Reticulated Platelets: A Promising Prognosis Marker in Cardiovascular Diseases

Reticulated Platelets Predict Cardiovascular Death and Adverse Events in Coronary Artery Disease: A Systematic Review and Meta-analysis

The term of reticulated platelets (RPs) was first used to describe a large synthesized platelets by Ingram and Coopersmith in 1970s.[1] In recent years, the role of these immature RPs has gained attention due to their prognostic contribution in cardiovascular diseases, especially in coronary artery disease (CAD).[2]

Approximately 100 billion new platelets must be produced daily from megakaryocytes to maintain a physiological platelet count, and the production rate is even higher in patients with sympathetic stress, inflammation, and in several acute and chronic diseases including acute myocardial infarction.[3] [4] Indeed, platelet abnormalities have also been implicated in adverse outcomes in other noncardiovascular conditions.[5] [6] Various pathophysiological mechanisms for the adverse impact of activated platelets have recently been reviewed.[7] [8] [9]

RPs are newly formed platelets (immature) with high granule content and a residual amount of megakaryocyte-derived mRNA.[10] [11] Immature platelets have been shown to adhere more in response to collagen exposure, contain higher concentration of adenosine triphosphate, adenosine monophosphate and glycogen, synthesize more prothrombotic factors such as thromboxane A2 leading to increased platelet activation, carry higher glycoprotein IIb/IIIa complex expression, which enhances platelet aggregation, and to be less responsive to antiplatelet therapy.[12] [13] RPs also seem to play an important role in the coagulation cascade, through enhanced participation in pro-thrombinase complex assembly, as indicated by the higher expression of both surface-bound α-granule factor V and factor X upon stimulation with thrombin.[14] These properties make larger platelets functionally more active. Regarding the relationship between RPs and cardiovascular outcomes seen in various studies,[12] in a transcriptome analysis of RPs, a significant enrichment of prothrombotic genes has been detected in RPs providing the first biological explanation for their pro-aggregatory phenotype and indicating a causal association of RPs with adverse cardiovascular events ([Fig. 1]).[11]

Fig. 1 Biological and prognostic features of reticulated platelets (RPs).

In this current issue of Thrombosis and Haemostasis, Bongiovanni and colleagues[15] conducted an interesting systematic review and meta-analysis, including seven studies of acute coronary syndrome (ACS) and chronic coronary syndrome (CCS) patients, trying to clarify the current evidence of the role of RPs in these disorders. In 2,213 patients included, the authors observed a significant higher risk association in patients with high rate of RPs for major adverse cardiovascular and cerebrovascular events (odds ratio [OR]: 2.67 [1.87–3.81], I² = 43.8%) and cardiovascular mortality (OR: 2.09 [1.36–3.22], I² = 40.4%) compared to patients with a low rate of RPs. No significant associations were observed for the components of myocardial infarction, stroke, urgent revascularization, or risk of bleeding. Several studies and systematic reviews also investigated the relationship between RPs and short- and long-term cardiovascular outcomes in patients with ACS or CCS. Cesari et al[10] measured RP levels at the time of presentation with ACS and found that RP levels independently predicted cardiovascular mortality at 1 year (OR: 4.15, 95% CI: 1.24–13.91, p = 0.02). Faber et al[12] conducted a systematic review of 42 studies and observed a positive association between measures of immature platelets and major adverse cardiovascular events in acute and stable ischemic heart disease, both in short-term and long-term follow-up. Recently, Zhao et al[16] also performed a systematic review and meta-regression analysis of six cohort studies of patients with ACS and CCS and the pooled results indicated that elevated RPs were associated with a higher risk of composite cardiovascular events (risk ratio [RR]: 2.26; 95% CI: 1.72–2.98) and cardiovascular death (RR: 2.33; 95% CI: 1.66–3.28). However, a specific study of patients with CCS provided contrasting results. Pedersen et al[17] analyzed 900 stable CAD patients and the markers of immature platelets did not predict future cardiovascular events during a 3-year follow-up.

