the invention will now be described by way of example with reference to the accompanying experimental data and drawings in which : fig1 . cross - linking of pecam - 1 inhibits collagen - stimulated platelet aggregation . ( a ) pecam - 1 cross - linking on platelet surfaces results in its tyrosine phosphorylation and does not stimulate platelet aggregation . ( i ) pecam - 1 was immunoprecipitated from washed human platelets under resting conditions or following pecam - 1 cross - linking . proteins were separated by sds - page and immunoblotted to detect phosphotyrosine residues ( upper panel ). immunoprecipitation was verified by reprobing for pecam - 1 ( lower panel ). ( ii ) pecam - 1 was cross - linked on washed platelets and aggregation was monitored using optical aggregometry . ( b ) ( i ) platelets were incubated with isotype - matched control igg for 5 minutes before the addition of f ( ab ′) 2 cross - linker for 90 seconds and then were stimulated with collagen ( 100 mm ( mg / ml )). aggregation was monitored using optical aggregometry . ( ii ) cross - linking of pecam - 1 inhibits collagen - stimulated platelet aggregation . platelets were stimulated with collagen ( 100 mm ( mg / ml )) for 90 seconds with and without first cross - linking pecam - 1 and platelet aggregation monitored by optical aggregometry . data are representative of 3 separate experiments . tyr ( p ), tyrosine phosphorylation ; pecam - 1 xl , pecam - 1 cross - linking . fig2 . pecam - 1 cross - linking inhibits gpvi - and thrombin receptor - stimulated platelet aggregation . ( a ) platelets were stimulated with cvx at ( i ) 15 nm ( ng / ml ) and ( ii ) 62 . 5 nm ( ng / ml ) with and without first cross - linking pecam - 1 , and aggregation responses were monitored by optical aggregation ( arrow indicates the addition of cvx ). ( b ) platelets were stimulated with thrombin at ( i ) 0 . 05 u / ml and ( ii ) 0 . 1 u / ml with and without prior cross - linking of pecam - 1 . platelet aggregation was monitored by optical aggregometry ( arrow indicates the addition of thrombin ). treatment of platelets with isotype - matched control igg before stimulation with cvx ( 15 nm ( ng / ml )) and thrombin ( 0 . 05 u / ml ) is shown in a ( i ) and b ( i ), respectively . data are representative of 3 separate experiments . pecam - 1 xl , pecam - 1 cross - linking . fig3 . platelet - dense granule secretion is inhibited by pecam - 1 signalling . platelets were loaded with [ 3 h ] 5 - ht before stimulation with cvx ( 62 . 5 nm [ ng / ml ]) or thrombin ( 0 . 1 u / ml ). where required , pecam - 1 was cross - linked prior to the addition of an agonist . secretion of [ 3 h ] 5 - ht into cell medium was measured using scintillation spectrometry . [ 3 h ] 5 - ht release is ex - pressed as a percentage of total tissue content after subtraction of basal secretion values . results represent mean +/− se ( n = 3 ). student &# 39 ; s t - test was used to compare pecam - 1 cross - linked and non - cross - linked sample for statistical significance . (* indicates p - values & lt ; 0 . 05 . pecam - 1 xl , pecam - 1 cross - linking fig4 . pecam - 1 signaling inhibits platelet protein tyrosine phosphorylation . platelet lysates were prepared in laemmli buffer from nonstimulated platelets ( stirred with buffer alone ) and platelets stimulated for 90 seconds with cvx ( 15 nm [ ng / ml ]) or thrombin ( 0 . 05 u / ml ). before stimulation , pecam - 1 was cross - linked in some samples ( a ) or platelets were incubated with isotype - matched control igg and cross - linker f ( ab ′) 2 ( b ). proteins were separated by sds - page under reducing conditions and immunoblotted to detect protein tyrosine phosphorylation . pecam - 1 xl , pecam - 1 cross - linking fig5 . pecam - 1 cross - linking inhibits the mobilization of calcium from intracellular stores . fura - 2 am loaded human platelets were stimulated with either cvx or thrombin , and the mobilization of calcium was measured fluorometrically ( arrow indicates the addition of agonist ). panel a ) treatment of platelets with control igg and cross - linker f ( ab ′) 2 has no effect on cvx - and thrombin - stimulated calcium mobilization . fig6 : panels b i and ii ) show representative calcium responses for stimulation with cvx at 62 . 