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

Functions of natural killer cells

Natural killer (NK) cells are effector lymphocytes of the innate immune system that control several types of tumors and microbial infections by limiting their spread and subsequent tissue damage. Recent research highlights the fact that NK cells are also regulatory cells engaged in reciprocal interactions with dendritic cells, macrophages, T cells and endothelial cells. NK cells can thus limit or exacerbate immune responses. Although NK cells might appear to be redundant in several conditions of immune challenge in humans, NK cell manipulation seems to hold promise in efforts to improve hematopoietic and solid organ transplantation, promote antitumor immunotherapy and control inflammatory and autoimmune disorders.

Sequence analysis of the SRGN, AP3B1, ARF6, and SH2D1A genes in familial hemophagocytic lymphohistiocytosis

In the present study, DNA sequencing of the genes SRGN, ARF6, AP3B1, and SH2D1A was performed in a well defined cohort of 18 families with familial hemophagocytic lymphohistiocytosis (FHL). A heterozygous nucleotide change (C > T) in the 3'untranslated region of the SRGN gene and a monoallelic 3-base pair deletion (c.2409_2411delGAA) in exon 21 of the AP3B1 gene were observed in two different families. Additionally, two novel polymorphisms, one in intron 17 of AP3B1 and another in intron 2 of SH2D1A were identified. We conclude that mutations in SRGN, ARF6, and AP3B1 are not likely to be common in patients fulfilling the FHL criteria.

Generation and maintenance of Listeria-specific CD8+ T cell responses in perforin-deficient mice chronically infected with LCMV

Disruption of the perforin gene results in primary immunodeficiency and an increased susceptibility to opportunistic pathogens. Perforin-deficient (PKO) mice fail to clear primary lymphocytic choriomeningitis virus (LCMV) Armstrong, resulting in persistent infection and functional exhaustion of virus-specific CD8+ T cells. CD8+ T cell responses to Listeria monocytogenes (LM) challenge within the first week after LCMV infection were diminished in both WT and PKO mice, and correlated with enhanced bacterial clearance. However, bacterial challenge at later time points generated similar CD8 T cell responses in both groups of mice. The phenotype and function of pre-existing LM-specific memory CD8+ T cells were maintained in persistently infected PKO mice. Thus persistent LCMV infection, as a result of perforin deficiency, results in dysfunction of the virus-specific CD8+ T cell response but does not compromise the host's ability to maintain pre-existing memory CD8+ T cells or to generate new memory CD8+ T cell responses against other pathogens.

Macrophage activation syndrome in rheumatic disease: what is the role of the antigen presenting cell?

Macrophage Activation Syndrome (MAS), alternatively referred to as secondary hemophagocytic lymphohistiocytosis (HLH), is a complication of many rheumatic diseases, most commonly Systemic Juvenile Idiopathic Arthritis (SJIA). MAS consists of a fulminant picture of pan-cytopenia, hectic high fevers, hepatosplenomegaly, lymphadenopathy, rash, and central nervous systemic inflammation. It can arise from genetic defects in the cytotoxic effector response of CD8+ T-cells, resulting in an inability to terminate antigen presentation, which in turn leads to uncontrolled immune activation. However, in the case of most rheumatic diseases, no such defect in cytotoxic killing is present. Little is known about what the contributions from the antigen presenting cells are in the pathogenesis of MAS. In fact, macrophages may be playing a regulatory, anti-inflammatory in MAS. We review the proposed pathogenesis of MAS/HLH, what role macrophages may play in the disease, and the relationship of MAS to its most common associated rheumatic disease, SJIA.

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.

Nationwide survey of hemophagocytic lymphohistiocytosis in Japan

Hemophagocytic lymphohistiocytosis (HLH), a disorder of the mononuclear phagocyte system, can be classified into two distinct forms: primary HLH (FHL) and secondary HLH. To clarify the epidemiology and clinical outcome for each HLH subtype, we conducted a nationwide survey of HLH in Japan. Since 799 patients were diagnosed in 292 institutions of Japan between 2001 and 2005, the annual incidence of HLH was estimated as 1 in 800,000 per year. Among them, 567 cases were actually analyzed in this study. The most frequent subtype was Epstein-Barr virus (EBV)-associated HLH, followed by other infection- or lymphoma-associated HLH. Age distribution showed a peak of autoimmune disease- and infection-associated HLH in children, while FHL and lymphoma-associated HLH occurred almost exclusively in infants and the elderly, respectively. The 5-year overall survival rate exceeded 80% for patients with EBV- or other infection-associated HLH, was intermediate for those with FHL or B-cell lymphoma-associated HLH, and poor for those with T/NK cell lymphoma-associated HLH (<15%). Although this nationwide survey establishes the heterogeneous characteristics of HLH, the results should be useful in planning prospective studies to identify the most effective therapy for each HLH subtype.

