Hemophagocytic Lymphohistiocytosis (HLH)

Overview and Epidemiology

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory syndrome characterized by uncontrolled activation of cytotoxic lymphocytes and macrophages, overwhelming cytokine release, and multisystem organ dysfunction. HLH is subclassified as primary (familial/genetic) and secondary (acquired). Primary HLH typically presents in infancy and early childhood due to biallelic defects in genes required for cytotoxic granule-mediated killing. Secondary HLH occurs at any age and is most often triggered by infections (notably EBV), malignancy, or autoimmune disease (macrophage activation syndrome, MAS).

Incidence estimates vary with geography and ascertainment method; genetic forms are rare (on the order of 1:50,000 in some cohorts), whereas secondary HLH is likely underdiagnosed and reported as a few cases per million annually in population series.

Pathophysiology

HLH results from failure of immune regulation leading to persistent antigen‑driven activation of CD8+ T cells and NK cells, defective cytotoxic function, and sustained macrophage activation. Central mechanisms:

• Defective cytotoxicity: mutations in PRF1, UNC13D, STX11, STXBP2 and other genes impair perforin/granzyme delivery or vesicle trafficking, preventing normal apoptosis of antigen‑presenting cells.
• Hypercytokinemia: markedly elevated IFN‑γ, TNF, IL‑1β, IL‑6, IL‑18 and soluble IL‑2 receptor (sCD25) drive systemic inflammation and organ injury.
• Hemophagocytosis: activated macrophages engulf erythrocytes, leukocytes and platelets; this finding supports the diagnosis but may be absent early.
• Organ damage: endothelial activation, coagulopathy, hepatic injury (hepatitis, cholestasis, fulminant liver failure), and central nervous system infiltration are common end‑organ consequences.

Clinical Manifestations (focus for pediatric gastroenterology)

Presentation can be acute and fulminant or subacute. Key features:

• Systemic: prolonged high fevers, profound malaise, failure to thrive in infants.
• Hematologic: cytopenias affecting ≥2 lineages (anemia, thrombocytopenia, neutropenia), coagulopathy, hypofibrinogenemia, bleeding tendency.
• Hepatosplenomegaly and hepatic dysfunction: marked hepatomegaly, cholestatic or hepatocellular enzyme elevations, hyperbilirubinemia, synthetic dysfunction, and risk of acute liver failure—critical for pediatric gastroenterologists to recognize early.
• Laboratory hallmarks: very high ferritin (often >10,000 ng/mL but variable), hypertriglyceridemia, elevated transaminases, elevated lactate dehydrogenase (LDH), and high soluble CD25 (sIL‑2R).
• Neurologic: irritability, seizures, altered consciousness, CSF pleocytosis or elevated protein, and MRI white‑matter abnormalities.
• Other: rash, lymphadenopathy, respiratory or renal failure depending on severity and triggers.

Diagnosis

Early recognition is essential because delay increases mortality. Suspect in any infant/child presenting with ALF. Use either identification of a pathogenic HLH mutation or fulfillment of clinical criteria (commonly HLH‑2004): five of eight criteria are required when genetic testing is unavailable or negative.

HLH‑2004 Diagnostic Criteria (summary)	Comments for Clinical Use
Fever	Persistent, high spiking fevers despite usual antimicrobials
Splenomegaly	Often marked; correlate with ultrasound if exam uncertain
Cytopenias (≥2 lineages)	Hemoglobin, platelets, neutrophils affected; consider marrow exam if unexplained
Hypertriglyceridemia and/or hypofibrinogenemia	Triglycerides ≥265 mg/dL; fibrinogen ≤150 mg/dL suggest DIC/consumption
Hemophagocytosis	Bone marrow, spleen, lymph node, or liver biopsy; absence does not rule out HLH
Low/absent NK‑cell function	Specialized testing; helpful when available
Ferritin ≥500 ng/mL	Markedly elevated ferritin increases specificity; trend useful
Elevated soluble CD25 (sIL‑2R)	Reflects T‑cell activation; high values support diagnosis

Recommended diagnostic workup: CBC with differential, peripheral smear, coagulation panel, fibrinogen, ferritin, triglycerides, liver panel, LDH, creatinine, blood cultures, EBV PCR and other viral studies, blood/urine/CSF as clinically indicated, soluble CD25, NK function (if available), bone marrow aspirate/biopsy to assess for hemophagocytosis and malignancy, and targeted genetic testing for familial HLH genes when suspicion is high.

Treatment

Treatment goals: rapidly suppress hyperinflammation, treat or remove triggers, provide organ support, and for genetic disease proceed to curative hematopoietic cell transplantation (HCT) once disease controlled.

Initial and induction therapy

• Immunochemotherapy (HLH‑94/HLH‑2004 backbone): dexamethasone with etoposide for induction in severe or progressive cases; intrathecal therapy if CNS disease is present per protocol.
• Cyclosporine A: used in some protocols for consolidation or maintenance.

Targeted and biologic therapies

• Anti‑cytokine agents: anakinra (IL‑1 receptor antagonist) widely used for MAS/secondary HLH and to avoid myelotoxicity in fragile patients; tocilizumab (anti‑IL‑6) used selectively.
• Anti‑IFN‑γ therapy: emapalumab (anti‑IFN‑γ) is approved for refractory or primary HLH and can be life‑saving in select patients.
• B‑cell targeted therapy: rituximab for EBV‑associated HLH to reduce viral reservoir.

Definitive therapy for genetic HLH

Hematopoietic cell transplantation (HCT) is the only curative therapy for primary HLH and certain genetic predispositions; optimal timing is after adequate disease control. Donor selection and transplant conditioning require subspecialist coordination.

Supportive care

• ICU‑level support as needed (ventilation, renal replacement).
• Blood product transfusion and correction of coagulopathy.
• Aggressive infection surveillance and prophylaxis when immunosuppressed.
• Careful attention to hepatic function: avoid hepatotoxic agents when possible, manage cholestasis and synthetic failure, and consult transplant hepatology early for severe liver failure.

Prognosis and Follow‑up

Prognosis varies by etiology and response to therapy. Without therapy HLH is frequently fatal. Modern protocols and HCT have markedly improved survival for familial disease, but mortality remains significant for malignancy‑associated and refractory cases.

Long‑term follow‑up should address:

• Relapse surveillance (clinical and laboratory monitoring, ferritin trends).
• Post‑HCT complications (graft failure, GVHD, infections).
• Neurologic sequelae and developmental follow‑up for children with CNS involvement.
• Ongoing management of organ dysfunction including hepatic sequelae; engage multidisciplinary teams early.

References