Autoimmune Lymphoproliferative Syndrome (ALPS)

Introduction

Autoimmune lymphoproliferative syndrome, also known as Canale-Smith syndrome, is a disorder of abnormal lymphocyte apoptosis. It is characterized by the accumulation of polyclonal populations of T cells.

Pathophysiology And Genetics

The disease primarily results from a failure of programmed cell death in lymphocytes.

• Genetic Variants:
  • The most common defect is a germline or somatic pathogenic variant in the FAS gene.
  • The FAS gene encodes a cell surface receptor of the tumor necrosis factor receptor superfamily (TNFRSF6).
  • Under normal conditions, stimulation of this receptor by its ligand produces programmed cell death.
  • Other causative gene variants include FASLG and CASP10.
• Cellular Mechanism:
  • Persistent survival of lymphocytes leads to immune dysregulation and subsequent autoimmunity.
  • The disorder is marked by the accumulation of double-negative T cells (DNTs).
  • DNTs are T cells that express CD3 and $\alpha$/$\beta$ antigen receptors but lack both CD4 and CD8 co-receptors (CD3+ TCR$\alpha$/$\beta$+ CD4- CD8-).
  • These accumulated T cells respond poorly to antigens or mitogens.
  • They also fail to produce essential growth or survival factors, such as interleukin-2.

Clinical Manifestations

Patients typically present in the first year of life. Most affected individuals become symptomatic by 5 years of age.

Lymphoproliferation

• Chronic, persistent, or recurrent lymphadenopathy is a hallmark.
  • Lymph node enlargement can be striking.
• Splenomegaly is a consistent finding and may lead to hypersplenism.
• Hepatomegaly is observed in approximately 50% of patients.
• The lymphoproliferative process may regress over time.

Autoimmune Features

Unlike lymphoproliferation, autoimmune features do not regress. They are characterized by frequent exacerbations and recurrences.

• Autoimmune cytopenias are highly prevalent.
  • These include Coombs-positive hemolytic anemia, thrombocytopenia, and mild neutropenia.
  • ALPS is a recognized cause of Evans syndrome (concurrent immune thrombocytopenia and immune hemolytic anemia).
• Other systemic autoimmune manifestations can occur.
  • These include urticaria, uveitis, glomerulonephritis, and hepatitis.
  • Patients may also develop vasculitis, panniculitis, arthritis, or premature ovarian failure.
  • Central nervous system involvement can present as seizures, encephalopathy, transverse myelitis, or Guillain-Barré syndrome.

Malignancy Risk

• There is a significantly increased risk for malignancies.
• Patients are prone to developing Hodgkin and non-Hodgkin lymphomas.
• Solid-tissue tumors of the thyroid, skin, heart, or lung may also occur.

Diagnosis And Laboratory Evaluation

Diagnosis relies on clinical findings, flow cytometry, and genetic testing.

• Flow cytometry is utilized to identify the expanded population of DNT cells.
• Functional genetic analysis typically reveals a heterozygous variant in the TNFRSF6 gene.
• Laboratory findings may show hypergammaglobulinemia involving immunoglobulin G (IgG) and immunoglobulin A (IgA).

Revised Diagnostic Criteria

A definitive diagnosis requires the presence of both required criteria plus one primary accessory criterion. A probable diagnosis requires both required criteria plus one secondary accessory criterion.

Management

Therapy focuses on controlling the lymphoproliferation and the autoimmune complications.

• Immunosuppression:
  • Rapamycin (sirolimus) is highly effective.
  • It frequently controls both the adenopathy and the autoimmune cytopenias.
  • Mycophenolate mofetil is another potential targeted therapy.
• Definitive And Directed Therapies:
  • Stem cell transplantation remains a potential option for treating the severe autoimmune manifestations.
  • If malignancies develop, they are treated using standard oncologic protocols.