<p>Protective immunity against <i>Plasmodium falciparum</i> may be obtained after repeated exposure to infection. Several studies indicate that immunity against the blood stages of the <i>P. Falciparum</i> infection is mainly antibody mediated. Protective antibodies may act either on their own, mediate antibody-dependent phagocytosis and/or cell-mediated neutralization of parasites. This thesis describes several aspects of humoral immune responses to <i>P. falciparum</i> infection in individuals of different age groups, different genetic background and with different degrees of malaria exposure.</p><p>Several target antigens for antibody-mediated inhibition of parasite growth or invasion have been identified. One such antigen is Pf332, which appears on the surface of parasitized erythrocytes at late trophozoite and schizont stage. This surface exposure makes the antigen a possible target for opsonizing antibodies. We optimized an <i>in vitro</i> assay for studying cellmediated parasite neutralization in the presence of Pf332-reactive antibodies. Our data demonstrate that, Pf332 specific antibodies are able to inhibit parasite growth on their own and in cooperation with human monocytes.</p><p>The <i>P. falciparum</i> parasites have evolved several mechanisms to evade the host neutralizing immune responses. In this thesis, we show that freshly isolated<i> P. falciparum </i>parasites from children living in a malaria endemic area of Burkina Faso were less sensitive for growth inhibition <i>in vitro</i> by autologous immunoglobulins (Ig) compared with heterologous ones. Analyses of two consecutive isolates taken 14 days apart, with regard to genotypes and sensitivity to growth inhibition <i>in vitro</i>, did not give any clear-cut indications on possible mechanisms leading to a reduced inhibitory activity in autologous parasite/antibody combinations. The frequent presence of persisting parasite clones in asymptomatic children indicates that the parasite possesses as yet undefined mechanisms to evade neutralizing immune responses.</p><p>Transmission reducing measures such insecticide treated nets (ITNs) have been shown to be effective in reducing morbidity and mortality from malaria. However, concerns have been raised that ITNs usage could affect the acquisition of malaria immunity. We studied the effect of the use of insecticide treated curtains (ITC) on anti-malarial immune responses of children living in villages with ITC since birth. The use of ITC did neither affect the levels of parasite neutralizing immune responses nor the multiplicity of infection. These results indicate that the use of ITC does not interfere with the acquisition of anti-malarial immunity in children living in a malaria hyperendemic area.</p><p>There is substantial evidence that the African Fulani tribe is markedly less susceptible to malaria infection compared to other sympatrically living ethnic tribes. We investigated the isotypic humoral responses against<i> P. falciparum</i> asexual blood stages in different ethnic groups living in sympatry in two countries exhibiting different malaria transmission intensities, Burkina Faso and Mali. We observed higher levels of the total malaria-specific-IgG and its cytophilic subclasses in individuals of the Fulani tribe as compared to non-Fulani individuals. Fulani individuals also showed higher levels of antibodies to measles antigen, indicating that the intertribal differences are not specific for malaria and might reflect a generally activated immune system in the Fulani.</p>

