Differential Diagnosis for Elevated Interleukin-2 (IL-2) Levels
Elevated IL-2 levels most commonly indicate active T-cell immune activation and should prompt evaluation for hemophagocytic lymphohistiocytosis (HLH), active autoimmune disease, chronic active EBV infection (CAEBV), or iatrogenic causes from IL-2 immunotherapy.
Primary Diagnostic Considerations
1. Hemophagocytic Lymphohistiocytosis (HLH)
This is the most critical diagnosis to exclude given its high mortality if untreated. Soluble IL-2 receptor (sCD25) ≥2400 U/mL is one of the eight diagnostic criteria for HLH 1. The soluble IL-2 receptor has demonstrated excellent diagnostic performance with an area under the curve of 0.90 for adult HLH, superior to ferritin (AUC 0.78) 1.
Key clinical features to assess:
- Fever and splenomegaly
- Cytopenias affecting ≥2 cell lines (hemoglobin <90 g/L, platelets <100×10⁹/L, neutrophils <1.0×10⁹/L)
- Hypertriglyceridemia (≥3.0 mmol/L) and/or hypofibrinogenemia (≤1.5 g/L)
- Ferritin ≥500 μg/L (often >10,000 μg/L)
- Hemophagocytosis on bone marrow examination 1
2. Active Autoimmune Diseases
Rheumatoid Arthritis (RA): Serum IL-2 levels correlate positively with disease activity (DAS28 score) and rheumatoid factor titers 2. Multiple regression analysis confirms IL-2 as an independent factor affecting RA disease activity. Elevated IL-2 associates with increased Th17/Treg ratios and abnormally elevated NK cell counts in active disease 2.
Adult-Onset Still's Disease (AOSD): IL-2 levels increase during active disease as part of a Th1-predominant cytokine profile, alongside interferon-γ and TNF-α 3. Look for the classic triad: high-spiking quotidian fevers (>39°C), salmon-pink evanescent rash, and arthritis/arthralgias.
Systemic Lupus Erythematosus (SLE): Consider when ANA and anti-dsDNA antibodies are present, though liver involvement is uncommon in pure SLE 4.
3. Chronic Active Epstein-Barr Virus (CAEBV) Infection
CAEBV demonstrates activated T cells with high EBV DNA levels and increased expression of both Th1 and Th2 cytokines, including IL-2 5. Some patients show increased IL-10 activity suggesting Th2 predominance 5.
Diagnostic approach:
- Measure EBV DNA in plasma (significantly elevated)
- Identify which cell type harbors EBV (T-cells or NK-cells via flow cytometry)
- Assess for IM-like chronic illness, hemophagocytic lymphohistiocytosis, or lymphoproliferative disease
- Note: Cells infected with EBV in CAEBV are typically CD8-negative 5
4. Iatrogenic: IL-2 Immunotherapy
High-dose IL-2 therapy for metastatic melanoma or renal cell carcinoma causes predictable IL-2 elevation 6. Monitor for toxicities:
- Renal: serum creatinine increases, oliguria (<0.5 mL/kg/hour)
- Neurological: personality changes, confusion, hallucinations (NOT ICANS—do not treat with steroids) 6
- Cardiovascular: hypotension, capillary leak syndrome
- Hold IL-2 if creatinine increases by 100% from baseline or reaches 2.5-2.9 mg/dL 6
5. Drug-Induced Sarcoidosis-Like Reaction (DISR)
In melanoma patients on immune checkpoint inhibitors, DISR occurs in approximately 9% of cases 7. Soluble IL-2 receptor levels are markedly elevated and serve as a reliable diagnostic marker when histology is unavailable 7. This distinguishes DISR from tumor progression without requiring invasive mediastinoscopy.
6. Idiopathic Nephrotic Syndrome
IL-2 levels increase during proteinuria and normalize during remission in adults with idiopathic nephrotic syndrome and children with minimal change disease or FSGS 8. This pattern reflects widespread cellular immunity disorder rather than being causally related to proteinuria 8.
7. Cardiovascular Disease
Stable angina patients show significantly elevated IL-2 and soluble IL-2 receptor levels compared to controls, though interestingly, unstable angina patients do not demonstrate this elevation 9. The mechanism involves T-lymphocyte activation in atherosclerotic plaques.
Algorithmic Diagnostic Approach
Step 1: Assess for life-threatening conditions first
- Calculate HScore if HLH suspected (includes fever, organomegaly, cytopenias, ferritin, triglycerides, fibrinogen, AST, hemophagocytosis) 1
- Measure soluble IL-2 receptor (sCD25) levels
Step 2: Evaluate for active autoimmune disease
- Check inflammatory markers (ESR, CRP), autoantibodies (ANA, RF, anti-CCP, anti-dsDNA)
- Calculate disease activity scores (DAS28 for RA)
- Assess for AOSD clinical triad
Step 3: Rule out infectious triggers
- EBV serology and plasma EBV DNA quantification
- Consider other viral triggers if CAEBV suspected 5
Step 4: Review medication history
- Current or recent IL-2 immunotherapy
- Immune checkpoint inhibitors (evaluate for DISR with imaging and sIL-2R) 7
Step 5: Assess for organ-specific manifestations
- Proteinuria and nephrotic syndrome features
- Cardiac symptoms with stable angina pattern
Critical Pitfalls to Avoid
- Do not dismiss elevated IL-2 as nonspecific inflammation—it indicates active T-cell activation requiring systematic evaluation
- Do not treat IL-2-induced neurotoxicity with steroids—this is NOT ICANS and steroids are contraindicated 6
- Do not confuse DISR with tumor progression in cancer patients on immunotherapy—check sIL-2R levels before changing treatment 7
- Do not overlook HLH in patients with hyperferritinemia and cytopenias—soluble IL-2 receptor is more specific than ferritin alone 1