Modeling the Early Stages of Autoimmunity Through the Study of Immunological Thrombocytopenic Purpura and Juvenile Lupus

The development of an autoimmune disease (AID) requires several years. In systemic lupus erythematosus (SLE), anti-nuclear antibodies (ANA) appear several years before the onset of the disease. However, it is difficult to identify the early mechanisms of autoimmunity in humans as most patients' samples are obtained at the time of diagnosis. The investigators hypothesize that immune thrombocytopenic purpura (ITP) in children provides a unique model for understanding the initial mechanisms leading to autoimmunity. Children with ITP have common immune system dysfunctions leading to the production of anti-platelet antibodies. However, despite this supposedly common dysfunction, the course and degree of loss of tolerance of the immune system are very heterogeneous: the disease is transient in 75 to 80% of cases, and 15 to 20% of children with ITP have ANA and their course is usually persistent or chronic. This subgroup of children with hematological AID, ITP ANA+ meets the criteria for a broader form of AID: pre-lupus. The investigators have recently shown that 16% of these children with pre-lupus / ITP-ANA+ progressed to complete SLE within a median of 3.8 years. Why do some children have transient or persistent ITP? limited or systemic ITP? The investigators aim to describe the shared and/or unique immunological pathways involved at diagnosis in children with ITP ANA- (transient or persistent), with ITP ANA+ (pre-lupus condition, transient or persistent), and in patients with SLE (persistent by definition) In this exploratory, prospective, bi-centric cohort study, the investigators will include 70 children with ITP and 20 children with SLE newly diagnosed, over 36 months with a 3-month follow-up for children with ITP-ANA- and SLE and a 48-month follow-up for children with ITP-ANA+. ANA positivity will be defined by a titer ≥ 1/160. Blood samples will be collected at inclusion and at 3 months (bio-collection), and further each 6 months for children with ITP-ANA+. ITP status will be defined at 3 months: transient or persistent. The investigators will analyze the signaling pathways (B and T lymphocytes, cytokines, disruption of tolerance and inactivation of X) involved in these different pathophysiological distinct subgroups