5 nm ( ng / ml ) and 15 nm ( ng / ml ), respectively , with and without first cross - linking pecam - 1 . traces are representative of 3 separate experiments . panel ( iii ) shows pecam - 1 - induced percentage reduction in peak levels of intracellular calcium stimulated by 62 . 5 , 31 . 25 , and 15 nm ( ng / ml ) cvx . data presented represents mean +/− se ( n = 3 ). fig7 : panels c i , and ii ) show representative calcium responses for stimulation with thrombin at 0 . 1 u / ml and 0 . 05 u / ml , respectively , with and without first cross - linking pecam - 1 . traces are representative of 3 separate experiments . panel ( iii ) pecam - 1 - induced percentage reduction in peak levels of intracellular calcium stimulated by 0 . 5 , 0 . 1 , and 0 . 05 u / ml thrombin . data represent mean +/− se ( n = 3 )). pecam - 1 xl , pecam - 1 cross - linking . platelet endothelial cell adhesion moleule - 1 pecam - 1 is a 130 kda membrane - spanning glycoprotein whose expression is restricted to several haematopoietic cell types including platelets , monocytes , neutrophils , certain t - lymphocytes and also vascular endothelial cells . the functions of pecam - 1 are diverse and include angiogenesis , vasculogenesis , integrin regulation , transendothelial migration of leukocytes , and t - and b - cell antigen receptor function , although the role of this molecule in platelets is presently unclear . when pecam - 1 was cloned it was assigned to the family of cell adhesion molecules on the basis of structural similarities . pecam - 1 is involved in adhesion although much attention has been recently been directed to studying its ability to participate in signal transduction . the cytoplasmic tail of pecam - 1 contains a conserved motif called an immunoreceptor tyrosine based inhibitory motif ( itim ), which underlies its signaling properties and is shared by a growing family of inhibitory receptors . these include the immunoglobulin g receptor fcγriib , the killer inhibitory receptors ( kir ) and signal regulatory proteins ( sirps ), although pecam - 1 is the only itim - bearing receptor that has been reported to be expressed in platelets . it has therefore been proposed that pecam - 1 should be assigned to the ig - itim family of receptors . the ligand binding properties of pecam - 1 are complex . it has the capacity for homophilic interactions , and also heterophilic interactions with a number of molecules that include integrin α v β 3 and cd38 , pecam - 1 becomes phosphorylated on tyrosine residues in response to a variety of stimuli that include pecam - 1 cross - linking , activation of the high affinity receptor for immunoglobulin e ( fcεri ), shear and oxidative stress . the inventors have recently found that platelet activation via the collagen receptor glyoprotein gpvi ( gpvi ) and thrombin receptors result in pecam - 1 tyrosine phosphorylation which is not dependent on platelet aggregation and secretion , although tyrosine phosphorylation is enhanced by aggregation . the tyrosine residues that become phosphorylated in pecam - 1 have been mapped and fall within the itim . phosphorylated itims recruit signalling molecules such as the tyrosine phosphatases shp - 1 and shp - 2 that bind to the motif via src - homology 2 domain interactions . indeed , both shp - 1 and shp - 2 have been shown to associate with tyrosine phosphorylated pecam - 1 , and pecam - 1 itim phosphopeptides activate these phosphatases in vitro . generally , these protein tyrosine phosphatases exhibit inhibitory effects by counteracting tyrosine kinase - dependent pathways , although shp - 2 has been shown to positively