Defective natural killer cell function in patients with hemophagocytic lymphohistiocytosis and in first degree relatives

Tolerance and autoimmunity: lessons at the bedside of primary immunodeficiencies

The recent progress in the genetic characterization of many primary immunodeficiencies (PIDs) allows for a better understanding of immune molecular and cellular mechanisms. The present chapter discusses associations between PIDs and autoimmune diseases (AIDs) in this new light. PIDs are classified according to the frequency of association with AIDs, defining four groups of conditions: systematic (more than 80% of all patients), strong (10-80%), mild (less than 10%), and absent (no available descriptions). Several general conclusions could be drawn: (1) pathological autoimmune (AI) manifestations are very frequently associated with PIDs, indicating that, contrary to conventional notions, antimicrobial protection and natural tolerance to body tissues share many basic mechanisms; (2) in some gene defects, association is so strong that one could speak of "monogenic" AIDs; (3) basic types of PIDs are selectively associated with AID of a particular set of target tissues; (4) while for some gene defects, current theory satisfactorily explains pathogenesis of the corresponding AID, other situations suggest extensive gaps in the present understanding of natural tolerance; and (5) not exceptionally, observations on the AI phenotype for the same gene defect in mouse and man are not concordant, perhaps owing to the limited genetic diversity of mouse models, often limited to a single mouse strain. Overall, clinical observations on PID support the new paradigm of "dominant" tolerance to self-components, in which AID owes to deficits in immune responses (i.e., in regulatory mechanisms), rather than from excessive reactivity.

Novel spectrum of perforin gene mutations in familial hemophagocytic lymphohistiocytosis in ethnic Omani patients

Familial hemophagocytic lymphohistiocytosis (FHL) is an autosomal recessive immune disorder, characterized by fever, hepatosplenomegaly, pancytopenia, hypertriglyceridemia, hypofibrinogenemia, markedly elevated levels of inflammatory cytokines, and impaired cytotoxic activity of lymphocytes. FHL is often fatal in early infancy. Histologic features include organ infiltration by activated macrophages and lymphocytes. Four genetic loci (FHL1, 2, 3, and 4) have been identified, of which FHL2 involves mutations in the perforin gene and is present in 20-50% of patients with FHL. We herein report the first comprehensive molecular analysis of 16 unrelated cases of FHL in ethnic Omanis. Using direct DNA sequencing analysis in 11 families, seven different mutations were identified in the coding region of the perforin gene, of which five were novel. Perforin gene defects do not seem to be involved in one-third of the cases of FHL in ethnic Omanis.

Hemophagocytic lymphohistiocytosis and related disorders

PURPOSE OF REVIEW

The rate of diagnosis of hemophagocytic lymphohistiocytosis, a genetically heterogeneous and, frequently, rapidly fatal autosomal recessive disorder of immune regulation, is increasing worldwide. Awareness has grown through the Histiocyte Society and the publication of newly-recognized genetic causes. I summarize current knowledge regarding the pathophysiology, diagnosis and treatment of hemophagocytic lymphohistiocytosis.

RECENT FINDINGS

Genetic defects leading to life-threatening hemophagocytic syndromes have recently been described. Two autosomal recessive gene defects underlie 40-50% of primary (familial) cases worldwide: perforin, the major immune cytotoxic protein, and MUNC 13-4, a protein involved in exocytosis of perforin-bearing cytotoxic granules during apoptosis. Related autosomal recessive defects of secretory cytotoxic lysosomes - LYST 1 (Chediak-Higashi syndrome), Rab27A (Griscelli syndrome), and X-linked lymphoproliferative disorder - also carry a very high risk of fatal hemophagocytic lymphohistiocytosis. Concurrently, treatment protocols involving multiagent immunomodulatory therapy followed by allogeneic hematopoeitic cell transplantation have been tested. With immunomodulatory treatment, 75% of children with hemophagocytic lymphohistiocytosis are symptomatically improved after 2 months of therapy. Disease-free survival after allogeneic hematopoeitic cell transplantation currently ranges from 50 to 70%.

SUMMARY

Bench and clinical research have advanced understanding of the pathophysiology of hemophagocytic lymphohistiocytosis and related disorders, and significantly improved clinical outcomes during the past decade.

HLH-2004: Diagnostic and therapeutic guidelines for hemophagocytic lymphohistiocytosis

In HLH-94, the first prospective international treatment study for hemophagocytic lymphohistiocytosis (HLH), diagnosis was based on five criteria (fever, splenomegaly, bicytopenia, hypertriglyceridemia and/or hypofibrinogenemia, and hemophagocytosis). In HLH-2004 three additional criteria are introduced; low/absent NK-cell-activity, hyperferritinemia, and high-soluble interleukin-2-receptor levels. Altogether five of these eight criteria must be fulfilled, unless family history or molecular diagnosis is consistent with HLH. HLH-2004 chemo-immunotherapy includes etoposide, dexamethasone, cyclosporine A upfront and, in selected patients, intrathecal therapy with methotrexate and corticosteroids. Subsequent hematopoietic stem cell transplantation (HSCT) is recommended for patients with familial disease or molecular diagnosis, and patients with severe and persistent, or reactivated, disease. In order to hopefully further improve diagnosis, therapy and biological understanding, participation in HLH studies is encouraged.

Familial and acquired hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening condition of severe hyperinflammation caused by the uncontrolled proliferation of activated lymphocytes and histiocytes secreting high amounts of inflammatory cytokines. Cardinal signs and symptoms are prolonged fever, hepatosplenomegaly and pancytopenia. Characteristic biochemical markers include elevated triglycerides, ferritin and low fibrinogen. HLH occurs on the basis of various inherited or acquired immune deficiencies. Impaired function of natural killer (NK) cells and cytotoxic T-cells (CTL) is shared by all forms of HLH. Genetic HLH occurs in familial forms (FHLH) in which HLH is the primary and only manifestation, and in association with the immune deficiencies Chédiak-Higashi syndrome 1 (CHS 1), Griscelli syndrome 2 (GS 2) and x-linked lymphoproliferative syndrome (XLP), in which HLH is a sporadic event. Most patients with acquired HLH have no known underlying immune deficiency. Both acquired and genetic forms are triggered by infections, mostly viral, or other stimuli. HLH also occurs as a complication of rheumatic diseases (macrophage activation syndrome) and of malignancies. Several genetic defects causing FHLH have recently been discovered and have elucidated the pathophysiology of HLH. The immediate aim of therapy in genetic and acquired HLH is suppression of the severe hyperinflammation, which can be achieved with immunosuppressive/immunomodulatary agents and cytostatic drugs. Patients with genetic forms have to undergo stem cell transplantation to exchange the defective immune system with normally functioning immune effector cells. In conclusion, awareness of the clinical symptoms and of the diagnostic criteria of HLH is crucial in order not to overlook HLH and to start life-saving therapy in time.

