The spleen in malaria: the role of barrier cells

Plasmodium vivax tryptophan-rich antigen reduces type I collagen secretion via the NF-κBp65 pathway in splenic fibroblasts

BACKGROUND

The spleen plays a critical role in the immune response against malaria parasite infection, where splenic fibroblasts (SFs) are abundantly present and contribute to immune function by secreting type I collagen (collagen I). The protein family is characterized by Plasmodium vivax tryptophan-rich antigens (PvTRAgs), comprising 40 members. PvTRAg23 has been reported to bind to human SFs (HSFs) and affect collagen I levels. Given the role of type I collagen in splenic immune function, it is important to investigate the functions of the other members within the PvTRAg protein family.

METHODS

Protein structural prediction was conducted utilizing bioinformatics analysis tools and software. A total of 23 PvTRAgs were successfully expressed and purified using an Escherichia coli prokaryotic expression system, and the purified proteins were used for co-culture with HSFs. The collagen I levels and collagen-related signaling pathway protein levels were detected by immunoblotting, and the relative expression levels of inflammatory factors were determined by quantitative real-time PCR.

RESULTS

In silico analysis showed that P. vivax has 40 genes encoding the TRAg family. The C-terminal region of all PvTRAgs is characterized by the presence of a domain rich in tryptophan residues. A total of 23 recombinant PvTRAgs were successfully expressed and purified. Only five PvTRAgs (PvTRAg5, PvTRAg16, PvTRAg23, PvTRAg30, and PvTRAg32) mediated the activation of the NF-κBp65 signaling pathway, which resulted in the production of inflammatory molecules and ultimately a significant reduction in collagen I levels in HSFs.

CONCLUSIONS

Our research contributes to the expansion of knowledge regarding the functional role of PvTRAgs, while it also enhances our understanding of the immune evasion mechanisms utilized by parasites.

Dysfunction of CD169+ macrophages and blockage of erythrocyte maturation as a mechanism of anemia in Plasmodium yoelii infection

Plasmodium parasites cause malaria with disease outcomes ranging from mild illness to deadly complications such as severe malarial anemia (SMA), pulmonary edema, acute renal failure, and cerebral malaria. In young children, SMA often requires blood transfusion and is a major cause of hospitalization. Malaria parasite infection leads to the destruction of infected and noninfected erythrocytes as well as dyserythropoiesis; however, the mechanism of dyserythropoiesis accompanied by splenomegaly is not completely understood. Using Plasmodium yoelii yoelii 17XNL as a model, we show that both a defect in erythroblastic island (EBI) macrophages in supporting red blood cell (RBC) maturation and the destruction of reticulocytes/RBCs by the parasites contribute to SMA and splenomegaly. After malaria parasite infection, the destruction of both infected and noninfected RBCs stimulates extramedullary erythropoiesis in mice. The continuous decline of RBCs stimulates active erythropoiesis and drives the expansion of EBIs in the spleen, contributing to splenomegaly. Phagocytosis of malaria parasites by macrophages in the bone marrow and spleen may alter their functional properties and abilities to support erythropoiesis, including reduced expression of the adherence molecule CD169 and inability to support erythroblast differentiation, particularly RBC maturation in vitro and in vivo. Therefore, macrophage dysfunction is a key mechanism contributing to SMA. Mitigating and/or alleviating the inhibition of RBC maturation may provide a treatment strategy for SMA.

Mouse Models for Unravelling Immunology of Blood Stage Malaria

Malaria comprises a spectrum of disease syndromes and the immune system is a major participant in malarial disease. This is particularly true in relation to the immune responses elicited against blood stages of Plasmodium-parasites that are responsible for the pathogenesis of infection. Mouse models of malaria are commonly used to dissect the immune mechanisms underlying disease. While no single mouse model of Plasmodium infection completely recapitulates all the features of malaria in humans, collectively the existing models are invaluable for defining the events that lead to the immunopathogenesis of malaria. Here we review the different mouse models of Plasmodium infection that are available, and highlight some of the main contributions these models have made with regards to identifying immune mechanisms of parasite control and the immunopathogenesis of malaria.

Protection and Alleviated Inflammation Induced by Virus-like Particle Vaccines Containing Plasmodium berghei MSP-8, MSP-9 and RAP1

Virus-like particles (VLP) are a highly efficient vaccine platform used to present multiple antigenic proteins. Merozoite surface protein 8 (MSP-8), 9 (MSP-9) and rhoptry-associated protein 1 (RAP1) of Plasmodium berghei are the important proteins in erythrocyte invasion and the replication of parasites. In this study, we generated three VLPs expressing MSP-8, MSP-9 or RAP1 together with influenza virus matrix protein M1 as a core protein, and the protection and alleviated inflammation induced by VLP immunization were investigated. Mice were immunized with a mixture of three VLPs, MSP-8, MSP-9 and RAP1, and challenge-infected with P. berghei. As a result, VLPs immunization elicited higher levels of P. berghei or VLPs-specific IgG antibody responses in the sera upon boost compared to that upon prime and naive. Upon challenge infection with P. berghei, higher levels of CD4+ T cell and memory B cell responses in the spleen were also found in VLPs-immunized mice compared to non-immunized control. Importantly, VLP immunization significantly alleviated inflammatory cytokine responses (TNF-α, IFN-γ) both in the sera and spleen. VLP vaccine immunization also assisted in diminishing the parasitic burden in the peripheral blood and prolonged the survival of immunized mice. These results indicated that a VLPs vaccine containing MSP-8, MSP-9 and RAP1 could be a vaccine candidate for P. berghei infection.

Plasmodium falciparum erythrocyte membrane protein 1 variants induce cell swelling and disrupt the blood-brain barrier in cerebral malaria

Cerebral malaria (CM) is caused by the binding of Plasmodium falciparum-infected erythrocytes (IEs) to the brain microvasculature, leading to inflammation, vessel occlusion, and cerebral swelling. We have previously linked dual intercellular adhesion molecule-1 (ICAM-1)- and endothelial protein C receptor (EPCR)-binding P. falciparum parasites to these symptoms, but the mechanism driving the pathogenesis has not been identified. Here, we used a 3D spheroid model of the blood-brain barrier (BBB) to determine unexpected new features of IEs expressing the dual-receptor binding PfEMP1 parasite proteins. Analysis of multiple parasite lines shows that IEs are taken up by brain endothelial cells in an ICAM-1-dependent manner, resulting in breakdown of the BBB and swelling of the endothelial cells. Via ex vivo analysis of postmortem tissue samples from CM patients, we confirmed the presence of parasites within brain endothelial cells. Importantly, this discovery points to parasite ingress into the brain endothelium as a contributing factor to the pathology of human CM.

The spleen: "epicenter" in malaria infection and immunity

The spleen is a complex secondary lymphoid organ that plays a crucial role in controlling blood‐stage infection with Plasmodium parasites. It is tasked with sensing and removing parasitized RBCs, erythropoiesis, the activation and differentiation of adaptive immune cells, and the development of protective immunity, all in the face of an intense inflammatory environment. This paper describes how these processes are regulated following infection and recognizes the gaps in our current knowledge, highlighting recent insights from human infections and mouse models.

