Platelet factor 4 mediates inflammation in experimental cerebral malaria

Beta interferon suppresses the development of experimental cerebral malaria

Cerebral malaria (CM) is a major complication of Plasmodium falciparum infection, particularly in children. The pathogenesis of cerebral malaria involves parasitized red blood cell (RBC)-mediated vascular inflammation, immune stimulation, loss of blood-brain barrier integrity, and obstruction of cerebral capillaries. Therefore, blunting vascular inflammation and immune cell recruitment is crucial in limiting the disease course. Beta interferon (IFN-β) has been used in the treatment of diseases, such as multiple sclerosis (MS) but has not yet been explored in the treatment of CM. Therefore, we sought to determine whether IFN-β also limits disease progression in experimental cerebral malaria (ECM). Plasmodium berghei-infected mice treated with IFN-β died later and showed increased survival, with improved blood-brain barrier function, compared to infected mice. IFN-β did not alter systemic parasitemia. However, we identified multiple action sites that were modified by IFN-β administration. P. berghei infection resulted in increased expression of chemokine (C-X-C motif) ligand 9 (CXCL9) in brain vascular endothelial cells that attract T cells to the brain, as well as increased T-cell chemokine (C-X-C motif) receptor 3 (CXCR3) expression. The infection also increased the cellular content of intercellular adhesion molecule 1 (ICAM-1), a molecule important for attachment of parasitized RBCs to the endothelial cell. In this article, we report that IFN-β treatment leads to reduction of CXCL9 and ICAM-1 in the brain, reduction of T-cell CXCR3 expression, and downregulation of serum tumor necrosis factor alpha (TNF-α). In addition, IFN-β-treated P. berghei-infected mice also had fewer brain T-cell infiltrates, further demonstrating its protective effects. Hence, IFN-β has important anti-inflammatory properties that ameliorate the severity of ECM and prolong mouse survival.

Immunomodulatory mediators in platelet transfusion reactions

Our appreciation of the roles that platelets play in vascular biology is constantly expanding. One of the major roles of platelets is in initiating and accelerating immune responses. Platelet transfusion may be associated with adverse inflammatory outcomes manifested as fever, discomfort, tachycardia, and respiratory issues. This may in part be due to immune mediators either expressed by activated platelets or released into the platelet media during platelet storage. This review will highlight some more recent knowledge gained regarding the platelet storage lesion and potential mediators of platelet transfusion reactions.

The role of the CXC chemokines platelet factor-4 (CXCL4/PF-4) and its variant (CXCL4L1/PF-4var) in inflammation, angiogenesis and cancer

Chemokines are chemotactic cytokines which recruit leukocytes to inflammatory sites. They also affect tumor development and metastasis by acting as growth factor, by attracting pro- or anti-tumoral leukocytes or by influencing angiogenesis. Platelet factor-4 (CXCL4/PF-4) was the first chemokine shown to inhibit angiogenesis. CXCL4L1/PF-4var, recently isolated from thrombin-stimulated platelets, differing from authentic CXCL4/PF-4 in three carboxy-terminally located amino acids, was found to be more potent than CXCL4/PF-4 in inhibiting angiogenesis and tumor growth. Both glycosaminoglycans (GAG) and CXCR3 are implicated in the activities of the PF-4 variants. This report reviews the current knowledge on the role of CXCL4/PF-4 and CXCL4L1/PF-4var in physiological and pathological processes. In particular, the role of CXCL4/PF-4 in cancer, heparin-induced thrombocytopenia and atherosclerosis is described.

Platelets as initiators and mediators of inflammation at the vessel wall

Platelets are dynamic cells with activities that extend beyond thrombosis including an important role in initiating and sustaining vascular inflammation. A role for platelets has been described in many physiologic and pathophysiologic processes such as atherosclerosis, stem cell trafficking, tumor metastasis, and arthritis. Platelet activation at sites of an intact inflamed endothelium contributes to vascular inflammation and vascular wall remodeling. Platelets secrete a wide array of preformed and synthesized inflammatory mediators upon activation that can exert significant local and systemic effects. This review will focus on the role of platelet derived mediators in vascular inflammation and vascular wall remodeling.

