Cytokines inhibit the development of liver schizonts of the malaria parasite Plasmodium berghei in vivo

Cytokine gene polymorphisms implicated in the pathogenesis of Plasmodium falciparum infection outcome

Cytokines play a critical role in the immune mechanisms involved in fighting infections including malaria. Polymorphisms in cytokine genes may affect immune responses during an infection with Plasmodium parasites and immunization outcomes during routine administration of malaria vaccines. These polymorphisms can increase or reduce susceptibility to this deadly infection, and this may affect the physiologically needed balance between anti-inflammatory and pro-inflammatory cytokines. The purpose of this review is to present an overview of the effect of selected cytokine gene polymorphisms on immune responses against malaria.

The distinctive role of membrane fibrinogen-like protein 2 in the liver stage of rodent malaria infections

Viral infection often induce the expression of murine fibrinogen-like protein 2 (mFGL2) triggering immune coagulation, which causes severe liver pathogenesis via increased fibrin deposition and thrombosis in the microvasculature. We aimed to investigate the role of mFGL2 in the liver stage of malaria infections. We reveal that infection with malaria sporozoites also induces increased expression of mFGL2 and that this expression is primarily located within the liver Kupffer and endothelial cells. In addition, we report that inhibition of FGL2 has no significant effect on immune coagulation but increases the expression of inflammatory cytokines in the livers of infected mice. Interestingly, FGL2 deficiency had no significant impact on the development of liver stage malaria parasites or the pathogenesis of the infected liver. In contrast to viral infections, we conclude that mFGL2 does not contribute to either parasite development or liver pathology during these infections, revealing the unique features of this protein in liver-stage malaria infections.

Genetic variation in the immune system and malaria susceptibility in infants: a nested case-control study in Nanoro, Burkina Faso

BACKGROUND

Genetic polymorphisms in the human immune system modulate susceptibility to malaria. However, there is a paucity of data on the contribution of immunogenetic variants to malaria susceptibility in infants, who present differential biological features related to the immaturity of their adaptive immune system, the protective effect of maternal antibodies and fetal haemoglobin. This study investigated the association between genetic variation in innate immune response genes and malaria susceptibility during the first year of life in 656 infants from a birth cohort survey performed in Nanoro, Burkina Faso.

METHODS

Seventeen single nucleotide polymorphisms (SNPs) in 11 genes of the immune system previously associated with different malaria phenotypes were genotyped using TaqMan allelic hybridization assays in a Fluidigm platform. Plasmodium falciparum infection and clinical disease were documented by active and passive case detection. Case-control association analyses for both alleles and genotypes were carried out using univariate and multivariate logistic regression. For cytokines showing significant SNP associations in multivariate analyses, cord blood supernatant concentrations were measured by quantitative suspension array technology (Luminex).

RESULTS

Genetic variants in IL-1β (rs1143634) and FcγRIIA/CD32 (rs1801274)-both in allelic, dominant and co-dominant models-were significantly associated with protection from both P. falciparum infection and clinical malaria. Furthermore, heterozygote individuals with rs1801274 SNP in FcγRIIA/CD32 showed higher IL-1RA levels compared to wild-type homozygotes (P = 0.024), a cytokine whose production is promoted by the binding of IgG immune complexes to Fcγ receptors on effector immune cells.

CONCLUSIONS

These findings indicate that genetic polymorphisms in genes driving innate immune responses are associated to malaria susceptibility during the first year of life, possibly by modulating production of inflammatory mediators.

Hepatic Inflammation Confers Protective Immunity Against Liver Stages of Malaria Parasite

Deciphering the mechanisms by which Plasmodium parasites develop inside hepatocytes is an important step toward the understanding of malaria pathogenesis. We propose that the nature and the magnitude of the inflammatory response in the liver are key for the establishment of the infection. Here, we used mice deficient in the multidrug resistance-2 gene (Mdr2-/-)-encoded phospholipid flippase leading to the development of liver inflammation. Infection of Mdr2-/- mice with Plasmodium berghei ANKA (PbANKA) sporozoites (SPZ) resulted in the blockade of hepatic exo-erythrocytic forms (EEFs) with no further development into blood stage parasites. Interestingly, cultured primary hepatocytes from mutant and wild-type mice are equally effective in supporting EEF development. The abortive infection resulted in a long-lasting immunity in Mdr2-/- mice against infectious SPZ where neutrophils and IL-6 appear as key effector components along with CD8+ and CD4+ effector and central memory T cells. Inflammation-induced breakdown of liver tolerance promotes anti-parasite immunity and provides new approaches for the design of effective vaccines against malaria disease.