The pathology of the spleen in lethal canine babesiosis caused by Babesia rossi

To provide useful information based on the macropathology, histopathology and immunohistochemical investigation in the spleens of dogs with Babesia rossi infection. Control spleens were collected from four healthy dogs euthanized for welfare reasons. Nine dogs that died naturally because of a mono-infection with Babesia rossi were selected for the diseased group. One haematoxylin-and-eosin-stained section of splenic tissue from each of the infected and control dogs was examined under the light microscope. Immunohistochemical markers were applied to characterize different immunocyte populations. The application of analytic software enabled semi-quantitative comparison of leucocyte subpopulations. Routine splenic histopathology revealed diffuse intermingling of white and red pulp from infected dogs with a clear loss of distinction between these zones. Immunohistochemistry revealed an increase in the proportion of tissue resident and bone marrow origin macrophages in the infected spleens. Apart from a few remnant lymphocytes within the peri-arteriolar lymphatic sheaths and follicles, the majority of the immunocytes redistributed to the red pulp, supporting the observation of white and red pulp intermingling. The majority of our findings are in agreement with histomorphological descriptions of the spleen in a variety of noncanid mammalian hosts with lethal malaria or babesiosis.

Plasmodium falciparum malaria and invasive bacterial co-infection in young African children: the dysfunctional spleen hypothesis

Children with recent or acute malaria episodes are at increased risk of invasive bacterial infections (IBI). However, the exact nature of the malaria-IBI association is still unclear. Young children have an age-related spleen immunologic immaturity, mainly due to the still ongoing development of the marginal zone (MZ) B cell subset. By mounting a rapid antibody response against encapsulated bacteria, these cells are critical for the defence against highly pathogenic microorganisms that do not elicit classical T cell-dependent responses. There is increasing evidence that the anatomy of the spleen becomes disorganized during malaria infection, with complete dissolution of the MZ and apoptosis of MZ B cells. Correspondingly, a reduction in the frequency of the peripheral equivalent of the MZ B cells has been found in malaria endemic areas. A remarkable similarity exists in IBI susceptibility between African children with malaria and hyposplenic or splenectomized patients. However, studies specifically assessing the immune function of the spleen in controlling bacterial infections in young children with malaria are scarce.

Here, it is hypothesized that Plasmodium falciparum malaria infection constitutes a detrimental factor in the still immature spleen function of young children, resulting in a factually hyposplenic state during malaria episodes, putting children with malaria at a high risk to develop life-threatening bacterial infections. Studies to confirm or reject this hypothesis are greatly needed, as well as the development of affordable and feasible tools to assess the immune spleen function against encapsulated bacteria in children with malaria.

Role of chitotriosidase (chitinase 1) under normal and disease conditions

Mammalian chitinases belong to the glycosyl hydrolase 18 family based on structural homology and the family includes a large number of bacterial and eukaryotic chitinases. Among the mammalian chitinases, chitotriosidase (CHIT1) and acidic mammalian chitinase (AMCase) are capable of hydrolyzing the β-(1, 4)-linkage between the adjacent N-acetyl glucosamine residues of chitin. CHIT1 is one of the most abundantly secreted proteins, being mainly produced by activated macrophages and epithelial cells. CHIT1 plays a pivotal role in the context of infectious disease including malaria and fungi infections as a host defense towards chitin in pathogen's cell structure and as a diagnostic marker of disease. In contrast, CHI1 released by activated Kupffer cells in liver could induce hepatic fibrosis and cirrhosis. Increased serum levels of CHIT1 were observed in patients with many disorders, including Gaucher's disease, bronchial asthma, and atherosclerosis. Therefore, CHIT1 seems to have dual (regulatory and pathogenic) roles depending on the disease and producing cell types during the inflammatory conditions.

Hemozoin (malarial pigment) directly promotes apoptosis of erythroid precursors

Severe malarial anemia is the most common syndrome of severe malaria in endemic areas. The pathophysiology of chronic malaria is characterised by a striking degree of abnormal development of erythroid precursors (dyserythropoiesis) and an inadequate erythropoietic response in spite of elevated levels of erythropoietin. The cause of dyserythropoiesis is unclear although it has been suggested that bone-marrow macrophages release cytokines, chemokines or lipo-peroxides after exposure to hemozoin, a crystalloid form of undigested heme moieties from malarial infected erythrocytes, and so inhibit erythropoiesis. However, we have previously shown that hemozoin may directly inhibit erythroid development in vitro and the levels of hemozoin in plasma from patients with malarial anemia and hemozoin within the bone marrow was associated with reduced reticulocyte response. We hypothesized that macrophages may reduce, not enhance, the inhibitory effect of hemozoin on erythropoiesis. In an in vitro model of erythropoiesis, we now show that inhibition of erythroid cell development by hemozoin isolated from P. falciparum is characterised by delayed expression of the erythroid markers and increased apoptosis of progenitor cells. Crucially, macrophages appear to protect erythroid cells from hemozoin, consistent with a direct contribution of hemozoin to the depression of reticulocyte output from the bone marrow in children with malarial anemia. Moreover, hemozoin isolated from P. falciparum in vitro inhibits erythroid development independently of inflammatory mediators by inducing apoptotic pathways that not only involve activation of caspase 8 and cleavage of caspase 3 but also loss of mitochondrial potential. Taken together these data are consistent with a direct effect of hemozoin in inducing apoptosis in developing erythroid cells in malarial anemia. Accumulation of hemozoin in the bone marrow could therefore result in inadequate reticulocytosis in children that have adequate levels of circulating erythropoietin.

Fibroblastic reticular cell infection by hemorrhagic fever viruses

Viral hemorrhagic fevers (VHFs) often cause high mortality with high infectivity, multiorgan failure, shock and hemorrhagic diathesis. Fibroblastic reticular cells (FRCs) within secondary lymphoid organs provide a supporting scaffold to T-lymphocyte areas. These cells regulate the movement of various immune cells and soluble molecules that promote T-lymphocyte homeostasis. We previously reported Ebola virus infection of FRCs, but ascribed little significance to this finding. Here, we studied infection of FRCs by Ebola, Marburg and Lassa viruses. We demonstrate that FRCs, or the extracellular ‘conduit’ of the fibroblastic reticulum of nonhuman primates, are targets of Ebola, Marburg and Lassa viruses. Furthermore, we observed that FRC damage correlates temporally and spatially with lymphocyte damage and that FRCs serve as nidi of fibrin deposition. In addition, we show that nonhuman primate FRCs express p75 NGF receptor and tissue transglutaminase. Our data suggest that viral infection of FRCs may be crucial to the immunological dysfunction and coagulopathy characteristic of VHFs. We further propose that p75 NGF receptor and tissue transglutaminase may be involved in FRC-associated dysfunction during the course of infection.

Microfluidic approaches to malaria pathogenesis

Malaria is a major poverty-related human infectious disease of the world. Over a billion individuals are under threat and several million die from malaria every year. The nature of disease, especially fatal disease, has been the subject of many studies. The consensus is that parasite-induced cytoadherance of red blood cells precipitates capillary blockage and inflammatory responses in affected organs. Reduced deformability of infected erythrocytes may also contribute to disease. What is not very clear is why people with significant parasite burdens display large variations in disease outcomes. Technologies which allow a detailed description of the cytoadherance properties of infected erythrocytes in individual patients, and which allow a complete description of the flow capabilities of red blood cell populations in that patient, would be very useful. Here we review the recent introduction of microfluidic technology to study malaria pathogenesis, including the fabrication processes. The devices are cheap, versatile, portable and require very small patient samples. With greater use in research laboratories and field sites, we eventually expect microfluidic methods to play important roles in malaria diagnosis, as well as prognosis.

