Flow cytofluorimetric detection and immunophenotyping of platelet-monocyte complexes in peripheral blood

Activated platelets aggregate with monocytes by binding membrane bound molecules. Platelet-monocyte interaction is considered to underlie pathophysiological mechanisms bridging thrombosis and inflammation. Detection and analysis of platelet-monocyte complexes (PMC) provide means for revealing their physiological and pathogenetic roles and are instrumental in the diagnostics of various pathological conditions including obstetric complications. The aim of the study was to develop the method of quantitative determination of peripheral blood PMC, that preserve phenotypic features of platelets and monocytes, and to reveal their changes by ex vivo analysis. The suggested procedure includes immediate fixation of blood sample, immunocytochemical staining with fluorochrome-conjugated specific antibodies against markers of activation and differentiation followed by lysis of erythrocytes, and flow cytometric analysis. Fourteen samples of peripheral blood from patients with history of pregnancy complication were obtained in first trimester of ongoing pregnancy and analyzed. It was demonstrated that quantitative and qualitative in vivo characteristics of PMC remained unchanged in fixed samples, whereas the number of PMC and expression levels of the markers of platelet and monocyte activation dramatically increased in the unfixed blood. The set of monoclonal antibodies and gating strategies, used in this study, ensure phenotyping and evaluation of percentage/absolute count of PMC in the total monocyte population (CD45⁺CD14⁺) and in the subpopulations of classical (CD14⁺CD16^-), intermediate (CD14⁺CD16⁺), and non-classical (CD14^lowCD16⁺) monocytes. This approach provides insight into the participation of different monocyte subsets in the formation of PMC and their roles in physiological and pathophysiological processes. In some samples, elevated PMC proportion was observed, accompanied by significant increase in the expression of platelet activation marker CD62P and decrease in the expression of its monocytic ligand CD162. These changes suggested altered activation of PMC and their participation in the pathophysiological mechanisms of some pregnancy complications. Immunophenotyping of PMC affords an opportunity to characterize their proinflammatory, procoagulant and adhesive properties; these results can be used for research and diagnostics. In particular, the method is suitable for detection and phenotyping of PMC in pregnancy complications and other pathological conditions associated with the disorders of hemostasis and thrombosis.

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