Potential therapeutic role of natural killer cells in cancer

Cancer immunotherapy is a growing field that aims at restoring and enhancing immune function to combat oncogenic conditions. One target of this field is natural killer (NK) cells. Part of innate immunity, NK cells are able to kill tumor cells without previous priming. Results from stem cell transplants containing alloreactive donor NK cells and in vitro work have evidenced a great antitumor potential. In addition, NK cells are likely to interact with dendritic cells, potent antigen-presenting cells, thus forming a bridge between innate and adaptive immunity. This review aims to provide an overview of NK cells with particular emphasis on properties that can and are being targeted in order to potentiate the antitumor activity of these cells.

The diagnostic significance of soluble CD163 and soluble interleukin-2 receptor α-chain in macrophage activation syndrome and untreated new-onset systemic juvenile idiopathic arthritis

OBJECTIVE

Macrophage activation syndrome is characterized by an overwhelming inflammatory reaction driven by excessive expansion of T cells and hemophagocytic macrophages. Levels of soluble interleukin-2 receptor alpha (sIL-2Ralpha) and soluble CD163 (sCD163) may reflect the degree of activation and expansion of T cells and macrophages, respectively. This study was undertaken to assess the value of serum sIL-2Ralpha and sCD163 in diagnosing acute macrophage activation syndrome complicating systemic juvenile idiopathic arthritis (JIA).

METHODS

Enzyme-linked immunosorbent assay was used to assess sIL-2Ralpha and sCD163 levels in sera from 7 patients with acute macrophage activation syndrome complicating systemic JIA and 16 patients with untreated new-onset systemic JIA. The results were correlated with clinical features of established macrophage activation syndrome, including ferritin levels.

RESULTS

The median level of sIL-2Ralpha in the patients with macrophage activation syndrome was 19,646 pg/ml (interquartile range [IQR] 18,128), compared with 3,787 pg/ml (IQR 3,762) in patients with systemic JIA (P = 0.003). Similarly, the median level of sCD163 in patients with macrophage activation syndrome was 23,000 ng/ml (IQR 14,191), compared with 5,480 ng/ml (IQR 2,635) in patients with systemic JIA (P = 0.017). In 5 of 16 patients with systemic JIA, serum levels of sIL-2Ralpha or sCD163 were comparable with those in patients with acute macrophage activation syndrome. These patients had high inflammatory activity associated with a trend toward lower hemoglobin levels (P = 0.11), lower platelet counts, and significantly higher ferritin levels (P = 0.02). Two of these 5 patients developed overt macrophage activation syndrome several months later.

CONCLUSION

Levels of sIL-2Ralpha and sCD163 are promising diagnostic markers for macrophage activation syndrome. They may also help identify patients with subclinical macrophage activation syndrome.

Prolonged course of familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis is usually diagnosed in the first 2 years of life and, if untreated, is rapidly fatal. We describe a 10-year-old boy with a 9-year history of prolonged fever and progressive hepatosplenomegaly who was diagnosed as having hemophagocytic lymphohistiocytosis 2, being homozygote to a previously described mutation in the PRF1 gene, and cured by the HLH-2004 protocol and allogenic bone marrow transplantation. This unique case emphasizes the heterogeneity of this disease and the diversity of its clinical presentations.

The hemophagocytic syndrome: titrating continuous hemofiltration to the degree of lactic acidosis

In 3 cases of severe multiple organ failure due to hemophagocytic lymphohistiocytosis (HLH) in children, the authors demonstrate the utility of continuous hemofiltration in attenuating the consequences of excess cytokine activity, with therapy titrated to the degree of lactic acidosis. HLH was diagnosed in 3 encephalopathic children with multiple organ failure, elevated ferritin (49,396-237,582 pmol/L; or 21,983-105,733 ng/mL), elevated serum triglyceride, and depressed cell lines. One had a known malignancy, one had EBV-associated lymphoproliferative disease, and one was previously healthy. Continuous hemofiltration was initiated, with the ultrafiltrate production rate and countercurrent dialysate flow titrated to metabolic acidosis as reflected by the serum lactate (maximum 3.5 mmol/L or 31.6 mg/dL). Hemofiltration was titrated upward until lactic acidosis resolved, through clearance of lactate or interruption of excess cytokine-driven activity; maximum prescription was 2000 mL/h ultrafiltrate production plus 2500 mL/h dialysate flow. Stability was achieved with hemofiltration, then substantial resolution occurred with treatment according to the HLH-94 protocol (dexamethasone, cyclosporin, VP-16, intrathecal methotrexate). One child succumbed to candidiasis. Another made a full recovery. A third succumbed to his primary malignancy. HLH should be suspected in unexplained or unresolving multiple organ failure. Titration of hemofiltration based on measurable parameters of cellular metabolism (e.g., lactate, base deficit) may stabilize the child with metabolic acidosis long enough to allow proper diagnosis and institution of definitive therapy. Hemofiltration is not a panacea but rather a stabilizing mechanism, with poorly understood effects on interstitial water and solute flux, that facilitates recovery over weeks, not days.

Fulminant neonatal liver failure in siblings: probable congenital hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (HLH) is an autosomal recessive disorder of immune regulation characterized by fever, splenomegaly, cytopenia, hypertriglyceridemia, hypofibrinogenemia, and hyperferritinemia. Although presentation usually occurs during the first 2 years of life, congenital presentation is rare. We report siblings with a presumptive diagnosis of familial HLH who presented with hydrops fetalis and severe hepatic involvement ultimately resulting in their deaths. This report emphasizes the difficulty of confirming the diagnosis of HLH. However, establishing the diagnosis has important implications for genetic counseling and family planning. HLH should be considered in the setting of perinatal liver failure. The immunologic basis of the disease is incompletely understood but testing for natural killer cell function, and perforin defects may be helpful in establishing a diagnosis. HLH can be treated with chemotherapy, immunotherapy, and stem cell transplantation.