Structural insight into epitopes in the pregnancy-associated malaria protein VAR2CSA

Pregnancy-associated malaria is caused by Plasmodium falciparum malaria parasites binding specifically to chondroitin sulfate A in the placenta. This sequestration of parasites is a major cause of low birth weight in infants and anemia in the mothers. VAR2CSA, a polymorphic multi-domain protein of the PfEMP1 family, is the main parasite ligand for CSA binding, and identification of protective antibody epitopes is essential for VAR2CSA vaccine development. Attempts to determine the crystallographic structures of VAR2CSA or its domains have not been successful yet. In this study, we propose 3D models for each of the VAR2CSA DBL domains and we show that regions in the fold of VAR2CSA inter-domain 2 and a PfEMP1 CIDR domain seem to be homologous to the EBA-175 and Pkα-DBL fold. This suggests that ID2 could be a functional domain. We also identify regions of VAR2CSA present on the surface of native VAR2CSA by comparing reactivity of plasma containing anti-VAR2CSA antibodies in peptide array experiments before and after incubation with native VAR2CSA. By this method we identify conserved VAR2CSA regions targeted by antibodies that react with the native molecule expressed on infected erythrocytes. By mapping the data onto the DBL models we present evidence suggesting that the S1+S2 DBL sub-domains are generally surface-exposed in most domains, whereas the S3 sub-domains are less exposed in native VAR2CSA. These results comprise an important step towards understanding the structure of VAR2CSA on the surface of CSA-binding infected erythrocytes.

Individuals living in areas with high Plasmodium falciparum transmission acquire immunity to malaria over time and adults have markedly reduced risk of getting severe disease. However, pregnant women constitute an important exception, and they become more susceptible to malaria during pregnancy. This so called pregnancy-associated malaria (PAM) has severe consequences for both mother and child, and a vaccine would save hundreds of thousands of lives each year. PAM is caused by P. falciparum–infected red blood cells that bind to receptors in the placenta. By binding to the placental tissue, the parasites avoid being filtered though the spleen where they would have been killed. The protein mediating this placental binding is a very large multidomain and variant protein named VAR2CSA. Using structural modeling of VAR2CSA and antibody reagents from women who have had PAM, we show that antibodies tend to bind in similar regions, on one side of the individual VAR2CSA domains. In addition, we show that highly conserved parts of this variant protein are accessible for antibodies. This finding correlates with epidemiological data showing that woman acquire immunity towards PAM relatively fast, and the identification of these epitopes is thus a major step towards a protective vaccine.

Mobilization of endothelial progenitor cells in fracture healing and distraction osteogenesis

INTRODUCTION

Fracture healing and distraction osteogenesis (DO) are unique postnatal bone formation processes, and neovascularization is critically required for successful bone regeneration. We investigated endothelial progenitor cell (EPC) mobilization during bone regeneration, and the possible contribution of EPCs to increased vascularization and new bone formation, especially in DO.

METHODS

Mouse tibia fracture and rat tibia DO models were used in this study. The proportion of EPCs among the peripheral and splenic mononuclear cells (MNCs) was determined by examining the endothelial lineage staining characteristics and EPC cell surface markers. Messenger RNA expression of molecules related to EPC mobilization and homing at the fracture site were analyzed by ribonuclease protection assay and reverse-transcription polymerase chain reaction. In the rat tibia DO model, we measured blood flow during DO, and determined the distribution of ex vivo-expanded and intravenously-infused EPCs.

RESULTS

The proportion of EPCs among the peripheral and splenic MNCs increased after fracture, peaked on post-fracture day 3, and returned to basal levels during the healing period. Messenger RNA expression of EPC mobilizing cytokines such as vascular endothelial growth factor (VEGF), stem cell factor, monocyte chemoattractant protein-1, and stromal cell-derived factor-1, were upregulated at the fracture callus. The plasma VEGF levels peaked prior to the increase in the EPC proportion. Adhesion molecules involved in EPC homing were expressed at the fracture callus. In the DO model, the temporal pattern of the increase in the EPC proportion was similar to that in the fracture healing model, but the EPC proportion increased again during the distraction and consolidation phases. The distraction gap was relatively ischemic during the distraction phase and blood flow increased profusely later in the consolidation phase. The number of EPCs homing to the bone regeneration site in the DO model correlated with the number of transplanted EPCs in a dose-dependent manner.

CONCLUSIONS

These findings suggest that signals from the bone regeneration site mobilize EPCs from the bone marrow into the peripheral circulation. Increased EPC mobilization and homing may contribute to neovascularization and thus to new bone formation in fracture healing and DO.

Plasmodium berghei: parasite clearance after treatment with dihydroartemisinin in an asplenic murine malaria model

Clinical reports indicate that malaria-infected asplenic patients have a reduced capacity for parasite clearance despite intensive antimalarial therapy. The aim of this study was to evaluate the efficacy of dihydroartemisinin in an asplenic murine malaria model. Mice were inoculated with Plasmodium berghei parasitised erythrocytes and received a single dose of dihydroartemisinin 56 h later, at 2-5% parasitaemia. Haematology, liver biochemistry and histopathology of key organs were performed to evaluate organ response to malaria infection. The nadir parasitaemia occurred 20 h after dihydroartemisinin administration, falling 2.8- to 6.0-fold and 2.7- to 6.9-fold in asplenic and intact mice, respectively, (10-100 mg/kg). Histopathology indicated increased stimulation of liver function/activity during malaria infection of asplenic mice (as compared to intact mice). Overall efficacy of single-dose dihydroartemisinin treatment in asplenic mice was similar to intact mice although the rate of recrudescence in asplenic mice was significantly greater than intact mice at 30 and 100 mg/kg. The asplenic murine malaria model could be used in pre-clinical studies of splenic function and clearance of malaria parasites, pathophysiological studies or antimalarial drug efficacy in asplenia.

New insights into the cell biology of the marginal zone of the spleen

In the marginal zone of the spleen the bloodstream passes through an open system of reticular cells and fibers in which various myeloid and lymphoid cells are located. Macrophages in this region are well equipped to recognize pathogens and filter the blood by virtue of unique combinations of pattern recognition receptors. They interact with a specific set of B cells that can be found only in the marginal zone and that are able to react rapidly to bacterial antigens in particular. This combination of strategically located cells is an important factor in our defense against blood-borne pathogens. New data on the development of the marginal zone itself and the marginal zone B cells are reviewed and discussed in light of the function of the spleen in host defense.