Defective cytotoxic lymphocyte degranulation in syntaxin-11 deficient familial hemophagocytic lymphohistiocytosis 4 (FHL4) patients

Familial hemophagocytic lymphohistiocytosis (FHL) is typically an early onset, fatal disease characterized by a sepsislike illness with cytopenia, hepatosplenomegaly, and deficient lymphocyte cytotoxicity. Disease-causing mutations have been identified in genes encoding perforin (PRF1/FHL2), Munc13-4 (UNC13D/FHL3), and syntaxin-11 (STX11/FHL4). In contrast to mutations leading to loss of perforin and Munc13-4 function, it is unclear how syntaxin-11 loss-of-function mutations contribute to disease. We show here that freshly isolated, resting natural killer (NK) cells and CD8(+) T cells express syntaxin-11. In infants, NK cells are the predominant perforin-containing cell type. NK cells from FHL4 patients fail to degranulate when encountering susceptible target cells. Unexpectedly, IL-2 stimulation partially restores degranulation and cytotoxicity by NK cells, which could explain the less severe disease progression observed in FHL4 patients, compared with FHL2 and FHL3 patients. Since the effector T-cell compartment is still immature in infants, our data suggest that the observed defect in NK-cell degranulation may contribute to the pathophysiology of FHL, that evaluation of NK-cell degranulation in suspected FHL patients may facilitate diagnosis, and that these new insights may offer novel therapeutic possibilities.

Immunotherapy of familial hemophagocytic lymphohistiocytosis with antithymocyte globulins: a single-center retrospective report of 38 patients

OBJECTIVES

Familial hemophagocytic lymphohistiocytosis is a genetically determined condition that is characterized by unremitting CD8 T lymphocyte and macrophage activation and leads to death in the absence of therapy. On the basis of the immunologic pathophysiology of familial hemophagocytic lymphohistiocytosis, we propose a therapy with a combination of antithymocyte globulins with corticosteroids, cyclosporin A, and intrathecal injections of methotrexate.

METHODS

We retrospectively analyzed the outcome of antithymocyte globulin-based therapy that was performed in 38 consecutive patients who had familial hemophagocytic lymphohistiocytosis and were treated in a single center between 1991 and 2005. Overall, they received 45 courses of antithymocyte globulin (5-10 mg/kg per day for 5 days).

RESULTS

This regimen was associated with infections after 10 of 45 courses of antithymocyte globulin. There were 6 events after 11 antithymocyte globulin courses given as second-line therapy against 4 after 34 antithymocyte globulin courses in patients who were treated primarily with antithymocyte globulin. Antithymocyte globulin administration led to rapid and complete response of familial hemophagocytic lymphohistiocytosis in 73% of cases, partial response in 24%, and no response only once. When hematopoietic stem cell transplantation was performed early after complete or partial response induction, it led to a high rate of cure, in 16 of 19 cases. Overall survival was 21 of 38 with 4 toxic deaths.

CONCLUSION

Antithymocyte globulin based immunotherapy of familial hemophagocytic lymphohistiocytosis is efficient and carries an acceptable toxicity when used as a first treatment of familial hemophagocytic lymphohistiocytosis.

Recognition and management of macrophage activation syndrome in juvenile arthritis

PURPOSE OF REVIEW

Macrophage activation syndrome is a life-threatening complication seen predominantly in children with systemic onset juvenile idiopathic arthritis. It accounts for a significant amount of the morbidity and mortality seen with systemic onset juvenile idiopathic arthritis.

RECENT FINDINGS

In this article, we will look at the new developments in the diagnosis, classification, pathogenesis and management of macrophage activation syndrome in systemic onset juvenile idiopathic arthritis patients.

SUMMARY

More work is needed to further elucidate the pathophysiology of macrophage activation syndrome in systemic onset juvenile idiopathic arthritis. This would be the key to early diagnosis using more sensitive criteria and better management.

Use of rituximab in conjunction with immunosuppressive chemotherapy as a novel therapy for Epstein Barr virus-associated hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis is a rare, life-threatening complication of Epstein Barr virus (EBV) infection. Current treatments are directed at reducing virus-induced immune dysregulation. Addition of agents that eliminate EBV-infected B cells may improve therapeutic efficacy. On the basis of the observations that the anti-CD-20 monoclonal antibody rituximab reduces disease burden in individuals with EBV-associated lymphoproliferative disorders, we treated a patient with severe EBV-hemophagocytic lymphohistiocytosis using a combination of rituximab and chemotherapy. This patient demonstrated a rapid clinical response and an 18-fold reduction in EBV viral load within 24 hours of receiving rituximab. He remains free of disease 8 months after completing treatment.

CD4+CD25+ T regulatory cells dominate multiple immune evasion mechanisms in early but not late phases of tumor development in a B cell lymphoma model

Tumors use a complex set of direct and indirect mechanisms to evade the immune system. Naturally arising CD4(+)CD25(+)FoxP3(+) T regulatory (Treg) cells have been implicated recently in tumor immune escape mechanism, but the relative contribution of these cells to overall tumor progression compared with other immune evasion mechanisms remains to be elucidated. Using the A20 B cell lymphoma as a transplantable tumor model, we demonstrate that this tumor employs multiple direct (expression of immunoinhibitory molecule PD-L1, IDO, and IL-10, and lack of expression of CD80 costimulatory molecule) and indirect (down-regulation of APC function and induction of Treg cells) immune evasion mechanisms. Importantly, Treg cells served as the dominant immune escape mechanism early in tumor progression because the physical elimination of these cells before tumor challenge resulted in tumor-free survival in 70% of mice, whereas their depletion in animals with established tumors had no therapeutic effect. Therefore, our data suggest that Treg cells may serve as an important therapeutic target for patients with early stages of cancer and that more vigorous combinatorial approaches simultaneously targeting multiple immune evasion as well as immunosurveillance mechanisms for the generation of a productive immune response against tumor may be required for effective immunotherapy in patients with advanced disease.

