Analysis of natural killer-cell function in familial hemophagocytic lymphohistiocytosis (FHL): defective CD107a surface expression heralds Munc13-4 defect and discriminates between genetic subtypes of the disease

Natural killer (NK) cells from patients with familial hemophagocytic lymphohistiocytosis because of PRF1 (FHL2, n = 5) or MUNC13-4 (FHL3, n = 8) mutations were cultured in IL-2 prior to their use in various functional assays. Here, we report on the surface CD107a expression as a novel rapid tool for identification of patients with Munc13-4 defect. On target interaction and degranulation, FHL3 NK cells displayed low levels of surface CD107a staining, in contrast to healthy control subjects or perforin-deficient NK cells. B-EBV cell lines and dendritic cell targets reveal the FHL3 NK-cell defect, whereas highly susceptible tumor targets were partially lysed by FHL3 NK cells expressing only trace amounts of Munc13-4 protein. Perforin-deficient NK cells were completely devoid of any ability to lyse target cells. Cytokine production induced by mAb-crosslinking of triggering receptors was comparable in patients and healthy control subjects. However, when cytokine production was induced by coculture with 721.221 B-EBV cells, FHL NK cells resulted in high producers, whereas control cells were almost ineffective. This could reflect survival versus elimination of B-EBV cells (ie, the source of NK-cell stimulation) in patients versus healthy control subjects, thus mimicking the pathophysiologic scenario of FHL.     

A prospective evaluation of degranulation assays in the rapid diagnosis of familial hemophagocytic syndromes

Familial hemophagocytic lymphohistiocytosis (FHL) is a life-threatening disorder of immune regulation caused by defects in lymphocyte cytotoxicity. Rapid differentiation of primary, genetic forms from secondary forms of hemophagocytic lymphohistiocytosis (HLH) is crucial for treatment decisions. We prospectively evaluated the performance of degranulation assays based on surface up-regulation of CD107a on natural killer (NK) cells and cytotoxic T lymphocytes in a cohort of 494 patients referred for evaluation for suspected HLH. Seventy-five of 77 patients (97%) with FHL3-5 and 11 of 13 patients (85%) with Griscelli syndrome type 2 or Chediak-Higashi syndrome had abnormal resting NK-cell degranulation. In contrast, NK-cell degranulation was normal in 14 of 16 patients (88%) with X-linked lymphoproliferative disease and in 8 of 14 patients (57%) with FHL2, who were identified by diminished intracellular SLAM-associated protein (SAP), X-linked inhibitor of apoptosis protein (XIAP), and perforin expression, respectively. Among 66 patients with a clinical diagnosis of secondary HLH, 13 of 59 (22%) had abnormal resting NK-cell degranulation, whereas 0 of 43 had abnormal degranulation using IL-2-activated NK cells. Active disease or immunosuppressive therapy did not impair the assay performance. Overall, resting NK-cell degranulation below 5% provided a 96% sensitivity for a genetic degranulation disorder and a specificity of 88%. Therefore, degranulation assays allow a rapid and reliable classification of patients, benefiting treatment decisions.     

Genetic subtypes of familial hemophagocytic lymphohistiocytosis: correlations with clinical features and cytotoxic T lymphocyte/natural killer cell functions

Mutations of the perforin (PRF1) and MUNC13-4 genes distinguish 2 forms of familial hemophagocytic lymphohistiocytosis (FHL2 and FHL3, respectively), but the clinical and biologic correlates of these genotypes remain in question. We studied the presenting features and cytotoxic T lymphocyte/natural killer (CTL/NK) cell functions of 35 patients for their relationship to distinct FHL subtypes. FHL2 (n = 11) had an earlier onset than either FHL3 (n = 8) or the non-FHL2/FHL3 subtype lacking a PRF1 or MUNC13-4 mutation (n = 16). Deficient NK cell activity persisted after chemotherapy in all cases of FHL2, whereas some patients with FHL3 or the non-FHL2/FHL3 subtype showed partial recovery of this activity during remission. Alloantigen-specific CTL-mediated cytotoxicity was deficient in FHL2 patients with PRF1 nonsense mutations, was very low in FHL3 patients, but was only moderately reduced in FHL2 patients with PRF1 missense mutations. These findings correlated well with Western blot analyses showing an absence of perforin in FHL2 cases with PRF1 nonsense mutations and of MUNC13-4 in FHL3 cases, whereas in FHL2 cases with PRF1 missense mutations, mature perforin was present in low amounts. These results suggest an association between the type of genetic mutation in FHL cases and the magnitude of CTL cytolytic activity and age at onset.     