Correlation between phenotypic heterogeneity and gene mutational characteristics in familial hemophagocytic lymphohistiocytosis (FHL)

BACKGROUND

Classification of familial hemophagocytic lymphohistiocytosis (FHL) into FHL2, FHL3, and other subtypes based on genetic abnormalities has recently become possible. We studied the phenotypic differences among these subtypes in Japan.

METHODS

Forty patients clinically diagnosed with FHL were analyzed. Perforin abnormality was screened by flow cytometric analysis and/or DNA sequencing in these patients, and those without perforin abnormalities were further examined for the presence of mutations in the Munc13-4 gene by DNA sequencing. The correlation between clinical features and genetic subtypes was investigated.

RESULTS

Of the 40 HLH patients, 11 showed perforin gene mutations (classified as FHL2) and ten had Munc13-4 gene mutations (FHL3), but neither mutation was noted in 19 patients (non-FHL2/3). Although the majority of the patients developed the disease before the age of 1 year, the onset in three FHL2 patients with missense mutations was late (7, 11, and 12 years). Incidence of deficient natural killer cell activity was higher in FHL2 patients (9/9 FHL2, 4/9 FHL3, and 6/17 non-FHL2/3; P = 0.005). The serum levels of ferritin and soluble interleukin-2 receptor were significantly higher in FHL2 patients with nonsense perforin mutations compared to other subgroups (P < or = 0.05). Epstein-Barr virus infection was involved in 8 of the 40 HLH patients: one FHL2, one FHL3, and six non-FHL2/3.

CONCLUSIONS

Although clinical features of FHL3 appear to be homogeneous, the heterogeneous clinical features of FHL2 depend upon the nature of perforin gene mutations. Characterization of the non-FHL2/3 group with regard to FHL1 or other novel gene mutations remains to be conducted.

Chronic fatigue syndrome is associated with diminished intracellular perforin

Chronic fatigue syndrome (CFS) is an illness characterized by unexplained and prolonged fatigue that is often accompanied by abnormalities of immune, endocrine and cognitive functions. Diminished natural killer cell cytotoxicity (NKCC) is a frequently reported finding. However, the molecular basis of this defect of in vitro cytotoxicy has not been described. Perforin is a protein found within intracellular granules of NK and cytotoxic T cells and is a key factor in the lytic processes mediated by these cells. Quantitative fluorescence flow cytometry was used to the intracellular perforin content in CFS subjects and healthy controls. A significant reduction in the NK cell associated perforin levels in samples from CFS patients, compared to healthy controls, was observed. There was also an indication of a reduced perforin level within the cytotoxic T cells of CFS subjects, providing the first evidence, to our knowledge, to suggest a T cell associated cytotoxic deficit in CFS. Because perforin is important in immune surveillance and homeostasis of the immune system, its deficiency may prove to be an important factor in the pathogenesis of CFS and its analysis may prove useful as a biomarker in the study of CFS.

Hemophagocytic lymphohistiocytosis: diagnosis, pathophysiology, treatment, and future perspectives

Hemophagocytic lymphohistiocytosis (HLH) is a rare life-threatening disease in which the immune system becomes overactive due to its inability to effectively respond to infections and/or shut down the immune response to such infections. The discovery of genetic defects in the secretory pathway of natural killer (NK) cells and cytotoxic T cells in some patients with this disease has raised important questions of the role of cytotoxic cells in the control of infections and in immune regulation. This review will give a brief overview of the clinical presentation and accepted treatment of HLH. Furthermore, it will give an in-depth review into the known genetic defects and current knowledge of the pathophysiology of this disorder, and will highlight recent evidence suggesting that cytotoxic defects in CD4+ T regulatory cells may contribute to the pathogenesis of HLH.

Transient familial haemophagocytic lymphohistiocytosis reactivation post-CD34 haematopoietic stem cell transplantation

Familial haemophagocytic lymphohistiocytosis (FHLH) is a genetic disorder caused by defective lymphocyte cytotoxicity, resulting in impaired lymphocyte homeostasis and macrophage infiltration of solid tissues and bone marrow, with extensive haemophagocytosis. It is invariably fatal unless treated by allogeneic haematopoietic stem cell transplantation (HSCT). In a retrospective analysis of 11 cases of FHLH, transplanted in one centre between January 1999 and December 2003, it was found that host T cell expansion occurred early after HSCT in a setting of a viral infection (cytomegalovirus and Epstein-Barr virus respectively) in two cases who received T cell-depleted HSCT. Transient recurrence of clinical and biological manifestations of FHLH was observed, despite evidence for donor cell engraftment. Secondary development of donor T cells led to stable mixed chimaerism and sustained remission of FHLH. Detection of host-derived T cells soon after HSCT in a patient with FHLH should thus not mistakenly be taken as a manifestation of graft rejection.

Subtyping of natural killer cell cytotoxicity deficiencies in haemophagocytic lymphohistocytosis provides therapeutic guidance

The familial form of haemophagocytic lymphohistiocytosis (HLH) is a fatal disease, with allogeneic stem cell transplantation (SCT) being the only curative treatment. In contrast, patients with secondary (infection-associated) HLH usually do not require SCT. Since it often is difficult to distinguish primary and secondary HLH, we wanted to identify a tool that provides guidance on whether SCT is required. The clinical outcome of 65 HLH patients was analysed in relation to the recently reported four types of defects in natural killer (NK)-cell cytotoxicity in HLH. None (0%) of the 36 patients with NK-cell deficiency type 3 attained a sustained (1-year) remission after stopping therapy without receiving SCT, in contrast to 45% (13/29) non-type 3 patients (P < 0.001). Most type 3 patients (22/36) underwent SCT (14/22, 64% are alive), whereas 11 of 14 that did not receive SCT died, and the three others had received HLH-therapy during the last year of follow-up. Of 54 patients analysed for perforin expression and/or mutation, the five with perforin deficiency were all type 3 patients. The data suggests that HLH patients with NK-cell deficiency type 3 will probably require SCT to survive. Thus, NK-cell deficiency classification may provide valuable guidance in judging whether an HLH-patient needs SCT.

Does systemic-onset juvenile idiopathic arthritis belong under juvenile idiopathic arthritis?

