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Parkinson's Disease and the Immune System.
This is a detailed subject, and what it means for your situation depends on your history. Dr. Glowney is glad to work through it with you.
More than dopamine loss.
Parkinson's disease has long been taught as a disorder of dopamine: neurons in the substantia nigra die, movement suffers, and levodopa replaces what is missing. That account is accurate and incomplete. It explains the symptoms but not the cause, and it offers no reason why a disease that begins decades before diagnosis should progress the way it does.
A large body of work now places the immune system inside the disease process rather than alongside it. Immune cells are not simply reacting to dying neurons; in at least some patients they appear to arrive before the neurons die. If that is correct, it changes what a disease-modifying therapy would have to do, and it explains why cell-based approaches have attracted serious attention.
What follows is a summary of that evidence and an honest assessment of where the treatment research actually stands.
The antigen: a familiar protein in an unfamiliar shape.
Alpha-synuclein is a normal synaptic protein that regulates neurotransmitter release. Under ordinary conditions it folds against cell membranes in a stable helical shape. In Parkinson's disease it misfolds into sheet-rich aggregates that form oligomers, protofibrils, and ultimately Lewy bodies, and the intermediate oligomers are both the most toxic and the most capable of seeding further misfolding in neighboring cells.
The immunological point is that shape and chemical modification determine whether the immune system reacts. Nitration, phosphorylation at serine 129, ubiquitination, and truncation all change how the protein is seen. In 2017, Sulzer and colleagues showed that specific alpha-synuclein fragments act as genuine antigens, presented on the same MHC alleles that epidemiology had already linked to Parkinson's risk, and capable of driving both helper and cytotoxic T-cell responses in patients.
Critically, the immune response is directed largely at the modified protein rather than the normal one. In animal models, T cells respond to nitrated alpha-synuclein but not to the native form. This has a practical consequence: a therapy aimed at normal alpha-synuclein may simply miss the species the immune system is actually reacting to.
Why the gut keeps coming up.
Patients with Parkinson's disease have measurably different intestinal bacteria. Studies of colonic tissue and stool have found fewer of the butyrate-producing organisms associated with an anti-inflammatory environment and more organisms associated with inflammation, along with genetic signatures pointing toward increased bacterial endotoxin production.
The taxonomy varies between cohorts, and diet, transit time, and levodopa all confound it. The functional consequence is more consistent. Short-chain fatty acids (acetate, propionate, butyrate) maintain the intestinal barrier, restrain inflammation, and reduce immune cell migration toward the brain. When they fall, the barrier loosens and trafficking is disinhibited at the same time.
A leaky barrier lets bacterial products reach gut-associated lymphoid tissue, the immune sampling apparatus of the intestinal wall, which responds with inflammatory signaling that has effects well beyond the gut. One receptor pathway is especially relevant: TLR2, working with TLR1, recognizes misfolded alpha-synuclein directly. The gut is therefore not only a source of general inflammation but a plausible site where the specific protein at the center of the disease first encounters the immune system.
In mice engineered to overexpress alpha-synuclein, gut bacteria are required for motor deficits and brain inflammation to develop, and colonizing them with microbiota from Parkinson's patients worsens their impairment. These are model-system findings and should not be read as proof of causation in humans, but they establish that microbial signals are sufficient to drive the process.
The vagus nerve as a route - for some patients.
Braak's staging work observed that the earliest brain lesions in Parkinson's disease appear in the dorsal motor nucleus of the vagus, and that alpha-synuclein deposits can be found in the nerve plexuses of the gut wall. That suggested a physical route: misfolded protein traveling from gut to brainstem along the vagus nerve.
The cleanest experimental test injected pathological alpha-synuclein fibrils into the gut wall of mice. Pathology appeared first in the dorsal motor nucleus, then spread up the brainstem, and only much later reached the substantia nigra, with dopaminergic cell loss and motor and non-motor symptoms following the same sequence. Cutting the vagus nerve prevented all of it.
This route does not apply to everyone. Current models describe two trajectories: a body-first pattern beginning in the enteric nervous system, and a brain-first pattern beginning in the olfactory bulb or amygdala. Imaging supports the distinction, patients with REM sleep behavior disorder before motor onset show the peripheral signature, while those without it show nigral changes first.
That has a direct implication for research. If only a subset of patients follows a gut-initiated course, treatments aimed at gut or systemic inflammation should not be expected to work uniformly across all patients, and trials that do not account for this will dilute real effects.
When the blood-brain barrier opens.
The brain was once considered immunologically privileged. It is better described as immunologically regulated, and in Parkinson's disease that regulation fails. Inflammatory cytokines IL-1β, IL-6, IL-17, interferon-γ, TNF-α - reduce the tight junction proteins claudin-5 and occludin that seal the barrier. Astrocytes contribute directly by releasing vascular endothelial growth factor A, which further strips those junctions. Chemokine receptors increase and actively direct immune cells toward the brain. Bacterial endotoxin arriving from the gut raises barrier permeability and impairs regulatory T-cell function.