regulate growth factor receptor signalling . immunoreceptor tyrosine - based activatory motif -( itam ) bearing receptors have been shown to have a critical place in the regulation of platelet function . indeed the collagen receptor gpvi - fcr γ - chain complex signals through an itam present on the cytoplasmic tail of the fcr γ - chain . several studies in other cell systems have provided evidence of an antagonistic relationship between itam and itim containing receptors when expressed on the same cell . an example of this is the receptor for igg fcγriib ( itim ) that negatively regulates cell activation stimulated by fcγriia ( itam ). pecam - 1 was stimulated through cross - linking using antibodies directed to the extracellular domain of the receptor . this strategy was the chosen as the most specific manner to activate pecam - 1 , and activation was confirmed since cross - linking stimulated its tyrosine phosphorylation . pecam - 1 cross - linking stimulates tyrosine phosphorylation ( fig1 ) and association of shp - 2 ( not shown ) but does not itself cause platelet activation . tyrosine phosphorylation of pecam - 1 on cross - linking was shown not to be dependent on integrin α iib β 3 engagement . cross - linking pecam - 1 for 90 s prior to stimulation with collagen caused inhibition of platelet aggregation . at lower concentrations of collagen , aggregation was inhibited completely , but even at very high concentrations of collagen ( 100 μg / ml ) pecam - 1 activation caused a substantial inhibition of aggregation . since collagen is also able to bind other receptors on the platelet , including the integrin α 2 β 1 , the effect of pecam - 1 cross - linking on gpvi - mediated platelet aggregation using the specific agonist cvx was examined . similar results were obtained , with complete inhibition of aggregation at lower concentrations of agonist ( 15 ng / ml ) and partial effects at higher concentrations ( 62 . 5 ng / ml ), indicating that pecam - 1 is a potent inhibitor of gpvi - mediated ( itam ) platelet activation . experiments were coducted to determine whether the inhibitory effect of pecam - 1 is restricted to signalling via itam containing receptors . on the contrary , pecam - 1 cross - linking was also found to inhibit thrombin - stimulated platelet aggregation . although no inhibitory effect of pecam - 1 was observed at moderate concentrations of thrombin ( 0 . 5 and 1 u / ml , results now shown ) dramatic levels of inhibition were observed at very high concentrations of collagen ( 100 μg / ml ). pecam - 1 mediated inhibition of thrombin - stimulated aggregation was observed only a lower thrombin concentrations ( complete inhibition with 0 . 05 u / ml and slight inhibition at 0 . 1 u / ml ). showing that pecam - 1 activation inhibits thrombin - stimulated platelet aggregation less efficiently than collagen - stimulated aggregation . the effect of pecam - 1 cross - linking on platelet aggregation stimulated by other g protein - coupled receptor agonists was also examined . aggregation in response to low concentrations of the thromboxane mimetic u46619 were also reduced by pecam - 1 signalling . in addition , preliminary work suggests that adp - induced platelet aggregation at low agonist concentrations may also be affected . the inventors have demonstrated that activation of pecam - 1 signalling by antibody - mediated cross - linking results in inhibition of collagen - mediated activation . they have also shown that the effects of the gpvi - selective agonist convulxin , are inhibited by activation of pecam - 1 signalling . thrombin - stimulated activation has also been shown to be inhibited by the activation of pecam - 1 signalling , indicating that the inhibitory effects of pecam - 1 are not restricted to the inhibition of itam - containing receptor signalling pathways . this is consistent with pecam - 1 performing a negative regulatory role in the control of platelet activation stimulated by both itam - and non - itam - containing receptor agonists . even though previous studies by for example duncan g s , andrew d p , takimoto h , et al : genetic evidence for functional redundancy of platelet / endothelial cell adhesion molecule - 1 ( pecam - 1 ): cd31 - deficient mice reveal pecam - 1 - dependent and pecam - 1 - independent functions . j . immun . 