Hemophagocytic syndromes

Hemophagocytic syndromes (hemophagocytic lymphohistiocytosis, HLH) represent a severe hyperinflammatory condition with the cardinal symptoms prolonged fever, cytopenias, hepatosplenomegaly, and hemophagocytosis by activated, morphologically benign macrophages. Biochemical markers include elevated ferritin and triglycerides, and low fibrinogen. Whereas in children several inherited immune deficiencies may lead to this syndrome, most adults with HLH have no known underlying immune defect. Nevertheless, impaired function of natural killer (NK) cells and cytotoxic T-cells (CTL) is characteristic for both genetic and acquired forms of HLH. Frequent triggers are infectious agents, mostly viruses of the herpes group. Malignant lymphomas, especially in adults, may be associated with HLH. A special form of HLH in rheumatic diseases is called macrophage-activation syndrome. Initially HLH may masquerade as a normal infection since all symptoms, even though less pronounced, may also be found in immune competent patients. Patients with HLH, however, cannot control the hyperinflammatory response which, if untreated, is fatal in genetic cases and in a high percentage of acquired cases. Awareness of the clinical symptoms and of the diagnostic criteria of HLH is important to start life-saving therapy with immunosuppressive/immunomodulatory agents in time.

Human granulocytic anaplasmosis and macrophage activation

Patients with human granulocytic anaplasmosis present with fever, thrombocytopenia, leukopenia, and an elevated aspartate transaminase level. Clinical and histopathologic features of severe disease suggest macrophage activation. Twenty-nine patients with human granulocytic anaplasmosis had higher ferritin, interleukin-10, interleukin-12 p70, and interferon- gamma levels than did control subjects matched for age and sex; severity correlated with triglyceride, ferritin, and interleukin-12 p70 levels. Several severely affected patients had cases that fulfilled macrophage activation syndrome diagnostic criteria. Macrophage activation and excessive cytokine production may belie tissue injury associated with Ananplasma phagocytophilum infection.

Jinx, an MCMV susceptibility phenotype caused by disruption of Unc13d: a mouse model of type 3 familial hemophagocytic lymphohistiocytosis

Mouse cytomegalovirus (MCMV) susceptibility often results from defects of natural killer (NK) cell function. Here we describe Jinx, an N-ethyl-N-nitrosourea–induced MCMV susceptibility mutation that permits unchecked proliferation of the virus, causing death. In Jinx homozygotes, activated NK cells and cytotoxic T lymphocytes (CTLs) fail to degranulate, although they retain the ability to produce cytokines, and cytokine levels are markedly elevated in the blood of infected mutant mice. Jinx was mapped to mouse chromosome 11 on a total of 246 meioses and confined to a 4.60–million basepair critical region encompassing 122 annotated genes. The phenotype was ascribed to the creation of a novel donor splice site in Unc13d, the mouse orthologue of human MUNC13-4, in which mutations cause type 3 familial hemophagocytic lymphohistiocytosis (FHL3), a fatal disease marked by massive hepatosplenomegaly, anemia, and thrombocytopenia. Jinx mice do not spontaneously develop clinical features of hemophagocytic lymphohistiocytosis (HLH), but do so when infected with lymphocytic choriomeningitis virus, exhibiting hyperactivation of CTLs and antigen-presenting cells, and inadequate restriction of viral proliferation. In contrast, neither Listeria monocytogenes nor MCMV induces the syndrome. In mice, the HLH phenotype is conditional, which suggests the existence of a specific infectious trigger of FHL3 in humans.

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.

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.

Perforin-mediated target-cell death and immune homeostasis

The granule exocytosis pathway of cytotoxic lymphocytes is crucial for immune surveillance and homeostasis. The trafficking of granule components, including the membrane-disruptive protein perforin, to the immunological synapse leads to the delivery of granule proteases (granzymes) into the target cell and its destruction through apoptosis. Several independent molecular abnormalities associated with defects of either granule trafficking or perforin function can cause cytotoxic lymphocyte dysfunction. In humans, inherited perforin mutations result in severe immune dysregulation that manifests as familial haemophagocytic lymphohistiocytosis. This Review describes recent progress in defining the structure, function, biochemistry and cell biology of perforin.

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.

Hemophagocytic macrophages constitute a major compartment of heme oxygenase expression in sepsis

Schaer DJ, Schaer CA, Schoedon G, Imhof A, Kurrer MO.

Hemophagocytic macrophages constitute a major compartment of heme oxygenase expression in sepsis.

