Letter: Malarial parasites in mammalian platelets

Plasmodium and mononuclear phagocytes

Plasmodium, the causative agent of malaria, initially multiplies inside liver cells and then in successive cycles inside erythrocytes, causing the symptoms of the disease. In this review, we discuss interactions between the extracellular and intracellular forms of the Plasmodium parasite and innate immune cells in the mammalian host, with a special emphasis on mononuclear phagocytes. We overview here what is known about the innate immune cells that interact with parasites, mechanisms used by the parasite to evade them, and the protective or detrimental contribution of these interactions on parasite progression through its life cycle and pathology in the host.

Platelets present antigen in the context of MHC class I

Platelets are most recognized for their vital role as the cellular mediator of thrombosis, but platelets also have important immune functions. Platelets initiate and sustain vascular inflammation in many disease conditions, including arthritis, atherosclerosis, transplant rejection, and severe malaria. We now demonstrate that platelets express T cell costimulatory molecules, process and present Ag in MHC class I, and directly activate naive T cells in a platelet MHC class I-dependent manner. Using an experimental cerebral malaria mouse model, we also demonstrate that platelets present pathogen-derived Ag to promote T cell responses in vivo, and that platelets can be used in a cell-based vaccine model to induce protective immune responses. Our study demonstrates a novel Ag presentation role for platelets.

Rodent blood-stage Plasmodium survive in dendritic cells that infect naive mice

Plasmodium spp. parasites cause malaria in 300 to 500 million individuals each year. Disease occurs during the blood-stage of the parasite's life cycle, where the parasite is thought to replicate exclusively within erythrocytes. Infected individuals can also suffer relapses after several years, from Plasmodium vivax and Plasmodium ovale surviving in hepatocytes. Plasmodium falciparum and Plasmodium malariae can also persist after the original bout of infection has apparently cleared in the blood, suggesting that host cells other than erythrocytes (but not hepatocytes) may harbor these blood-stage parasites, thereby assisting their escape from host immunity. Using blood stage transgenic Plasmodium berghei-expressing GFP (PbGFP) to track parasites in host cells, we found that the parasite had a tropism for CD317(+) dendritic cells. Other studies using confocal microscopy, in vitro cultures, and cell transfer studies showed that blood-stage parasites could infect, survive, and replicate within CD317(+) dendritic cells, and that small numbers of these cells released parasites infectious for erythrocytes in vivo. These data have identified a unique survival strategy for blood-stage Plasmodium, which has significant implications for understanding the escape of Plasmodium spp. from immune-surveillance and for vaccine development.

Platelet microparticles: a new player in malaria parasite cytoadherence to human brain endothelium

Cerebral malaria (CM) is characterized by accumulation of circulating cells within brain microvessels, among which platelets play an important role. In vitro, platelets modulate the cytoadherence of Plasmodium falciparum-parasitized red blood cells (PRBCs) to brain endothelial cells. Here we show for the first time that platelet microparticles (PMPs) are able to bind to PRBCs, thereby transferring platelet antigens to the PRBC surface. This binding is largely specific to PRBCs, because PMPs show little adherence to normal red blood cells. PMP adherence is also dependent on the P. falciparum erythrocyte membrane protein 1 variant expressed by PRBCs. PMP binding to PRBCs decreases after neutralization of PRBC surface proteins by trypsin or after treatment of PMPs with a mAb to platelet-endothelial cell adhesion molecule-1 (CD31) and glycoprotein IV (CD36). Furthermore, PMP uptake is a dynamic process that can be achieved by human brain endothelial cells (HBECs), inducing changes in the endothelial phenotype. Lastly, PMPs dramatically increase PRBC cytoadherence to HBECs. In conclusion, our study identifies several mechanisms by which PMPs may participate in CM pathogenesis while interacting with both PRBCs and HBECs. PMPs thereby provide a novel target for antagonizing interactions between vascular cells that promote microvascular sludging and blood brain barrier alteration during CM.

