Cellular & Molecular Immunology

Background: Patients receiving anti-TNF treatment for chronic inflammatory disease display impaired antibody responses, but it remains unclear how immune memory formation is affected. We evaluated antibody responses and memory B cells (Bmem) after COVID-19 booster vaccination in inflammatory bowel disease (IBD) patients receiving anti-TNF treatment. Methodology: Blood was sampled at baseline, 1, and 6 months after WH1/BA.5 bivalent or XBB.1.5 monovalent vaccination from 27 IBD patients receiving intravenous anti-TNF and 44 controls. Neutralizing antibodies were measured using an infectious virus assay. SARS-CoV-2 spike receptor binding domain (RBD)-specific serum IgG was quantified by ELISA, and RBD-specific Bmem were immunophenotyped by flow cytometry using recombinant proteins from ancestral, Omicron BA.1, BA.5, XBB.1.5, and JN.1 variants. Results: Serum IgG to vaccine RBD and neutralizing antibodies in patients increased pre to 1 month post-vaccination, but were lower than controls. Ancestral-, BA.5- and XBB.1.5-specific Bmem increased after vaccination but were significantly lower in patients than controls. Within RBD-specific Bmem, frequencies of recently activated CD21lo cells were increased after vaccination, and were higher in patients than controls. Fewer antigen-specific Bmem in patients expressed IgG4, and more expressed IgG3 or IgD following vaccination. Following vaccination, more RBD-specific Bmem recognized multiple viral variants. However, patients had fewer Bmem that could bind to subvariants than controls. Conclusion: Antibody and Bmem responses to COVID-19 booster vaccination in anti-TNF-treated IBD patients displayed reduced capacity, durability and cross-reactivity, suggesting impaired immune memory for protection against breakthrough infection. This supports the recommendation for annual booster vaccination to prevent severe disease and viral spread.

IgM is the most ancient antibody isotype, playing an important role in both circulatory and mucosal immune responses across vertebrates, yet structural characterization of its polymeric forms is limited outside of mammals. Here, we report the cryo-electron microscopy structure of mallard duck secretory (S) IgM at 3.37-[A] resolution. The structure revealed a pentameric core globally similar to human SIgM, supporting the view that pentameric IgM is subject to strong evolutionary constraints. However, compared to mammalian structures, we observed species-specific differences at molecular interfaces. Surface plasmon resonance binding assays characterizing secretory component (SC)-IgM interactions supported structural observations and, when compared to IgA binding, revealed isotype-specific contributions from the avian SC N-terminal extension. Together, these findings establish a comparative structural framework for polymeric IgM across vertebrates and provide insight into how avian SIgM-specific features may support mucosal immunity in birds.

Substantial inter-individual variation in susceptibility to viral infection persists despite widespread vaccination, and its immunological basis remains poorly understood. We investigated how innate and adaptive immune responses contribute to susceptibility to SARS-CoV-2 infection during the early COVID-19 pandemic. We compared two groups of vaccinated individuals who either remained uninfected or became infected during the first Omicron wave. Blood samples were collected at baseline and 24 weeks later. Peripheral blood mononuclear cells (PBMCs) and polymorphonuclear neutrophils (PMNs) were isolated and stimulated with the TLR7/8 agonist R848 to assess innate responses. PBMCs were stimulated with SARS-CoV-2 peptide pools and highly purified inactivated viruses (ancestral and Omicron BA.1) to evaluate adaptive immunity. Prior to infection, individuals in the infected group exhibited reduced CD4 and CD8 T cells proliferative responses, alongside with increased TNF production across all stimulation conditions, despite largely comparable immune phenotypes, indicating a pre-existing functional immune deficit. Following infection, T-cell proliferation and IFN-gamma production were partially restored in response to viral antigens, although responses to Omicron BA.1 remained suboptimal. This functional deficit was accompanied by heightened inflammatory activity, including increased TNF and IFN-gamma production, elevated anti-nucleocapsid IgG3 levels, higher frequencies of B cells and myeloid cells, reduced circulating interferon-inducible T-cell Alpha Chemoattractant (I-TAC) concentrations, and a modest impairment in PMN IL-8 responses. Notably, these alterations were detectable prior to infection and persisted thereafter, indicating that they represent determinants rather than consequences of viral infection. Importantly, beyond differences in the magnitude of immune responses, protection was associated with the degree of functional coordination within the humoral compartment, as reflected by the relationship between Spike-binding antibodies and neutralizing activity. Together, these results demonstrate that susceptibility to Omicron infection is linked to a pre-existing and persistent functional immune imbalance affecting both innate and adaptive arms of immunity.