'Science is the systematic classification of experience' George Henry Lewes (1817-78), English philosopher, critic, dramatist, scientist. Juvenile idiopathic arthritis (JIA) is prevalent in about 1 in 1000 children. The earliest formal description of this disease was by Sir George Frederick Still in 1897. This work was done when he was a registrar at the Hospital for Sick Children, Great Ormond Street, London. In this initial description of 19 patients he identified three patterns of arthritis, one of which came to be known later as Still's disease [now known as systemic-onset juvenile idiopathic arthritis (SoJIA)]. Over the next few decades it came to be appreciated that one form of arthritis in children is very different and dominated by the presence of systemic manifestations. Over the last two decades several paediatric rheumatologists have come together to classify juvenile arthritis for purposes of better disease identification and research. All along, the systemic form of juvenile arthritis was always recognized as belonging to a distinct group; in fact for several decades (and even now in some countries) the systemic form of juvenile arthritis was referred to as Still's disease. In this article we will attempt to highlight the reasons why we feel that SoJIA is perhaps not best retained in the company of JIA.

Perforin and lymphohistiocytic proliferative disorders

Perforin is critical for cytotoxicity mediated by granules present in natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). Perforin-deficient mice have impaired cytotoxicity by NK cells and CTLs, resulting in failure to control infections with certain viruses or bacteria. Infection of perforin-deficient mice with lymphocytic choriomeningitis virus results in haemophagocytic lymphohistiocytosis and elevated levels of pro-inflammatory cytokines. Mutations throughout the perforin gene have been identified in patients with familial haemophagocytic lymphohistiocytosis (FHL) type 2. These patients present with fever, hepatosplenomegaly, pancytopenia, have marked elevations of T-helper type 1 and type 2 cytokines, and have impaired NK cell and CTL cytotoxicity. A number of infectious pathogens have been implicated as triggering the onset of disease. Identification of mutations in perforin as the cause of FHL should allow prenatal diagnosis of the disorder. While stem cell transplantation is curative, gene therapy might be effective in the future.

Natural killer cell dysfunction is a distinguishing feature of systemic onset juvenile rheumatoid arthritis and macrophage activation syndrome

Macrophage activation syndrome (MAS) has been reported in association with many rheumatic diseases, most commonly in systemic juvenile rheumatoid arthritis (sJRA). Clinically, MAS is similar to hemophagocytic lymphohistiocytosis (HLH), a genetic disorder with absent or depressed natural killer (NK) function. We have previously reported that, as in HLH, patients with MAS have profoundly decreased NK activity, suggesting that this abnormality might be relevant to the pathogenesis of the syndrome. Here we examined the extent of NK dysfunction across the spectrum of diseases that comprise juvenile rheumatoid arthritis (JRA). Peripheral blood mononuclear cells (PBMC) were collected from patients with pauciarticular (n = 4), polyarticular (n = 16), and systemic (n = 20) forms of JRA. NK cytolytic activity was measured after co-incubation of PBMC with the NK-sensitive K562 cell line. NK cells (CD56⁺/T cell receptor [TCR]-αβ^-), NK T cells (CD56⁺/TCR-αβ⁺), and CD8⁺ T cells were also assessed for perforin and granzyme B expression by flow cytometry. Overall, NK cytolytic activity was significantly lower in patients with sJRA than in other JRA patients and controls. In a subgroup of patients with predominantly sJRA, NK cell activity was profoundly decreased: in 10 of 20 patients with sJRA and in only 1 of 20 patients with other JRA, levels of NK activity were below two standard deviations of pediatric controls (P = 0.002). Some decrease in perforin expression in NK cells and cytotoxic T lymphocytes was seen in patients within each of the JRA groups with no statistically significant differences. There was a profound decrease in the proportion of circulating CD56^bright NK cells in three sJRA patients, a pattern similar to that previously observed in MAS and HLH. In conclusion, a subgroup of patients with JRA who have not yet had an episode of MAS showed decreased NK function and an absence of circulating CD56^bright population, similar to the abnormalities observed in patients with MAS and HLH. This phenomenon was particularly common in the systemic form of JRA, a clinical entity strongly associated with MAS.

Pathogenesis of hemophagocytic syndrome (HPS)

Hemophagocytic syndrome (HPS) is a clinicopathologic entity characterized by increased proliferation and activation of benign macrophages with hemophagocytosis throughout the reticuloendothelial system. Uncontrolled T-lymphocyte activation is responsible for increased T(H)1 cytokines secretion such as IFN-gamma, IL-12 and IL-18 that promotes macrophage activation. Genetic defects specific for cytotoxic T lymphocytes (CTL) and natural killer (NK) cells have been identified in patients with primary HPS that are responsible for altered cell death and apoptosis induction or target killing. HPS may be secondary to malignancy, infection or autoimmune disease, and mechanisms involved are poorly understood. However, in adult-onset Still's disease, juvenile chronic arthritis and probably systemic lupus erythematosus, IL-18 might play a role in initiating macrophage activation.

Hemophagocytic lymphohistiocytosis in children: abdominal US findings within 1 week of presentation

PURPOSE

To evaluate the ultrasonographic (US) findings at presentation in a series of children who received a diagnosis of hemophagocytic lymphohistiocytosis (HLH) at a single institution.

MATERIALS AND METHODS

All available results of imaging studies of children who received a diagnosis of HLH between January 1985 and June 2000 were retrieved. For nine patients, abdominal US images obtained within 1 week of presentation to the hospital were reviewed retrospectively by two radiologists who were aware of the diagnosis and of the original interpretation of each study. US images were examined for evidence of splenomegaly, hepatomegaly, ascites, gallbladder wall thickening, increased periportal echogenicity, lymphadenopathy, pleural effusion, and nephromegaly. Any other abnormalities were also recorded. Differences in interpretation were resolved by consensus. The patients ranged in age from 2 months to 4(1/2) years. The male-to-female ratio was 5:4.

RESULTS

Findings at presentation included splenomegaly (in eight of the nine children), hepatomegaly (in seven children), ascites (in six children), gallbladder wall thickening (in six children), increased periportal echogenicity (in three children), lymphadenopathy (in three children), and pleural effusion (in two children). Miscellaneous findings in individual patients included coarse hepatic echotexture with a single 9-mm hypoechoic focus in the liver, multiple hypoechoic foci in the spleen, nephromegaly, gallstone, increased renal cortical echogenicity, and mural thickening of the duodenum.

