agent that exploits the altered permeability of Plasmodium-infected red cells as a means to killing the parasite and highlights a chemical reagent that may prove useful in high throughput screening of compounds for inhibitors of the channel.

Cutting edge: identification of hepatitis C virus-specific CD8+ T cells restricted by donor HLA alleles following liver transplantation

By necessity, human liver transplantation is performed across HLA barriers. As a result, intracellular infection of the allograft presents a unique immunologic challenge for the recipient's immune system. In this study, we describe the presence of HLA-A2-restricted, hepatitis C virus (HCV)-specific CD8+ T cells in liver transplant recipients in whom the allograft is HLA-A2 positive and the recipient is HLA-A2 negative. These memory-effector T cells are recipient derived and recognize HCV peptide uniquely in the context of HLA-A2. Furthermore, these cells were absent before the transplant, suggesting that the allograft is capable of selectively expanding naive CD8+ T cells. The in vitro specificity to donor HLA allele-restricted CD8+ T cells suggests that these cells may function to control HCV spread in the allograft.

The Development of Functional CD8 T Cell Memory afterListeria monocytogenesInfection Is Not Dependent on CD40

The immunologic requirements for generating long-lived protective CD8 T cell memory remain unclear. Memory CD8 populations generated in the absence of CD4 Th cells reportedly have functional defects, and at least a subset of CD8 T cells transiently express CD40 after activation, suggesting that direct CD4-CD8 T cell interactions through CD40 may influence the magnitude and functional quality of memory CD8 populations. To ascertain the role of CD40 in such direct T cell interactions, we investigated CD8 T cell responses in CD40-/- mice after infection with Listeria monocytogenes, an intracellular bacterium that induces APC activation and thus priming of CD8 T cells independently of CD4 Th cell help through CD40. In this study we show that memory CD8 T cells generated in CD40-deficient mice show in vivo cytotoxicity and cytokine production equivalent to CD8 memory T cells from wild-type mice. Upon secondary Listeria infection, CD40-/- memory CD8 T cells expand to greater numbers than seen in wild-type mice. These results indicate that CD40 ligation on CD8 T cells, although reportedly a part of CD8 T cell memory development in an H-Y-directed response, is not needed for the development of functional memory CD8 T cell populations after Listeria infection.

Listeria monocytogenes as a vaccine vector: virulence attenuation or existing antivector immunity does not diminish therapeutic efficacy

The bacterium L. monocytogenes is a proposed vaccine carrier based upon the observation that this pathogen replicates within the intracytoplasmic environment facilitating delivery of Ag to the endogenous Ag processing and presentation pathway with subsequent stimulation of peptide specific MHC class I-restricted CD8(+) effector cells. In this report, we evaluate virulence-attenuated strains of Listeria monocytogenes as vaccine vectors and examine whether existing antivector (antilisterial) immunity limits or alters its efficacy as a therapeutic cancer vaccine. Following immunization with virulence-attenuated mutants, we found that the effectiveness of L. monocytogenes as a recombinant cancer vaccine remains intact. In addition, we found that antibiotic treatment initiated 24 or 36 h following therapeutic immunization with recombinant L. monocytogenes allows full development of the antitumor response. We also demonstrate that the vaccine vector potential of L. monocytogenes is not limited in animals with existing antilisterial immunity. For these latter studies, mice previously immunized with wild-type L. monocytogenes were infused with melanoma cells and then 5 days later challenged with recombinant tumor Ag expressing L. monocytogenes. Collectively, these results add additional support for the use of L. monocytogenes as a vaccine vector and underscore its potential to be used repeatedly for stimulation of recall responses concomitant with primary cell-mediated responses to newly delivered heterologous tumor-associated epitopes.

Protection of interferon-gamma knockout mice against Listeria monocytogenes challenge following intramuscular immunization with DNA vaccines encoding listeriolysin O

In this study we evaluated the efficacy of DNA vaccination of IFN-gamma knockout (GKO) mice against Listeria monocytogenes, as these immunodeficient mice are highly susceptible to infection with low numbers of this intracellular bacterial pathogen. Following intramuscular immunization of BALB/c GKO mice with plasmid DNA constructs encoding recombinant forms of the L. monocytogenes hemolysin, listeriolysin O (LLO), we detected the in vivo induction of a LLO(91-99) peptide-specific, protective immune CTL response equivalent to that observed following similar DNA vaccination of normal BALB/c mice. The observed protection represented greatly enhanced immunity for the GKO host, suggesting that DNA vaccination may provide a useful vaccine alternative for certain immunocompromised host populations.

Antileishmanial drug development: exploitation of parasite heme dependency

A rational approach in the search for new antiparasitic drugs is the exploitation of biochemical differences between the parasite and its mammalian host. One specific example in the case of Leishmania relates to the biosynthesis of heme, a critical prosthetic group for proteins involved in metabolism and electron transport. Like all Trypanosomatids, Leishmania parasites require heme or pre-formed porphyrins for survival because they lack several key enzymes in the heme biosynthetic pathway. Considering their specific nutritional requirements, we speculated that they would be particularly sensitive to the effects of heme-complexing xanthones. In this report, we document the antileishmanial activity of selected nitrogenated xanthones and correlate drug potency with heme affinity. In vitro tests demonstrated that 3,6-bis-omega-diethylaminoamyloxyxanthone, C5, was at least 100 times more active than pentamidine against intracellular amastigotes of Leishmania mexicana. Our findings provide practical guidance for optimizing the antileishmanial activity of the xanthone pharmacophore to better exploit parasite heme salvage processes.

