A single amino acid change, A91V, leads to conformational changes that can impair processing to the active form of perforin

Hyperinflammatory syndrome, natural killer cell function and genetic polymorphisms in the pathogenesis of severe dengue

BACKGROUND

Severe dengue, characterized by shock and organ dysfunction is driven by an excessive host immune response. We investigated the role of hyperinflammation in dengue pathogenesis.

METHODS

Patients recruited into an observational study were divided into three plasma leak severity grades. Hyperinflammatory biomarkers were measured at 4 time-points. Frequencies, activation and cytotoxic potential of NK cells were analysed by flow cytometry. RNA was extracted from sorted CD56+ NK cells and libraries prepared using SMART-Seq and sequenced using HiSeq3000 (Illumina).

RESULTS

69 patients were included (grade 0: 42, grade 1: 19, grade 2: 8 patients). Patients with grade 2 leakage had higher biomarkers than grade 0, including higher peak ferritin levels (83.3% vs 45.2%) and H scores (median 148.5 vs 105.5). NK cells from grade 2 patients exhibited decreased expression of perforin and granzyme B and activation markers. RNA sequencing revealed three SNPs in NK cell functional genes associated with more severe leakage; NK cell lectin-like receptor K1 gene (KLRK1) and PRF1 gene.

CONCLUSIONS

Features of hyperinflammation are associated with dengue severity, including higher biomarkers, impaired NK cell function and polymorphisms in genes NK cell cytolyitc function genes (KLRK1 and PRF-1). Trials of immunomodulatory therapy in these patients is now warranted.

Associations between PRF1 Ala91Val polymorphism and risk of hemophagocytic lymphohistiocytosis: a meta-analysis based on 1366 subjects

BACKGROUND

Perforin (PRF1) gene mutation can cause the onset of hemophagocytic lymphohistiocytosis (HLH). It has reported that PRF1 Ala91Val polymorphism was related with HLH risk. In the meta-analysis, we aim to evaluate the association between PRF1 Ala91Val polymorphism and HLH risk.

METHODS

We accomplished a meta-analysis of six published case-control studies including 391 patients with HLH and 975 controls. We evaluated the quality of each study through Newcastle-Ottawa Scale (NOS). Data analysis was performed with Stata software.

RESULTS

In general, all studies were of high quality (NOS score higher than 7). There were statistically significant between the PRF1 Ala91Val polymorphism and HLH risk though the pooled analysis [for Ala/Val vs. Ala/Ala: pooled odds ratio (OR) = 3.22, 95% confidence interval (CI) 1.08-9.56, P = 0.035, random model; for Ala/Val + Val/Val vs. Ala/Ala: pooled OR = 2.96, 95% CI 1.14-7.69, P = 0.025, random model]. Furthermore, sensitivity analysis also revealed a relationship between PRF1 Ala91Val polymorphism and HLH risk (for Ala/Val vs. Ala/Ala: pooled OR = 5.236, 95% CI 2.72-10.08, P < 0.000, I² = 12.1%, P_{heterogeneity} = 0.332; for Ala/Val + Val/Val vs. Ala/Ala, pooled OR = 4.856, 95% CI 2.66-8.85, P < 0.000, I² = 5.9%, P_{heterogeneity} = 0.373). Funnel plot and Egger's test did not indicate obvious published bias (P = 0.841 for Ala/Val vs. Ala/Ala; P = 0.284 for Ala/Val + Val/Val vs. Ala/Ala).

CONCLUSION

This meta-analysis indicated that PRF1 Ala91Val polymorphism affects the factor for developing HLH and future studies of PRF1 Ala91Val on the onset of HLH will be guaranteed.

Highways to hell: Mechanism-based management of cytokine storm syndromes

Since the first textbook devoted to cytokine storm syndromes (CSSs) was published in 2019, the world has changed dramatically and the term’s visibility has broadened. Herein, we define CSSs broadly to include life/organ-threatening systemic inflammation and immunopathology regardless of the context in which it occurs, recognizing that the indistinct borders of such a definition limit its utility. Nevertheless, we are focused on the pathomechanisms leading to CSSs, including impairment of granule-mediated cytotoxicity, specific viral infections, excess IL-18, and chimeric antigen receptor T-cell therapy. These mechanisms are often reflected in distinct clinical features, functional tests, and/or biomarker assessments. Moreover, these mechanisms often indicate specific, definitive treatments. This mechanism-focused organization is vital to both advancing the field and understanding the complexities in individual patients. However, increasing evidence suggests that these mechanisms interact and overlap. Likewise, the utility of a broad term such as “cytokine storm” is that it reflects a convergence on a systemic inflammatory phenotype that, regardless of cause or context, may be amenable to “inflammo-stabilization.” CSS research must improve our appreciation of its various mechanisms and their interactions and treatments, but it must also identify the signs and interventions that may broadly prevent CSS-induced immunopathology.

Prevalence and disease predisposition of p.A91V perforin in an aged population of European ancestry

In a population-based analysis including a large database restricted to patients over age 70, the authors demonstrate that the A91V polymorphism in the familial hemophagocytic lymphohistiocytosis–related gene is a nonpathological polymorphism that confers no increase in cancer, death, or immunopathology.

Macrophage Activation Syndrome Versus Hemophagocytic Lymphohistiocytosis: A Diagnostic Dilemma in a Patient With Still's Disease and Epstein-Barr Virus Viremia

Macrophage activation syndrome (MAS) and hemophagocytic lymphohistiocytosis (HLH) are two overlapping, potentially fatal syndromes classified by disorganization and malfunction of the immune system that results in wide spread inflammation and end-organ damage. We present the case of a 22-year-old female with both underlying adult-onset still’s disease and active Epstein-Barr virus (EVB) viremia who presented with criteria for MAS/HLH. She ultimately improved on an immunosuppressive regimen, and during follow-up was also found to be heterozygote carrier for a known genetic mutation that has been associated with “primary” HLH. This case thus highlights the clinical spectrum of HLH/MAS, the different treatment approaches, and the new investigations into the relationship between primary and secondary HLH.