Lymphocyte entry is not leakage. It is a receptor-mediated program: adhesion molecules on brain capillary endothelium capture circulating T cells and walk them across. The barrier in Parkinson's disease is not simply weakened; it is actively recruiting.
This cuts both ways, and the second direction is the basis of an entire therapeutic strategy. A barrier open enough to admit damaging cells is also open enough to admit regulatory ones, which is the implicit premise behind intravenous immune-modulating treatments.
Microglia and astrocytes: the local engine.
Microglia are the brain's resident immune cells. Autopsy studies of Parkinson's brains show them activated and clustered around alpha-synuclein aggregates, alongside infiltrating T cells and monocytes.
Activated microglia release TNF-α, IL-1β, IL-6, and reactive oxygen species, which damage neurons, which release more aggregated protein, which activates more microglia. The loop sustains itself. Worse, aggregated alpha-synuclein binding to microglial receptors actually suppresses their ability to clear it, so the protein disables the machinery that would remove it while amplifying the inflammation that promotes further aggregation.
Two details matter for therapy. First, activated microglia increase their display of MHC molecules, which is what makes neurons visible to cytotoxic T cells. Second, this is reversible in principle: neurons die when exposed to inflammatory microglia alone, but survive when anti-inflammatory microglia are present too. The goal of immune-modulating treatment is therefore to shift the balance, not to shut microglia down.
Astrocytes play a dual role, they regulate the barrier, and when they accumulate alpha-synuclein they can act as antigen-presenting cells. Together with microglia, this means T cells can be reactivated inside the brain itself with no return trip to peripheral lymph nodes required. Regulatory T cells push astrocytes the other way, toward producing growth factors that support neurons.
T cells and the timing problem.
This is the most consequential evidence, and the timing is what makes it so. A systematic autopsy study examined the substantia nigra across the full spectrum of disease stages. At the earliest stage, before any detectable alpha-synuclein aggregation and before measurable neuron loss, there was already substantial infiltration of cytotoxic CD8 T cells, many in direct physical contact with dopaminergic neurons. Many were tissue-resident memory cells, suggesting established local surveillance rather than transient visitation. CD4 T cells showed no comparable pattern. The authors concluded that cytotoxic T-cell attack may initiate neuronal death, not merely accompany it.
The target is capable of being attacked. Human dopaminergic neurons express MHC class I, the molecule that presents antigen to CD8 T cells, and expression is inducible by interferon-γ, by signals from activated microglia, and by high internal dopamine and oxidative stress. In laboratory conditions, MHC class I expressing dopaminergic neurons are destroyed by cytotoxic T cells when the right antigen is present.
Put together, a modified self-protein with proven antigen activity, presented on risk-associated MHC alleles, to cytotoxic cells sitting in contact with neurons that can be induced to display it. That is a reasonably complete autoimmune mechanism.
On the other side, regulatory T cells, which normally restrain this, are reduced in number and function in Parkinson's patients, and their deficit correlates with motor severity. This makes regulatory T-cell support the most mechanistically justified immune target in the disease. A phase 1 trial of GM-CSF (sargramostim) increased regulatory T-cell number and function with modest motor improvement and corresponding changes on brain imaging, though with a meaningful side-effect burden.
Two different things are called "cell therapy".
These approaches are routinely discussed together. They share almost nothing - not mechanism, route, target, dosing, or the endpoints by which they should be judged. Conflating them has caused real confusion.
Immune modulation: Mesenchymal stromal cells, given intravenously, are not meant to engraft or become neurons. They act through secreted signals that shift the immune environment. Dosing is repeated, the plausible effect is on rate of progression, and the right endpoints are progression slopes and inflammatory biomarkers measured over years.
Cell replacement: Dopaminergic progenitor cells, delivered surgically into the putamen, are meant to engraft, mature, and restore dopamine signaling. It is a one-time surgical implant. The plausible effect is improved motor function and better "ON" time, and the right endpoints are motor scales and symptom diaries.
A third category sits apart: minimally manipulated preparations made from the patient's own bone marrow, such as bone marrow aspirate concentrate. This is the approach we use at Boulder Biologics. We draw the marrow, minimally process it in our on-site lab, and give it by IV and, in selected cases, through the nose (intranasally). It contains mesenchymal stromal (stem) cells, and no donor cells are used. It follows the immune-modulation logic above, without surgery, and its cell makeup and dose differ from those of laboratory-expanded products.
Donor cells or the patient's own?
This question is usually argued on cost and logistics. Argued on immunology, it looks different.