Transforming growth factor-beta profiles correlate with clinical symptoms and parameters of haemostasis and inflammation in a controlled human malaria infection

BACKGROUND

After a controlled human malaria infection (CHMI), presentation of clinical signs and symptoms and host responses is heterogeneous. Transforming growth factor-beta (TGF-β) is the first serum cytokine that changes in malaria-naïve volunteers after CHMI. We studied a possible relation between TGF-β changes, pro-inflammatory cytokines, activation of haemostasis and endothelial cells and clinical symptoms.

METHODS

A panel of cytokines including TGF-β, and markers of activation of haemostasis and endothelial cells were measured in blood samples of 15 volunteers at baseline before CHMI and during CHMI at day of treatment. The change of the parameters on the day of treatment was examined for a significant alteration during infection.

RESULTS

Nine of 15 volunteers showed a significant decrease in TGF-β compared to baseline, with concomitant increased concentrations of D-dimer (p = 0.012), Von Willebrand factor (p = 0.017), IL-6 (p = 0.012) and IFN-γ (0.028) and a significantly decreased platelet count (p = 0.011). In contrast, 6 of 15 volunteers showed sustained or increased TGF-β concentrations without change in the aforementioned parameters. The sustained responders presented with less moderate and severe clinical symptoms than the negative responders (p = 0.036) and had a higher baseline lymphocyte count (p = 0.026). TGF-β concentrations did not correlate with the parasitaemia on day of treatment.

CONCLUSION

Early decreases of serum TGF-β might function a marker for a pro-inflammatory host response and downstream clinical symptoms and pathology during CHMI.

The TLR2 is activated by sporozoites and suppresses intrahepatic rodent malaria parasite development

TLRs (Toll-like receptors) play an important role in the initiation of innate immune responses against invading microorganisms. Although several TLRs have been reported to be involved in the innate immune response against the blood-stage of malaria parasites, the role of TLRs in the development of the pre-erythrocytic stage is still largely unknown. Here, we found that sporozoite and its lysate could significantly activate the TLR2, and induce macrophages to release proinflammatory cytokines, including IL-6, MCP-1 and TNF-α, in a TLR2-dependent manner. Further studies showed that sporozoite and its lysate could be recognized by either TLR2 homodimers or TLR2/1 and TLR2/6 heterodimers, implicating the complexity of TLR2 agonist in sporozoite. Interestingly, the TLR2 signaling can significantly suppress the development of the pre-erythrocytic stage of Plasmodium yoelii, as both liver parasite load and subsequent parasitemia were significantly elevated in both TLR2- and MyD88-deficient mice. Additionally, the observed higher level of parasite burden in TLR2^−/− mice was found to be closely associated with a reduction in proinflammatory cytokines in the liver. Therefore, we provide the first evidence that sporozoites can activate the TLR2 signaling, which in turn significantly inhibits the intrahepatic parasites. This may provide us with novel clues to design preventive anti-malaria therapies.

Plasmodium cellular effector mechanisms and the hepatic microenvironment

Plasmodium falciparum malaria remains one of the most serious health problems globally. Immunization with attenuated parasites elicits multiple cellular effector mechanisms capable of eliminating Plasmodium liver stages. However, malaria liver stage (LS) immunity is complex and the mechanisms effector T cells use to locate the few infected hepatocytes in the large liver in order to kill the intracellular LS parasites remain a mystery to date. Here, we review our current knowledge on the behavior of CD8 effector T cells in the hepatic microvasculature, in malaria and other hepatic infections. Taking into account the unique immunological and lymphogenic properties of the liver, we discuss whether classical granule-mediated cytotoxicity might eliminate infected hepatocytes via direct cell contact or whether cytokines might operate without cell–cell contact and kill Plasmodium LSs at a distance. A thorough understanding of the cellular effector mechanisms that lead to parasite death hence sterile protection is a prerequisite for the development of a successful malaria vaccine to protect the 40% of the world’s population currently at risk of Plasmodium infection.

