Blood platelets, well known for their role in hemostasis, are abnormally activated in patients suffering from systemic lupus erythematosus (SLE), but also from other immunomediated diseases (scleroderma, vasculitis, myositis, Gougerot-Sjögren's and rheumatoid arthritis) in cases of high disease activity. Once activated, platelets express adhesion molecules such as P-selectin on their surface, enabling them to interact physically with immune cells. In a recent work, we identified that activated platelets from lupus patients interact with regulatory T cells and block their regulatory function, thus participating in the deregulated activation of the immune system in SLE. In addition, inhibition of platelet-immune cell interactions by an anti-P-selectin antibody improved LES symptoms in two mouse models. The aim of this work is to investigate other potential platelet-immune cell interactions in patients with SLE, in comparison with other autoimmune diseases (systemic scleroderma, ANCA vasculitides, inflammatory myositis, Gougerot-Sjögren syndrome and rheumatoid arthritis). This study could lead to a better understanding of the role of platelets in the pathophysiology of autoimmune diseases, identify new biomarkers of activity, and assess the potential of new therapeutic avenues in these diseases, such as platelet targeting.