Post-acute infection syndromes (PAIS) follow viral syndromes including post-acute sequelae of COVID19 (PASC) which complicates 10-25% of SARS-CoV-2 infections. These syndromes lack precise explanatory mechanisms. We studied 173 human saliva proteomes during respiratory viral syndromes, seeking associations between 44 clinically-relevant protein expression patterns and subsequent sequelae counts. Exploratory models adjusted by extensive clinical annotations found interactions between 23 acutely-responsive proteins and SARS-CoV-2 infection that inversely predicted subsequent neurocognitive sequelae. An overlapping 19 acutely-responsive proteins during any acute respiratory viral syndrome inversely predicted general fatigue-related sequelae. Altogether, 29 proteins, derived from interferon stimulated genes (ISG), were uniformly beneficial, including 13 predictive of both neurocognitive and general sequelae. The proteins suggested both shared early pathobiology and virus-specific protective responses that shaped resolution of acute disease and different PAIS. Acutely elevated protective ISG proteins associated with reduced post-viral symptoms identify investigational starting points for novel mechanisms, diagnostics and therapeutics for PASC and PAIS.

Nipah virus (NiV) is a highly pathogenic zoonotic paramyxovirus with epidemic potential. Despite the threat NiV poses, no therapeutics are licensed to treat infection. Studies have shown that monoclonal antibodies (mAb) can protect animals against NiV and the related Hendra virus (HeV). The best studied mAb, m102.4, has been used to treat infected patients on a compassionate basis, and has entered clinical trials. However, there is a need to define additional mAbs with therapeutic potential, which could be combined with m102.4 to improve neutralising potency and breadth. Here, we isolated five high affinity mAbs from an mRNA immunised pig, which bound the G glycoprotein derived from NiV Malaysia strain (NiV-M), and one of which (mAb A2) also bound HeV G. Aligned with this, all mAbs neutralised NiV-M pseudovirus but only mAb A2 neutralised pseudovirus representing the NiV Bangladesh (NiV-B) strain. mAb A2 and the most potent NiV-M neutralising mAb, C1, showed minimal competition with each other and m102.4, suggesting recognition of non-overlapping epitopes. Single-particle cryogenic electron microscopy of the NiV-M G receptor binding domain complexed to A1 and C2 Fab fragments revealed distinct epitopes that did not overlap with the receptor-binding site, targeted by m102.4, suggesting action through steric impedance of receptor binding or interference downstream of receptor engagement. Inoculation of mAb A2 to hamsters did not provide complete protection against NiV-B challenge (60% survival), however, a split dose of mAb A2 and m102.4 provided the same protection as m102.4 alone (100% survival). Collectively, these data demonstrate the potential of the porcine model for isolation of therapeutic candidate mAbs, which contribute both to our understanding of the NiV G antigenic landscape, and the development of mAb combinations, that exert complementary mechanisms of neutralisation, for therapeutic intervention.

