Platelet-derived chemokines: pathophysiology and therapeutic aspects

The identification of chemokines in blood platelets has strengthened our view of these cells as participants in immune host defense. Platelet chemokines representing prestored and rapidly releasable proteins may play a major role as first-line inflammatory mediators. This is evident from their capability to recruit early inflammatory cells such as neutrophil granulocytes and monocytes and even to exhibit direct antimicrobial activity. However, insight is growing that platelet chemokines may be also long-term regulators, e.g., by activating T lymphocytes, by modulating the formation of endothelium and even thrombocytopoiesis itself. This review deals with the individual and cooperative functionality of platelet chemokines, as well as their potential as a basis for therapeutic intervention in the pathology of inflammation, infection, allergy and tumors. Within this context, therapeutic strategies based on the use of antibodies, modified chemokines, chemokine-binding proteins and chemokine receptor antagonists as well as first clinical studies will be addressed.     

The cellular immune response to heat shock proteins.

T lymphocytes, which are central to almost every immune response, frequently recognize microbial hsp60. Such cells could provide an early defense mechanism against pathogenic microbes. However, T cells also recognize epitopes of hsp60 shared by microbe and host. Not only conventional alpha/beta T cells respond to hsp60; gamma/delta T cells do so, as well. In fact, certain gamma/delta T cells seem to have a particular preference for this molecule. Recognition of stressed host cells expressing hsp60 could facilitate the scavenger function of the T cell system. On the other hand, such recognition could be involved in autoimmune disease.     

Bacterial resistance mechanisms against host defense peptides

Host defense peptides and proteins are important components of the innate host defense against pathogenic microorganisms. They target negatively charged bacterial surfaces and disrupt microbial cytoplasmic membranes, which ultimately leads to bacterial destruction. Throughout evolution, pathogens devised several mechanisms to protect themselves from deleterious damage of host defense peptides. These strategies include (a) inactivation and cleavage of host defense peptides by production of host defense binding proteins and proteases, (b) repulsion of the peptides by alteration of pathogen’s surface charge employing modifications by amino acids or amino sugars of anionic molecules (e.g., teichoic acids, lipid A and phospholipids), (c) alteration of bacterial membrane fluidity, and (d) expulsion of the peptides using multi drug pumps. Together with bacterial regulatory network(s) that regulate expression and activity of these mechanisms, they represent attractive targets for development of novel antibacterials.     

Platelet–bacterial interactions

Many bacteria are capable of interacting with platelets and inducing platelet aggregation. This interaction may be a direct interaction between a bacterial surface protein and a platelet receptor or may be an indirect interaction where plasma proteins bind to the bacterial surface and subsequently bind to a platelet receptor. However, these interactions usually do not trigger platelet activation as a secondary co-signal is also required. This is usually due to specific antibody bound to the bacteria interacting with FcγRIIa on the platelet surface. Secreted bacterial products such as gingipains and lipopolysaccharide may also be capable of triggering platelet activation.     

Host recognition by toxigenic plant pathogens

Certain fungal pathogens release host-selective (or host-specific) toxins (HST) as a host recognition factor during spore germination at the infection site on plants. Prior to penetration of the pathogen into its host, the released toxin specifically binds to a putative receptor on the host cells and initiates signaling mechanisms leading to pleiotropic effects on cells. Of these, the crucial one negates the general and inducible defense reactions of the cells. This is accomplished by a signal from the HST, which is transduced through a path way at or near the step of plasma membrane modulation, which is directly or indirectly triggered by the HST. This mechanism operates even though the toxin may affect mitochondria or chloroplasts as the primary target organelle. The fungal spore is able to penetrate the so-called 'narcotized cell' and completes the initial colonization of the host. The host recognition process may take place without necessitating host cell death, even in the case of perthophytic parasites. At the molecular level, HST-mediated recognition of the host by a pathogen requires strict stereochemical precision like a lock and key.     

Signal perception in plant pathogen defense

Highly sensitive and specific recognition systems for microbial pathogens are essential for disease resistance in plants. Structurally diverse elicitors from various pathogens have been identified and shown to trigger plant defense mechanisms. Elicitor recognition by the plant is assumed to be mediated by receptors. Plant receptors for fungus-derived elicitors appear to reside preferentially in the plasma membrane, whereas viral and bacterial elicitors may enter the plant cell and are perceived intracellularly. Receptor activation initiates an intracellular signal transduction cascade leading to stimulation of a characteristic set of plant defense responses. Isolation of plant elicitor receptors and their encoding genes is expected to provide significant information on the molecular basis of signal perception and intracellular signal generation in plant-pathogen interactions.     