Protective immunity against Plasmodium falciparum may be obtained after repeated exposure to infection. Several studies indicate that immunity against the blood stages of the P. Falciparum infection is mainly antibody mediated. Protective antibodies may act either on their own, mediate antibody-dependent phagocytosis and/or cell-mediated neutralization of parasites. This thesis describes several aspects of humoral immune responses to P. falciparum infection in individuals of different age groups, different genetic background and with different degrees of malaria exposure. Several target antigens for antibody-mediated inhibition of parasite growth or invasion have been identified. One such antigen is Pf332, which appears on the surface of parasitized erythrocytes at late trophozoite and schizont stage. This surface exposure makes the antigen a possible target for opsonizing antibodies. We optimized an in vitro assay for studying cellmediated parasite neutralization in the presence of Pf332-reactive antibodies. Our data demonstrate that, Pf332 specific antibodies are able to inhibit parasite growth on their own and in cooperation with human monocytes. The P. falciparum parasites have evolved several mechanisms to evade the host neutralizing immune responses. In this thesis, we show that freshly isolated P. falciparum parasites from children living in a malaria endemic area of Burkina Faso were less sensitive for growth inhibition in vitro by autologous immunoglobulins (Ig) compared with heterologous ones. Analyses of two consecutive isolates taken 14 days apart, with regard to genotypes and sensitivity to growth inhibition in vitro, did not give any clear-cut indications on possible mechanisms leading to a reduced inhibitory activity in autologous parasite/antibody combinations. The frequent presence of persisting parasite clones in asymptomatic children indicates that the parasite possesses as yet undefined mechanisms to evade neutralizing immune responses. Transmission reducing measures such insecticide treated nets (ITNs) have been shown to be effective in reducing morbidity and mortality from malaria. However, concerns have been raised that ITNs usage could affect the acquisition of malaria immunity. We studied the effect of the use of insecticide treated curtains (ITC) on anti-malarial immune responses of children living in villages with ITC since birth. The use of ITC did neither affect the levels of parasite neutralizing immune responses nor the multiplicity of infection. These results indicate that the use of ITC does not interfere with the acquisition of anti-malarial immunity in children living in a malaria hyperendemic area. There is substantial evidence that the African Fulani tribe is markedly less susceptible to malaria infection compared to other sympatrically living ethnic tribes. We investigated the isotypic humoral responses against P. falciparum asexual blood stages in different ethnic groups living in sympatry in two countries exhibiting different malaria transmission intensities, Burkina Faso and Mali. We observed higher levels of the total malaria-specific-IgG and its cytophilic subclasses in individuals of the Fulani tribe as compared to non-Fulani individuals. Fulani individuals also showed higher levels of antibodies to measles antigen, indicating that the intertribal differences are not specific for malaria and might reflect a generally activated immune system in the Fulani.

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2007. / Härtill 4 uppsatser.

Diss. (sammanfattning) Stockholm : Karol. inst., 2004. / Härtill 4 uppsatser.

Thesis (M.C.T.M. (Tropical Pediatrics))--Mahidol University, 2004.

Thesis (Ph.D.)--Uniformed Services University of the Health Sciences, 2006 / Typescript (photocopy)

Cerebral malaria is characterised by an accumulation of infected erythrocytes in the microvasculature of the brain. Plasmodium falciparum infected erythrocytes have been shown to bind to a Human Brain Endothelial Cell line (HBEC-5i) in vitro. This provides a model for the investigation of interactions between P. falcuparum and human brain endothelium. Currently neither the parasite adhesion ligands on infected erythrocytes, nor the host endothelial cell receptors necessary for this interaction have been identified. In this work, the identity of the host receptor on brain endothelial cells was addressed by binding assays of selected and unselected parasites on a wide range of malaria-associated host molecules. The identity of the parasite ligand was investigated by microarray analysis of parasites after selection for cytoadherence to HBEC-5i. The hypothesis being tested was that the gene encoding the parasite cytoadherence ligand would show significant upregulation in selected compared to unselected paarasites. The P. falciparum laboratory strains 3D7, HB3 and IT/FCR3 were selected for binding to HBEC-5i using a panning assay. Compared to unselected parasites, HBEC-5i selected parasites showed a distinct phenotype with reduced platelet-mediated clumping. There was no significant increase in binding of parasites to any of the known endothelial cytoadherence receptors for P. falciparum after selection on HBEC-5i. Binding inhibition assays with various antibodies and soluble receptors did not greatly block the adhesion of parasites to HBEC-5i except for heparin. Altogether, the receptor(s) mediating the interation with HBEC-5i remains unknown. In order to carry out transcriptional analysis of selected and unselected paarasites form all three parasite strains, it was necessary to update the existing microarray chip which is based on the 3D7 genome. This is because each parasite train has a unique repertoire of variant surface antigens (VSAs) including var, rif and stevor genes. Therefore, to fully analysis HB3 and IT genomes. Unique oligonnucleotide probes were then designed for each new sequence and the 3D7-based microarray chip was updated. Transcriptional analysis was then carried out on selected and unselected parasites of all strains. Microarray data clearly indicated that the most highly upregulated genes after selection were group A or group A-like var genes (HB3var3, 3D7_PFDOO2Oc, ITvar7 and ITvar19), showing 11 to over 100 fold upregulation in selected parasites. The rif gene adjacent to the upregulated var gene was also highly expressed. To a lesser extent some exported proteins like RESA-1, PfEMP3 or PHIST family members also showed increased transcription in HBEC-selected parasites (2-3 fold upregulation). Reverse transcriptase-PCR confirmed the upregulation of group A var genes in selected parasites, suggessted that the group A PfEMP1 variants are major candidate ligands for parasite binding to HBEC-5i. These findings are consistent with previous work showing an association between Group A var genes and cerebral malaria.

