Hemophagocytic Lymphohistiocytosis and Infections in Children: An Update

Hemophagocytic lymphohistiocytosis (HLH), also referred to as macrophage activation syndrome (MAS), is a rare and life-threatening hyperinflammatory disorder characterized by excessive immune activation. The syndrome arises from a dysregulated cytokine cascade, leading to uncontrolled inflammation, multiorgan dysfunction and high mortality if left untreated. Infections are frequently implicated as triggers, particularly in individuals with underlying genetic predispositions. Early recognition is paramount, as prompt intervention targeting both the hyperinflammatory state and the inciting infection when possible is essential for improving outcomes. This review synthesizes current evidence on the pathophysiology, diagnostic criteria, and therapeutic strategies for HLH in children, with a focus on the pivotal role of infections in disease initiation and progression. CLASSIFICATION AND DEMOGRAPHICS Traditionally, HLH has been classified as either primary (familial) or secondary (acquired). However, contemporary understanding highlights distinct pathological mechanisms underlying different HLH subtypes, each requiring tailored management approaches.1,2 Familial HLH (fHLH) results from genetic defects impairing cytotoxic function, typically presenting in infancy with severe manifestations. MAS-HLH occurs in the context of autoinflammatory diseases, such as systemic juvenile idiopathic arthritis or systemic lupus erythematosus, where macrophage activation predominates. Malignancy-associated HLH, though rare in children, is most frequently linked to lymphomas. Infection-associated HLH (iHLH) represents a significant proportion of pediatric cases and may unmask underlying inborn errors of immunity. Additionally, immune effector-associated HLH has emerged as a complication of immunotherapies, such as CAR-T cell therapy. iHLH accounts for approximately half of pediatric HLH cases,3 though infections can exacerbate all subtypes. For instance, up to 65% of MAS-HLH flares are triggered by concurrent infections.3 The demographic distribution of HLH is influenced by geographic variations in infectious triggers. Epstein–Barr virus (EBV) is a predominant cause in East Asia,4 while leishmaniasis is frequently implicated in Southern Europe.5 Tropical infections, though likely underreported, contribute to HLH in endemic regions. Data from some global regions, such as Africa, remain limited, highlighting the need for further epidemiological studies. Pathogenesis and Genetics The pathophysiology of HLH centers on defective cytotoxic lymphocyte function, leading to uncontrolled immune activation. In healthy individuals, CD8+ T cells and natural killer (NK) cells induce apoptosis of infected or dysregulated macrophages, maintaining immune homeostasis. In HLH, genetic or acquired defects in cytolytic pathways result in persistent macrophage activation and excessive cytokine release, including interleukin (IL)-1β, IL-6, IL-12, IL-18 and tumor necrosis factor-alpha.6 IL-12 and IL-18 stimulate cytotoxic T cells, which in turn secrete IL-2 and interferon-gamma (IFN-γ).6 IFN-γ further amplifies this inflammatory cascade, creating a self-sustaining “cytokine storm.”6,7 Despite compensatory anti-inflammatory responses, such as elevated IL-10, the feedback loop remains unchecked, culminating in tissue damage and multiorgan failure. Cytokine profiles vary across HLH subtypes, offering diagnostic and therapeutic insights. EBV-driven HLH is characterized by markedly elevated IFN-γ, whereas MAS-HLH demonstrates prominent IL-6 and IL-18 elevation, being more driven by activation of macrophages and the inflammasome, with T cell activation being secondary.8 In contrast, fHLH exhibits profound cytotoxic dysfunction with relatively lower IFN-γ levels. Genetic studies have identified mutations in key cytolytic genes (eg, PRF1, UNC13D and STXBP2) as causative in fHLH (Table, Supplemental Digital Content 1, https://links.lww.com/INF/G446).4 Notably, hypomorphic mutations