The kinetics of uptake and accumulation of 3,6-bis-omega-diethylamino-amyloxyxanthone by the human malaria parasite Plasmodium falciparum

Malarial parasites rely on the digestion of hemoglobin during the intra-erythrocytic stage. The enzymatic degradation of hemoglobin yields amino acids for parasite survival, and free heme which is detoxified by conversion to an aggregate of dimeric heme known as hemozoin. Xanthones have been found to subvert this process by formation of soluble drug-heme complexes. We have optimized the simple hydroxyxanthone structure to include side chains with protonatable nitrogen atoms to enhance interaction with the propionate groups of heme and to target the drug to the parasite digestive vacuole. One member of this optimized class of compounds, 3,6-bis-omega-diethylaminoamyloxyxanthone (C5), was used as a prototype for mechanistic studies. By HPLC analysis we demonstrate that the drug accumulates in the digestive vacuole from 5 to approximately 33,000 microM within 1 h of exposure to parasitized red cells. Confocal fluorescence microscopy was used to visualize the accumulation process directly and to document the colocalization of the drug with the acidophilic dye, LysoTracker Red.

Optimization of xanthones for antimalarial activity: the 3,6-bis-omega-diethylaminoalkoxyxanthone series

Hydroxyxanthones have been identified as novel antimalarial agents. The compounds are believed to exert their activity by complexation to heme and inhibition of hemozoin formation. Modification of the xanthone structure was pursued to improve their antimalarial activity. Attachment of R-groups bearing protonatable nitrogen atoms was conducted to enhance heme affinity through ionic interactions with the propionate side chains of the metalloporphyrin and to facilitate drug accumulation in the parasite food vacuole. A series of 3,6-bis-omega-diethylaminoalkoxyxanthones with side chains ranging from 2 to 8 carbon atoms were prepared and evaluated. Measurement of heme affinity for each of the derivatives revealed a strong correlation (R(2) = 0.97) between affinity and antimalarial potency. The two most active compounds in the series contained 5- and 6-carbon side chains and exhibited low nanomolar 50% inhibitory concentration (IC(50)) values against strains of chloroquine-susceptible and multidrug-resistant Plasmodium falciparum in vitro. Both of these xanthones exhibit stronger heme affinity (8.26 x 10(5) and 9.02 x 10(5) M(-1), respectively) than either chloroquine or quinine under similar conditions and appear to complex heme in a unique manner.

Frequencies of HCV-specific effector CD4+ T cells by flow cytometry: correlation with clinical disease stages

Hepatitis C virus (HCV) is the leading cause of chronic hepatitis, affecting approximately 2% of the world's population. The immune mechanisms responsible for the highly variable natural history in a given individual are unknown. We used a multiparameter flow cytometric technique to functionally and phenotypically characterize HCV-specific effector T cells in the peripheral blood of 32 individuals with different stages of hepatitis C disease (resolved, mild chronic, advanced chronic) and normal controls. We found the highest frequencies of virus-specific effector cells with an activated memory phenotype (CD45RO+CD69+) in subjects who had resolved HCV infection, either spontaneously or with antiviral therapy. Effector cells from patients with resolved infection produced Th1 type cytokines following stimulation with nonstructural antigens (NS3 and NS4), whereas effector cells from chronically infected patients produced Th1 type cytokines predominantly following stimulation with the HCV core antigen. Stimulation with superantigen staphylococcal enterotoxin (SEB) induced the same levels of cytokine production in the different patient groups. Among the HCV-seropositive patients, viral load inversely correlated with the Th1 effector cell response to NS3. Interleukin (IL)-4 was produced only in response to the control antigens, but not in response to the HCV recombinant proteins. Taken together, these findings suggest that a vigorous HCV-specific CD4+ Th1 response, particularly against the nonstructural proteins of the virus, may be associated with viral clearance and protection from disease progression. Prospective studies using this new flow cytometric assay will be required to determine whether antiviral therapy modifies the frequency, specificity, and function of these virus-specific effector cells.

Lack of expansion of major histocompatibility complex class Ib-restricted effector cells following recovery from secondary infection with the intracellular pathogen Listeria monocytogenes

Sublethal infection of BALB/c mice with the intracellular bacterial pathogen Listeria monocytogenes leads to the development of antilisterial immunity with concurrent stimulation of major histocompatibility complex (MHC) class Ia- and Ib-restricted CD8+ effector T cells. Secondary L. monocytogenes infection is followed by an accelerated increase in the number of Listeria-specific CD8+ cells and rapid clearance of the bacterium from the murine host. Recovery from secondary infection is associated with increased levels of effector cell function, as measured by gamma interferon secretion following coculture of immune cells with L. monocytogenes infected APCs in vitro, as well as antilisterial cytotoxicity, as measured by effector cell recognition of L. monocytogenes-infected target cells. We assessed the frequency of L. monocytogenes-specific MHC class I-restricted cells following secondary infection by ELISPOT assays utilizing coculture of immune cells with L. monocytogenes-infected antigen-presenting cells that express MHC class Ia and/or Ib molecules. We found that the antilisterial Qa-1b (MHC class Ib)-restricted effector subset is not detected as an expanded population following secondary infection compared to the frequency of this effector population as measured following recovery from primary infection. This is in contrast to the frequency of antilisterial H2-Kd (MHC class Ia)-restricted effector cells, which following recovery from secondary infection are detected as an expanded population, and appears to undergo a substantial expansion event 3 to 4 days post-secondary infection. These results are consistent with the conclusion that although Listeria-specific MHC class Ib-restricted effector cells are present following recovery from secondary infection, this subset does not appear to undergo the expansion phase that is detected for the MHC class Ia-restricted effector cell response.

