The research, led by James M. Hennegan and colleagues at the University of Southampton, focused on Fc gamma receptor IIb, the only inhibitory member of the Fc gamma receptor family that is conserved across species. These receptors normally serve as molecular sentinels of the immune system, binding the Fc domain of IgG antibodies and calibrating immune cell responses through a balance of activating and inhibitory signals. FcγRIIb restrains immune activation via its immunoreceptor tyrosine-based inhibitory motif, or ITIM, which recruits intracellular phosphatases when engaged. Crucially, the receptor is not confined to the periphery. Previous work has documented its expression on both neuronal and microglial membranes within the central nervous system, particularly under conditions of proteopathic stress, and earlier in vitro studies had shown that fibrillar alpha-synuclein binds directly to FcγRIIb, triggering downstream SHP-1/2-c-Src signalling that facilitates the protein’s internalisation.

What remained unresolved was whether this receptor genuinely governs alpha-synuclein spread in a living brain, rather than merely in a culture dish. To answer this question, the team turned to the preformed fibril, or PFF, model of synucleinopathy, in which synthetic fibrils of recombinant human alpha-synuclein are injected into the dorsal striatum of mice. The fibrils, fragmented by sonication into particles smaller than 50 nanometres and validated by transmission electron microscopy and Thioflavin-T fluorimetry, seed the aggregation of the brain’s own alpha-synuclein, generating Lewy body-like pathology that spreads along anatomically connected circuits in a manner that closely mirrors the human disease. The researchers compared wild-type C57BL/6 mice with mice in which the Fcgr2b gene had been deleted, and confirmed at both the RNA and protein levels that the knockout abolished receptor expression in the brain and spleen without altering other Fc gamma receptor family members or the mice’s own production of alpha-synuclein.

The results were striking. In primary cortical neurons cultured from embryonic tissue, exposure to preformed fibrils produced abundant phosphorylated alpha-synuclein, the pathological modification found at serine 129 that defines Lewy bodies, in wild-type cultures. Neurons lacking FcγRII showed an 88 percent reduction in this pathological staining, with far fewer inclusions in both the cell bodies and the neuritic arbours. When the team moved into living animals, injecting fibrils unilaterally into the striatum, the difference was even more dramatic. Thirty days after injection, wild-type mice had already developed robust phosphorylated alpha-synuclein pathology in the ipsilateral substantia nigra pars compacta, whereas knockout mice showed a near-complete absence of staining. By ninety days, wild-type brains were riddled with inclusions across the prefrontal cortex, primary motor cortex, entorhinal cortex, amygdala and substantia nigra, with muted deposition appearing even in the contralateral hemisphere, consistent with trans-hemispheric propagation.

The knockout mice were largely spared this anatomical cascade. Quantitative image analysis revealed reductions in pathological burden ranging from more than 70 percent in the ipsilateral substantia nigra to nearly 99 percent in the prefrontal and motor cortices, indicating that the receptor is not merely permissive but fundamental for efficient seeding and trans-neuronal propagation of the pathology along synaptically connected pathways. Distribution maps generated from the immunohistochemical data visualised the stark divergence between the two genotypes, with the wild-type hemisphere dotted with pathological signal and the knockout hemisphere almost clean. Control injections of monomeric, non-fibrillar alpha-synuclein produced no pathology in either genotype, confirming the specificity of the fibril-seeding response.

The protective effect extended beyond protein aggregation to the neurons themselves. Neuroinflammation is a central component of Parkinson’s pathogenesis, and in wild-type mice the fibril injection provoked a pronounced rise in microglial density, marked by increased Iba1 staining, and elevated expression of FcγRI, the high-affinity IgG receptor that appears on activated microglia. In knockout mice, this response was substantially blunted, with roughly a 72 percent reduction in Iba1-positive microglial density and a 29 percent decrease in FcγRI expression in the injected striatum at ninety days. The authors interpret this dampened gliosis as a secondary consequence of the reduced alpha-synuclein burden, suggesting that the receptor’s primary contribution to disease may be neuron-intrinsic, with impaired fibril internalisation limiting the downstream activation of glial cells.