Munc18b/STXBP2 is required for platelet secretion

Platelets are vital for hemostasis because they release their granule contents in response to vascular damage. Platelet exocytosis is mediated by soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), whose interactions are governed by regulators, eg, Sec/Munc18 proteins. These proteins chaperone syntaxin t-SNAREs and are required for exocytosis. Platelets contain 3 Munc18 isoforms: Munc18a, Munc18b, and Munc18c. We report that Munc18b is the major isoform and is required for platelet secretion. Familial hemophagocytic lymphohistiocytosis type 5 (FHL5) is caused by defects in the Munc18b/STXBP2 gene. We confirm a previous report showing that platelets from FHL5 patients have defective secretion. Serotonin, ADP/ATP, and platelet factor 4 release was profoundly affected in the 2 biallelic patients and partially in a heterozygous patient. Release of lysosomal contents was only affected in the biallelic platelets. Platelets from the FHL5 biallelic patients showed decreased Munc18b and syntaxin-11 levels were significantly reduced; other syntaxins were unaffected. Munc18b formed complexes with syntaxin-11, SNAP-23, and vesicle-associated membrane protein-8 in human platelets. Other potential secretion regulators, Munc13-4 and Rab27, were also found associated. These data demonstrate a key role for Munc18b, perhaps as a limiting factor, in platelet exocytosis and suggest that it regulates syntaxin-11.     

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.     

Distinct mutations in STXBP2 are associated with variable clinical presentations in patients with familial hemophagocytic lymphohistiocytosis type 5 (FHL5)

Familial hemophagocytic lymphohistiocytosis (FHL) is a genetically determined hyperinflammatory syndrome caused by uncontrolled immune response mediated by T-lymphocytes, natural killer (NK) cells, and macrophages. STXBP2 mutations have recently been associated with FHL5. To better characterize the genetic and clinical spectrum of FHL5, we analyzed a cohort of 185 patients with suspected FHL for mutations in STXBP2. We detected biallelic mutations in 37 patients from 28 families of various ethnic origins. Missense mutations and mutations affecting 1 of the exon 15 splice sites were the predominant changes detectable in this cohort. Patients with exon 15 splice-site mutations (n = 13) developed clinical manifestations significantly later than patients with other mutations (median age, 4.1 year vs 2 months) and showed less severe impairment of degranulation and cytotoxic function of NK cells and CTLs. Patients with FHL5 showed several atypical features, including sensorineural hearing deficit, abnormal bleeding, and, most frequently, severe diarrhea that was only present in early-onset disease. In conclusion, we report the largest cohort of patients with FHL5 so far, describe an extended disease spectrum, and demonstrate for the first time a clear genotype-phenotype correlation.     