Objectives: Uncontrolled macrophage activation with hemophagocytosis is a distinctive feature of hemophagocytic syndromes (HPS). We examined whether lympho-histiocytic infiltration of the bone marrow and liver, as well as hemo-/erythrophagocytosis also occurs during sepsis and whether this process could account for the increased production of anti-inflammatory heme-oxygenase (HO-1) products observed during sepsis. Methods: Hemophagocytosis and expression of CD163, HO-1, ferritin as well as CD8 and granzyme-B were examined in post-mortem bone marrow samples from 28 patients with sepsis and from eight control patients. Results: Comparison of samples from non-septic patients with samples from patients with fatal sepsis revealed that the latter group displayed dense lympho-histiocytic bone marrow infiltration with CD163⁺/HO-1⁺/ferritin⁺ macrophages as well as with CD8⁺ and granzyme-B⁺ T-cells. Hemophagocytosis with prominent phagocytosis of erythroid cells was readily apparent in septic patients, implying that this process is a likely stimulus for the up-regulation of macrophage HO-1 expression. Conclusions: Lympho-histiocytic activation with hemophagocytosis is a shared pathophysiologic mechanism in HPS and sepsis. Furthermore, the association of hemophagocytosis with an increase in HO-1 expression may indicate a novel role for this apparently futile process as a negative regulator of inflammation.

Canine hemophagocytic histiocytic sarcoma: a proliferative disorder of CD11d+ macrophages

Histiocytic disorders of dogs include histiocytoma, localized histiocytic sarcoma (HS), disseminated HS (malignant histocytosis), and the reactive histiocytoses: cutaneous and systemic. A common element to these diseases is proliferation of dendritic cells (DC) of either Langerhans cell (epithelial DC) or interstitial DC lineage. In this report, 17 dogs with hemophagocytic HS are described. Breeds affected included Bernese Mountain Dog (6), Golden Retriever (4), Rottweiler (3), Labrador Retriever (2), a mixed-breed dog, and a Schnauzer, which were from 2.5 to 13 years old. The dogs presented with Coombs negative responsive anemia in 16/17 dogs (94%), thrombocytopenia in 15/17 dogs (88%), hypoalbuminemia in 16/17 dogs (94%), and hypocholesterolemia in 11/16 dogs (69%). All dogs died or were euthanized. The clinical course ranged from 2 to 32 weeks (mean 7.1 weeks). Diffuse splenomegaly with ill-defined masses was consistently present. Microscopic lesions were prevalent in spleen, liver, lung, and bone marrow. Metastasis occurred by insidious intravascular invasion with minimal mass formation. Histiocytes were markedly erythrophagocytic and accompanied by foci of extramedullary hemopoiesis. Cytologically, the histiocytes varied from well differentiated to atypical, with atypia more prevalent in spleen than bone marrow. These tumors arose from splenic red pulp and bone marrow macrophages, which expressed major histocompatibility complex class II and the beta2 integrin, CD11d. They had low and/or inconsistent expression of CD1 and CD11c, which are dominantly expressed by canine nonhemophagocytic HS of DC origin. Canine histiocytic proliferative diseases now encompass proliferation of all members of the myeloid histiocytic lineage: Langerhans cells, interstitial DC, and macrophages.

Perforin and Granzymes Have Distinct Roles in Defensive Immunity and Immunopathology

Successful control of viral infection requires the host to eliminate the infecting pathogen without causing overt immunopathology. Here we showed that perforin (Prf1) and granzymes (Gzms) have distinct roles in defensive immunity and immunopathology in a well-established model of viral infection. Both Prf1 and Gzms drastically affected the outcome of murine cytomegalovirus (MCMV) infection. Viral titres increased markedly in both Prf1(-/-) and Gzma(-/-)Gzmb(-/-) mice, but Gzma(-/-)Gzmb(-/-) mice recovered and survived infection, whereas Prf1(-/-) mice did not. Indeed, infected Prf1-deficient hosts developed a fatal hemophagocytic lymphohistiocytosis (HLH)-like syndrome. This distinction in outcome depended on accumulation of mononuclear cells and T cells in infected Prf1(-/-) mice. Importantly, blocking experiments that clearly identified tumor necrosis factor-alpha (TNF-alpha) as the principal contributor to the lethality observed in infected Prf1(-/-) mice provided support for the clinical potential of such an approach in HLH patients whose disease is triggered by viral infection.