Evaluation of medicinal turpentine used for the prevention of bovine babesiosis in southern KwaZulu-Natal and the eastern Free State

Medicinal turpentine has been used extensively in the eastern Free State and KwaZulu-Natal provinces of South Africa with reportedly excellent results. It is believed that it is able to prevent and treat babesiosis (redwater) in cattle. Redwater is an often-fatal disease in cattle and results in losses of large numbers every year in South Africa. This study was initiated in an attempt to investigate the validity of the use of the turpentine as a medicinal agent. Using a semi in vitro screening assay, Babesia caballi grown in primary equine erythrocytes was exposed to various concentrations of turpentine in comparison to diminazene and imidocarb. The turpentine had no parasiticidal effect following direct exposure. During the recovery phase, the previously exposed parasites appeared to grow more slowly than the controls. In comparison, diminazene and imidocarb were 100% effective in killing the parasites. In a subsequent tolerance study in adult cattle (n = 6) at 1x (2 mL), 3x and 5x the recommended dose, the product was non-toxic. Irritation was noted at the injection site with the higher dose. The only major finding on clinical pathology was a general increase in globulins, without a concurrent change in native babesia antibody titres. It was concluded that it is unlikely that medicinal turpentine is an effective treatment against babesiosis.

Genetics of Sub-Saharan African Human Population: Implications for HIV/AIDS, Tuberculosis, and Malaria

Sub-Saharan Africa has continued leading in prevalence and incidence of major infectious disease killers such as HIV/AIDS, tuberculosis, and malaria. Epidemiological triad of infectious diseases includes susceptible host, pathogen, and environment. It is imperative that all aspects of vertices of the infectious disease triad are analysed to better understand why this is so. Studies done to address this intriguing reality though have mainly addressed pathogen and environmental components of the triad. Africa is the most genetically diverse region of the world as well as being the origin of modern humans. Malaria is relatively an ancient infection in this region as compared to TB and HIV/AIDS; from the evolutionary perspective, we would draw lessons that this ancestrally unique population now under three important infectious diseases both ancient and exotic will be skewed into increased genetic diversity; moreover, other evolutionary forces are also still at play. Host genetic diversity resulting from many years of malaria infection has been well documented in this population; we are yet to account for genetic diversity from the trio of these infections. Effect of host genetics on treatment outcome has been documented. Host genetics of sub-Saharan African population and its implication to infectious diseases are an important aspect that this review seeks to address.

Cytotoxic markers associate with protection against malaria in human volunteers immunized with Plasmodium falciparum sporozoites

BACKGROUND

Immunization of healthy volunteers by bites from Plasmodium falciparum-infected mosquitoes during chloroquine chemoprophylaxis (hereafter, chemoprophylaxis and sporozoites [CPS] immunization) induces sterile protection against malaria. CPS-induced protection is mediated by immunity against pre-erythrocytic stages, presumably at least partially by cytotoxic cellular responses. We therefore aimed to investigate the association of CPS-induced cytotoxic T-cell markers with protection.

METHODS

In a double-blind randomized controlled trial, we performed dose titration of CPS immunization followed by homologous challenge infection in 29 subjects. Immune responses were assessed by in vitro restimulation of peripheral blood mononuclear cells and flow cytometry.

RESULTS

Dose-dependent complete protection was obtained in 4 of 5 volunteers after immunization with bites from 45 P. falciparum-infected mosquitoes, in 8 of 9 volunteers with bites from 30, and in 5 of 10 volunteers with bites from 15 (odds ratio [OR], 5.0; 95% confidence interval [CI], 1.5-17). Completely protected subjects had significantly higher proportions of CD4 T cells expressing the degranulation marker CD107a (OR, 8.4; 95% CI, 1.5-123; P = .011) and CD8 cells producing granzyme B (OR, 11; 95% CI, 1.9-212; P = .004) after P. falciparum restimulation.

CONCLUSIONS

These data underline the efficiency of CPS immunization to induce sterile protection and support a possible role for cytotoxic CD4 and CD8 T-cell responses in pre-erythrocytic immunity.

CLINICAL TRIALS REGISTRATION

NCT01218893.

The host genetic diversity in malaria infection

Populations exposed to Plasmodium infection develop genetic mechanisms of protection against severe disease. The clinical manifestation of malaria results primarily from the lysis of infected erythrocytes and subsequent immune and inflammatory responses. Herein, we review the genetic alterations associated with erythrocytes or mediators of the immune system, which might influence malaria outcome. Moreover, polymorphisms in genes related to molecules involved in mechanisms of cytoadherence and their influence on malaria pathology are also discussed. The results of some studies have suggested that the combinatorial effects of a set of genetic factors in the erythrocyte-immunology pathway might be relevant to host resistance or susceptibility against Plasmodium infection. However, these results must be interpreted with caution because of the differences observed in the functionality and frequency of polymorphisms within different populations. With the recent advances in molecular biology techniques, more robust studies with reliable data have been reported, and the results of these studies have identified individual genetic factors for consideration in preventing severe disease and the individual response to treatment.

