Journal of Experimental Medicine

Cerebral malaria is a severe neurological complication of Plasmodium falciparum infection. Damage of the blood-brain barrier (BBB) and endothelial dysfunction are established drivers of the disease pathology, however, whether astrocytes, a major constituent of the BBB, influence the disease outcome remains unclear. Using the murine model of experimental cerebral malaria (ECM), we show that astrocytes decisively regulate the outcome of ECM and the deubiquitinating enzyme OTUD7B in astrocytes fosters the disease. Mice lacking astrocytic OTUD7B showed reduced brain pathology and were protected from ECM compared with wildtype littermate controls. Transcriptomic profiling of ex vivo-isolated astrocytes revealed reduced proinflammatory chemokines and cytokines in the absence of OTUD7B. Plasmodium infection-associated microvesicles triggered a pro-inflammatory response in astrocytes, which was dependent on OTUD7B. Mechanistically, OTUD7B cleaved K48-linked ubiquitin chains from TRAF3 and TRAF6 upon stimulation with microvesicles or activation of TLR3/TLR9 by plasmodial nucleic acids. The OTUD7B-dependent TRAF3 and TRAF6 stabilization led to sustained NF-{kappa}B and p38 MAP kinase signaling and CXCL10 expression. Therapeutic silencing of CNS Otud7b or Cxcl10 expression after disease onset protected mice from ECM, identifying the cerebral OTUD7B-Cxcl10 axis as an attractive therapeutic target.

Food provides nutrients that are selectively absorbed by the intestine, but, at the same time, may contain elements that challenge the intestinal barrier and induce post-prandial inflammation (PPI). How PPI is controlled in order to avoid pathological perturbation of homeostasis remains unclear. Here, we report that during fasting, enterocytes increase their absorptive potential and oxidative metabolism, a program that is largely reversed upon food intake of lipids that perturb the intestinal barrier and induce PPI. Such perturbation is countered by ILC3s, in the absence of which PPI increases, program reversal does not occur, and enterocytes engage into excessive oxidative metabolism. This enterocyte state leads to critical hypoglycemia as a consequence of decreased glucose absorption and increased insulinemia, recapitulating the pathological situation found in patients suffering from intestinal damage and sepsis. We hereby uncover a critical function for ILC3s in maintaining enterocyte homeostasis upon challenging food intake.

Fibroblasts play a dual role in shaping tissue homeostasis and immune responses during inflammatory perturbations. Manipulating fibroblast behavior has therefore emerged as a promising strategy for autoimmune diseases. Here, through integrated multimodal single-cell transcriptomic and proteomic profiling of synovial tissue combined with prospective clinical data from 54 patients with rheumatoid arthritis, we identify C-X-C motif chemokine 12 (CXCL12)hi Apolipoprotein C1 (APOC1)+ fibroblasts as a pathogenic cell population driving refractory synovitis. CXCL12hi APOC1+ fibroblasts construct local niche in spatial coordinates with plasmablasts via the CXCL12-CXCR4 axis. APOC1 orchestrates senescent inflammatory cancer-associated fibroblast(iCAF)-like properties of this cluster through activation of the STAT3-C/EBP pathway. Therapeutic elimination of senescent cells, either alone or in combination with TNF inhibition, significantly ameliorates experimental arthritis. Together, these findings uncover a mechanistic basis for treatment resistance in rheumatoid arthritis and highlight senescent iCAF-like fibroblasts as a promising therapeutic target.

Systemic vaccination induces serum antibodies and circulating memory B cells but provides limited protection in the upper respiratory tract, where many respiratory pathogens initiate infection. How systemic memory B cells contribute to mucosal immunity remains unclear. Using multiparametric flow cytometry, single-cell RNA and V(D)J sequencing, and functional analyses of paired blood and nasal/oropharyngeal samples, we characterized human B cells across systemic and mucosal compartments. Swab-derived B cells transcriptionally overlap with circulating activated memory B cells while exhibiting distinct features of activation, tissue retention, and spontaneous IgA/IgG secretion. Approximately 6% of mucosal B-cell clones were shared with blood, indicating systemic-mucosal connectivity. Both infection and vaccination expanded two circulating antigen-specific activated memory B cells subsets, whereas antigen-specific B cells accumulated in the upper respiratory tract only following local inflammation. The finding that B-cell recruitment is reactive rather than preemptive may explain the limited efficacy of parenteral vaccines and provides a rationale for developing integrated systemic-mucosal vaccination strategies.