Nephrotic syndrome associated with hemophagocytic syndrome

Hemophagocytic syndrome (HPS) is defined by bone marrow and organ infiltration by activated, nonmalignant macrophages, which phagocytose blood cells. The clinical spectrum of HPS is broad, but renal involvement has rarely been investigated. We report a previously unknown renal manifestation of HPS: nephrotic syndrome. This multicentric retrospective study included patients fulfilling the following criteria: (i) no history of nephropathy; (ii) HPS diagnosis with histologic evidence of hemophagocytosis; (iii) occurrence of nephrotic syndrome during HPS; and (iv) available renal histology. Using the same criteria, we also searched the literature for additional cases. We identified nine patients retrospectively and found two additional cases in the literature (five males and six females, whose mean age was 34 +/- 27 years). Black African patients predominated (63.6%). HPS was due to lymphoma (six cases), infectious disease (three cases), and autoimmune disease (one case), and was primary in one patient. Acute renal failure was associated with nephrotic syndrome in 10/11 cases. Renal histology showed acute tubular necrosis associated with collapsing glomerulopathy in five patients (all Africans with negative human immunodeficiency virus serology), minimal change glomerulopathy in four, and thrombotic-microangiopathy with abnormal podocytes in two. Death occurred in seven cases. Nephrotic syndrome should be included among the renal complications of HPS with acute renal failure. We postulate that abnormal T-cell activation and/or high pro-inflammatory cytokine levels during HPS might cause podocyte injuries, especially among African patients with a susceptible genetic background.

Défaut d’exocytose des granules lytiques

An in vivo disturbance of lymphocyte homeostasis occurs during the course of the hemophagocytic syndrome (HS). HS is a severe and often fatal syndrome resulting from potent and uncontrolled activation and proliferation of T-lymphocytes, mainly polyclonal CD8 lymphocytes, leading to excessive macrophage activation, high level of proinflammatory cytokine production and multiple deleterious effects. The onset of HS characterizes several inherited disorders in humans. In most of these conditions, the molecular defect impairs the granule-dependent cytotoxic activity of lymphocytes, thus highlighting the determinant role of this function in driving back the immune system to a state of equilibrium following infection. Several lines of evidence suggest that an increase in the expansion phase rather than a decrease in the contraction phase of the CD8+ T cells population characterizes the HS. Failure to kill antigen presenting cells through a transaction mechanism of cytotoxic cells should favor a sustained response, although the mechanism may be more complex than simple decrease of antigen load. Defect in the granule dependent cytotoxic function of lymphocytes result from perforin mutation in familial hemophagocytic lymphohistiocytosis type 2, from Munc13-4 (UNC13D) mutation in familial hemophagocytic lymphohistiocytosis type 3, from Rab27a mutation in Griscelli syndrome type 2, and from CHS/LYST mutation in Chediak-Higashi syndrome. The characterization of the molecular causes leading to these conditions identified Rab27a and Munc13-4 as two critical effectors of the exocytic machinery, required for the terminal transport/docking or priming of the cytotoxic granules, respectively. Different members of the Rab and Munc13 family of proteins are also used in neurotransmitter release at the neurological synapse, highlighting the similarity of the mechanisms regulating both secretory pathways. Future investigations regarding HS will continue to elucidate this exocytic pathway machinery and improve our understanding of how it finely regulates the immune response, an area that is likely to be useful for therapeutic intervention.

Reactive hemophagocytic syndrome complicating the treatment of inflammatory bowel disease

BACKGROUND AND AIMS

Reactive hemophagocytic syndrome (RHS) is a rare disease in which inappropriately activated macrophages consume bone marrow-derived cells. Most cases are associated with infection in the setting of immunodeficiency. The widespread use of immunosuppressive therapy in the treatment of inflammatory bowel disease (IBD) places patients with Crohn's disease and ulcerative colitis at risk of this complication. No concerted effort has been made to alert gastroenterologists of this condition, and treatment recommendations are lacking. The aims of this study were to describe the clinical and laboratory features of RHS associated with IBD and to review diagnostic criteria, treatment options, and pathogenesis.

MATERIALS AND METHODS

Clinical and laboratory data were pooled from the clinical practice of the investigators and from published cases. Descriptive statistics were performed.

RESULTS AND CONCLUSIONS

Seven cases of RHS complicating the treatment of IBD were identified. All patients were on immunosuppressive therapy, with nearly half taking >1 agent. All patients presented with fever, leukopenia, anemia, and hyperferritinemia. Infection by a member of the herpesvirus family or an intracellular pathogen precipitated RHS in 6 of 7 patients. The mortality rate was 29%. The diagnosis of RHS should be considered in patients with IBD taking immunosuppressive therapy who present with fever and cytopenia. Evaluation should begin with a serum ferritin. In patients with a serum ferritin > or =10,000 ng/mL, a bone marrow biopsy should be performed to confirm hemophagocytosis. If the initial evaluation is negative, then clinical suspicion should be maintained until the episode resolves.

Spectrum and clinical implications of syntaxin 11 gene mutations in familial haemophagocytic lymphohistiocytosis: association with disease-free remissions and haematopoietic malignancies

OBJECTIVE

To determine the frequency and spectrum of mutations in the gene encoding syntaxin 11 (STX11) in familial haemophagocytic lymphohistiocytosis (FHL), a rare autosomal recessive disorder of immune dysregulation characterised by a defect in natural killer cell function.

METHODS

Mutational analysis of STX11 by direct sequencing was done in 28 FHL families that did not harbour perforin mutations, previously identified in some FHL patients. A detailed investigation of clinical features of these patients was also undertaken.

