, C1q induces a rapid up-regulation of P-selectin and modulates collagen-and collagen-related peptidetriggered activation in human platelets, 2010, Immunobiology, (215), 12, 987-995 The aim of this study was to further characterize the effects of C1q on platelets, by quantifying the platelet surface expression of P-selectin (CD62P) and monitoring the formation of platelet-neutrophil aggregates. Using flow cytometry, we found that C1q dosedependently triggered a rapid but moderate and transient up-regulation of P-selectin already within 5 seconds of C1q exposure. Pre-incubation with an antibody directed against gC1qR significantly inhibited (with 57 % compared to control) the up-regulation, whereas an antibody towards the αIIβI-integrin showed no effect. Stimulation with C1q did not change the cytosolic calcium-levels, as measured with the fluorescent ratiometric probe Fura-2, however, a protein kinase C inhibitor (GF109203x) blocked the C1q-induced P-selectin expression. Furthermore, pre-incubation of platelets with C1q diminished both the collagen as well as the collagen-related peptide induced up-regulation of P-selectin, most evident after 90 seconds of stimulation. This indicates that C1q may regulate platelet activation via the GPVI receptor, which is a novel finding. Moreover, C1q antagonized the collagen-induced formation of platelet-neutrophil aggregates, indicating a reduced interaction between platelet P-selectin and neutrophil P-selectin glycoprotein ligand-1(PSGL-1/CD162). In summary, C1q induces a moderate rapid platelet P-selectin expression, modulates subsequent collagen and collagen-related peptide stimulation of platelets, and inhibits the formation of platelet-4 neutrophil aggregates. These immuno-regulatory effects of C1q may have a crucial role in innate immunity and inflammation.5