NLRP3 is an innate immune sensor of a broad range of stimuli, which upon activation forms a multiprotein inflammasome complex triggering caspase-1 activation, IL-1{beta} and IL-18 maturation, and inflammatory cell death. The canonical NLRP3 activation pathway has been well characterized from a structural perspective. It involves the association of NLRP3 with membranes in the form of inactive oligomeric "cage" complexes, which, upon activation, convert to an active oligomeric NLRP3 disc. NLRP3 structural rearrangements during non-classical NLRP3 activation pathways, however, remain unknown. Here, we report a novel mode of NLRP3 activation utilized by the NLRP3 homolog from zebrafish. The cryo-EM structure of zebrafish NLRP3 shows that, unlike human NLRP3, it forms disc-shaped heptamers that undergo further trimerization, resulting in a 21-mer oligomeric arrangement. Surprisingly, a single zebrafish NLRP3 heptamer cannot arrange its PYD domains into a PYD helix and therefore requires a trimer of heptamers to form a PYD filament that enables ASC oligomerization. Furthermore, zebrafish NLRP3 does not associate with the Golgi network, nor does it form inactive "cage" oligomers or interact with NEK7. Thus, our data demonstrate an ancestral non-canonical structural mechanism of NLRP3 activation, which may shed light on alternative NLRP3 activation pathways present in humans.

Regulatory T cells (Tregs) prevent autoimmunity through suppressive functions largely programmed by the transcription factor FOXP3. Healthy humans express approximately equivalent levels of two major alternatively spliced isoforms of FOXP3: a full-length version containing all coding exons (FOXP3-FL) and a version lacking exon 2 (FOXP3-{Delta}E2). However, sole FOXP3-{Delta}E2 expression causes lethal IPEX syndrome, and the FOXP3-{Delta}E2 isoform is elevated in several autoimmune diseases. These observations strongly suggest defects in suppression by FOXP3-{Delta}E2 Tregs which we investigated here using Foxp3-{Delta}E2 mice. In an influenza virus infection model, Foxp3-{Delta}E2 mice had an increased magnitude of the CD8+ T cell response during acute and memory formation phases of infection. Transcriptomic and chromatin accessibility analyses of homeostatic Foxp3-{Delta}E2 Tregs revealed impaired Treg programming, including reduced expression of inhibitory molecules such as Il2ra and chemokine receptors. Decreased cell surface CD25 expression on Foxp3-{Delta}E2 Tregs was associated with reduced IL-2 responsiveness in Foxp3-{Delta}E2 Tregs and, reciprocally, increased IL-2 responsiveness in CD8+ T cells from Foxp3-{Delta}E2 mice. Additionally, altered chemokine receptor expression resulted in diminished localization of Foxp3-{Delta}E2 Tregs to the T cell zone of the inflamed lymph node. Thus, Treg programming by the Foxp3-{Delta}E2 isoform impairs suppressive function, resulting in failure to restrain CD8+ T cells and aberrant immune responses.

During ageing, hematopoietic stem cells (HSCs) have reduced regenerative potential, skewed differentiation toward the myeloid lineage, and heightened susceptibility to clonal expansion and malignancy. While epigenetic alterations are well documented, the impact of aging on higher-order 3D chromatin architecture remains poorly understood. Here, we examined the 3D genome organisation of aged murine HSCs using in-situ Hi-C then integrated this with gene expression and chromatin accessibility data to build HiC-informed gene regulatory networks (GRNs). Aged HSCs display erosion of topologically associating domain (TAD) boundaries, A/B compartment switching, and reorganised enhancer-promoter loops associated with lineage-inappropriate gene expression. Our GRN analysis identifies a hierarchy of transcription factors, including a c-Maf-Lyl1-Mnt axis that orchestrates the transition from a youthful to aged state and a Gfi1-Sox4 axis in young HSCs that regulates Bach1. This study provides a structural blueprint for aging HSCs and defines specific regulatory targets for potential reprogramming interventions to restore hematopoietic youthfulness.