Seasonal influenza A viruses cause significant global morbidity each year. Although vaccination remains the primary preventive strategy, effectiveness is often reduced by antigenic drift. This challenge is particularly pronounced for influenza A(H3N2), which has required eight vaccine updates over the past decade. Here, we present a computational framework to engineer broadly reactive influenza A(H3N2) vaccines, using protein language models to generate novel hemagglutinin (HA) sequences and a machine learning model to predict antigenic distance from circulating strains. In a proof-of-concept study, seven HA candidates designed using sequence data from 2013-2018 were evaluated in mice against contemporary and subsequently circulating viruses. Two candidates elicited protective levels of reactive antibodies, robust H3-specific antibody-secreting cell responses, and cross-neutralization against contemporary clades and drifted 2019-2020 strains. These findings demonstrate that an integrated generation-selection strategy can enhance vaccine coverage across current and future A(H3N2) seasons and may be applicable to other influenza subtypes.

Introduction: C3 glomerulopathy (C3G) is driven by fluid-phase alternative complement pathway dysregulation, with emerging evidence linking glomerular neutrophil infiltration to disease severity. Neutrophil extracellular traps (NETs) are implicated in other forms of glomerulonephritis. However, their participation in the pathogenesis of C3G remains undefined. Methods: Kidney biopsies from 33 patients with C3G (15 with dense deposit disease [DDD] and 18 with C3 glomerulonephritis [C3GN]) were compared with 15 anti-neutrophil cytoplasmic antibody associated vasculitis (AAV) biopsies as a neutrophil-rich disease control in this retrospective cross-sectional study. Glomerular neutrophils and NETs were identified using immunofluorescence, staining for myeloperoxidase, citrullinated histone H3, peptidyl arginine deiminase-4, and DNA. Supervised machine learning was used to quantify glomerular NET formation, and the data were correlated with kidney function at time of biopsy using linear regression. Results: Intraglomerular NETs were abundant and detected in the majority of glomeruli in C3G biopsies. Compared with AAV, C3G showed a significantly higher fraction of neutrophils forming NETs, despite similar neutrophil counts per glomerulus. NET abundance was similar in DDD and C3GN. In exploratory analyses, a greater proportion of glomeruli containing NETs was associated with lower kidney function (estimated glomerular filtration rate) at biopsy, and this association remained significant after adjustment for age, C3G subtype, and interstitial fibrosis. Conclusions: These observations demonstrate that intraglomerular NETs are a common and prominent observation in C3G and are associated with reduced kidney function at biopsy. These findings raise the possibility that NET deposition in glomeruli is a previously unrecognized driver of glomerular injury in C3G.

Malaria vaccines provide waning protection from disease that is correlated with the production of antibodies to the repeat region of the circumsporozoite protein (CSP). CSP-based vaccines display limited durability in malaria-naive individuals yet are even less effective in malaria-experienced individuals, suggesting the generation of non-optimal humoral immunity in response to both infection and vaccination. To address this hypothesis, we performed a cross-species, comprehensive analysis of B cell responses to CSP after Plasmodium infection or immunization, focusing our analysis on the repeat and C-terminus domains included in malaria subunit vaccines. Herein we demonstrate that the repetitive nature of the protective region of the CSP protein independently impacts the differentiation of the CSP-specific B cells, impinging on their ability to recall for multiple subsequent exposures.

Hybridoma screening in semi-solid medium typically employs antigens labeled with visible fluorophores (e.g., FITC, AF488) to enable single-step identification of antibody-secreting clones. However, conventional chemical conjugation via NHS-esters or isothiocyanate groups frequently modifies lysine residues located within epitopes, potentially abrogating antibody recognition of these critical regions. Here, we describe a SpyTag SpyCatcher-based site-specific labeling strategy that circumvents epitope damage during semi-solid medium screening. A 16-amino-acid SpyTag was genetically fused to the C-terminus of the target antigen, enabling covalent conjugation to an sfGFP SpyCatcher fluorescent probe. In semi-solid medium supplemented with SpyTag-antigen and sfGFPSpyCatcher, positive hybridoma clones were readily identified by distinct fluorescent halos, whereas negative clones showed no detectable signal. Notably, the site-specific method yielded a significantly higher frequency of fluorescence-positive clones compared to the conventional AF488-labeled antigen method, suggesting that epitope preservation enhances screening recovery. Furthermore, this approach did not impair hybridoma growth or final clone positivity, offering a simple, rapid, and epitope-compatible method for monoclonal antibody screening.

