Cloak and dagger: the case for adult onset still disease and hemophagocytic lymphohistiocytosis

Adult onset Still’s disease (AOSD) is a rare systemic inflammatory disorder of unknown etiology. Systemic onset juvenile idiopathic arthritis (SoJIA) is the preferred nomenclature of Still’s disease. Strong association with so-called macrophage activation syndrome (MAS) may provide a clue to the understanding of the distinctive pathogenetic features of SoJIA. MAS is a severe, potentially life-threatening complication characterized by the excessive activation of well-differentiated macrophages. It is more appropriately named autoimmune disease associated reactive hemophagocytic lymphohistiocytosis (ReHLH), a subset of a histiocytic disorder: class II histiocytosis hemophagocytic lymphohistiocytosis (HLH). The relation of SoJIA with HLH is still under debate. We propose that MAS, HLH, SoJIA, and AOSD are indeed the same disease, in different clinical presentations that may be classified based on severity and laboratory findings, but with essentially the same physiopathogenesis. We propose that the case described by Hong & Lee (Rheumatol Int 2008) was actually an AOSD-associated MAS/RHS/ReHLH fulminant disease.     

A Recurrent Case of Adult-onset Still's Disease with Concurrent Acalculous Cholecystitis and Macrophage Activation Syndrome/Hemophagocytic Lymphohistiocytosis Successfully Treated with Combination Immunosuppressive Therapy

We herein report the case of 21-year-old female diagnosed with adult-onset Still''s disease (AOSD) three years earlier who presented with fever and right upper abdominal pain. She was diagnosed with acute acalculous cholecystitis (AAC) based on hepatic dysfunction, elevated C-reactive protein, and gallbladder wall thickening on abdominal ultrasound. Based on the presence of pancytopenia, hyperferritinemia, and hemophagocytosis by a bone marrow examination, she was diagnosed with macrophage activation syndrome (MAS)/hemophagocytic lymphohistiocytosis (HLH) which was refractory to glucocorticoid pulse therapy. The combination of intravenous cyclosporine A with glucocorticoids was able to successfully control the disease activity of AOSD-related AAC and MAS/HLH.     

Murine typhus complicated by sHLH mimicking adult-onset Still's disease

IntroductionAdult-onset Still's disease (AOSD) is a rare multisystemic disorder and a diagnostic challenge for physicians because of the wide range of differential diagnoses. Common features of AOSD and secondary hemophagocytic lymphohistiocytosis (sHLH) could favour diagnostic uncertainty, in particular in case of infection-related sHLH.ObservationA 61-year-old man was admitted to our internal medicine department for suspected AOSD. He reported a 2-week history of sudden onset fever, headaches, myalgia, sore throat, diarrhoea, and an erythematous macular rash of the trunk as well as petechial purpuric lesions on both legs on return from Reunion Island. Laboratory tests found cytopenia, hepatic cytolysis, hypertriglyceridaemia, and hyperferritinaemia. Hemophagocytosis was diagnosed on bone marrow aspiration in favour of the diagnosis of secondary hemophagocytic lymphohistiocytosis (sHLH). Subcutaneous anakinra (100 mg) was initiated to treat sHLH with favourable course. Oral doxycycline was added 3 days later because of atypical features for AOSD diagnosis such as diarrhoea, hypergammaglobulinaemia, and doubtful serologies for Rickettsia and Coxiella. Three weeks later, Rickettsia typhi serology was checked again and revealed an increase in IgG titer > 4 times that confirmed the diagnosis of murine typhus. A diagnosis of murine typhus complicated by sHLH was retained, successfully treated by anakinra and doxycycline.ConclusionOur observation shows that AOSD diagnosis has to be stringent due to the many differential diagnoses, particularly infection complicated by sHLH, which may be rare. It is important to consider murine typhus in patients returning from endemic areas, such as La Reunion or other tropical areas, when they present fever of unknown origin with non-specific clinical features. Moreover, this case illustrates the effectiveness of IL-1 blockers as a treatment for symptomatic sHLH without severity criteria, regardless of the aetiology.     