Convergent pathways of the hyperferritinemic syndromes

Hyperferritinemia and pronounced hemophagocytosis help distinguish a subset of patients with a particularly inflammatory and deadly systemic inflammatory response syndrome. Two clinically similar disorders typify these hyperferritinemic syndromes: hemophagocytic lymphohistiocytosis (HLH) and macrophage activation syndrome (MAS). HLH is canonically associated with a complete disturbance of perforin/granzyme-mediated cytotoxicity, whereas MAS occurs in the context of the related rheumatic diseases systemic juvenile idiopathic arthritis and adult-onset Still's disease, with associated IL-1 family cytokine activation. In practice, however, there are accumulating lines of evidence for innate immune dysregulation in HLH as well as partial impairments of cytotoxicity in MAS, and these mechanisms likely represent only a fraction of the host and environmental factors driving hyperferritinemic inflammation. Herein, we present new findings that highlight the pathogenic differences between HLH and MAS, two conditions that present with life-threatening hyperinflammation, hyperferritinemia and hemophagocytosis.

Haemophagocytic lymphohistiocytosis complicating pembrolizumab treatment for metastatic breast cancer in a patient with the PRF1A91V gene polymorphism

BACKGROUND

Immune checkpoint inhibitor therapy is a modern breakthrough in medical oncology, but it can precipitate inflammatory and autoimmune adverse effects. Among the most serious of these toxicities is haemophagocytic lymphohistiocytosis (HLH), a life-threatening disorder of unbridled immune activation that results in injury to multiple organ systems.

OBJECTIVE

Description of a case of pembrolizumab-associated HLH in a patient with a proposed underlying genetic risk factor for its occurrence.

METHODS AND RESULTS

We describe a patient with aggressive metastatic breast cancer who developed HLH while undergoing experimental treatment with pembrolizumab, resulting in critical illness and multiorgan system failure. Pembrolizumab discontinuation and high-dose corticosteroids were effective in managing HLH. Subsequent next-generation sequencing of 15 genes associated with HLH revealed a germline polymorphism in perforin-1 (PRF1), PRFA91V, that may have predisposed the patient to develop HLH. The patient has had no evidence of malignancy for 2 years following recovery despite receiving no further cancer-directed treatment.

CONCLUSIONS

HLH is a rare but serious complication of immune checkpoint blockade. Patients with underlying hypomorphic alleles in PRF1 may be predisposed to develop this toxicity. Further studies are necessary to confirm a possible link between perforin gene mutations and immune checkpoint blockade-associated HLH.

Recommendations for the Use of Etoposide-Based Therapy and Bone Marrow Transplantation for the Treatment of HLH: Consensus Statements by the HLH Steering Committee of the Histiocyte Society

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome requiring aggressive immunosuppressive therapy. Following 2 large international studies mainly targeting pediatric patients with familial disease and patients without underlying chronic or malignant disease, the HLH-94 protocol is recommended as the standard of care when using etoposide-based therapy by the Histiocyte Society. However, in clinical practice, etoposide-based therapy has been widely used beyond the study inclusion criteria, including older patients and patients with underlying diseases (secondary HLH). Many questions remain around these extended indications and published reports do not address several practical issues. To tackle these concerns, the HLH Steering Committee of the Histiocyte Society decided to issue guidance for use of the HLH-94 protocol. The group convened in a structured consensus finding process to define recommendations that are based largely on expert opinion backed up by available data from the literature. The recommendations address all main elements of HLH-94 including corticosteroids, cyclosporin, etoposide, intrathecal therapy, and hematopoietic stem cell transplantation (HSCT) and consider various forms of HLH and all age groups. Aspects covered include indications, applications, dosing, side effects, duration of therapy, salvage therapy, and HSCT. These recommendations aim to provide a framework to guide treatment decisions in this severe disease.

Perforin proteostasis is regulated through its C2 domain: supra-physiological cell death mediated by T431D-perforin

The pore forming, Ca²⁺-dependent protein, perforin, is essential for the function of cytotoxic lymphocytes, which are at the frontline of immune defence against pathogens and cancer. Perforin is a glycoprotein stored in the secretory granules prior to release into the immune synapse. Congenital perforin deficiency causes fatal immune dysregulation, and is associated with various haematological malignancies. At least 50% of pathological missense mutations in perforin result in protein misfolding and retention in the endoplasmic reticulum. However, the regulation of perforin proteostasis remains unexplored. Using a variety of biochemical assays that assess protein stability and acquisition of complex glycosylation, we demonstrated that the binding of Ca²⁺ to the C2 domain stabilises perforin and regulates its export from the endoplasmic reticulum to the secretory granules. As perforin is a thermo-labile protein, we hypothesised that by altering its C2 domain it may be possible to improve protein stability. On the basis of the X-ray crystal structure of the perforin C2 domain, we designed a mutation (T431D) in the Ca²⁺ binding loop. Mutant perforin displayed markedly enhanced thermal stability and lytic function, despite its trafficking from the endoplasmic reticulum remaining unchanged. Furthermore, by introducing the T431D mutation into A90V perforin, a pathogenic mutation, which results in protein misfolding, we corrected the A90V folding defect and completely restored perforin's cytotoxic function. These results revealed an unexpected role for the Ca²⁺-dependent C2 domain in maintaining perforin proteostasis and demonstrated the possibility of designing perforin with supra-physiological cytotoxic function through stabilisation of the C2 domain.

