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Abstract: Current hemophagocytic lymphohistiocytosis 2004 (HLH-2004)-based diagnostic criteria for familial hemophagocytic lymphohistiocytosis (FHL) are based on expert opinion. Here, we performed a case-control study to test and possibly improve these criteria. We also developed 2 complementary expert opinion-based diagnostic strategies for FHL in patients with signs/symptoms suggestive of HLH, based on genetic and cellular cytotoxicity assays. The cases (N = 366) were children aged <16 years with verified familial and/or genetic FHL (n = 341) or Griscelli syndrome type 2 (n = 25); 276 from the HLH-94/HLH-2004 databases and 90 from the Italian HLH Registry. All fulfilled the HLH-94/HLH-2004 patient inclusion criteria. Controls were 374 children with systemic-onset juvenile idiopathic arthritis (sJIA) and 329 + 361 children in 2 cohorts with febrile infections that could be confused with HLH and sepsis, respectively. To provide complete data sets, multiple imputations were performed. The optimal model, based on 17 variables studied, revealed almost similar diagnostic thresholds as the existing criteria, with accuracy 99.1% (sensitivity 97.1%; specificity 99.5%); the original HLH-2004 criteria had accuracy 97.4% (sensitivity 99.0%; specificity 97.1%). Because cellular cytotoxicity assays here constitute a separate diagnostic strategy, HLH-2004 criteria without natural killer (NK)-cell function was also studied, which showed accuracy 99.0% (sensitivity, 96.2%; specificity, 99.5%). Thus, we conclude that the HLH-2004 criteria (without NK-cell function) have significant validity in their current form when tested against severe infections or sJIA. It is important to exclude underlying malignancies and atypical infections. In addition, complementary cellular and genetic diagnostic guidelines can facilitate necessary confirmation of clinical diagnosis., (© 2024 American Society of Hematology. Published by Elsevier Inc. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

Background: Homozygous or compound heterozygous mutations in JAGN1 cause severe congenital neutropenia. JAGN1-mutant patients present with severe early-onset bacterial infections and most have been described as low-responders to recombinant granulocyte colony-stimulating factor (G-CSF) therapy. In a murine, hematopoietic JAGN1 knockout model, which displays susceptibility to Candida albicans infection in the absence of neutropenia, treatment with granulocyte-macrophage-CSF (GM-CSF) was able to restore the functional defect of neutrophils., Patients: We present two unrelated patients with biallelic JAGN1 mutations, who were both treated with subcutaneous GM-CSF (sargramostim) after treatment failure to G-CSF. The first patient was an 18-year-old pregnant woman who received GM-CSF at 12 weeks of gestation up to a dose of 10 µg/kg/d for 7 days. The second patient was a 5-month-old girl who received GM-CSF for a total of 9 days at a dose of up to 20 µg/kg/d. GM-CSF did not increase neutrophil counts in our patients. Treatment was stopped when neutrophil numbers declined further, no beneficial effect was noticed, and patients presented with infections. No adverse effects were observed in either patient and the fetus. Both patients ultimately underwent successful hematopoietic stem cell transplantation., Discussion: Both patients showed a high recurrence rate of severe infections on G-CSF treatment. GM-CSF therapy did not ameliorate the clinical phenotype, in contrast to the improvement of neutrophil function observed in the JAGN1 mouse model. No major additional extra-hematopoietic manifestations were evident in our patients., Conclusion: In two unrelated patients, GM-CSF did not have any beneficial effect on neutrophil counts. Patients with JAGN1-mutant SCN with reduced G-CSF responsiveness and elevated infection rate should be evaluated early for stem cell transplantation., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision., (Copyright © 2024 Farmand, Aydin, Wustrau, Böhm, Ayuk, Escherich, Skokowa, Müller and Lehmberg.)