Most importantly from a therapeutic standpoint, the loss of FcγRII preserved the nigrostriatal dopaminergic system. Wild-type mice injected with fibrils showed a marked reduction in tyrosine hydroxylase-positive fibre density in the dorsal striatum and a significant loss of dopamine neuron cell bodies in the substantia nigra by ninety days, the classic signature of progressive neurodegeneration in this model. Knockout mice maintained fibre density and neuronal counts equivalent to monomer-injected controls at both time points. Behavioural testing, conducted monthly before the animals were killed, suggested functional correlates of this protection: wild-type fibril-injected mice displayed reduced locomotor activity and exploratory drive in the open field and increased descent latency in the pole test, while knockout mice performed normally across all paradigms. The authors caution that the small cohort sizes render these behavioural findings suggestive rather than definitive, but their directionality aligns with the histological preservation.

To translate these genetic findings into a pharmacological strategy, the team exploited transgenic mice expressing human FcγRIIb and tested two monoclonal antibodies, BI-1206 and BI-1607, that selectively antagonise the human receptor. One antibody carried a wild-type Fc domain, while the other incorporated an N297Q substitution rendering it Fc-null, allowing the researchers to distinguish effects of receptor blockade itself from those mediated by Fc-driven engagement of activating Fc gamma receptors on immune cells. The antibodies were co-injected intracranially with the fibrils and supplemented with systemic dosing at 10 milligrams per kilogram to maintain circulating levels. Thirty days later, both antibody formats had produced a roughly 90 percent decrease in phosphorylated alpha-synuclein burden in the striatum and substantia nigra relative to an isotype control, with comparable efficacy between the Fc-wild-type and Fc-null variants. This equivalence suggests that blockade works predominantly through steric interference with the fibril-binding interface on the receptor, preventing alpha-synuclein uptake and pathological templating at the injection site before spread can begin.

The implications reach well beyond the laboratory. The antibodies used in this study are already advancing through oncology clinical trials, where FcγRIIb-directed therapeutics are being evaluated in CD32b-positive B-cell lymphomas, establishing a safety, tolerability and pharmacodynamic footprint that could accelerate repurposing toward neurodegenerative indications. Because the strategy targets the cellular entry route for fibrils rather than the protein aggregates themselves, it offers a mechanistically distinct complement to conventional anti-alpha-synuclein immunotherapies, which have struggled in clinical trials. Other receptors implicated in alpha-synuclein uptake, including LAG3, APLP1, the alpha-3 subunit of the sodium-potassium ATPase and connexin 32, suggest that fibrils may engage multiple overlapping entry pathways, but the magnitude of protection seen here indicates FcγRIIb is an especially influential conduit.

The authors are careful to note the limitations of the work. The initial knockout experiments used only male animals, cohort sizes were small, and tyrosine hydroxylase staining alone cannot distinguish frank neuronal loss from reduced expression or dysfunction without an independent pan-neuronal counterstain. The prophylactic antibody paradigm also does not address therapeutic intervention once pathology is established, and intracerebral delivery does not recapitulate the pharmacokinetics of systemic administration. Nevertheless, the convergence of genetic deletion and antibody blockade on the same outcome, a profound impairment of alpha-synuclein seeding, propagation and neurotoxicity in vivo, marks FcγRIIb as a pivotal and druggable regulator of synucleinopathy, and raises the tangible prospect that a receptor once studied for its role in immune regulation and cancer immunotherapy may become a target for slowing the progression of Parkinson’s disease.