in genes linked to HLH may predispose individuals to late-onset HLH, supporting a “multi-hit” model in which genetic susceptibility combines with environmental triggers, such as infections, to precipitate disease.9 Intriguingly, heterozygous mutations in HLH-associated genes have been identified in fatal influenza cases,10 suggesting a broader role in severe hyperinflammatory responses. Moreover, heterozygous mutations in fHLH genes have been found at a higher rate in systemic juvenile idiopathic arthritis patients who progress to MAS-HLH, further supporting a “multi-hit” model.11 Infection and HLH Any infection can potentially trigger HLH, including viruses, bacteria, mycobacteria, parasites, and fungi (Table, Supplemental Digital Content 2, https://links.lww.com/INF/G446). An underlying susceptibility to infection or inability to eradicate an infection potentially contributes to the development of HLH. For example, neonates are more vulnerable to disseminated, serious herpes simplex virus (HSV) infection, and HSV is therefore a key triggering infection in neonatal HLH.12 This applies equally to inborn errors of immunity, where between 68% and 80% of cases of HLH have an underlying infectious trigger and may present with HLH before diagnosis.3,13 In severe combined immunodeficiency, where patients have a susceptibility to viral infections, adenovirus and cytomegalovirus were common causes of HLH with subsequent 63% mortality,13,14 whereas in chronic granulomatous disease patients with a phagocytic defect, bacterial and fungal causes are described as HLH triggers.14 Infections can act both as triggers for HLH and as secondary complications, including bacteremia, fungal infections and viral reactivation.2,12 This dual role stems from multifactorial mechanisms: the hyperactivated immune response in HLH disrupts the host’s protective barriers, such as mucosal integrity, while cytokine-driven myelosuppression results in cytopenias and impaired cell-mediated immunity, collectively weakening the host’s ability to combat pathogens.2 Thus, secondary infections recorded in HLH are a sequel of immune dysregulation, cytopenias and use of immunosuppressive therapies to control HLH. Invasive bacterial infections (eg, sepsis), fungal diseases and reactivated viral infections (eg, cytomegalovirus) significantly contribute to high morbidity and mortality rates.2,15,16 Therefore, comprehensive infection screening and prophylactic measures—including prophylaxis against Pneumocystis jirovecii pneumonia, antifungal agents and antiviral therapies—are critical components of HLH management to mitigate these risks.15,16 Viruses Many viruses have been described in HLH, as listed in Table, Supplemental Digital Content 2, https://links.lww.com/INF/G446. EBV is the infection most commonly associated with HLH in both adults and children.13,17 EBV-associated HLH can develop in the context of high viral load, acute infection, reactivation and chronic active infection, with the latter associated with worse outcomes.17,18 While acute HIV infection has been described as a trigger, most cases of HLH in HIV-infected patients are caused by other pathogens, in particular AIDS-defining infections such as Cryptococcus neoformans and P. jirovecii, further highlighting that the immune system’s inability to effectively manage an infection is an important factor in the HLH pathogenesis.19–21 Another notable viral infection associated with HLH is SARS-CoV-2, responsible for the COVID-19 pandemic. Primary acute SARS-CoV-2 infection is associated with a hyperinflammatory state in adults suffering from severe disease and postinfectious multisystem inflammatory syndrome in children. One systematic review found an HLH prevalence of 7% in severe SARS-CoV-2 infection in adults based on HScore,22 though only a handful of case reports describe HLH following acute SARS-CoV-2 infection in children, many with fHLH diagnoses, possibly reflecting the generally milder course of infection in childhood.23 Bacteria and Mycobacteria Both serious bacterial