SYNTAXIN-11 (STX11) is a SNARE protein that mediates the fusion of cytotoxic granules with the plasma membrane at the immunological synapses of CD8 T or NK cells. Autosomal recessive inheritance of deleterious STX11 variants impairs cytotoxic granule exocytosis, causing familial hemophagocytic lymphohistiocytosis type 4 (FHL-4). In several FHL-4 patients, we also observed hypogammaglobulinemia, elevated frequencies of naive B cells, and increased double-negative DN2:DN1 B cell ratios, indicating a hitherto unrecognized role of STX11 in humoral immunity. Detailed analysis of Stx11-deficient mice revealed impaired CD4 T cell help for B cells, associated with disrupted germinal center formation, reduced isotype class switching, and low antibody avidity. Mechanistically, Stx11-/- CD4 T cells exhibit impaired membrane fusion leading to reduced CD107a and CD40L surface mobilization and diminished IL-2 and IL-10 secretion. Our findings highlight a critical role of STX11 in SNARE-mediated membrane trafficking and vesicle exocytosis in CD4 T cells, important for successful CD4 T cell-B cell interactions. Deficiency in STX11 impairs CD4 T cell-dependent B cell differentiation and humoral responses., (© 2024 Kögl et al.)

Staphylococcus aureus ( S. aureus ) is a significant human pathogen, in particular in patients with an underlying medical condition. It is equipped with a large variety of virulence factors enabling both colonization and invasive disease. The spectrum of manifestation is broad, ranging from superficial skin infections to life-threatening conditions like pneumonia and sepsis. As a major cause of healthcare-associated infections, there is a great need in understanding staphylococcal immunity and defense mechanisms. Patients with inborn errors of immunity (IEI) frequently present with pathological infection susceptibility, however, not all of them are prone to S. aureus infection. Thus, enhanced frequency or severity of S. aureus infections can serve as a clinical indicator of a specific underlying immunological impairment. In addition, the analysis of immunological functions in patients with susceptibility to S. aureus provides a unique opportunity of understanding the complex interplay between staphylococcal virulence and host immune predisposition. While the importance of quantitatively and qualitatively normal neutrophils is widely known, less awareness exists about the role of specific cytokines such as functional interleukin (IL)-6 signaling. This review categorizes well-known IEI in light of their susceptibility to S. aureus and discusses the relevant associated pathomechanisms. Understanding host-pathogen-interactions in S. aureus infections in susceptible individuals can pave the way for more effective management and preventive treatment options. Moreover, these insights might help to identify patients who should be screened for an underlying IEI. Ultimately, enhanced understanding of pathogenesis and immune responses in S. aureus infections may also be of relevance for the general population., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (© 2024 Kurz, Lehmberg and Farmand.)

Abstract: Primary hemophagocytic lymphohistiocytosis (pHLH) is a life-threatening hyperinflammatory syndrome that develops mainly in patients with genetic disorders of lymphocyte cytotoxicity and X-linked lymphoproliferative syndromes. Previous studies with etoposide-based treatment followed by hematopoetic stem cell transplantation (HSCT) resulted in 5-year survival of 50% to 59%. Contemporary data are lacking. We evaluated 88 patients with pHLH documented in the international HLH registry from 2016-2021. In 12 of 88 patients, diagnosis was made without HLH activity, based on siblings or albinism. Major HLH-directed drugs (etoposide, antithymocyte globulin, alemtuzumab, emapalumab, ruxolitinib) were administered to 66 of 76 patients who were symptomatic (86% first-line etoposide); 16 of 57 patients treated with etoposide and 3 of 9 with other first-line treatment received salvage therapy. HSCT was performed in 75 patients; 7 patients died before HSCT. Three-year probability of survival (pSU) was 82% (confidence interval [CI], 72%-88%) for the entire cohort and 77% (CI, 64%-86%) for patients receiving first-line etoposide. Compared with the HLH-2004 study, both pre-HSCT and post-HSCT survival of patients receiving first-line etoposide improved, 83% to 91% and 70% to 88%. Differences to HLH-2004 included preferential use of reduced-toxicity conditioning and reduced time from diagnosis to HSCT (from 148 to 88 days). Three-year pSU was lower with haploidentical (4 of 9 patients [44%]) than with other donors (62 of 66 [94%]; P < .001). Importantly, early HSCT for patients who were asymptomatic resulted in 100% survival, emphasizing the potential benefit of newborn screening. This contemporary standard-of-care study of patients with pHLH reveals that first-line etoposide-based therapy is better than previously reported, providing a benchmark for novel treatment regimes., (© 2024 American Society of Hematology. Published by Elsevier Inc. All rights are reserved, including those for text and data mining, AI training, and similar technologies.)