Keratin gene mutations are often associated with inflammatory skin disorders, in which the ensuing immunopathology is generally attributed to disrupted barrier integrity and consequent microbial invasion. Here, we challenge this paradigm by demonstrating a role for keratin 76 (Krt76) in thymic central tolerance to skin-targeting autoimmune responses. We show that transfer of Krt76-/- thymic lobes under the kidney capsules of athymic recipients is sufficient to induce expansion of effector T cells in the secondary lymphoid organs, T cell skin infiltration, and autoantibody reactivity to both skin and oral mucosa tissue. Mechanistically, we demonstrate that loss of thymic Krt76 expression disrupts canonical differentiation of the thymic medulla and impacts the development of post-AIRE-expressing keratinocyte-like mimetic medullary epithelial cells (termed CorneoTECs). Notably, Krt76-expressing CorneoTECs differentially express a specific skin and oral mucosa-associated gene signature, including skin-specific tissue self-antigens (TSAs). Importantly, in the absence of Krt76 this skin and oral mucosa TSA signature is almost entirely lost, and T cell negative selection is affected. Collectively, these data highlight a heretofore unanticipated role for Krt76 in thymic central tolerance to skin and oral mucosatargeting T cells and suggest that loss-of-keratin-associated skin disorders could also include autoimmune pathologies.

Vaccine responses vary across individuals due to differences in ageing and health status. Using transcriptomic profiling, we analyzed early gene expression profiles after influenza (QIV) followed by pneumococcal (PCV13) vaccination in 148 participants spanning young, middle-aged, and older adults. The two vaccines induced distinct immune signatures: QIV elicited innate and interferon immune activation, while PCV13 triggered inflammation-based responses. Older adults showed weaker but similar transcriptomic profiles compared to young adults. Among older adults, frailty, in addition to age, was strongly associated with reduced innate responses. In addition, we identified associations between early-stage transcriptomic profiles and later-stage antibody responses for QIV; however, no such associations were observed for PCV13. Importantly, observed group differences arose not from altered immune modules but from differences in the magnitude of gene expression, paving the way for immune-boosting interventions to enhance early gene expression in at-risk populations.

Enterococci are opportunistic pathogens classified by the World Health Organization as high-priority microorganisms. They cause a broad spectrum of infections, and their intrinsic and acquired resistance to antimicrobials makes these infections particularly difficult to treat and eradicate. In Enterococcus faecalis, the most frequently isolated enterococcal pathogen in humans, antimicrobial resistance and innate immune evasion are largely driven by the Enterococcal Polysaccharide Antigen (EPA). This surface polymer underpins key virulence traits, including resistance to host defence mechanisms, reduced susceptibility to multiple classes of antimicrobials, and susceptibility to bacteriophage infection. EPA consists of a rhamnan backbone decorated with strain-specific substituents that are essential for its biological activity. Here, we show that epaB encodes the enzyme responsible for the first committed step in assembling the EPA rhamnan chain. Using NMR spectroscopy, we demonstrate that E. faecalis lacking epaB produces an EPA polymer composed solely of decorations directly anchored to the peptidoglycan, with no detectable rhamnan backbone. The absence of this rhamnan moiety profoundly alters cell wall architecture, as revealed by atomic force microscopy of the mutant cell walls. The epaB mutation also abolishes innate immune evasion and virulence in the zebrafish infection model, while conferring resistance to bacteriophages. Collectively, these findings demonstrate that both the rhamnan backbone and its decorations are required for EPAs full biological activity, establishing the structural and functional interdependence of these two components.

