Brain network dysfunction-including hyperexcitability, altered oscillations, and sleep disruption-is prominent in Alzheimer's disease (AD), but the contribution of vascular-neuroimmune processes to these alterations remains unclear. Here, we blocked the pro-inflammatory interaction of the blood protein fibrin with microglia using genetic (Fggγ390-396A mice) and antibody-based (5B8 and THN392) strategies to test its role in AD-related network dysfunction. The 5xFAD model of AD exhibited network hyperexcitability associated with oscillatory slowing, sleep states, and disrupted sleep-circadian rhythms. These deficits were largely attenuated by blocking fibrin-microglia interactions in 5xFAD;Fggγ390-396A mice. Notably, pharmacological interventions after disease onset with both anti-fibrin antibodies similarly attenuated these AD-related network deficits and behavioral abnormalities. We conclude that vascular-neuroimmune processes driven by fibrin-microglia interactions promote AD-related network dysfunction and that targeting the fibrin-microglia axis-currently under clinical evaluation with the humanized antibody THN391- represents a promising therapeutic strategy for AD. There is a companion manuscript submitted to bioRxiv (Yan et al., 2026).110.

Individuals with Down syndrome (DS) have an increased genetic risk of developing Alzheimer’s disease (AD), with most adults developing AD neuropathology in their 40s. Despite having a low frequency of systemic vascular risk factors such as hypertension and atherosclerosis, adults with DS display cerebrovascular pathology, including microbleeds, microinfarcts, and cerebral amyloid angiopathy. This suggests that blood-brain barrier (BBB) integrity may be compromised allowing the extravasation of blood proteins in the brain parenchyma. The blood coagulation factor fibrin promotes immune-mediated neurodegeneration and is a marker of BBB disruption in a wide range of neurological diseases. This study investigated the severity of fibrin deposition as a measure of BBB integrity in the brains of adults with DS and AD pathology (DSAD). We hypothesized that fibrin deposition is increased in DSAD in comparison to neurotypical controls without DS or AD. Fibrin immunoreactivity was assessed by free-floating immunohistochemistry in 30µm tissue sections from the occipital cortex from neurotypical controls (n = 12; 41-65 years old) and DSAD (n = 12; 46-66 years old). Using whole slide imaging, brain sections were digitized, and the severity of fibrin deposition was scored using Aperio Imagescope. Individuals with DSAD display significantly higher fibrin deposition in the white and grey matter of the occipital cortex in comparison to the age-matched neurotypical controls (p<0.0001). Neurotypical controls display minimal fibrin deposition in the brain parenchyma and perivascular space. However, compared to neurotypical controls, adults with DS at advanced stages of AD neuropathology display significant fibrin deposition in the occipital cortex, suggesting that the BBB may be compromised in this population. Funding : NIH U19AG068054, RF1AG079519, P30AG066519 and 23AARFD-1022715.

In the coming year, adults with Down syndrome (DS) will be included in Alzheimer's disease (AD) clinical trials involving anti-amyloid immunotherapies. Such therapies have been associated with adverse cerebrovascular events such as amyloid-related imaging abnormalities (ARIA). Moreover, cerebral amyloid angiopathy (CAA) is associated with an increased risk of ARIA. By the age of 40 years, people with DS exhibit AD neuropathology (DSAD) and cerebrovascular disease, including severe CAA, suggesting that individuals with DS may be at increased risk of ARIA. Thus, to better understand their cerebrovascular profile, our main objective is to characterize cerebrovascular pathology in individuals with DS, including blood-brain barrier (BBB) integrity and vascular morphology. Free-floating immunohistochemistry was used to label the basement membrane (BM), blood vessels, pericytes, and fibrin parenchymal deposition in the occipital cortex of adults with DSAD ( n  = 12) and age-matched neurotypical controls ( n  = 12). We measured basement membrane coverage, vessel length, and density, and the number of pericytes and string vessels (also known as collapsed capillaries or connective tissue strands with no endothelial cells) in lamina III-IV. Fibrin deposition in the brain parenchyma was visually scored using whole brain sections. Two-tailed unpaired t-tests, Mann-Whitney, and Fisher's exact test were used accordingly. We are analyzing the same measures in a second and larger autopsy cohort from young (1-33 years old, n  = 14) and old individuals with DS (42-70 years old, n  = 84), late-onset AD (75-90 years old, n  = 33), and age-matched neurotypical controls (2-90 years old, n  = 72). Compared to neurotypical controls, adults with DSAD displayed increased BM coverage [t(18)=2.145, p  = 0.045], vessel density [t(18)=5.277, p  <0.0001] and length [t(18)=3.43, p  = 0.003]. We also found an increased number of string vessels (U=13, p  = 0.0009) and pericytes [t(18)=3, p  = 0.005]. Pericyte per vessel density did not differ between groups [t(18)=1.813, p  = 0.086]. Finally, we found abundant fibrin deposition in DSAD brains compared to controls (3-fold increase, Fisher's exact test, two-tailed, p  = 0.0002). Adults with DS at late stages of AD neuropathology display vascular changes suggestive of vascular remodeling processes and BBB breakdown. The administration of anti-amyloid beta immunotherapies in this population must be carefully evaluated. Funding: NIH-U19AG068054, RF1AG079519, P30AG066519, 23AARFD-1022715.