Background: Leukocyte adhesion deficiency type 1 (LAD-1) is a congenital immunodeficiency leading to impaired trafficking of neutrophils to inflammation sites. Solitary or multiple pyoderma gangrenosum (PG)-like skin ulcers (PGLUs) have been reported previously in 13 children (aged 0.5-19 years) with LAD-1., Objective: Our aim was to report the case of a 10-year-old boy presenting with PGLUs as the first manifestation of LAD-1 treated with ustekinumab., Methods: We obtained in situ cytokine profiles., Results: PGLUs were triggered by cutaneous ringworm infection ( Trichophyton tonsurans ). Skin biopsy samples showed increased intralesional expression of IL-17A, Il-23, and IL-1β as compared with their expression in healthy controls. After an unsuccessful attempt at treatment with oral methylprednisolone, ustekinumab induced regression of the ulcerations, associated with complete normalization of the cytokine profile., Conclusions: PGLUs, triggered by ringworm infection, can be a late harbinger of LAD-1. Ustekinumab is a safe and effective therapeutic option for patients with LAD-1 and PGLUs while bridging the time until stem cell transplantation., (© 2024 The Author(s).)

Background: Lymphoproliferation and autoimmune cytopenias characterise autoimmune lymphoproliferative syndrome. Other conditions sharing these manifestations have been termed autoimmune lymphoproliferative syndrome-like diseases, although they are frequently more severe. The aim of this study was to define the genetic, clinical, and immunological features of these disorders to improve their diagnostic classification., Methods: In this prospective cohort study, patients were referred to the Center for Chronic Immunodeficiency in Freiburg, Germany, between Jan 1, 2008 and March 5, 2022. We enrolled patients younger than 18 years with lymphoproliferation and autoimmune cytopenia, lymphoproliferation and at least one additional sign of an inborn error of immunity (SoIEI), bilineage autoimmune cytopenia, or autoimmune cytopenia and at least one additional SoIEI. Autoimmune lymphoproliferative syndrome biomarkers were determined in all patients. Sanger sequencing followed by in-depth genetic studies were recommended for patients with biomarkers indicative of autoimmune lymphoproliferative syndrome, while IEI panels, exome sequencing, or genome sequencing were recommended for patients without such biomarkers. Genetic analyses were done as decided by the treating physician. The study was registered on the German Clinical Trials Register, DRKS00011383, and is ongoing., Findings: We recruited 431 children referred for autoimmune lymphoproliferative syndrome evaluation, of whom 236 (55%) were included on the basis of lymphoproliferation and autoimmune cytopenia, 148 (34%) on the basis of lymphoproliferation and another SoIEI, 33 (8%) on the basis of autoimmune bicytopenia, and 14 (3%) on the basis of autoimmune cytopenia and another SoIEI. Median age at diagnostic evaluation was 9·8 years (IQR 5·5-13·8), and the cohort comprised 279 (65%) boys and 152 (35%) girls. After biomarker and genetic assessments, autoimmune lymphoproliferative syndrome was diagnosed in 71 (16%) patients. Among the remaining 360 patients, 54 (15%) had mostly autosomal-dominant autoimmune lymphoproliferative immunodeficiencies (AD-ALPID), most commonly affecting JAK-STAT (26 patients), CTLA4-LRBA (14), PI3K (six), RAS (five), or NFκB (three) signalling. 19 (5%) patients had other IEIs, 17 (5%) had non-IEI diagnoses, 79 (22%) were unresolved despite extended genetics (ALPID-U), and 191 (53%) had insufficient genetic workup for diagnosis. 16 (10%) of 161 patients with a final diagnosis had somatic mutations. Alternative classification of patients fulfilling common variable immunodeficiency or Evans syndrome criteria did not increase the proportion of genetic diagnoses., Interpretation: The ALPID phenotype defined in this study is enriched for patients with genetic diseases treatable with targeted therapies. The term ALPID might be useful to focus diagnostic and therapeutic efforts by triggering extended genetic analysis and consideration of targeted therapies, including in some children currently classified as having common variable immunodeficiency or Evans syndrome., Funding: Deutsche Forschungsgemeinschaft under Germany's Excellence Strategy., Translation: For the German translation of the abstract see Supplementary Materials section., Competing Interests: Declaration of interests SE are members of a data monitoring committee for leniolisib (Pharming). SH and SE received research funding from Pharming. MA and IC are a full-time employees of UCB Pharma and are shareholders of UCB. SE received research support from UCB for this study, including the whole-genome trio analysis of 30 families. All other authors declare no competing interests., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Background: Elevated TCRαβ + CD4 - CD8 - double-negative T cells (DNT) and serum biomarkers help identify FAS mutant patients with autoimmune lymphoproliferative syndrome (ALPS). However, in some patients with clinical features and biomarkers consistent with ALPS, germline or somatic FAS mutations cannot be identified on standard exon sequencing (ALPS-undetermined: ALPS-U)., Objective: We sought to explore whether complex genetic alterations in the FAS gene escaping standard sequencing or mutations in other FAS pathway-related genes could explain these cases., Methods: Genetic analysis included whole FAS gene sequencing, copy number variation analysis, and sequencing of FAS cDNA and other FAS pathway-related genes. It was guided by FAS expression analysis on CD57 + DNT, which can predict somatic loss of heterozygosity (sLOH)., Results: Nine of 16 patients with ALPS-U lacked FAS expression on CD57 + DNT predicting heterozygous "loss-of-expression" FAS mutations plus acquired somatic second hits in the FAS gene, enriched in DNT. Indeed, 7 of 9 analyzed patients carried deep intronic mutations or large deletions in the FAS gene combined with sLOH detectable in DNT; 1 patient showed a FAS exon duplication. Three patients had reduced FAS expression, and 2 of them harbored mutations in the FAS promoter, which reduced FAS expression in reporter assays. Three of the 4 ALPS-U patients with normal FAS expression carried heterozygous FADD mutations with sLOH., Conclusion: A combination of serum biomarkers and DNT phenotyping is an accurate means to identify patients with ALPS who are missed by routine exome sequencing., (Copyright © 2023 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)