CONCLUSION

In the appropriate clinical setting, the differential diagnosis of a combination of hepatosplenomegaly, ascites, gallbladder wall thickening, increased periportal echogenicity, lymphadenopathy, and/or pleural effusion should include HLH.

Atypical features of familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (FHLH) is a rare, rapidly progressive disorder of early childhood characterized by uncontrolled activation of T cells and macrophages. Although perforin gene mutations have been described in a proportion of patients with FHLH, the genotype/phenotype correlation is still limited. Only a few patients with late onset clinical manifestations have been reported. The biochemical and immunologic alterations in the asymptomatic phase are not well known. We report on a family in which 2 fraternal twins both homozygous for a perforin mutation previously described as causative of the disease, markedly differed in phenotypic expression of FHLH. The twins also had a second novel heterozygous mutation. Natural killer (NK) activity was severely impaired in the patient and was normal in the asymptomatic fraternal twin. Our report highlights that FHLH may present after a long disease-free interval during which biochemical or immunologic alterations may be not evident, thus implying a role for interfering factors.

Macrophage activation syndrome and reactive hemophagocytic lymphohistiocytosis: the same entities?

PURPOSE OF THE REVIEW

One of the most perplexing features of systemic-onset juvenile rheumatoid arthritis is the association with macrophage activation syndrome, a life-threatening complication caused by excessive activation and proliferation of T cells and macrophages. The main purpose of the review is to summarize current understanding of the relation between macrophage activation syndrome and other clinically similar hemophagocytic disorders.

RECENT FINDINGS

Clinically, macrophage activation syndrome has strong similarities with familial and virus-associated reactive hemophagocytic lymphohistiocytosis. The better understood familial hemophagocytic lymphohistiocytosis is a constellation of rare, autosomal recessive immune disorders. The most consistent immunologic abnormalities in patients with familial hemophagocytic lymphohistiocytosis are decreased natural killer and cytotoxic cell functions. In approximately one third of familial hemophagocytic lymphohistiocytosis patients, these immunologic abnormalities are secondary to mutations in the gene encoding perforin, a protein that mediates cytotoxic activity of natural killer and cytotoxic CD8+ T cells. Several recent studies have suggested that profoundly depressed natural killer cell activity and abnormal levels of perforin expression may be a feature of macrophage activation syndrome in systemic-onset juvenile rheumatoid arthritis as well. Although it has been proposed that in both hemophagocytic lymphohistiocytosis and macrophage activation syndrome, natural killer and cytotoxic cell dysfunction may lead to inadequate control of cellular immune responses, the exact nature of such dysregulation and the relation between macrophage activation syndrome and hemophagocytic lymphohistiocytosis still remain to be determined.

Infection-associated hemophagocytic syndrome due to Pseudomonas aeruginosa in preterm infants

Infection-associated hemophagocytic syndrome due to bacteria is rare and its appearance in preterm babies is uncommon. The signs, symptoms, laboratory values, and clinical status of three preterm babies with infection-associated hemophagocytic syndrome due to Pseudomonas aeruginosa are described and differences between preterm and infant cases are discussed in this report.

Characteristic perforin gene mutations of haemophagocytic lymphohistiocytosis patients in Japan

Perforin gene (PRF1) mutations appear to occur in about 30% of patients with haemophagocytic lymphohistiocytosis (HLH). We tested perforin expression and gene mutations in 14 HLH patients and six patients with Epstein-Barr virus-associated HLH (EBV-HLH) in Japan. Five of the 14 HLH patients had perforin abnormalities. The presence of PRF1 genetic abnormality correlated well with the lack of perforin expression as determined by flow cytometry. Sequencing showed that four patients had a compound heterozygous mutation while the fifth patient had a homozygous mutation. Three of the mutations we detected were novel. In contrast, none of the six EBV-HLH patients showed perforin abnormalities. Our data, combined with the PRF1 mutations in three previously reported Japanese patients, suggest that the 1090-1091delCT and 207delC mutations of the perforin gene are frequently present in Japanese HLH patients (62.5% and 37.5% respectively). Examination of the geographical origins of the ancestors in the perforin-mutant HLH patients revealed that they mostly came from the Western part of Japan, suggesting that the present-day cases may largely derive from a common ancestor.

Natural killer cell dysfunction in patients with systemic-onset juvenile rheumatoid arthritis and macrophage activation syndrome

OBJECTIVES

To assess natural killer (NK) and cytotoxic functions in patients with systemic-onset juvenile rheumatoid arthrithis (soJRA) complicated by macrophage activation syndrome (MAS).

METHODS

NK cells (CD56+/TCRalphabeta-), NK T cells (CD56+/TCRalphabeta+) and CD8+ cells were assessed for perforin expression by flow cytometry. NK cytotoxic activity was measured after coincubation of mononuclear cells with an NK-sensitive K562 cell line.

RESULTS

Two major patterns of immunologic abnormalities were detected. Four of 7 patients had decreased NK activity, low NK cell numbers, and mildly increased levels of perforin expression in CD8+ and CD56+ cytotoxic cells. Three remaining patients with MAS, however, had decreased NK activity associated with low levels of perforin expression in all cytotoxic cell populations, a pattern indistinguishable from that in carriers of perforin-deficient familial hemophagocytic lymphohistiocytosis. Remarkably, two of these patients had previous episodes of MAS.

CONCLUSIONS

NK dysfunction is an immunologic abnormality common to both familial hemophagocytic lymphohistiocytosis and MAS of soJRA. The extent of NK cell abnormalities in soJRA needs to be further investigated.

Hemophagocytic syndrome in children: an important diagnostic consideration in fever of unknown origin

To study the evolution of hemophagocytic syndrome (HPS) in children, we performed a retrospective review of 19 patients (median age, 17.4 months) in whom an infectious diseases consultation was requested at Texas Children's Hospital during the period of September 1991 through September 2001. Clinical findings consistent with HPS most frequently presented during days 6-14 of illness, concomitant with laboratory abnormalities. Fever was present for a median of 19 days before the diagnosis of HPS. Elevated serum lactate dehydrogenase and ferritin levels were noted in all patients. An infectious agent was identified in 42% of patients; 16% were found to have immunologic or vasculitic disease. HPS is a rare but often fatal disease that can initially present as fever of unknown origin with varying clinical findings, and it can be recognized by physicians who are familiar with the evolution of HPS. It is likely that many of these cases remain undiagnosed because of the HPS's rapidly fatal course.

