How is Autoimmune Lymphoproliferative Syndrome (ALPS) diagnosed?

Diagnosis of Autoimmune Lymphoproliferative Syndrome (ALPS)

The diagnosis of ALPS requires the presence of two required criteria (chronic lymphadenopathy/splenomegaly and elevated double-negative T cells) plus at least one primary accessory criterion (defective lymphocyte apoptosis or pathogenic FAS pathway mutation). 1

Required Diagnostic Criteria

1. Chronic lymphadenopathy and/or splenomegaly

   • Must persist for ≥6 months
   • If lymphadenopathy is isolated, it must affect at least 2 nodal chains
   • Neoplastic and infectious causes must be excluded
   • Hepatomegaly may be present but is not sufficient in isolation

2. Elevated CD3+TCRαβ+CD4-CD8- double-negative T cells (DNT)

   • ≥1.5% of total lymphocytes OR
   • ≥2.5% of CD3+ lymphocytes
   • Must be measured in the setting of normal or elevated lymphocyte counts
   • Lymphopenia invalidates this criterion

Primary Accessory Criteria (one required for definitive diagnosis)

1. Defective lymphocyte apoptosis

   • Must be demonstrated in 2 separate assays
   • Acceptable tests include direct FAS activation or TCR restimulation

2. Pathogenic mutation

   • Somatic or germline mutation in FAS, FASLG, or CASP10 genes
   • FAS gene mutations account for approximately 85% of genetically confirmed cases 2

Secondary Accessory Criteria (one required for probable diagnosis)

1. Elevated biomarkers (any one of the following):

   • Plasma sFASL levels ≥200 pg/mL
   • Plasma IL-10 levels ≥20 pg/mL
   • Serum/plasma vitamin B12 levels ≥1500 ng/L
   • Plasma IL-18 levels ≥500 pg/mL

2. Characteristic histopathology

   • Requires review by an experienced hematopathologist
   • Typically shows florid paracortical expansion of double-negative T cells in lymph nodes 3

3. Combined autoimmune manifestations:

   • Autoimmune cytopenias (hemolytic anemia, thrombocytopenia, or neutropenia)
   • PLUS elevated IgG levels (polyclonal hypergammaglobulinemia)

4. Family history

   • Non-malignant/non-infectious lymphoproliferation with or without autoimmunity

Diagnostic Algorithm

1. Initial screening for patients with:

   • Persistent lymphadenopathy and/or splenomegaly
   • Autoimmune cytopenias
   • Recurrent fever without clear infectious cause 4

2. First-line testing:

   • Complete blood count with differential
   • Flow cytometry to quantify CD3+TCRαβ+CD4-CD8- DNT cells
   • Immunoglobulin levels (looking for hypergammaglobulinemia)
   • Biomarkers: vitamin B12, IL-10, IL-18, and sFASL levels

3. Confirmatory testing (if required criteria are met):

   • Lymphocyte apoptosis assays (in specialized centers)
   • Genetic testing for FAS, FASLG, and CASP10 mutations

Diagnostic Classification

• Definitive ALPS: Both required criteria + one primary accessory criterion
• Probable ALPS: Both required criteria + one secondary accessory criterion

Important Considerations and Pitfalls

• DNT levels between 1.0-1.5% of total lymphocytes may be seen in healthy individuals or as a reactive phenomenon in conditions like systemic lupus erythematosus 1
• DNT levels above 3% of total lymphocytes (or 5% of T cells) are rarely seen in conditions other than ALPS and are essentially pathognomonic 1
• The lymphocyte apoptosis assay is no longer considered essential for diagnosis, as patients with somatic FAS mutations or germline FASLG mutations may have normal FAS-induced apoptosis assays 1
• Differential diagnosis includes infections, autoimmune diseases, primary immunodeficiencies, and malignancies, particularly lymphoma 3, 5
• Flow cytometry must distinguish TCRαβ+ DNT cells from TCRγδ+ DNT cells or natural killer T cells, which can also be CD3+CD4-CD8- 1
• Each laboratory should develop age-adjusted reference ranges for DNT cells and report both percentage and absolute numbers 1

From a clinical perspective, patients with probable ALPS should be treated and monitored similarly to those with definitive diagnosis, but genetic or apoptosis assay-based diagnostic workup should be pursued whenever possible 1.