Microglia, the resident macrophages of the central nervous system, are recognized for their heterogeneity and integral role in brain function and diseases. In the context of high fat diet (HFD) feeding and obesity, microglia become overactive, acquiring a prevailing lipid associated microglial phenotype (also known as LAM). Yet, how microgliosis is induced and regulated remains unclear. Here we report a key role for the Complement 3a Receptor (C3aR), on HFD-induced hypothalamic gliosis and weight gain in mice. HFD consumption leads to elevated microglial expression of C3aR, which parallels widespread accumulation of reactive microglia, selectively in the hypothalamus. Conditional microglial C3aR deletion protects mice from HFD-induced hypothalamic reactive microgliosis. C3aR deletion or pharmacological antagonism opposes HFD-induced weight gain in male but not female mice. Mechanistically, we demonstrated that C3aR is essential for lipid-induced lipid droplet formation, and acquisition of a LAM molecular signature. In summary, we uncovered a previously unknown role for C3aR in the acquisition of a LAM signature driving diet-induced gliosis, identifying this receptor as a new viable therapeutic candidate for conditions associated with hypothalamic neuroinflammation.

How plasma cells (PCs) shape anti-tumor immunity is unclear. We hypothesized that conflicting prognostic associations reflect differences in immune context and PC ontogeny. We identify extrafollicular (EF)-PCs as an antibody-independent checkpoint that aborts priming by disabling the cDC1[->]CD8+ T-cell axis in tumor-draining lymph nodes (td-LNs). EF-PCs blunt cDC1 activation and CCR7-guided repositioning into T-cell zones, precluding formation of TCF1 stem-like CD8 T-cells. Depleting EF-PCs in vivo restores cDC1 trafficking, expands the stem-like reservoir, increases intratumoral CD8 infiltration, and restrains tumor growth; benefit is lost with CD8 T-cell ablation. Neither serum transfer nor Fc{gamma} receptor blockade reverses tumor control, supporting a non-canonical, antibody-independent mechanism. Across independent triple-negative breast cancer cohorts, we find EF-PC hyperplasia in td-LNs and tumors; and within immune-cold cases, EF-PC burden stratifies poor prognosis and metastatic risk. A cross-species EF-PC signature maps to a conserved PC-state across cancer types that is linked to poor outcome and immune-checkpoint blockade resistance. EF-PCs thus relocate the dominant failure point to td-LNs and offer a tractable upstream target to convert immune-cold tumors into immune-responsive disease.

Protein arginine methyltransferase 5 (PRMT5) is a synthetic lethal target in methylthioadenosine phosphorylase-deleted (MTAP-null) cancers. Second-generation MTA-cooperative PRMT5 inhibitors preferentially target MTAP-null cells while largely sparing MTAP-wildtype (MTAP-WT) cells, thereby improving tumor selectivity over first-generation PRMT5 inhibitors. Despite encouraging efficacy and safety signals in early clinical studies, the modest objective response rates (ORRs) observed with these inhibitors suggest that intrinsic or acquired resistance may limit their clinical benefit. Here, we investigated mechanisms of acquired resistance to the MTA-cooperative PRMT5 inhibitor BMS-986504/MRTX1719 in MTAP-null non-small cell lung cancer (NSCLC) cells and sought to identify therapeutic vulnerabilities that emerge upon resistance. Using multiple in vitro-derived resistant models, we found that acquired resistance was not fully explained by alterations in PRMT5 activity or reduced MTA levels. Instead, resistance was associated with collateral sensitivity to MEK inhibition and enrichment of MAPK-related transcriptional programs. Together, these findings identify MEK inhibition as an actionable collateral vulnerability in MTAP-null NSCLC cells that acquire resistance to PRMT5 inhibition.

Vaccination programs worldwide have effectively reduced the burden of childhood diseases, yet immune responses remain highly heterogeneous among individuals. While host characteristics such as age and sex are established determinants of vaccine immunogenicity, the timing of vaccination, specifically the calendar season of vaccination, remains largely underexplored. Although circadian rhythms are known to regulate daily immune function, evidence for long-term circannual patterns has been limited by the difficulty of collecting year-round vaccination data across diverse populations. Here, we show that the season of vaccination systematically shapes the immune response across a broad range of pediatric vaccines. By leveraging data from 96 randomized control trials worldwide, including over 48,000 children vaccinated against 14 pathogens, we demonstrate that immunogenicity after vaccination follows a pronounced latitudinal gradient, typically peaking during colder months in temperate regions and exhibiting distinct variability in the tropics. These findings suggest that the circadian human immune response might extend to a circannual scale, potentially synchronized by environmental cues. Incorporating the season of vaccination into the design of clinical trials and public health campaigns may optimize vaccine performance and enhance seroprotection.

