Connection between the Platelets and Immunity

1.Hemos 2.intro

Introduction

Platelets are a unique type of mammalian blood cell. They are anucleate molecules which play an integral role in host homeostasis, immune defence and repair. At sites of infection or injury in a host organism, platelets will be deployed rapidly where they will display their role in defence and signalling. Platelets are activated by agonists, of which there are several. Thrombin, collagen, ADP, platelet activating factor (PAF) and other factors all promote the activation of platelets. There are two types of platelet activation, primary and secondary, both of which lead to platelet aggregation. Primary platelet aggregation occurs when the specialised surface receptors of platelets bind to the negative charges of injured endothelial area (exposed collagen) whilst secondary platelet activation refers to activation of the platelets by their actin and microtubule system as well as by their secretion of coagulation factors. The potency of platelet agonist varies depending on the activation response

Hemostasis

Platelets' foremost role is hemostasis. It not only helps the animal from blood loss but it also prevents microorganisms from entering the animal which may cause disease. The role of Platelets in homeostasis would suggest evolution from more primitive multifunctional innate defensive cells as although highly differentiated they have inflammatory and antimicrobial systems and link clotting and immune cascades (A.S Weyrich et al). Hemostasis is crucial to animals, when a blood vessel of theirs becomes damaged or injured. It is the first stage of wound healing. Platelets are present in the blood circulation at a volume of 2-8 x 1011/l, depending on the species. They are unable to move themselves so they passively move with the blood circulation (page 131). After an injury to a blood vessel occurs, vascular constriction is the organisms initial reaction. Platelets are not usually attracted to endothelial surfaces. When an injury of a blood vessel happens, platelets combine with their specialized surface receptors to the negative charges of the endothelial regions. This is primary aggregation (page 200-201). Also, Von Willebrand factor, which is a multimeric glycoprotein released by the endothelial cells plays a key role in the adhesion of platelets to the subendothelium.(M. Hermmann et al). This allows platelet aggregation to increase. Secondary activation follows. The microtubule system which is located beneath the plasma membrane and the actin cytoskeleton system of the platelets activate the reaction (S. Severin et al). This causes white thrombosis to arise. This happens when by filopodia are produced which provides the framework for platelets to attach to each other and also collagen fibres. Endothelial production of prostacyclines and nitrogen monoxyde ceases. Their role is to prevent the production of factors responsible for aggregation. Accordingly, the thrombocytes can now produce their own stimulating factors. These include TXA2, serotonin, adenosine-diphosphates that will stimulate other thrombocytes increasing the aggregation as well as the secretion of coagulation factors (page 201 and 202). This shows how platelets are pivitol in hemostasis. Without them, a relatively innocuous injury to a blood vessel may be fatal. They help heal wounds and by doing this help to prevent harmful microorganisms which could be disease causing from entering the animal. If these disease causing microorganisms were to off got into the body the animal would have to endure processes such as the production of antibodies to fight the infection. These processes are energy consuming and put the animal under stress. Therefore platelet's role in hemostasis can be looked upon as a temporary mechanical barrier between the organism and the outer environment, reducing the threat of disease when an animal is wounded.

A.S Weyrich et al The evolving role of platelets in inflammation J.Thromb. Haemost., 1 (2003), pp. 1897-1905) http://www.jstor.org/discover/10.2307/30107584?searchUri=%2Faction%2FdoBasicSearch%3FQuery%3Dvon%2Bwillebrand%26Search%3DSearch%26gw%3Djtx%26prq%3Dplatelets%2Bvon%2Bwillebrand%26hp%3D25%26acc%3Doff%26aori%3Doff%26wc%3Don%26fc%3Doff&Search=yes&searchText=von&searchText=willebrand&uid=3738216&uid=2129&uid=2134&uid=2&uid=70&uid=4&sid=21102878826017 http://onlinelibrary.wiley.com/doi/10.1111/jth.12053/pdf

Platelets are rapidly targeted to sites of injury and infection

Due to their rapid deployment and activation platelets will display similar surveillance and information transfer abilities as those possessed by dendritic cells, mast cells and macrophages. These cells are pre-positioned in tissues to detect microbial invasion and wounding whilst providing advanced molecular intelligence and directives for subsequent responses of the immune system . Platelets undergo rapid homotypic aggregation in the blood and adhesion to subendothelial matrix proteins and Von Willebrand factor. These responses are crucial in the role of homeostastis . Previous studies ( P.B Maguire, D.J. Fitzgerald) would indicate that platelets also aggregate at sites of bacterial invasion, accumulate in inflammatory exudates, and target tissues subjected to antigen-antibody-mediated injury. This study would also suggest that by aggregating around the bacteria, platelets might promote their clearance from the blood. Further it indicates that adherent platelets are carried by migrating polymorphonuclear leukocytes (neutrophils) to extravascular sites of inflammation and that they might be able to migrate into extravascular tissue. With these abilities the platelets should have the capability of performing rapid targeting mechanism thus localizing platelets to both intravascular compartments and the extravascular milieu in incidents of injury, microbial invasion and inflammation Once activated, platelets interact with and transfer information to other cells of immune defence, such as monocytes or neutrophils. Should platelets accumulate in the vessels or in the extracellular space, they also possess the ability to signal mast cell, macrophages and dendritic cells. Simultaneously these tissue surveillance cells or sentinel cells can also transfer signals to the platelets through the production of substances such as Platelet Activating Fator (PAF). PAF is an inflammatory mediator that is recognized by a receptor on the platelet plasma membrane. Therefore platelets can both supply and respond to signals at early points of control in inflammation and immune progression.