Spirochete-platelet attachment and thrombocytopenia in murine relapsing fever borreliosis

Thrombocytopenia is common in persons infected with relapsing fever Borreliae. We previously showed that the relapsing fever spirochete Borrelia hermsii binds to and activates human platelets in vitro and that, after platelet activation, high-level spirochete-platelet attachment is mediated by integrin alpha IIb beta 3, a receptor that requires platelet activation for full function. Here we established that B hermsii infection of the mouse results in severe thrombocytopenia and a functional defect in hemostasis caused by accelerated platelet loss. Disseminated intravascular coagulation, immune thrombocytopenic purpura, or splenic sequestration did not play a discernible role in this model. Instead, spirochete-platelet complexes were detected in the blood of infected mice, suggesting that platelet attachment by bacteria might result in platelet clearance. Consistent with this, splenomegaly and thrombocytopenia temporally correlated with spirochetemia, and the severity of thrombocytopenia directly correlated with the degree of spirochetemia. Activation of platelets and integrin alpha IIb beta 3 were apparently not required for bacterium-platelet binding or platelet clearance because the bacterium-bound platelets in the circulation were not activated, and platelet binding and thrombocytopenia during infection of beta 3-deficient and wild-type mice were indistinguishable. These findings suggest that thrombocytopenia of relapsing fever is the result of platelet clearance after beta 3-independent bacterial attachment to circulating platelets.

Serum cytokine profiles in patients with Plasmodium vivax malaria: a comparison between those who presented with and without thrombocytopenia

One of the peculiar features of Plasmodium vivax malaria in South Korea is the surprisingly high frequency of thrombocytopenia. The mechanism by which this malaria-related thrombocytopenia develops and its role in the pathology and progress of human infection with P. vivax have not yet been completely understood. In the present study, the serum cytokine profiles of cases of P. vivax malaria who presented with thrombocytopenia were compared with those of similar cases who did not have thrombocytopenia at presentation. The subjects were the 94 consecutive cases of P. vivax malaria who presented at five hospitals in South Korea (all near the Demilitarized Zone) between May 2000 and October 2002, 47 of whom had thrombocytopenia at presentation. When mean values and (S.E.) were compared, the thrombocytopenic patients were found not only to be generally older than the non-thrombocytopenic [25.3 (1.1) v. 21.3 (0.18) years; P < 0.001] but also to have presented with higher serum concentrations of aspartate aminotransferase [77.6 (16.6) v. 32.3 (7.4) U/litre; P < 0.0001], alanine aminotransferase [96.7 (19.0) v. 44.7 (12.0) U/litre; P = 0.0001], interleukin-1 [49.9 (7.4) v. 23.7 (5.1) pg/ml; P < 0.001], interleukin-6 [174.9 (26.4) v. 57.3 (14.6) pg/ml; P = 0.001], interleukin-10 [308.2 (39.6) v. 137.9 (23.1) pg/ml; P < 0.002] and transforming growth factor-beta [1134.3 (387.5) v. 416.6 (183.8) pg/ml; P < 0.0001], and higher levels of parasitaemia [4345.7 (966.6) v. 1443.8 (222.7) parasites/microl; P = 0.03). The non-thrombocytopenic patients, however, had relatively high total leucocyte counts [5.8 (0.24) v. 5.4 (0.66) leucocytes/nl; P = 0.03]. The thrombocytopenia associated with P. vivax malaria in South Korea therefore appears to be associated with elevated serum concentrations of both pro- and anti-inflammatory cytokines. To define the role of each cytokine in the development of thrombocytopenia during the course of acute P. vivax malaria, further prospective studies are needed.