A possible explanation of these contradictory findings could be the definition of RPs/immature platelets and the lack of standardized test protocols. So, it makes hard to compare study results and limit the clinical utility of these cells as potential diagnostic and prognostic markers.[18] There are different factors to evaluate immature platelets: mean platelet volume (MPV), immature platelet count (IPC), and immature platelet fraction (IPF).[19] The automated determination of RPs using IPC and IPF appears to be the preferable quantification method for clinical use given its highly standardized testing approach, which allows the comparison of results between different cohorts.[18] However, MPV may be easier to measure and an increase in MPV has been noted in a number of patient groups with known CAD risk factors, such as smoking habit, diabetes mellitus, obesity, hypertension, and hypercholesterolemia but it is not equivalent to RPs.[20] MPV is a measure of platelet size but not all large platelets are RPs. The IPF—as an index of elevated platelet turnover—might be superior in predicting ischemic risk compared to MPV or specific platelet function tests.[21] These heterogeneity in platelet function test used could influence the results of the trials.

The other explanation is regarding the difference of platelet activation between ACS and CCS, highlighting immature platelets may play a more intensive and active role during an acute event (ACS or stroke) than in a chronic disease.

Platelets are an essential component of the thrombotic process that follows plaque rupture and leads to ACS or acute stroke.[22] Platelet function was observed not to be static during ischemia-reperfusion. Instead, during ischemia, regional platelet function increased and both regional and systemic platelet function increased during myocardial reperfusion.[23] Indeed, percutaneous coronary intervention further exacerbates the risk of thrombosis in the presence of dysfunctional endothelium, vulnerable plaques, and systemic prothrombotic state.[24] For that reason, optimal platelet inhibition is key for the prevention of ischemic complications in patients after coronary affection, particularly those undergoing percutaneous coronary intervention.[25]

It has been shown that patients with ACS have elevated levels of RPs and that elevated levels of these cells are associated with worse clinical outcomes and with reduced antiplatelet response to aspirin and thienopyridine treatment.[18] Detection of peripheral RPs reflects the increased platelet production from megakaryocytes in the bone marrow and hence the rate of platelet turnover.[21] [26] However, it is unknown if the lack of response to antiplatelets of RPs is related with the rate of platelet turnover or with intrinsic properties of RPs. In this current issue, Bongiovanni and colleagues in subgroup analyses showed consistent findings among those enrolling patients treated with dual- or single-antiplatelet therapy and irrespective of the use of potent P2Y12 inhibitors.[15]

Regarding the reduced action of aspirin in patients with RPs, one mechanism would be associated with higher turnover and early recovery of platelet function.[14] Another explanation is the observation that circulating RPs possess uninhibited COX-1 as well as COX-2 activity.[14] [27] Related to irreversible P2Y12 inhibitors, Guthikonda et al[28] evaluated the relationship between RPs and platelet function in patients with CCS taking aspirin and clopidogrel, and they observed that large platelets (RPs) exhibited increased reactivity despite dual-antiplatelet therapy, compared with smaller platelets. A similar association of impaired antiplatelet response and RP levels was demonstrated by Perl et al[29] in ACS patients, where high levels of RPs were associated with increased platelet reactivity despite prasugrel treatment but not for ticagrelor. For the reversible P2Y12-receptor inhibitor ticagrelor, no association of RPs and antiplatelet response could be demonstrated. In the same line, the effectiveness of the injectable P2Y12-receptor blocker cangrelor is not attenuated by RPs.[25] One reason for the difference between irreversible and reversible P2Y12 agents could be related with turnover because P2Y12-receptor inhibitors with little fluctuation of plasma concentrations such as cangrelor and ticagrelor might be better able to inhibit newly built platelets as compared with P2Y12-receptor inhibitors with a relatively short half-life of their active metabolites such as clopidogrel and prasugrel. However, a recent study conducted by Stratz et al[26] suggested that the impact of RPs on antiplatelet response is mostly independent of platelet turnover even for clopidogrel or prasugrel. The second possibility is related with intrinsic properties of RPs in the response to nonreversible P2Y12 agents.[30] However, no difference in terms of P2Y12 receptor expression has been reported in RPs and further investigations about the final effect of P2Y12 inhibitors over RPs are needed.

The role of RPs in cardiovascular diseases as a prognostic tool seems to be well established. The mechanisms underlying this condition and the efficacy of potent antiplatelet agents are an interesting window of opportunity to perform future clinical trials. However, the decision to include RPs as a prognosis biomarker in clinical routine is far for being agreed, mainly because it is needed to establish the optimal parameter and determine its cutoff. So, new research is needed to confirm the reproducibility and feasibility of the laboratory technique for RPs and the influence of different antiplatelets in patients with high RPs.

Publication History

Received: 04 September 2023

Accepted: 06 September 2023

Accepted Manuscript online:
07 September 2023

Article published online:
31 October 2023

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