Association of multispecific CD4(+) response to hepatitis C and severity of recurrence after liver transplantation

BACKGROUND & AIMS

After liver transplantation for hepatitis C virus (HCV), reinfection of the allograft invariably occurs. Indirect evidence suggests that the cellular immune response may play a central role. The purpose of this analysis was to determine the correlation between HCV-specific peripheral CD4(+) T-cell responses and the severity of recurrence after liver transplantation.

METHODS

Fifty-eight HCV-seropositive patients, including 43 liver transplant recipients with at least 1 year of histological follow-up, were studied. Peripheral blood mononuclear cells (PBMCs) were isolated from fresh heparinized blood and stimulated with either recombinant HCV antigens (core, E2, NS3, NS4, and NS5) or control antigens.

RESULTS

Fourteen (40%) of 35 patients with mild or no evidence of histological recurrence within their allografts responded to at least 1 of the HCV antigens. Eleven responded to NS3, 5 to all the nonstructural antigens, and 3 to the HCV core polypeptide alone. In contrast, in the 8 patients with severe HCV recurrence, no proliferation in response to any of the HCV antigens was seen (P = 0. 03) despite responses to the control antigens.

CONCLUSIONS

Despite immunosuppression, HCV-specific, major histocompatibility complex class II- restricted CD4(+) T-cell responses are detectable in patients with minimal histological recurrence after liver transplantation. In contrast, PBMCs from patients with severe HCV recurrence, despite being able to proliferate in response to non-HCV antigens, fail to respond to the HCV antigens. These findings suggest that the inability to generate virus-specific T-cell responses plays a contributory role in the pathogenesis of HCV-related graft injury after liver transplantation. It is hoped that further characterization of the immunoregulatory mechanisms related to recurrent HCV will provide the rationale for novel therapeutic strategies and diminish the incidence of inevitable graft loss.

Genetic Immunization of Mice Against Listeria monocytogenes Using Plasmid DNA Encoding Listeriolysin O

The development of protective immunity against many intracellular bacterial pathogens commonly requires sublethal infection with viable forms of the bacteria. Such infection results in the in vivo activation of specific cell-mediated immune responses, and both CD4+ and CD8+ T lymphocytes may function in the induction of this protective immunity. In rodent models of experimental infection with Listeria monocytogenes, the expression of protective immunity can be mediated solely by the immune CD8+ T cell subset. One major target Ag of Listeria-immune CD8+ T cells is the secreted bacterial hemolysin, listeriolysin O (LLO). In an attempt to generate a subunit vaccine in this experimental disease model, eukaryotic plasmid DNA expression vectors containing genes encoding either the wild-type or modified forms of recombinant LLO were generated and used for genetic vaccination of naive mice. Results of these studies indicate that the intramuscular immunization of mice with specifically designed plasmid DNA constructs encoding recombinant forms of LLO stimulates peptide-specific CD8+ immune T cells that exhibit in vitro cytotoxic activity. More importantly, such immunization can provide protective immunity against a subsequent challenge with viable L. monocytogenes, demonstrating that this experimental approach may have direct application in prevention of acute disease caused by intracellular bacterial pathogens.

Genetic immunization of mice against Listeria monocytogenes using plasmid DNA encoding listeriolysin O

The development of protective immunity against many intracellular bacterial pathogens commonly requires sublethal infection with viable forms of the bacteria. Such infection results in the in vivo activation of specific cell-mediated immune responses, and both CD4+ and CD8+ T lymphocytes may function in the induction of this protective immunity. In rodent models of experimental infection with Listeria monocytogenes, the expression of protective immunity can be mediated solely by the immune CD8+ T cell subset. One major target Ag of Listeria-immune CD8+ T cells is the secreted bacterial hemolysin, listeriolysin O (LLO). In an attempt to generate a subunit vaccine in this experimental disease model, eukaryotic plasmid DNA expression vectors containing genes encoding either the wild-type or modified forms of recombinant LLO were generated and used for genetic vaccination of naive mice. Results of these studies indicate that the intramuscular immunization of mice with specifically designed plasmid DNA constructs encoding recombinant forms of LLO stimulates peptide-specific CD8+ immune T cells that exhibit in vitro cytotoxic activity. More importantly, such immunization can provide protective immunity against a subsequent challenge with viable L. monocytogenes, demonstrating that this experimental approach may have direct application in prevention of acute disease caused by intracellular bacterial pathogens.