Elevated levels of IL-10 and G-CSF associated with asymptomatic malaria in pregnant women

In sub-Saharan Africa, approximately 30 million pregnant women are at risk of contracting malaria annually. Nearly 36% of healthy pregnant women receiving routine antenatal care tested positive for Plasmodium falciparum HRP-II antigen in Ghana. We tested the hypothesis that asymptomatic HRP II positive pregnant women expressed a unique Th1 and Th2 phenotype that differs from healthy controls. Plasma from healthy (n = 15) and asymptomatic (n = 25) pregnant women were evaluated for 27 biomarkers (IL-1b, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-12, IL-13, IL-15, IL- 17, Eotaxin, bFGF-2, G-CSF, GM-CSF, IFN-γ, IP-10, MCP-1, MIP-1α, MIP-1β, PDGF-bb, RANTES, TNF, and VEGF) associated with Th1 and Th2 cytokine homeostasis. IL-10 and G-CSF levels were elevated in the asymptomatic group when compared with the healthy group (P = .031 and .041, resp.). The median ratios of IL-1β:5, IL-1β:10, IL-1β:G-CSF, IL-1β:Eotaxin, IL-12:G-CSF, IL-15:10, IL-17:G-CSF, IL-17:Eotaxin, TNF:IL-4, TNF:IL-5, and TNF:G-CSF were significantly different among the two groups. Thus, asymptomatic malaria carriage may be linked to circulating levels of IL-10 and G-CSF.

Cerebrospinal fluid and serum biomarkers of cerebral malaria mortality in Ghanaian children

Background

Plasmodium falciparum can cause a diffuse encephalopathy known as cerebral malaria (CM), a major contributor to malaria associated mortality. Despite treatment, mortality due to CM can be as high as 30% while 10% of survivors of the disease may experience short- and long-term neurological complications. The pathogenesis of CM and other forms of severe malaria is multi-factorial and appear to involve cytokine and chemokine homeostasis, inflammation and vascular injury/repair. Identification of prognostic markers that can predict CM severity will enable development of better intervention.

Methods

Postmortem serum and cerebrospinal fluid (CSF) samples were obtained within 2–4 hours of death in Ghanaian children dying of CM, severe malarial anemia (SMA), and non-malarial (NM) causes. Serum and CSF levels of 36 different biomarkers (IL-1β, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-9, IL-10, IL-12 (p70), IL-13, IL-15, IL-17, Eotaxin, FGF basic protein, CRP, G-CSF, GM-CSF, IFN-γ, TNF-α, IP-10, MCP-1 (MCAF), MIP-1α, MIP-1β, RANTES, SDF-1α, CXCL11 (I-TAC), Fas-ligand [Fas-L], soluble Fas [sFas], sTNF-R1 (p55), sTNF-R2 (p75), MMP-9, TGF-β1, PDGF bb and VEGF) were measured and the results compared between the 3 groups.

Results

After Bonferroni adjustment for other biomarkers, IP-10 was the only serum biomarker independently associated with CM mortality when compared to SMA and NM deaths. Eight CSF biomarkers (IL-1ra, IL-8, IP-10, PDGFbb, MIP-1β, Fas-L, sTNF-R1, and sTNF-R2) were significantly elevated in CM mortality group when compared to SMA and NM deaths. Additionally, CSF IP-10/PDGFbb median ratio was statistically significantly higher in the CM group compared to SMA and NM groups.

Conclusion

The parasite-induced local cerebral dysregulation in the production of IP-10, 1L-8, MIP-1β, PDGFbb, IL-1ra, Fas-L, sTNF-R1, and sTNF-R2 may be involved in CM neuropathology, and their immunoassay may have potential utility in predicting mortality in CM.