Macrophage activation syndrome: characteristic findings on liver biopsy illustrating the key role of activated, IFN-gamma-producing lymphocytes and IL-6- and TNF-alpha-producing macrophages

Macrophage activation syndrome (MAS) is a rare and potentially fatal disorder, thought to result from uncontrolled activation and proliferation of T cells and excessive activation of macrophages. The term MAS designates a clinicopathologic entity that occurs in different hemophagocytic syndromes (HSs). Primary hemophagocytic lymphohistiocytosis (HLH) is recognized to have an immunogenetic basis, but in the secondary HS (also referred to as secondary HLH), the cause is unknown. The pathogenesis of the accelerated disease phase typical of MAS remains incompletely understood. This report describes the immunohistochemical findings on liver tissues from 5 children, each of whom presented with MAS in the context of a different type of HS. The data provide direct evidence for the involvement of activated CD8(+) lymphocytes through the production of interferon-gamma and of macrophages through hemophagocytosis and production of interleukin 6 and tumor necrosis factor-alpha, and underscore the view that MAS in different HSs share a common effector pathway.     

Hemophagocytic lymphohistiocytosis in COVID-19: Case reports of a stepwise approach

Rationale:The immunologic syndrome induced by severe acute coronavirus disease 2019 (COVID-19) is yet not fully understood. Typical patterns of clinical and laboratory features match secondary hemophagocytic lymphohistiocytosis (sHLH). However, the optimal approach to COVID-19 patients testing positive for sHLH is still unclear.Patient concerns:Three patients with COVID-19 are reviewed. All showed hyperinflammation and cytokine storm, necessitating intensive care treatment including mechanical ventilation.Diagnosis:Secondary hemophagocytic lymphohistiocytosis due to severe COVID-19; diagnosed via HScore.Interventions:A treatment regimen of methylprednisolone, pentaglobin, and anakinra was developed and administered.Outcomes:One patient survived the ICU stay. Two other patients, in whom sHLH was diagnosed too late, deceased.Lessons:A routine screening of COVID-19 patients for secondary HLH by using the HScore is feasible; especially those patients deteriorating clinically with no sufficient response to shock management might be at particular high risk. A stepwise therapeutic approach comprising corticosteroids, immunoglobulins and anakinra, accompanied by immunoadsorption, may dampen cytokine storm effects, and potentially reduce mortality.     

Clinical characteristics and follow-up analysis of adult-onset Still’s disease complicated by hemophagocytic lymphohistiocytosis

We evaluated clinical characteristics and prognosis for adult-onset Still’s disease (AOSD) complicated by hemophagocytic lymphohistiocytosis (HLH). We retrospectively identified cases of AOSD with (n?=?10) and without (n?=?305) HLH complications. We reviewed their medical records, completed follow-up through outpatient clinic and telephone interviews, and analyzed their clinical symptoms, signs, laboratory test results, treatments, and prognosis. More AOSD patients with HLH developed hepatomegaly, bleeding, serositis, and neurologic symptoms than those without HLH, and they more commonly presented with leukopenia, thrombocytopenia, severe anemia, severe liver function abnormalities, decreased fibrinogen, elevated immunoglobulin, and bone marrow hemophagocytosis. The ten patients with AOSD complicated by HLH were treated with high-dose steroids or pulse steroid therapy, and eight of them also received cytotoxic drugs, while biological agents showed poor response. Follow-up results indicated that AOSD patients overall had good prognosis, while those with HLH showed worse prognosis, including higher relapse and readmission rates and increased mortality. In patients with AOSD, unexplained decreased blood cells, severe liver dysfunction, and/or hemophagocytosis in the bone marrow should be considered as signs of HLH complication. Patients with AOSD complicated by HLH have worse prognosis and higher relapse rates compared to AOSD patients without HLH complications. Thus, these patients should undergo frequent and careful follow-up.     