Lactoferrin

Lactoferrin (LF) is a member of the transferrin family that is expressed and secreted by glandular epithelial cells and is found in the secondary granules of neutrophils. Originally viewed as an iron-binding protein in milk, with bacteriostatic properties, it is becoming increasingly evident that LF is a multifunctional protein to which several physiological roles have been attributed. These include regulation of iron homeostasis, host defense against a broad range of microbial infections, anti-inflammatory activity, regulation of cellular growth and differentiation and protection against cancer development and metastasis. While iron binding is likely central to some of the biological roles of LF, other activities, including specific interactions with mammalian receptors and microbial components, also contribute to the pleoitropic functional nature of this protein. In this article, recent advances in the understanding of these functions at the cellular and molecular level are discussed.     

Lactoferrin

Lactoferrin is an iron-binding glycoprotein of the transferrin family. Abundant expression and secretion of lactoferrin, in particular in milk and fluids of the digestive tract, are related to its implication in the first line of host defense. Lactoferrin is also a prominent component of the secondary granules of neutrophils (PMNs) and is released in infected tissues and blood during the inflammatory process. In addition to its direct antimicrobial properties, the abilities of lactoferrin to regulate the immune response and to protect against infection and septic shock have been described in numerous in vitro and in vivo studies. Although the cellular and molecular mechanisms that account for the modulation of the inflammatory and immune responses by lactoferrin are not yet totally elucidated, many are now established. At the cellular level, lactoferrin modulates the migration, maturation and function of immune cells. At the molecular level and in addition to iron binding, interactions of lactoferrin with a plethora of compounds, either soluble or membrane molecules, account for its modulatory properties. This paper reviews our current understanding of the cellular and molecular mechanisms that explain the regulatory properties of lactoferrin in host defence.     

Bacterial nonspecific acid phosphohydrolases: physiology, evolution and use as tools in microbial biotechnology

Bacterial nonspecific acid phosphohydrolases (NSAPs) are secreted enzymes, produced as soluble periplasmic proteins or as membrane-bound lipoproteins, that are usually able to dephosphorylate a broad array of structurally unrelated substrates and exhibit optimal catalytic activity at acidic to neutral pH values. Bacterial NSAPs are monomeric or oligomeric proteins containing polypeptide components with an M _r of 25 – 30 kDa. On the basis of amino acid sequence relatedness, three different molecular families of NSAPs can be distinguished, indicated as molecular class A, B and C, respectively. Members of each class share some common biophysical and functional features, but may also exhibit functional differences. NSAPs have been detected in several microbial taxa, and enzymes of different classes can be produced by the same bacterial species. Structural and phyletic relationships exist among the various bacterial NSAPs and some other bacterial and eucaryotic phosphohydrolases. Current knowledge on bacterial NSAPs is reviewed, together with analytical tools that may be useful for their characterization. An overview is also presented concerning the use of bacterial NSAPs in biotechnology. Received 21 November 1997; received after revision 10 March 1998; accepted 10 March 1998     

Host recognition by toxigenic plant pathogens

Certain fungal pathogens release host-selective (or host-specific) toxins (HST) as a host recognition factor during spore germination at the infection site on plants. Prior to penetration of the pathogen into its host, the released toxin specifically binds to a putative receptor on the host cells and initiates signaling mechanisms leading to pleiotropic effects on cells. Of these, the crucial one negates the general and inducible defense reactions of the cells. This is accomplished by a signal from the HSt, which is transduced through a path way at or near the step of plasma membrane modulation, which is directly or indirectly triggered by the HST. This mechanism operates even though the toxin may affect mitochondria or chloroplasts as the primary target organelle. The fungal spore is able to penetrate the so-called narcotized cell and completes the initial colonization of the host. The host recognition process may take place without necessitating host cell death, even in the case of perthophytic parasites. At the molecular level, HST-mediated recognition of the host by a pathogen requires strict stereochemical precision like a lock and key.     