Testosterone responsiveness of spleen and liver in female lymphotoxin β receptor-deficient mice resistant to blood-stage malaria

Disrupted signaling through lymphotoxin beta receptor (LTbetaR) results in severe defects of the spleen and even loss of all other secondary lymphoid tissues, making mice susceptible to diverse infectious agents. Surprisingly, however, we find that female LTbetaR-deficient mice are even more resistant to blood stages of Plasmodium chabaudi malaria than wild-type C57BL/6 mice. Higher resistance of LTbetaR-deficient mice correlates with an earlier onset of reticulocytosis, and the period of anemia is shorter. After surviving fulminant parasitemias of about 35%, mice develop long-lasting protective immunity against homologous rechallenge, with both spleen and liver acting as anti-malaria effectors. Testosterone suppresses resistance, i.e. all mice succumb to infections during or shortly after peak parasitemia. At peak parasitemia, testosterone does not essentially affect cellularity and apoptosis in the spleen, but aggravates liver pathology in terms of increased cell swelling, numbers of apoptotic and binucleated cells and reduced serum alkaline phosphatase levels, and conversely, reduces inflammatory lymphocytic infiltrates in the liver. In the spleen, hybridization of cDNA arrays identified only a few testosterone-induced changes in gene expression, in particular upregulation of INFgamma and IFN-regulated genes. By contrast, a much larger number of testosterone-affectable genes was observed in the liver, including genes involved in regulation of the extracellular matrix, in chemokine and cytokine signaling, and in cell cycle control. Collectively, our data suggest that testosterone dysregulates the inflammatory response in spleen and liver during their differentiation to anti-malaria effectors in malaria-resistant female LTbetaR-deficient mice, thus contributing to the testosterone-induced lethal outcome of malaria.

Variant genes and the spleen in Plasmodium vivax malaria

It is generally accepted that Plasmodium vivax, the most widely distributed human malaria, does not cytoadhere in the deep capillaries of inner organs and thus this malaria parasite must have evolved splenic evasion mechanism in addition to sequestration. The spleen is a uniquely adapted lymphoid organ whose central function is the selective clearance of cell and other particles from the blood, and microbes including malaria. Splenomegaly is a hallmark of malaria and no other disease seems to exacerbate this organ as this disease does. Besides this major selective clearance function however, the spleen is also an erythropoietic organ which, under stress conditions, can be responsible for close to 40% of the RBC populations. Data obtained in experimental infections of human patients with P. vivax showed that anaemia is associated with acute and chronic infections and it has been postulated that the continued parasitemia might have been sufficient to infect and destroy most circulating reticulocytes. We review here the basis of our current knowledge of variant genes in P. vivax and the structure and function of the spleen during malaria. Based on this data, we propose that P. vivax specifically adhere to barrier cells in the human spleen allowing the parasite to escape spleen-clearance while favouring the release of merozoites in an environment where reticulocytes, the predominant, if not exclusive, host cell of P. vivax, are stored before their release into circulation to compensate for the anaemia associated with vivax malaria.

Testosterone suppresses protective responses of the liver to blood-stage malaria

Testosterone induces a lethal outcome in otherwise self-healing blood-stage malaria caused by Plasmodium chabaudi. Here, we examine possible testosterone effects on the antimalaria effectors spleen and liver in female C57BL/6 mice. Self-healing malaria activates gating mechanisms in the spleen and liver that lead to a dramatic reduction in trapping activity, as measured by quantifying the uptake of 3-mum-diameter fluorescent polystyrol particles. However, testosterone delays malaria-induced closing of the liver, but not the spleen. Coincidently, testosterone causes an approximately 3- to 28-fold depression of the mRNA levels of nine malaria-responsive genes, out of 299 genes tested, only in the liver and not in the spleen, as shown by cDNA arrays and Northern blotting. Among these are the genes encoding plasminogen activator inhibitor (PAI1) and hydroxysteroid sulfotransferase (STA2). STA2, which detoxifies bile acids, is suppressed 10-fold by malaria and an additional 28-fold by testosterone, suggesting a severe perturbation of bile acid metabolism. PAI1 is protective against malaria, since disruption of the PAI1 gene results in partial loss of the ability to control the course of P. chabaudi infections. Collectively, our data indicate that the liver rather than the spleen is a major target organ for testosterone-mediated suppression of resistance against blood-stage malaria.

Early treatment during a primary malaria infection modifies the development of cross immunity

We have used a murine model to study the kinetics of cross-protection when a primary infection is halted at different times. We analysed how parasitaemia is modified during a second infection with the homologous parasite, a heterologous parasite, or a mixture of the two. In addition, possible mechanisms involved in cross-protection were analysed. Results show that treatment with pyrimethamine on day 5 during a primary infection with P. chabaudi AS (non-lethal), prevents the generation of cross-protection to a new challenge with lethal P. yoelii 17XL. In contrast, when treatment is on day 7, mice survive a P. yoelii infection. Differences between both groups suggest that in order for 'preimmune' mice to survive a lethal challenge, a predominantly TH2-type response is required, with a higher mRNA expression level of IL-4 and IL-10, and a lower mRNA expression of IFN-gamma. This work shows that an early treatment of a malaria infection produced by a non-lethal parasite drives the immune response towards a loss of cross-protection to further infections, in particular with more virulent parasites. This finding should be taken into account for the development of effective malaria vaccines.

Plasmodium chabaudi chabaudi infection in mice induces strong B cell responses and striking but temporary changes in splenic cell distribution

B cells and Abs play a key role in controlling the erythrocytic stage of malaria. However, little is known about the way the humoral response develops during infection. We show that Plasmodium chabaudi chabaudi causes major, but temporary changes in the distribution of leukocytes in the spleen. Despite these changes, an ordered response to infection develops, which includes vigorous extrafollicular growth of plasmablasts and germinal center formation. Early in the response, the lymphocytes in the T zone and follicles become widely spaced, and the edges of these compartments blur. This effect is maximal around the peak of parasitemia. Germinal centers are apparent by day 8, peak at day 20, and persist through day 60. Extrafollicular foci of plasmablasts are visible from day 4 and initiate a very strong plasma cell response. Initially, the plasma cells have a conventional red pulp distribution, but by day 10 they are unconventionally sited in the periarteriolar region of the white pulp. In this region they form clusters occupying part of the area normally filled by T cells. B cells are absent from the marginal zone for at least 30 days after the peak of infection, although flow cytometry shows their continued presence in the spleen throughout infection. Relatively normal splenic architecture is regained by day 60 of infection. These results show that the changes in splenic cell distribution are linked to the presence of parasites and do not seem to interfere with the development of the humoral response.

Reproducible infection of intact Aotus lemurinus griseimembra monkeys by Plasmodium falciparum sporozoite inoculation

Aotus lemurinus griseimembra is considered one of the best nonhuman primate species for malarial studies because of its susceptibility to infection by Plasmodium falciparum asexual blood stages. However, reproducible transmission of infective P. falciparum sporozoites by mosquito inoculation has been difficult to achieve even in splenectomized monkeys. Characterization of an Aotus-P. falciparum cyclical transmission model has become a top priority as a result of the significant progress toward the development of preerythrocytic malaria vaccines. Herein, we describe a reproducible model developed using intact A. lemurinus griseimembra monkeys intravenously inoculated with sporozoites from a monkey-adapted P. falciparum (Santa Lucia) strain and a wild Falciparum-Cali-Colombia-4 (FCC-4) strain. Sporozoites were obtained by salivary gland dissection of laboratory-reared Anopheles albimanus mosquitoes. Parasitemia was monitored by thick-smear microscopy, parasite lactate dehydrogenase (pLDH) determination, and mosquito xenodiagnosis. The last method proved to be the most sensitive method for monitoring parasitemias. Infection with the Santa Lucia strain showed a mean prepatent period of 16 days (range 6-21 days), whereas infection with the wild FCC-4 strain resulted in a 24-day prepatent period. Mean peak parasite density was approximately 900 parasites/microliter for both parasite strains. The prepatent period, the peak of parasitemia, and the duration of patency were independent of the size of the sporozoite inoculum and the presence of spleen in the host. This model is being successfully used to test the protective efficacy of P. falciparum preerythrocytic vaccine candidates.