Neuroinflammation is a central driver of tauopathy, yet the precise chemokines that orchestrate the inflammatory microenvironment remain elusive. Here, we report C-X-C motif chemokine ligand 10 (CXCL10) is markedly upregulated in the brains of tauopathy model mice, where it co-localizes with prominent tau pathology. Notably, genetic ablation of Cxcl10 in these mice significantly attenuates tau burden and extends the survival period, specifically in a female-dependent manner. Mechanistically, although Cxcl10 deficiency reduces the number of brain T cells in both sexes, this reduction does not correlate with the female-specific rescue of the phenotype. Furthermore, Cxcl10 deficiency did not alter glial cell activation or motor function, suggesting a sex-specific mechanism. We show CXCL10 is primarily produced by pathological glia, fostering a localized inflammatory microenvironment. Our findings identify CXCL10 as a key mediator of tau pathology and reveal a sex-dimorphic regulatory axis that operates independently of T cell and glial activation paradigms.

Human Fc receptor-like 5 (FCRL5) is a low-affinity IgG Fc receptor expressed on various B cell subsets and a potential therapeutic target. We discovered that commonly used Fc-silencing mutations, designed to prevent interactions between the Fc{gamma} receptors on immune cells and the Fc domain of therapeutic IgG, do not prevent binding to FCRL5. As a result, unintended interactions between Fc-silent therapeutic IgG and human B cells may occur. We isolated a well-expressed variant of the Fc-binding portion of human FCRL5 by directed evolution and used structural modeling to guide the engineering of a human IgG1 Fc variant with approximately 100-fold higher affinity for FCRL5, enabling us to produce FCRL5:Fc complexes in solution. Native mass spectrometry, size exclusion chromatography, and the crystal structure of the FCRL5- IgG1 Fc complex solved at 3.4 [A] indicate that the two proteins bind in a 1:1 stoichiometry. Furthermore, the structure revealed that FCRL5 binds to IgG1 Fc in a manner completely distinct from that of previously characterized Fc-binding proteins, such as Fc{gamma} receptors, explaining why most Fc-silencing mutations do not disrupt FCRL5 binding. We demonstrate that selective cross-linking of FCRL5 with the B cell receptor (BCR) in cis, using Fc-engineered antibodies with either physiological or enhanced FCRL5 affinity, inhibits Ca2+ flux in FCRL5-expressing B cells. We compare this effect with the selective co-ligation of Fc{gamma}RIIb with the BCR. Our work demonstrates that FCRL5 interacts with human IgG Fc in a distinctive manner and that engagement of FCRL5 by Fc-silent therapeutic IgG could influence B cell function.

Sarcoidosis is a heterogenous disease of unknown etiology characterized by non-caseating granulomas. Disease prevalence and presentation vary significantly by ancestry and ranges from acute, self-resolving disease to severe, chronic disease. Following previous reports suggesting B cells in the development and pathogenesis of sarcoidosis, we present here results of single-cell RNA sequencing, supporting B cell involvement in sarcoidosis through altered immediate early response, rewiring of MAPK signaling, and ancestry-specific preferential expansion of B cell receptors. Peripheral blood mononuclear cells were obtained from individuals of African or European Ancestry (AA and EA, respectively) including 48 healthy controls, 59 sarcoidosis patients, and 28 systemic lupus erythematosus (SLE) patients. SLE samples were used as a disease control. Differential expression analysis highlighted many differentially expressed genes (DEGs) with almost 5x more in the AA sarcoidosis versus AA control group compared to the EA sarcoidosis versus EA control group. B cells had the most DEGs of all cell types and expression patterns were similar between ancestries, however, sarcoidosis had an opposite transcription pattern than SLE, demonstrating an alternative immune response to acute activation than that seen in a prototypical autoinflammatory disease. This trend was maintained when examining specialized B cell subsets, with the most pronounced effect in the AA sarcoidosis versus AA control comparison. Our results strongly support further investigation of the role of humoral immune response in sarcoidosis and the potential to highlight patient groups likely to benefit from existing B cell therapies.

