Erythrocytes undergo a well-defined switch from fetal to postnatal circulation, which is mainly reflected by the stage-specific expression of hemoglobin chains. Perinatal alterations in thrombopoiesis are poorly understood. We assessed the ontogenesis of platelet phenotype and function from early prematurity to adulthood. We recruited 64 subjects comprising 7 extremely preterm (27-31 weeks gestational age), 25 moderately preterm (32-36 weeks), 10 term neonates, 8 infants (<2 years), 5 children (2-13 years), and 9 adults (>13 years). Blood was withdrawn at up to 3 different time points in neonates (t1: 0-2, t2: 3-7, and t3: 8-14 days after birth). We found that the expression levels of the major surface receptors for fibrinogen, collagen, vWF, fibronectin, and laminin were reduced but correlated with decreased platelet size, indicating a normal surface density. Although CD62P and CD63 surface exposure upon stimulation with TRAP-6, ADP, or U46619 was unaltered or only slightly reduced in neonates, GPIIb/IIIa inside-out and outside-in activation was blunted but showed a continuous increase until adulthood, correlating with the expression of the GPIIb/IIIa regulating tetraspanin CD151. Platelet subpopulation analysis using automated clustering revealed that neonates presented with a CD63+/PAC-1- pattern, followed by a continuous increase in CD63+/PAC-1+ platelets until adulthood. Our findings revealed that the number of platelet-monocyte and platelet-neutrophil aggregates, but not platelet-lymphocyte aggregates, is increased in neonates and that neonatal aggregate formation depends in part on CD62P activation. Our PLatelets In Neonatal Infants Study (PLINIUS) provides several lines of evidence that the platelet phenotype and function evolve continuously from neonates to adulthood.
Background : Infection with SARS-CoV-2 leads to an altered hemostatic system and Covid-19 associated coagulopathy (CAC). Platelet counts remain overall unaltered, but thromboembolic events are frequently reported. Studies on the contribution of platelets to CAC are emerging but still lacking precise cohort comparison and broad analyses of platelet markers. Aims : We aimed to analyze platelet receptor expression and function on platelets and biomarkers in platelet-poor plasma to investigate the role of platelets in the onset of critical progression of CAC. Methods : Extensive platelet function analyses were performed on 34 critically-ill patients with Covid-19 and data was compared to sepsis patients ( n = 24) and non-SARS-CoV-2 acute infection ( n = 18). Tests included PFA-200, aggregometry, flow cytometry and whole mount TEM. Plasma levels of TPO, sCD62P and sGPVI were determined by ELISA. For all patients, relatives, and for healthy controls ( n = 10) informed consent was obtained. Results : While platelet counts in patients of our Covid-19 cohort were expectably unaltered, platelet function was severely impaired in multiple assays. Platelets failed to aggregate in response to ADP or TRAP-6 and could not activate integrin response or release α-granules. The amount of platelet-leukocyte aggregates was markedly elevated, indicating previous platelet activation in line with higher levels of sCD62P and sGPVI. Remarkably, we observed platelet exhaustion in Covid-19 patients using whole mount TEM by means of a lack of dense granules corroborating with impaired uptake of mepacrine. Conclusions : Our data imply that SARS-CoV-2 infection leads to a sub-threshold activation of platelets in a way that they become activated already before critical disease progression, without being cleared from the circulation, which is in striking contrast to sepsis. The platelet pool appears to be exhausted with detrimental consequences for thrombus stability and the risk of thromboembolic events. The mere platelet count in Covid-19 does thus not reflect progression to CAC, whereas platelet function is of high prognostic relevance.
