Platelets are proven essential in haemostasis, however, they are now also increasingly recognized as cells with important immunomodulatory properties, e.g. through interaction with leukocytes and several species of bacteria and by release inflammatory mediators upon activation. Moreover, platelets express receptors involved in immunity and inflammation such as Fcγ‐receptor IIa, complement protein C1q‐receptors (gC1qR, cC1qR, CD93 and α2β1) and toll‐like receptors (TLR‐1, ‐2, ‐4, ‐6 and ‐9). C1q, C‐reactive protein (CRP) and TLRs are all pattern recognition molecules able to recognize non‐self structures and initiate an immune response. Uncontrolled or misdirected activation of platelets and the immune response is involved in the onset and progress of several conditions with an inflammatory component, such as coronary artery disease and autoimmune diseases.

Hence, the aims of the present thesis were to investigate the effects and q mechanisms of C1and CRP on platelet activation, and to clarify the intracellular signaling events provoked by TLR‐2 stimulation of platelets. Platelet interaction with immune complexes is poorly understood, however by utilizing well‐characterized model surfaces with adsorbed IgG and microscopy, we show that both C1q and CRP are able to inhibit FcγR‐mediated platelet adhesion and spreading. Using isolated platelets in suspension and flow cytometry, we also found that C1q triggers a rapid, moderate and transient up‐regulation of P‐selectin that is sensitive to blockade of gC1qR and protein kinase C (PKC), but not blockade of α2β1. Additionally, subsequent platelet activation by collagen or collagen‐related peptide (GPVI specific) is inhibited by C1q, suggesting a role for GPVI in C1q‐mediated regulation of collagen‐induced platelet activation. Whole blood studies revealed that C1q inhibits total cell aggregation, formation of platelet‐leukocyte aggregates, and potentiates the production of reactive oxygen species (ROS), all in a platelet‐dependent manner. Furthermore, using the TLR‐2/1 agonist Pam3CSK4 we found that TLR‐2/1‐activation of platelets is mediated via a P2X1‐dependent increase in intracellular free Ca2+, P2Y1 and P2Y12 –receptor ligation, and activation of cyclooxygenase. We also found that platelets express IRAK‐1, however, without being rapidly phosphorylated upon Pam3CSK4 stimulation and thus probably not involved in the early aggregation/secretion response. Furthermore, TLR‐2/6 stimulation does not lead to platelet activation but instead inhibits TLR‐2/1‐provoked activation. Taken together, these findings further strengthen the role of platelets as key players in inflammatory processes.