Human vascular endothelial cell adhesion receptors for Plasmodium falciparum-infected erythrocytes: roles for endothelial leukocyte adhesion molecule 1 and vascular cell adhesion molecule 1

Serum laminin in malaria

AIM

To determine serum laminin concentrations in patients with uncomplicated Plasmodium falciparum malaria.

METHODS

An enzyme linked immunosorbent assay (ELISA) was used to determine serum laminin concentrations in 54 patients with acute uncomplicated P falciparum malaria during and after treatment, and in 17 control subjects in Bangkok, Thailand.

RESULTS

Raised concentrations of soluble laminin were observed in patients (mean (SD) concentration 628 (225) ng/ml), compared with normal controls (490 (116) ng/ml), during the acute phase of the disease. During treatment, serum laminin concentrations decreased and returned to normal within three days. Serum laminin concentrations were correlated with parasite counts before treatment, and with the serum concentration of soluble intercellular adhesion molecule-1 (ICAM-1), soluble E-selectin, and soluble tumour necrosis factor receptor at 55 kilodaltons.

CONCLUSIONS

These findings are compatible with an increased production or release of laminin in P falciparum malaria, which could indicate a role for the subendothelial basement membrane in the pathogenesis of the disease.

Serum levels of adhesion molecules and thrombomodulin as indicators of vascular injury in severe Plasmodium falciparum malaria

Severe Plasmodium falciparum malaria is characterized by multiple organ involvement due to sequestration of infected erythrocytes in small vessels. Endothelial cell adhesion molecules play an important role in this interaction. During the course of a severe cerebral P. falciparum malaria infection we found very markedly elevated levels of the soluble adhesion molecules intercellular adhesion molecule-1, E-selectin, and vascular cell adhesion molecule-1, with a maximum increase of nine, seven, and eight times, respectively. These very high levels of soluble adhesion molecules point to an endothelial cell injury as an additional cause to physiological release or shedding due to receptor interactions. Soluble thrombomodulin (sTM) levels showed an extremely marked elevation up to 332 ng/ml (up to 13 times the normal value) as well. Malaria patients without severe organ involvement/cerebral manifestation showed only a mild elevation of sTM levels. TM is a parameter independent of the immunological system. It is regarded as a marker of vasculitis and endothelial cell destruction. Therefore, markedly elevated sTM levels document a substantial endothelial cell injury in severe malarial infection and may be of diagnostic and prognostic importance.

Malaria pathogenesis

Malaria is a disease caused by repeated cycles of growth of the parasite Plasmodium in the erythrocyte. Various cellular and molecular strategies allow the parasite to evade the human immune response for many cycles of parasite multiplication. Under certain circumstances Plasmodium infection causes severe anemia or cerebral malaria; the expression of disease is influenced by both parasite and host factors, as exemplified by the exacerbation of disease during pregnancy. This article provides an overview of malaria pathogenesis, synthesizing the recent field, laboratory, and epidemiological data that will lead to the development of strategies to reduce mortality and morbidity.

Rolling and stationary cytoadhesion of red blood cells parasitized by Plasmodium falciparum: separate roles for ICAM-1, CD36 and thrombospondin

Adhesion of parasitized erythrocytes to microvascular endothelium is a central event in the pathogenesis of severe falciparum malaria. We have characterized the adhesion of flowing parasitized red blood cells to three of the known endothelial receptors coated on plastic surfaces (CD36, intercellular adhesion molecule-1 (ICAM-1) and thrombospondin (TSP)), and also to cells bearing these receptors (human umbilical vein endothelial cells (HUVEC) and platelets). All of the surfaces could mediate adhesion at wall shear stress within the physiological range. The great majority of adherent parasitized cells formed rolling rather than static attachments to HUVEC and ICAM-1, whereas static attachments predominated for platelets, CD36 and TSP. Studies with monoclonal antibodies verified that binding the HUVEC was mainly via ICAM-1, and to platelets via CD36. Adhesion via ICAM-1 was least sensitive to increasing wall shear stress, but absolute efficiency of adhesion was greatest for CD36, followed by ICAM-1, and least for TSP. TSP did not give long-lasting adhesion under flow, whereas cells remained adherent to CD36 or ICAM-1. We propose that the different receptors may have complementary roles in modulating adhesion in microvessels. Initial interaction at high wall shear stress may be of a rolling type, mediated by ICAM-1 or other receptors, with immobilization and stabilization occurring via CD36 and/or TSP.

