the paradigm of intercellular cell signaling mechanisms has until recently been assumed to be limited to direct cell  cell contact and via local soluble factor release .
however , there has been increasing interest in the role of plasma membrane - derived small lipid / protein vesicles in cell  cell communication in the last decade .
microparticles ( mp ) are one such type of vesicles gaining more recognition as their biological roles are being explored .
although there is no standardized terminology for these small extracellular vesicles , the term , microparticle , usually refers to a subset of extracellular , cell membrane - derived vesicles ranging from 0.1 to 1  m in diameter , shed directly from the plasma membrane through the process of ectocytosis ( gasser and schifferli , 2004 ; cocucci et al . , 2009
although mp are shed from steady - state cells , elevated levels are observed in actively proliferating cells and cells activated by a wide range of stimuli , from mitogens and inflammatory mediators , to antigenically foreign compounds , such as pathogen components , as well as various other stress - inducing challenges in both in vitro and in vivo models ( ratajczak et al . , 2006 ;
in vivo , mp are most commonly found in blood , however , they are also observed in other bodily fluids , such as synovial fluid ( berckmans et al .
, 2005 ; distler et al . , 2005b ) and urine ( smalley et al . , 2008 ) .
microparticles are often compared with exosomes and apoptotic bodies ( table 1 ) , however , while superficial structural similarities exist between them , both the mechanism of mp formation and their subsequent composition are fundamentally different ( thry et al . , 2009 ; mause and weber , 2010 ; gyrgy et al . , 2011 ) .
mp shedding is a multi - enzyme - regulated process . under normal conditions , the asymmetric composition of the plasma membrane
is maintained by three principal translocase enzyme families of flippases , floppases , and scramblases , which actively maintain phosphatidylcholine ( pc ) and sphingomyelin ( sm ) on the external leaf , while phosphatidylserine ( ps ) and phosphatidylethanolamine ( pe ) are kept on the internal leaf . on cellular activation
, these enzymes are inhibited following an influx of ca into the cytoplasm , and consequently the normal phospholipid asymmetry is disrupted ( coltel et al . , 2006 ; piccin et al . , 2007 ; pap et al . ,
this is accompanied by the loss of actin  spectrin anchorages and the degradation of actin filaments by calpain .
these protrusions are pinched off as individual mp in a rho - associated kinase i ( rock - i)-dependent manner from the cell membrane , with ps and pe exposed on the outer surfaces .
in contrast , exosomes fall into the 50100  nm range and are formed by inward budding into the endosomal lumen of small intra - luminal vesicles ( ilv ) that aggregate to become multivesicular bodies ( mvb ) .
mvb subsequently fuse with the plasma membrane to release the ilv into the extracellular space as exosomes ( thry et al . ,
exosomes are enriched in cd63 , and major histocompatibility complex ( mhc ) i and ii molecules , but tend to have low levels of ps and pro - coagulation factors on the surface ( denzer et al .
, 2000 ; thry et al . , 2009 ; mause and weber , 2010 ; gyrgy et al . , 2011 ) .
apoptotic bodies , yet another type of subcellular vesicle , are much larger ( 14  m ) and are essentially fragments of cells produced in late stage apoptosis ( table 1 ) .
the process by which mp are produced results in their possessing many of the characteristics of the cell from which they originated .
this includes many of the surface molecules , such as integrins , receptor ligands , co - stimulatory molecules , as well as cytoplasmic contents , such as activated signaling molecules and even genetic materials e.g. , mrna ( baj - krzyworzeka et al . , 2006 ; aliotta et al . , 2010 ) and mirna ( yuan et al . , 2009 ) .
mp either attach and fuse to their target cell ( del conde et al . , 2005 ) , or are internalized and processed by pinocytic or phagocytic mechanisms ( dasgupta et al .
since mp carry biological molecules from their cells of origin , they may induce associated responses in their target cells , depending on the source of mp , the cell type on which they act and the route by which they are incorporated .
given that mp are produced in response to many stress conditions and are involved in several immunological processes , it seems likely that mp are involved in the regulation of the immune response against invading pathogens , however , their potential role in infectious diseases has not been extensively studied .
this review aims to consolidate current evidence on their biological role and venture opinion on their potential impact in infectious disease .
one of the hallmarks of bacterial , viral , fungal , or parasitic infection is the formation of an inflammatory milieu around infected or affected cells .
