Systemic histiocytoses in adults (Langerhans cell histiocytosis, Erdheim-Chester disease, and Rosai-Dorfman disease) are rare inflammatory disorders in which recent discoveries have identified a clonal origin, with activating mutations in the MAP kinase pathway, enabling access to targeted therapies. However, the mechanism by which these mutations induce an inflammatory profile in tissue histiocytes remains largely unknown.

Despite these advances, there is a clear need to refine diagnostic and prognostic classification, to identify the biological mechanisms involved in the onset and progression of these diseases, to develop new targeted strategies, and to establish minimally invasive monitoring methods (liquid biopsies).

This project aims to make a decisive contribution toward these goals.

Systemic histiocytoses are rare diseases with a clinical spectrum ranging from mild forms to severe, life-threatening multi-organ involvement. Numerous recent studies have identified somatic mutations in the MAP kinase pathway in tissue-infiltrating histiocytes, providing a better understanding of the disease pathophysiology and enabling access to more effective targeted therapies. However, due to the extreme rarity of these diseases, many unknowns remain.

These mutations do not appear to induce a proliferative oncogenic process, as seen in cancers where similar mutations have been identified. Instead, they seem to trigger a pro-inflammatory and pro-fibrotic immune response, ultimately leading to organ damage. The immune mechanisms induced by these mutations in histiocytes remain unexplored.

There are also currently no reliable data to accurately predict disease progression, including survival, treatment response, remission, or organ involvement.

It is therefore essential to establish patient cohorts to improve disease understanding and to identify effective diagnostic, prognostic, and predictive biomarkers-whether for standard treatment response or as potential theranostic markers (actionable by a specific treatment).

Inclusion Criteria:

• Age ≥ 18 years
• Patient followed for systemic histiocytosis in Internal Medicine Department 2 at Pitié-Salpêtrière Hospital
• Non-opposition to participation in the study

Exclusion Criteria:

• Pregnant or breastfeeding women
• Patients without French social security or covered by State Medical Aid (AME)
• Patients deprived of liberty by judicial or administrative decision, or under legal protection

Adult patients with systemic histiocytosis

The procedures carried out with the French data privacy authority (CNIL, Commission nationale de l'informatique et des libertés) do not provide for the transmission of the database, nor do the information and consent documents signed by the patients.

Consultation by the editorial board or interested researchers of individual participant data that underlie the results reported in the article after deidentification may nevertheless be considered, subject to prior determination of the terms and conditions of such consultation and in respect for compliance with the applicable regulations.

Beginning 3 months and ending 3 years following article publication. Requests out of these time frame can also be submitted to the sponsor

Researchers who provide a methodologically sound proposal.