RESULTS

Two different STX11 mutations were identified, one nonsense mutation and one deletion, affecting six of 34 children in four of 28 unrelated PRF1 negative families. Both mutations have been reported before. Three patients experienced long periods (> or = 1 year) in remission without specific treatment, which is very uncommon in this disease. Despite the milder phenotype, some children with STX11 mutations developed severe psychomotor retardation. Two of the six patients harbouring STX11 gene defects developed myelodysplastic syndrome (MDS) or acute myelogenous leukaemia (AML).

CONCLUSIONS

STX11 gene mutations were found in 14% of the PRF1 negative FHL families included in the present cohort. These results suggest that STX11 gene mutations may be associated with secondary malignancies (MDS/AML), and that there is segregation of specific clinical features in FHL patients with an underlying genotype.

Insufficient perforin expression in CD8+ T cells in response to hemagglutinin from avian influenza (H5N1) virus

Hypercytokinemia has been considered to be responsible for hemophagocytosis and multiorgan failure in severe human cases of H5N1 viral infection. In this study, we show a recombinant hemagglutinin (H5) from a H5N1 virus may suppress the perforin expression and reduce cytotoxicity of human CD8+ T cells to kill H5-bearing cells. The persistence of H5-presenting cells provides sustained stimulation and leads to a marked lymphoproliferation and IFN-gamma hyperproduction. We propose the IFN-gamma hyperproduction may explain macrophage overactivation and subsequent hypercytokinemia and hemophagocytosis in severe human cases of avian influenza.

Hematopoietic stem cell transplantation in hemophagocytic lymphohistiocytosis: a single-center report of 48 patients

OBJECTIVES

Familial hemophagocytic lymphohistiocytosis (FHLH) is a genetically determined disorder characterized by the early onset of fever, hepatosplenomegaly, central nervous system disease, thrombocytopenia, coagulation disorders, and hemophagocytosis. It is caused by genetic defects that impair T cell-mediated and natural cytotoxicity. Chemotherapy- or immunotherapy-based treatments can achieve remission. Hematopoietic stem cell transplantation (HSCT), however, is the only curative option, but optimal modalities and long-term outcome are not yet well known.

METHODS

We retrospectively analyzed the outcome of HSCT that was performed in 48 consecutive patients who had FHLH and were treated in a single center between 1982 and 2004.

RESULTS

The overall survival was 58.5% with a median follow-up of 5.8 years and extending to 20 years. A combination of active disease and haploidentical HSCT had a poor prognosis because in this situation, HLH disease is more frequently associated with graft failure. Twelve patients received 2 transplants because of graft failure (n = 7) or secondary graft loss that led to HLH relapse (n = 5). Transplant-related toxicity essentially consisted in veno-occlusive disease, which occurred in 28% of transplants and was associated with young age, haploidentical transplantation, and the use of antithymocyte globulin (ATG) in the conditioning regimen. A sustained remission was achieved in all patients with a donor chimerism > or = 20% of leukocytes. Long-term sequelae were limited, because only 2 (7%) of 28 patients experienced a mild neurologic disorder.

CONCLUSIONS

This survey demonstrates the long-term efficacy of HSCT as a cure of FHLH. HSCT preserves quality of life. It shows that HSCT should be performed as early as a complete remission has been achieved. Additional studies are required to improve the procedure and reduce its toxic effects.

Lethal hemophagocytic lymphohistiocytosis in Hermansky-Pudlak syndrome type II

Griscelli syndrome (GS) was diagnosed in a 2-year-old patient with oculocutaneous albinism and immunodeficiency, but sequencing of RAB27a revealed only a heterozygous mutation. Due to impaired natural killer (NK) and T-cell cytotoxicity implying a high risk of developing hemophagocytic lymphohistiocytosis (HLH), he was prepared for hematopoietic stem cell transplantation (HSCT). Unexpectedly, a severe bleeding episode occurred that led to the demonstration of disturbed platelet aggregation, reduced plateletdense granules, and impaired platelet degranulation. In combination with neutropenia, this suggested the diagnosis of Hermansky-Pudlak syndrome type II (HPSII) and a novel homozygous mutation in AP3B1 was detected. None of the 3 reported HPSII patients had developed HLH, and our patient seroconverted to Epstein-Barr virus (EBV) without clinical symptoms. HSCT was therefore withheld, and granulocyte-colony-stimulating factor (G-CSF) therapy was initiated and prevented further bacterial infections. At 3 years of age, however, the patient developed, without an obvious trigger, fulminant HLH that was resistant to therapy. This patient shows that careful clinical and molecular diagnosis is essential to differentiate the complex disorders of lysosomal trafficking. HPSII belongs to the group of familial hemophagocytic syndromes and may represent an indication for HSCT.