Three facts work against the assumption that the Parkinson's brain is a hospitable site for donor tissue. The barrier is already permeable to lymphocytes. The grafted cell type, dopaminergic neurons, including those grown from stem cells, can be induced to display MHC class I under exactly the inflammatory conditions found in the recipient brain. And the patient's cytotoxic T cells are already present in the substantia nigra, in contact with that cell type, from the earliest stages of disease.
Against that background, donor HLA mismatch adds an antigenic burden to a host already primed against the graft's lineage. The immunosuppression schedules used in the leading donor-cell programs, roughly 12 to 15 months, were chosen pragmatically, and they are short relative to how long a surgical graft is meant to last. The autologous programs have proceeded with no immunosuppression at all.
So, the immunologic argument favors using the patient's own cells. It is important to say clearly that this advantage is partial, for two reasons that advocacy for autologous approaches often omits.
First, a graft made from the patient carries the patient's genome, including any Parkinson's risk variants affecting lysosomal function or mitochondrial quality control. Autologous sourcing removes the rejection problem at the cost of reproducing the cell-intrinsic vulnerability.
Second, and more fundamentally, autologous sourcing does not solve the autoimmune problem, because the antigen is not a donor antigen. It is the patient's own modified alpha-synuclein, presented on the patient's own MHC. A perfectly matched graft expresses the same protein and the same MHC alleles. The long-term fetal graft literature makes this concrete; grafted neurons examined 11 to 16 years after transplantation had developed Lewy body pathology, indicating that the disease process continues and can affect new cells. Notably, only a small minority of grafted neurons were affected and most remained functional after a decade, with patients still reporting benefit, so this warrants caution rather than pessimism. But the principle holds, the host environment, not the graft's genetics, determines whether pathology returns.
The coherent conclusion is that autologous sourcing is immunologically preferable and immunologically insufficient. A graft addresses the deficit; it does not address the process that produced it. That is the strongest argument for eventually combining replacement with sustained immune modulation, a combination no current trial tests, and which should not be adopted in practice before it is tested.
Where the evidence actually stands.
The mechanistic case is strong. The treatment case is not yet made, and the honest summary is less encouraging than most coverage suggests.
Immune modulation: A phase 2 randomized trial of intravenous donor-derived mesenchymal stromal cells in 45 patients met its prespecified statistical threshold in the three-infusion arm, and treatment was safe and well tolerated. Two findings complicate it. The placebo group improved by nearly 15 points on the standard motor scale over 88 weeks, against an expected decline of roughly 2 points per year; a placebo response larger than the treatment difference. And the two-infusion arm performed worse than placebo, which the investigators attribute partly to variability between manufactured cell batches and which prompted their call for functional potency testing. Repeated donor exposure also produced new HLA antibodies in a minority of recipients, though no donor-specific antibodies were detected.
Cell replacement: The most advanced donor-cell program has reported three-year follow-up in 12 patients: no adverse events attributed to the therapy, imaging consistent with surviving grafts two years after immunosuppression was stopped, and motor improvements that were larger in the higher-dose group. A separate donor-derived program in Japan reported no serious adverse events and increased dopamine signaling in seven patients, and received conditional, time-limited national approval on that basis; a decision that drew substantive criticism precisely because seven uncontrolled patients cannot establish benefit or characterize risk. An autologous program has reported twelve-month results in eight patients without immunosuppression, with improvements in motor scores and daily "ON" time.
Every one of these replacement datasets is small, open-label, and uncontrolled. In a disease where surgery alone produces large expectation effects, that is a serious limitation, and the placebo response in the one properly controlled trial demonstrates exactly why. The numbers from different programs also cannot be compared directly, given differences in design, dose, population, and blinding.
What has to happen next.
The gap is in trial design, not in mechanism. Several things would close it.
Blinded, controlled trials: A registrational phase 3 trial of donor-derived dopaminergic cells is now enrolling roughly 100 patients, randomized against sham surgery with blinded immunosuppression. Its result will be more informative than the entire preceding literature combined.
Potency and consistency testing: The inverted dose-response in the mesenchymal cell trial cannot be interpreted without functional potency data. For autologous products, where every batch is a population of one, validated release criteria are not optional.
Durability past immunosuppression: Donor graft survival must be shown over a horizon matching patient life expectancy, not a 12-month drug schedule.
Immune monitoring: No trial has yet reported whether a cell therapy changes the alpha-synuclein-reactive T-cell population. The laboratory methods exist and the target antigens are known. Including these assays would turn an assumed mechanism into a measured one.
Patient selection: If gut-first and brain-first trajectories are genuinely distinct, trials should stratify for them using markers that are already clinically available.
Controlled trials are needed to provide this evidence. Until they report, no cell-based approach can accurately be described as altering the course of Parkinson's disease.
Selected sources.
For readers who wish to review the primary literature.
- Sulzer D, Alcalay RN, Garretti F, et al. T cells from patients with Parkinson's disease recognize α-synuclein peptides. Nature. 2017;546(7660):656-661.