Clinical features and treatment strategies of Epstein-Barr virus-associated hemophagocytic lymphohistiocytosis

Epstein-Barr virus (EBV) is the major triggering factor producing hemophagocytic syndrome or hemophagocytic lymphohistiocytosis (HLH). In this review, diagnostic problems, clinical and histopathological features, and treatment strategies of EBV-HLH have been described. In patients with EBV-HLH, the EBV-infected T cells or natural killer (NK) cells are mostly mono- or oligo-clonally proliferating, where hypercytokinemia plays a major role and causes hemophagocytosis, cellular damage and dysfunction of various organs. Although the majority of EBV-HLH cases develop in apparently immunocompetent children and adolescents, it also occurs in association with infectious mononucleosis, chronic active EBV infection, familial HLH, X-linked lymphoproliferative disease, lymphoproliferative disease like peripheral T-cell lymphoma and NK cell leukemia. In terms of treatment, special therapeutic measures are required to control the cytokine storm generated by EBV and to suppress proliferating EBV-genome-containing cells, because the clinical courses are often fulminant and result in a poor outcome.

Human natural killer cell deficiencies and susceptibility to infection

There are a surprisingly large number of human natural killer (NK) cell deficiency states that provide insight into the role of NK cells in defense against human infectious disease. Many disorders associated with NK cell defects are caused by single gene mutations and, thus, give additional understanding concerning the function of specific molecules in NK cell development and activities. A resounding theme of NK cell deficiencies is susceptibility to herpesviruses, suggesting that unexplained severe herpesviral infection should raise the possibility of an NK cell deficit.

Hemophagocytic lymphohistiocytosis is associated with deficiencies of cellular cytolysis but normal expression of transcripts relevant to killer-cell-induced apoptosis

In 65 patients with hemophagocytic lymphohistiocytosis (HLH), we found an as yet undescribed heterogeneity of defects in cellular cytotoxicity when assay conditions were modified by the incubation time, the presence of mitogen, or interleukin-2 (IL-2). The standard 4-hour natural killer (NK) test against K562 targets was negative in all patients. In patients deficient in type 1 (n = 21), type 2 (n = 5), and type 4 (n = 8) HLH, negative NK function could be reconstituted by mitogen, by IL-2, or by prolongation of the incubation time (16 hours), respectively. Most patients (n = 31) displayed the type 3 defect, defined by a lack of any cellular cytotoxicity independent of assay variations. The characteristic hypercytokinemia also concerned counterregulatory cytokines, such as proinflammatory interferon-gamma (IFN-gamma), simultaneously elevated with suppressive IL-10 in 38% of types 1-, 2-, and 4-deficient patients and in 71% of type 3-deficient patients. Elevated IFN-gamma alone correlated with high liver enzymes, but sCD95-ligand and sCD25 did not-though these markers were expected to indicate the extent of histiocytic organ infiltration. Outcome analysis revealed more deaths in patients with type 3 deficiency (P =.017). Molecular defects were associated with homozygously mutated perforin only in 4 patients, but other type 3 patients expressed normal transcripts of effector molecules for target-cell apoptosis, including perforin and granzyme family members, as demonstrated by RNase protection analysis. Thus, target-cell recognition or differentiation defects are likely to explain this severe phenotype in HLH. Hyperactive phagocytes combined with NK defects may imply defects on the level of the antigen-presenting cell.

Low natural killer activity and central nervous system disease as a high-risk prognostic indicator in young patients with hemophagocytic lymphohistiocytosis

BACKGROUND

Familial hemophagocytic lymphohistiocytosis HLH (FHL) is fatal, unless patients are rescued with hematopoietic stem cell transplantation (SCT). Although the molecular identification of FHL now is possible at least in part from perforin gene study, many cases escape detection or never are tested due to the lack of specific hallmarks, making diagnosis difficult. To the authors' knowledge, it remains to be determined whether persistently low natural killer cell (NK) activity and a high incidence of central nervous system (CNS) disease increase the probability of FHL.

METHODS

The authors analyzed 42 HLH patients age < 2 years, 13 of whom developed overt CNS disease and 5 of whom demonstrated persistently deficient NK activity (Group 1). The remaining 24 patients had no CNS disease and had NK activity of moderate decrease to within the normal range (Group 2).

RESULTS

In Group 1, CNS symptoms were detected in 6 cases within 1 month and between 4.5-9 months in 6 other patients. In these cases, spotty lesions demonstrating a high T2 signal in the white matter were noted on brain magnetic resonance imaging. The survival was significantly poor for patients in Group 1 unless they were rescued with SCT, which was performed in 5 of the 13 patients with CNS disease and in all 5 patients with persistent NK activity deficiency. SCT was successful in 9 patients, with no CNS sequelae reported after the transplantation. Conversely, the prognosis of the 24 patients in Group 2 was better and only 1 patient required SCT.

CONCLUSIONS

Very young HLH patients (age < 2 years) who are at high risk of fatal FHL with persistently deficient NK activity and/or overt CNS disease require appropriate SCT to reverse CNS disease and achieve a complete cure.

Functional consequences of perforin gene mutations in 22 patients with familial haemophagocytic lymphohistiocytosis

Familial haemophagocytic lymphohistiocytosis (FHL), an inherited form of haemophagocytic lymphohistiocytosis (HLH) syndrome, is characterized by the overwhelming activation of T lymphocytes and macrophages invariably leading to death in the absence of treatment. FHL is a heterogeneous autosomal recessive disorder, with one known causative gene which codes for perforin, a cytotoxic effector protein. In this study, we have characterized the genotype and phenotype of 14 unrelated families with perforin deficiency. Four new missense mutations of the perforin gene were identified. In every case, perforin gene mutations led to undetectable intracellular perforin expression within cytotoxic cells, while some residual T-cell cytotoxic activity could be associated with certain missense mutations. Clinical and biological analyses did not differentiate between patients with nonsense or missense mutations, although age at diagnosis, which tended to be similar within members of the same family, was delayed in patients from two families belonging to the second group. In one case, consequences of perforin deficiency, diagnosed at birth, could be assessed prior to onset of clinical manifestations. No evidence for T-cell activation could be shown, suggesting that an exogenous event is required to trigger the disease manifestation. Control assessment of perforin expression and cytotoxic assays by lymphocytes from young children led to the conclusion that perforin content of natural killer cells could be a reliable diagnostic test at any age. Altogether, these data enabled a better characterization of perforin deficiency and its consequences, and defined reliable diagnostic tools.