Stromal cell derived factor 1 synthesis by spleen cells in rodent malaria, and the effects of in vivo supplementation of SDF-1alpha and CXCR4 receptor blocker

The mechanisms of malaria parasite clearance in the host are not well understood, but are ascribed to the intact spleen, the site for parasite clearance. The infection induces a huge increase in spleen volume and cellularity. There is, however, a lack of studies on the splenic production of chemokines, which are small proteins that control homing and activation of immune cells and must be crucial for organized tissue growth. We studied the spleen cell production of SDF-1, a primordial chemokine of the CXCL12 class, through mRNA Reverse transcriptase and polymerase chain reaction of both isoforms, alpha and beta, in lethal (Plasmodium berghei ANKA) and non-lethal recrudescent malaria (Plasmodium chabaudi CR) in BALB/c and C57BL/6 mouse strains. In non-lethal P. chabaudi malaria in C57BL/6 mice, SDF-1alpha mRNA production clearly peaked before the control of parasitemia, a fact not observed in the same mouse strain infected with lethal P. berghei, when this production was lower and without peaks. The infection of BALB/c mice infected with the same Plasmodium species led to a similar evolution of parasitemia and also chemokine production, albeit at lower levels. SDF-1beta synthesis was more constant and regular during both infections, presenting some variation but usually occurring at all the experimental times. Supplementation of lethal models with SDF-1alpha i.p., at the time when endogenous stromal cell chemokine production peaked in non-lethal models, induced a clear reduction in parasitemia, probably with prolonged host survival. Blocking SDF-1 action by administration of T-140, a CXCR4 receptor blocker, caused an increase in circulating parasites in the usually benign non-lethal P. chabaudi malaria in C57BL/6 mice, mainly at recrudescence of parasitemia. These data suggest that SDF-1alpha production in the spleen plays an important role in rodent malaria, and its supplementation was found to partially correct defects in the control of malaria in lethal models.

Cerebral malaria in mice: interleukin-2 treatment induces accumulation of gammadelta T cells in the brain and alters resistant mice to susceptible-like phenotype

In this study, we report that infection with Plasmodium yoelii 17XL, a lethal strain of rodent malaria, does not result in death in the DBA/2 strain of mice. In contrast to BALB/c mice, DBA/2 mice developed significantly less parasitemia and never manifested symptoms of cerebral malaria (CM) on infection with this parasite. Moreover, the histological changes evident in the brain of susceptible BALB/c were absent in DBA/2 mice. Interestingly, the resistant DBA/2 mice when treated with recombinant interleukin (IL)-2, were found to develop CM symptoms and the infection became fatal by 6 to 8 days after infection. This condition was associated with an augmented interferon-gamma and nitric oxide production. Unexpectedly, IL-10 levels were also elevated in IL-2-treated DBA/2 mice during late stage of infection (at day 6 of infection) whereas the inverse relationship between IL-10 and interferon-gamma or nitric oxide was maintained in the early stage of infection (at day 3 after infection). The level of tumor necrosis factor-alpha production was moderately increased in the late phase of infection in these mice. Histology of brain from IL-2-treated mice demonstrated the presence of parasitized erythrocytes and infiltration of lymphocytes in cerebral vessels, and also displayed some signs of endothelial degeneration. Confocal microscopical studies demonstrated preferential accumulation of gammadelta T cells in the cerebral vessels of IL-2-treated and -infected mice but not in mice treated with IL-2 alone. The cells recruited in the brain were activated because they demonstrated expression of CD25 (IL-2R) and CD54 (intercellular adhesion molecule 1) molecules. Administration of anti-gammadelta mAb prevented development of CM in IL-2-treated mice until day 18 after infection whereas mice treated with control antibody showed CM symptoms by day 6 after infection. The information concerning creating pathological sequelae and death in an otherwise resistant mouse strain provides an interesting focus for the burden of pathological attributes on death in an infectious disease.

Granulocyte-macrophage colony-stimulating factor-deficient mice have impaired resistance to blood-stage malaria

The contribution of granulocyte-macrophage colony-stimulating factor (GM-CSF), a hematopoietic and immunoregulatory cytokine, to resistance to blood-stage malaria was investigated by infecting GM-CSF-deficient (knockout [KO]) mice with Plasmodium chabaudi AS. KO mice were more susceptible to infection than wild-type (WT) mice, as evidenced by higher peak parasitemia, recurrent recrudescent parasitemia, and high mortality. P. chabaudi AS-infected KO mice had impaired splenomegaly and lower leukocytosis but equivalent levels of anemia compared to infected WT mice. Both bone marrow and splenic erythropoiesis were normal in infected KO mice. However, granulocyte-macrophage colony formation was significantly decreased in these tissues of uninfected and infected KO mice, and the numbers of macrophages in the spleen and peritoneal cavity were significantly lower than in infected WT mice. Serum levels of gamma interferon (IFN-gamma) were found to be significantly higher in uninfected KO mice, and the level of this cytokine was not increased during infection. In contrast, IFN-gamma levels were significantly above normal levels in infected WT mice. During infection, tumor necrosis factor alpha (TNF-alpha) levels were significantly increased in KO mice and were significantly higher than TNF-alpha levels in infected WT mice. Our results indicate that GM-CSF contributes to resistance to P. chabaudi AS infection and that it is involved in the development of splenomegaly, leukocytosis, and granulocyte-macrophage hematopoiesis. GM-CSF may also regulate IFN-gamma and TNF-alpha production and activity in response to infection. The abnormal responses seen in infected KO mice may be due to the lack of GM-CSF during development, to the lack of GM-CSF in the infected mature mice, or to both.

Spleen of Dasypus hybridus (Mammalia, dasypodidae): a light and electron microscopic study

Armadillos are relictual mammals important as models for biomedical studies. They contain adaptative and primitive characteristics in both anatomical and physiological aspects. In this study we describe the splenic histology and cytology of the "mulita," Dasypus hybridus. Organ samples were processed for light and electron microscopy study. The microanatomy of the organ samples as well as their different cell types are described. The spleen is non-sinusoidal, with the typical arrangement for storage functions. White pulp is lightly diffuse. Red pulp is a meshwork of circulating, immunocompetent and hemopoietic cells. Differences with other studied members of the group are discussed. The general structure of the organ agrees with the semi-fossorial habit of the species. Persistence of myeloid activity in the adult suggests the existence of specific inductive functions of the stroma. Better knowledge of this fact may give further insight on the phylogeny of hemopoiesis.