Existing antilisterial immunity does not inhibit the development of a Listeria monocytogenes-specific primary cytotoxic T-lymphocyte response

Infection of BALB/c mice with Listeria monocytogenes stimulates an antilisterial immune response evident by the appearance of H2-Kd-restricted CD8(+) cytotoxic T lymphocytes (CTLs) specific for the nanomer peptides amino acids (aa) 91 to 99 of listeriolysin O (LLO 91-99) and aa 217 to 225 of the p60 molecule (p60 217-225). We have introduced point mutations at anchor residues within LLO 91-99 (92F) or p60 217-225 (218F), and BALB/c mice infected with L. monocytogenes strains containing these point mutations do not develop CTLs specific for LLO 91-99 or p60 217-225, respectively. We have used these strains to test whether primary CTL responses against L. monocytogenes-derived determinants can be stimulated within an environment of existing antilisterial immunity. We found that the development of a primary L. monocytogenes-specific CTL response is not altered by existing immunity to L. monocytogenes. For example, primary immunization with the p60 218F strain of L. monocytogenes followed by a secondary immunization with wild-type L. monocytogenes results in stimulation of p60 217-225-specific CTLs at primary response levels and LLO 91-99-specific effectors at levels consistent with a memory CTL response. Similarly, primary immunization with the 92F strain of L. monocytogenes followed by a secondary immunization with wild-type L. monocytogenes results in stimulation of LLO 91-99-specific CTLs at primary response levels and p60 217-225-specific effectors at levels consistent with a memory CTL response. These results provide additional support for the use of L. monocytogenes as a recombinant vaccine vector and show that antivector immunity does not inhibit the development of a primary CTL response when the epitope is delivered by L. monocytogenes as the vaccine strain.

Listeria monocytogenes-infected hepatocytes are targets of major histocompatibility complex class Ib-restricted antilisterial cytotoxic T lymphocytes

Subclinical infection of BALB/c mice with the intracellular bacterial pathogen Listeria monocytogenes results in the development of protective antilisterial immunity. L. monocytogenes can infect hepatocytes, and antilisterial cytotoxic T lymphocytes (CTL) lyse Listeria-infected hepatocytes in a major histocompatibility complex (MHC) class Ia-restricted manner. It remained to be determined whether L. monocytogenes-infected hepatocytes are susceptible to MHC class Ib-restricted cytolysis. In this study, we showed that hepatocytes express MHC class Ib molecule Qa-1(b) mRNA and protein. We further showed that Listeria-infected hepatocytes are susceptible to MHC class Ib-restricted cytolysis, since C57BL/6-derived Listeria-infected hepatocytes were lysed by BALB/c-derived antilisterial CTL. These results establish that Listeria-infected hepatocytes are susceptible to cytolysis by MHC class Ib restricted Listeria-specific CTL.

MHC class Ib-restricted cells contribute to antilisterial immunity: evidence for Qa-1b as a key restricting element for Listeria-specific CTLs

Subclinical infection of BALB/c mice with the intracellular pathogen Listeria monocytogenes results in the development of MHC class Ia- and Ib-restricted CTLs. L. monocytogenes-infected TAP-/- bone marrow macrophage targets are not lysed by MHC class Ia- or Ib-restricted CTLs, showing a requirement for transport of peptides into the endoplasmic reticulum for development of the MHC class Ib-peptide target. L. monocytogenes-infected B6.Tla(a)-derived bone marrow macrophages (Kb Qa-1a) are not lysed by BALB/c (Kd Qa-1b)-derived antilisterial CTLs, confirming an earlier finding that the Ib-restricting element is T region encoded. We have further determined that Qa-1b is a restricting element for antilisterial CTLs using L. monocytogenes-infected Qa-1b-transformed mouse L cells as well as human-derived HeLa cells as target populations. These L. monocytogenes-infected Qa-1b-transformed cell lines are lysed by BALB/c (Qa-1b)- or C57BL/6 (Qa-1b)-derived antilisterial CTLs, but are not lysed by B6.AKM (Qa-1a)-derived antilisterial CTLs. Using L. monocytogenes-infected targets, we found that MHC class Ia- and Ib-restricted CTLs are evident within 4 days following infection, peak on day 5 following infection, and although Ib-restricted CTLs disappear by day 6 postinfection, la-restricted antilisterial CTL activity can still be detected. These results demonstrate that Qa-1b is a restricting element for antilisterial CTLs, and expression of the MHC class Ib-presented target at the cell surface is TAP dependent. In addition, these results show that following L. monocytogenes infection, MHC class Ib-restricted CTLs are evident in vivo.

MHC class Ib-restricted cells contribute to antilisterial immunity: evidence for Qa-1b as a key restricting element for Listeria-specific CTLs

Subclinical infection of BALB/c mice with the intracellular pathogen Listeria monocytogenes results in the development of MHC class Ia- and Ib-restricted CTLs. L. monocytogenes-infected TAP-/- bone marrow macrophage targets are not lysed by MHC class Ia- or Ib-restricted CTLs, showing a requirement for transport of peptides into the endoplasmic reticulum for development of the MHC class Ib-peptide target. L. monocytogenes-infected B6.Tla(a)-derived bone marrow macrophages (Kb Qa-1a) are not lysed by BALB/c (Kd Qa-1b)-derived antilisterial CTLs, confirming an earlier finding that the Ib-restricting element is T region encoded. We have further determined that Qa-1b is a restricting element for antilisterial CTLs using L. monocytogenes-infected Qa-1b-transformed mouse L cells as well as human-derived HeLa cells as target populations. These L. monocytogenes-infected Qa-1b-transformed cell lines are lysed by BALB/c (Qa-1b)- or C57BL/6 (Qa-1b)-derived antilisterial CTLs, but are not lysed by B6.AKM (Qa-1a)-derived antilisterial CTLs. Using L. monocytogenes-infected targets, we found that MHC class Ia- and Ib-restricted CTLs are evident within 4 days following infection, peak on day 5 following infection, and although Ib-restricted CTLs disappear by day 6 postinfection, la-restricted antilisterial CTL activity can still be detected. These results demonstrate that Qa-1b is a restricting element for antilisterial CTLs, and expression of the MHC class Ib-presented target at the cell surface is TAP dependent. In addition, these results show that following L. monocytogenes infection, MHC class Ib-restricted CTLs are evident in vivo.