- Galiano-Landeira J, Torra A, Vila M, Bové J. CD8 T cell nigral infiltration precedes synucleinopathy in early stages of Parkinson's disease. Brain. 2020;143(12):3717-3733.
- Cebrián C, Zucca FA, Mauri P, et al. MHC-I expression renders catecholaminergic neurons susceptible to T-cell-mediated degeneration. Nat Commun. 2014;5:3633.
- Keshavarzian A, Green SJ, Engen PA, et al. Colonic bacterial composition in Parkinson's disease. Mov Disord. 2015;30(10):1351-1360.
- Sampson TR, Debelius JW, Thron T, et al. Gut microbiota regulate motor deficits and neuroinflammation in a model of Parkinson's disease. Cell. 2016;167(6):1469-1480.e12.
- Kim S, Kwon SH, Kam TI, et al. Transneuronal propagation of pathologic α-synuclein from the gut to the brain models Parkinson's disease. Neuron. 2019;103(4):627-641.e7.
- Horsager J, Andersen KB, Knudsen K, et al. Brain-first versus body-first Parkinson's disease: a multimodal imaging case-control study. Brain. 2020;143(10):3077-3088.
- Gendelman HE, Zhang Y, Santamaria P, et al. Evaluation of the safety and immunomodulatory effects of sargramostim in a randomized, double-blind phase 1 clinical Parkinson's disease trial. NPJ Parkinsons Dis. 2017;3:10.
- Schiess MC, Suescun J, Martinez-Lemus JD, et al. Allogeneic bone marrow-derived mesenchymal stem cells for Parkinson's disease: a randomized trial. Mov Disord. Published online September 1, 2025. doi:10.1002/mds.70028
- Tabar V, Sarva H, Lozano AM, et al. Phase I trial of hES cell-derived dopaminergic neurons for Parkinson's disease. Nature. 2025;641(8064):978-983.
- Sawamoto N, Doi D, Nakanishi E, et al. Phase I/II trial of iPS-cell-derived dopaminergic cells for Parkinson's disease. Nature. 2025;641(8064):971-977.
- Li JY, Englund E, Holton JL, et al. Lewy bodies in grafted neurons in subjects with Parkinson's disease suggest host-to-graft disease propagation. Nat Med. 2008;14(5):501-503.
- Kordower JH, Chu Y, Hauser RA, Freeman TB, Olanow CW. Lewy body–like pathology in long-term embryonic nigral transplants in Parkinson's disease. Nat Med. 2008;14(5):504-506.
- Du X, Akter S, Oludipe DB, et al. Adaptive immunity in the pathogenesis and treatments of Parkinson's disease. NeuroImmune Pharmacol Ther. 2025;4(2):273-284.
Regulatory Disclaimer
Regulatory status
- No stem cell or cellular therapy is FDA-approved for Parkinson's disease.
- One conditional approval has been granted in Japan, and it is not available to patients in the United States.
- The cell-based approaches described in this article are investigational.
- No claims are made regarding cure, neuroregeneration, or disease modification.
- Outcomes vary; no benefit is guaranteed.
We describe these treatments only as far as the published evidence goes.
This article is provided for educational purposes and is not a recommendation or medical advice.
Terminology and scientific accuracy
Consistent with FDA guidance and current scientific consensus, we avoid claims that mesenchymal stromal cells differentiate into functional brain cells in vivo. Any potential benefit is described as paracrine signaling, immune modulation, and support for endogenous repair, consistent with the prevailing literature.
References to "stem cells" in this article include manufactured and expanded stem cell products studied in clinical trials elsewhere. The cellular therapy offered at Boulder Biologics uses cellular populations contained within autologous bone marrow–derived biologic material, not manufactured or expanded stem cell products.
Autologous vs donor-derived cells
The FDA has raised particular concern regarding allogeneic (donor-derived) stem cell products, including umbilical cord, placental, and amniotic products, marketed outside of approved clinical trials.
At Boulder Biologics:
- Only autologous (patient-derived) cellular material is used
- No donor cells are administered
- No culture expansion, genetic modification, or manufacturing of cells occurs
Investigational nature and informed consent
- Patients are informed that this approach is experimental.
- Clinical response is variable and unpredictable.
- Some patients may experience no benefit.
- Long-term efficacy data for Parkinson's disease are not yet established.
Participation is voluntary, and treatment is undertaken only after a comprehensive informed-consent process.
Anyone considering participation in a clinical trial should discuss it with their treating neurologist.
Intranasal delivery, investigational
As with Long COVID-associated cognitive dysfunction, intranasal delivery is explored as a route to allow biologic signaling molecules to access the central nervous system via olfactory and trigeminal pathways. Intranasal delivery remains investigational and is not FDA-approved for neurological disease.