Hemophagocytic lymphohistiocytosis (HLH) has been described for decades in association with malignancies (M-HLH). While its mechanism is unknown, M-HLH has a poor prognosis, ranging from 10% to 30% overall survival. Mature T-cell lymphomas, diffuse large B-cell lymphoma, and Hodgkin lymphoma, with or without viral co-triggers such as Epstein-Barr virus, are among the most frequent underlying entities. Most M-HLH cases occur at the presentation of malignancy, but they may also occur during therapy as a result of immune compromise from chemotherapy (HLH in the context of immune compromise, IC-HLH) and (typically) disordered response to infection or after immune-activating therapies (Rx-HLH, also known as cytokine release syndrome, CRS). IC-HLH typically occurs months after diagnosis in the context of fungal, bacterial, or viral infection, though it may occur without an apparent trigger. Rx-HLH can be associated with checkpoint blockade, chimeric antigen receptor T-cell therapy, or bispecific T-cell engaging therapy. Until recently, M-HLH diagnosis and treatment strategies were extrapolated from familial HLH (F-HLH), though optimized diagnostic and therapeutic treatment strategies are emerging., (© 2024. The Author(s), under exclusive license to Springer Nature Switzerland AG.)

Backgr Ound: T-cell receptor excision circle (TREC)-based newborn screening (NBS) for severe combined immunodeficiencies (SCID) was introduced in Germany in August 2019., Methods: Children with abnormal TREC-NBS were referred to a newly established network of Combined Immunodeficiency (CID) Clinics and Centers. The Working Group for Pediatric Immunology (API) and German Society for Newborn Screening (DGNS) performed 6-monthly surveys to assess the TREC-NBS process after 2.5 years., Results: Among 1.9 million screened newborns, 88 patients with congenital T-cell lymphocytopenia were identified (25 SCID, 17 leaky SCID/Omenn syndrome (OS)/idiopathic T-cell lymphocytopenia, and 46 syndromic disorders). A genetic diagnosis was established in 88%. Twenty-six patients underwent hematopoietic stem cell transplantation (HSCT), 23/26 within 4 months of life. Of these, 25/26 (96%) were alive at last follow-up. Two patients presented with in utero onset OS and died after birth. Five patients with syndromic disorders underwent thymus transplantation. Eight syndromic patients deceased, all from non-immunological complications. TREC-NBS missed one patient, who later presented clinically, and one tracking failure occurred after an inconclusive screening result., Conclusion: The German TREC-NBS represents the largest European SCID screening at this point. The incidence of SCID/leaky SCID/OS in Germany is approximately 1:54,000, very similar to previous observations from North American and European regions and countries where TREC-NBS was implemented. The newly founded API-CID network facilitates tracking and treatment of identified patients. Short-term HSCT outcome was excellent, but NBS and transplant registries will remain essential to evaluate the long-term outcome and to compare results across the rising numbers of TREC-NBS programs across Europe., (© 2023. The Author(s).)

