Immunologic thrombocytopenic purpura (ITP) in children is a pre-lupus condition if associated with the presence of anti-nuclear antibodies (ANA), providing a unique model for understanding the natural history of autoimmunity, particularly that of systemic lupus erythematosus (SLE). The investigators will describe the shared and/or unique immunological pathways involved at diagnosis in 70 children with ITP and in 20 children with SLE, and compare them between ITP-ANA- (more often transient), ITP-ANA+ (pre-lupus condition, more often persistent) and SLE

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

• Inclusion criteria:

  • For patients :

    • Child or adolescent with newly diagnosed ITP or SLE according to the specific definitions of ITP or SLE, prior to any treatment,
    • Over 1 and under 18 years of age at diagnosis, weighing more than 7 kg.
    • Written consent from parents or guardians,
    • Patient affiliated to a social security scheme.

  • For controls :

    • Over 1 and under 18 years of age at diagnosis, weighing more than 7 kg.
    • Follow-up in the day hospital at the Bordeaux University Hospital, for a condition that does not affect the immune system
    • Matched on age,
    • Written consent from parents or guardians,
    • Patient affiliated to a social security scheme

• Exclusion criteria:

  • For patients :

    • ITP secondary to a known cause: previous or concomitant immune deficiency, bone marrow or organ transplantation, other autoimmune disease, Evans syndrome (autoimmune hemolytic anemia or autoimmune neutropenia present at ITP diagnosis) or cancer with immunosuppressive therapy.
    • Treatment with immunomodulation or immunosuppressants (including immunoglobulins, corticoids, hydroxychloroquine), started prior to inclusion (day of sampling).
    • Pregnant women, women in labour and breastfeeding women

  • For controls :

    • Suffering from an immunological disease,
    • Infection within fifteen days prior to inclusion,
    • Immunomodulatory therapy.
    • Pregnant women, women in labour and breastfeeding women