infections and more indolent ones can be associated with HLH. Bacterial sepsis may evolve into HLH, and so it is important to monitor for HLH features that may develop, such as cytopenias and splenomegaly, especially if there is a poor response to initial management. Ferritin can be used as an initial screening tool,3 though it will be raised to a lesser extent in neonates.24 Notably, some patients with mycobacterial-associated HLH developed the condition without prior infectious symptoms or clinical signs, despite being asymptomatic before HLH onset.25 Fungi and Parasites Leishmaniasis, a parasitic disease caused by Leishmania species, is a well-recognized trigger and critical differential diagnosis for HLH. Visceral leishmaniasis (VL) exhibits overlapping clinical and laboratory features with HLH, including fever, hepatosplenomegaly and hemophagocytosis observed in bone marrow biopsies. Notably, VL progresses to secondary HLH in approximately one-third of hospitalized patients, underscoring the diagnostic challenge.5 A history of travel to or residence in endemic regions—such as southern Europe, South Asia or parts of Africa—should heighten suspicion for VL, as timely diagnosis and treatment with amphotericin B are essential to prevent fatal outcomes. Diagnosis Timely diagnosis of HLH demands a high clinical suspicion, particularly in children with severe, persistent or treatment-resistant infections. Classic features, such as unremitting fever, cytopenias and hepatosplenomegaly, should prompt immediate evaluation. Elevated ferritin—a first-line, accessible biomarker—is highly suggestive of HLH. A threshold of 500 μg/L is outlined in the HLH-2004 criteria, though ferritin may be less markedly elevated in neonates and higher thresholds have been recommended to improve specificity.24,26 Structured diagnostic frameworks, including the HLH-94, HLH-2004 protocols and HScore, integrate clinical, laboratory and histopathological findings (Table 1, and Table, Supplemental Digital Content 1, https://links.lww.com/INF/G446). However, hemophagocytosis on bone marrow biopsy, despite being a namesake feature of HLH, lacks diagnostic specificity and sensitivity.4,15 TABLE 1. - Diagnostic Scores for Diagnosing HLH HLH-94 criteria HLH-2004 criteria Revisited HLH-2024 criteria MAS score HScore Set of 5 diagnostic criteria should be fulfilled before establishing the diagnosis. Diagnosis can be established based on 1 or 2 is fulfilled. The diagnosis of HLH can be established if at least 1 of either 1, 2 or 3 below is fulfilled. Criteria used to diagnose MAS in patients with known or suspected systemic juvenile idiopathic arthritis. Adult score validated in some child cohorts. Cut off value for HLH diagnosis: 169. Clinical Criteria: Persistent or intermittent fever. Splenomegaly Laboratory Criteria: Cytopenia affecting at least 2 of 3 lineages (hemoglobin <90 g/L, platelets <100 × 109/L and neutrophils <1.0 × 109/L)Hypertriglyceridemia (fasting triglycerides ≥2.0 mmol/L) and/or hypofibrinogenemia (fibrinogen ≤1.5 g/L)Histopathologic Criteria: Haemophagocytosis in bone marrow, spleen or lymph nodes and no evidence of malignancy. 1. Molecular diagnosis consistent with HLH2. Diagnostic criteria (5 of the 8 following criteria):• Fever• Splenomegaly• Cytopenias in ≥2 lineages Hemoglobin <9 g/dL, in neonates <10 g/dLPlatelet count <100 × 103/mLNeutrophil count <1 × 103/mL• Hypertriglyceridemia ≥3.0 mmol/L (>265 mg/dL) and/or hypofibrinogenemia (<150 mg/dL)• Haemophagocytosis in bone marrow or spleen, liver or lymph node.