Murine gamma delta T lymphocytes elicited during Plasmodium yoelii infection respond to Plasmodium heat shock proteins

gamma delta T cells accumulate during Plasmodium infections in both murine and human malarias. The biological role of these cells and the antigens that they recognize are not clearly understood, although recent findings indicate that gamma delta T cells in general influence both innate and antigen-specific adaptive host responses. We examined the accumulation of gamma delta T cells elicited during infection with virulent and avirulent Plasmodium yoelii parasites in relatively susceptible and resistant strains of mice. Our results indicated that in nonlethal malaria infections, gamma delta T cells comprise a larger proportion of splenic T cells than in lethal infections and that only a live infection is capable of inducing an increase in the percentage of gamma delta T cells in vivo. Furthermore, we demonstrate that gamma delta T cells elicited during a P. yoelii infection respond by proliferation in vitro to P. falciparum heat shock proteins (HSPs) of 60 and 70 kDa, suggesting a possible immunological involvement of parasite HSPs in this arm of the cellular immune response during malarial infection in mice.

Neurobiology of cerebral malaria and African sleeping sickness

This review is aimed at emphasizing the need for basic neuroscience research on two tropical diseases, malaria and sleeping sickness (African trypanosomiasis), that still represent major health problems and in which severe involvement of the nervous system is frequently the direct cause of death. The life cycles of the two parasites, the protozoan Plasmodium and Trypanosoma brucei, which are the causative agents of malaria and sleeping sickness, respectively, are briefly reviewed. The historical contribution to the pathogenesis and therapy of malaria by a renowned pioneer in neuroscience, Camillo Golgi, is pointed out. The different strategies for survival in the host by the intracellular Plasmodium and the extracellular African trypanosomes are summarized; such strategies include sites favorable for hiding or replication of the parasites in the host, antigenic variation, and interactions with the cytokine network of the host. In particular, tumor necrosis factor-alpha and interferon-gamma may play a role in these infections. The parasites may paradoxically interact with cytokines to their benefit. However, cytokine receptors are expressed on neuronal subsets sensitive to cytokine action, and stimulation of these subsets may cause neuronal dysfunctions during the infections. Finally, the clinical symptoms of cerebral malaria and African trypanosomiasis and research aiming at deciphering their pathogenetic mechanisms that could affect the nervous system at a molecular level are described. The need for neuroscientists in this endeavor is emphasized.

Novel gene expressed in spleen cells mediating acquired testosterone-resistant immunity to Plasmodium chabaudi malaria

We report the identification of a novel mouse cDNA encoding IAP38, a putative plasma membrane protein of 38 kDa in splenic macrophages, B cells and T cells. The expression of iap38 is induced by blood-stage infections of Plasmodium chabaudi malaria and is testosterone-sensitive in non-immune mice. However, when mice have acquired testosterone-resistant immunity to P. chabaudi, there is an about 40-fold increase in the expression of iap38, which has then largely lost its responsiveness to infection and testosterone. The gene iap38 is suggested to be involved in imparting spleen cells the ability to mediate testosterone-resistant immunity to P. chabaudi malaria.

Does the mechanism of protection from falciparum malaria by red cell genetic disorders involve a switch to a balanced TH1/TH2 cytokine production mode?

The mechanism of protection from falciparum malaria by red cell genetic disorders still remains controversial. Decreased survival of parasites in variant red cells has previously been proposed. However, in vitro experiments were not conclusive and do not seem sufficient to explain the substantial degree of in vivo protection afforded to red cell genetic trait carriers. Evidence has recently been accumulating in favour of enhancement of the host immune response by these genetic traits. Malaria-infected variant red cells undergo modifications to their antigenicity which lead to accelerated and selective removal of early blood-stage parasites by splenic macrophages, resulting in fewer parasites reaching schizogony. Consequently there will be alterations in antigen processing, presentation and recognition which could explain the differences observed in T-cell responses between trait carriers and normal individuals. It is suggested that exposure to a lower dose of early parasite-stage antigens rather than the exoantigens of late mature schizonts could lead during primary and subsequent secondary infections to differentiation of T-helper cells into balanced TH1/TH2 subsets that promote protection, reversing the susceptibility to the fatal complications of falciparum malaria.

Trafficking of Plasmodium chabaudi adami-infected erythrocytes within the mouse spleen

Plasmodium chabaudi adami causes a nonlethal infection in mice. We found that crisis, the time of rapidly dropping parasitemia, was abrogated by splenectomy, indicating the role of spleen in parasite killing. The factors that mediate spleen-dependent immunity are not known. An earlier study in Plasmodium berghei-infected rats showed an association between increased clearance of heat-treated erythrocytes and the onset of crisis [Wyler, D. J., Quinn, T. C. & Chen, L.-T. (1982) J. Clin. Invest. 67, 1400-1404]. To determine the potential effects of different vascular beds in parasite killing, we studied the distribution of parasitized erythrocytes and bacteria in the spleens of P. chabaudi adami-infected mice during precrisis (a period of rising parasitemia) and during crisis. After intravenous injection, bacteria were localized predominantly in the marginal zone. In contrast, parasitized erythrocytes were found in the red pulp. We also found that during precrisis, a time of no immunity, the uptake of radiolabeled infected erythrocytes by the spleen was increased, not decreased. These data imply that no change occurs in the flow of parasitized erythrocytes through the spleen during the transition to an immune state (crisis). Our observations suggest that immune effector mechanisms, not circulatory changes, account for spleen-dependent parasite killing during a P. chabaudi adami infection in mice.

Tissue distribution and structure of barrier cells in the hematopoietic and lymphoid organs of salmonids

BACKGROUND

Barrier cells have been recognized as a discrete group of fibroblastic- or myofibroblastic-like cells located in the lymphoid and hematopoietic organs of mammals. This paper reports the results of a morphological study of the main lymphoid organs of three salmonid species, in which cells structurally similar to the mammalian barrier cells were observed in healthy animals.

METHODS

The spleen, kidney, and thymus of fingerlings of rainbow trout, Oncorhynchus mykiss, and Coho salmon, Oncorhynchus kisutch, and of adult brown trout, Salmo trutta fario, were processed for electron microscopy study using various fixation methods. Semithin sections were used for the Periodic Acid-Schiff (PAS) staining technique, and for the demonstration of the endogenous peroxidase activity.

RESULTS

The kidney and spleen of all the species contained a variable, but usually low, number of electron-dense, elongated, and branched cells, ultrastructurally similar to the mammalian barrier cells. They also occurred in the thymus of some brown trout and Coho salmon, but not of rainbow trout. The electron density of this cell type was present after the various types of fixation procedures. They show numerous ribosomes, well-developed secretory organelles, electron-clear vesicles, large granules, and microfilaments. In all the salmonid species, barriers cells were positive for PAS staining and for endogenous peroxidase activity. A small number of barrier cells were in mitosis. In the different organs barrier cells appeared as isolated cells, or forming syncytial networks. They were found lining the blood sinusoids of the splenic red pulp and of the renal hematopoietic tissue, in the periellipsoidal sheaths, and closely associated with erythropoietic and plasmacytopoietic foci.

CONCLUSIONS

Our results demonstrate the presence of cells closely resembling mammalian barrier cells in the hematopoietic and lymphoid organs of salmonids. The structure and tissue distribution of the salmonid barrier cells are discussed in relation to the functional roles described for this cell type in mammals.

