Autoimmune lymphoproliferative syndrome (ALPS) represents a failure of apoptotic mechanisms to maintain lymphocyte homeostasis, permitting accumulation of lymphoid mass and persistence of autoreactive cells that often manifest in childhood with chronic nonmalignant lymphadenopathy, hepatosplenomegaly, and recurring multilineage cytopenias. Cytopenias in these patients can be the result of splenic sequestration as well as autoimmune complications manifesting as autoimmune hemolytic anemia, immune-mediated thrombocytopenia, and autoimmune neutropenia. More than 300 families with hereditary ALPS have now been described; nearly 500 patients from these families have been studied and followed worldwide over the last 20 years by our colleagues and ourselves. Some of these patients with FAS mutations affecting the intracellular portion of the FAS protein also have an increased risk of B-cell lymphoma. The best approaches to diagnosis, followup, and management of ALPS, its associated cytopenias, and other complications resulting from infiltrative lymphoproliferation and autoimmunity are presented. This trial was registered at www.clinicaltrial.gov as #NCT00001350. (Blood. 2011;118(22): 5741-5751)
IntroductionAutoimmunity results from failure of self-tolerance, which can be further divided into central and peripheral tolerance. Central tolerance is fostered by apoptosis through elimination of autoreactive lymphocytes in generative lymphoid organs (the bone marrow and thymus), whereas mechanisms of peripheral tolerance include anergy, deletion by apoptosis, and suppression by regulatory T cells to avoid autoimmunity and tissue damage. 1 Apoptosis, the intrinsic program of cell death, is triggered by receptor-ligand interactions at the cell surface (the extrinsic pathway) or by the activation of mitochondrial proteins (the intrinsic pathway), leading to processing and activation of caspases and their downstream targets. Lymphocyte apoptosis mediated by the cell surface receptor FAS plays a pivotal role in lymphocyte homeostasis. FAS (also termed CD95/APO1) is a member of the tumor necrosis factor receptor superfamily of proteins that directly trigger apoptosis to maintain lymphocyte homeostasis and peripheral immune tolerance and prevent autoimmunity. 2,3 It is a membrane-bound molecule that is highly expressed, not only on activated B and T lymphocytes but also present in other cells, such as hepatocytes. FAS is present on the cell surface as a preformed trimer. 4 Its binding with FAS ligand leads to conformational changes on the intracellular portion of FAS protein triggering rapid recruitment of the death domain of the adaptor protein FADD (Fas-associated death domain) to the homologous death domain in the cytoplasmic tail of FAS. This is followed by the recruitment of procaspases 8 or 10 through the interaction of their death-effecter domains with the amino termini of FADD. The resulting Fas/FADD/caspase complex is termed the death-inducing signaling complex that incites a further cascade of caspase activation culminati...