Perforin gene variation influences survival in childhood acute lymphoblastic leukemia

Although a growing body of data links mutations in the perforin gene with increased susceptibility to hematologic malignancies, no studies discuss their influence on the clinical course of such diseases. The present study examines the impact of perforin gene variation on the clinical outcome in acute lymphoblastic leukemia (ALL) patients. The study enrolled 312 children aged 1-18 years, treated for ALL. PRF1 gene variants were analyzed through direct DNA sequencing. Variation in rs885822 was found to be associated with overall survival: patients carrying the GG genotype demonstrated a significantly increased risk of death compared to those carrying the A allele, independently of ALL risk groups (HR 3.13, 95%CI 1.16-7.8, p = 0.014). The effect was even more pronounced in high-risk ALL patients (p = 0.006). On the other hand, the presence of the rs35947132 minor A allele was slightly protective with regard to overall prognosis (p = 0.047). No differences in relapse-free survival were observed with regard to genotypes. The results of the study may imply that perforin gene variation has a role in modifying mortality in childhood ALL.

Missense mutations in the perforin (PRF1) gene as a cause of hereditary cancer predisposition

Perforin, a pore-forming toxin released from secretory granules of NK cells and CTLs, is essential for their cytotoxic activity against infected or cancerous target cells. Bi-allelic loss-of-function mutations in the perforin gene are invariably associated with a fatal immunoregulatory disorder, familial haemophagocytic lymphohistiocytosis type 2 (FHL2), in infants. More recently, it has also been recognized that partial loss of perforin function can cause disease in later life, including delayed onset FHL2 and haematological malignancies. Herein, we report a family in which a wide range of systemic inflammatory and neoplastic manifestations have occurred across three generations. We found that disease was linked to two missense perforin gene mutations (encoding A91V, R410W) that cause protein misfolding and partial loss of activity. These cases link the partial loss of perforin function with some solid tumors that are known to be controlled by the immune system, as well as haematological cancers. Our findings also demonstrate that perforin gene mutations can contribute to hereditary cancer predisposition.

Haemophagocytic lymphohistiocytosis in adults: a multicentre case series over 7 years

Haemophagocytic lymphohistiocytosis (HLH) is a syndrome of uncontrolled immune activation that has gained increasing attention over the past decade. Although classically known as a familial disorder of children caused by mutations that affect cytotoxic T-cell function, an acquired form of HLH in adults is now widely recognized. This is often seen in the setting of malignancy, infection or rheumatological disorders. We performed a retrospective review across 3 tertiary care centres and identified 68 adults with HLH. The average age was 53 years (range 18-77 years) and 43 were male (63%). Underlying disorders included malignancy in 33 patients (49%), infection in 22 (33%), autoimmune disease in 19 (28%) and idiopathic HLH in 15 (22%). Patients were treated with disease-specific therapy and immunomodulatory agents. After a median follow-up of 32·2 months, 46 patients had died (69%). The median overall survival was 4 months (95% CI: 0·0-10·2 months). Patients with malignancy had a worse prognosis compared to those without (median survival 2·8 months versus 10·7 months, P = 0·007). HLH is a devastating disorder with a high mortality. Further research is needed to improve treatment and outcomes.

Genetic predisposition to hemophagocytic lymphohistiocytosis: Report on 500 patients from the Italian registry

BACKGROUND

Hemophagocytic lymphohistiocytosis (HLH) is a rare life-threatening disease affecting mostly children but also adults and characterized by hyperinflammatory features. A subset of patients, referred to as having familial hemophagocytic lymphohistiocytosis (FHL), have various underlying genetic abnormalities, the frequencies of which have not been systematically determined previously.

OBJECTIVE

This work aims to further our understanding of the pathogenic bases of this rare condition based on an analysis of our 25 years of experience.

METHODS

From our registry, we have analyzed a total of 500 unselected patients with HLH.

RESULTS

Biallelic pathogenic mutations defining FHL were found in 171 (34%) patients; the proportion of FHL was much higher (64%) in patients given a diagnosis during the first year of life. Taken together, mutations of the genes PRF1 (FHL2) and UNC13D (FHL3) accounted for 70% of cases of FHL. Overall, a genetic diagnosis was possible in more than 90% of our patients with FHL. Perforin expression and the extent of degranulation have been more useful for diagnosing FHL than hemophagocytosis and the cytotoxicity assay. Of 281 (56%) patients classified as having "sporadic" HLH, 43 had monoallelic mutations in one of the FHL-defining genes. Given this gene dosage effect, FHL is not strictly recessive.

CONCLUSION

We suggest that the clinical syndrome HLH generally results from the combined effects of an exogenous trigger and genetic predisposition. Within this combination, different weights of exogenous and genetic factors account for the wide disease spectrum that ranges from HLH secondary to severe infection to FHL.

Late-onset severe chronic active EBV in a patient for five years with mutations in STXBP2 (MUNC18-2) and PRF1 (perforin 1)

Severe chronic active Epstein-Barr virus (CAEBV) disease is defined as a severe progressive illness lasting 6 months or longer with infiltration of tissues with EBV-positive lymphocytes, markedly elevated levels of EBV DNA in the blood, and no known immunodeficiency such as HIV. These patients usually have fever, splenomegaly, lymphadenopathy, and may have markedly elevated EBV antibody titers to viral capsid antigen. Although the cause of most cases of severe CAEBV is unknown, one well-documented case was associated with compound heterozygous mutations in PRF1 (perforin 1). Here we report a patient with prolonged severe CAEBV who underwent bone marrow transplant for his disease and subsequently was found to have compound heterozygous mutations in STXBP2 (MUNC18-2) as well as a heterozygous mutation in PRF1 (perforin 1).

How I treat hemophagocytic lymphohistiocytosis in the adult patient

Hemophagocytic lymphohistiocytosis (HLH) is a devastating disorder of uncontrolled immune activation characterized by clinical and laboratory evidence of extreme inflammation. This syndrome can be caused by genetic mutations affecting cytotoxic function (familial HLH) or be secondary to infectious, rheumatologic, malignant, or metabolic conditions (acquired HLH). Prompt recognition is paramount and, without early treatment, this disorder is frequently fatal. Although HLH is well described in the pediatric population, less is known about the appropriate work-up and treatment in adults. Here, we review the clinical characteristics, diagnosis, and treatment of HLH in adults.