Syntaxin binding mechanism and disease-causing mutations in Munc18-2

Mutations in either syntaxin 11 (Stx11) or Munc18-2 abolish cytotoxic T lymphocytes (CTL) and natural killer cell (NK) cytotoxicity, and give rise to familial hemophagocytic lymphohistiocytosis (FHL4 or FHL5, respectively). Although Munc18-2 is known to interact with Stx11, little is known about the molecular mechanisms governing the specificity of this interaction or how in vitro IL-2 activation leads to compensation of CTL and NK cytotoxicity. To understand how mutations in Munc18-2 give rise to disease, we have solved the structure of human Munc18-2 at 2.6 Å resolution and mapped 18 point mutations. The four surface mutations identified (R39P, L130S, E132A, P334L) map exclusively to the predicted syntaxin and soluble N-ethylmaleimide–sensitive factor accessory protein receptor binding sites of Munc18-2. We find that Munc18-2 binds the N-terminal peptide of Stx11 with a ∼20-fold higher affinity than Stx3, suggesting a potential role in selective binding. Upon IL-2 activation, levels of Stx3 are increased, favoring Munc18-2 binding when Stx11 is absent. Similarly, Munc18-1, expressed in IL-2–activated CTL, is capable of binding Stx11. These findings provide potential explanations for restoration of Munc18-Stx function and cytotoxicity in IL-2–activated cells.Cytotoxic T lymphocytes (CTL) and natural killer (NK) cells perform a crucial role in host defense, destroying virally infected and tumourigenic cells. CTL or NK recognition of a target cell triggers polarization of secretory lysosomes, containing the cytotoxic proteins perforin and granzymes, toward the immunological synapse formed between the two cells. Fusion of secretory lysosomes with the CTL plasma membrane releases the cytotoxic proteins and destroys the targeted cell.Granule secretion is critical for CTL and NK cytotoxicity. Mutations that disrupt this step give rise to profound immunodeficiencies, including familial hemophagocytic lymphohistiocytosis (FHL), which is characterized by fever, cytopenia, and hepatosplenomegaly, symptoms caused by hyperactive CTL and NK (1, 2). To date, four genetic loci have been associated with FHL. Disease-causing mutations have been mapped onto the genes encoding the pore-forming protein perforin (FHL type 2), the secretory regulator Munc13-4 (FHL type 3), the soluble N-ethylmaleimide–sensitive factor accessory protein receptor (SNARE) protein syntaxin 11 (Stx11, FHL type 4), and Munc18-2 (FHL type 5) (3–7).The molecular mechanisms underlying the killing defect seen in CTL and NK from FHL2 and FHL3 patients have been relatively straightforward to understand, as loss of perforin (FHL2) or inhibition of granule secretion (FHL3) prevents target cell lysis (3, 4). However, understanding the molecular basis of FHL4 or FHL5 mutations has been complicated by the finding that cytotoxicity is often restored by in vitro activation of NK or CTL with IL-2, required for culture of these cells (5–8), limiting investigations and raising the question as to how a genetic deficiency can be overcome by in vitro culture.Munc18-2 belongs to the Sec1/Munc18-like (SM) protein family, whose members are all ∼600 residues long and are involved in regulation of SNARE-mediated membrane fusion events (9, 10). The two closest homologs of Munc18-2 are Munc18-1, which is crucial for neurotransmitter secretion in neurons (11, 12), and Munc18-3, which is more widely expressed and is involved in Glut4 translocation (13). Stx11 is a member of the syntaxin-family of SNARE proteins, comprised of an N-terminal peptide (N peptide) followed by an autonomously folded, three-helical bundle (H_ABC domain) and a single helical SNARE motif. However, although most syntaxins associate with membranes through a C-terminal transmembrane domain, Stx11 is unusual in that it contains a cysteine-rich region at its C terminus, allowing for putative palmitoylation and membrane association (14).Munc18 family proteins regulate SNARE-mediated membrane fusion by binding syntaxins and SNARE complexes, and this is reflected in their structures. All SM proteins analyzed to date adopt an arch-shaped structure formed by three distinct domains (15–19). The cavity within the arch forms the major interface for syntaxin and most likely SNARE complex binding (15, 17, 20). However, the N-terminal peptide of syntaxins extends from this cavity and binds at a spatially distinct site on Munc18 proteins, formed by an acidic groove, a basic region and a hydrophobic pocket (16, 21, 22).Munc18 family proteins have been found to regulate SNARE-mediated membrane fusion both positively and negatively. The cocrystal structure of Stx1A with rat Munc18-1 revealed a “closed” Stx1A conformation, with the SNARE helix bound back on the H_ABC domain, clasped inside the central cavity of Munc18-1 (17, 20). This structural model explains how overexpression of Munc18 proteins might impair secretion by locking syntaxins in an inactive state. Conversely, the ability of the same region in Munc18-1 to bind to fully assembled SNARE complexes explains how Munc18 proteins might also facilitate vesicle docking and catalyze membrane fusion (23–26). Indeed, the yeast homolog Sec1p only binds SNARE complexes and not monomeric syntaxins (27, 28).The functional importance of the N peptide binding site on the surface of Munc18 proteins is not entirely clear. For syntaxin binding, the N peptide is crucial for the interaction between Munc18-3 and Stx4 (29), and between Munc18-2 and Stx3 (30) but not for the interaction between Stx1A and Munc18-1 (22). It has been proposed that the N peptide initiates contact between syntaxins and Munc18 proteins and in this way may lead to high-affinity binding of the full-length syntaxin molecule (31). In polarized epithelial cells the N peptide has also been found to determine which Munc18 isoform is bound and where the syntaxin localizes (32). However, nothing is known about the role of the N peptide in the selection of syntaxin binding when two different syntaxins are both able to bind the same Munc18 protein.An essential role for the N peptide in supporting fusion in vitro has been shown in several studies (24, 33, 34). One recent study has proposed that the role of the N peptide is to facilitate the transition of Munc18-bound syntaxins from a “closed” fusion-incompetent to an “open” conformation that allows SNARE complex formation (31, 35), although a subsequent study provides an alternative hypothesis (36). Although gene complementation in Caenorhabditis elegans in vitro studies (37, 38) and studies in cultured neurons (36) support a critical functional role for the N peptide interaction with Munc18 proteins, conflicting reports exist (39, 40).Although the exact roles of Munc18-2 and Stx11 in CTL and NK are not known, it seems likely that these proteins function together because Munc18-2–deficient patients (FHL5) show decreased levels of Stx11 and the two proteins can be coprecipitated from cell lysates (5, 7, 41).Studies on FHL4 and FHL5 have been hampered by the fact that CTL and NK, which need to be cultured in IL-2, often show restored cytotoxicity (5, 8, 42), suggesting that IL-2 activation can restore Munc18-Stx function. The molecular basis for this is completely unexplored.In this study we ask how Munc18-2 and Stx11 function is linked in CTL and NK, and whether the Stx11 N peptide plays a functional role in Munc18-2 binding. We have solved the crystal structure of human Munc18-2 to 2.6 Å resolution and mapped point mutations that lead to FHL5. Our study identifies four disease-causing surface mutations in Munc18-2, all of which map to either the syntaxin or SNARE binding domains. Using biophysical techniques we reveal that the syntaxin N peptide interaction is likely to be important for the selection of Stx11 over Stx3 by Munc18-2. Furthermore, we analyzed changes in syntaxin protein levels that occur upon activation of resting NK with IL-2, and propose a molecular mechanism for the restoration of cytotoxicity in FHL4 and FHL5 upon IL-2 activation.     