Acquired immunity to an intracellular pathogen: immunologic recognition of L. monocytogenes-infected cells

Listeria monocytogenes (L. monocytogenes) is a pathogenic bacterium, and subclinical infection in mice is utilized as a prototypic model to investigate the development and expression of acquired resistance to facultative intracellular organisms. A key virulence factor of L. monocytogenes is the hemolysin listeriolysin O (LLO), and BALB/c mice immunized with hemolysin-secreting strains of L. monocytogenes develop specific acquired resistance, while mice immunized with hemolysin-negative strains or non-viable preparations of L. monocytogenes do not develop a protective immune response. Adoptive transfer studies show that L. monocytogenes-immune CD8+ T cells mediate acquired resistance. The L. monocytogenes-immune CD8+ population is cytotoxic, and target cells infected with hemolysin-secreting strains of L. monocytogenes are lysed, while target cells infected with hemolysin-negative strains or non-viable preparations of L. monocytogenes are not lysed. MHC class Ia and Ib molecules present L. monocytogenes-derived peptides, and we have identified Qa-Ib, a T-region-encoded MHC class Ib molecule, as a restriction element for L. monocytogenes-specific CD8+ CTL. MHC class Ib-restricted CTL are stimulated following infection with L. monocytogenes and are a significant component of the total MHC class I-restricted CTL population. These findings support the observation that cytoplasmic L. monocytogenes-derived antigens are endogenously processed and presented in association with MHC class Ia and Ib molecules to CD8+ effector cells, and that both populations of effector cells contribute to the immune response to this intracellular pathogen.

Potentiation of an antimalarial oxidant drug

In a previous report we described the synergistic antimalarial interaction between two structurally similar compounds, rufigallol and exifone. To explain this phenomenon, we proposed that exifone is transformed inside the parasitized erythrocyte into a xanthone with potent antimalarial properties. We speculated that the transformation process was induced by the prooxidant activity of rufigallol. On the basis of this model we hypothesized that exifone would act synergistically with other oxidant drugs. In the present study we have found a similar synergistic interaction between exifone and ascorbic acid (vitamin C) against both chloroquine-susceptible and multidrug-resistant strains of Plasmodium falciparum. The prooxidant activity of ascorbic acid against Plasmodium-infected erythrocytes is believed to result from an intraerythrocytic Fenton reaction occurring in the acidic food vacuole of the parasite. The hydroxyl radicals produced during this process are believed to attack exifone, which undergoes cyclodehydration to become 2,3,4,5,6-pentahydroxyxanthone (X5). Evidence presented to support this "xanthone hypothesis" includes the demonstration that the exifone ==> X5 transformation occurs readily in vitro under mildly acidic conditions in the presence of iron, ascorbic acid, and oxygen.

Xanthones as antimalarial agents; studies of a possible mode of action

We recently demonstrated that 2,3,4,5,6-pentahydroxyxanthone (X5) inhibits the in vitro growth of both chloroquine-sensitive and multidrug-resistant strains of P. falciparum. To study the molecular basis of its antimalarial action, we tested X5 and selected hydroxyxanthone analogs as inhibitors of in vitro heme polymerization in a low ionic strength phosphate solution at mildly acidic pH. We found that addition of 1 Eq. of X5 resulted in complete inhibition of polymerization in this system whereas addition of up to 40 Eqs. of standard antimalarial compounds (chloroquine, primaquine, quinacrine, artemisinin and methylene blue) had no such effect although these compounds did co-precipitate with heme. The antimalarial potency of the hydroxyxanthones correlated well with their ability to inhibit in vitro heme polymerization in our assay, suggesting that these compounds exert their antimalarial action by preventing hemozoin formation. Based on the observed structure-activity relationships, we propose a model displaying possible interactions between hydroxyxanthones and heme.

Experimental allergic encephalomyelitis and vaccination-induced resistance in DA rats

In this report, we show that DA rats (RT1a haplotype) immunized with myelin basic protein (MBP)-CFA develop and recover from an ascending paralysis, with the course and severity of clinical disease similar to the kinetics observed with MBP-CFA-immunized Lewis rats. Experimental allergic encephalomyelitis (EAE) can be adoptively transferred with MBP-stimulated immune spleen cells, with onset of paralysis 4 days following transfer and complete recovery 3-4 days later. To determine if the vaccination-induced resistance response could develop in the DA rat strain, which has previously been shown to occur only in the Lewis rat, we selected a MBP-specific T-cell line by standard methods from DA rats immunized previously with MBP-CFA. The DA T-cell line was encephalitogenic, and DA recipients developed and recovered from T-cell line-mediated paralytic disease. Following recovery from T-cell line-mediated disease, DA recipients were resistant to subsequent disease induction following MBP-CFA challenge, a response consistent with T-cell vaccination, as observed previously in Lewis rats. Analysis of the proliferative response of the DA T-cell line showed that the encephalitogenic fragment was within the 40-67 region of MBP, with no response to the 85-97 fragment. The 85-97 fragment, which is a minor encephalitogenic determinant for the Lewis strain, also appears to be a minor encephalitogenic epitope for DA rats. These results show that the vaccination-induced resistance response occurs in the DA rat strain and that this phenomenon is not unique to the Lewis rat model.