Introduction: Inborn errors of immunity (IEI) are characterized by a dysfunction of the immune system leading to increased susceptibility to infections, impaired immune regulation and cancer. We present a unique consanguineous family with a history of Hodgkin lymphoma, impaired EBV control and a late onset hemophagocytic lymphohistiocytosis (HLH)., Methods and Results: Overall, family members presented with variable impairment of NK cell and cytotoxic T cell degranulation and cytotoxicity. Exome sequencing identified homozygous variants in RAB27A , FBP1 ( Fructose-1,6-bisphosphatase 1 ) and ACAD9 ( Acyl-CoA dehydrogenase family member 9 ). Variants in RAB27A lead to Griscelli syndrome type 2, hypopigmentation and HLH predisposition., Discussion: Lymphoma is frequently seen in patients with hypomorphic mutations of genes predisposing to HLH. We hypothesize that the variants in FBP1 and ACAD9 might aggravate the clinical and immune phenotype, influence serial killing and lytic granule polarization by CD8 T cells. Understanding of the interplay between the multiple variants identified by whole exome sequencing (WES) is essential for correct interpretation of the immune phenotype and important for critical treatment decisions., Competing Interests: KS is an employee of AstraZeneca and has stock ownership and/or stock options or interests in the company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Brauer, Maruta, Lisci, Strege, Oschlies, Nakamura, Böhm, Lehmberg, Brandhoff, Ehl, Parvaneh, Klapper, Fukuda, Griffiths, Hennies, Niehues and Ammann.)

Most hereditary forms of hemophagocytic lymphohistiocytosis (HLH) are caused by defects of cytotoxicity, including the vesicle trafficking disorder Griscelli syndrome type 2 (GS2, RAB27A deficiency). Deficiency of the mitogen-activated protein kinase activating death domain protein (MADD) results in a protean syndrome with neurological and endocrinological involvement. MADD acts as a guanine nucleotide exchange factor for small guanosine triphosphatases, including RAB27A. A homozygous splice site mutation in MADD was identified in a female infant with syndromic features, secretory diarrhea, and features of HLH. Aberrant splicing caused by this mutation leads to an in-frame deletion of 30 base pairs and favors other aberrant variants. Patient natural killer (NK) cells and cytotoxic T cells showed a severe degranulation defect leading to absent perforin-mediated cytotoxicity. Platelets displayed defective adenosine triphosphate secretion, similar to that in GS2. To prove causality, we introduced a CRISPR/Cas9-based MADD knockout in the NK cell line NK-92mi. MADD-deficient NK-92mi cells showed a degranulation defect and impaired cytotoxicity similar to that of the patient. The defect of cytotoxicity was confirmed in another patient with MADD deficiency. In conclusion, RAB27A-interacting MADD is involved in vesicle release by cytotoxic cells and platelets. MADD deficiency causes a degranulation defect and represents a novel disease predisposing to an HLH phenotype., (© 2023 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