Variations of the perforin gene in patients with chronic inflammatory demyelinating polyradiculoneuropathy

Perforin (PRF) has a key role in the function of cytotoxic T and natural killer cells. Rare variations of PRF1 predispose to autoimmunity. Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is an autoimmune disease of the peripheral nervous system, involving defective lymphocyte apoptosis. The aim of this study was to investigate the role of PRF1 in CIDP. The entire coding region of PRF1 was sequenced in 94 patients and 158 controls. We found three missense variations leading to amino acid substitutions and one nonsense variation resulting in a premature stop codon. All variations would decrease PRF activity. Their overall frequency was significantly higher in patients than in controls (odds ratio (OR)=4.47). The most frequent variation was p.Ala91Val (OR=3.92) previously associated with other autoimmune diseases. Clinical analysis showed that PRF1 variations were more frequent in relapsing patients and in patients displaying axonal damage. These data suggest that PRF1 variations may influence CIDP development and course.

Patients with Griscelli syndrome and normal pigmentation identify RAB27A mutations that selectively disrupt MUNC13-4 binding

BACKGROUND

Familial hemophagocytic lymphohistiocytosis (FHL) is a rare and often fatal disorder characterized by defective cellular cytotoxicity and hyperinflammation, and the only cure known to date is hematopoietic stem cell transplantation. Mutations in RAB27A, LYST, and AP3B1 give rise to FHL associated with oculocutaneous albinism, and patients with FHL are usually only screened for mutations in these genes when albinism is observed. A number of patients with FHL and normal pigmentation remain without a genetic diagnosis.

OBJECTIVE

We asked whether patients with FHL with immunodeficiency but with normal pigmentation might sometimes have mutations that affected cellular cytotoxicity without affecting pigmentation.

METHODS

We carried out mutation analysis of RAB27A, LYST, and AP3B1 in patients with FHL with pigment dilution, as well as a cohort with no clinical evidence of pigment dilution but no mutations in the other known FHL-related genes (PRF1, STXBP2, and UNC13D).

RESULTS

We identify patients with Griscelli syndrome type 2 with biallelic mutations in RAB27A in the absence of albinism. All 6 patients carried mutations at amino acids R141, Y159, or S163 of Rab27a that disrupt the interaction of Rab27a with Munc13-4, without impairing the interaction between melanophilin and Rab27a.

CONCLUSION

These studies highlight the need for RAB27A sequencing in patients with FHL with normal pigmentation and identify a critical binding site for Munc13-4 on Rab27a, revealing the molecular basis of this interaction.

Familial hemophagocytic lymphohistiocytosis: when rare diseases shed light on immune system functioning

The human immune system depends on the activity of cytotoxic T lymphocytes (CTL), natural killer (NK) cells, and NKT cells in order to fight off a viral infection. Understanding the molecular mechanisms during this process and the role of individual proteins was greatly improved by the study of familial hemophagocytic lymphohistiocytosis (FHL). Since 1999, genetic sequencing is the gold standard to classify patients into different subgroups of FHL. The diagnosis, once based on a clinical constellation of abnormalities, is now strongly supported by the results of a functional flow-cytometry screening, which directs the genetic study. A few additional congenital immune deficiencies can also cause a resembling or even identical clinical picture to FHL. As in many other rare human disorders, the collection and analysis of a relatively large number of cases in registries is crucial to draw a complete picture of the disease. The conduction of prospective therapeutic trials allows investigators to increase the awareness of the disease and to speed up the diagnostic process, but also provides important functional and genetic confirmations. Children with confirmed diagnosis may undergo hematopoietic stem cell transplantation, which is the only cure known to date. Moreover, detailed characterization of these rare patients helped to understand the function of individual proteins within the exocytic machinery of CTL, NK, and NKT cells. Moreover, identification of these genotypes also provides valuable information on variant phenotypes, other than FHL, associated with biallelic and monoallelic mutations in the FHL-related genes. In this review, we describe how detailed characterization of patients with genetic hemophagocytic lymphohistiocytosis has resulted in improvement in knowledge regarding contribution of individual proteins to the functional machinery of cytotoxic T- and NK-cells. The review also details how identification of these genotypes has provided valuable information on variant phenotypes.

An A91V SNP in the perforin gene is frequently found in NK/T-cell lymphomas

NK/T-cell lymphoma (NKTCL) is the most frequent EBV-related NK/T-cell disease. Its clinical manifestations overlap with those of familial haemophagocytic lymphohistiocytosis (FHLH). Since PERFORIN (PRF1) mutations are present in FHLH, we analysed its role in a series of 12 nasal and 12 extranasal-NKTCLs. 12.5% of the tumours and 25% of the nasal-origin cases had the well-known g.272C>T(p.Ala91Val) pathogenic SNP, which confers a poor prognosis. Two of these cases had a double-CD4/CD8-positive immunophenotype, although no correlation was found with perforin protein expression. p53 was overexpressed in 20% of the tumoral samples, 80% of which were of extranasal origin, while none showed PRF1 SNVs. These results suggest that nasal and extranasal NKTCLs have different biological backgrounds, although this requires validation.

Human immunodeficiency syndromes affecting human natural killer cell cytolytic activity

Natural killer (NK) cells are lymphocytes of the innate immune system that secrete cytokines upon activation and mediate the killing of tumor cells and virus-infected cells, especially those that escape the adaptive T cell response caused by the down regulation of MHC-I. The induction of cytotoxicity requires that NK cells contact target cells through adhesion receptors, and initiate activation signaling leading to increased adhesion and accumulation of F-actin at the NK cell cytotoxic synapse. Concurrently, lytic granules undergo minus-end directed movement and accumulate at the microtubule-organizing center through the interaction with microtubule motor proteins, followed by polarization of the lethal cargo toward the target cell. Ultimately, myosin-dependent movement of the lytic granules toward the NK cell plasma membrane through F-actin channels, along with soluble N-ethylmaleimide-sensitive factor attachment protein receptor-dependent fusion, promotes the release of the lytic granule contents into the cleft between the NK cell and target cell resulting in target cell killing. Herein, we will discuss several disease-causing mutations in primary immunodeficiency syndromes and how they impact NK cell-mediated killing by disrupting distinct steps of this tightly regulated process.