1999 ; 162 : 3022 - 3030 , and patil s , newman d k , newman p j : pecam - 1 serves as an inhibitory receptor that modulates platelet responses to collagen . blood . 2001 ; 97 : 1727 - 1732 , have reported that thrombin and adp stimulated platelet aggregation in pecam - 1 - deficient mouse platelets is normal . the observation that itam - mediated signalling is inhibited to a higher degree than non - itam - mediated signalling is further supported by experiments in platelets where pecam - 1 was co - ligated with fcγriia ( an itam - containing immunoglobulin g receptor ). co - ligation results in inhibition of fcγriia - mediated platelet aggregation and intracellular calcium mobilisation stimulated by receptor cross - linking . the inhibitory effects of pecam - 1 on stimulation with agonists such as the collagen , cvx , thrombin , and the thromboxane mimetic u46619 , suggest that pecam - 1 inhibits primary signalling events and also secondary stimulation by factors released by activated platelets . it is possible , therefore , that inhibition of receptors that cross - talk with gpvi - mediated signalling may contribute to the inhibitory actions of pecam - 1 on collagen - and convulxin - stimulated activation . this may also explain why the inhibitory functions of pecam - 1 on gpvi - mediated activation are more effective than on thrombin - mediated activation . having established that pecam - 1 cross - linking inhibits platelet function , the effect of this on some aspects of signal transduction was examined . platelet activation was found to be accompanied by a concomitant inhibition of platelet protein tyrosine phosphorylation and decreased levels of calcium mobilisation from intracellular stores . platelet activation by the collagen receptor gpvi is dependent on tyrosine kinases , and consequently is associated with the rapid tyrosine phosphorylation of a wide variety of platelet proteins . stimulation with thrombin results in protein tyrosine phosphorylation , but to a lesser degree . cross - linking pecam - 1 , which in itself does not alter protein tyrosine phosphorylation levels , inhibits substantially the level of tyrosine phosphorylation that is induced by subsequent stimulation with cvx or thrombin . this is consistent with the reduction in aggregation and secretion observed . the identities of the phosphoproteins whose phosphorylation is reduced on pecam - 1 cross - linking is currently under investigation . underlying the pecam - 1 mediated inhibition of platelet activation , is a significant level of inhibition of calcium release from intracellular stores . as seen with aggregation assays , at lower concentrations of cvx and thrombin ( 15 ng / ml ) ad 0 . 05 u / ml , respectively ) pecam - 1 signalling inhibits release almost completely , where a partial effect is observed at higher agonist concentrations . calcium mobilisation is stimulated through the intracellular generation of inositol 1 , 4 , 5 - trisphosphate ( ip 3 ) from phosphatidylinositol 4 , 5 - bisphosphate by phospholipase c . it is well established that in platelets , stimulation with collagen leads to phosphorylation and activation of plcγ2 isoform , and thrombin signalling regulates plc β . pecam - 1 cross - linking results in a substantial and significant reduction in total inositol phosphates stimulated by cvx and thrombin . which suggests that pecam - 1 exerts its effects on collagen - and thrombin - stimulated signalling either upstream of the plc isoforms , or on these enzymes themselves . materials — horm - chemie collagen ( collagen - fibres from equine