What is hemophagocytic lymphohistiocytosis (HLH) and what are its diagnostic criteria and first‑line treatment?

Hemophagocytic Lymphohistiocytosis (HLH)

HLH is a life-threatening hyperinflammatory syndrome caused by excessive immune activation with uncontrolled macrophage and T-lymphocyte proliferation, diagnosed when 5 of 8 HLH-2004 criteria are met, and treated first-line with dexamethasone plus etoposide. 1

What is HLH?

HLH represents a threshold phenomenon where combined genetic and environmental factors trigger overwhelming interferon-gamma production and pathologic immune activation 2. Rather than a simple binary classification, it exists on a spectrum:

• Primary (familial) HLH: Caused by genetic mutations affecting cytotoxic T-cell and NK-cell function (e.g., PRF1, UNC13D, STX11) 1
• Secondary HLH: Triggered by infections (especially EBV, CMV, HIV), malignancies (particularly T/NK-cell lymphomas), or autoimmune diseases 1

In adults, the mean age at onset is 49 years, with infections being the most common trigger overall, though malignancy-associated HLH increases with age 1.

Diagnostic Criteria

Diagnosis requires meeting 5 of 8 HLH-2004 criteria OR having a molecular diagnosis consistent with HLH 1:

1. Fever
2. Splenomegaly
3. Cytopenias (≥2 lineages):
   • Hemoglobin <90 g/L
   • Platelets <100 × 10⁹/L
   • Neutrophils <1.0 × 10⁹/L
4. Hypertriglyceridemia and/or hypofibrinogenemia:
   • Triglycerides ≥3.0 mmol/L (≥265 mg/dL)
   • Fibrinogen ≤1.5 g/L
5. Hemophagocytosis in bone marrow, spleen, or lymph nodes
6. Low/absent NK-cell activity
7. Ferritin ≥500 μg/L
8. Soluble CD25 (sIL-2 receptor) ≥2400 U/mL

Critical Diagnostic Pearls

• Hyperferritinemia is the key screening marker: Ferritin >10,000 μg/L is >90% sensitive and specific in children; in adults, levels of 7,000-10,000 μg/L should strongly raise suspicion, though specificity is lower 1
• Soluble CD25 has superior diagnostic performance in adults compared to ferritin (AUC 0.90 vs 0.78) 1
• Treatment should not be delayed if clinical suspicion is high, even if 5 criteria are not yet fulfilled 1
• Serial bone marrow aspirates may be needed if hemophagocytosis is not initially demonstrated 1

The HScore is an alternative validated tool using weighted parameters (immunosuppression status, temperature, organomegaly, cytopenias, ferritin, triglycerides, fibrinogen, AST, hemophagocytosis) 1.

First-Line Treatment

The standard first-line regimen is dexamethasone plus etoposide, based on the HLH-94/2004 protocols 1, 3, 4:

Initial Therapy Protocol

• Dexamethasone: 10 mg/m² (or prednisolone 1-2 mg/kg)
• Etoposide: Standard dosing with weekly reevaluation
  • Dose reduction required for renal impairment
  • No dose reduction needed for isolated hyperbilirubinemia/elevated transaminases 1
• Cyclosporine A: Added upfront in HLH-2004 protocol 4

Treatment Duration and Monitoring

• Many patients require 8 weeks of etoposide 1
• Weekly reevaluation is essential to assess need for continued therapy 1
• Monitor ferritin, sCD25, cell counts, and trigger-specific markers (e.g., EBV DNA) 1

Adjunctive Therapies

• IVIG (up to 1.6 g/kg over 2-3 days) may be added for anti-inflammatory effects 1
• Intrathecal therapy (methotrexate + corticosteroids) for CNS involvement 4

Treatment Modifications by Clinical Context

For EBV-HLH with rapid deterioration: Start etoposide immediately without delay 1

For less severe or improving disease: A conservative approach with short-course corticosteroids ± IVIG may be justified 1

For malignancy-associated HLH: Early administration of lymphoma-directed therapy is critical when feasible, as outcomes are poorest in this subgroup 1, 5, 6

For critically ill patients with multiorgan failure: Consider non-myelosuppressive regimens (anakinra, ruxolitinib, IVIG) to avoid worsening cytopenias while controlling inflammation 3, 6

For macrophage activation syndrome (HLH secondary to autoimmune disease): If steroids fail, add IL-1 inhibition (anakinra) and/or cyclosporine A 3

Critical Management Considerations

Infection Prophylaxis is Mandatory

Broad antimicrobial prophylaxis against Pneumocystis jirovecii and fungi is required due to severe T-cell depletion from both disease and treatment 1. Antiviral prophylaxis is also recommended 1.

Identify and Treat the Trigger

A meticulous search for underlying triggers must continue despite ongoing HLH treatment 1. This includes:

• Comprehensive infectious workup (especially EBV, CMV, HIV)
• Malignancy evaluation (lymphoma screening, bone marrow biopsy)
• Autoimmune/autoinflammatory disease assessment

Hematopoietic Stem Cell Transplant

HSCT is recommended for:

• Patients with familial HLH or confirmed genetic mutations 1, 4
• Severe, persistent, or reactivated disease 4
• Should occur once adequate (not perfect) disease control is achieved 2

Common Pitfalls

• Delaying treatment while awaiting all 5 criteria: Start therapy based on clinical judgment if HLH is strongly suspected 1
• Using etoposide during "HLH during chemotherapy": Use sparingly as bone marrow recovery is essential for immune reconstitution; prioritize corticosteroids ± IVIG 1
• Missing occult malignancy: Lymphoma is the leading cause of malignancy-associated HLH in adults and requires ongoing surveillance 1, 5
• Inadequate infection prophylaxis: Secondary infections are a major cause of mortality 1

Prognosis

Mortality ranges from 20-88% in adults, primarily from refractory HLH, secondary infections, and progression of underlying triggers 1. Malignancy-associated HLH has the worst prognosis of all subgroups 1. Early recognition and prompt treatment initiation are critical for improving outcomes 7, 3.