Symptoms of Clostridioides difficile infection (CDI) are primarily caused by two major protein toxins, toxin A (TcdA) and toxin B (TcdB). In addition, approximately 5-30% of C. dif[fi]cile strains produce a third toxin, C. difficile binary toxin (CDT), which is has been associated with enhanced virulence and severe disease. CDT consists of an enzymatic component CDTa, and a binding and translocation component CDTb, which mediates the delivery of CDTa into host cells. CDTb contains two receptor-binding domains, RBD1 and RBD2. Recent structural studies suggest that RBD2 plays a critical role in the formation and stabilization of the di-heptameric CDTb assembly required for efficient intoxication of host cells. In this study, we evaluated the immunogenicity and protective potential of RBD1 and RBD2 using in silico, in vitro and in vivo approaches. Sequence analysis demonstrated that RBD2 is highly conserved among diverse CDT-producing C. difficile ribotypes and toxinotypes. Immunization of mice with RBD2, but not RBD1 conferred effective protection against direct CDT challenge. Moreover, RBD2 immunization protected hamsters against infection with a CDT-only-producing C. difficile strain (DSM 101085; TcdA-TcdB-CDT). Mechanistically, anti-RBD2 serum, but not anti-RBD1 serum, effectively neutralized CDT-mediated cytotoxicity, as demonstrated by inhibition of cell rounding in Vero cells. Collectively, these findings identify RBD2 as a promising vaccine antigen targeting CDT and provide functional evidence supporting its critical role in CDT-mediated host-cell intoxication. Incorporation of RBD2 into multivalent C. difficile vaccines may broaden protection against hypervirulent, CDT-producing strains.

The increasing incidence of H5N1 influenza virus transmission from animal species to humans has heightened concerns about an imminent H5N1 pandemic. Prior studies using recombinant hemagglutinin and neuraminidase proteins have reported age-dependent cross-reactivity to H5N1, attributed to immune imprinting from an individual's first influenza virus exposure. However, whether this pattern holds when using whole inactivated virus (WIV), capturing antibodies against diverse viral proteins, and is stable over time remains unknown. We therefore aimed to determine whether H5N1 cross-reactivity of pre-existing antibodies to whole virus follows an age-dependent or imprinting-specific pattern, and whether this pattern is stable over a five-year period. To this end, we measured serum antibody levels in adolescents, adults and seniors by ELISA using whole inactivated H5N1 virus as antigen rather than purified proteins. Detectable, albeit generally low, levels of H5N1-reactive antibodies were present in most individuals, irrespective of age. Comparison of antibody levels against H5N1 with those to five historical influenza virus strains revealed a consistent positive correlation between H5N1-reactive antibodies and responses to the H1N1pdm09 strain A/California/7/2009 (CA), across all age groups. Using unbiased clustering of antibody titers against H5N1, CA, and the H3N2 strain A/Perth/16/2009 (PE), we identified seven distinct age-transcending antibody profiles. These profiles covered individuals with varying titers to all three included viruses but also identified individuals with high anti-CA levels, yet low anti-H5N1 levels and vice versa. Moreover, despite stable antibody levels over a five-year interval in the study population, individual antibody levels and profiles fluctuated considerably over this period. Taken together, our results confirm the presence of H5N1-reactive antibodies in human sera and their association with previously circulating strains. However, they also caution against inferring antibody levels against a new strain based solely on responses to antigenically related strains and highlight the limitations of extrapolating immune status from single timepoint measurements.