CDK4/6 inhibitors are standard-of-care for metastatic estrogen receptor-positive (ER+) breast cancer, yet the development of resistance remains a significant clinical hurdle. While CDK4/6 inhibitors are primarily recognized for their ability to induce cytostasis, their role in modulating innate immune responses remains poorly defined. Here, we demonstrated that CDK4/6i treatment remodels the tumor cell surface to favor recognition and elimination by Natural Killer (NK) cells. Using a diverse biobank of patient-derived organoids (PDOs), we found that CDK4/6 inhibition robustly upregulated the adhesion molecule ICAM-1 and the NKG2D stress ligands (ULBP2/5/6 and MICA/B). This NK-engaging cell surface phenotype was driven by a bifurcated signaling network: NF-{kappa}B signaling orchestrated ICAM-1 induction, while the PI3K/mTOR pathway regulated the expression of stress ligands. Functional assays confirmed that these ligands were indispensable for NK cell-mediated elimination of breast cancer cells. In vivo studies using ER+ PDX models revealed that a brief seven-day primer treatment with the CDK4/6 inhibitor abemaciclib was sufficient to sensitize tumors to NK cell therapy, significantly inhibiting tumor growth and prolonging survival. We also observed efficacy with a concurrent dosing strategy that delayed the onset of acquired resistance. These findings provide a mechanistic rationale for combining CDK4/6 inhibitors with NK cell therapy. This "prime and kill" approach offers a promising strategy to overcome therapeutic resistance and improve outcomes for patients with metastatic ER+ breast cancer.

Respiratory viral infections in early childhood are major drivers of acute morbidity and long-term airway disease, yet how distinct viruses remodel the pediatric nasal mucosa at cellular resolution remains unresolved. Here, we generated a single-cell RNA sequencing atlas of 335,174 nasal epithelial and immune cells from 132 children under five years of age with SARS-CoV-2, rhinovirus, or respiratory syncytial virus (RSV) infection, alongside uninfected controls. Mapping viral transcripts to individual cells revealed virus-specific infected epithelial states: an NF-kB-responsive ciliated subset in SARS-CoV-2 and a previously undescribed KRT17+ squamous-like subset in RSV. We delineated divergent mucosal response programs, including a robust interferon (IFN) response in SARS-CoV-2, an IL-13-responsive secretory program in rhinovirus, and heightened inflammatory and cytotoxic immune activation in RSV. In RSV, specific immune subsets and elevated IFN-response signatures were associated with disease severity, whereas rhinovirus-induced wheeze was marked by expansion of a CST1+ goblet cell subset. Integration of asthma genome-wide association data with our atlas revealed a KRT13+ hillock-like squamous epithelial subset enriched for expression of childhood-onset asthma risk loci. Finally, we demonstrate that this resource enables high-resolution annotation of independent pediatric cohorts in Kolkata, India and rural Bangladesh. Together, this atlas establishes a comprehensive view of antiviral immunity in the pediatric nasal mucosa and defines virus-specific mucosal immune programs relevant to disease severity and asthma risk in early life.

Genome-wide association studies (GWAS) have advanced the understanding of germline susceptibility in common cancers, yet rare malignancies remain underexplored due to limited sample sizes. To address this gap, we conducted large-scale GWAS across 20 rare cancer types and meta-analyzed results from three cohorts: two clinically sequenced cancer center cohorts and an independent population biobank, comprising over 480,000 individuals. We identified nine novel genome-wide significant susceptibility loci with moderate to large effect sizes that replicated across cohorts in eight rare malignancies, including myelodysplastic syndromes (MDS), germ cell tumors, gastrointestinal stromal tumor (GIST), gastrointestinal neuroendocrine tumors, anal cancer (ANSC), non-melanoma skin cancer, mesothelioma, and hepatobiliary cancer. Among the strongest associations were loci in MDS near API5 (OR = 2.21, p = 1.06x10-8), in GIST near SLC6A18 and TERT (OR = 1.91, p = 8.20x10-50), and in ANSC near HLA-DQA2 (OR = 1.58, p = 5.50x10-18). The GIST risk variant was enriched in tumors harboring somatic KIT mutations (OR = 2.21, p = 6.5x10-4) and was associated with worse survival among carriers with KIT-mutant tumors (hazard ratio = 4.06, p = 0.015), implicating germline-somatic interplay in tumor initiation and progression. The ANSC risk variant was associated with HPV infection (OR = 1.44, p = 3.19x10-5), supporting a host-viral interaction in HPV-driven tumorigenesis. The MDS risk variant at the API5 locus was associated with altered neutrophil counts, suggesting a role in hematopoietic dysregulation in disease pathogenesis. We further identified novel, independent associations with mesothelioma, GIST, and hepatobiliary cancer at the 5p15.33 locus encompassing TERT, consistent with pleiotropic genetic effects at a core telomere-maintenance gene. Collectively, these findings demonstrate that integrating clinically ascertained sequencing cohorts with population biobanks substantially enhances germline discovery in rare cancers, enabling identification of high-confidence susceptibility loci and facilitating downstream biological interpretation through linked somatic, viral, and clinical data. This framework provides a scalable approach for characterizing inherited susceptibility across diverse rare malignancies.