Hemophagocytic Lymphohistiocytosis: Clinical Analysis of 103 Adult Patients

To investigate the clinical features of adult patients with hemophagocytic lymphohistiocytosis (HLH) and to explore possible risk factors for death, we retrospectively reviewed the medical records of 103 adult HLH patients hospitalized from 1997 to 2012. We analyzed the underlying diseases, clinical characteristics, 1aboratory findings, outcomes, and prognostic factors. The most common cause of HLH was hematologic malignancies (n = 49), followed by infectious diseases (n = 24) and autoimmune disorders (n = 14); 24 cases were of unknown etiology. Eight patients had a combination of underlying diseases. HLH was clinically characterized by high fever (96.1%), splenomegaly (79.6%), hepatomegaly (65.0%), lymphadenopathy (53.4%), proteinuria (31.1%), skin rash (25.2%), gastrointestinal hemorrhage (14.6%), disseminated intravascular coagulation (13.6%), increased creatinine (7.8%), and central nervous system involvement (12.6%) including altered mental status (9.7%) and cranial hemorrhage (2.9%). Laboratory abnormalities included cytopenia (99.0%), serum ferritin >500 ug/L (98.4%), liver dysfunction (98.1%), hypertriglyceridemia (88.5%), hemophagocytosis in bone marrow smear (87.4%), and hypofibrinogenemia (60.9%).In addition to the treatment they received for the underlying causes, patients received therapy for HLH consisting of corticosteroids, immunosuppressive drugs, and intravenous immunoglobulin. Twenty-six patients (25.2%) recovered after treatment, and 19 of them achieved long-term remission during follow-up. Seventy-seven patients (74.8%) died because of tumor, sepsis, multiple organ failure, or HLH-related organ hemorrhage and coagulopathy. The deceased patients were more likely to be older at disease onset, male, and to present with splenomegaly and thrombocytopenia, compared to the survivors. Treatment for the underlying diseases combined with corticosteroids, immunosuppressive agents, and immunoglobulin therapy may improve the prognosis of HLH. More attention should be paid to high-risk patients to prevent the development of serious complications associated with HLH.Key words/Abbreviations: hemophagocytic lymphohistiocytosis, lymphoma, autoimmune diseases, clinical manifestation, prognosis, risk factor, AOSD = adult-onset Still disease, CNS = central nervous system, DIC = disseminated intravascular coagulation, CMV = cytomegalovirus, EBV = Epstein-Barr virus, HLH = hemophagocytic lymphohistiocytosis, IVIg = intravenous immunoglobulin, MAS = macrophage-activation syndrome, NK = natural killer, RA = rheumatoid arthritis, SLE = systemic lupus erythematosus     

Haemophagocytic lymphohistiocytosis and Epstein–Barr virus: a complex relationship with diverse origins,expression and outcomes

Epstein–Barr virus (EBV) is a ubiquitous herpesvirus with rare but severe potential for lymphoproliferative complications. EBV is associated with a variety of presentations of haemophagocytic lymphohistiocytosis (HLH). HLH is a life-threatening hyperinflammatory syndrome that can occur in patients with genetic defects associated with dysregulation of the immune response (familial HLH) or arise in patients with underlying infection or malignancy (non-familial or secondary HLH). EBV can both serve as the incidental trigger of familial HLH or as the driving factor in patients with selective inherited vulnerability (e.g. X-linked lymphoproliferative disease). Alternatively, acute infection can idiosyncratically cause non-neoplastic HLH in patients without inherited predisposition (i.e. secondary HLH), while EBV-associated T/natural killer (NK)-cell lymphoproliferative disorders and lymphomas can cause neoplasia-associated HLH. The present review will discern between EBV-associated familial and non-familial HLH and highlight diagnostic and therapeutic considerations. Non-familial EBV-associated HLH is a major diagnostic dilemma, as it represents a diverse spectrum of disease ranging from highly curable (non-neoplastic EBV-HLH) to indolent but incurable (chronic active EBV) to acutely fatal (systemic EBV-positive T-cell lymphoma of childhood). Increased clinical awareness and understanding of this rare and potentially devastating subset of EBV-related complications is desperately needed to improve survival for patients with neoplasia-associated HLH.     

Reactive Hemophagocytic Syndrome in Adult-Onset Still Disease: Clinical Features,Predictive Factors,and Prognosis in 21 Patients