Low-density lipoprotein and its effect on human blood platelets

Events leading to hyperactivity of human blood platelets are accompanied by an enhanced risk of atherosclerosis and arterial thrombosis. Lipoprotein disorders affect platelet functions, and hypersensitive platelets are observed in various stages of hyperlipidemia. Low-density lipoprotein (LDL), a circulating complex of lipids and proteins that is increased in hypercholesterolemia, enhances platelet function and increases sensitivity of platelets to several naturally occurring agonists. LDL sensitizes platelets via binding of apoB-100 to a receptor on the platelet membrane and via transfer of lipids to the platelet membrane. The receptor that mediates binding of LDL to the platelet and initiates subsequent intracellular signaling cascades has not yet been identified. Modification of native LDL generates a platelet-activating particle, and this interaction might contribute to the development of the atherosclerotic plaque. Lysophosphatidic acid is formed upon mild oxidation of LDL and is responsible for subsequent platelet activation induced by the modified LDL particle. Thus, LDL changes the functions of platelets via a broad spectrum of interactions.     

The invasion-associated type III secretion system of Salmonella typhimurium: common and unique features

Several bacterial pathogens make use of a specialized protein secretion system to inject effector proteins into host cells. This system, commonly referred to as type III secretion, is always associated with phenotypes related to intimate interactions between the pathogen and its respective host cells. The enteric pathogen Salmonella typhimurium utilizes a type III secretion system to invade nonphagocytic intestinal epithelial cells. Whereas the invasion-associated type III system of S. typhimurium has evolved to perform a specific function, many of the components of this system are conserved among the type III systems of other bacterial pathogens. This review will discuss the common and unique features of the S. typhimurium system in relation to the type III systems of other human pathogens. Topics discussed include the phenotypes associated with various type III systems, the genetic loci encoding these systems, the components of the type III secretion apparatus, the effector proteins and the mechanisms by which they enter host cells as well as the mechanisms used to regulate the expression of type III systems.     

Platelets and viruses: an ambivalent relationship

Thrombocytopenia is a frequent complication of viral infections providing evidence that interaction of platelets with viruses is an important pathophysiological phenomenon. Multiple mechanisms are involved depending on the nature of the viruses involved. These include immunological platelet destruction, inappropriate platelet activation and consumption, and impaired megakaryopoiesis. Viruses bind platelets through specific receptors and identified ligands, which lead to mutual alterations of both the platelet host and the viral aggressor. We have shown that HIV-1 viruses are internalized specifically in platelets and megakaryocytes, where they can be either sheltered, unaltered (with potential transfer of the viruses into target organs), or come in contact with platelet secretory products leading to virus destruction and facilitated platelet clearance. In this issue, we have reviewed the various pathways that platelets use in order to interact with viruses, HIV and others. This review also shows that more work is still needed to precisely identify platelet roles in viral infections, and to answer the challenge of viral safety in platelet transfusion.     

MAP kinase signalling: interplays between plant PAMP- and effector-triggered immunity

In plants, mitogen-activated protein kinase (MAPK) cascades are involved in regulating many biological processes including immunity. They relay signals from membrane-residing immune receptors to downstream components for defense activation. Arabidopsis MPK3/6 and MPK4 are activated in two parallel MAPK cascades during PAMP-triggered immunity. MPK3/6 have been implicated in the activation of various immune responses and their inactivation leads to compromised defense against pathogens. On the other hand, the MEKK1-MKK1/2-MPK4 cascade plays critical roles in basal resistance. Disruption of this MAPK cascade results in constitutive defense responses mediated by the NB-LRR protein SUMM2. Interestingly, SUMM2 guards the MEKK1-MKK1/2-MPK4 cascade activity indirectly through monitoring the phosphorylation status of CRCK3, which is a substrate of MPK4. From the pathogens’ side, a number of effectors are shown to target various components of MAPK cascades in plants. Inactivation of MPK4 by the Pseudomonas effector HopAI1 triggers SUMM2-mediated immunity. Together, these findings suggest intricate interplays between PAMP-triggered immunity and effector-triggered immunity via MAPK signaling.     