pathogen - associated molecular pattern ( pamp ) recognition receptors ( prr ) , such as toll - like receptors , certain classes of rna helicases , like the retinoic acid inducible gene - i ( rig - i ) , and nod - like receptors ( nlr ) , specifically detect components of the invading pathogen and trigger a cascade of intracellular signaling events that lead to the activation of transcription to initiate the first wave of innate immune responses ( creagh and oneill , 2006 ; seth et al . , 2006 ) .
however , endotoxins , produced by bacteria during infection , or the sheer load of replicating intracellular virus would undoubtedly cause stress in affected cells
. indeed , generation of mp may be closely associated with the initiation of apoptosis and necrosis ( ardoin et al . , 2007 ) .
whether stress - inducible proteins , such as hypoxia - inducible factor 1 ( hif-1 ) , heat - shock factor ( hsf-1 ) , heat - shock protein ( hsp ) family , nuclear factor b ( nf-b ) , jun nh2-terminal kinase ( jnk ) , and p53 ( ohiro et al . , 2003 ; razorenova et al . , 2005 ; thompson and locarnini , 2007 ; zinkernagel et al .
, 2007 ; de maio , 2011 ; li et al . , 2011 ; rawat and mitra , 2011 ; zhang et al . , 2011 ) and
mhc class i polypeptide - related sequence a and b ( mica , micb ; stern - ginossar and mandelboim , 2009 ) are involved , remains to be determined .
nevertheless , a broad array of pro - inflammatory cytokines , such as tumor necrosis factor ( tnf ) , interleukin-1 ( il-1 ) , il-6 , and type 1 interferons ( ifn - i ) are produced in response to infection ( medzhitov and horng , 2009 ) , and many of these induce mp shedding in the absence of infection ( combes et al . , 1999 ; sheremata et al .
2008 ) . not surprisingly , therefore , both stimulation in vitro with lipopolysaccharide ( lps ; aharon et al . , 2008 ; bernimoulin et al . ,
2009 ) and intravenous injection of bacterial endotoxin in vivo ( zubairova et al . , 2006 ; wang et al . ,
furthermore , elevated blood mp levels are also observed in septic patients ( nieuwland et al . , 2000 ; mostefai et al . ,
2008 ) , in human immunodeficiency virus ( hiv ) patients ( aupeix et al . , 1997 ) and ebola virus - infected macaques ( geisbert et al . , 2003 ) , as well as malaria patients and plasmodium - inoculated laboratory animals ( combes et al . , 2004 , 2005 ; penet et al . , 2008 ; couper et al . , 2010 ; pankoui mfonkeu et al . ,
many activated immune cells not necessarily infected by pathogens , such as monocytes , macrophages , dendritic cells ( dc ) , and lymphocytes , produce mp when activated by relevant stimuli .
mp have been shown to be produced by monocytes and macrophages stimulated with a23187 ( cerri et al . ,
, 2006 ; aharon et al . , 2008 ) , and starvation ( aharon et al . , 2008 ) , by dc stimulated with lps ( obregon et al .
, 2006 , 2009 ) , by t cells stimulated with agonistic cd3 antibodies , fas ligand , heat , actinomycin - d , staurosporine , pha , pma ( scanu et al . , 2008 ) , and by neutrophils stimulated with fmlp and c5a ( gasser and schifferli , 2004 ) . in addition , mp are produced by cells isolated from patients with autoimmune disease , such as diabetes ( martin et al . , 2004 ) , as well as vascular disease ( tesse et al . , 2005 ) .
the wide variety of possible stimuli producing mp in the absence of infection makes it difficult to predict pathways leading to mp production in infectious disease , or predict their function in this context and few experimental models utilize infectious pathogens .
nevertheless , these studies provide valuable clues to what possible responses might occur in infection .
as mentioned above , elevated blood mp levels have been reported in models of sepsis , viral infection , and cerebral malaria .
because of the generalized inflammatory conditions that such events generally create , it seems reasonable to propose this would be the case for most , if not all infections accompanied by significant hematogenous spread , i.e. , during parasitemia , bacteremia , viremia or fungemia .
furthermore , due to the propensity of mp to carry contents derived from their cells of origin , it is likely that they contain antigens , either present or induced in infected cells .