Malaria afflicts 300-500 million people in the world and the mortality ranges from 1-2 million, children in Africa being the most susceptible. With a vaccine not being available against malaria and the front line drugs such as chloroquine and antifolates registering widespread parasite resistance, the challenge of malaria treatment is a formidable task. While, research to discover new drugs has become essential, it has also become necessary to identify therapeutic strategies in the short-term. One approach is to examine whether known drugs used for other applications can be used to treat malaria. A second strategy is to look for natural compounds for antimalarial activity either singly or in combination. Combination therapy has assumed considerable importance in the context of artemisinin derivatives being the sole, tested, efficacious antimalarials left in the basket. A combination therapy with artemisinin derivative may prevent recrudescence due to monotherapy, extend the life of the drug and perhaps bring down the cost of therapy as well. A primary requirement to embark on such studies is to assess the status of drug resistance to the front line drugs in use. In India, chloroquine is still used as the front line drug for malaria therapy. Although, there have been indications and sporadic reports on the development of chloroquine resistance in the country, there has not been a detailed molecular or clinical evaluation for resistance. Keeping all these considerations in mind, the objectives of the present study are as follows: 1. Evaluation of chloroquine resistance inP.falciparum isolates from patients using Pfcrt-mutation as marker. 2. Evaluation of the anti-tubercular drugs, rifampicin and isonicotinic acid hydrazide (INH) for antimalarial activity. 3. Evaluation of curcumin from turmeric singly and in combination with α,β- arteether for antimalarial acitivity. Chapter I deals with the review of literature pertaining to scenario of available antimalarials, efforts to discover new antimalarials based on new drug targets, mechanisms of drug resistance and strategies for combination therapies. Chapter II deals with an evaluation of Pfcrt mutation in clinical samples of P.falciparum malaria in India. After several false starts to find molecular markers to identify chloroquine resistance, mutations in the Pfcrt gene of P.falciparum, K76T mutation in particular, has been shown to correlate very well with chloroquine resistance in culture. A study of 109 P.falciparum – infected blood samples from different parts of India has revealed that close to 95% of the isolates carry the K76T mutation. This was shown on the basis of susceptibility to ApoI restriction digestion of the PCR product covering this region (264 nt) and DNA sequencing of the PCR product. Interestingly, the resistant haplotype in this region of 72-76 amino acids was found to be mostly SVMNT, except for 4 samples with CVIET haplotype. SVMNT has all along been considered to be of South American origin, where as CVIET is of South East Asian/African origin. Subsequent studies by another group in the country has also shown that the Pfcrt - K76T mutation is seen at least in 85% of the cases and in addition to the dominant SVMNT haplotype, newer haplotypes are also seen. The present study has also included an analysis of N86Y mutation in the Pfmdr1 gene based on susceptibility to Afl III restriction enzyme digestion and DNA sequencing of the PCR product (603 nt). Pfmdr1 mutations have been extensively studied in literature for possible correlation to CQR. The net conclusion is that it does not contribute directly to CQR but may have an indirect correlation. It has been shown in Mali that there is very good correlation between Pfcrt - K76T mutation and Pfmdr1 - N86Y mutation in the P.falciparum isolates. However, in the present study with Indian isolates only around 30% of the samples were found to carry the Pfmdr1 - N86Y mutation. While, further studies on the clinical relevance of the extensive Pfcrt mutation seen in the Indian isolates are needed, it is clear that the genetic change towards chloroquine resistance has already taken place in the Indian context. Chapter III is devoted to a study of the antimalarial effects of the anti-tubercular drugs, rifampicin and INH. This is on the basis that rifampicin is an inhibitor of prokaryotic and mitochondrial/chloroplast RNA polymerase. P.falciparum harbors the apicoplast, a remnant of chloroplast with a 35kb DNA. It is known that the β, β’- subunits of the apicoplast RNA polymerase are coded by the apicoplast DNA. There is a report that rifampicin is a slow acting antimalarial in cases of P.vivax -nfection. INH is known to act by inhibiting the enoyl-ACP reductase and β - hydroxy ACP synthase in M.tuberculosis. While, M.tuberculosis is known to manifest Fab I and Fab II pathways of fatty acid biosynthesis, it has recently been shown that P.falciparum manifests the FabII (discrete