Perforin: a key pore-forming protein for immune control of viruses and cancer

Perforin (PFN) is the key pore-forming molecule in the cytotoxic granules of immune killer cells. Expressed only in killer cells, PFN is the rate-limiting molecule for cytotoxic function, delivering the death-inducing granule serine proteases (granzymes) into target cells marked for immune elimination. In this chapter we describe our current understanding of how PFN accomplishes this task. We discuss where PFN is expressed and how its expression is regulated, the biogenesis and storage of PFN in killer cells and how they are protected from potential damage, how it is released, how it delivers Granzymes into target cells and the consequences of PFN deficiency.

Perforin and human diseases

Natural killer (NK) cells and cytotoxic T lymphocytes (CTL) use a highly toxic pore-forming protein perforin (PFN) to destroy cells infected with intracellular pathogens and cells with pre-cancerous transformations. However, mutations of PFN and defects in its expression can cause an abnormal function of the immune system and difficulties in elimination of altered cells. As discussed in this chapter, deficiency of PFN due to the mutations of its gene, PFN1, can be associated with malignancies and severe immune disorders such as familial hemophagocytic lymphohistiocytosis (FHL) and macrophage activation syndrome. On the other hand, overactivity of PFN can turn the immune system against autologous cells resulting in other diseases such as systemic lupus erythematosus, polymyositis, rheumatoid arthritis and cutaneous inflammation. PFN also has a crucial role in the cellular rejection of solid organ allografts and destruction of pancreatic β-cells resulting in type 1 diabetes. These facts highlight the importance of understanding the biochemical characteristics of PFN.

Perforinopathy: a spectrum of human immune disease caused by defective perforin delivery or function

Congenital perforin deficiency is considered a rare cause of human immunopathology and immune dysregulation, and classically presents as a fatal illness early in infancy. However, we propose that a group of related disorders in which killer lymphocytes deliver only partially active perforin or a reduced quantum of wild-type perforin to the immune synapse should be considered part of an extended syndrome with overlapping but more variable clinical features. Apart from the many rare mutations scattered over the coding sequences, up to 10% of Caucasians carry the severely hypomorphic PRF1 allele C272 > T (leading to A91V mutation) and the overall prevalence of the homozygous state for A91V is around 1 in 600 individuals. We therefore postulate that the partial loss of perforin function and its clinical consequences may be more common then currently suspected. An acute clinical presentation is infrequent in A91V heterozygous individuals, but we postulate that the partial loss of perforin function may potentially be manifested in childhood or early adulthood as “idiopathic” inflammatory disease, or through increased cancer susceptibility – either hematological malignancy or multiple, independent primary cancers. We suggest the new term “perforinopathy” to signify the common functional endpoints of all the known consequences of perforin deficiency and failure to deliver fully functional perforin.

Structural and functional analysis of perforin mutations in association with clinical data of familial hemophagocytic lymphohistiocytosis type 2 (FHL2) patients

Perforin plays a key role in the immune system via pore formation at the target cell membrane in the elimination of virus-infected and transformed cells. A vast number of observed mutations in perforin impair this mechanism resulting in a rare but fatal disease, familial hemophagocytic lymphohistiocytosis type 2 (FHL2). Here we report a comprehensive in silico structural analysis of a collection of 76 missense perforin mutations based on a proposed pore model. In our model, perforin monomers oligomerize having cyclic symmetry in consistent with previously found experimental constraints yet having flexibility in the size of the pore and the number of monomers involved. Clusters of the mutations on the model map to three distinct functional regions of the perforin. Calculated stability (free energy) changes show that the mutations mainly destabilize the protein structure, interestingly however, A91V polymorphism, leads to a more stable one. Structural characteristics of mutations help explain the severe functional consequences on perforin deficient patients. Our study provides a structural approach to the mutation effects on the perforin oligomerization and impaired cytotoxic function in FHL2 patients.

Diagnostic evaluation of patients with suspected haemophagocytic lymphohistiocytosis

Haemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome characterized by severely disturbed immune homeostasis. It can affect all age groups. Diagnostic evaluation of the patient with suspected HLH has to address three main questions: (i) does the patient have HLH? There is no simple diagnostic test, but a number of clinical and laboratory criteria define this clinical syndrome. (ii) Can a trigger be identified? A variety of infections, malignant or autoimmune diseases can contribute to the disturbed immune homeostasis with important consequences for treatment. (iii) Does the patient suffer from a genetic disease predisposing to HLH? Recent advances in the understanding of the genetic and pathophysiological basis of HLH have enabled a better and more rapid answer to this question, which is relevant for prognosis and the decision to perform haematopoietic stem cell transplantation. This review summarizes the current diagnostic approach to the patient with HLH.