Immunohistochemical characteristics of chicken spleen ellipsoids using newly established monoclonal antibodies

Ellipsoids, the extra-vasculature sites surrounding penicilliary capillaries of the chicken spleen, play critical roles in the immune response and also in the clearance of pathogens or other particles. The meshwork of ellipsoids is formed by fibroblastic reticular cells. To characterize ellipsoidal reticular cells, a series of monoclonal antibodies against the chicken spleen have been developed. Of these antibodies, CSA-1 antibody reacts with fibroblastic reticular cells in ellipsoids and with endothelial cells. The reticular nature of positive cells in ellipsoids is indicated by immuno-electron microscopy, and by double staining with anti-heat-shock protein 47 kDa (hsp47) antibody. The reaction of CSA-1 with reticular cells is limited in ellipsoids; CSA-1 does not react with reticular cells in other lymphoid organs. These findings indicate that ellipsoidal reticular cells share the antigen with endothelial cells. Ontogenic studies reveal that, on embryonic day 18, the development of ellipsoids is completed, penicilliary capillaries become fenestrated, and CSA-1 expression in ellipsoids begins. These findings suggest that CSA-1 is expressed on the cell surface of ellipsoidal reticular cells once they are exposed to blood flow.

PfEMP3 and HRP1: co-expressed genes localized to chromosome 2 of Plasmodium falciparum

A malarial protein, Plasmodium falciparum erythrocyte membrane protein 3 (PfEMP3), has been recently characterized as a high-molecular-mass component (approx. 315 kDa) localized to the erythrocyte membrane of knob-bearing (K+), cytoadherent (C+) mature stages of P. falciparum-parasitized erythrocytes (PE) [Pasloske et al., Mol. Biochem. Parasitol. 59 (1993) 59-72]. Knobless (K-), non-cytoadherent (C-) parasites of the same strain were shown to lack the PfEMP3 gene. In view of the biological importance of the knobby and cytoadherent phenotypes with regard to parasite virulence, we extended the analysis of PfEMP3 and its gene product to other K+/K- and C+/C- parasites. Previously, other studies have shown that the malarial protein, knob-associated histidine-rich protein 1 (HRP1), is also strongly correlated with knob expression. Here, we show that PfEMP3 and HRP1 were absent from all the K- parasites tested, including the Palo Alto (PA) K-C+ strain. This result demonstrates that PfEMP3 and HRP1 are not essential for cytoadherence. PfEMP3 was localized to chromosome 2 of the K+ parasites, within no more than 130 kb of HRP1, between the telomere and HRP1. Stage-specific analysis of the mRNA for HRP1 and PfEMP3 indicated maximal transcription of the genes in ring-stage parasites, with little or no mRNA present during the mature parasite stages. Analysis of PfEMP3 and HRP1 by immunofluorescence assay (IFA) revealed identical staining patterns of fixed PE at all stages of the asexual life cycle. Hence, PfEMP3 and HRP1 are adjacent to each other in chromosome 2, co-expressed temporally and their gene products co-localized to the PE membrane.

Malaria, the red cell, and the endothelium

Erythrocytes infected with mature stages of Plasmodium falciparum malaria adhere to vascular endothelial cells in postcapillary venules of several organs. In some patients, infected cells also form rosettes with uninfected erythrocytes. The special pathology of acute cerebral malaria appears to result from excessive adherence of infected cells in cerebral vessels coupled with occlusion of cerebral blood flow in microvessels by infected cell rosettes. Several endothelial cell proteins have been identified as potential receptors for infected erythrocyte adherence to vascular endothelium, including thrombospondin, CD36, intercellular adhesion molecule-1 (ICAM-1), vascular adhesion molecule-1 (VCAM-1), and endothelial leukocyte adhesion molecule-1 (ELAM-1). The receptor on infected erythrocytes that mediates adhesion to endothelial cells has been identified as a very large malarial protein on infected cells called PfEMP1. PfEMP1 has been shown to bind to CD36 and thrombospondin in vitro. Antibody-mediated blockade or reversal of infected erythrocyte adherence to vascular endothelium is postulated not only to decrease the pathology of blood-stage malaria, but also to lead to infected cell destruction and clearance, especially in the spleen. PfEMP1 is therefore a prime candidate malarial protein for inclusion in a multicomponent asexual malaria vaccine.

Predominance of infected reticulocytes in the peripheral blood of CD4+ T-cell-depleted mice chronically infected with Plasmodium chabaudi chabaudi

The distribution of Plasmodium chabaudi chabaudi AS among normocytes and reticulocytes in the peripheral blood of NIH mice undergoing a primary infection was determined from brilliant cresyl blue/Giemsa's stained thin blood films. During the early stages of infection in normal mice, parasites were found exclusively in normocytes. The presence of parasites in reticulocytes was limited to a period of severe immune pressure on the parasites, peak parasitaemia and the ensuing "crisis" phase, at which time the rapid production of new erythrocytes in response to the anaemia in these mice resulted in a high reticulocyte count. Later, during the recrudescence, parasites inhabited normocytes only. Thus, in immunologically competent animals, P. c. chabaudi AS showed no absolute preference for either mature or immature erythrocytes. In marked contrast, in chronically infected CD4+ T-cell-depleted mice, this malaria parasite apparently displayed a pronounced predilection for reticulocytes. During an unremitting parasitaemia of 2.8%-3.9% during days 13-60 post infection, all parasites were found in reticulocytes, even though these comprised only 25% of the total erythrocyte count. The possible reasons for this reversal in preference for the type of erythrocyte inhabited by asexual malaria parasites are discussed.

The importance of T cell homing and the spleen in reaching a balance between malaria immunity and immunopathology: the moulding of immunity by early exposure to cross-reactive organisms

It takes a number of years to develop clinical immunity to malaria and malaria pathology is also most evident a number of years after birth. T cells are known to play an important role in defence from malaria parasites but may also contribute to the disease symptoms associated with malaria. T cells which react against malaria parasites have arisen through stimulation with organisms which cross-react with malaria or through exposure to the malaria parasites themselves and express a memory phenotype (CD45Ro+, CD45Ra-, CD4+). T clones which have arisen through exposure to cross-reactive organisms may be expected to home to the tissues where initial exposure occurred as determined by tissue-specific adhesion molecules on the lymphocyte surface. Such tissues may not be appropriate to parasite killing and localization of T cells in such sites may contribute to the immunopathology of malaria. The sharp increase in immunity and decline in pathology observed in later childhood in malaria endemic areas may result from an increase in the number of T cells induced by the parasite itself (as opposed to cross-reactive organisms). Such T cells may not have a preferential trafficking to other organs and may be more likely to circulate through the spleen. Splenic changes may also allow more malaria-specific T cells to concentrate in the spleen and may facilitate interactions between T cells, monocytes, neutrophils and parasites resulting in parasite death. Whereas cytokines secreted by parasite-reactive T cells in all locations may contribute to cerebral malaria and other forms of pathology, cytokines in the spleen at least, should directly contribute to parasite death.

Malaria: becoming more specific about non-specific immunity

For the hundreds of millions of people presently infected with malaria, survival may depend on relatively non-specific immune effector mechanisms. Progress has been made in understanding the anti-parasitic properties of tumor necrosis factor-alpha, interferon-gamma and nitric oxide, in defining the parasite toxins that induce tumor necrosis factor-alpha production, and in exploring the role of cytokines and adhesion molecules in the pathogenesis of cerebral malaria.