Hemophagocytic lymphohistiocytosis presenting with thrombocytopenia in the newborn

Hemophagocytic lymphohistiocytosis (HLH) may present with thrombocytopenia during the newborn period. Three neonates (one term and two preterm) presented during the newborn period with thrombocytopenia. Transient recovery occurred in two newborns. The diagnosis of HLH was made after the recurrence of thrombocytopenia and the clinical symptoms at 5 and 7 weeks. The third infant was a premature baby diagnosed at 8 days of age after manifesting the clinical and laboratory features of HLH. All three neonates were treated with chemotherapy and responded well. After hematologic and clinical remission was achieved, the two newborns received hematopoietic stem cell transplantation from allogeneic donors. The third neonate is currently receiving chemotherapy. Persistent or recurrent thrombocytopenia of undetermined cause during the neonatal period should raise the suspicion of HLH, even though other symptoms or signs are not yet evident.

Perforin expression in cytotoxic lymphocytes from patients with hemophagocytic lymphohistiocytosis and their family members

Mutations in the perforin gene have been described in some patients with hemophagocytic lymphohistiocytosis (HLH), but the role of perforin defects in the pathogenesis of HLH remains unclear. Four-color flow cytometric analysis was used to establish normal patterns of perforin expression for control subjects of all ages, and patterns of perforin staining in cytotoxic lymphocytes (natural killer [NK] cells, CD8(+) T cells, CD56(+) T cells) from patients with HLH and their family members were studied. Eleven unrelated HLH patients and 19 family members were analyzed prospectively. Four of the 7 patients with primary HLH showed lack of intracellular perforin in all cytotoxic cell types. All 4 patients showed mutations in the perforin gene. Their parents, obligate carriers of perforin mutations, had abnormal perforin-staining patterns. Analysis of cytotoxic cells from the other 3 patients with primary HLH and remaining family members had normal percentages of perforin-positive cytotoxic cells. On the other hand, the 4 patients with Epstein-Barr virus-associated HLH typically had depressed numbers of NK cells but markedly increased proportions of CD8(+) T cells with perforin expression. Four-color flow cytometry provides diagnostic information that, in conjunction with evidence of reduced NK function, may speed the identification of life-threatening HLH in some families and direct further genetic studies of the syndrome.

Recent developments in the management of haemophagocytic lymphohistiocytosis

Over the past two decades, the underlying pathophysiology of haemophagocytic lymphohistiocytosis (HLH) (synonyms: haemophagocytic syndrome, macrophage activation syndrome) has been well recognised. Cytokine storm plays a major role, which derives from an inappropriate immune reaction caused by proliferating and activated T-cell or natural killer (NK) cells associated with macrophage activation and inadequate apoptosis of immunogenic cells. Many biological parameters reflecting activity of disease or response to treatment have been identified, in particular, serum ferritin has been confirmed to be one of the markers for HLH. The common types of HLH consist of non-hereditary (acquired) infection-associated disease such as Epstein-Barr virus (EBV)-haemophagocytic lymphohistiocytosis (HLH) and hereditary (familial) disease such as FHL, in which, at the molecular level, dysfunctional perforin was clarified. Regarding the therapeutic strategies, prompt differential diagnosis of underlying disease is essential and choice of treatment should be based on the risk (low or high) of prognosis, where either cyclosporin A, steroids or iv. immunoglobulin (IVIG) may be indicated as initial treatment for low-risk patients, with etoposide-containing regimens for high-risk patients. Significant improvement of prognosis has been obtained by incorporating intensive supportive care at the disease onset and prompt introduction of immunosuppressants to control cytokine storm. Subsequent immunochemotherapy and haemopoietic stem cell transplantation have contributed significantly to further improve survival of hereditary and refractory HLH patients.

Mutations in apoptosis genes: a pathogenetic factor for human disease

Cell death by apoptosis is exerted by the coordinated action of many different gene products. Mutations in some of them, acting at different levels in the apoptosis process, have been identified as cause or contributing factor for human diseases. Defects in the transmembrane tumor necrosis factor receptor 1 (TNF-R1) lead to the development of familial periodic fever syndromes. Mutations in the homologous receptor Fas (also named CD95; Apo-1) are observed in malignant lymphomas, solid tumors and the autoimmune lymphoproliferative syndrome type I (ALPS I). A mutation in the ligand for Fas (Fas ligand; CD95 ligand, Apo-1 ligand), which induces apoptosis upon binding to Fas, was described in a patient with systemic lupus erythematodes and lymphadenopathy. Perforin, an other cytotoxic protein employed by T- and NK-cells for target cell killing, is mutated in chromosome 10 linked cases of familial hemophagocytic lymphohistiocytosis. Caspase 10, a representative of the caspase family of proteases, which plays a central role in the execution of apoptosis, is defect in autoimmune lymphoproliferative syndrome type II (ALPS II). The intracellular pro-apoptotic molecule bcl-10 is frequently mutated in mucosa-associated lymphoid tissue (MALT) lymphomas and various non-hematologic malignancies. The p53, an executioner of DNA damage triggered apoptosis, and Bax, a pro-apoptotic molecule with the ability to perturb mitochondrial membrane integrity, are frequently mutated in malignant neoplasms. Anti-apoptotic proteins like bcl-2, cellular-inhibitor of apoptosis protein 2 (c-IAP2) and neuronal apoptosis inhibitory protein 1 (NAIP1) are often altered in follicular lymphomas, MALT lymphomas and spinal muscular atrophy (SMA), respectively. This article reviews the current knowledge on mutations of apoptosis genes involved in the pathogenesis of human diseases and summarises the gradual transformation of discoveries in apoptosis research into benefits for the clinical management of diseases.