• Low or no NK cell activity• Ferritin ≥500 ng/mL• sCD25 ≥2400 1. A molecular diagnosis consistent with FHL in a patient with signs/symptoms suggestive of HLH2. Functional cellular findings consistent with FHL in a patient with signs/symptoms suggestive of HLH3. Clinical diagnostic criteria for FHL with at least 5 of the 7 criteria below fulfilled• Fever ≥38.5°C• Splenomegaly (≥2 cm below the costal margin)• Cytopenias [affecting ≥2/3 lineages in the peripheral blood: hemoglobin <90 g/L; platelets <100 × 109/L; neutrophils <1.0 × 109/L(in infants <4 weeks old: hemoglobin <100 g/L)]• Hypertriglyceridemia and/or hypofibrinogenemia: fasting triglycerides ≥3.0 mmol/L and fibrinogen ≤1.5 g/L• Haemophagocytosis• Ferritin ≥500 μg/L• sCD25 (ie, soluble interleukin-2 receptor) ≥2400 U/mL Ferritin >684 ng/mL and 2 of the following:1. Platelet count ≤ 181 × 109/L2. Aspartate aminotransferase >48 U/L3. Triglycerides >156 mg/dL4. Fibrinogen ≤360 mg/dL Ferritin (ng/mL)0 (<2000), 35 (2000–6000) or 50 (>6000)Known underlying immunosuppression0 (no) or 18 (yes)Temperature (°C)0 (<38.4), 33 (38.4–39.4) or 49 (>39.4)Fibrinogen (gm/liter)0 (>2.5) or 30 (≤2.5)Serum glutamic oxaloacetic transaminase (IU/L)0 (<30) or 19 (≥30)Haemophagocytosis features on bone marrow aspirate0 (no) or 35 (yes)Organomegaly0 (no), 23 (hepatomegaly or splenomegaly) or 38 (hepatomegaly and splenomegaly)No. of cytopenias0 (1 lineage), 24 (2 lineages) or 34 (3 lineages)Triglyceride (mmoles/L)0 (<1.5), 44 (1.5–4) or 64 (>4) 16 26 1 3 27 FHL, familial haemphoagocytic lymphohistiocytosis; sCD25: soluble culture of differentiation 25. Advancements in diagnostic approaches now emphasize genetic testing and soluble CD25 (sIL-2R) assays, which improve sensitivity compared with older criteria reliant on NK cell functional assays. Notably, NK cell activity results may be confounded by severe pancytopenia or immunosuppression, and revised guidelines exclude these assays to enhance reliability.1 HLH can also manifest in isolated organs, such as the central nervous system (CNS) or liver, necessitating vigilance even in atypical presentations.3 Isolated CNS-HLH, for instance, may present with unexplained neurological symptoms, requiring prompt magnetic resonance imaging and, in select cases, brain biopsy for confirmation.28 Similarly, hepatic HLH underscores the importance of imaging to delineate organ involvement. A comprehensive diagnostic workup should pursue 2 parallel objectives: identifying the underlying HLH trigger and evaluating for concurrent infections. In infants, genetic testing and lymphocyte subset analysis are critical to exclude primary immunodeficiencies. Older children with recurrent or atypical infections should undergo evaluation for underlying immune dysregulation. Malignancy, though uncommon in pediatric HLH, must be excluded, particularly in cases with poor treatment response. Treatment HLH represents a complex interplay of genetic predisposition, immune dysregulation and infectious triggers. Advances in understanding its pathophysiology have led to more precise diagnostic and therapeutic strategies. Future research should focus on biomarker-driven therapies and personalized approaches to improve outcomes in this devastating syndrome. Aggressive management of infection is essential. Therapeutic Approaches Treatment should be patient-specific, multidisciplinary and 2 pronged: to manage both the underlying cause and trigger, and may include anti-infectives, immunosuppressive therapies, chemotherapy, biologics and hematopoietic stem cell transplant (HSCT). A multidisciplinary team should involve hematology, immunology, infectious diseases, rheumatology and intensive care, with challenging cases discussed at a regional or national level if feasible. Anti-Infectives Removing the infectious trigger in HLH is essential to switch off the hyperinflammatory feedback loop and, in some cases, is sufficient to successfully treat.5,24 This should be tailored to the infection; for example, acyclovir in HSV-driven HLH. Empiric broad-spectrum antibiotics should also be considered, given the immunosuppressed state in HLH and potential difficulties distinguishing HLH from severe sepsis. Antifungal, P. jirovecii and antiviral prophylaxis should be considered depending on treatment risk and are part of the HLH-94 protocol and recent trials with targeted therapies.3,16,29 Immunosuppressives/Immunomodulators Corticosteroids, particularly dexamethasone, form the cornerstone of HLH therapy across most protocols due to their broad immunosuppressive