Perforin-deficient CD8+ T cells mediate fatal lymphocytic choriomeningitis despite impaired cytokine production

Intracerebral (i.c.) infection with lymphocytic choriomeningitis virus (LCMV) is one of the most studied models for virus-induced immunopathology, and based on results from perforin-deficient mice, it is currently assumed that fatal disease directly reflects perforin-mediated cell lysis. However, recent studies have revealed additional functional defects within the effector T cells of LCMV-infected perforin-deficient mice, raising the possibility that perforin may not be directly involved in mediating lethal disease. For this reason, we decided to reevaluate the role of perforin in determining the outcome of i.c. infection with LCMV. We confirmed that the expansion of virus-specific CD8(+) T cells is unimpaired in perforin-deficient mice. However, despite the fact that the virus-specific CD8(+) effector T cells in perforin-deficient mice are broadly impaired in their effector function, these mice invariably succumb to i.c. infection with LCMV strain Armstrong, although a few days later than matched wild-type mice. Upon further investigation, we found that this delay correlates with the delayed recruitment of inflammatory cells to the central nervous system (CNS). However, CD8(+) effector T cells were not kept from the CNS by sequestering in infected extraneural organ sites such as liver or lungs. Thus, the observed dysfunctionality regarding the production of proinflammatory mediators probably results in the delayed recruitment of effector cells to the CNS, and this appears to be the main explanation for the delayed onset of fatal disease in perforin-deficient mice. However, once accumulated in the CNS, virus-specific CD8(+) T cells can induce fatal CNS pathology despite the absence of perforin-mediated lysis and reduced capacity to produce several key cytokines.

Aberrant maturation of mutant perforin underlies the clinical diversity of hemophagocytic lymphohistiocytosis

Missense mutations in perforin, a critical effector of lymphocyte cytotoxicity, lead to a spectrum of diseases, from familial hemophagocytic lymphohistiocytosis to an increased risk of tumorigenesis. Understanding of the impact of mutations has been limited by an inability to express human perforin in vitro. We have shown, for the first time to our knowledge, that recombinant human perforin is expressed, processed appropriately, and functional in rat basophilic leukemia (RBL) cells following retroviral transduction. Subsequently, we have addressed how perforin missense mutations lead to absent perforin detection and impaired cytotoxicity by analyzing 21 missense mutations by flow cytometry, immunohistochemistry, and immunoblot. We identified perforin missense mutations with partial maturation (class 1), no apparent proteolytic maturation (class 2), and no recognizable forms of perforin (class 3). Class 1 mutations exhibit lytic function when expressed in RBL cells and are associated with residual protein detection and variable cytotoxic function in affected individuals, suggesting that carriers of class 1 alleles may exhibit more subtle immune defects. By contrast, class 3 mutations cause severely diminished perforin detection and cytotoxicity, while class 2 mutations have an intermediate phenotype. Thus, the pathologic mechanism of perforin missense mutation likely involves a protein dosage effect of the mature protein.

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.

Perforin-dependent elimination of dendritic cells regulates the expansion of antigen-specific CD8+ T cells in vivo

The lifespan and survival of dendritic cells (DC) in vivo are potentially critical to the expansion of T cell immune responses. We have previously reported that DC loaded with specific antigen are rapidly eliminated by cytotoxic T lymphocytes (CTL) in vivo, but the site, mechanism, and consequences of DC elimination were not defined. In this article we show that DC elimination in vivo occurs in a perforin-dependent manner and does not require IFN-gamma or the presence of CD4(+)CD25(+) regulatory T cells. Most importantly, failure to eliminate DC had profound consequences on the CTL immune response. Perforin-deficient mice showed a progressive increase in the numbers of antigen-specific CD8(+) T cells after repeated immunizations with DC. In contrast, in control mice the number of antigen-specific CD8(+) T cells did not notably increase with repeated immunizations. Lastly, we also show that CTL-mediated elimination of DC occurs in peripheral tissues but not in the lymph node. Our data suggest that CTL act as "gatekeepers" that control access of antigen-loaded DC into the lymph node, thereby preventing continued expansion of antigen-specific T cells.

Apoptosis: a basic biological phenomenon with wide-ranging implications in human disease

Apoptosis is a highly regulated process of cell deletion and plays a fundamental role in the maintenance of tissue homeostasis in the adult organism. Numerous studies in recent years have revealed that apoptosis is a constitutive suicide programme expressed in most, if not all cells, and can be triggered by a variety of extrinsic and intrinsic signals. Many human diseases can be attributed directly or indirectly to a derangement of apoptosis, resulting in either cell accumulation, in which cell eradication or cell turnover is impaired, or cell loss, in which the apoptotic programme is inadvertently triggered. In addition, defective macrophage engulfment and degradation of cell corpses may also contribute to a dysregulation of tissue homeostasis. An increased understanding of the signalling pathways that govern the execution of apoptosis and the subsequent clearance of dying cells may thus yield novel targets for therapeutic intervention in a wide range of human maladies.

Chronic exposure to benzene results in a unique form of dysplasia

Hematotoxicity following chronic benzene exposure has been recognized for over a century, although the mechanism remains unknown. We describe a novel form of bone marrow dysplasia in 23 workers exposed to high concentrations of benzene. Distinguishing features of benzene-induced dysplasia include: marked dyserythropoiesis, eosinophilic dysplasia and abnormal cytoplasmic granulation of neutrophilic precursors. Hematophagocytosis, stromal degeneration and bone marrow hypoplasia are also seen. Severe bone marrow dysplasia is frequently accompanied by clonal T cell expansion and alterations in T lymphocyte subsets. No clonal cytogenetic abnormalities were observed. These results suggest that autoimmune-mediated bone marrow injury is an early or predisposing event in the pathogenesis of benzene-induced persistent hematopoietic disease.