plasmodium parasite antigens were detected in the mp purified from the plasma of infected animals ( couper et al . , 2010 ) .
this is unlikely to be unique to parasitic infections , as infecting viruses inevitably become an integral part of the cell , inducing a range of virus - specified proteins crucial for their replication and these could become incorporated into the mp contents , either specifically or non - specifically . thus , mp could remotely convey viral antigens via the blood stream to professional antigen - presenting cells ( apc ) , such as macrophages and dc residing centrally in the spleen and liver without the necessity for their direct interaction with infected cells .
both organs normally have high throughputs from the systemic circulation , increasing the likelihood that mp will interact with apc . from a host point of view , this maintains a desirable separation between the peripheral site of infection and the site of immune induction .
additionally , tnf - stimulated , endothelial cell - derived mp have the capacity to induce activation and maturation of plasmacytoid dc , although not conventional dc ( angelot et al . , 2009 ) .
together , such remote interactions could increase the probability of detecting infection early , enabling prompt initiation of innate and adaptive responses by apc responding to mp in the blood .
indeed , it has been shown that mp produced by mycobacterium - infected macrophages were able to activate and present antigen to naive t cells ( ramachandra et al . , 2010 ) , thus obviating the necessity for direct apc and t cell contact to generate a t cell response . in these experiments , mp were better at inducing t cell proliferation than exosomes .
( 2010 ) also demonstrated the activation of bone marrow - derived macrophages , with upregulation of cd40 and tnf expression .
this raises the further possibility that mp may facilitate the mobilization of subsets recruited from the bone marrow during infection ( king et al .
, 2007 ; getts et al . , 2008 ) . as well as accepting mp by fusion , dc also produce mp following activation .
lps - induced mp from dc can transfer antigen to recipient dc , which can present this to t cells ( obregon et al .
. other cellular membrane function can be transferred by mp from activated to nave t cells via the tetraspanin , cd81 , important for adhesion and lymphocyte maturation ( fritzsching et al .
, 2002 ; quast et al . , 2011 ) . furthermore , cd154 platelet - derived mp introduced into cd154 mice triggers cd4 t cell - dependent production of antigen - specific igg and germinal center formation , suggesting that with the right surface molecules mp can fully activate the adaptive immune response ( sprague et al . , 2008 ) .
indeed , mp transfer could explain the rapid exchange of t cell receptors ( tcr ) observed in the early stages of t cell activation to expand antigen - specific t cell clones ( chaudhri et al . , 2009 ) .
irrespective , it seems likely that mp containing pathogen antigens along with the necessary co - stimulatory molecules can activate t cells , bypassing cell  cell contact , thus enabling a wider , more efficient initiation , amplification and/or modulation of early adaptive immune responses .
microparticles may also contain pro - inflammatory cytokines , such as il-1 , providing an alternative or additional pathway for cytokine secretion into the extracellular space to modulate local cytokine / chemokine signaling ( mackenzie et al .
depending on their mode of manufacture and cellular release , it seems highly likely that other cytokines / chemokines , may make use of this pathway ( garcia et al .
, 2005 ; dean et al . , 2009 ) . certainly , t cell - derived mp have been shown to activate monocytes ( distler et al .
, 2005a ; huber et al . , 2007 ; scanu et al . , 2008 ) and monocytes activated with mp from pha- or
pma - stimulated blood t cells produce tnf , il-1 , and soluble interleukin-1 receptor antagonist ( sil-1ra ; scanu et al . , 2008 ) .
mp derived from t cells and monocytes also induce nf-b - dependent expression of matrix metalloproteinases ( mmp ; distler et al . , 2005b ) , as well as some pro - inflammatory cytokines and chemokines such as il-6 , il-8 , macrophage chemoattractant protein ( mcp)-1 , mcp-2 , and regulated upon activation , normal t cell expressed and secreted ( rantes ) in synovial fibroblasts ( berckmans et al . , 2005 ; distler et al . , 2005b ) .
monocyte / macrophage - derived mp also upregulate inflammatory mediator synthesis by epithelial cells ( cerri et al . , 2006 ) .
not surprisingly , under certain conditions mp can also induce anti - inflammatory il-10 expression in monocytes ( kppler et al . , 2006 ) and transforming growth factor 1 ( tgf-1 ) in macrophages ( sadallah et al .