Subject of Research: The role of the FcγRIIb receptor in alpha-synuclein propagation and its blockade as a therapeutic strategy for Parkinson's disease

Article Title: Deletion or immunotherapeutic blockade of FcγRIIb (CD32b) impairs α-Syn propagation in vivo

Article References: Hennegan, J. M., Hurley, M. J., Colley, M., Cox, K. L., Douglas, L. R., Duriez, P. J., Oldham, R. J., Cragg, M. S., Frendéus, B., Roghanian, A., & Teeling, J. L. (2026). Deletion or immunotherapeutic blockade of FcγRIIb (CD32b) impairs α-Syn propagation in vivo. Acta Neuropathologica, 152(1), Article 40. https://doi.org/10.1007/s00401-026-03082-7

Image Credits: AI Generated

DOI: 10.1007/s00401-026-03082-7

Keywords: Parkinson's disease, alpha-synuclein, FcγRIIb, CD32b, preformed fibrils, neurodegeneration, monoclonal antibodies, microglia, substantia nigra, prion-like propagation, immunotherapy, synucleinopathy

Amyotrophic lateral sclerosis is characterized by the adult-onset degeneration of the upper and lower motor neurons, the nerve cells that command voluntary movement. The disease typically begins in a focal region of the nervous system and then spreads contiguously, producing progressive paralysis and, ultimately, death from respiratory failure. Mutations in the gene encoding the free radical scavenging enzyme superoxide dismutase-1 are a well-established cause of the disease and are found in roughly one to nine percent of all patients. Since the SOD1 gene was first linked to familial ALS in 1993, more than 235 coding mutations have been catalogued. Most of these mutations are inherited as dominant traits, but the most prevalent of them all, D90A, behaves differently: disease develops primarily in individuals who carry two copies of the mutation, one inherited from each parent.

The central mystery that has driven this field for years concerns how the disease spreads. Cytosolic inclusions containing aggregated SOD1 are a pathological hallmark of ALS, both in patients and in transgenic animal models expressing mutant human SOD1. Using a technique called binary epitope mapping, the Umeå University team, led by researchers including Caitlin Henne, Isabelle Sigfridsson, Thomas Brännström, Stefan Marklund, Per Zetterström and Peter Andersen, previously discovered that two structurally different strains of human SOD1 aggregates, designated A and B, can arise in mice. Strain A forms in most mutant models, whereas homozygous D90A mice characteristically produce the distinct strain B, alongside strain A. Critically, when seed preparations of either strain are injected into the spinal cords of recipient mice expressing a human SOD1 transgene, the aggregates propagate in a templated fashion, spreading through the nervous system and precipitating premature, fatal motor neuron disease that closely resembles human ALS.

Earlier experiments had already demonstrated that seeds prepared from the central nervous systems of patients carrying the aggressive G127X truncation mutation could transmit strain A aggregation and disease to mice. But those patients carry a destabilized, inactive protein present only in minute quantities in the nervous system. The D90A mutation presents a very different challenge: the D90A protein is molecularly stable, retains wild-type-like enzymatic activity, and accumulates at high concentrations in the central nervous system. Patients homozygous for D90A typically survive more than a decade after onset, with a median of fourteen years, and their spinal ventral horns become profoundly degenerated with massive motor neuron loss. Seeds prepared from such tissue were therefore expected to contain only vanishingly small amounts of seeding-competent material, raising real doubt about whether transmission would be detectable at all.

The researchers addressed this question by preparing seeds from the ventral horns, including the entire lamina IX region, of six patients homozygous for D90A who had died of ALS. The preparation protocol involved homogenization in buffer containing detergent and guanidinium chloride, followed by ultracentrifugation through dense iohexol cushions that pelleted very large proteinaceous complexes. Quantitative analysis revealed that the seeds contained only picogram amounts of aggregated SOD1 per microliter, diluted within roughly fifty thousand times more protein from other ventral horn components. One microliter of each seed was then inoculated stereotactically into the lumbar ventral horn of the left side of the spinal cord in one-hundred-day-old, still asymptomatic mice carrying the human SOD1 G85R transgene, a slow model of disease in which aggregation arises spontaneously in late life.