How plants recognize pathogens and defend themselves

Plants have an innate immunity system to defend themselves against pathogens. With the primary immune system, plants recognize microbe-associated molecular patterns (MAMPs) of potential pathogens through pattern recognition receptors (PRRs) that mediate a basal defense response. Plant pathogens suppress this basal defense response by means of effectors that enable them to cause disease. With the secondary immune system, plants have gained the ability to recognize effector-induced perturbations of host targets through resistance proteins (RPs) that mediate a strong local defense response that stops pathogen growth. Both primary and secondary immune responses in plants depend on germ line-encoded PRRs and RPs. During induction of local immune responses, systemic immune responses also become activated, which predispose plants to become more resistant to subsequent pathogen attacks. This review gives an update on recent findings that have enhanced our understanding of plant innate immunity and the arms race between plants and their pathogens. Received 24 June 2007; received after revision 18 July 2007; accepted 15 August 2007     

Human endoglin as a potential new partner involved in platelet–endothelium interactions

Complex interactions between platelets and activated endothelium occur during the thrombo-inflammatory reaction at sites of vascular injuries and during vascular hemostasis. The endothelial receptor endoglin is involved in inflammation through integrin-mediated leukocyte adhesion and transmigration; and heterozygous mutations in the endoglin gene cause hereditary hemorrhagic telangiectasia type 1. This vascular disease is characterized by a bleeding tendency that is postulated to be a consequence of telangiectasia fragility rather than a platelet defect, since platelets display normal functions in vitro in this condition. Here, we hypothesize that endoglin may act as an adhesion molecule involved in the interaction between endothelial cells and platelets through integrin recognition. We find that the extracellular domain of human endoglin promotes specific platelet adhesion under static conditions and confers resistance of adherent platelets to detachment upon exposure to flow. Also, platelets adhere to confluent endothelial cells in an endoglin-mediated process. Remarkably, Chinese hamster ovary cells ectopically expressing the human αIIbβ3 integrin acquire the capacity to adhere to myoblast transfectants expressing human endoglin, whereas platelets from Glanzmann’s thrombasthenia patients lacking the αIIbβ3 integrin are defective for endoglin-dependent adhesion to endothelial cells. Furthermore, the bleeding time, but not the prothrombin time, is significantly prolonged in endoglin-haplodeficient (Eng ^+/?) mice compared to Eng ^+/+ animals. These results suggest a new role for endoglin in αIIbβ3 integrin-mediated adhesion of platelets to the endothelium, and may provide a better understanding on the basic cellular mechanisms involved in hemostasis and thrombo-inflammatory events.     

Der Einfluss von Papaverin auf Funktionen der Blutplättchen

Summary The adhesiveness and the ADP-induced aggregation of human blood platelets as well as the agglomeration and viscous metamorphosis initiated by thrombin was inhibited by papaverin. The release of biogenic amines and ATP from rabbit blood platelets induced by thrombin or other proteolytic enzymes was diminished. Also eupaverin and ethylpapaverin have an inhibitory effect on the platelet functions.     

Reelin is a platelet protein and functions as a positive regulator of platelet spreading on fibrinogen

Abnormalities of platelet functions have been linked to reelin-impaired neuronal disorders. However, little attention has been given to understanding the interplay between reelin and platelet. In this study, reelin was found to present in the human platelets and megakaryocyte-like leukemic cells. Reelin-binding assays revealed that extracellular reelin can interact with platelets through the receptor belonging to the low density lipoprotein receptor gene family. The reelin-to-platelet interactions enhance platelet spreading on fibrinogen concomitant with the augmentation of lamellipodia formation and F-actin bundling. In contrast, reelin has no effect on integrin αIIbβ3 activation and agonist-induced platelet aggregation. Molecular analysis revealed that the up-regulation of Rac1 activity and the inhibition of protein kinase C δ-Thr505 phosphorylation are important for reelin-mediated enhancement of platelet spreading on fibrinogen. These findings demonstrate for the first time that reelin is present in platelets and the reelin-to-platelet interactions play a novel role in platelet signaling and functions.     

Activation of GCN2 upon HIV-1 infection and inhibition of translation

Higher eukaryotic organisms have a variety of specific and nonspecific defense mechanisms against viral invaders. In animal cells, viral replication may be limited through the decrease in translation. Some viruses, however, have evolved mechanisms that counteract the response of the host. We report that infection by HIV-1 triggers acute decrease in translation. The human protein kinase GCN2 (eIF2AK4) is activated by phosphorylation upon HIV-1 infection in the hours following infection. Thus, infection by HIV-1 constitutes a stress that leads to the activation of GCN2 with a resulting decrease in protein synthesis. We have shown that GCN2 interacts with HIV-1 integrase (IN). Transfection of IN in amino acid-starved cells, where GCN2 is activated, increases the protein synthesis level. These results point to an as yet unknown role of GCN2 as an early mediator in the cellular response to HIV-1 infection, and suggest that the virus is able to overcome the involvement of GCN2 in the cellular response by eliciting methods to maintain protein synthesis.