tendons ) was purchased from nycomed ( munich , germany ). convulxin was purified from the venom of the rattlesnake ( crotalus durissus terrificus ) as described previously . the thromboxane mimetic u46619 and adp was purchased from sigma ( poole , uk ). anti - phosphotyrosine monoclonal antibody ( 4g10 ) was from upstate biotechnology ( tcs biologicals , buckinghamshire , uk ). anti - pecam - 1 antibodies : monoclonal antibody hcl / 6 was from serotec ( oxford , uk ); polyclonal anti - pecam - 1 ( c - 20 ) and monoclonal antibody ab468 were from autogen bioclear ltd . ( wiltshire , uk ); monoclonal antibody pecam 1 . 3 was kindly provided by professor peter newman ( the blood centre of woutheastem wisconsin , milwaukee , wis ., usa ). control mouse igg , was purchased from sigma ( poole , uk ). monoclonal antibody iv . 3 was purified from hybridoma cell culture medium and f ( ab ′) 2 fragments generated by pepsin digestion using reagents purchased from pierce ( perbio scientific , chester , uk ). horseradish peroxidase - conjugated secondary antibodies and the enhanced chemiluminescence detection system were purchased from amersham pharmacia biotech ( buckinghamshire , uk ). fura - 2 am was from molecular probes ( cambridge bioscience , cambridge , uk ). preparation and stimulation of platelets : human platelets from drug - free volunteers were prepared on the day of the experiment by differential centrifugation and suspended in modified tyrodes - hepes buffer ( 134 mm nacl , 0 . 34 mm na 2 hpo 4 , 2 . 9 mm kcl , 12 mm nahco 3 , 20 mm hepes , 5 mm glucose , 1 mm mgcl 2 , ph7 . 3 ) to a density of 2 × 10 8 cells / ml . for protein precipitation experiments , platelets were resuspended at 8 × 10 8 cells / ml in buffer containing 1 mm egta to prevent aggregation . stimulation of platelets ( 450 μl ) with coliagen , convulxin ( cvx ) and thrombin ( delivered in 50 μl ) was performed at 37 ° c . in an optical platelet aggregometer ( chrono - log corporation ) with continuous stirring ( 1200 rpm ). pecam - 1 activation was stimulated by incubation with anti - pecam - 1 antibodies ( ab468 , 1 μ / ml or pecam 1 . 3 , 10 μg / ml ) for 5 minutes , followed by incubation for 90 s with f ( ab ′) 2 fragments of anti - igg secondary antibodies ( 30 μg / ml ) to cross - link . mab ab468 was generated against full length pecam - 1 ectodomain . control experiments were performed using an irrelevant isotype - matched antibody in place of ab468 or pecam - 1 . 3 and used at the same concentration . in some experiments the low affinity receptor for igg was blocked by incubation with a saturating concentration ( 1 μg / ml ) of f ( ab ′) 2 fragments of mabiv . 3 for 5 minutes . saturating concentrations of mabiv . 3 antibody were established by determining the concentration of f ( ab ′) 2 fragment that completely inhibited subsequent whole igg - mediated fcγriia cross - linking and platelet activation . platelet aggregation was determined by optical aggregometry . immunoprecipitation studies : platelet stimulation was terminated by the addition of an equal volume of ice cold lysis buffer ( 2 % [ v / v ] nonidet p40 , 20 mm tris , 300 mm nacl , 10 mm edta , 1 mm phenylmethylsulfonyl fluoride , 2 mm na 3 vo 4 , 10 μg / ml leupeptin , 10 μg / ml aprotinin , 1 μg / ml pepstatin a , ph7 . 3 ). detergent - insoluble debris was removed and the lysates were pre - cleared by mnixing with protein a - sepharose for 1 hour at 4 ° c . ( 20 μl of a 50 % [ w / v ] suspension of protein a - sepharose of tris - buffered saline - tween [ tbs - t : 20 mm tris , 137 mm nacl , 0 . 1 % [ v / v ] tween 20 , ph7 . 6 ). protein a - sepharose was removed from the lysates before the addition of anti - pecam - 1 antibody ( hcl / 6 , 1 μg ). following rotation at 4 ° c . for 1 hour , 0 . 5 μl secondary antiserum was added ( rabbit anti - mouse igg ) and mixed for a further 30 min . 