Vascular dysfunction and subsequent innate immune activation are key players of neurodegenerative, retinal, and inflammatory diseases, including Alzheimer's disease (AD), multiple sclerosis (MS), diabetic retinopathy (DR), and age-related macular degeneration (AMD). At sites of vascular damage, conversion of the blood coagulation protein fibrinogen to fibrin exposes a cryptic inflammatory epitope, γ377–395, which can bind CD11b/CD18 and CD11c/CD18 complement receptors on microglia, macrophages, and dendritic cells. Genetic targeting of the fibrin γ377–395 epitope or its pharmacologic inhibition with the mouse monoclonal antibody 5B8 protects from inflammation and neurodegeneration in AD and MS mouse models. Here, we present the development of THN391, a first-in-class humanized antibody, to neutralize fibrin toxicity without adverse anticoagulant effects for the treatment of neurodegenerative, retinal, and inflammatory diseases. THN391 was affinity matured with 100-fold greater affinity than 5B8, engineered to lack Fc effector function, and have improved developability properties for clinical use. THN391 blocks the interaction of fibrin with CD11b/c and does not bind fibrinogen nor interfere with coagulation, consistent with the crystal structure of its binding interface to the γ377–395 epitope. THN391 and its Fc wild-type counterpart THN313 showed preclinical efficacy in experimental autoimmune encephalomyelitis (EAE) mouse models of MS and in a rodent model of retinal disease. Both THN391 and THN313 reduced demyelination, inflammatory foci, and clinical scores in EAE, demonstrating that anti-fibrin γ377-395 antibodies function as pure antagonists, blocking fibrin from activating CD11b/c complement receptors. THN391 was as effective as the standard of care vascular endothelial growth factor (VEGF) antagonists in reducing laser-induced neovascular lesions in a rat model of neovascular macular degeneration. Taken together, these results support the clinical development of THN391 for neurological diseases and ophthalmic indications.

Cerebellar injury in preterm infants with central nervous system (CNS) hemorrhage results in lasting neurological deficits and an increased risk of autism. The impact of blood-induced pathways on cerebellar development remains largely unknown, so no specific treatments have been developed to counteract the harmful effects of blood after neurovascular damage in preterm infants. Here, we show that fibrinogen, a blood-clotting protein, plays a central role in impairing neonatal cerebellar development. Longitudinal MRI of preterm infants revealed that cerebellar bleeds were the most critical factor associated with poor cerebellar growth. Using inflammatory and hemorrhagic mouse models of neonatal cerebellar injury, we found that fibrinogen increased innate immune activation and impeded neurogenesis in the developing cerebellum. Fibrinogen inhibited sonic hedgehog (SHH) signaling, the main mitogenic pathway in cerebellar granule neuron progenitors (CGNPs), and was sufficient to disrupt cerebellar growth. Genetic fibrinogen depletion attenuated neuroinflammation, promoted CGNP proliferation, and preserved normal cerebellar development after neurovascular damage. Our findings suggest that fibrinogen alters the balance of SHH signaling in the neurovascular niche and may serve as a therapeutic target to mitigate developmental brain injury after CNS hemorrhage.

The brain-vascular-immune interface has emerged as a dynamic player in brain physiology and disease. We propose integrating vascular risk factors with genetic susceptibility as the nexus for the discovery of mechanisms and therapies for neuroinflammation, neurodegeneration, and neurorepair across polygenic neurologic diseases.