The results were striking despite the technical odds. Seeds from two of the six patients, designated A1 and A2, significantly shortened the survival of the recipient mice compared with non-inoculated controls. Mice receiving the A2 seed developed fatal paralysis and their spinal cords showed strain A aggregation patterns by binary epitope mapping, indicating that the patient’s ventral horn had contained strain A aggregates. The A1 seed told a subtler story. The two shortest-lived mice in that group displayed unmistakable strain B patterns, whereas the longer-lived mice in the same group showed strain A patterns, which arise spontaneously in the G85R model. This suggests the A1 patient harbored strain B aggregates, which propagate roughly thirty percent more slowly than strain A, and that the spontaneous strain A aggregation eventually overwhelmed any later-seeded B aggregation in the surviving animals. Confocal immunohistochemistry confirmed the biochemical findings, revealing both strain A and strain B aggregates in the tissue of the two most short-lived A1-inoculated mice, while all other inoculated mice showed strain A alone.

The specificity of these effects was rigorously controlled. Nine different seed preparations from four neurologically normal individuals, prepared with three distinct protocols, produced lifespans indistinguishable from non-inoculated mice, as did seeds from control C57BL/6 mice. Notably, the postmortem interval was significantly shorter in the control group than in the ALS group, a factor that should have favored, rather than undermined, seeding activity in the controls, since seeding-competent material is sensitive to proteolytic degradation. The pattern of disease onset also told a coherent story: mice receiving the active seeds overwhelmingly developed hindleg symptoms first, consistent with aggregation initiating at the lumbar inoculation site, and the aggregates were found to have spread along the neuraxis in terminally ill animals, exactly as expected from a prion-like propagation process.

An intriguing aspect of the findings is that the researchers detected no obvious difference in the total quantity of detergent-insoluble SOD1 aggregates between the two active seeds and the four inactive ones. This implies that the prion-active aggregates represent only a subfraction of the total insoluble SOD1 in the tissue, meaning that total aggregate burden alone is a poor predictor of biological activity. What matters is the structural nature of the aggregates present. This has practical implications: understanding which aggregate structures are seeding-competent could inform the selection of epitopes for antibody-based therapies and other structure-dependent interventions. It may also help explain why ALS phenotypes vary so dramatically between different SOD1 mutations, since different aggregate strains could propagate at different rates and produce different disease courses.

Perhaps the most compelling comparison in the study is between the two patient mutations whose seeds have now been shown to transmit disease. The truncated G127X protein is inactive, disordered, and present in only minute amounts, yet its seeds transmitted strain A aggregation and rapidly progressive disease. The D90A protein is stable, active, and abundant, and its seeds transmitted both strain A and strain B aggregation in patients with a uniform, slowly progressive clinical phenotype. That aggregates from two mutations with such radically different biochemical properties and clinical courses both prove capable of templating their misfolding in recipient animals provides the strongest evidence yet that prion-like propagation of misfolded SOD1 is the primary pathogenic mechanism in SOD1-linked ALS. While the current sample of two transmitting patients is too small to draw firm conclusions about which clinical phenotypes map to which strains, the researchers note that the strain B-transmitting seed came from the patient with the earliest onset and shortest symptomatic disease, a pattern consistent with prior observations in transgenic mice. The work opens a path toward diagnosing and, ultimately, intercepting these pathological protein strains before they march through the nervous system.

Subject of Research: Prion-like transmission of SOD1 aggregate strains from D90A ALS patient tissue to transgenic mice

Article Title: Seeds from ALS patients homozygous for the SOD1 D90A mutation transmit two types of SOD1 aggregation and motor neuron disease

Article References: Henne, C., Sigfridsson, I., Brännström, T., Forsberg, K. M. E., Marklund, S. L., Zetterström, P., & Andersen, P. M. (2026). Seeds from ALS patients homozygous for the SOD1 D90A mutation transmit two types of SOD1 aggregation and motor neuron disease. Acta Neuropathologica, 152(1), Article 27. https://doi.org/10.1007/s00401-026-03078-3

Image Credits: AI Generated

DOI: 10.1007/s00401-026-03078-3

Keywords: ALS, SOD1, D90A mutation, prion-like propagation, protein aggregation, motor neuron disease, binary epitope mapping, aggregate strains, transgenic mice, amyotrophic lateral sclerosis, Seeds, patients