Rapid diagnosis of FHL3 by flow cytometric detection of intraplatelet Munc13-4 protein

Familial hemophagocytic lymphohistiocytosis (FHL) is a potentially lethal genetic disorder of immune dysregulation that requires prompt and accurate diagnosis to initiate life-saving immunosuppressive therapy and to prepare for hematopoietic stem cell transplantation. In the present study, 85 patients with hemophagocytic lymphohistiocytosis were screened for FHL3 by Western blotting using platelets and by natural killer cell lysosomal exocytosis assay. Six of these patients were diagnosed with FHL3. In the acute disease phase requiring platelet transfusion, it was difficult to diagnose FHL3 by Western blot analysis or by lysosomal exocytosis assay. In contrast, the newly established flow cytometric analysis of intraplatelet Munc13-4 protein expression revealed bimodal populations of normal and Munc13-4-deficient platelets. These findings indicate that flow cytometric detection of intraplatelet Munc13-4 protein is a sensitive and reliable method to rapidly screen for FHL3 with a very small amount of whole blood, even in the acute phase of the disease.     

Hematopoietic stem cell transplantation with reduced intensity conditioning from a family haploidentical donor in an infant with familial hemophagocytic lymphohistocytosis

Allogeneic hematopoietic stem cell transplantation is the only curative method for patients with familial hemophagocytic lymphohistiocytosis (FHL). We present a case of a 3-month-old girl with Munc13-4 mutation (FHL3), who underwent bone marrow transplantation (BMT) from her human leukocyte antigen-haploidentical mother following reduced intensity conditioning (RIC) with fludarabine, melphalan, and busulfan. Engraftment after BMT was generally uneventful, with only mild acute graft versus host disease. Munc13-4 protein was restored following BMT, and she is well and free of disease 14 months after BMT. These results suggest that BMT with RIC from a family haploidentical donor may sufficiently restore immune regulation in infants, while lessening treatment-related mortality and long-term sequelae.     