25 μl protein a - sepharose suspension was added to each sample and mixing continued for 1 hour before washing the sepharose pellet in lysis buffer followed by a wash with tbs - t , and the addition of laemmli sample - treatment buffer . proteins were separated by sds - page under reducing conditions using 10 % gels and transferred to polyvinylidene difluoride membranes by semi - dry western blotting . immunoblotting : membranes were blocked by incubation in 10 % ( w / v ) bovine serum albumin ( bsa ) dissolved din tbs - t . primary and secondary antibodies were diluted in tbs - t containing 2 % ( w / v ) bsa , and incubated with membranes for 1 hour at room temperature . blots were washed for 2 hours in tbs - t following each incubation with antibodies for 1 hour at room temperature , and then developed using an enhanced chemiluminescence detection system . primary antibodies were used at a concentration of 1 μg / ml ( anti - phosphotyrosine , 4g10 ; anti - pecam - 1 , c - 20 ) and horseradish peroxidase - conjugated secondary antibodies were diluted 1 : 10000 . 5 - hydroxytryptamine ( 5 - ht ) secretion assay : platelets were loaded with [ 3 h ] 5 - ht by incubation with 0 . 5 μci / ml of platelet - rich plasma for 1 h at 37 ° c . platelets were prepared from the platelet - rich plasma as described above . stimulation of platelets was terminated by addition of an equal volume of 6 % glutaraldehyde and microcentrifugation , and the level of [ 3 h ] 5 - ht release into the supernatant was determined by scintillation spectrometry . [ 3 h ] 5 - ht release was expressed as a percentage of the total tissue content following subtraction of release under basal conditions . measurement of [ ca 2 + ] 1 by spectrofluorimetry : washed human platelets ( prepared as above ) were incubated at 2 × 10 9 cells / ml in calcium free tyrodes - hepes buffer with 3 μm fura - 2 am for 45 min . platelets were washed once and resuspended at 2 × 10 8 cells / ml in modified tyrodes - hepes buffer . stimulation of platelets ( 450 μl ) in the presence of 2 mm egta with cvx and thrombin ( delivered in 50 μl ) was performed with constant stirring at 37 ° c . in a luminescence spectrophotometer ( ls - 50b , perkin elmer ) with excitation wavelengths of 340 nm and 380 nm . fluorescence emission was measured at a wavelength of 510 nm . where required , pecam - 1 was cross - linked prior to stimulation with agonist as described above . the ration of emission values ( excitation at 340 / 380 nm ) was calculated and converted to calcium concentration using flwinlab software ( perkin elmer ) utilising the equation [ ca 2 + ] 1 = k d ×( r − r min )/ r max − r )× sfb ( where r is emission ratio value ( 340 / 380 nm ). r max the maximum 340 / 380 ratio , was determined by lysing platelets with 25 μm digitonin in the presence of 1 mm cacl 2 . the r min 340 / 380 ratio was obtained by addition of 2 mm egta . k d is the dissociation constant of the fura - 2 / ca 2 + complex ( 224 nm ) and sfb is the fluorescence ratio at 340 / 380 nm of r min and r max . statistical analysis : determination of statistical significance was performed using student &# 39 ; s paired t - test . the results are expressed as the mean +/− standard error of the mean ( s . e . m .). cross - linking pecam - 1 inhibits collagen - stimulated platelet aggregation : an antagonistic relationship has been reported between itim - and itam - containing receptors when expressed in the same cell . since the platelet collagen receptor gpv1 signals through an itam present on the fcr γ - chain with which it is associated , the inventors investigated the effect of pecam - 1 signalling on platelet activation with collagen . pecam - 1 was activated by incubation with antibodies specific for the ectodomain of pecam - 1 ( ab468 [ fig1 ] or pecam - 1 . 3 [ not shown ]) and cross - linked with a secondary antibody [ f ( ab ′) 2 fragment ]. this resulted in increased tyrosine phosphorylation of the protein , and did not result in stimulation of platelet aggregation ( fig1 a ). tyrosine phosphorylation of pecam - 1 was maintained on cross - linking in the presence of egta ( 1 mm ), rgds peptide ( 0 . 