Platelets contribute to allograft rejection through glutamate receptor signaling

Platelets recruit leukocytes and mediate interactions between leukocytes and endothelial cells. Platelets have been long described as markers of transplant rejection, but the contribution of platelets to transplant rejection has not been critically examined. We demonstrate in this study that following T cell initiation of allograft rejection, platelets contribute to T cell recruitment and increased plasma inflammatory mediators and accelerate T cell-meditated skin graft rejection. Prior work from our laboratory has shown that platelets secrete glutamate when activated, which then induces platelet thromboxane production by signaling through platelet-expressed ionotropic glutamate receptors. Glutamate receptor antagonists therefore represent, to our knowledge, novel inhibitors of platelet-accelerated inflammation. We have found that plasma glutamate is increased in mice that receive skin grafts and that mice treated with glutamate receptor antagonists have improved graft survival and decreased plasma thromboxane, platelet factor 4 (CXCL4), and IFN-γ. Taken together, our work now demonstrates that subsequent to T cell initiation of skin graft rejection, platelets contribute to further T cell recruitment and that by blunting glutamate-mediated platelet activation, graft survival is improved.

Thrombocytopenia in early malaria is associated with GP1b shedding in absence of systemic platelet activation and consumptive coagulopathy

Thrombocytopenia develops early in malaria, but the underlying mechanisms remain incompletely understood. We studied the aetiology of malaria-associated thrombocytopenia in volunteers experimentally infected with Plasmodium falciparum malaria, in Indonesian malaria patients and in ex vivo studies. In experimental human malaria, the decrease in platelet counts was associated with a concurrent rise in young platelets (immature platelet fraction) and thrombopoietin. D-dimer concentrations were moderately elevated without a prolongation in the activated partial thromboplastin time or decrease in fibrinogen. There was no increase in expression of the platelet surface markers CD62P, PAC-1 and CD63 and in plasma concentrations of the platelet factors P-selectin, CXCR4, CXCL7, RANTES and CD40L. In contrast, concentrations of soluble glycoprotein-1b (sGP1b), the external domain of the platelet receptor for von Willebrand factor (VWF), increased early. Indonesian malaria patients also had elevated concentrations of sGP1b, which correlated with VWF concentrations. Finally, incubation of platelets with parasitized erythrocytes in vitro failed to induce platelet aggregation or activation. We concluded that neither compromised platelet production nor platelet activation or consumptive coagulopathy were responsible for the early thrombocytopenia in malaria. We hypothesize that the increase in sGP1b concentrations results from VWF-mediated GP1b shedding; a process that may prevent excessive adhesion of platelets and parasitized erythrocytes.

Our expanding view of platelet functions and its clinical implications

Platelets are the primary cell mediator of thrombosis. A deficiency of platelets can result in severe bleeding defects. "Overactive" platelets contribute to life-threatening outcomes in diseases such as heart attack, stroke, and cancer. The use of platelet inhibitors for thrombosis prevention must therefore seek a delicate balance between inhibiting platelet activation and an associated increased bleeding risk. There are currently few platelet inhibitors clinically available, making the search for novel anti-platelet drug targets a major research priority. Several newly identified pathways of platelet activation may hold hope in this area. In addition, important roles for platelets beyond hemostasis have been discovered. Platelets have recently been described as mediators of diverse inflammatory diseases such as arthritis, hepatitis, malaria, and atherosclerosis. This has heightened the need to broaden our understanding of platelet functions and platelet-derived inflammatory mediators. It has also heightened interest in a continued search for new platelet inhibitors and presents new opportunities for platelet inhibitors to be used in a wide array of disease treatment strategies.

Trafficking of immune cells in the central nervous system

The CNS is an immune-privileged environment, yet the local control of multiple pathogens is dependent on the ability of immune cells to access and operate within this site. However, inflammation of the distinct anatomical sites (i.e., meninges, cerebrospinal fluid, and parenchyma) associated with the CNS can also be deleterious. Therefore, control of lymphocyte entry and migration within the brain is vital to regulate protective and pathological responses. In this review, several recent advances are highlighted that provide new insights into the processes that regulate leukocyte access to, and movement within, the brain.

Platelet factor 4 regulation of monocyte KLF4 in experimental cerebral malaria

Cerebral malaria continues to be a difficult to treat complication of Plasmodium falciparum infection in children. We have shown that platelets can have major deleterious immune functions in experimental cerebral malaria (ECM). One of the platelet derived mediators we have identified as particularly important is platelet factor 4/CXCL4. Our prior work demonstrated that PF4(-/-) mice are protected from ECM, have reduced plasma cytokines, and have reduced T-cell trafficking to the brain. We now show that PF4 drives monocyte cytokine production in a Kruppel like factor 4 (KLF4) dependent manner. Monocyte depleted Plasmodium berghei infected mice have improved survival, and KLF4 is greatly increased in control, but not monocyte depleted mice. PF4(-/-) mice have less cerebral monocyte trafficking and no change in KLF4 expression. These data indicate that PF4 induction of monocyte KLF4 expression may be an important step in the pathogenesis of ECM.