enzymes) pathway. Thus, it was considered possible that INH may also inhibit the fatty acid biosynthetic pathway of P.falciparum leading to inhibition of phospohlipid and membrane biosynthesis. Studies were, therefore, carried out with rifampicin, INH and the combination on the survival of P.falciparum in culture and P.berghei in mice. With P.falciparum, growth was followed by measuring3[H]-Hypoxanthine incorporation and slide detection of parasites using Giemsa stain. The results indicate that while, rifampicin inhibits P.falciparum growth with an IC50 around 25nM, and INH fails to show any effect even at 200µM concentration. The combination of rifampicin (25nM) and INH (100µM) shows enhanced killing effect. In view of these results, studies were undertaken in mice infected with P.berghei. After 72 hr infection, the mice were orally fed with rifampicin (500 µg/40 g body weight) or INH (1 mg/40 g body weight) or a combination of the two orally for 5 days, starting on day 3. Apart from parasite clearance in blood, protection against mortality is a good index, since all the infected mice die in about 7-8 days. The results indicate that rifampicin leads to around 50% protection and INH treatment gives around 10% protection. However, the combination gives around 83% protection with complete clearance of the parasite in blood. Short- term treatment of infected mice with drugs and an assay of rpoB/C transcription in the parasite using appropriate PCR primers reveal a striking inhibition in combination treatment. Again, when such parasites were put into short-term culture and32P- incorporation into phospholipids was measured, there was striking inhibition with combination treatment. Thus, the results indicate that a combination of rifampicin and INH has potent antimalarial activity in P.berghei-infected mice. The results are dramatic in this case when compared to the results obtained with P.falciparum culture. It is not clear whether the differences are due to differences in action in vitro vs in vivo or due to differences in susceptibility between P.falciparum and P. berghei to the treatment provided. Chapter IV deals with the antimalarial activity of curcumin (diferuloyl methane) from turmeric singly or in combination with artemesinin or its derivative. Curcumin is reported to have a wide variety of biochemical effects and its anti-cancer activity is under serious investigation. There is an earlier report that curcumin shows antimalarial activity against chloroquine-sensitive P.falciparum. In the present study, curcumin was tested against a chloroquine-resistant culture of P.facliparum and it inhibits growth with an IC50 of 5-8 µM. When P.berghei-infected mice were orally fed with curcumin for 5 days, there was delay in the development of parasitemia, with about 30% of the animals protected against mortality by day 28. For reasons mentioned earlier curcumin was tested in combination with artemisinin/derivative in P.falciparum culture and P.berghei in mice. The results indicate that artemisinin and curcumin have an additive inhibitory effect on P.falciparum growth, based on a detailed analysis of the isobolograms. In terms of the mechanism of action, curcumin treatment leads to accumulation of45Ca in the parasite cytoplasm. It also has a striking inhibitory effect on32P-incorporation into parasite proteins and phospholipids, suggesting an interference with phosphorylation mechanisms. None of these effects are seen under artemisinin treatment, which has been reported to specifically inhibit PfATP6 (Ca ATPase) in P.falciparum. In view of the possible different modes of action of artemisinin and curcumin, the combination was tested in P.berghei-infected mice. The infected mice received a single injection of α,β-arteether and 3 oral doses of curcumin (5mg/30g body weight). Curcumin treatment was found to dramatically delay the onset of parasitemia seen in animals treated with α,β-arteether alone due to recrudescence. In particular, a combination with a single injection of α,β-arteether (750µg or 1.5mg/30g body weight) followed by 3 oral doses of curcumin leads to complete prevention of recrudescence and 100% protection against mortality. Several combinations with artemisinin derivative are under investigation and they all suffer from toxic side effects, pharmacokinetic mismatch, known resistance to the combining partner and high cost. It is felt that this artemisinin derivative curcumin combination could prove superior in view of the fact that no resistance is known to curcumin and is safe even at very high doses used in the human. Both the drugs are eliminated fast and curcumin is a cheap chemical and available in plenty from natural source (turmeric). In view of these positive attributes, a clinical trial with this combination is recommended. 121

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2006. / Härtill 4 uppsatser.

Diss. (sammanfattning) Stockholm : Stockholms universitet, 2008. / Härtill 5 uppsatser.