The immune status of Kupffer cells profoundly influences their responses to infectiousPlasmodium berghei sporozoites

Multi-factorial immune mechanisms underlie protection induced with radiation-attenuated Plasmodia sporozoites (gamma-spz). Spz pass through Kupffer cells (KC) before invading hepatocytes but the involvement of KC in protection is poorly understood. In this study we investigated whether gamma-spz-immune KC respond to infectious spz in a manner that is distinct from the response of naive KC to infectious spz. KC were isolated from (1) naive, (2) spz-infected, (3) gamma-spz-immune, and (4) gamma-spz-immune-challenged C57BL/6 mice and examined for the expression of MHC class I and II, CD40 and CD80/CD86, IL-10 and IL-12 responses and antigen-presenting cell (APC) function. KC from gamma-spz-immune-challenged mice up-regulated class I and costimulatory molecules and produced elevated IL-12p40, relative to naive KC. In contrast, KC from naive mice exposed to infectious spz down-modulated class I and IL-12p40 was undetectable. Accordingly, KC from spz-infected mice had reduced APC function, while KC from gamma-spz-immune-challenged mice exhibited augmented APC activity. The nearly opposite responses are consistent with the fact that spz challenge of gamma-spz-immune mice results in long-lasting sterile protection, while infection of naive mice always results in malaria.

Inhibition of Plasmodium yoelii blood-stage malaria by interferon alpha through the inhibition of the production of its target cell, the reticulocyte

The effect of a recombinant hybrid human interferon alpha (IFN-alpha) (which cross-reacts with murine cells) on C57BL/6 mice infected with Plasmodium yoelii sporozoites or parasitized erythrocytes was determined. IFN-alpha did not inhibit the development of the parasite in the liver, but it did reduce the blood parasite load and the hepatosplenomegaly induced by the infection in mice injected with blood-stage parasites. The extent of anemia in IFN-alpha-treated and control mice was similar, despite the lower parasite load in the IFN-alpha-treated mice. The reduced blood parasite load in IFN-alpha-treated mice was associated with reduced erythropoiesis and reticulocytosis. As reticulocytes are the preferred target cells for the strain of P yoelii used (P yoelii yoelii 265 BY), it was postulated that the inhibition of reticulocytosis in IFN-alpha-treated mice was causally related to the observed decreased blood parasite load. This was supported by the finding that IFN-alpha inhibited a different strain of P yoelii (17X clone A), which also displays a tropism for reticulocytes, but not a line of Plasmodium vinckei petteri, which infects only mature red blood cells. As human malaria species also display different tropism for reticulocytes, these findings could be relevant for people coinfected with multiple Plasmodium species or strains or coinfected with Plasmodium and virus. (Blood. 2001;97:3966-3971)

Differential Th1/Th2 cytokine patterns in chronic arthritis: interferon gamma is highly expressed in synovium of rheumatoid arthritis compared with seronegative spondyloarthropathies

OBJECTIVE

To investigate possible differences in Th1 and Th2 cytokine mRNA expression in the synovial tissue (ST) of patients with rheumatoid arthritis (RA) and seronegative spondyloarthropathies (SpA) with diagnostic and/or pathogenic interest.

METHODS

Eleven RA patients and 14 SpA patients (10 with undifferentiated spondyloarthropathy (USpA), two with ankylosing spondylitis (AS) and two with psoriatic arthritis (PsA)) were included. Th1 (interferon gamma, interleukin 2) and Th2 (interleukin 4, interleukin 5 and interleukin 10) cytokine mRNA levels from arthritic knee ST were quantified by using an optimised polymerase chain reaction method with a computerised analysis system. Protein levels of proinflammatory cytokines (interleukin 1, tumour necrosis factor alpha and interleukin 6) in synovial fluid were quantified with a specific ELISA test.

RESULTS

Th1 cytokines were detected in all of RA ST samples in contrast with 58% (interferon gamma) and 71% (interleukin 2) of SpA samples. Th2 cytokines were expressed in 90% of RA ST samples, but the findings in SpA were interleukin 10 in 90%, interleukin 4 in 60% and interleukin 5 in 40% of ST samples. However, when the mRNA levels of each cytokine were quantified and corrected for T cell mRNA levels, only interferon gamma levels were significantly higher in RA than in SpA (p<0.003). Thus, the Th1/Th2 cytokine ratio in RA was fivefold that of SpA. Synovial fluid interleukin 1beta concentrations were higher in RA than in SpA (p<0. 05); there were also higher synovial fluid levels of tumour necrosis factor alpha in RA than in SpA, but without statistical significance.

CONCLUSION

This study has detected both Th1 and Th2 cytokine gene expression in ST from RA and SpA patients. Synovium interferon gamma mRNA levels and SF interleukin 1beta protein levels were significantly higher in RA than in SpA, so reflecting the known proinflammatory activity of interferon gamma through macrophage activation. Thus, the Th1 (interferon gamma)/Th2 (interleukin 4) ratio is significantly higher in RA than in SpA ST. These data confirm previous studies on ST Th1/Th2 balance in RA and extend previous work in comparing ST RA with subgroups of SpA distinct of ReA.