Testosterone-unresponsiveness of existing immunity against Plasmodium chabaudi malaria

Testosterone (Te) is known to suppress immunity and to increase host susceptibility to many parasites. This study investigates the action of Te on immunity acquired against blood-stages of the malaria parasite Plasmodium chabaudi in female mice of the inbred strain C57BL/10. Our data show: (i) About 90% of mice infected with 10(6) P. chabaudi-infected erythrocytes are able to develop protective immune mechanisms which become evident in self-healing the infection. The capability of self-healing is lost when mice are pretreated with Te for 3 weeks. (ii) Mice which have self-healed infections acquire immunity to homologous rechallenge. Concomitantly, mice become Te-unresponsive in that their acquired immunity is not suppressible by Te-treatment. (iii) Flow cytometry reveals that Te-pretreatment entails an increase of CD8+ cells and a decrease of Ig+ cells by about 4% in spleens of non-immune mice. In immune mice, however, there is a Te-unresponsiveness of the percental distribution of splenic cell populations. (iv) Adoptive transfer experiments indicate that immunity is conferred by spleen cells, presumably non-T-cells. These cells are Te-unresponsive, since they exert their effect in Te-pretreated mice in the presence of Te. (v) Te-unresponsive immunity can be also transferred by serum, especially the IgG-fraction, obtained from immune mice. Our data demonstrate that Te prevents the development of protective immunity against P. chabaudi infections. However, when once established, protective immunity becomes unresponsive to Te. Our data suggest that the effector mechanisms of protective immunity involve Te-unresponsive B cells secreting protective IgG-antibodies.

Testosterone-induced diminution of two peptides in spleen cells from testosterone-immunosuppressed B10 mice

High-performance liquid chromatography (HPLC) is used to detect testosterone (T)-sensitive peptides in spleen cells isolated from female C57BL/10 mice immunosuppressed against Plasmodium chabaudi malaria by T treatment. Two peaks with retention times of about 25 min and 34 min, respectively, were identified to be diminished by about 52% and 47%, respectively, in spleen cells from T-treated mice compared to those from untreated mice. Amino acid sequencing revealed that the 24 min peak consisted of the dipeptide Met-Phe and the 34 min peak contained a degradative fragment of the alpha-chain of hemoglobin. Our data suggest that the immunosuppressive T treatment of B10 mice induces a perturbation of erythrophagocytosis in spleens.

Cells in the marginal zone of the spleen

The marginal zone of the spleen forms an intriguing area in which a variety of cell types are combined. Several of these cell types seem to have a fixed position in the marginal zone, such as the marginal zone macrophages, the marginal metallophilic macrophages at the inner border, and, to a lesser extent, the marginal zone B cells. For other cell types--T lymphocytes, small B cells, and dendritic cells--the marginal zone is only a temporary residence. It is this combination of relatively sessile cell populations and the continuous influx and passing of bloodborne immunocompetent cells that turn the marginal zone into a dynamic area, particularly apt for antigen processing and recognition. In no other lymphoid organ can such a unique combination of cells and functions be found. The opening of the arterial blood stream in the marginal sinuses results in a reduction of the velocity of the blood stream, and antigens are initially screened in the marginal zone. To this, extremely potent phagocytic cells, the marginal zone macrophages, are present which can take up and phagocytize large foreign particles, such as bacteria and effete red blood cells. Further filtration of the blood takes place in the filtration beds of the red pulp. The marginal zone macrophages express membrane receptors for bacterial polysaccharides which lead to efficient phagocytosis, probably even in the absence of prior opsonization. Antigenic fragments produced this way can be taken up by dendritic cells that enter the spleen by the blood as part of a mobile surveillance immune system. Dendritic cells present antigen to T cells in the outer area of the T cell-dependent PALS, leading to clustering and enrichment of antigen-specific T cells. Antigens in the marginal zone can also directly associate with memory B cells thought to reside here for longer times, having intimate contact with the marginal zone macrophages. B memory cells then migrate into the PALS and present antigen to T cells. The marginal zone therefore functions not only as an area of initial filtration and phagocytosis of antigens from the blood, but also as a site of lymphocyte emigration. Some of the incoming T and B lymphocytes in the recirculating pool enter the white pulp from the marginal zone. The underlying force and selective molecular mechanisms that guide this migration are unknown. Both B and T lymphocytes recirculate through the outer PALS area on their way to the follicles and the inner PALS, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Testosterone-induced abrogation of self-healing of Plasmodium chabaudi malaria in B10 mice: mediation by spleen cells

This study investigates the suppressive effect of testosterone (Te) on the self-healing of Plasmodium chabaudi malaria in female mice of the strain C57BL/10, and, in particular, the possible role of spleen cells in mediating this Te effect. Our data show the following. (i) About 80% of B10 mice infected with 10(6) P. chabaudi-infected erythrocytes are capable of self-healing the infections. This capability is progressively impaired and finally abrogated after pretreating the B10 mice with Te for 3 weeks. (ii) The spleen is Te responsive. This becomes evident in a reduction of total spleen cells from 1.05 x 10(8) to 0.54 x 10(8) on average after Te treatment for 3 weeks. Moreover, Te treatment causes an increase in the relative proportion of CD8+ cells by about 4% and a decrease of Ig+ cells by about 4.5%, as revealed by flow cytometry. (iii) Spleen cells mediate the suppressive Te effect as revealed by adoptive transfer experiments. The percentage of self-healing mice dramatically decreases to about 8% when they receive, just prior to infection, nucleated spleen cells isolated from mice treated with Te for 3 weeks. This suppressive effect can be transferred by T cells in particular but also by non-T cells, though to a lesser extent. (iv) The adoptively transferred cells mediate their suppressive effect on self-healing only if the recipient mice receive Te during infection. Our data suggest that spleen cells become functionally changed by the Te treatment for 3 weeks. Particularly T cells, but also non-T cells, gain P. chabaudi-specific suppressive activities, and the cells require a Te-induced factor(s) to mediate these activities.

Cellular mechanisms in immunity to blood stage infection

We studied mechanisms of immunity to blood stage infection in the mouse malarias Plasmodium vinckei and Plasmodium yoelii 17X. Infection with P. vinckei was uniformly lethal, whereas P. yoelii 17X caused a self-limited, nonlethal infection. Transfer of immune CD4+ T cells conferred protection against P. yoelii in nude mice. Previous studies by others had suggested that immunity to P. yoelii may be related to MHC class I expression on reticulocytes and found that CD8+ T cells alone transferred protection in immunodeficient mice. However, in our experiments, immune CD8+ T cells failed to transfer protection. In the P. vinckei system, both B cell-deficient and immunologically intact mice developed immunity to P. vinckei after parasite infection and drug cure. In vivo depletion of CD4+ T cells abrogated immunity in these immune mice. Adoptive transfer of CD4+ T cells failed to protect nude or normal mice from P. vinckei infection, but the transfer of immune CD4+ T cells reconstituted immunity in CD4-depleted immune mice. Splenectomy of immune mice resulted in the complete loss of immunity. Despite the fact that immunity to P. vinckei could be achieved with live parasite infection and drug cure, immunization of mice with killed P. vinckei with various adjuvants failed to protect mice from live challenge. In contrast, immunization with killed P. vinckei antigens in combination with attenuated Salmonella typhimurium SL3235 induced a high degree of protective immunity. These results suggest that induction of immunity against virulent malarias requires both induction of CD4+ T cells and certain splenic alterations caused by parasite infection or S. typhimurium.(ABSTRACT TRUNCATED AT 250 WORDS)