Cell adhesion molecules in inflammation and immunity: relevance to periodontal diseases

Inflammatory and immune responses involve close contact between different populations of cells. These adhesive interactions mediate migration of cells to sites of inflammation and the effector functions of cells within the lesions. Recently, there has been significant progress in understanding the molecular basis of these intercellular contacts. Blocking interactions between cell adhesion molecules and their ligands has successfully suppressed inflammatory reactions in a variety of animal models in vivo. The role of the host response in periodontal disease is receiving renewed attention, but little is known of the function of cell adhesion molecules in these diseases. In this review we summarize the structure, distribution, and function of cell adhesion molecules involved in inflammatory/immune responses. The current knowledge of the distribution of cell adhesion molecules is described and the potential for modulation of cell adhesion molecule function is discussed.

Molecular mechanisms of sequestration in malaria

Cell surface molecules have received intense attention in recent years because of the central roles they play at the interface between the external environment and the cellular interior. Their functions include adhesion to other cells or extracellular matrices, protection against hostile physical, chemical and biological agents and the transport of metabolites into and out of the cell. In addition, cell surface molecules transduce signals across the cell membrane, relaying information inwards and presenting altered characteristics to the exterior as the environment changes.

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.

Rosetting

Why do some individuals get severe falciparum malaria while others don't? Rosetting (the binding of uninfected erythrocytes to Plasmodium falciparum-infected erythrocytes), together with endothelial cytoadherence, has been shown to play a crucial role in the obstruction of the microvosculoture in P. falciparum malaria. Here, Mats Wahlgren, Victor Fernandez, Carin Scholonder and Johan Carlson review the literature surrounding rosetting.

Cytokines and T-cell response in malaria

The intracellular protozoan Plasmodium sp induces a complex immune response which sometimes implies serious pathological effects for the host. According to in vitro studies and epidemiological surveys, several effector mechanisms are displayed against plasmodial blood stages and a large interaction between humoral and cell-mediated immunity is presumed to occur among protected individuals. The key role of T cells in the antiplasmodial immune response is now well established, but all the regulatory heterogenous mechanisms are not yet fully known. An increasing body of data shows a dual role during malaria attack for some cytokines released by monocytes and macrophages (TNF, IL-1, IL-6) or by T cells (IFN-gamma, lymphotoxin (LT), IL-4). The importance of some plasmodial proteins in the cytokine-induced pathology and the stimulation of a preferential TH1 or TH2 mediated immune response to achieve protective immunity against Plasmodium sp are discussed.

Amplification of cytoadherence in cerebral malaria: towards a more rational explanation of disease pathophysiology

Cerebral malaria in man and in mice is the consequence of a cascade of events, involving the production of toxins by the parasite and cytokines by the host, and eventually leading to the amplification of the expression of the receptors for cytoadherence on brain capillary endothelial cells. Variations in the intrinsic characteristics of parasite isolates or the genetic make-up of the host and the degree of antimalarial immunity can modulate this sequence of events. A working hypothesis is proposed in which two features of the parasite, the ability to cytoadhere and to produce toxins, are clearly dissociated and where the amplification of cytoadherence receptors is considered crucial. This hypothesis, illustrated by new data from human malaria and rodent models, suggests that cerebral malaria may occur when these features occur together during an infection, while not necessarily within the same parasite clone.

Parasite virulence factors during falciparum malaria: rosetting, cytoadherence, and modulation of cytoadherence by cytokines