Platelet kinetics and other hematological profiles in experimental Plasmodium falciparum infection: a comparative study between Saimiri and Aotus monkeys

Levels of platelets and other hematological values were monitored in 21 Saimiri and 12 Aotus monkeys over a period of three weeks post-infection with monkey-adapted Indochina CDC-1 strain of Plasmodium falciparum. In both Saimiri sciureus boliviensis and Aotus nancymai karyotype-1 monkeys the severest thrombocytopenia was observed at 14 days post-infection coinciding with peak parasitemia, neutropenia, lymphocytosis, and anemia associated with severe hemoglobinemia and elevated fibrinogen degeneration products(FDP's). MCH and MCV profiles in Aotus monkeys decreased with ascending parasitemia. In contrast, these parameters in Saimiri were characterized by a significant compensatory increase correlating with parasitemia. In general, thrombocytopenia was one of the earliest clinical manifestations of the infection with the platelets returning to normal levels shortly after peak parasitemia at 14 days. Platelet kinetics had a strong correlation with hematologic and parasitologic values in the Aotus model. No consistent associations were observed between platelet kinetics and other parameters in the Saimiri model. These data indicate that the Aotus model for malaria is more predictable than the Saimiri. Further, platelet turnover rates and recovery provide a useful prognostic parameter during malaria infection. The results are discussed in relation to the value of the two species of monkeys as models for the pathogenesis of human malaria.

Host immune response and pathological expression in malaria: possible implications for malaria vaccines

Recent progress in parasite immunobiology has led to the identification of several plasmodial antigens representing the target of the protective antibody response of the infected host. As a consequence, some of these antigens have been envisaged as potential malaria vaccines in man. However, in spite of these achievements, the fine mechanisms which lead to the development of a state of partial protective immunity or to the triggering of immunopathology during malaria infection are not yet fully understood. Thus, it may be appropriate to evaluate the relative importance of individual host immune responsiveness to parasite epitopes involved in the induction of immunity, or of some immunologically mediated adverse reactions such as glomerulonephritis, anaemia, thrombocytopenia, and cerebral syndrome.

Enhanced parasitization of platelets by Plasmodium berghei yoelii

In previous studies plasmodia have been found by electron microscopy within human platelets naturally infected with Plasmodium vivax and within platelets of mice infected intraperitoneally with P. berghei. In both situations the number of parasitized platelets was low. An enhancement of platelet parasitization was attempted in order to study in greater detail the mechanisms and implications of such a phenomenon. Various in vitro incubation mixtures of normal mouse platelets and free merozoites of the 17X strain of P. berghei yoelii failed to produce any recognizable parasitization of platelets. In vivo, however, large numbers of invaded platelets were obtained by the use of massive intraperitoneal inocula of plasmodia (5 X 10(8) infected erythrocytes). By the 5th day of infection the proportion of parasitized platelets was 13.2 times higher in the animals receiving the large dose than in those receiving the regular passage inoculum (6 X 10(6) infected erythrocytes). Ultrastructural study of 266 intrathrombocytic parasites over eight days of infection failed to show schizogonic maturation beyond the trophozoite state.

Cultivation of the erythrocytic stages of Plasmodium berghei in Leydig cell tumor cultures

Twelve different established cell-lines were used in attempts to cultivate the erythrocytic stages of Plasmodium berghei, P. vinckei vinckei, P. coatneyi or P. knowlesi. Intracellular parasites were seen in only mouse Leydig cell testicular tumor (LCT) cultures inoculated with red cells infected with P. berghei. Intracellular parasites were present at 15 to 96 h after inoculation, being most numerous at 36 h. Most intracellular stages were rings, trophozoites, schizonts and merozoites; gametocytes were few in number and present only at 36 and 48 h. Intracellular parasites were normal in general morphology and staining characteristics at 15 to 48 h, but were abnormal after 72 h. Infected host cells exhibited progressive nuclear and cytoplasmic degenerative changes, which ultimately resulted in death of the cell. Uninfected cells appeared normal. The ability of parasites in LCT cultures to produce infections upon injection into mice was similar to that obtained with control cultures without LCT cells.