Familial hemophagocytic lymphohistiocytosis may present during adulthood: clinical and genetic features of a small series

Familial Hemophagocytic lymphohistiocytosis (FHL) is a rare immune deficiency with defective cytotoxic function. The age at onset is usually young and the natural course is rapidly fatal if untreated. A later onset of the disease has been sporadically reported even in adolescents and adults. We report the results of our retrospective data collection of all cases diagnosed with FHL at an age of 18 years or older and enrolled in the Italian Registry of HLH. All cases were diagnosed with FHL based on evidence of genetic defect in one FHL-related gene. A total of 11 patients were diagnosed with FHL. They were 9 males and 2 females, from 10 unrelated families; their age ranged between 18 and 43 years (median, 23 years). Family history was unremarkable in eight families at the time of the diagnosis. Their genetic diagnoses are: FHL2 (n = 6), FHL3 (n = 2), FHL5 (n = 1), XLP1 (n = 2). Clinical, molecular and functional data are described. These data confirm that FHL may present beyond the pediatric age and up to the fifth decade. FHL2 due to perforin defect is the most frequently reported subtype. Adult specialists should consider FHL in the differential diagnosis of patients with cytopenia and liver or central nervous system disorders, especially when a lymphoproliferative disease is suspected but eventually not confirmed. FHL may turn to be fatal within a short time course even in adults. This risk, together with the continuous improvement in the transplant technique, especially in the area of transplant from matched unrelated donor, resulting in reduced treatment related mortality, might suggest a wider use of SCT in this population. Current diagnostic approach allows prompt identification of patients by flow-cytometry screening, then confirmed by the genetic study, and treatment with chemo-immunotherapy followed by stem cell transplantation.

Protecting a serial killer: pathways for perforin trafficking and self-defence ensure sequential target cell death

Considerable progress has been made in understanding how cytotoxic lymphocytes use the highly toxic pore-forming protein perforin to eliminate dangerous cells, while remaining refractory to lysis. At least two mechanisms jointly preserve the killer cell: the C-terminal residues of perforin dictate its rapid export from the endoplasmic reticulum (ER), whose milieu otherwise favours pore formation; perforin is then stored in secretory granules whose acidity prevent its oligomerisation. Following exocytosis, perforin delivers the proapoptotic protease, granzyme B, into the target cell by disrupting its plasma membrane. Although the precise mechanism of perforin/granzyme synergy remains controversial, the recently defined crystal structure of the perforin monomer and cryo-electron microscopy (EM) of the entire pore suggest that passive transmembrane granzyme diffusion is the dominant proapoptotic mechanism.

Analyses of the PRF1 gene in individuals with hemophagocytic lymphohystiocytosis reveal the common haplotype R54C/A91V in Colombian unrelated families associated with late onset disease

Familial hemophagocytic lymphohistiocytosis (FHL), is a rare autosomal recessive disorder characterized by an impairment of cytotoxic cells and uncontrolled activation of macrophages. This study presents the first description of four patients with FHL type 2 in Latin America. Patient 1 fulfilled the disease diagnostic criteria since 2 months of age, whereas patients 2, 3 and 4 exhibited the typical manifestations of the disease only later in their childhood. The PRF1 genetic analysis in these patients revealed two previously reported mutations: L17fsx50 and R54C. Interestingly, seven out of the 8 alleles evaluated here in patients carried the haplotype R54C/A91V, suggesting that this is a highly frequent FHL type 2 allele in Colombia. This haplotype confers residual cytotoxic function leading to late onset disease. Therefore, this report highlights the remarkable complexity of FHL diagnostic, emphasizing the importance of the genetic characterization of the disease.

Inherited defects causing hemophagocytic lymphohistiocytic syndrome

Hemophagocytic lymphohistiocytosis (HLH) manifests as the uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. Molecular studies of individuals with HLH have demonstrated in most of these conditions a critical role of granule-dependent cytotoxic activity in the regulation of lymphocyte homeostasis, and have allowed the characterization of key effectors regulating cytotoxic granule release. The cytolytic process may now be considered a multistep process, including cell activation; the polarization of cytotoxic granules toward the conjugated target cell; the tethering, priming, and fusion of the cytotoxic granules with the plasma membrane; and the release of their contents (perforin and granzymes) into the intercellular cleft, leading to target cell death. Cytolytic cells have a second effector function involving the production of cytokines, principally γ-interferon, which is secreted independently of the exocytosis cytotoxic granule pathway. An analysis of the mechanisms underlying HLH has identified γ-interferon as a key cytokine inducing uncontrolled macrophage activation, and thus represents a potential therapeutic target.

Familial haemophagocytic lymphohistiocytosis: advances in the genetic basis, diagnosis and management

Familial haemophagocytic lymphohistiocytosis (FHL) is a rare autosomal recessive disorder of immune dysregulation associated with uncontrolled T cell and macrophage activation and hypercytokinaemia. The incidence of FHL is 0·12/100·000 children born per year, with a male to female ratio of 1:1. The disease is classified into six different types based on genetic linkage analysis and chromosomal localization; five specific genetic defects have been identified, which account for approximately 90% of all patients. Type 1 is due to an as yet unidentified gene defect located on chromosome nine. Type 2 is caused by mutations in the perforin (PRF1) gene, type 3 by mutations in the Munc-13-4 (UNC13D) gene, type 4 by mutations in the syntaxin 11 (STX11) gene and the recently described type 5 due to mutations in the gene encoding syntaxin binding protein 2 (STXBP-2). The incidence of the five types varies in different ethnic groups. The most common presenting features are pyrexia of unknown origin, pronounced hepatosplenomegaly and cytopenias. Neurological features tend to present later and are associated with poor prognosis. Absent or decreased lymphocyte cytotoxicity is the cellular hallmark of FHL. Biochemical features such as hyperferritinaemia, hypertriglyceridaemia and hypofibrinogenaemia are usually present, along with high levels of soluble interleukin 2 receptor in the blood and cerebrospinal fluid. Bone marrow aspirate may demonstrate the characteristic haemophagocytes, but initially is non-diagnostic in two-thirds of patients. Established international clinical, haematological and biochemical criteria now facilitate accurate clinical diagnosis. The disease is fatal unless a haematopoietic stem cell transplant (HSCT) is performed. The introduction of HSCT has dramatically improved the prognosis of the disease. However, the mortality of the disease is still significantly high and a number of challenges remain to be addressed. Active disease at the time of the transplant is the major significant poor prognostic factor. Delayed diagnosis, after irreversible organ damage has occurred, especially neurological damage, disease reoccurrence and pre-transplant mortality, remain a concern.

