BACKGROUND: X-linked lymphoproliferative syndrome-2 (XLP-2), caused by XIAP deficiency, is an inborn error of immunity classified in the context of immune dysregulation. Although initially described in patients with EBV-associated HLH, the clinical spectrum of the syndrome has broadened as additional cases have been reported. CASE PRESENTATION: The patient was born with hydrops fetalis. Additionally, he presented with cardiac defects (VSD, PDA), pancytopenia, hepatitis, and markedly elevated ferritin levels. He had an older brother with similar clinical findings, including hydrops fetalis, who died on the fourth day of life. Whole-exome sequencing revealed a novel hemizygous c.679T>C (p.Cys227Arg) variant of uncertain significance in exon 2 of the XIAP gene. Functional studies revealed defective XIAP expression and activity. The patient also fulfilled the diagnostic criteria for HLH. He underwent three HSCT procedures, with the last one being successful. Fifteen months after HSCT, he developed refractory seizures consistent with West syndrome, which progressively worsened his neurological condition. Although the precise cause of the seizures remains unclear, both drug exposure and CNS infection may have contributed. CONCLUSION: The novel XIAP variant appears to cause a severe, very early-onset phenotype, even prenatally. This case is notable as the first report of hydrops fetalis in a patient with XIAP deficiency, representing a previously undescribed loss-of-function variant.

Abstract: Primary hemophagocytic lymphohistiocytosis (HLH) is mainly caused by biallelic variants in genes disrupting cytotoxic natural killer (NK) cell and T-cell function (PRF1, UNC13D, STX11, STXBP2, RAB27A, and LYST). A “pathway defect accumulation” model proposes that heterozygous variants in multiple familial hemophagocytic lymphohistiocytosis (FHL) genes (digenic or multigenic inheritance) may increase susceptibility, but its significance remains debated. We assessed the prevalence and clinical relevance of digenic/multigenic FHL genotypes in a German FHL cohort (1987-2023) and the UK Biobank (UKB; 469 589 participants). We analyzed (1) variants classified as disease mutations in the Human Gene Mutation database (HGMD); (2) common variants such as PRF1 p.Ala91Val and p.Asn252Ser; and (3) additional variants previously reported in digenic HLH and explored phenotypic associations using codes of the International Classification of Diseases, 10th Revision for possible HLH-related conditions. In the German cohort, among 635 individuals sequenced for >1 FHL gene, no symptomatic patient with abnormal NK/cytotoxic T-lymphocyte degranulation carried digenic/multigenic heterozygous variants. In UKB, 575 individuals carried digenic FHL genotypes (0.1% prevalence), without enrichment for HLH-associated phenotypes (odds ratio, 0.95; P = 1). Four individuals carried trigenic genotypes; none had HLH-related diagnoses. Several HGMD-labeled pathogenic variants were observed biallelically in asymptomatic adults, suggesting potential misclassification. Including PRF1 p.Ala91Val and p.Asn252Ser increased digenic variant carriers to 2590 but did not cause phenotype enrichment. Digenic heterozygous FHL variants are relatively common in the general population but do not confer increased FHL risk. Many reported pathogenic FHL variants may be benign. These findings argue against classifying multigenic heterozygous carriers as at risk and support integrating population data into variant interpretation.