To determine virulence factors of isolates of Plasmodium falciparum and the potential role of cytokines in cerebral malaria, 46 Malagasy patients presenting with cerebral (n = 10), severe (n = 10), and uncomplicated (n = 26) malaria were enrolled in a study. The capacity of 21 of 46 P. falciparum isolates to form rosettes in vitro and to adhere to human umbilical vein endothelial cells (HUVECs) that express intercellular adhesion molecule-1 receptors and to C32 amelanotic melanoma cells that express mainly CD36 receptors was investigated together with the effects of tumor necrosis factor alpha (TNF-alpha), granulocyte macrophage-colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-6 alone and in two-by-two combinations on the cytoadherence of infected erythrocytes to HUVECs. Plasma levels of these cytokines were also measured in the patients at admission. The percentage of rosette formation was higher for the isolates from patients with cerebral (n = 6; 19.5%) and severe (n = 6; 30.5%) malaria than for those from patients with uncomplicated malaria (n = 9; 5%) (P < 0.002). The cytoadherence properties of the isolates did not differ among the three groups whatever the target cell used, but adherence to melanoma cells was systematically higher than that to HUVECs. Adhesion to HUVECs was increased more after TNF-alpha stimulation than after GM-CSF, IL-3, or IL-6 stimulation (P < 0.01). Only the combination of TNF-alpha and IL-3 enhanced cytoadherence more than TNF-alpha used alone (P < 0.02). No difference in the modulation of cytoadherence by cytokines was found in relation to the severity of the disease. TNF-alpha and IL-6 levels in peripheral blood were higher in the patients with cerebral and severe malaria than in the patients with uncomplicated malaria (P < 0.005). Most of the patients' sera contained little or no IL-3 or GM-CSF. Our results challenge the role of intercellular adhesion molecule-1 as the principal receptor mediating the cytoadherence of P. falciparum-infected erythrocytes and contrast with data obtained in the murine model.

Adhesion of parasitized red blood cells to cultured endothelial cells: a flow-based study of isolates from Gambian children with falciparum malaria

Adhesion of parasitized red blood cells to vascular endothelium is thought to play an important role in the development of the ischaemic complications associated with severe falciparum malaria. Using a novel, flow-based assay, we have investigated the adhesion of parasitized red blood cells to formalin-fixed human umbilical vein endothelial cells (HUVEC), for isolates obtained from 32 Gambian subjects with mild or severe falciparum malaria. Red cells infected with wild strains of Plasmodium falciparum were able to adhere to HUVEC under physiologically relevant flow conditions, but the level of adhesion was highly variable, ranging from 1 to 688 adherent cells per mm2 of HUVEC. Within isolates, some adherent parasitized cells remained stationary, whilst other formed less stable interactions and rolled slowly over the cell surface. There was no significant difference in adhesion of parasitized cells between isolates obtained from mild or severe cases of malaria, although a subset of isolates did show very high levels of adhesion. The results suggest that there is not a simple relationship between the adhesion of parasitized cells to cultured endothelial cells (presumably via the receptor ICAM-1) and the clinical severity of the disease, although variation in microvascular adhesion in vivo may still be a determinant of ischaemic complications.

Protection, pathogenesis and phenotypic plasticity in Plasmodium falciparum malaria

Why does Plasmodium falciparum cause severe illness in some but not all infections? How is clinical immunity acquired? These questions have intrigued investigators since the clinical epidemiology of malaria was first described. The search for answers to both questions has highlighted the changes that take place at the surface of infected red blood cells during the last half of the erythrocytic cycle. These changes specify the antigenic and adhesive or cytoadherence phenotypes for the infected cell. Now the antigenic and adhesive phenotypes appear to be linked and together undergo clonal variation. In this article David Roberts, Beverley-Ann Biggs, Graham Brown and Christopher Newbold explain how clonal phenotypic variation and the linkage between adhesive and antigenic types contribute to our understanding of naturally acquired immunity and of pathogenesis of severe malaria.

Immunity to blood stages of malaria

Those developmental stages of malaria parasites that infect erythrocytes are responsible for the severe morbidity and mortality associated with this disease. The nature and specificity of the slowly acquired immunity seen in endemic populations remain to be defined, but significant progress has been made recently in identifying specific blood-stage proteins, characterizing immune responses to them, and exploring the dynamics of non-specific host responses to infection.

Adhesion molecules in cell interactions

During a successful immune response, several families of adhesion molecules participate in a cascade of binding events that lead to the binding of leukocytes, both to each other and to cell types such as the endothelium and epithelium. A central theme emerging from recent studies is that the function of an adhesion receptor cannot be inferred from its expression alone; rather, adhesion receptors are 'selected' to perform distinct effector functions based on their cell-background and factors present in the local microenvironment. Thus, adhesion receptors expressed on different cell-types may find themselves in different states of 'activation-readiness' and may be further selected by prevailing conditions in the microenvironment to bind tissue-specific ligands and mediate leukocyte effector functions such as homing or transendothelial migration.