Hemophagocytic Lymphohistiocytosis (HLH) Causes and Associations
Primary (Genetic) HLH
- Hereditary defects that predispose to primary HLH include familial HLH types 2-5, which are caused by mutations affecting perforin, UNC13D, STX11, and STXBP2, according to the European Hematology Association 1
- Griscelli syndrome type 2 is a genetic defect that impairs cytotoxic granule function in NK cells and cytotoxic T lymphocytes, leading to defective killing of target cells and failure to contract the immune response, as stated by the American Society of Hematology 2
- X-linked lymphoproliferative syndromes types 1 and 2 (XLP1, XLP2) are associated with EBV-triggered HLH and lymphoma development, with XLP1 being particularly associated with these conditions, according to the National Institutes of Health 2, 1
Secondary (Acquired) HLH
Infectious Triggers
- Viral infections, particularly Epstein-Barr virus (EBV) and cytomegalovirus (CMV), are the most frequent triggers of secondary HLH, as reported by the Centers for Disease Control and Prevention 3, 4
- Invasive fungi and bacterial infections can also trigger HLH, especially in patients with chemotherapy-induced immunosuppression, according to the Infectious Diseases Society of America 4
Malignancy-Associated HLH
- T-cell and NK-cell lymphomas or leukemias are the most common malignancy associations with HLH, as stated by the American Society of Clinical Oncology 3, 4
- Diffuse large B-cell lymphoma (DLBCL) and Hodgkin lymphoma are also associated with HLH, according to the European Society for Medical Oncology 3, 4
HLH During Chemotherapy
- Chemotherapy-induced immunosuppression creates vulnerability to infections that trigger HLH, with invasive fungi and bacterial infections playing a substantial role, as reported by the American Society of Hematology 4
Important Clinical Considerations
- In male patients with lymphoma and EBV-driven HLH, genetic or flow cytometric analysis for XLP1 should be strongly considered, as 24% of XLP1 patients develop malignancy, usually Hodgkin lymphoma, according to the National Institutes of Health 2, 1
- Hypomorphic biallelic perforin mutations confer predisposition to both late-onset HLH and hematologic malignancies, particularly BCR-ABL positive acute lymphoblastic leukemia, as stated by the European Hematology Association 2, 1
- Co-triggers are common, with viral infections acting as co-triggers in malignancy-associated HLH, and it is often difficult to differentiate between malignancy-triggered HLH and HLH during chemotherapy when both malignancy reactivation and infection coexist, according to the American Society of Hematology 4
Hemophagocytic Lymphohistiocytosis Diagnosis and Management
Definition and Classification
- Hemophagocytic lymphohistiocytosis (HLH) represents a spectrum of hyperinflammatory disorders resulting in excessive immune activation with characteristic clinical and laboratory findings 5
- HLH is classified into two main types: primary (genetic/familial) HLH and secondary (acquired) HLH, with secondary HLH being more common in adults and triggered by infections, malignancies, or autoimmune/autoinflammatory disorders 5, 6
Pathophysiology
- Secondary HLH can be triggered by infections, particularly viruses like EBV and CMV, malignancies, especially lymphomas, and autoimmune/autoinflammatory conditions 5, 7
Clinical Manifestations
- Cardinal clinical features of HLH include persistent high fever, hepatosplenomegaly, bi- or trilineage cytopenias, and neurological symptoms 5
- Laboratory abnormalities include elevated ferritin, hypertriglyceridemia, hypofibrinogenemia, and elevated soluble CD25 (IL-2 receptor alpha chain) 5
Diagnosis
- The HLH-2004 diagnostic criteria include molecular diagnosis consistent with HLH or at least 5 of the following 8 criteria: fever, splenomegaly, cytopenias, hypertriglyceridemia and/or hypofibrinogenemia, hemophagocytosis, low or absent NK cell activity, ferritin ≥500 μg/L, and elevated soluble CD25 6, 7, 8
Management
- The American Society of Hematology recommends first-line treatment for most secondary HLH cases to include high-dose corticosteroids and simultaneous treatment of the underlying cause 9
- Second-line options for inadequate response to corticosteroids include cyclosporine A, anakinra, or tocilizumab 9
Prognosis
- Mortality remains high, especially in adults with malignancy-associated HLH, with factors associated with higher mortality including shock at ICU admission, platelet count <30 g/L, and HLH associated with T-cell lymphomas 9, 10, 11
- Early recognition and prompt treatment are essential to prevent irreversible organ damage and improve survival 6, 9
Emerging Therapies
Hemophagocytic Lymphohistiocytosis (HLH) Diagnosis and Management
Diagnostic Criteria
- The patient meets at least 5 of 8 HLH-2004 diagnostic criteria, which is sufficient for diagnosis, including cytopenias affecting ≥2 lineages, hypertriglyceridemia, and hyperferritinemia 12
- The key clinical feature is persistent fever and worsening cytopenias despite appropriate doxycycline treatment for confirmed Ehrlichia infection, indicating a secondary hyperinflammatory syndrome 13