BACKGROUND:Inborn errors of immunity are traditionally understood as monogenic germline disorders. However, somatic mosaicism can also result in immune-mediated diseases, mimicking inborn errors of immunity. While early postzygotic mosaicism is the predominant mechanism, genetic variants causing a selective advantage to hematopoietic progenitors and/or mature immune cells may cause immune dysregulation and initiate disease at any age. Somatic mosaicism for STAT5BN642H was linked to severe allergic disease in infancy, but its full clinical spectrum and underlying mechanisms remain incompletely defined. OBJECTIVE:We elucidated how somatic N642H mutations of the STAT5B gene shape lineage-specific mosaicism, immune cell function, and clinical phenotypes. METHODS:We investigated 3 new patients-including one adult-with STAT5BN642H mosaicism using deep sequencing, flow and mass cytometry, and functional immune assays. Mutant cell distribution was mapped across blood lineages. A mouse model with mosaic STAT5BN642H mutation in hematopoietic stem cells was generated to study clonal dynamics and immune phenotypes. RESULTS:Patients displayed variable lineage mosaicism correlating with two predominant clinical outcomes: early-onset severe atopy with hypereosinophilia, and autoimmune-lymphoproliferative immunodeficiency with expansions of CD8 and γδ T cells. Functional studies revealed enhanced IL-2-mediated proliferation, effector differentiation, and oligoclonal T-cell expansions. In mice, a few mutant hematopoietic stem cells reproduced the patient lineage-skewed immune landscapes with variable growth advantage of mutant cells across hematopoietic development and recapitulated patient T-cell phenotypes. Targeted mTOR inhibition successfully controlled lymphoproliferation in patients. CONCLUSION:A single somatic variant in a few stem cells can remodel hematopoiesis, generating variable immune mosaics and heterogeneous immune disease.

ABSTRACT:Genetic screening for severe congenital immunohematological diseases offers potential for early intervention, particularly through preemptive allogeneic hematopoietic stem cell transplantation (HSCT). However, the clinical value of such screening depends on precise prognostic predictions based on genotype-phenotype correlations and/or functional confirmation. We investigated familial hemophagocytic lymphohistiocytosis type 2 (FHL2), caused by PRF1 variants. Specifically, we evaluated the clinical significance of the frequent PRF1 A91V variant, if present in trans with a predicted loss-of-function (pLOF) PRF1 variant, defined as "disease mutation" listed in the Human Gene Mutation Database. We combined clinical and functional data from our hemophagocytic lymphohistiocytosis (HLH)-network registry with UK Biobank data to evaluate disease penetrance and clinical outcomes. Among 52 individuals with A91V/pLOF genotype in the registry, 39 (72%) showed FHL2-related manifestations with mean onset at 20 years. Four patients had recurrent disease, 15 received transplantation, and 14 died. Among 14 individuals with A91V/pLOF genotype identified by family screening (mean age, 29 years), however, only 1 was symptomatic. Moreover, among 21 A91V/pLOF carriers identified in 200 000 UK Biobank participants, 12 with genotypes identical to symptomatic registry patients, none had developed HLH by age 73 years. Premature stop pLOF alleles appeared more penetrant than missense variants, but functional data including perforin expression or cytotoxicity failed to predict disease manifestation. Our combined registry and population-based approach reveals significant variability in disease penetrance and severity among PRF1 A91V/pLOF carriers, with no clear association between genotype, functional data, and clinical outcomes. This complexity illustrates the challenges of genetic screening and highlights the need for careful clinical decision-making regarding preemptive HSCT in asymptomatic carriers.

Primary hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory syndrome caused by inborn errors of cytotoxicity. Patients with biallelic PRF1 null mutations (encoding perforin) usually develop excessive immune cell activation, hypercytokinemia, and life-threatening immunopathology in the first 6 months of life, often without an apparent infectious trigger. In contrast, perforin-deficient (PKO) mice only develop HLH after systemic infection with lymphocytic choriomeningitis virus (LCMV). We hypothesized that restricted microbe-immune cell interactions due to specific pathogen-free (SPF) housing might explain the need for this specific viral trigger in PKO mice. To investigate the influence of a "wild" microbiome in PKO mice, we fostered PKO newborns with Wildling microbiota ('PKO-Wildlings') and monitored them for signs of HLH. PKO-Wildlings survived long-term without spontaneous disease. Also, systemic infection with vaccinia virus did not reach the threshold of immune activation required to trigger HLH in PKO-Wildlings. Interestingly, after infection with LCMV, PKO-Wildlings developed an altered HLH pattern. This included lower IFN-γ serum levels along with improved IFN-γ-driven anemia, but more elevated levels of IL-17 and increased liver inflammation compared with PKO-SPF mice. Thus, wild microbiota alone is not sufficient to trigger HLH in PKO mice, but host-microbe interactions shape inflammatory cytokine patterns, thereby influencing manifestations of HLH immunopathology.