Familial hemophagocytic lymphohistiocytosis: too little cell death can seriously damage your health

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare and fatal disease of early childhood characterized by a non-malignant accumulation of activated T lymphocytes and histiocytes in the reticuloendothelial system. Moreover, immune system derangement, with prominent hypercytokinemia and low or absent cytotoxic T and natural killer (NK) cell activity, is a consistent feature of this autosomal recessive disorder. Recent work has demonstrated that the degree of spontaneous caspase activation in FHL lymphocytes is attenuated in vitro whereas Fas-mediated caspase activation and apoptosis induction remains unmitigated, and FHL can thus be distinguished from the related chronic disorder of immune regulation termed autoimmune lymphoproliferative syndrome or ALPS. However, subsequent studies have identified mutations in the gene encoding perforin, a cytotoxic granule constituent required for apoptotic killing of target cells, in a number of FHL patients. Hence, the underlying defect in FHL may be conceived of as a lack of apoptosis triggering within the immune system, rather than apoptosis resistance per se. These observations represent an important step in our understanding of the pathogenesis of FHL and also serve to emphasize the pivotal role of cellular (perforin-based) cytotoxicity in the regulation of immune homeostasis.

Spectrum of perforin gene mutations in familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (FHL) is an autosomal recessive disease of early childhood characterized by nonmalignant accumulation and multivisceral infiltration of activated T lymphocytes and histiocytes (macrophages). Cytotoxic T and natural killer (NK) cell activity is markedly reduced or absent in these patients, and mutations in a lytic granule constituent, perforin, were recently identified in a number of FHL individuals. Here, we report a comprehensive survey of 34 additional patients with FHL for mutations in the coding region of the perforin gene and the relative frequency of perforin mutations in FHL. Perforin mutations were identified in 7 of the 34 families investigated. Six children were homozygous for the mutations, and one patient was a compound heterozygote. Four novel mutations were detected: one nonsense, two missense, and one deletion of one amino acid. In four families, a previously reported mutation at codon 374, causing a premature stop codon, was identified, and, therefore, this is the most common perforin mutation identified so far in FHL patients. We found perforin mutations in 20% of all FHL patients investigated (7/34), with a somewhat higher prevalence, approximately 30% (6/20), in children whose parents originated from Turkey. No other correlation between the type of mutation and the phenotype of the patients was evident from the present study. Our combined results from mutational analysis of 34 families and linkage analysis of a subset of consanguineous families indicate that perforin mutations account for 20%-40% of the FHL cases and the FHL 1 locus on chromosome 9 for approximately 10%, whereas the major part of the FHL cases are caused by mutations in not-yet-identified genes.

Splenectomy in haemophagocytic lymphohistiocytosis: report of histopathological changes with CD19+ B-cell depletion and therapeutic results

The pathogenesis of haemophagocytic lymphohistiocytosis (HLH) in children without a known familial pattern of inheritance is often difficult to establish. Splenic enlargement, one of the main clinical findings in this disorder, has led to the use of splenectomy for uncontrollable coagulopathy, persistent cytopenia or both. This procedure is also thought to be a useful tool in making a differential diagnosis in cases of the immunochemotherapy-resistant HLH. We report here five cases of splenectomized childhood HLH, in which subsets of mononuclear spleen cells were analysed either by flow cytometry or immunohistochemistry, and the results were compared with those from cases of hereditary spherocytosis (controls). There was a statistically significant depletion of CD19+ B cells in the HLH cases (3.8 +/- 3.2% vs. 52.6 +/- 4.5%, P < 0. 0001) associated with an increase of T cells in three cases and of natural killer cells in another. The histopathological findings included atrophic white pulps, B-cell depletion with fibrosis and haemosiderosis in all five cases. Despite temporary therapeutic benefits, three of the HLH patients had a rapidly deteriorating post-splenectomy course and all three eventually died. These results demonstrate striking depletion of B cells in the enlarged spleens of children with HLH, which may be an intrinsic feature of HLH pathogenesis. Further study is needed to establish the therapeutic value of splenectomy in this disease.

Haemophagocytic lymphohistiocytosis in X-linked severe combined immunodeficiency

Haemophagocytic lymphohistiocytosis (HLH) is characterized by destruction of haematopoietic elements, and is associated with a variety of manifestations including immune abnormalities. We describe an infant with HLH who had no evidence of infection or malignancy. He had markedly reduced natural killer (NK) and T-cell numbers and mitogen responses, consistent with severe combined immune deficiency. Western blot and flow cytometry analyses revealed an absence of interleukin (IL)-2 receptor gamma (gamma common) chain expression and a transition (C --> T) at nucleotide 684 in the gamma common gene. This novel case highlights the need for a thorough evaluation of immunological phenotype and genotype in patients with HLH.

Perforin gene defects in familial hemophagocytic lymphohistiocytosis

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare, rapidly fatal, autosomal recessive immune disorder characterized by uncontrolled activation of T cells and macrophages and overproduction of inflammatory cytokines. Linkage analyses indicate that FHL is genetically heterogeneous and linked to 9q21.3-22, 10q21-22, or another as yet undefined locus. Sequencing of the coding regions of the perforin gene of eight unrelated 10q21-22-linked FHL patients revealed homozygous nonsense mutations in four patients and missense mutations in the other four patients. Cultured lymphocytes from patients had defective cytotoxic activity, and immunostaining revealed little or no perforin in the granules. Thus, defects in perforin are responsible for 10q21-22-linked FHL. Perforin-based effector systems are, therefore, involved not only in the lysis of abnormal cells but also in the down-regulation of cellular immune activation.

The histiocytoses of infancy

The histiocytoses comprise a rare and incompletely understood group of diseases that are characterized by an abnormal proliferation of histiocytes, mononuclear phagocytic, and antigen-presenting cells belonging to the reticuloendothelial system. The two major types of histiocytoses, which often present during infancy and require significant medical attention, are Langerhans cell histiocytosis and primary hemophagocytic lymphohistiocytosis. Their clinical manifestations frequently overlap, complicating their distinction, but a prompt and accurate diagnosis is essential to deliver the optimal treatment and maximize the chances for a favorable outcome. For Langerhans cell histiocytosis, careful risk stratification is critical for the appropriate administration of therapy. Patients with good prognostic factors may need only observation as their disease spontaneously regresses or minimal intervention. Poor prognostic factors mandate more intensive treatment. Patients with primary hemophagocytic lymphohistiocytosis require chemotherapeutic induction of their disease into a state of remission followed by stem cell transplantation, which currently offers the only known cure. Improvements on current therapies for the histiocytoses will depend on continued advances in the understanding of these enigmatic diseases.