Lymphophagocytic astrocytosis
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Lymphophagocytic histiocytosis
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"hemophagocytic lymphohistiocytosis"[MeSH Terms]
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Here is a comprehensive overview of Hemophagocytic Lymphohistiocytosis (HLH):
Hemophagocytic Lymphohistiocytosis (HLH)
Definition
HLH is a life-threatening syndrome of massive, uncontrolled hyperinflammation driven by unrelenting activation of CD8+ cytotoxic T lymphocytes and macrophages, leading to multiorgan damage — particularly in the liver, bone marrow, and CNS. It exists in a primary (genetic) and a secondary (acquired) form with a convergent clinical phenotype resembling septic shock.
— Robbins, Cotran & Kumar Pathologic Basis of Disease; Harrison's 22E
Classification
Primary HLH (Mendelian/Familial)
Caused by homozygous or compound heterozygous null mutations in genes controlling cytotoxic granule synthesis, trafficking, and function:
| Gene Defect | Syndrome |
|---|---|
| PRF1 (perforin) | FHL2 — ~30% of familial cases |
| UNC13D (Munc13-4) | FHL3 — ~30% |
| STX11 (syntaxin 11) | FHL4 — ~10% |
| STXBP2 (Munc18-2) | FHL5 — ~20% |
| SH2D1A | XLP1 |
| BIRC4 (XIAP) | XLP2 |
| RAB27A | Griscelli syndrome type 2 |
| LYST | Chédiak-Higashi syndrome |
Secondary HLH (Acquired)
- Infection-associated: Viruses (~70% of secondary cases) — EBV most common (40% of viral cases); also CMV, hepatitis viruses, influenza, COVID-19; bacteria (TB, Staphylococcus, Rickettsia); parasites (visceral leishmaniasis, malaria); fungi (Histoplasma)
- Malignancy-associated: Lymphoma is the trigger in >50% of adult cases
- Autoimmune-associated (MAS-HLH): Systemic JIA, adult-onset Still's, SLE, vasculitis
- Transplant-associated
Primary HLH mostly affects children; secondary HLH is far more common in adults.
— Harrison's 22E, Goldman-Cecil Medicine
Pathobiology
Under normal conditions, CTLs and NK cells release perforin-containing cytolytic granules to kill antigen-expressing target cells → target cell elimination → downregulation of the immune response (activation-induced cell death).
In HLH, this cytotoxic mechanism fails. Infected or antigen-bearing cells persist, causing continuous stimulation of CD8+ T cells and NK cells. These cells then switch to their secondary effector mechanism — massive cytokine secretion, especially IFN-γ, which is the master activator of macrophages. The result:
- Uncontrolled macrophage activation → hemophagocytosis (macrophages engulf RBCs, WBCs, platelets, and their precursors)
- Massive release of TNF-α, IL-6, IL-12, and IL-18 → cytokine storm
- Systemic inflammatory response → multiorgan failure
In adults with secondary HLH, ~15% harbor heterozygous hypomorphic alleles in familial HLH genes (most commonly the PRF1 A92V polymorphism, present in 5–8% of the general population), which may predispose to secondary HLH under a "second hit."
— Goldman-Cecil Medicine; Robbins Pathologic Basis of Disease
Clinical Features
| Feature | Details |
|---|---|
| Fever | High, unremitting; nearly universal |
| Hepatosplenomegaly | Prominent in both children and adults |
| Cytopenias | Bicytopenia required (thrombocytopenia most frequent; also anemia, neutropenia) |
| Coagulopathy | Low fibrinogen, DIC, severe bleeding risk |
| Liver involvement | Elevated transaminases, conjugated hyperbilirubinemia, elevated GGT; may mimic acute liver failure |
| CNS involvement | ~1/3 of children: seizures, decreased consciousness, meningismus, cranial nerve palsies; less common in adults |
| Rash | Non-specific maculopapular, petechiae, purpura |
| Lymphadenopathy | ~50% of children |
Children (primary HLH): Median onset 3–6 months; prominent CNS involvement; median survival without treatment 1–2 months.
Adults (secondary HLH): Present at any age; more often sepsis-like picture; CNS involvement less frequent; lymphoma is the most common trigger.
Diagnosis (HLH-2004 Criteria)
Diagnosis requires molecular diagnosis of familial disease OR ≥5 of these 8 criteria:
- Fever
- Splenomegaly
- Cytopenias of ≥2 lineages (Hgb <90 g/L, platelets <100×10⁹/L, neutrophils <1×10⁹/L)
- Hypertriglyceridemia (fasting TG ≥3.0 mmol/L) and/or hypofibrinogenemia (≤1.5 g/L)
- Hemophagocytosis on bone marrow, spleen, lymph node, or liver biopsy
- Low/absent NK cell activity
- Hyperferritinemia (ferritin ≥500 μg/L)
- Elevated soluble CD25 (sIL-2R) ≥2,400 U/mL
Key caveat: Hemophagocytosis in the bone marrow is neither sufficient nor required — it may be absent early in the course. Serial examinations may be needed. In sepsis vs. HLH, IFN-γ is markedly elevated in HLH while IL-6 dominates in sepsis — a useful distinguishing cytokine pattern.
— Goldman-Cecil Medicine; Harrison's 22E
Treatment
Algorithm:
HLH-94 Protocol (first-line for primary/severe HLH):
- Dexamethasone 10 mg/m²/day × 2 weeks, tapering over 8 weeks
- Etoposide 150 mg/m² twice weekly × 2 weeks, then weekly
- Intrathecal methotrexate (for neurological involvement)
- Cyclosporine A added in HLH-2004 (from day 1, trough ~200 μg/L)
Secondary HLH — treat the underlying trigger:
- Malignancy: Etoposide-containing chemotherapy immediately
- EBV-HLH: Mild/moderate → glucocorticoids ± IVIG ± cyclosporine; severe → etoposide + dexamethasone + rituximab (anti-CD20); etoposide dosing less intense than in primary HLH (75–100 mg/m² weekly in age <40; 50–75 mg/m² weekly in age ≥40)
- Autoimmune/MAS: Immunosuppression + disease-specific therapy
- Emerging therapies: JAK1 inhibitors (targeting the IFN-γ pathway), IL-1/IL-6 inhibitors, emapalumab (anti-IFN-γ)
Curative therapy: Allogeneic HSCT — mandatory for familial HLH and refractory/relapsed disease. Without treatment, familial HLH is uniformly fatal. With treatment ± HSCT, ~50% survival, though neurologic sequelae (psychomotor retardation, epilepsy) affect ~1/3 of survivors.
— Harrison's 22E; Goldman-Cecil Medicine; Robbins Basic Pathology
Key Points to Remember
- HLH is a clinical syndrome, not a disease — always search for the underlying trigger
- The unifying mechanism is failure of CTL/NK cytotoxicity → IFN-γ-driven macrophage activation
- Ferritin >500 μg/L is sensitive but not specific; ferritin >10,000 μg/L is highly suggestive
- sCD25 (sIL-2R) is a powerful biomarker of disease activity
- In critically ill patients who don't respond to antibiotics, think HLH
- EBV is the single most common trigger across all forms
- JAK inhibition (targeting JAK1/IFN-γ axis) is an emerging treatment strategy
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