Expression and cytokine regulation of immune recognition elements by normal human biliary epithelial and established liver cell lines in vitro

BACKGROUND/AIMS

Biliary epithelial cells are targets of immune-mediated attack in conditions such as primary biliary cirrhosis and allograft rejection. This has been attributed to the ability of biliary epithelial cells to express ligands for T cell receptors. We aimed to investigate the expression of immune recognition elements and the effects of pro-inflammatory and anti-inflammatory cytokines on cell surface phenotypes of normal human biliary epithelial cells and established human liver-derived (PLC/PRF/5, HepG2, Hep3B and CC-SW) lines.

METHODS

Cells were cultured in the presence or absence of cytokines for 72 h, and expression of cell surface molecules was assessed by flow cytometry and immunofluorescence.

RESULTS

All cell lines expressed MHC class I, ICAM-1 (CD54), LFA-3 (CD58) and EGF receptor, and all but Hep3B expressed Fas/Apo-1 (CD95). Unlike hepatocyte-derived cell lines, biliary epithelial cells and CC-SW expressed CD40 and CD44. As expected, IFNgamma and TNFalpha upregulated expression of ICAM-1, MHC class I and MHC class II, particularly in biliary epithelial cells. TGFbeta downregulated these molecules and downregulated CD95 on biliary epithelial cells, but upregulated LFA-3. The Th2 cytokines had little effect, although IL-4 upregulated CD95 expression on biliary epithelial cells. IFNgamma upregulated CD40 expression on biliary epithelial cells, CC-SW and HepG2.

CONCLUSIONS

These findings imply that biliary epithelial cells may be capable of interacting with activated T lymphocytes via CD40 and LFA-3, which are thought to be important T cell accessory ligands for T cell activation in a B7-independent manner. Sensitivity to pro-inflammatory cytokines and expression of CD95 may explain why biliary epithelial cells are primary targets for autoimmune attack.

Susceptibility to Plasmodium berghei infection in rats is modulated by the acute phase response

Brown Norway (BN) and Sprague Dawley (SD) rats are known to differ in their susceptibility to infection with sporozoites of Plasmodium berghei, as measured by the density of liver schizonts. Because of the known inhibitory effect of non-specific immunomodulators on schizont development, we compared some aspects of the acute phase response in these two rat strains. LPS induced IL-6 production was measured in supernatants of spleen cells and peritoneal macrophages of both strains. SD rats, which are the least susceptible to P. berghei sporozoites, showed significantly higher IL-6 production by macrophages from both sources. When LPS was administered in vivo, SD rats also had a significantly higher IL-6 response. Hepatocytes from both strains were cultured in the presence of IL-6. After three days of culture, alpha 2-Macroglobulin concentrations in the supernatants of SD hepatocytes were much higher than those from BN rats. Kupffer cell depletion in both BN and SD rats was correlated with a significant increase in liver schizont density, but did not abrogate the difference in susceptibility. From these results we conclude that the higher cytokine production capacity of SD rats compared to BN rats, may contribute to the difference in susceptibility to P. berghei sporozoites between these strains, but that other yet unknown factors are also involved.

The role of Kupffer cells in the clearance of malaria sporozoites from the circulation

In the past decade, one of the most intriguing subjects in understanding the mechanism of malaria infection has been explanation of the role of Kupffer cells. These liver cells, which play an important part in the body's defense against infection, seemed to have on essential supportive role in the homing o f sporozoites. Do Kupffer cells favor the establishment of primary malaria infection? Extensive research has revealed much, but still not everything we need to know about the sporozoite-Kupffer cell affair.

Kupffer cell elimination enhances development of liver schizonts of Plasmodium berghei in rats

We investigated the development of exoerythrocytic forms (EEF) of Plasmodium berghei in livers of normal and macrophage-depleted Brown Norway rats. Macrophages were depleted by use of liposome-encapsulated dichloromethylene diphosphonate. Upon inoculation of sporozoites, macrophage-depleted rats had significantly larger numbers of EEF than untreated rats. We also investigated the effect of macrophage impairment by silica treatment on the development of EEF and confirmed that silica induces a significant reduction of EEF development. Intravenous administration of silica induced high levels of interleukin-6 in plasma within a few hours. The seemingly contradictory results for EEF development may be explained by our previous observation that interleukin-6 strongly inhibits sporozoite penetration and EEF development in vivo. We conclude that in experimental infections with sporozoites, Kupffer cells inhibit rather than enhance EEF development.