Severe imbalance of IL-18/IL-18BP in patients with secondary hemophagocytic syndrome

Hemophagocytic syndrome (HPS) is characterized by an uncontrolled and poorly understood activation of T-helper 1 (Th-1) lymphocytes and macrophages. We studied 20 patients with HPS secondary to infections, autoimmune disease, lymphoma, or cancer and observed that the concentrations of serum interleukin 18 (IL-18), a strong inducer of Th-1 responses, interferon gamma (IFN-gamma) production, and stimulation of macrophages and natural killer (NK) cells were highly increased in HPS but not in control patients. In contrast, concentrations of its natural inhibitor, the IL-18 binding protein (IL-18BP), were only moderately elevated, resulting in a high level of biologically active free IL-18 in HPS (4.6-fold increase compared with controls; P < .001). Free IL-18 but not IL-12 concentrations significantly correlated with clinical status and the biologic markers of HPS such as anemia (P < .001), hypertriglyceridemia, and hyperferritinemia (P < .01) and also with markers of Th-1 lymphocyte or macrophage activation, such as elevated concentrations of IFN-gamma and soluble IL-2 and tumor necrosis factor alpha (TNF-alpha) receptor concentrations. Despite high IL-18 elevation, in vitro NK-cell cytotoxicity was severely impaired in HPS patients, in part due to NK-cell lymphopenia that was observed in a majority of patients but also secondary to an intrinsic NK-cell functional deficiency. We concluded that a severe IL-18/IL-18BP imbalance results in Th-1 lymphocyte and macrophage activation, which escapes control by NK-cell cytotoxicity and may allow for secondary HPS in patients with underlying diseases.

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.

Primary hemophagocytic syndromes point to a direct link between lymphocyte cytotoxicity and homeostasis

Hemophagocytic syndrome (HS) is a severe and often fatal syndrome resulting from potent and uncontrolled activation and proliferation of T-lymphocytes, leading to excessive macrophage activation and multiple deleterious effects. The onset of HS characterizes several inherited disorders in humans. In each condition, the molecular defect impairs the granule-dependent cytotoxic activity of lymphocytes, thus highlighting the determinant role of this function in driving the immune system to a state of equilibrium following infection. It has also been shown that some of the proteins required for lytic granule secretion are required for melanocyte function, leading to associated hypopigmentation in these conditions. This review focuses on several effectors of this secretory pathway, recently identified, because their defects cause these disorders, and discusses their role and molecular interactions in granule-dependent cytotoxic activity.

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.

Familial and acquired hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening condition characterized by uncontrolled hyperinflammation on the basis of various inherited or acquired immune deficiencies. Cardinal symptoms are prolonged fever, hepatosplenomegaly and cytopenias. Central nervous system (CNS) symptoms are common. Biochemical markers include elevated triglyceride and ferritin, high levels of the alpha chain of the soluble interleukin-2 receptor and low fibrinogen. Impaired function of natural killer (NK) cells and cytotoxic T-cells (CTL) is a characteristic of 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 (CHS), Griscelli syndrome (GS) and X-linked lymphoproliferative syndrome (XLP), in which secondary HLH occurs sporadically. Most patients with acquired HLH have no known underlying immune deficiency. Both acquired and genetic forms are triggered by infections, mostly viruses, or other stimuli. HLH also occurs as a complication of rheumatic diseases (macrophage activation syndrome) and of malignancies. The recent discovery of several genetic defects causing FHLH as well as the identification of the genes responsible for CHS, GS and XLP have underscored the role of granule (perforin/granzymes)-mediated cytotoxicity in both the killing of infected cells and the termination of the immune response. The immediate aim of therapy is suppression of the increased inflammatory response by immunosuppressive/immunomodulatory agents and cytotoxic drugs. Genetic cases can only be cured with stem cell transplantation. Awareness of the clinical symptoms and of diagnostic criteria for HLH is crucial to starting life-saving therapy in time.

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.

Human T Regulatory Cells Can Use the Perforin Pathway to Cause Autologous Target Cell Death

Cytotoxic T lymphocytes and natural killer cells use the perforin/granzyme pathway to kill virally infected cells and tumor cells. Mutations in genes important for this pathway are associated with several human diseases. CD4(+) T regulatory (Treg) cells have emerged as important in the control of immunopathological processes. We have previously shown that human adaptive Treg cells preferentially express granzyme B and can kill allogeneic target cells in a perforin-dependent manner. Here, we demonstrate that activated human CD4(+)CD25(+) natural Treg cells express granzyme A but very little granzyme B. Furthermore, both Treg subtypes display perforin-dependent cytotoxicity against autologous target cells, including activated CD4(+) and CD8(+) T cells, CD14(+) monocytes, and both immature and mature dendritic cells. This cytotoxicity is dependent on CD18 adhesive interactions but is independent of Fas/FasL. Our findings suggest that the perforin/granzyme pathway is one of the mechanisms that Treg cells can use to control immune responses.