Objective: Hemophagocytic lymphohistiocytosis is a hyperinflammatory syndrome that often requires critical care support and remains difficult to diagnose. These guidelines are meant to aid in the early recognition, diagnosis, supportive care, and treatment of patients with hemophagocytic lymphohistiocytosis in ICUs., Data Sources: The literature searches were performed with PubMed (MEDLINE)., Study Selection: Keywords and medical subject headings terms for literature search included "macrophage activation syndrome," hemophagocytic lymphohistiocytosis," and "hemophagocytic syndrome.", Data Extraction: The Histiocyte Society developed these consensus recommendations on the basis of published reports and expert opinions with level of evidence provided for each recommendation. They were endorsed by the Society of Critical Care Medicine., Data Synthesis: Testing for hemophagocytic lymphohistiocytosis should be initiated promptly in all patients admitted to ICUs with an unexplained or disproportionate inflammatory response, especially those with rapid clinical deterioration. Meeting five or more of eight hemophagocytic lymphohistiocytosis 2004 diagnostic criteria serves as a valuable diagnostic tool for hemophagocytic lymphohistiocytosis. Early aggressive critical care interventions are often required to manage the multisystem organ failure associated with hemophagocytic lymphohistiocytosis. Thorough investigation of the underlying triggers of hemophagocytic lymphohistiocytosis, including infections, malignancies, and autoimmune/autoinflammatory diseases, is essential. Early steroid treatment is indicated for patients with familial hemophagocytic lymphohistiocytosis and is often valuable in patients with acquired hemophagocytic lymphohistiocytosis (i.e., secondary hemophagocytic lymphohistiocytosis) without previous therapy, including macrophage activation syndrome (hemophagocytic lymphohistiocytosis secondary to autoimmune/autoinflammatory disease) without persistent or relapsing disease. Steroid treatment should not be delayed, particularly if organ dysfunction is present. In patients with macrophage activation syndrome, whose disease does not sufficiently respond, interleukin-1 inhibition and/or cyclosporine A is recommended. In familial hemophagocytic lymphohistiocytosis and severe, persistent, or relapsing secondary macrophage activation syndrome, the addition of prompt individualized, age-adjusted etoposide treatment is recommended., Conclusions: Further studies are needed to determine optimal treatment for patients with hemophagocytic lymphohistiocytosis in ICUs, including the use of novel and adjunct therapies., Competing Interests: Dr. Horne serves as a speaker for Swedish Orphan Biovitrum (SOBI) and Novartis; Dr. Jordan serves as a consultant for Novimmune and SOBI; Dr. Hines receives research funding from Incyte and the Histiocytosis Association; Drs. K. Beutel, Lehmberg, and Henter serve as consultants for SOBI; Dr. La Rosée serves as a speaker and consultant for SOBI and Novartis. Dr. Hines’ institution received funding from the American Lebanese Syrian Associated Charities and Incyte. Drs. Hines, Beutel, Hays, and Henter disclosed the off-label product use of extracorporeal life support, Etoposide, dexamethasone, anakinra, cyclosporine, cyclophosphamide, alemtuzumab, Tocilizumab, ruxolitinib, plasmapheresis, cytokine adsorption, IV immunoglobulin, and methylprednisolone. Drs. Beutel, Horne, Lachmann, Machowicz, La Rosée, and Henter received funding from SOBI Adboard. Dr. Beutel received funding from Biotest AG. Dr. Horne’s institution received funding from Novartis. Dr. van Laar disclosed government work. Dr. Shakoory disclosed that there is a Cooperative and Development Research Agreements between her section at National Institutes of Health (NIH) and SOBI. Dr. Zinter received funding from the NIH American Thoracic Society grant (K23HL146936); he received support for article research from the NIH. Dr. Henter’s institution received funding from the Swedish Children’s Cancer Foundation and the Stockholm County Council (ALF project). The remaining authors have disclosed that they do not have any potential conflicts of interest. The above competing interests and Histiocyte Society involvement did not influence the content of these guidelines., (Copyright © 2022 by the Society of Critical Care Medicine and Wolters Kluwer Health, Inc. All Rights Reserved.)

Hemophagocytic lymphohistiocytosis (HLH; hemophagocytic syndrome) is a rare syndrome of potentially fatal, uncontrolled hyperinflammation. Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is indicated in primary, recurrent or progressive HLH, but information about its outcomes in the adult population is limited. We obtained data about 87 adult (≥18 years of age) patients retrospectively reported to the EBMT. The median survival time was 13.9 months. The three and five-year overall survival (OS) was 44% (95% CI 33-54%). Among 39 patients with a follow-up longer than 15 months, only three died. Relapse rate was 21% (95% CI 13-30%), while NRM reached 36% (95% CI 25-46%). Younger patients (<30 years of age) had better prognosis, with an OS of 59% (95% CI 45-73%) at three and five years vs 23% (95% CI 8-37%) for older ones. No difference in survival between reduced and myeloablative conditioning was found. To our knowledge, this is the largest report of adult HLH patients who underwent allo-HSCT. Patients who survive the first period after this procedure can expect a long disease-free survival. Both reduced intensity and myeloablative conditioning have therapeutic potential in adult HLH., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)