Reduced perforin expression in systemic juvenile idiopathic arthritis is restored by autologous stem-cell transplantation

OBJECTIVES: Familial haemophagocytic lymphohistiocytosis (FHL) is a disorder characterized by deficient cytotoxic T-cell function and activated macrophages, owing to a defect in the perforin gene and absent perforin expression. Because symptoms of patients with systemic juvenile idiopathic arthritis (sJIA) are sometimes clinically very similar to those with FHL, we studied whether perforin expression in sJIA patients would be reduced also. METHODS: We determined the perforin expression levels on two subsets of CD8(+) cells (CD8(+)CD28(-)CD45RA(-) and CD8(+)CD28(-)CD45RA(+)) and natural killer (NK) cells from patients with sJIA under conventional treatment as well as before and after autologous stem-cell transplantation (ASCT). RESULTS: CD45RA(-) cytotoxic effector cells of sJIA patients (n=13) express significantly lower levels of perforin than polyarticular juvenile idiopathic arthritis (pJIA, n=9) patients [sJIA mean fluorescence intensity (MFI) 34.6; pJIA MFI 98.0] or control donors (MFI 124.6, n=5). A similar pattern was seen in the CD45RA(+) subset. Also NK cells from sJIA patients expressed significantly less intracellular perforin (sJIA MFI 398.4; controls MFI 972.4). In four patients with sJIA who were treated with ASCT, a clear increase in perforin expression was found at 12 months after ASCT in both cytotoxic effector cell subsets (CD45RA(-) subset before ASCT MFI 13.2; 12 months after ASCT MFI 172.3). CONCLUSION: We conclude that perforin expression can be severely reduced in sJIA. This finding may implicate defective cytotoxicity and haemophagocytosis and could thus explain why sJIA may be complicated by macrophage activation syndrome. ASCT leads to a reconstitution of the (T cell) immune system with a normal expression of perforin.     

Review of hemophagocytic lymphohistiocytosis (HLH) in children with focus on Japanese experiences

Hemophagocytic lymphohistiocytosis (HLH) is characterized by fever and hepatosplenomegaly associated with pancytopenia, hypertriglyceridemia and hypofibrinogenemia. Increased levels of cytokines and impaired natural killer activity are biological markers of HLH. HLH can be classified into two distinct forms, including primary HLH, also referred to as familial hemophagocytic lymphohistiocytosis (FHL), and secondary HLH. Although FHL is an autosomal recessive disorder typically occurring in infancy, it is important to clarify that the disease may also occur in older patients. It is now considered that FHL is a disorder of T-cell function; moreover, clonal proliferation of T lymphocytes is observed in a few FHL patients, and cytotoxicity of these T lymphocytes for target cells is usually impaired. In 1999, perforin gene (PRF1) mutation was identified as a cause of 20-30% of FHL (FHL2) cases. In Japan, two specific mutations of PRF1 were also detected. Furthermore, in 2003, MUNC13-4 mutations were identified in some non-FHL2 patients (FHL3). Identification of other genes responsible for remaining cases is a major concern. Hematopoietic stem cell transplantation (HSCT) has been established as the only accepted curative therapy for FHL. Thus, appropriate diagnosis and prompt treatment with HSCT are necessary for FHL patients. Genetic analysis for PRF1 and MUNC13-4 and functional assay of cytotoxic T lymphocytes are recommended to be performed in each patient. In those patients displaying impaired cytotoxic function but lacking genetic defects, samples should be employed for identification of unknown genes. In the near future, an entire pathogenesis should be clarified in order to establish appropriate therapies including immunotherapy, HSCT and gene therapy.     

2B4 inhibits NK-cell fratricide

2B4 (CD244) and its ligand, CD48, are expressed on all natural killer (NK) cells. In studies using 2B4-deficient, CD48-deficient, or wild-type NK cells with blocking antibodies, we found that in the absence of 2B4-CD48 interactions, activated murine NK cells kill each other. We also show that NK-NK fratricide in the absence of 2B4-CD48 interaction is dependent on perforin both in vitro and in vivo. 2B4 has been reported to have activating, costimulatory, and inhibitory functions on murine NK cells. 2B4-mediated inhibition of NK-cell fratricide explains some of the paradoxes of 2B4 function reported in studies of murine NK cells. We show that in the absence of 2B4 signaling, activated NK cells have defective cytotoxicity and proliferation because of fratricide and not due to the absence of a 2B4-dependent activation signal.     