Life-threatening thrombotic events and neurological symptoms are prevalent in COVID-19 and are persistent in patients with long COVID experiencing post-acute sequelae of SARS-CoV-2 infection1-4. Despite the clinical evidence1,5-7, the underlying mechanisms of coagulopathy in COVID-19 and its consequences in inflammation and neuropathology remain poorly understood and treatment options are insufficient. Fibrinogen, the central structural component of blood clots, is abundantly deposited in the lungs and brains of patients with COVID-19, correlates with disease severity and is a predictive biomarker for post-COVID-19 cognitive deficits1,5,8-10. Here we show that fibrin binds to the SARS-CoV-2 spike protein, forming proinflammatory blood clots that drive systemic thromboinflammation and neuropathology in COVID-19. Fibrin, acting through its inflammatory domain, is required for oxidative stress and macrophage activation in the lungs, whereas it suppresses natural killer cells, after SARS-CoV-2 infection. Fibrin promotes neuroinflammation and neuronal loss after infection, as well as innate immune activation in the brain and lungs independently of active infection. A monoclonal antibody targeting the inflammatory fibrin domain provides protection from microglial activation and neuronal injury, as well as from thromboinflammation in the lung after infection. Thus, fibrin drives inflammation and neuropathology in SARS-CoV-2 infection, and fibrin-targeting immunotherapy may represent a therapeutic intervention for patients with acute COVID-19 and long COVID.

Abstract Background Traumatic brain injury (TBI) causes significant blood-brain barrier (BBB) breakdown, resulting in the extravasation of blood proteins into the brain. The impact of blood proteins, especially fibrinogen, on inflammation and neurodegeneration post-TBI is not fully understood, highlighting a critical gap in our comprehension of TBI pathology and its connection to innate immune activation. Methods We combined vascular casting with 3D imaging of solvent-cleared organs (uDISCO) to study the spatial distribution of the blood coagulation protein fibrinogen in large, intact brain volumes and assessed the temporal regulation of the fibrin(ogen) deposition by immunohistochemistry in a murine model of TBI. Fibrin(ogen) deposition and innate immune cell markers were co-localized by immunohistochemistry in mouse and human brains after TBI. We assessed the role of fibrinogen in TBI using unbiased transcriptomics, flow cytometry and immunohistochemistry for innate immune and neuronal markers in Fggγ390–396A knock-in mice, which express a mutant fibrinogen that retains normal clotting function, but lacks the γ390–396 binding motif to CD11b/CD18 integrin receptor. Results We show that cerebral fibrinogen deposits were associated with activated innate immune cells in both human and murine TBI. Genetic elimination of fibrin-CD11b interaction reduced peripheral monocyte recruitment and the activation of inflammatory and reactive oxygen species (ROS) gene pathways in microglia and macrophages after TBI. Blockade of the fibrin-CD11b interaction was also protective from oxidative stress damage and cortical loss after TBI. Conclusions These data suggest that fibrinogen is a regulator of innate immune activation and neurodegeneration in TBI. Abrogating post-injury neuroinflammation by selective blockade of fibrin’s inflammatory functions may have implications for long-term neurologic recovery following brain trauma.

OBJECTIVE:X-linked adrenoleukodystrophy (ALD) is caused by mutations in ABCD1, a peroxisomal gene. More than half of males with an ABCD1 mutation develop inflammatory cerebral demyelination (cALD), but underlying mechanisms remain unknown and therapies are limited. We sought to develop and characterize a mouse model of cALD to facilitate study of disease mechanisms and therapy development. METHODS:We used immunoassays and immunohistochemistry to assess novel (interleukin 18 [IL-18]) and established molecular markers in cerebrospinal fluid (CSF) and postmortem brain tissue from cALD patients. We generated a cALD phenotype in Abcd1-knockout mice using a 2-hit method that combines cuprizone and experimental autoimmune encephalomyelitis models. We then used magnetic resonance imaging (MRI) and immunohistochemistry to assess the fidelity of cALD molecular markers in the mice. RESULTS:Human and mouse cALD lesions shared histologic features of myelin phagocytosis, myelin loss, abundant microglial activation, T and B-cell infiltration, and astrogliosis. Compared to wild-type controls, Abcd1-knockout mice displayed more cerebral demyelination, blood-brain barrier disruption, and perivascular immune cell infiltration. This enhanced inflammatory response was associated with higher levels of fibrin deposition, oxidative stress, demyelination, and axonal injury. IL-18 immunoreactivity co-localized with perivascular monocytes/macrophages in both human and mouse brain tissue. In cALD patients, CSF IL-18 levels correlated with MRI lesion severity. INTERPRETATION:Our results suggest loss of Abcd1 function in mice predisposes to more severe blood-brain barrier disruption, cerebral inflammation driven by the infiltration of peripheral immune cells, demyelination, and axonal damage, replicating human cALD features. This novel mouse model could shed light on cALD mechanisms and accelerate cALD therapy development. ANN NEUROL 2025;97:296-312.