, mp can even be pro - apoptotic , especially in macrophages , which can undergo apoptosis upon uptake ( distler et al .
for example , neutrophil - derived mp specifically upregulate the expression of tgf-1 , which helps to suppress the inflammatory response ( distler et al .
the mp used in these models induce macrophage apoptosis , possibly because the mp were generated from apoptotic cells , which might have transferred the apoptotic signals to the macrophages ( bing et al . , 2008 ) .
this is similar to the behavior of macrophages to dampen inflammation after ingesting apoptotic bodies , to prevent over - reaction ( savill et al . , 2002 ) .
however , these studies did not use pathogens and macrophages activated by plasmodium were in contrast highly pro - inflammatory ( couper et al . ,
, mp might contain factors induced or expressed by the infecting organism that could impede or abrogate the early induction of an effective immune response .
for example , influenza a ns1 protein expressed intracellularly has the capacity to suppress ifn - i production and inhibit apoptosis to enable better viral replication ( pauli et al . , 2008 ; wang et al . ,
several flavivirus subunits interfere with ifn - signaling at various levels and in different ways ( reviewed in daffis et al .
such molecules , carried into cells yet to be infected could arguably give the infecting organism a survival advantage , enabling greater levels of replication in the host for longer periods of time , thus increasing the probability of transmission to vectors and/or to the next host .
in the case of arboviruses , considering the relatively short times that organisms spend in the blood stream , even small increases in concentration and time spent in the blood stream could potentially subtend a significant survival advantage .
the inflammatory conditions created by mp could in principle play important roles in controlling infection and pathogen clearance .
how crucial these roles are in the immune system remain unclear , but as one of many factors that regulate the immune response against invading pathogens , mp could significantly influence the outcome for the host , both during the initial  knockdown  phase of controlling pathogen and in the ongoing adaptive response to it .
pro - inflammatory cytokines induced by mp would act to manage and contain the infection , while chemoattractant proteins would send signals to recruit leukocytes to the site of infection to eliminate the pathogen .
indeed , a further interesting characteristic of mp is their ability to upregulate adhesion molecules such as intercellular adhesion molecule-1 ( icam - i ; berckmans et al . , 2005 ) in target cells and promote monocyte recruitment ( mause et al . , 2005 ) .
along with ability of mp to induce mmp expression and tnf release , the endothelium becomes more permeable for immune active proteins and leukocytes to access the site of infection .
although this is an important step to enable pathogen clearance , it can be a double - edged sword in certain infectious diseases . in cases of lethal flavivirus encephalitis ,
in which immunopathology is one of the prominent features of pathogenesis , excessive monocyte infiltration of the central nervous system ( cns ) , and the accompanying pro - inflammatory cytokine spike are a major cause of mortality ( king et al . , 2007 ; getts et al . , 2008 ) .
we hypothesize that during viremia preceding encephalitis , flavivirus infecting endothelium , and blood monocytes induce mp , both as a consequence of infection and via tnf released by infected cells ( cheng et al . ,
brain barrier , potentially enabling virus access to the cns ( wang et al . , 2004 ) .
mp from tnf - activated endothelium could further activate uninfected monocytes and endothelium , while mp from infected cells could carry viral antigen , as well as activation signals to the liver and spleen .
early antigen presentation would likely advantage the host in generating anti - viral immune responses . however , wnv - infected cns neurons recruit activated ( ccr2 ) monocytes from the bone marrow to the brain via ccl2 ( getts et al . , 2008 ) . here ,
tnf- and mp - mediated activation of cns endothelium would facilitate macrophage margination , adhesion , and diapedesis in response to the chemokine gradient , thus potentially increasing numbers of monocytes recruited to the brain and exacerbating immunopathology ( figure 1 ) . putative role of mp in immunopathogenesis of flavivirus encephalitis .
after local replication of virus at the site of infection , the ensuing viremia results in endothelial and monocyte infection .
mp produced as a result of infection and via the consequent tnf secretion , are transported in the blood stream . on the one hand , mp from tnf - activated endothelium could further activate uninfected monocytes and endothelium at remote sites , such as in the brain , while mp from infected cells could carry viral antigen and activation signals to liver and spleen .
