Abt-Letterer-Siwe disease is the most aggressive and acute form of Langerhans Cell Histiocytosis (LCH). It is a multisystem, rapidly progressive histiocytic disorder that predominantly affects infants and young children under the age of 2. The disease takes its name from three researchers who described it independently: Arthur Abt, Erich Letterer, and Sture Siwe.
Pathogenesis
The disease originates from clonal Langerhans cells that proliferate abnormally and infiltrate multiple organs. These cells are normally antigen-presenting dendritic cells found in the skin and mucous membranes. Although the precise mechanism of aberrant proliferation has not been fully elucidated, the BRAF V600E mutation is identified in the majority of cases. This finding strongly suggests that the disease represents a neoplastic rather than a purely reactive process.
Clinical Presentation
Abt-Letterer-Siwe disease typically presents with multisystem involvement and follows a rapidly deteriorating course.
Skin involvement is the most frequent and earliest finding. Lesions resemble seborrheic dermatitis and appear as greasy, erythematous, petechial or purpuric papules predominantly over the scalp, retroauricular areas, and trunk.
Hepatic and splenic involvement leads to hepatosplenomegaly, liver failure, hypoalbuminemia, and coagulopathy.
Bone marrow infiltration results in pancytopenia, with anemia, thrombocytopenia, and neutropenia occurring concurrently.
Pulmonary involvement manifests as interstitial infiltrates, cysts, and pneumothorax, and may progress to respiratory failure in severe cases.
Lymph node involvement causes generalized lymphadenopathy.
Osseous lesions present as osteolytic defects, although isolated bone involvement is less characteristic of this particular form compared to other LCH subtypes.
Diagnosis
Diagnosis requires biopsy with histopathological confirmation. The diagnostic criteria include the following:
Light microscopy reveals the characteristic notched, coffee bean–shaped nuclei of Langerhans cells along with abundant pale cytoplasm and occasional eosinophils in the background.
Immunohistochemistry demonstrates positivity for CD1a, CD207 (Langerin), and S-100. Positivity for CD1a and Langerin is considered sufficient to confirm the diagnosis in current practice.
Electron microscopy can demonstrate Birbeck granules — tennis racket–shaped cytoplasmic organelles pathognomonic of Langerhans cells — although this is no longer considered mandatory given the widespread availability of Langerin immunostaining.
BRAF V600E mutation analysis has become standard practice for both diagnostic confirmation and treatment planning purposes.
Treatment
The disease requires systemic chemotherapy. Current standard protocols include the following:
Vinblastine and prednisolone combination therapy constitutes the first-line regimen and is administered within the framework of the LCH-III and LCH-IV international protocols.
Cytarabine and cladribine (2-CDA) combination is used in cases with central nervous system involvement or in patients unresponsive to first-line therapy.
BRAF inhibitors (vemurafenib, dabrafenib) have demonstrated promising results in BRAF V600E-positive refractory cases and are increasingly being incorporated into clinical practice through ongoing trials.
Allogeneic hematopoietic stem cell transplantation is considered in high-risk, relapsed, or refractory cases where conventional chemotherapy has failed.
Prognosis
Prognosis varies considerably depending on risk group and treatment response. Mortality remains high in cases involving risk organs — namely the liver, spleen, and bone marrow. In patients who respond to treatment, long-term sequelae such as diabetes insipidus, neurodegeneration, and growth retardation may develop. With early diagnosis and modern treatment protocols, five-year survival rates have improved substantially over the past decade.