Blood protein extravasation through a disrupted blood–brain barrier and innate immune activation are hallmarks of neurological diseases and emerging therapeutic targets. However, how blood proteins polarize innate immune cells remains largely unknown. Here, we established an unbiased blood-innate immunity multiomic and genetic loss-of-function pipeline to define the transcriptome and global phosphoproteome of blood-induced innate immune polarization and its role in microglia neurotoxicity. Blood induced widespread microglial transcriptional changes, including changes involving oxidative stress and neurodegenerative genes. Comparative functional multiomics showed that blood proteins induce distinct receptor-mediated transcriptional programs in microglia and macrophages, such as redox, type I interferon and lymphocyte recruitment. Deletion of the blood coagulation factor fibrinogen largely reversed blood-induced microglia neurodegenerative signatures. Genetic elimination of the fibrinogen-binding motif to CD11b in Alzheimer’s disease mice reduced microglial lipid metabolism and neurodegenerative signatures that were shared with autoimmune-driven neuroinflammation in multiple sclerosis mice. Our data provide an interactive resource for investigation of the immunology of blood proteins that could support therapeutic targeting of microglia activation by immune and vascular signals.

Central nervous system (CNS) lymphoma is an extranodal non-Hodgkin B-cell lymphoma characterized by malignant lymph tissue arising in the brain or spinal cord, associated with inflammation and blood-brain barrier (BBB) disruption. Although BBB disruption is known to occur in patients with CNS lymphoma, a direct link between these two has not been shown. Herein, abundant deposition of the blood coagulation protein fibrinogen around B-cell lymphoma was detected in CNS lymphoma patients and in the CNS parenchyma in an orthotopic mouse model. Functional enrichment analysis of unbiased cerebrospinal fluid proteomics of CNS B-cell lymphoma patients showed that coagulation protein networks were highly connected with tumor-associated biological signaling pathways. In vivo two-photon imaging demonstrated that lymphoma growth was associated with BBB disruption, and in vitro experiments identified a role for fibrinogen in promoting lymphoma cell adhesion. Overall, these results identify perivascular lymphoma clustering at sites of fibrinogen deposition, and suggest that fibrinogen may be a target for pharmacologic intervention in metastatic B-cell lymphoma associated with BBB disruption.

;'Insights into the molecular and immunologic pathogenesis of primary CNS lymphomas are essential for meaningful progress in therapy. Tumor-associated macrophages represent the dominant infiltrating leukocyte and there are few established insights into their phenotypes and role in this disease. While upregulation of Th2 cytokines IL-4 and IL-10 in the microenvironment has been demonstrated, the relative roles of M1 and M2 macrophages in contributing to CNS lymphoma pathogenesis has not been elucidated. To date, there is also no information regarding the relative contributions of brain resident microglia and infiltrating macrophages and their interactions with lymphoma. Additional key questions include the identification of factors that mediate both immune cell chemotaxis in CNS lymphomas, as well as the relationship between myeloid cell infiltration and T-cell mediated immune surveillance and immunosuppression.

Abstract Extrinsic inhibitors at sites of blood–brain barrier disruption and neurovascular damage contribute to remyelination failure in neurological diseases. However, therapies to overcome the extrinsic inhibition of remyelination are not widely available and the dynamics of glial progenitor niche remodelling at sites of neurovascular dysfunction are largely unknown. By integrating in vivo two-photon imaging co-registered with electron microscopy and transcriptomics in chronic neuroinflammatory lesions, we found that oligodendrocyte precursor cells clustered perivascularly at sites of limited remyelination with deposition of fibrinogen, a blood coagulation factor abundantly deposited in multiple sclerosis lesions. By developing a screen (OPC-X-screen) to identify compounds that promote remyelination in the presence of extrinsic inhibitors, we showed that known promyelinating drugs did not rescue the extrinsic inhibition of remyelination by fibrinogen. In contrast, bone morphogenetic protein type I receptor blockade rescued the inhibitory fibrinogen effects and restored a promyelinating progenitor niche by promoting myelinating oligodendrocytes, while suppressing astrocyte cell fate, with potent therapeutic effects in chronic models of multiple sclerosis. Thus, abortive oligodendrocyte precursor cell differentiation by fibrinogen is refractory to known promyelinating compounds, suggesting that blockade of the bone morphogenetic protein signalling pathway may enhance remyelinating efficacy by overcoming extrinsic inhibition in neuroinflammatory lesions with vascular damage.