The cell-intrinsic complement system has emerged as an important orchestrator of a variety of cell-physiological processes, with complement components interacting with intracellular effector systems to regulate cellular responses to pathogens or noxious stimuli. For instance, intracellular C5 signaling through a mitochondrial C5a receptor (C5aR1) controls IL-1{beta} production in human monocytes and macrophages. Here, we investigated whether cell-intrinsic C3 similarly regulates inflammatory responses in macrophages. In LPS-stimulated C3 knockout THP-1-derived macrophages, interferon (IFN)-{beta} production was increased, accompanied by elevated expression of interferon-stimulated genes and enhanced secretion of IFN-induced cytokines and chemokines. C3-deficient cells showed increased phosphorylation of IRF3 at Ser396 and a stabilization of the interaction between IRF3 and TBK1, along with enhanced IRF3 dimerization and nuclear translocation. TBK1 phosphorylation was unaffected, indicating that C3 limits IRF3-TBK1 complex formation rather than upstream TBK1 activation. Small-molecule inhibitors of complement factors B and D restored full-length C3 abundance in LPS-stimulated primary human macrophages, consistent with inhibition of the C3 convertase. It also reduced LPS-induced IFN-{beta} production in primary human macrophages and THP-1 cells, suggesting that full-length, uncleaved C3 suppresses IFN-{beta} production. Collectively, these findings identify cell-intrinsic C3 as a suppressor of IFN-{beta} production in human macrophages, highlighting the importance of the cell-intrinsic complement system in fine-tuning inflammatory responses to pathogens.

Antigen presenting cells (APC) can bind HIV-1 and subsequently trans infect CD4+ T cells. In comparison to direct (cis) infection of CD4+ T cells by free virus, APC-mediated HIV-1 trans infection is significantly more efficient and requires lower virus titers. As such, B cell-mediated HIV-1 trans infection of CD4+ T cells, particularly in secondary lymphoid organs (SLO) where B and CD4+ T cells interact frequently in and around B cell follicles, represents an efficient pathway for establishing and maintaining the latent HIV-1 reservoir. The molecular events involved in HIV-1 binding to B cells and transfer to CD4+ T cells are poorly understood. B cells are exposed to various activation signals in SLO including CD40 ligand (CD40L), interleukin-4 (IL-4), interferon-g (IFN-g), and B cell activating factor (BAFF). Here, we treated B cells with these different signals, or combination of signals, to identify those that facilitate HIV-1 binding to B cells and trans-infection of CD4+ T cells, and the mechanisms involved. We found that CD40L/IL-4 stimulated B cells are highly efficient mediators of HIV-1 trans infection of CD4+ T cells due to their enhanced capacity to bind HIV-1. Single cell RNA sequencing of differentially stimulated B cell populations revealed that CD40L/IL-4 stimulation significantly induced expression of the C-type lectin CD205. Confocal microscopy revealed that HIV-1 and CD205 co-localized on CD40L/IL-4 stimulated B cells, and antibody blocking of CD205 on these cells significantly reduced HIV-1 binding. Taken together, this study identifies CD205 as a critical receptor on B cells that facilitates HIV-1 binding and the transfer of virus to CD4+ T cells. Insight into the role of B cell mediated HIV-1 trans infection of CD4+ T cells is critical to optimizing the effectiveness of HIV-1 therapies in SLO.

Klebsiella pneumoniae (Kp) is a WHO high-priority pathogen for vaccine development, yet previous efforts failed largely because key protective immune mechanisms remain unclear. Here we show that protective immunity conferred by mucosal mRNA vaccines (but not parenteral) require neither serum IgG nor airway secretory IgA, but instead depends on a previously unrecognized lung-resident CD8IL-17 T-cells (Tc17) that rapidly recruits neutrophils/macrophages to eliminate bacteria. To therapeutically harness this paradigm, we developed INSPIRE, a machine learning-engineered exosome platform incorporating donor-screened, miRNA-bioactive backbones (miR-21-mediated airway barrier penetration and miR-155-associated dendritic-cell activation through SOCS1/Inpp5d axis) and computationally designed peptides that boosts 11.6-fold mRNA encapsulation and 3-fold dendritic-cell cross-presentation. Intranasal INSPIRE-mRNA vaccination confers near-complete protection against clinically relevant Kp strains while intramuscular counterparts fail (below ~30% survival). Leveraging pIgR-/- and IL-17-/- mice coupled with T-cell depletions, we demonstrate the protection is Tc17-dependent. This work overturns the antibody-centric dogma and redefines a non-canonical Tc17-correlate for extracellular bacterial pneumonia.