Cell-Cell interactions involving platelets

Activated platelets participate in crucial cell-cell interactions and provide signals in the immune continuum . The following are examples of cellular interactions with the potential to induce, amplify or modify multiple inflammatory or adaptive immune events.

A. Activated human platelets adhere to and signal monocytes, forming mixed cellular aggregates which will induce expression of inflammatory gene products.

B. Activated platelets synthesize and release Interleukin-1beta, which can then signal endothelial cells and other target cells. A consequence of signalling by this mechanism is the expression of genes that code for adhesion molecules and chemokines that mediate targeting and local activation of Neutrophils and Monocytes.

C. Platelets signal maturation and activation of dendritic cells in vitro. This may prove to a key cellular interaction that enables platelets to modify T- and B-Lymphocyte functions in response to experimental viral challenge in vitro.

Human Platelets can activate peripheral Blood B Cells and increase production of Immunoglobins

Platelets represent a link between hemostasis, inflammation and tissue repair. Their role in immune response and inflammation mainly involves many molecules such as Toll-Like receptors,CD40 and CD154. A study to examine the involvement of platelets in CD154 (as they are the main purveyors of this molecule) dependent immune response was carried out in order to understand the interactions between platelets and peripheral B-Lymphocytes. The results of this experiment revealed that in coculture, platelets and B-lymphocytes were mutually activated, demonstrated by the increase expression of platelet CD62p and B-cell CD86. The Platelet/B-Cell interactions were also accompanied by changes in membrane expression of CD40 and CD154 by both platelets and B-Lymphocytes. IL 12p70 and IL8 gene transcription were also significantly reduced, which was attributable to B-Cells. Conversely there was a significant, platelet-dependent reduction of sCD154 and CCL5 mRNA expression. After a 3-Day incubation period B-Cells increased their production of IgG1, IgG2, IgG3 but not IgG4, IgA or IgM. This data illustrates the potentially important role of platelets in the adaptive immune response. Platelets have an immunoregulatory role that might be applied clinically in multitransfused patients .

Platelet Mediated Modulation of Adaptive Immunity

Platelets play a major role in immunity through hemostasis and insuring structural composition of the vessel wall. Recently scientists have discovered platelets have a broader purpose than initially thought in the role of immunity. Platelets are also significantly important in the modulation of inflammation.

A molecule of great influence is CD154 which platelets express upon activation. Through CD154 expression, activated CD4+ T cells provide the signals required for the differentiation of T cell dependent B lymphocyte responses. A process that includes isotype switching. Yang and Wilson 1996, Renshaw et al. 1994, and Caux et al. 1994. CD154 expression on activated CD4+ T cells augment the DC maturation process. A requirement for cross priming and enhancing cytotoxic CD8+ T cells responses through the classical pathway. Ridge et al. 1998, Schoenberger et al. 1998and Bennett et al. 1998. The following experiments prove these theorys and therefore the importance of platelet derived CD154 in immunity. Adoptive transfer studies demonstrate that platelets activated physiologically with adenoviral infection (splenic and hepatic inflammation) are sufficient via CD154 for the induction of isotype switching by B cells and augmentation of the CD8+ T cell response during a viral infection Normal mice depleted of platelets prior to adenovirus infection exhibit decreased efficiency of production of adenovirus specific IgG underscoring the importance of platelets in the generation of an optimal adaptive immune response. Cd154 only present on activated platelets In previous experimental studies (Henn et al 1998, Henn et al. 2001, Ahmad et al. 2001, Diacovoet al 1994, Bombelo et al. 1998, Geba et al. 1996, Collins et al. 1994, Gawaz et al. 2000 and Barry et al. 1998) it was proven that platelet derived CD154 was only present in activated platelets. (platelet activation discussed previously) In the experiment, unactivated platelets and platelets activated with thrombin or collagen were tested for surface CD154. Results show that only activated platelets expressed surface CD 154. CD154 causes Dentritic cell maturation In later experiments, bone marrow derived dentritic cells were generated and incubated with activated platelets. The production of interleukins which are produced naturally by dendritic cells in response to antigenic stimulation was measured. The results show how activated platelets induced the interleukin production by the DC’s in a CD154 dependent mannor. CD154 induces B cell isotype switching The ability of B lymphocytes to switch from the production of IgM/IgD to IgG or other Ig isotypes requires CD40 activation (Renshaw et al. 1994). B cell activation in the absence of CD40 ligation results in hyper IgM syndrome with minimal switching to IgG secretion. As documented by Zhang et al. 2001 and Jaffee et al. 1992 mice injected with andenovirus result in splenic and hepatic inflammation. With this knowledge, experiments were carried out to assess the ability of platelet derived CD154 to induce B lymphocyte isotype switching in mice deficient in CD154 Adenovirus was injected intravenously into a CD154 deficient mouse. Activated platelets from normal mice were injected and then adenovirus specific Ig isotypes were measured. Therefore the transfer of normal activated platelets was sufficient to induce isotype switching Isotype switching was abrogated by anti- CD154. Although adenovirus specific IgM was produced IgG was not, therefore linking CD154 to the isotype switch to IgG. CD154 modulates CD 8+ T cell responses In the experiment, platelets derived from normal an CD154 derived mice were transferred in CD154 derived mice hosts. The mice were then challenged iv with Ad5-mOVA. Ten days later the spleens were isolated and ova-specific lytic activity was measured. Results show that there is adenovirus induced cytolytic T lymphocyte activity in CD154 derived mice , however it show lytic activity equivilant to mice receiving diluents without platelet transfer. Results in mice receiving normal platelets containing CD145 on the surface exhibit enhanced CTL activity. These findings indicate that platelet derived CD154 is sufficient to modulate CD8+ T cell responses.