Disease Pathophysiology and Management
- Ehrlichia chaffeensis is a documented trigger for secondary HLH, involving uncontrolled activation of cytotoxic T lymphocytes, NK cells, and macrophages, resulting in a cytokine storm with progressive tissue damage despite treating the underlying infection 13
- Critical management principle: patients infected by pathogens targeting the monocyte-macrophage system may develop HLH, and while antimicrobial treatment is essential, immunosuppression may be required when HLH criteria are met and the patient continues to deteriorate 13
Treatment Approach
- Immediate actions required include continuing doxycycline to treat the underlying Ehrlichia infection, and initiating HLH-directed therapy with high-dose corticosteroids, such as dexamethasone or methylprednisolone 12, 13
- Consider adding IVIG in severe cases, and monitor response every 12 hours, including ferritin levels, cell counts, triglycerides, and clinical status 12, 13
- Escalate treatment if there is an inadequate response to corticosteroids, with options including cyclosporine A, anakinra, or tocilizumab as second-line agents, and etoposide-based therapy if refractory 13
Evaluation and Management of Tick‑borne Rickettsial Infections and HLH Screening in Patients with Prolonged Fever
Clinical Presentation of Tick‑borne Rickettsial Diseases
- Fever, thrombocytopenia, mildly elevated liver enzymes, and mild splenomegaly are typical manifestations of ehrlichiosis and anaplasmosis; reactive lymphocytes are less prominent than neutropenia. (CDC MMWR Recommendations) 14, 15, 16
- A history of outdoor or wooded‑area exposure or a tick bite should prompt consideration of these infections even when the patient does not recall a bite. (CDC MMWR Recommendations) 15
Diagnostic Testing
Hematology and Peripheral Smear
- Examine the peripheral blood smear for intracellular morulae to identify ehrlichiosis. (CDC MMWR Recommendations) 14, 15
Exclusion of Bacterial Sepsis
- Obtain blood cultures to rule out bacterial sepsis in any febrile patient. (CDC MMWR Recommendations) 14, 15
Coagulation Assessment
- Perform coagulation studies (PT, APTT, INR, D‑dimer) to exclude disseminated intravascular coagulation and thrombotic thrombocytopenic purpura. (CDC MMWR Recommendations) 14
HLH Screening Parameters (per HLH‑2004)
- Fever > 38.5 °C persisting > 7 days and mild splenomegaly are among the HLH criteria to be evaluated early. (CDC MMWR Recommendations) 15
Serology and Molecular Testing for Tick‑borne Pathogens
- Order Ehrlichia and Anaplasma serology with PCR when exposure history or residence in endemic areas is present. (CDC MMWR Recommendations) 14, 15
Initial Supportive Care
- Provide hydration and antipyretics for symptomatic relief in febrile patients. (CDC MMWR Recommendations) 15
- Monitor vital signs every 4–6 hours to detect hemodynamic instability promptly. (CDC MMWR Recommendations) 15
Specific Antimicrobial Therapy
- Treat confirmed or highly suspected ehrlichiosis with doxycycline 100 mg orally or intravenously twice daily for 7–14 days, while continuing surveillance for possible HLH development. (CDC MMWR Recommendations) 14
Clinical Pitfalls
- Do not overlook geographic or exposure history; tick‑borne diseases (ehrlichiosis, anaplasmosis) and dengue can mimic other febrile illnesses and require pathogen‑specific antimicrobial therapy. (CDC MMWR Recommendations) 14, 15
Strength of evidence: Not explicitly graded in the cited MMWR recommendations.
Atypical Lymphocytosis in Immunocompromised and Malignancy‑Associated Settings
Infectious Triggers in Immunocompromised Hosts
- Invasive fungal and bacterial infections in patients with chemotherapy‑induced immunosuppression can precipitate atypical lymphocytosis as part of secondary hemophagocytic lymphohistiocytosis (HLH). 17
- Chemotherapy‑induced immunosuppression creates vulnerability to infections that trigger atypical lymphocytosis and secondary HLH. 17
Malignancy‑Associated Causes
- T‑cell and natural‑killer‑cell lymphomas or leukemias account for roughly 35 % of adult HLH cases that present with atypical lymphocytosis. 17
- Diffuse large B‑cell lymphoma (DLBCL) is the leading trigger in Western countries, representing about 32 % of B‑cell lymphoma‑associated atypical lymphocytosis cases. 17
- Hodgkin lymphoma contributes to approximately 6 % of malignancy‑associated HLH cases with atypical lymphocytosis. 17
- Solid tumors can cause paraneoplastic atypical lymphocytosis, although they represent only about 3 % of malignancy‑associated cases. 17
Iatrogenic and Transplant‑Related Factors
- Post‑transplant immunosuppression, especially after kidney transplantation, is linked to infection‑triggered atypical lymphocytosis and HLH. 17
- The same post‑transplant setting is further supported by additional evidence (see citation 18).
Co‑trigger Dynamics
- Viral infections frequently act as co‑triggers in malignancy‑associated atypical lymphocytosis, complicating the distinction between primary and secondary causes. 17