Elimination of the listeriolysin O-directed immune response by conservative alteration of the immunodominant listeriolysin O amino acid 91 to 99 epitope

A major H2-Kd-presented epitope for antilisterial cytotoxic T lymphocytes (CTLs) is the nanomer peptide which corresponds to the amino acid 91 to 99 (aa91-99) sequence from listeriolysin O (LLO). Although the LLO sequence contains at least five additional nanomer peptides which also satisfy the H2-Kd binding motif, aa91-99 is the only LLO-derived target peptide that is recognized by antilisterial CTLs following infection of BALB/c mice with Listeria monocytogenes. In order to investigate further the immunodominance of the LLO aa91-99 epitope following endogenous processing of LLO, we introduced a point mutation in hly (the gene for LLO) which results in a conservative Y-to-F substitution for the anchor residue at position 2 within the aa91-99 sequence. This "92F" L. monocytogenes mutant produces biologically active LLO and is phenotypically indistinct from wild-type L. monocytogenes in terms of intracellular growth in vitro and virulence in vivo. BALB/c mice actively immunized with the 92F L. monocytogenes mutant are protected against challenge with wild-type L. monocytogenes. Antilisterial CTLs from mice immunized with the 92F mutant lyse targets infected with L. monocytogenes; however, these CTLs do not lyse target cells pulsed with either the LLO aa91-99 peptide, other LLO-derived peptides which satisfy the H2-Kd binding motif, or a peptide corresponding to the LLO aa91-92F-99 sequence. Target cells pulsed with the LLO aa91-92F-99 peptide are, however, lysed by wild-type LLO aa91-99-specific cytotoxic cells. Thus, a conservative amino acid change in the first anchor residue of the immunodominant aa91-99 sequence of LLO eliminates the induction of the cytotoxic cell response to this epitope as well as to any of the other candidate LLO-derived peptides which fit the H2-Kd binding motif. The lack of anti-LLO-specific CTLs following immunization with the 92F mutant does not appear, however, to influence the protective antilisterial immune response.

T-cell vaccination prevents EAE effector cell development but does not inhibit priming of MBP responsive cells

Cell recipients which have recovered from adoptively transferred Experimental Allergic Encephalomyelitis (EAE) mediated by encephalitogenic T-cell lines do not develop clinical disease following subsequent challenge with myelin basic protein (MBP) emulsified in CFA (MBP-CFA), a recipient response termed vaccination. The immune mechanism(s), which accounts for the vaccination-induced resistance response, is not known. We have used an adoptive transfer system to investigate the point(s) of control within the pathway of EAE effector cell development from MBP-specific naive precursors that prevents clinical disease in T-cell line vaccinated, MBP-CFA challenged Lewis rats. Although EAE effector cells do not develop in T-cell line vaccinated recipients, our data shows that MBP precursor cells are primed in T-cell line vaccinated MBP-CFA challenged animals, and these MBP-specific precursor cells can be stimulated in culture to the EAE effector cell level. MBP-memory cells also arise in T-cell line vaccinated MBP-CFA challenged donors, as demonstrated by the early and rapid onset of EAE in MBP-CFA challenged recipients of lymphnode cells from T-cell line vaccinated MBP-CFA challenged donors. We also found that it was possible to adoptively transfer resistance to MBP-CFA challenge using spleen cells from donors previously vaccinated with encephalitogenic T-cells. These results show that although EAE effector cells do not develop in T-cell line vaccinated animals, T-cell vaccination does not inhibit the initial MBP precursor cell response and does not prevent the development of MBP memory cells.

Cytotoxic-T-lymphocyte responses to epitopes of listeriolysin O and p60 following infection with Listeria monocytogenes

In order to test the influence of the cell surface density of a specific H2-Kd-presented epitope on the subsequent level of the cytotoxic-T-lymphocyte (CTL) response directed against the epitope, we investigated the CTL response to two secreted products of Listeria monocytogenes from mice immunized with viable L. monocytogenes. We determined the response to the H2-Kd-presented amino acid 91 to 99 (aa91-99) immunodominant peptide of listeriolysin O (LLO) and to the aa217-225 immunodominant peptide of p60. The p60-derived peptide appears at the cell surface as an H2-Kd-complexed peptide at a level sixfold higher than that of LLO aa91-99. CTL frequency analysis of anti-LLO- or anti-p60-specific CTLs from mice immunized with wild-type L. monocytogenes showed that the numbers of immune spleen cell-derived CTLs specific for the two peptides were essentially equivalent. We have also found that Listeria-specific CTL populations lyse target cells pulsed with the p60 aa217-225 peptide with a magnitude of the lytic response markedly less than that for targets pulsed with the LLO aa91-99 peptide. Additionally, immunization with mutants of L. monocytogenes which do not stimulate anti-LLO-specific CTLs does not alter the CTL frequency of anti-p60-specific effector cells, with levels of anti-p60-specific CTLs similar to those seen in mice immunized with wild-type L. monocytogenes. These results suggest that the relative cell surface density of major histocompatibility complex class I-presented L. monocytogenes-derived epitopes is but one of the criteria which determine the magnitude of the cytotoxic effector cell response that develops in antilisterial immunity.