Characterization of ovine Toll-like receptor 9 protein coding region, comparative analysis, detection of mutations and maedi visna infection

One of the major roles of innate immunity system is the recognition and the determination of the nature of the antigen. This ability is encompassed by specific receptors as Toll-like receptors (TLRs). TLR9 recognizes bacterial and viral CpG motifs, while their potent immunostimulation effect seems to be promising for lentiviral therapies. Recent studies, however, show the presence of a big polymorphism within the TLR genes and the linkage between substitutions and susceptibility to various infections. Moreover, different recognition ability seems to be utilized by different species and possibly breeds. In this study, we characterized the protein coding region of ovine TLR9 gene. By using comparative analysis of two closely related species and humans, we suggest, which characteristics of protein could be responsible for altered recognition. Furthermore, analyzing the presence of the substitutions, we show the intraspecies polymorphism and its possible implications, while attempting to define the association of discovered substitutions with the maedi visna infection.

Inherited defects in lymphocyte cytotoxic activity

The granule-dependent cytotoxic activity of lymphocytes plays a critical role in the defense against virally infected cells and tumor cells. The importance of this cytotoxic pathway in immune regulation is evidenced by the severe and often fatal condition, known as hemophagocytic lymphohistiocytic syndrome (HLH) that occurs in mice and humans with genetically determined impaired lymphocyte cytotoxic function. HLH manifests as the occurrence of uncontrolled activation of T lymphocytes and macrophages infiltrating multiple organs. In this review, we focus on recent advances in the characterization of effectors regulating the release of cytotoxic granules, and on the role of this cytotoxic pathway in lymphocyte homeostasis and immune surveillance. Analysis of the mechanisms leading to the occurrence of hemophagocytic syndrome designates gamma-interferon as an attractive therapeutic target to downregulate uncontrolled macrophage activation, which sustains clinical and biological features of HLH.

Perforin: structure, function, and role in human immunopathology

The secretory granule-mediated cell death pathway is the key mechanism for elimination of virus-infected and transformed target cells by cytotoxic lymphocytes. The formation of the immunological synapse between an effector and a target cell leads to exocytic trafficking of the secretory granules and the release of their contents, which include pro-apoptotic serine proteases, granzymes, and pore-forming perforin into the synapse. There, perforin polymerizes and forms a transmembrane pore that allows the delivery of granzymes into the cytosol, where they initiate various apoptotic death pathways. Unlike relatively redundant individual granzymes, functional perforin is absolutely essential for cytotoxic lymphocyte function and immune regulation in the host. Nevertheless, perforin is still the least studied and understood cytotoxic molecule in the immune system. In this review, we discuss the current state of affairs in the perforin field: the protein's structure and function as well as its role in immune-mediated diseases.

Mutations of perforin gene in Chinese patients with acute lymphoblastic leukemia

To address whether mutations and single nucleotide polymorphisms (SNPs) in perforin gene (PRF1) are correlated with acute lymphoblastic leukemia (ALL) in Chinese, we screened mutations in codon region of PRF1 in 111 ALL patients, and correlated the results with patients' immunophenotype, karyotype and fusion genes. Four novel monoallelic missense and two novel monoallelic synonymous mutations (G198R, R225Q, D486G, R509K, S388S and Q540Q) were identified in 9 B-ALL, of whom 7 cases carried BCR-ABL gene, one carried MLL-AF4 fusion gene, and one lost two chromosomes. Our results suggest that mutations in PRF1 may play a role in the pathogenesis of B-ALL.

Perforin deficiency and susceptibility to cancer

Cytotoxic lymphocytes (CLs) are the killer cells that destroy intracellular pathogen-infected and transformed cells, predominantly through the cytotoxic granule-mediated death pathway. Soluble cytotoxic granule components, including pore-forming perforin and pro-apoptotic serine proteases, granzymes, synergize to induce unscheduled apoptosis of the target cell. A complete loss of CL function results in an aggressive immunoregulatory disorder, familial hemophagocytic lymphohistiocytosis, whereas a partial loss of function seems to be a factor strongly predisposing to hematological malignancies. This review discusses the pathological manifestations of CL deficiencies due to impaired perforin function and describes novel aspects of perforin biology.

Mutations in the perforin gene can be linked to macrophage activation syndrome in patients with systemic onset juvenile idiopathic arthritis

OBJECTIVE

Macrophage activation syndrome (MAS) in systemic onset juvenile idiopathic arthritis (SoJIA) is considered to be an acquired form of familial haemophagocytic lymphohistiocytosis (fHLH). FHLH is an autosomal recessive disorder, characterized by diminished NK cell function and caused by mutations in the perforin gene (PRF1) in 20-50% of patients. Interestingly, SoJIA patients display decreased levels of perforin in NK cells and diminished NK cell function as well. Here, we analysed PRF1 and its putative promoter in SoJIA patients with or without a history of MAS.

METHODS

DNA of 56 SoJIA patients (41 Italian and 15 Dutch) was isolated. Of these, 15 (27%) had a confirmed history of MAS. We sequenced PRF1 and 1.5 kb of the 5'-upstream region. DNA sequence variations in the promoter region were functionally tested in transfection experiments using a human NK cell line.

RESULTS

We detected a previously undescribed sequence variation (-499 C > T) in the promoter of PRF1 in 18% of the SoJIA patients. However, transfection experiments did not show functional implications of this variation. Secondly, we found that 11 of 56 (20%) SoJIA patients were heterozygous for missense mutations in PRF1. In particular, we found a high prevalence of the Ala91Val mutation, a variant known to result in defective function of perforin. Interestingly, the prevalence of Ala91Val in SoJIA patients with a history of MAS (20%) was increased compared with SoJIA patients without MAS (9.8%). One SoJIA patient, heterozygous for Ala91Val, showed profound decreased perforin levels at the time of MAS.