Blood clots are a central feature of coronavirus disease-2019 (COVID-19) and can culminate in pulmonary embolism, stroke, and sudden death. However, it is not known how abnormal blood clots form in COVID-19 or why they occur even in asymptomatic and convalescent patients. Here we report that the Spike protein from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) binds to the blood coagulation factor fibrinogen and induces structurally abnormal blood clots with heightened proinflammatory activity. SARS-CoV-2 Spike virions enhanced fibrin-mediated microglia activation and induced fibrinogen-dependent lung pathology. COVID-19 patients had fibrin autoantibodies that persisted long after acute infection. Monoclonal antibody 5B8, targeting the cryptic inflammatory fibrin epitope, inhibited thromboinflammation. Our results reveal a procoagulant role for the SARS-CoV-2 Spike and propose fibrin-targeting interventions as a treatment for thromboinflammation in COVID-19. One-Sentence Summary SARS-CoV-2 spike induces structurally abnormal blood clots and thromboinflammation neutralized by a fibrin-targeting antibody. ### Competing Interest Statement KA is a co-founder, scientific advisor and board director of Therini Bio. KA is the inventor of the 5B8 patent. KA, JKR, MM and WCG are inventors on a patent application for 5B8 use in COVID-19. KA, JKR, MM, EGS and WCG are inventors on a patent application related to Spike-induced thromboinflammation model. KA and JKR are inventors on fibrin assay patents. Their interests are managed in accordance with their respective institutions conflict of interest policy.

Microglial surveillance is a key feature of brain physiology and disease. Here, we found that G i -dependent microglial dynamics prevent neuronal network hyperexcitability. By generating Mg PTX mice to genetically inhibit G i in microglia, we show that sustained reduction of microglia brain surveillance and directed process motility induced spontaneous seizures and increased hypersynchrony after physiologically evoked neuronal activity in awake adult mice. Thus, G i -dependent microglia dynamics may prevent hyperexcitability in neurological diseases.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Alzheimer's disease (AD) is characterized by amyloid β (Aβ) deposition, tau pathology, cerebrovascular damage, and microglia‐mediated neuroinflammation. Cerebrovascular damage includes breakdown of the blood‐brain barrier leading to fibrinogen deposition in the AD brain. Fibrinogen is a blood coagulation factor that upon conversion to fibrin binds to the CD11b/CD18 receptor and induces microglial activation. Here, we show that genetic disruption of fibrinogen interaction with the CD11b/CD18 receptor in 10‐month‐old 5XFAD mice crossed with Fggγ390–396A knock‐in mice, which lack the CD11b binding site, have less neuropathology and microglial activation than 5XFAD littermates. Comparative histological analyses revealed reductions in neuronal and presynaptic terminal loss, Iba‐1+ microglia, immunostaining for the lysosomal marker LAMP‐1, and Aβ plaque burden in the hippocampus of 5XFAD:Fggγ390–396A mice. Thus, genetic inhibition of fibrinogen interaction with CD11b/CD18 receptor reduces multiple AD‐relevant neuropathological alterations in 5XFAD mice.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Extracellular vesicles (EVs) are emerging as potent mediators of intercellular communication with roles in inflammation and disease. In this study, we examined the role of EVs from blood plasma (pEVs) in an experimental autoimmune encephalomyelitis mouse model of central nervous system demyelination. We determined that pEVs induced a spontaneous relapsing-remitting disease phenotype in MOG35-55-immunized C57BL/6 mice. This modified disease phenotype was found to be driven by CD8+ T cells and required fibrinogen in pEVs. Analysis of pEVs from relapsing-remitting multiple sclerosis patients also identified fibrinogen as a significant portion of pEV cargo. Together, these data suggest that fibrinogen in pEVs contributes to the perpetuation of neuroinflammation and relapses in disease.