5 mm ) and the γ - chain peptide of fibrinogen ( 100 μm ) added separately or altogether ( not shown ). this , together with the fact that these experiments were performed on washed platelets indicates that the tyrosine phosphorylation of pecam - 1 on cross - linking is not dependent on integrin α iib β 3 engagement . the effect of pecam - 1 cross - linking for 90 s prior to stimulation with collagen was found to have a marked inhibitory effect on collagen - stimulated platelet aggregation . at lower concentrations of collagen ( e . g . 10 μg / ml ), cross - linking of pecam - 1 before agonist addition completely abolished aggregation ( data not shown ). fig1 b ( ii ) shows the marked inhibitory effect of pecam - 1 cross - linking on a very high concentration of collagen ( 19 μg / ml ). the use of an isotype - matched igg control and cross - linker f ( ab ′) 2 , had no effect of pecam - 1 tyrosine phosphorylation ( not shown ) and collagen - stimulated platelet aggregation [ fig1 b ( i )]. results are representative of 3 separate experiments . similar results were obtained using the alternative anti - pecam - 1 antibody pecam 1 . 3 . in some experiments the low affinity receptor for igg fcγriia was blocked prior to pecam - 1 cross - linking and agonist stimulation using a saturating concentration of f ( ab ′) 2 fragments of mabiv . 3 . the inhibitory effect of pecam - 1 cross - linking was unaltered under these conditions . this clearly indicates that the inhibitory effect of pecam - 1 using antibodies is not due to activation of fcγriia . pecam - 1 cross - linking inhibits gpvi - and thrombin receptor - mediated platelet aggregation : given the marked effect of pecam - 1 signalling on collagen - mediated platelet aggregation the inventors investigated whether this effect was restricted to gpvi - mediated signalling only . gpvi was stimulated using the selective agonist cvx , a protein purified from the venom of the rattlesnake crotalis durissus terrificus . aggregation stimulated with 15 ng / ml cvx was completely inhibited by prior activation of pecam - 1 [ fig2 a ( i )], and a partial inhibitory effect was observed at higher concentrations of cvx [ 31 . 25 and 62 . 5 ng / ml , not shown and in fig2 a ( ii ), respectively ]. similar results were observed on stimulation of platelets with the g protein coupled receptor agonist thrombin . complete inhibition of aggregation at 90 s stimulation was observed at a thrombin concentration of 0 . 05 u / ml [ fig2 b ( i ) and a partial effect at 0 . 1 u / ml fig2 b ( ii )]. the use of an isotype - matched igg control and cross - linker f ( ab ′) 2 had no effect on cvx - or thrombin - stimulated platelet aggregation [ fig2 a ( i ) and 2 b ( i )]. no inhibitory effect of pecam - 1 activation was observed at higher concentrations of thrombin ( e . g . 0 . 5 and 1 u / ml , data not shown ). results are representative of 5 separate experiments . similar results were obtained using the alternative anti - pecam - 1 antibody pecam - 1 . 3 , and when fcγriia was blocked prior to pecam - 1 cross - linking . platelet secretion is inhibited by pecam - 1 signalling : platelet activation is accompanied by secretion from dense - granules . dense granule secretion was assessed by measuring the release of [ 3 h ] 5 - ht from pre - loaded washed platelets . fig3 shows the results of experiments to determine the effect of pecam - 1 cross - linking on [ 3 h ] 5 - ht secretion . a significant reduction in secretion was observed in platelets where pecam - 1 was activated before stimulation with cvx ( 81 . 9 % +/− 2 . 9 to 39 . 8 % +/− 4 . 1 , p = 0 . 02 , n = 3 ) or thrombin ( 70 . 9 % +/− 4 . 6 to 37 . 0 % +/− 8 . 0 , p = 0 . 01 , n = 3 ). the use of an isotype - matched igg control and cross - linker f ( ab ′) 2 had no effect on cvx - or thombin - stimulated dense granule secretion ( not shown ). experiments performed in the presence of mab iv . 