CONCLUSIONS

These findings suggest that PRF1 mutations play a role in the development of MAS in SoJIA patients.

Proteolytic activation of the cytotoxic phenotype during human NK cell development

NK cells induce apoptosis in target cells via the perforin-mediated delivery of granzyme molecules. Cytotoxic human NK cells can be generated by IL-15-mediated differentiation of CD34(+) cells in vitro and these cultures have been used extensively to analyze the development of the NK cell surface phenotype. We have used NK cell differentiation in vitro together with protease-deficient human NK cells to analyze the acquisition of the cytotoxic phenotype. Granzymes are synthesized as inactive zymogens and are proteolytically activated by the cysteine protease cathepsin C. Cathepsin C is also synthesized as a zymogen and activated by proteolysis. We show that human NK cells generated in vitro undergo granule exocytosis and induce the caspase cascade in target cells. IL-15 and stem cell factor (IL-15 plus SCF) induced the expression of the granzyme B and perforin genes and the activation of cathepsin C and granzyme B zymogens. Perforin activation is also mediated by a cysteine protease and IL-15 plus SCF-mediated differentiation was accompanied by perforin processing. However, cathepsin C-deficient human NK cells revealed that perforin processing could occur in the absence of cathepsin C activity. The combination of IL-15 plus SCF is therefore sufficient to coordinate the development of the NK cell surface phenotype with the expression and proteolytic activation of the cytotoxic machinery, reflecting the central role of IL-15 in NK cell development.

Variations of the perforin gene in patients with multiple sclerosis

Perforin is involved in cell-mediated cytotoxicity and mutations of its gene (PRF1) cause familial hemophagocytic lymphohistiocytosis (FLH2). PRF1 sequencing in 190 patients with multiple sclerosis and 268 controls detected two FLH2-associated variations (A91V, N252S) in both groups and six novel mutations (C999T, G1065A, G1428A, A1620G, G719A, C1069T) in patients. All together, carriers of these variations were more frequent in patients than in controls (phenotype frequency: 17 vs 9%, P=0.0166; odds ratio (OR)=2.06, 95% confidence interval (CI): 1.13-3.77). Although A91V was the most frequent variation and displayed a trend of association with multiple sclerosis (MS) in the first population of patients and controls (frequency of the 91V allele: 0.076 vs 0.043, P=0.044), we used it as a marker to confirm PRF1 involvement in MS and assessed its frequency in a second population of 966 patients and 1520 controls. Frequency of the 91V allele was significantly higher in patients than in controls also in the second population (0.075 vs 0.058%, P=0.019). In the combined cohorts of 1156 patients and 1788 controls, presence of the 91V allele in single or double dose conferred an OR=1.38 (95% CI=1.10-1.74). These data suggest that A91V and possibly other perforin variations indicate susceptibility to MS.

Variations of the perforin gene in patients with type 1 diabetes

OBJECTIVE

Perforin plays a key role in cell-mediated cytotoxicity. Mutations of its gene, PRF1, cause familial hemophagocytic lymphohistiocytosis but have also been associated with lymphomas and the autoimmune/lymphoproliferative syndrome. The aim of this work was to investigate the role of PRF1 variations in type 1 diabetes.

RESEARCH DESIGN AND METHODS

We typed for the N252S and A91V variations in an initial population of 352 type 1 diabetic patients and 816 control subjects and a second population of 365 patients and 964 control subjects. Moreover, we sequenced the coding sequence and intron-exons boundaries in 200 patients and 300 control subjects.

RESULTS

In both cohorts, allelic frequency of N252S was significantly higher in patients than in control subjects (combined cohorts: 1.5 vs. 0.4%; odds ratio 6.68 [95% CI 1.83-7.48]). Sequencing of the entire coding region detected one novel mutation in one patient, causing a P477A amino acid change not detected in 199 patients and 300 control subjects. Typing for HLA-DQA1 and DQB1 alleles showed that type 1 diabetes-predisposing DQ alpha/DQ beta heterodimers were less frequent in patients carrying N252S or P477A than in those carrying wild-type PRF1. We previously found that natural killer (NK) activity is not decreased in most N252S heterozygotes, but we detected one whose NK activity was normal at the age of 12 but strikingly low in early childhood. Here, we discovered that NK function was low in three heterozygotes in early childhood, one homozygous adult, and in the subject carrying P477A.

CONCLUSIONS

These data suggest that N252S and possibly other PRF1 variations are susceptibility factors for type 1 diabetes development.

Precursor B-cell acute lymphoblastic leukemia presenting with hemophagocytic lymphohistiocytosis

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory syndrome which can be an inherited congenital disorder or can develop secondary to malignancy, infection, or autoimmune disease. Secondary HLH due to malignancy occurs most commonly with T or NK-cell lymphoid neoplasms. HLH with B-cell malignancies is less common and HLH has rarely been described in association with precursor B-cell acute lymphoblastic leukemia (B-ALL). We report three cases of HLH associated with B-ALL and review 17 cases of ALL-associated HLH previously reported in the literature.

A common fold mediates vertebrate defense and bacterial attack

Proteins containing membrane attack complex/perforin (MACPF) domains play important roles in vertebrate immunity, embryonic development, and neural-cell migration. In vertebrates, the ninth component of complement and perforin form oligomeric pores that lyse bacteria and kill virus-infected cells, respectively. However, the mechanism of MACPF function is unknown. We determined the crystal structure of a bacterial MACPF protein, Plu-MACPF from Photorhabdus luminescens, to 2.0 angstrom resolution. The MACPF domain reveals structural similarity with poreforming cholesterol-dependent cytolysins (CDCs) from Gram-positive bacteria. This suggests that lytic MACPF proteins may use a CDC-like mechanism to form pores and disrupt cell membranes. Sequence similarity between bacterial and vertebrate MACPF domains suggests that the fold of the CDCs, a family of proteins important for bacterial pathogenesis, is probably used by vertebrates for defense against infection.