Thrombosis Research 2014:133:3-4. Invited commentary

Soluble P-selectin: The next step

Andrew D Blann PhD FRCPath

Haemostasis, Thrombosis and Vascular Biology Unit

University of Birmingham Centre for Cardiovascular Sciences

Department of Medicine

City Hospital, Birmingham, B18 7QH, UK.

Contact details

Dr A Blann

Tel/fax 00 44 121 507 5076

Email

Key words

Platelets, thrombosis, soluble P selectin, VTE

It has long been recognised that platelets are implicated in both mortality and morbidity in the pathophysiology of a variety of conditions, including coronary artery disease [1-3], diabetes [4], cancer [5], stroke [6], and deep vein thrombosis [7]. The success of aspirin in reducing thrombotic events, and the prognostic value of platelet volume, numbers and function in predicting cardiovascular death [7-10] are further evidence of the importance of this cell. Consequently, the ability to accurately identify those subjects at risk of thrombosis, and possibly those experiencing an acute thrombotic event, is highly sought-after if risk reduction and successful treatment with anticoagulants and antithrombotics are to be achieved. The assessment of inappropriate platelet activity in vivo is one route to identifying these at-risk people.

Although powerful tools for the dissection of platelet biology, techniques such as flow cytometry and whole blood and purified platelet aggregation, are slow and technically cumbersome in an assessment is demanded urgently. An alternative is plasma markers of platelet function, measurable with relative ease and rapidity by immunoassay, of which several models exist (such as near patient testing for INR and d-dimers). Over the decades, molecules such as platelet factor 4, thromboxane-beta2 and beta-thromboglobulin have generated interest as potential tools for assessing platelet activity [11-13]. However it is fair to say that these molecules have failed to take a leading place in routine clinical aspects of the pathophysiology of this cell (i.e. thrombosis), especially in an emergency setting. More recently, other aspect of platelet biology, such as components of the membrane of the platelet and its organelles, including CD40L, GpIIb/IIIa, GpV, GpVI and P-selectin have all received attention [13,14]. It has been argued that sections of these molecules that have been shed and/or cleaved from the membrane (hence becoming soluble) are markers of platelet function and turnover, and of these, there is a considerable weight of literature on soluble P-selectin (sPsel).

Originally described by McEver, Wagner, the Furies, and others as GMP-140 and PADGEM, P-selectin was found to be an adhesion molecule (CD62P) component of the membrane of the alpha granule (and, incidentally, of the membrane of the Weibel-Palade body), and that alpha-degranulation resulted in increased surface expression and subsequent presence in the plasma [15-18]. Increased sPsel in different animal models of thrombosis and in a variety of thrombotic and cardiovascular diseases and their risk factors [18-21] led us to the hypothesis that it had potential as a clinical marker of platelet activation [22], a viewpoint which is gaining acceptance [13,23-25]. Furthermore, there is increasing evidence, the most powerful being an animal model [26], that increased sPsel not merely marks excessive platelet activity, but directly promotes thrombosis. Interfering with P-selectin binding reduces thrombosis in several animal models [27-29]. Of course, all of this is fascinating, but what does it do for the practising clinician? - and what tools does it give him or her to help their patients?

One of the difficulties in the accurate diagnosis of a life-threatening acute coronary syndrome (ACS) is differentiating alternative diagnoses for the chest pain, such as those with a gastric and/or musculoskeletal aetiology. Although platelet activation is likely in ACS and not in the differential diagnoses, increased sPsel in a patient in whom an ACS is suspected may in fact be due to numerous other factors such as sub-clinical arthritis or an occult tumour. But the whole point about the work of Body et al [30] is that they have found that low levels of sPsel are likely to be indicative of not having a heart attack - so that it can exclude a lot of people from an inappropriate diagnosis, much as a low d-dimer results effectively excludes deep vein thrombosis or pulmonary embolism. They used sPsel measurement alongside standard indices of troponin and an ‘ischaemic’ ECG, finding that a model combining all three had 97.6% sensitivity, 52.8% specificity, and 99.0% negative predictive value for acute myocardial infarction. They concluded that the use of this model could obviate the need for hospital admission in 44.2% of patients, 2.5% of whom would be expected to develop an acute coronary episode. But this situation is different once the ACS has been diagnosed and the patient moved to a coronary care unit. Here, sPsel measured in blood taken 24 to 96 hours after an ACS is unable to predict those who will die or will suffer a further ACS [31]. But given the fact that an ACS patient will be heavy loaded with antiplatelets and anticoagulants, this is hardly surprising.

And what of venous thrombosis? Several groups have shown in case-control studies that raised levels of sPsel are present in the blood of subjects who have suffered a venous thromboembolic event (VTE) up to several years in the past [32-39]. But probably more interestingly, some have shown in follow-up studies in patients with cancer and in others that have already suffered one VTE, that increased levels of sPsel predict those who are more likely to suffer an event [40,41]. In this issue of the Journal, Antonopoulos et al [42] have pooled all this data into a meta-analysis which demonstrates unequivocally that sPsel is increased after a VTE. Not included in this meta-analysis is the report of Vandy et al [43], who again showed increased sPsel in those suffering a VTE. Although it could be said that it is hardly surprising that those with a VTE have raised sPsel, there are implications. Leaving aside the position of cancer, where raised sPsel predicts VTE [40], if it is indeed the case that platelet-over activity is a cause of VTE, then why is it that standard treatment focuses on anticoagulation, not on anti-platelet therapy? It is established that vitamin K antagonism is more effective than aspirin in reducing the risk of primary and secondary VTE, so what is it that raised sPsel adds to diagnosis and treatment?

The answer lies not in sPsel alone, but in sPsel in combination with other markers, as demonstrated by Body et al in the diagnosis of ACS [30]. The statistical power of the studies of Ramacciotti et al [39] and Vandy et al [43] allowed these authors to construct models that improved the sensitivity, specificity, positive predictive value and negative predictive value of existing screening methods for VTE. The current gold-standard laboratory tool in VTE diagnosis is the determination of d-dimer by a simple near-patient method such as latex agglutination, which provides to front line staff a result with a minimum of delay [44]. If the findings described above are to be translated from a research setting to being able to provide active help to the health care professional faced with a diagnostic problem in a patient with chest pain or a painful swollen leg, then a rapid method for sPsel needs to be available, as it is for d-dimer. The current method, enzyme linked immunosorbent assay, is too slow and unwieldy, but a latex agglutinating method may be possible. Our colleagues in industry should give us technique such as this.

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