3 to block the fc receptor fcγriia produced similar results . pecam - 1 inhibits platelet protein tyrosine phosphorylation : the effect of pecam - 1 cross - linking on gpvi - and thrombin receptor - stimulated signalling was also investigated . platelets were stimulated with cvx ( 15 ng / ml ) or thrombin ( 0 . 05 u / ml ) with or without prior cross - linking of pecam - 1 for 90 s . whole cell protein tyrosine phosphorylation levels were determined by immunoblotting . cross - linking pecam - 1 alone had no effect on basal platelet tyrosine phosphorylation levels ( fig4 ). stimulation with cvx ( 15 ng / ml ) or thrombin ( 0 . 05 u / ml ) caused an increase in the level of tyrosine phosphorylation of a broad range of proteins . in those samples where pecam - 1 signalling was stimulated by cross - linking before incubation with cvx or thrombin , total tyrosine phosphorylation was reduced ( fig4 ). the use of an isotype - matched igg control and cross - linker f ( ab ′) 2 had no detectable effect on cvx - or thrombin - stimulated total tyrosine phosphorylation levels . pecam - 1 inhibits the mobilisation of calcium from intracellular stores : stimulation of the collagen receptor gpvi and thrombin receptors leads to rapid intracellular mobilisation of calcium , an effect that is essential for secretion and aggregation . intracellular calcium levels were measured fluorometrically using the calcium sensitive dye fura - 2 am . experiments were performed in the presence of 2 mm egta to prevent the entry of extracellular calcium . stimulation of platelets with cvx and thrombin resulted in a rapid increase in the levels of intracellular calcium that declined over a period of approximately 5 minutes . the incubation of platelets with control antibody ad cross - linker f ( ab ′) 2 caused no change in cvx - and thrombin - stimulated intracellular calcium mobilisation ( fig5 ). the cross - linking of pecam - 1 for 90 s before incubation with cvx ( 62 . 5 ng . ml ) or thrombin ( 0 . 1 u / ml ) resulted in a markedly reduced level of calcium mobilisation [ fig6 b ( i ) and 7 c ( i )]. at the lower concentrations of agonists used ( cvx , 15 ng / ml ; thrombin , 0 . 05 u / ml ) calcium mobilisation was almost abolished [ fig6 b ( ii ) and 7 c ( ii )]. the effect of pecam - 1 cross - linking on the reduction of peak intracellular calcium levels for a range of agonist concentrations is shown in fig6 b ( iii ) and 7 c ( iii ). a reduction of at least 50 % in calcium mobilisation was observed at all of the concentrations of cvx and thrombin tested . similar results were obtained using the alternative anti - pecam - 1 antibody pecam - 1 . 3 . furthermore , experiments performed in the absence of extracellular egta indicate that pecam - 1 cross - linking does not inhibit agonist - induced influx of calcium ( data not shown ).
activation of pecam - 1 with , for example , a small molecule or antibody derivative provides a new therapeutic route for modifying platelet activation and thrombus formation which has utility in the treatment or prevention of cardiovascular diseases such as thrombosis , stroke , and vascular occlusion and in the treatment or prevention of homeostasis disorders . this treatment approach activates a general inhibitory mechanism . a screen for activators of pecam - 1 include pecam - 1 , an ectodomain of pecam - 1 , the cytoplasmic tail of pecam - 1 , the itim of pecam - 1 , an active site of pecam - 1 , a recombinant extracellular domain of pecam - 1 , or a part of derivative thereof and ways for detecting activation or cross - linking or phosphorylation or tyronsine phosphorylation of pecam - 1 , an ectodomain of pecam - 1 , the cytoplasmic tail of pecam - 1 , the ipim of pecam - 1 , an active site of pecam - 1 , a recombinant extracellular domain of pecam - 1 , or a part or derivative thereof .