activity and efficacy in diverse HLH subtypes. Dexamethasone is favored for its superior CNS penetration, attributed to its longer half-life and higher cerebrospinal fluid concentrations, a critical consideration in neuroinflammatory HLH manifestations.30 Vigilant monitoring for corticosteroid-related complications—such as increased rates of infection, hypertension, hyperglycemia and posterior reversible encephalopathy syndrome—is imperative during treatment.30 Intravenous immunoglobulin and plasmapheresis serve as adjunctive therapies in select cases. Intravenous immunoglobulin is widely accessible, carries a favorable safety profile, and does not interfere with malignancy evaluations or exacerbate immunosuppression, though robust evidence supporting its efficacy in HLH remains limited.3 Plasmapheresis, while less commonly utilized, offers a theoretical benefit by rapidly removing pro-inflammatory cytokines from circulation. It has demonstrated utility as a bridging therapy in severe HLH or as cotherapy to stabilize patients before definitive treatment.31 Chemotherapies Early initiation of chemotherapy regimens, including etoposide and/or ciclosporin, is critical for improving survival in patients with fHLH, severe EBV-driven HLH and malignancy-associated HLH, as emphasized by EULAR 2022 guidelines.3 The HLH-94 and HLH-2004 protocols both utilize dexamethasone, etoposide and ciclosporin, with the key difference being the timing of ciclosporin administration—introduced at treatment onset in HLH-2004. However, studies found no significant improvement in response rates or overall survival between the 2 protocols.26,32 Both therapies carry substantial risks: etoposide is associated with myelosuppression, while ciclosporin may induce hypertension or posterior reversible encephalopathy syndrome, necessitating rigorous monitoring for secondary infections and toxicities.30 Biologics, Including Novel Therapies Anakinra, an IL-1 receptor antagonist, has become a promising therapeutic option for MAS-HLH, iHLH and immune effector-associated HLH. Its appeal stems from a favorable safety profile in sepsis, limited immunosuppressive effects, rapid onset of action and supporting clinical evidence.3,33 These attributes have led to its inclusion in the 2022 EULAR guidelines as a potential early intervention for undifferentiated HLH. Targeted therapies are now tailored to disease subtypes and cytokine profiles. For example: 1. Rituximab, a B-cell-depleting demonstrates high efficacy in EBV-driven an is for fHLH and potential in other hyperinflammatory therapies for 1. a cytokine and has efficacy as in a dual and an IL-18 are in cases with elevated such as an has been successfully used in clinical trials are these both as and in with is a treatment for patients with fHLH and or recurrent secondary HLH, particularly in cases with underlying genetic defects or persistent to to defective immune cells with healthy stem cells, immune and disease The timing of is with outcomes observed when is disease control protocols etoposide and or targeted have survival rates by organ while with survival in However, such as infections and immune necessitating and guidelines for all fHLH cases and select secondary HLH patients with strategies with agents (eg, are to further for Future therapy as a for fHLH, particularly in patients with mutations in genes such as PRF1, or which cytotoxic lymphocyte functional genes or mutations therapy to immune cell without the of disease or associated with trials have been including safety and the of genetic therapy treatment for fHLH, offering a with HLH represents a complex interplay of genetic predisposition, immune dysregulation and infectious triggers. Advances in understanding its pathophysiology have led to more precise diagnostic and therapeutic strategies. Infections remain central to both the initiation and of HLH, necessitating monitoring and management. Future research should focus on biomarker-driven therapies and personalized approaches to improve outcomes in this devastating syndrome.