Hermansky-Pudlak syndrome (HPS), a rare heterogeneous autosomal recessive disorder, is characterized by oculocutaneous albinism (OCA) and a bleeding diathesis due to a defect regarding melanosomes and platelet delta (δ)-granule secretion. Interestingly, patients with HPS type 2 (HPS-2) or HPS type 10 (HPS-10) present additionally with an immunological defect. We investigated three patients (IP1, IP2, and IP3) who suffer from a bleeding diathesis. Platelet aggregometry showed impaired platelet function and flow cytometry revealed a severely reduced platelet CD63 expression hinting to either a defect of platelet delta granule secretion or a decreased number of delta granules in these patients. However, only IP3 presents with an apparent OCA. We performed panel sequencing and identified a homozygous deletion of exon 6 in DTNBP1 for IP3. Western analysis confirmed the absence of the encoded protein dysbindin confirming the diagnosis of HPS-7. Interestingly, this patient reported additionally recurrent bacterial infections. Analysis of lymphocyte cytotoxicity showed a slightly reduced NK-degranulation previously documented in a more severe form in patients with HPS-2 or HPS-10. IP1 is carrier of two compound heterozygous variants in the HPS3 gene (c.65C > G and c.1193G > A). A homozygous variant in HPS5 (c.760G > T) was identified in IP2. The novel missense variants were classified as VUS (variant of uncertain significance) according to ACMG guidelines. For IP1 with the compound heterozygous variants in HPS3 a specialized ophthalmological examination showed ocular albinism. HPS3 and HPS5 encode subunits of the BLOC-2 complex and patients with HPS-3 or HPS-5 are known to present with variable/mild hypopigmentation.

Glucocorticoids (GC), synthesized by the 11β-hydroxylase (Cyp11b1), control excessive inflammation through immunosuppressive actions. The skin was proposed to regulate homeostasis by autonomous GC production in keratinocytes. However, their immunosuppressive capacity and clinical relevance remain unexplored. Here, we demonstrate the potential of skin-derived GC and their role in the regulation of physiological and prevalent inflammatory skin conditions. In line with 11β-hydroxylase deficiency in human inflammatory skin disorders, genetic in vivo Cyp11b1 ablation and long-term GC deficiency in keratinocytes primed the murine skin immune system resulting in spontaneous skin inflammation. Deficient skin GC in experimental models for inflammatory skin disorders led to exacerbated contact hypersensitivity and psoriasiform skin inflammation accompanied by decreased regulatory T cells and the involvement of unconventional T cells. Our findings provide insights on how skin homeostasis and pathology are critically regulated by keratinocyte-derived GC, emphasizing the immunoregulatory potential of endogenous GC in the regulation of epithelial immune microenvironment.

Hemophagocytic lymphohistiocytosis (HLH) is a disorder of uncontrolled immune activation with distinct clinical features including fever, cytopenia, splenomegaly, and sepsis-like symptoms. In a young adolescent patient a novel germline GATA2 variant (NM_032638.5 (GATA2): c.177C>G, p.Tyr59Ter) was discovered and had resulted in non-tuberculous mycobacterial (NTM) infection and aggressive HLH. Strikingly, impaired degranulation of cytotoxic T-lymphocytes (CTL) and natural killer (NK)-cells was detected in CD107a-analyses. The affected patient was treated with HLA-matched unrelated alloHSCT, and subsequently all hematologic and infectious abnormalities including HLH and NTM resolved. This case supports early alloHSCT in GATA2 deficiencies as curative approach regardless of active NTM infection. Future studies on GATA2 c.177C>G, p.Tyr59*Ter might unravel its potential role in cytotoxic effector cell function and its contribution to HLH pathogenesis.