Hemophagocytic syndrome (HPS) is a potentially life-threatening complication of systemic inflammatory disorders. Adult-onset Still disease (AOSD) is one of the systemic autoimmune diseases associated with reactive hemophagocytic syndrome (RHS). This study aimed to evaluate the characteristic findings, predictive factors, and prognosis of RHS in patients with AOSD.We retrospectively evaluated 109 patients diagnosed with AOSD and reviewed their clinical data and laboratory findings, including the biopsy results of 21 AOSD patients with RHS. Moreover, data from 17 hemophagocytic lymphohistiocytosis (HLH) patients evaluated during the same period were compared with those from the RHS patients.Twenty-one patients (19.3%) developed RHS during the course of AOSD, and only 7 patients (6.4%) were confirmed by bone marrow, liver, or lymph node biopsy. AOSD patients with RHS showed significantly higher frequencies of splenomegaly, hepatomegaly, and lymphadenopathy than did those without RHS. Moreover, patients with RHS showed significantly higher relapse rates than those without RHS (61.9% vs 18.2%, P < 0.001). Possible triggering factors inducing hemophagocytosis were detected in 16 of 21 RHS patients (76.2%): disease flare in 12 patients (75%), infection in 3 patients (18.8%), and drug use in 1 patient (6.3%). AOSD patients with RHS showed higher frequencies of leukopenia, anemia, thrombocytopenia, hypoalbuminemia, hypofibrinogenemia, hypertriglyceridemia, hyperferritinemia, and elevated lactate dehydrogenase levels than did those without RHS. Multivariate logistic regression with forward selection procedure showed that low platelet count (<121,000/mm³), anemia, and hepatomegaly were independent predictors of RHS. Patients with definite RHS and those with probable RHS showed comparable results. Although RHS is a life-threatening complication of AOSD, long-term prognosis was observed to be similar in patients with and those without RHS. Compared to RHS patients, HLH patients had poor prognosis, such as higher death rates (52.9% vs 9.5%, P = 0.005).RHS can be considered when an AOSD patient shows at least 2 of the following 3 findings: low platelet count, anemia, and hepatomegaly. Diagnostic confirmation by biopsy may not be essential if typical clinical findings of RHS are present. Moreover, prognosis of RHS was better than that of HLH diagnosed by the presence of trilineage cytopenia at admission.     

Two‐faced haemophagocytic lymphohistiocytosis: comparative review of two cases of adult haemophagocytic lymphohistiocytosis

Haemophagocytic lymphohistiocytosis (HLH) is a rare but potentially fatal disease of children and adults. Cytokine dysfunction, uncontrolled accumulation of activated T‐cells and histiocytes, and the inability to terminate the immune response lead to the clinical manifestations of extreme inflammation and end‐organ damage. HLH is notoriously underreported because of its ability to mimic many other common diseases. Here, we outline two cases of HLH, one primary and the other secondary, to highlight some of the differences and to discuss therapeutic principles and emerging concepts.     

Hemophagocytic syndrome in a child with severe Crohn's disease and familial Mediterranean fever

Hemophagocytic lymphohistiocytosis (HLH) is a rare, potentially fatal, severe condition of hyperinflammation caused by the uncontrolled proliferation of activated lymphocytes and histiocytes secreting high amounts of inflammatory cytokines. Here we report a fatal hemophagocytic syndrome in a 11-year-old boy with a diagnosis of both Crohn's disease receiving immunosuppressive therapy and familial Mediterrenean fever. It is important to evaluate the patients with inflammatory bowel disease receiving immunosuppressive therapy presenting with unexplained fever, cytopenia, progression of organomegaly and biochemical changes for the investigation of HLH for diagnosis and treatment.     

Haematopoietic stem cell transplantation in haemophagocytic lymphohistiocytosis

Haemophagocytic lymphohistiocytosis (HLH) poses major therapeutic challenges, and the primary inherited form, familial haemophagocytic lymphohistiocytosis (FHL), is usually fatal. We evaluated, including Cox regression analysis, survival in 86 children (29 familial) that received HLH-94-therapy (etoposide, dexamethasone, ciclosporin) followed by allogeneic stem cell transplantation (SCT) between 1995 and 2000. The overall estimated 3-year-survival post-SCT was 64% [confidence interval (CI) = +/-10%] (n = 86); 71 +/- 18% in those patients with a matched related donor (MRD, n = 24), 70 +/- 16% with a matched unrelated donor (MUD, n = 33), 50 +/- 24% with a family haploidentical donor (haploidentical, n = 16), and 54 +/- 27% with a mismatched unrelated donor (MMUD, n = 13). After adjustment for potential confounding factors, estimated odds ratios (OR) for mortality were 1.93 (CI =0.61-6.19) for MUD, 3.31 (1.02-10.76) for haploidentical, and 3.01 (0.91-9.97) for MMUD, compared with MRD. In children with active disease after 2-months of therapy (n = 43) the OR was 2.75 (1.26-5.99), compared with inactive disease (n = 43). In children with active disease at SCT (n = 37), the OR was 1.80 (0.80-4.06) compared with inactive disease (n = 49), after adjustment for disease activity at 2-months. Mortality was predominantly transplant-related. Most HLH patients survived SCT using MRD or MUD, and survival with partially mismatched donors was also acceptable. Patients that responded well to initial pretransplant-induction therapy fared best, but some persisting HLH activity should not automatically preclude performing SCT.     