Molecular analysis of the methylprednisolone-mediated inhibition of NK-cell function: evidence for different susceptibility of IL-2- versus IL-15-activated NK cells

Steroids have been shown to inhibit the function of fresh or IL-2-activated natural killer (NK) cells. Since IL-15 plays a key role in NK-cell development and function, we comparatively analyzed the effects of methylprednisolone on IL-2- or IL-15-cultured NK cells. Methylprednisolone inhibited the surface expression of the major activating receptors NKp30 and NKp44 in both conditions, whereas NK-cell proliferation and survival were sharply impaired only in IL-2-cultured NK cells. Accordingly, methylprednisolone inhibited Tyr phosphorylation of STAT1, STAT3, and STAT5 in IL-2-cultured NK cells but only marginally in IL-15-cultured NK cells, whereas JAK3 was inhibited under both conditions. Also, the NK cytotoxicity was similarly impaired in IL-2- or IL-15-cultured NK cells. This effect strictly correlated with the inhibition of ERK1/2 Tyr phosphorylation, perforin release, and cytotoxicity in a redirected killing assay against the FcRgamma(+) P815 target cells upon cross-linking of NKp46, NKG2D, or 2B4 receptors. In contrast, in the case of CD16, inhibition of ERK1/2 Tyr phosphorylation, perforin release, and cytotoxicity were not impaired. Our study suggests a different ability of IL-15-cultured NK cells to survive to steroid treatment, thus offering interesting clues for a correct NK-cell cytokine conditioning in adoptive immunotherapy.     

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.     

Mutations affecting mRNA splicing are the most common molecular defect in patients with familial hemophagocytic lymphohistiocytosis type 3

Mutations of UNC13D have been described in patients affected by familial hemophagocytic lymphohistiocytosis (FHL3). The Munc13-4 protein contributes to the priming of the secretory granules. Mutation in this gene results in defective cellular cytotoxicity and the familial hemophagocytic lymphohistiocytosis clinical picture. Among reported mutations, few are predicted to impair splicing. Yet, functional impact of these mutations has not been addressed. We identified 18 out of 31 familial hemophagocytic lymphohistiocytosis families showing at least one mutation responsible for splicing error. We identified some known and three novel splicing mutations: one falls at the acceptor site of exon 11 and 2 are deep intronic mutations in IVS1 and in IVS30. We demonstrated that these deep intronic mutations affect regulatory sequences causing aberrant splicing. We report that UNC13D mutations leading to splicing errors represent the majority of mutations observed in familial hemophagocytic lymphohistiocytosis. This finding has implications for designing strategies for analysis of the families with suspected familial hemophagocytic lymphohistiocytosis.     

Temperature sensitivity of human perforin mutants unmasks subtotal loss of cytotoxicity,delayed FHL,and a predisposition to cancer

The pore-forming protein perforin is critical for defense against many human pathogens and for preventing a catastrophic collapse of immune homeostasis, manifested in infancy as Type 2 familial hemophagocytic lymphohistiocytosis (FHL). However, no evidence has yet linked defective perforin cytotoxicity with cancer susceptibility in humans. Here, we examined perforin function in every patient reported in the literature who lived to at least 10 years of age without developing FHL despite inheriting mutations in both of their perforin (PRF1) alleles. Our analysis showed that almost 50% of these patients developed at least 1 hematological malignancy in childhood or adolescence. The broad range of pathologies argued strongly against a common environmental or viral cause for the extraordinary cancer incidence. Functionally, what distinguished these patients was their inheritance of PRF1 alleles encoding temperature-sensitive missense mutations. By contrast, truly null missense mutations with no rescue at the permissive temperature were associated with the more common severe presentation with FHL in early infancy. Our study provides the first mechanistic evidence for a link between defective perforin-mediated cytotoxicity and cancer susceptibility in humans and establishes the paradigm that temperature sensitivity of perforin function is a predictor of FHL severity.     