Background: Transplant arterial injury is an underlying feature of acute organ transplant rejection and is a main cause of late heart transplant failure. The role of the gut microbiota, and especially specific microbial components of this community, in controlling immune responses that cause this aspect of rejection is poorly understood. Methods: We utilized a murine aortic interposition model of transplant arterial injury to investigate the role of the gut commensal bacteria, Akkermansia muciniphila, in controlling immune responses in transplant arteries. Results: Early life treatment of female mice with broad spectrum antibiotics, which delayed colonization of the intestinal tract with bacteria until after weaning, led to the development of dysbiosis in adults that was characterized by the absence of A. muciniphila. This was related to an elevation in systemic levels of CCL2 and a reduction in the immunomodulatory short-chain fatty acid, propionate. When transplant arterial injury was examined, there was more arterial injury indicative of acute rejection and increased intimal thickening reflective of transplant arteriosclerosis in grafts from dysbiotic mice compared to controls. Dysbiosis also increased macrophage accumulation early after transplantation in dysbiotic mice. Notably, restoring A. muciniphila in the gut microbiota of dysbiotic mice through voluntary oral administration in infants ameliorated macrophage-mediated transplant arterial injury. Conclusions: A. muciniphila is an immunomodulatory component of the gut microbiota that protects against vascular injury and pathology in organ transplantation.

Mechanisms shaping population vulnerability to typhoon-related mortality remain poorly understood. Constructing a Bayesian spatio-temporal model, we linked 12.9 million deaths across Japan from 2010 to 2019 to population-weighted typhoon wind exposure and assessed effect modification by income, natural hazard vulnerability and healthcare access. Typhoon exposure was associated with 2,426 cumulative excess deaths [95% credible interval: 139, 4,632] among adults [&ge;]70 years, with mortality increasing within 0-1 weeks of exposure and more strongly in areas with limited healthcare access and greater hazard vulnerability. Among individuals <70 years, cumulative excess mortality was uncertain [781 deaths; -309 to 1,900], but delayed mortality increases were concentrated in lower-income and landslide-prone areas. These distinct patterns suggest that typhoon mortality reflects an interaction between acute exposure, demographic ageing and geographically uneven adaptive capacity, highlighting the need to incorporate local vulnerability into climate-resilient health systems.

Background VEXAS syndrome is a late onset autoinflammatory disease caused by somatic UBA1 mutations and characterized by heterogeneous systemic and hematologic manifestations. We aimed to describe all identified Belgian cases through a national multicenter cohort. Methods We conducted a retrospective study across four Belgian tertiary centers. Clinical, biological, genetic, therapeutic, and outcome data were collected using standardized anonymized case report forms. Analyses were descriptive. Results Twenty-one male patients were identified between January 2018 and May 2025. General symptoms such as Fatigue, weight loss and sweating occurred in 95% of cases. The most frequent manifestations were cutaneous (85.7%), hematologic (76.2%), articular (66.7%), thromboembolic (57.1%), chondritis (42.9%), ophthalmologic (38.1%), pulmonary (38.1%). Other manifestations also included vasculitis (61.9%). At diagnosis, 95% had anemia, macrocytic in 57%, and 28.6% had thrombocytopenia. Corticosteroids were the main first line therapy. Second line treatments included anti IL 6 agents (46.7%), JAK inhibitors (20%), and azacitidine (14.3%). Complete remission occurred in 50% of patients receiving anti IL 6 therapy and in 33% treated with either JAK inhibitors or azacitidine. Two patients underwent allogeneic stem cell transplantation, one died from infectious complications. Twenty six infectious episodes were recorded, including opportunistic infections. Six patients (28.6%) died during follow-up, four from infectious complications. Conclusion This first Belgian national cohort confirms the clinical heterogeneity of VEXAS syndrome and highlights substantial infectious morbidity and mortality. Access to targeted second-line therapies, particularly anti IL-6 agents and JAK inhibitors, remains challenging despite apparent clinical benefit.