The recent approval of KRAS inhibitors supports the therapeutic value of targeting mutant KRAS cancers. However, clinical efficacy is hindered by both primary and treatment-associated acquired resistance. We applied a CRISPR-Cas9 loss-of-function screen and identified loss of KEAP1 as a resistance mechanism to the KRASG12D-selective inhibitor MRTX1133 and the RAS(ON) multi-selective inhibitor RMC-7977 in pancreatic cancer models. RNA-sequencing analyses revealed a KEAP1KO transcriptome that is distinct from the ERK-, MYC-, and YAP/TAZ-TEAD-dependent transcriptional programs that drive KRAS inhibitor resistance, demonstrating a distinct mechanism of resistance. We then established a PDAC KEAP1-deficient (PKD) gene signature that was enriched in patients and preclinical models insensitive to KRAS inhibitor treatment. Finally, we observed that KEAP1-deficient cells exhibited elevated glutamine metabolism, and combination treatment with the glutamine antagonist DRP-104 (sirpiglenastat) enhanced KRAS inhibitor suppression of pancreatic and lung tumors. SIGNIFICANCEKEAP1 loss is associated with reduced response to KRAS inhibitor therapy. We demonstrate that KEAP1 loss-associated resistance can be overcome by pharmacologic inhibition of the KEAP1 loss-induced glutamine dependency, establishing a combination to enhance RAS inhibitor clinical efficacy.

Circadian clocks impose temporal architecture on signaling networks, and disruption of this architecture predisposes to cancer metastasis. Through direct pharmacological and genetic perturbation of core clock components, we establish that circadian desynchronization in mouse lung fibroblasts eliminates temporal migration gating, creating constitutive motility responses to TNF- and TGF-{beta}, and identify YAP/TEAD as the obligate, non-redundant convergence node through which clock-regulated ECM mechanical signals and cytokine-driven transcriptional programs jointly drive cellular motility. Chronic jet lag (CJL) in mice generates nocturnal TNF- elevation (ZT12-21) that drives sustained matrix metalloprotease expression while simultaneously reorganizing Hippo and TGF-{beta} signaling toward temporal convergence during daytime hours, enabling YAP/TEAD-dependent transcription to synergize with TGF-{beta} signaling and drive epithelial-to-mesenchymal transition (EMT) programs that normally remain temporally restricted. Functional validation demonstrates CJL doubles metastatic colonization incidence (40% to 90%) following B16F10 melanoma inoculation. Critically, established metastases amplify these molecular changes: metastatic burden under CJL creates maximal TGF-{beta} expression (ZT15-21), constitutive YAP activity, and sustained EMT marker expression, while eliminating M1/M2 macrophage temporal organization. Analysis of TCGA-SKCM metastatic melanoma datasets confirms that clock-disrupted human tumors exhibit selective strengthening of YAP/TAZ, EMT, and inflammatory pathway coupling, establishing that this convergence architecture is conserved in human disease. Together, these findings demonstrate that circadian disruption transforms from a facilitator of initial metastatic colonization into a driver of progressive metastatic burden by eliminating the temporal segregation that normally constrains pro-metastatic programs to discrete, non-overlapping windows, creating self-perpetuating cycles wherein pathway convergence facilitates colonization and established tumors amplify pro-metastatic signaling to maintain permissive microenvironmental conditions.

53BP1 nuclear bodies are dynamic and are endowed with properties that resemble biomolecular condensates, but molecular determinants that underlie transition of 53BP1 oligomerisation to its higher-order assembly at DNA double-strand breaks (DSBs) remain to be established. We found that 53BP1 condensation is stimulated by phosphatidylinositol 3-phosphates (PI(3)Ps) in vitro, and is effected via its C-terminal phospho-binding BRCTs. Consistently, mutational inactivation of 53BP1 BRCTs not only compromised PI(3)P binding, but also suppressed its ability to undergo optodroplet formation in vivo. We further show that swift 53BP1 oligomerisation following DNA damage precedes its stable assembly on DSB-flanking chromatin, requires its BRCT domain, and is suppressed by sequestration of nuclear PI(3)Ps. Taken together, we propose that PI(3)P binding underlies maturation of 53BP1 higher-order assembly on the damaged chromatin.