Fatal adenovirus serotype 21 infection associated with hemophagocytic lymphohistiocytosis and multiorgan failure

Recent reports describe an increase in the incidence of fatal adenovirus infections. Several severe cases have been linked to adenovirus serotype 21. The exact etiology for this unexpectedly high mortality remains unknown. We report the case of a patient with severe adenovirus serotype 21 pneumonia resulting in hemophagocytic lymphohistiocytosis (HLH) with acute respiratory distress syndrome and rapidly progressive multiorgan dysfunction syndrome (MODS). HLH describes a cytokine storm due to uncontrolled accumulation of activated T-lymphocytes and activated histiocytes. This results in organ infiltration with these cells, and subsequent hemophagocytosis of erythrocytes, leukocytes and platelets. In its most severe form, HLH leads to a sepsis-like picture and MODS. The association between adenovirus 21 and HLH may at least in part explain the recently observed increase in incidence of fatal adenoviral infections. We suggest that HLH should be considered in cases of severe adenoviral infection. If HLH is present, aggressive treatment is warranted.     

Clinical analysis of macrophage activation syndrome in pediatric patients with autoimmune diseases

Macrophage activation syndrome (MAS) belongs to secondary hemophagocytic lymphohistiocytosis (HLH) syndrome. It is usually associated with rheumatic diseases. We retrospectively reviewed our hospital's medical records of 102 HLH/MAS patients from the past 20 years. Demographics, clinical data, treatment, and outcomes were analyzed. Among 102 patients, eight patients with underlying juvenile systemic lupus erythematous (two patients), mixed connective tissue disease (one patient), primary anti-phospholipid syndrome (one patient), and systemic type juvenile rheumatoid arthritis (sJRA; four patients) with 13 episodes of MAS were studied. Clinical manifestations of MAS included fever (100 %), hepatosplenomegaly (77 %), lymphadenopathy (38 %), skin rash (62 %), and neurological involvement (31 %). Laboratory features included leukopenia (54 %), anemia (46 %), thrombocytopenia (77 %), jaundice (27 %), hypofibrinogenemia (40 %), decreased erythrocyte sedimentation rate (67 %), and elevated liver enzymes (77 %), lactate dehydrogenase (100 %), ferritin (88 %), triglycerides (91 %), C-reactive protein (85 %), plasma D-dimer (50 %), and hemophagocytosis in bone marrow (83 %). The Epstein-Barr virus and adenovirus infection triggered MAS in two patients with sJRA. Methylprednisolone pulse therapy was effective in two out of three patients, and high-dose intravenous immunoglobulin (IVIG) was effective in two out of six patients. Patients with sJRA responded well to corticosteroids and cyclosporine. Complications included opportunistic infection with Pneumocystis jiroveci, multiple organ failure, and intensive care unit myopathy. The mortality rate was one out of eight (12.5 %). Our results showed that MAS could be fatal and complicate various pediatric autoimmune diseases. It generally has a good response to corticosteroids and IVIG. Prompt recognition and timely treatment can result in good outcomes.     

Macrophage activation syndrome II/II

Macrophage activation syndrome (MAS) is a rare systemic disorder which results from uncontrolled activation and proliferation of T cells and excessive activation of macrophages. Primary haemophagocytic lymphohistiocytosis (HLH) is recognized as having a genetic basis, but the secondary haemophagocytic syndrome (HS), also referred to as MAS, occurs in a number of autoimmune disorders including systemic onset juvenile idiopathic arthritis, systemic lupus erythematosus (SLE), adult onset Still's disease and other disorders. In this second of the two part series, the clinical features and management are described.     

Benefit and a possible risk of tocilizumab therapy for adult-onset Still’s disease accompanied by macrophage-activation syndrome

We report a 57-year-old female case of intractable adult-onset Still’s disease (AOSD). Initial high-dose prednisolone therapy was ineffective, and macrophage-activation syndrome (MAS) manifested after one session of additional tocilizumab therapy. After successful treatment for MAS with lipo-dexamethasone and cyclosporin, tocilizumab therapy aided in the rapid reduction of the therapeutic steroid dose. Tocilizumab may be useful for maintenance therapy for AOSD, although its efficacy is unclear for the highly active phase of the disease.     