Potentiation of the antimalarial agent rufigallol

We have discovered a remarkable synergistic antimalarial interaction between rufigallol and the structurally similar compound exifone. The synergistic effects were produced in chloroquine-susceptible and chloroquine-resistant clones of Plasmodium falciparum. The degree of potentiation as estimated by standard isobolar analysis was approximately 60-fold for experiments initiated with asynchronous parasites. The most pronounced synergism was observed in experiments with synchronized trophozoite-infected erythrocytes, in which the degree of synergy was at least 300-fold. While the mechanism underlying this drug potentiation remains unresolved, it is hypothesized that rufigallol acts in pro-oxidant fashion to produce oxygen radicals inside parasitized erythrocytes. These radicals would attack exifone, thereby initiating its transformation into a more potent compound, a xanthone.

Adoptive transfer of experimental allergic encephalomyelitis: recipient response to myelin basic protein-reactive lymphocytes

We have used adoptive transfer of myelin basic protein (MBP)-reactive lymphocytes in the Lewis rat model of experimental allergic encephalomyelitis (EAE) to identify stages of effector cell development and to investigate the nature of the subsequent recipient response to the transferred cells. Depending on the timing of cell collection, lymph node cells (LNC) obtained from MBP-CFA (MBP emulsified in complete Freund's adjuvant)-immunized donors may directly transfer clinical disease; however, independent of disease development, recipients of LNC develop early onset of clinical disease following immunization of the recipients with MBP-CFA, consistent with the presence of MBP-memory cells in the LNC transfer inoculum. Similarly obtained spleen cells do not directly transfer disease and do not contain MBP-memory cells (as defined by the early onset of clinical disease following MBP-CFA challenge). Spleen cells adoptively transfer clinical disease only following in vitro culture stimulation with antigen or selected mitogens. Recipients of the primary culture-derived encephalitogenic spleen cells also develop an accelerated onset of clinical disease following MBP-CFA challenge, indicative of the presence of MBP-memory cells, and are not vaccinated. Encephalitogenic T cell lines adoptively transfer clinical disease, and in most cases recipients are vaccinated to MBP-CFA-induced active disease, but remain susceptible to adoptively transferred disease. Co-transfer of encephalitogenic T cell line cells with MBP-reactive lymph node or encephalitogenic spleen cells does not alter the vaccination response. We have found that during the process of T cell line development, the vaccinating phenotype is acquired following the second antigen stimulation cycle. These studies also demonstrate that regulation induced by T cell vaccination blocks the development of effector cells from precursor cells and that such regulation is also equally effective in blocking disease development in recipients which have increased numbers of memory cells. Thus, the response to T cell vaccination, once established, is fully capable of inhibiting the development of effector cells from increased numbers of precursor/memory cells, a response that would be needed in the clinical application of vaccination-induced resistance.

An H2-T MHC class Ib molecule presents Listeria monocytogenes-derived antigen to immune CD8+ cytotoxic T cells

Mouse spleen T cells can adoptively transfer immunity to Listeria monocytogenes; this activity was markedly enhanced by stimulation with Con A in vitro before transfer. The enhanced and prolonged protection against L. monocytogenes in vivo was correlated with enhanced lysis in vitro of target cells infected with strains of L. monocytogenes that produce listeriolysin O (LLO). One of the targets of such cytotoxic cells from BALB/c (H2d) mice was a peptide that corresponded to amino acids 91 to 99 (p91-99) of the LLO molecule, which satisfies the binding motif of H2-Kd. Listeria-immune CD3+CD8+, but not CD3+CD8-, cells could also lyse H-2-incompatible, infected target cells. Immune cells from C57BL/6 (H2b) mice lysed allogeneic H-2d target cells infected with L. monocytogenes or a Bacillus subtilis transformant that secretes LLO, but did not lyse targets pulsed with p91-99. This H2-unrestricted cytolysis was therefore directed at a fragment of the LLO molecule other than p91-99. Listeria-infected bone marrow macrophages from congenic and recombinant strains of mice were lysed only when they shared the H2-T region or were Qa1-compatible with the immune cytotoxic cells; sharing of the H2-D, Q, or M region was insufficient. Thus, the immune response to L. monocytogenes included cytolytic CD8+ cells that recognized endogenously processed Listeria-derived Ags in the context of the class Ia H2-K molecule, as well as a class Ib H2-T molecule.

An H2-T MHC class Ib molecule presents Listeria monocytogenes-derived antigen to immune CD8+ cytotoxic T cells

Mouse spleen T cells can adoptively transfer immunity to Listeria monocytogenes; this activity was markedly enhanced by stimulation with Con A in vitro before transfer. The enhanced and prolonged protection against L. monocytogenes in vivo was correlated with enhanced lysis in vitro of target cells infected with strains of L. monocytogenes that produce listeriolysin O (LLO). One of the targets of such cytotoxic cells from BALB/c (H2d) mice was a peptide that corresponded to amino acids 91 to 99 (p91-99) of the LLO molecule, which satisfies the binding motif of H2-Kd. Listeria-immune CD3+CD8+, but not CD3+CD8-, cells could also lyse H-2-incompatible, infected target cells. Immune cells from C57BL/6 (H2b) mice lysed allogeneic H-2d target cells infected with L. monocytogenes or a Bacillus subtilis transformant that secretes LLO, but did not lyse targets pulsed with p91-99. This H2-unrestricted cytolysis was therefore directed at a fragment of the LLO molecule other than p91-99. Listeria-infected bone marrow macrophages from congenic and recombinant strains of mice were lysed only when they shared the H2-T region or were Qa1-compatible with the immune cytotoxic cells; sharing of the H2-D, Q, or M region was insufficient. Thus, the immune response to L. monocytogenes included cytolytic CD8+ cells that recognized endogenously processed Listeria-derived Ags in the context of the class Ia H2-K molecule, as well as a class Ib H2-T molecule.