Atypical familial hemophagocytic lymphohistiocytosis due to mutations in UNC13D and STXBP2 overlaps with primary immunodeficiency diseases

Background

Familial hemophagocytic lymphohistiocytosis is a genetic disorder of lymphocyte cytotoxicity that usually presents in the first two years of life and has a poor prognosis unless treated by hematopoietic stem cell transplantation. Atypical courses with later onset and prolonged survival have been described, but no detailed analysis of immunological parameters associated with typical versus atypical forms of familial hemophagocytic lymphohistiocytosis has been performed.

Design and Methods

We analyzed disease manifestations, NK-cell and T-cell cytotoxicity and degranulation, markers of T-cell activation and B-cell differentiation as well as Natural Killer T cells in 8 patients with atypical familial hemophagocytic lymphohistiocytosis due to mutations in UNC13D and STXBP2.

Results

All but one patient with atypical familial hemophagocytic lymphohistiocytosis carried at least one splice-site mutation in UNC13D or STXBP2. In most patients episodes of hemophagocytic lymphohistiocytosis were preceded or followed by clinical features typically associated with immunodeficiency, such as chronic active Epstein Barr virus infection, increased susceptibility to bacterial infections, granulomatous lung or liver disease, encephalitis or lymphoma. Five of 8 patients had hypogammaglobulinemia and reduced memory B cells. Most patients had a predominance of activated CD8⁺ T cells and low numbers of Natural Killer T cells. When compared to patients with typical familial hemophagocytic lymphohistiocytosis, NK-cell cytotoxicity and NK-cell and CTL degranulation were impaired to a similar extent. However, in patients with an atypical course NK-cell degranulation could be partially reconstituted by interleukin-2 and cytotoxic T-cell cytotoxicity in vitro was normal.

Conclusions

Clinical and immunological features of atypical familial hemophagocytic lymphohistiocytosis show an important overlap to primary immunodeficiency diseases (particularly common variable immunodeficiency and X-linked lymphoproliferative syndrome) and must, therefore, be considered in a variety of clinical presentations. We show that degranulation assays are helpful screening tests for the identification of such patients.     

The PP2A inhibitor SET regulates granzyme B expression in human natural killer cells

The ability of natural killer (NK) cells to kill malignant or infected cells depends on the integration of signals from different families of cell surface receptors, including cytokine receptors. How such signals then regulate NK-cell cytotoxicity is incompletely understood. Here we analyzed an endogenous inhibitor of protein phosphatase 2A (PP2A) activity called SET, and its role in regulating human NK-cell cytotoxicity and its mechanism of action in human NK cells. RNAi-mediated suppression of SET down-modulates NK-cell cytotoxicity, whereas ectopic overexpression of SET enhances cytotoxicity. SET knockdown inhibits both mRNA and protein granzyme B expression, as well as perforin expression, whereas SET overexpression enhances granzyme B expression. Treatment of NK cells with the PP2A activator 1,9-dideoxy-forskolin also inhibits both granzyme B expression and cytotoxicity. In addition, pretreatment with the PP2A inhibitor okadaic acid rescues declining granzyme B mRNA levels in SET knockdown cells. Down-modulation of SET expression or activation of PP2A also decreases human NK-cell antibody-dependent cellular cytotoxicity. Finally, the induction of granzyme B gene expression by interleukin-2 and interleukin-15 is inhibited by SET knockdown. These data provide evidence that granzyme B gene expression and therefore human NK-cell cytotoxicity can be regulated by the PP2A-SET interplay.     

Fatal immune dysregulation due to a gain of glycosylation mutation in lymphocyte perforin

Mutations in the perforin gene (PRF1) are a common cause of the fatal immune dysregulation disorder, familial hemophagocytic lymphohistiocytosis (type 2 FHL, FHL2). Here we report a female infant born with biallelic PRF1 mutations: a novel substitution, D49N, and a previously identified in-frame deletion, K285del. We assessed the effects of each mutation on the cytotoxicity of human NK cells in which the expression of endogenous perforin was ablated with miR30-based short hairpin (sh) RNAs. Both mutations were detrimental for function, thereby explaining the clinically severe presentation and rapidly fatal outcome. We demonstrate that D49N exerts its deleterious effect by generating an additional (third) N-linked glycosylation site, resulting in protein misfolding and degradation in the killer cell. Our data provide a rationale for treating some cases of type 2 familial hemophagocytic lymphohistiocytosis, based on the pharmacologic inhibition or modification of glycosylation.