成人斯蒂尔病与巨噬细胞活化综合征关联性的临床分析

目的 探讨成人斯蒂尔病(AOSD)与巨噬细胞活化综合征(MAS)的关系.方法 选择AOSD组为78例资料完整的AOSD;MAS组是从26例有组织学证据的噬血细胞综合征的随访治疗中确定11例为风湿免疫疾病相关的噬血细胞综合征.对以上患者的临床表现和实验室资料进行分析.结果 在AOSD组78例中,有9例(占12%)在使用治疗之前可以诊断为MAS,但无噬血组织学依据.在11例有噬血现象的MAS中,AOSD 6例,脂膜炎2例,系统性红斑狼疮、皮肌炎、系统性血管炎各1例.脾脏肿大、白细胞减低、贫血、血小板下降、高甘油三酯是AOSD出现MAS的相关临床指标.结论 AOSD继发MAS的现象比较常见,严重者可以有组织学的噬血表现.AOSD出现脾脏增大、血细胞降低时,需要作MAS的相关检查,包括骨髓检查以及甘油三酯、纤维蛋白原、自然杀伤(NK)细胞活性等,以便及时诊断MAS.     

An animal model of hemophagocytic lymphohistiocytosis (HLH): CD8+ T cells and interferon gamma are essential for the disorder

Hemophagocytic lymphohistiocytosis (HLH) is a rare disorder with familial and acquired forms. The familial form is associated with mutations in the perforin gene and both forms are associated with severe defects in lymphocyte cytotoxic function. We examined perforin-deficient mice as a model of HLH in order to gain insight into this poorly understood disorder. While these mice do not spontaneously develop HLH-like symptoms, we found that they manifest all of the features of HLH after infection with lymphocytic choriomeningitic virus (LCMV). Following LCMV infection, perforin-deficient mice develop fever, splenomegaly, pancytopenia, hypertriglyceridemia, hypofibrinogenemia, and elevation of multiple serum cytokine levels, and hemophagocytosis is evident in many tissues. Investigation into how this phenotype develops has revealed that CD8+ T cells, but not natural killer (NK) cells, are necessary for the development of this disorder. Cytokine neutralization studies have revealed that interferon gamma (IFNgamma) is uniquely essential as well. Finally, the excessive amount of IFNgamma seen in affected mice appears to be driven by increased antigen presentation to CD8+ T cells. These studies provide insight into the pathophysiology of HLH, and provide new targets for specific therapeutic intervention in this fatal disorder.     

Macrophage activation syndrome and cytokine-directed therapies

Macrophage activation syndrome (MAS) is an episode of overwhelming inflammation that occurs most commonly in children with systemic juvenile idiopathic arthritis (SJIA). It is characterized by expansion and activation of T lymphocytes and hemophagocytic macrophages and bears great similarity to hemophagocytic lymphohistiocytosis (HLH). This disorder has substantial morbidity and mortality, and there is frequently a delay in recognition and initiation of treatment. Here, we will review what is known about the pathogenesis of MAS and, in particular, its similarities to HLH. The development of MAS is characterized by a cytokine storm, with the elaboration of numerous pro-inflammatory cytokines. We will examine the evidence for various cytokines in the initiation and pathogenesis of MAS and discuss how new biologic therapies may alter the risk of MAS. Finally, we will review current treatment options for MAS and examine how cytokine-directed therapy could serve as novel treatment modalities.     

Mutations of the hemophagocytic lymphohistiocytosis-associated gene UNC13D in a patient with systemic juvenile idiopathic arthritis

The clinical syndromes of hemophagocytic lymphohistiocytosis (HLH) and macrophage activation syndrome (MAS) are both characterized by dysregulated inflammation with prolonged fever, hepatosplenomegaly, coagulopathy, hematologic cytopenias, and evidence of hemophagocytosis in the bone marrow or liver. While HLH is either inherited or acquired, children with severe rheumatic diseases, most notably systemic juvenile idiopathic arthritis, are at risk for MAS. The phenotypic similarity between HLH and MAS raises the possibility that they share common pathogenetic mechanisms. Familial forms of HLH have been attributed to mutations in the genes encoding perforin (PRF1) and Munc13-4 (UNC13D), among others, and are characterized by defective cytotoxic lymphocyte function. While some patients with systemic JIA have decreased levels of perforin protein expression and natural killer (NK) cell function, mutations of HLH-associated genes in patients with systemic JIA have not been reported. We report the case of an 8-year-old girl with systemic JIA without MAS who was found to have compound heterozygous mutations of UNC13D and reduced NK cell cytotoxic function. This case broadens the range of clinical phenotypes attributable to UNC13D mutations and offers new insights into the etiology and pathogenesis of systemic JIA.