Antilisterial immunity includes specificity to listeriolysin O (LLO) and non-LLO-derived determinants

Subclinical infection of BALB/c mice with virulent Listeria monocytogenes leads to the generation of Listeria-specific T-cell populations required for the expression of protective immunity. The L. monocytogenes-produced hemolysin listeriolysin O (LLO) is a virulence factor which appears to be crucial for the induction of protective antilisterial immunity. Analysis of the specificity of antilisterial cytotoxic cells from Listeria-immune BALB/c donors has shown a dominant response to an epitope corresponding to amino acids 91 to 99 of LLO. Demonstration of antilisterial T cells with specificity to non-LLO-derived epitopes has been difficult to achieve because of the requirement of LLO in facilitating escape of the bacteria to the cytoplasm of the host cell and the apparent dominance of an anti-LLO response in antilisterial immunity. In this study we show that antilisterial immunity also includes specificity to non-LLO-derived determinants. We used as an immunogen an LLO- mutant of L. monocytogenes which expresses the hemolysin perfringolysin O (PFO). The LLO- PFO+ L. monocytogenes mutant possesses invasive properties similar to those of wild-type L. monocytogenes and escape from the phagocytic vacuole because of the activity of PFO. We found that J774 target cells infected with the LLO- PFO+ L. monocytogenes mutant were lysed by antilisterial cytotoxic T cells obtained from BALB/c mice immunized with wild-type L. monocytogenes. In addition, BALB/c mice immunized with the LLO- PFO+ L. monocytogenes mutant were immune to challenge with LLO+ wild-type L. monocytogenes, a finding indicative of protective antilisterial immunity specific to Listeria-derived epitopes other than LLO. Spleen cells from BALB/c mice immunized with the LLO- PFO+ L. monocytogenes mutant adoptively transferred antilisterial protection to a subsequent challenge with wild-type L. monocytogenes. This splenocyte population also contained cytotoxic cells which lysed target cells infected with either the LLO- PFO+ L. monocytogenes mutant or wild-type LLO+ L. monocytogenes but did not lyse target cells infected with an LLO-expressing Bacillus subtilis transformant. These results establish that during the immune response to L. monocytogenes, immune splenocytes with specificity for LLO and other, non-LLO-derived epitopes develop. These non-LLO epitopes serve as targets for antilisterial cytotoxic cells and for lymphocytes which adoptively transfer antilisterial immunity.

Intracytoplasmic growth and virulence of Listeria monocytogenes auxotrophic mutants

The intracellular growth of several auxotrophic mutants of Listeria monocytogenes was examined in cell culture, and virulence was evaluated in mice by intravenous injection of log-phase bacteria. L. monocytogenes transposon insertion mutants requiring either uracil, phenylalanine, glycine, proline, or nicotinic acid for growth were fully virulent and grew similarly to the parental strain as shown by their growth rates in cell culture. Those requiring all three aromatic amino acids (phenylalanine, tryptophan, and tyrosine) or adenine were 1.5 log10 less virulent than the wild type. A threonine auxotroph, which showed enhanced growth in the presence of threonine-containing peptides as compared with that in the presence of free threonine, was approximately 1 log10 less virulent than the wild type. When host cells were deprived of specific amino acids required by both the host cell and L. monocytogenes, the bacteria continued to grow intracellularly. These studies suggest that the cytoplasm of eucaryotic cells behaves like rich medium, facilitating the growth of an intracellular bacterial pathogen with complex growth requirements. In addition, results related to amino acid deprivation during intracellular growth and specific extracellular growth requirements of a threonine auxotroph suggest that L. monocytogenes may utilize intracellular peptides as a source of amino acids.

Parasite lactate dehydrogenase as an assay for Plasmodium falciparum drug sensitivity

This report compares the use of the lactate dehydrogenase (pLDH) assay with 3H-hypoxanthine incorporation and Giemsa microscopy for the evaluation of anti-malaria drug inhibition of the growth of P. falciparum in vitro. The inhibition profiles and IC50 determinations of the pLDH assay were directly comparable to those determined by the radioactive uptake and microscopic methods. Furthermore, the pLDH culture sensitivity assay is reproducible, easily interpreted, rapid and inexpensive to perform, suggesting field applicability.

Measurement of the lactate dehydrogenase activity of Plasmodium falciparum as an assessment of parasitemia

This report describes an enzyme assay for the detection of Plasmodium falciparum. The assay is based on the observation that the lactate dehydrogenase (LDH) enzyme of P. falciparum has the ability to rapidly use 3-acetyl pyridine NAD (APAD) as a coenzyme in the reaction leading to the formation of pyruvate from lactate. Human red blood cell LDH carries out this reaction at a very slow rate in the presence of APAD. We measured the development of APADH and found that the formation of this product could establish the basis of an assay that detected the presence of P. falciparum from in vitro cultures at parasitemia levels of 0.02%. We also had occasion to use this assay with clinical samples. We found a correlation between levels of parasitemia and the activity of parasite LDH. Parasite LDH (pLDH) activity could be measured in blood hemolysates and in plasma and serum from patients with malaria. We used the serum assay for pLDH and followed the level of pLDH in a patient with cerebral malaria prior to antimalarial treatment and during the recovery period. From these initial studies, it is evident that the measurement of pLDH has a correlation with parasitemia and may offer a method that can be developed into a simple test for the detection of Plasmodium parasitemia.