early antigen presentation would likely advantage the host by generating anti - viral immune responses . on the other hand ,
endothelial activation may enable increased migration of monocytes into the brain in response to ccl2 secreted from infected neurons . while this immigration may initially enable early viral clearance , over - exuberant immigration of activated monocytes that differentiate into macrophages may be immunopathological for neurons via toxic soluble factors produced .
this does not exclude a possible  trojan horse  scenario , in which virus may also further gain access to the brain via the cerebral endothelium and/or via infected immigrant monocytes during encephalitis .
brain barrier in cerebral malaria via monocyte , tnf , and mp activities ( combes et al . ,
moreover , in mice deficient in the floppase , atp - binding cassette transporter a1 , which modulates ps distribution on the outer plasma membrane leaflet , mp production is reduced in cerebral malaria , abrogating neurological signs in this model , and presenting the possibility for pharmacological modulation of mp to ameliorate immunopathology ( combes et al . , 2005 ; penet et al . ,
2008 ) . immunological aspects aside , mp could potentially also be used directly as transport by viruses , including flavivirus , influenza , and herpes simplex virus ( hsv ) that propagate via a process of ectocytosis from the plasma membrane of an infected cell . in a model of hiv-1 , the glycome profile and
the budding signal of the virion were similar to that of the mp secreted in conjunction , suggesting that the mechanism for viral propagation was similar to that of the formation of mp ( krishnamoorthy et al .
even if mp formation selectively excludes budding virions , the mp may still potentially contain unpackaged viral genome and other viral proteins necessary for replication in the target cell . in either scenario ,
mp may be used by viruses as  trojan horses  to spread to other cells .
indeed , hiv-1 induces mp that  express  cd45 , cd80 , cd86 , as well as mhc - i and ii ( esser et al . , 2001 ) , thus , potentially masking themselves as being of  host  origin .
in addition , hiv also induces ccr5-containing mp and can transfer this receptor to other peripheral blood mononuclear cells ( pbmc ) deficient of endogenous ccr5 , enabling infection of these cells ( mack et al . , 2000 ) .
however , the success of allogeneic transplantation of 32/32 ccr5 stem cells in keeping the patient hiv free , indicates that much more remains to be understood about these processes ( htter et al . , 2009 ) .
as with all emerging areas , a number of technical issues remain to be fully resolved .
for example , the accurate determination of mp size and phenotype , their clear distinction from and relationship to , other membrane vesicles , and their pure separation for functional experimental purposes , remain a significant challenge ( gyrgy et al . , 2011 ) .
while mp clearly have likely roles in homeostasis and inflammation , few studies have to date directly addressed their involvement in infectious disease .
thus , many questions remain to be answered , perhaps most important among which are , whether the production of mp as a programmatic response to infection confers an overall species survival advantage . at the mechanistic level ,
also , how cellular decision - making occurs to produce mp , what they should contain , and whether production of the full range of membranous vesicles is required and/or produced kinetically to accommodate host requirements from initial infection to disease resolution , remains to be elucidated . as with most defenses against invading pathogens , on the one hand , the transfer of phenotype and function by mp may be of considerable benefit to the host , in the early warning and initiating of immune defenses at both innate and adaptive levels .
on the other , however , this may exacerbate immune responses leading to immune pathology , or be commandeered by the invading pathogen for its own survival .
the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
 despite their clear relationship to immunology , few existing studies have examined the potential role of microparticles ( mp ) in infectious disease . 
 mp have a different size range from exosomes and apoptotic bodies , with which they are often grouped and arise by different mechanisms in association with inflammatory cytokine action or stress on the source cell . 
 infection with pathogens usually leads to the expression of a range of inflammatory cytokines and chemokines , as well as significant stress in both infected and uninfected cells . 
 it is thus reasonable to infer that infection - associated inflammation also leads to mp production . 
 mp are produced by most of the major cell types in the immune system , and appear to be involved at both innate and adaptive levels , potentially serving different functions in each . 
 thus , they do not appear to have a universal function ; instead their functions are source- or stimulus - dependent , although likely to be primarily either pro- or anti - inflammatory . 
 we argue that in infectious diseases , mp may be able to deliver antigen , derived from the biological cargo acquired from their cells of origin , to antigen - presenting cells . 
 another potential benefit of mp would be to transfer and/or disseminate phenotype and function to target cells . 
 however , mp may also potentially be manipulated , particularly by intracellular pathogens , for survival advantage . 