Microbial induction of vascular pathology in the CNS

The central nervous system (CNS) is a finely tuned organ that participates in nearly every aspect of our day-to-day function. Neurons lie at the core of this functional unit and maintain an active dialogue with one another as well as their fellow CNS residents (e.g. astrocytes, oligodendrocytes, microglia). Because of this complex dialogue, it is essential that the CNS milieu be tightly regulated in order to permit uninterrupted and efficient neural chemistry. This is accomplished in part by anatomical barriers that segregate vascular components from the cerebral spinal fluid (CSF) and brain parenchyma. These barriers impede entry of noxious materials and enable the CNS to maintain requisite protein and ionic balances for constant electrochemical signaling. Under homeostatic conditions, the CNS is protected by the presence of specialized endothelium/epithelium, the blood brain barrier (BBB), and the blood-CSF barrier. However, following CNS infection these protective barriers can be comprised, sometimes resulting in severe neurological complications triggered by an imbalance or blockage of neural chemistry. In some instances, these disruptions are severe enough to be fatal. This review focuses on a selection of microbes (both viruses and parasites) that compromise vascular barriers and induce neurological complications upon gaining access to the CNS. Emphasis is placed on CNS diseases that result from a pathogenic interplay between host immune defenses and the invading microbe.

CXC chemokine ligand 4 (Cxcl4) is a platelet-derived mediator of experimental liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide. Platelets are involved in liver damage, but the underlying molecular mechanisms remain elusive. Here, we investigate the platelet-derived chemokine (C-X-C motif) ligand 4 (CXCL4) as a molecular mediator of fibrotic liver damage. Serum concentrations and intrahepatic messenger RNA of CXCL4 were measured in patients with chronic liver diseases and mice after toxic liver injury. Platelet aggregation in early fibrosis was determined by electron microscopy in patients and by immunohistochemistry in mice. Cxcl4(-/-) and wild-type mice were subjected to two models of chronic liver injury (CCl(4) and thioacetamide). The fibrotic phenotype was analyzed by histological, biochemical, and molecular analyses. Intrahepatic infiltration of immune cells was investigated by fluorescence-activated cell sorting, and stellate cells were stimulated with recombinant Cxcl4 in vitro. The results showed that patients with advanced hepatitis C virus-induced fibrosis or nonalcoholic steatohepatitis had increased serum levels and intrahepatic CXCL4 messenger RNA concentrations. Platelets were found directly adjacent to collagen fibrils. The CCl(4) and thioacetamide treatment led to an increase of hepatic Cxcl4 levels, platelet activation, and aggregation in early fibrosis in mice. Accordingly, genetic deletion of Cxcl4 in mice significantly reduced histological and biochemical liver damage in vivo, which was accompanied by changes in the expression of fibrosis-related genes (Timp-1 [tissue inhibitor of matrix metalloproteinase 1], Mmp9 [matrix metalloproteinase 9], Tgf-beta [transforming growth factor beta], IL10 [interleukin 10]). Functionally, Cxcl4(-/-) mice showed a strongly decreased infiltration of neutrophils (Ly6G) and CD8(+) T cells into the liver. In vitro, recombinant murine Cxcl4 stimulated the proliferation, chemotaxis, and chemokine expression of hepatic stellate cells.

CONCLUSION

The results underscore an important role of platelets in chronic liver damage and imply a new target for antifibrotic therapies.

Cerebral malaria: why experimental murine models are required to understand the pathogenesis of disease

Cerebral malaria is a life-threatening complication of malaria infection. The pathogenesis of cerebral malaria is poorly defined and progress in understanding the condition is severely hampered by the inability to study in detail, ante-mortem, the parasitological and immunological events within the brain that lead to the onset of clinical symptoms. Experimental murine models have been used to investigate the sequence of events that lead to cerebral malaria, but there is significant debate on the merits of these models and whether their study is relevant to human disease. Here we review the current understanding of the parasitological and immunological events leading to human and experimental cerebral malaria, and explain why we believe that studies with experimental models of CM are crucial to define the pathogenesis of the condition.

Platelet power: sticky problems for sticky parasites?

Platelets might have a crucial role in the pathogenesis of both human and rodent malarias by assisting in the sequestration of infected erythrocytes within the cerebral vasculature. However, recent elegant work by McMorran et al. suggests that they are also involved in innate protection during the early stages of infection. Here, we discuss the implications of their important findings in the context of immunity to malaria.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.