The Future of Immune Protection for Islet Replacement Therapy
I. in the I.S.L.E.T. framework series
Introduction
Say what you will about the IRA, but you can’t deny that their correspondence had a certain literary quality to it. So much so that, in 2024, one of their threats ended up floating around Facebook as an inspirational quote:
While originally directed at Margaret Thatcher after a failed assassination attempt, this threat (and erstwhile Facebook inspo) perfectly describes the relationship between the immune system and transplanted islets in type 1 diabetes (T1D).
Immune Rejection Mechanisms in Islet Replacement for T1D
The immune system has many ways to kill transplanted cells, means and methods and pathways of unmatched persistence. In islet replacement for T1D, there are several overlapping, mutually reinforcing immune rejection mechanisms in play.
To understand ways of protecting against this attack, it’s useful to understand how it happens: through allorejection — the immune system’s response to cells that aren’t its own — and then the recurrent autoimmune response characteristic of the disease.
First, there is allorejection, which itself comes in multiple waves: innate and adaptive. In innate allo-rejection, natural killer (NK) cells recognize “non-self” cells, punching holes in the new cells’ walls and pouring in self-destruct enzymes, while monocytes release inflammatory chemicals (cytokines) toxic to beta cells. Then there is the adaptive wave of allorejection. This is where T cells read the foreign cells’ surface protein ID (HLA) to kill on sight, and B cells produce antibodies, sticky Y-shaped proteins that work like a flare, calling in complement killers and the immune cells that grab the tags and pull the cell apart. The antibody arm is the hardest to stop; it works even beneath the drugs built to restrain T cells.
And then comes recurrent autoimmunity. The immune system has already killed these cells once and has proven itself more than willing to do it again (and again) in people with T1D. The autoreactive T cells that killed the beta cells the first time still have an affinity for killing beta cells. While the T-cell arm, like allorejection, works through HLA presentation (which will matter later), the antibody arm targets autoantigens (insulin, GAD65, IA-2, ZnT8, and TSPAN7) that pop up like little flares on the surface of beta cells, attracting immune killing like moths to a light.
Strategies for Immune Protection in Islet Replacement for T1D
This section sorts immune protection strategies onto a spectrum. At one end is modifying the person receiving the cells. At the other end is modifying the cells themselves.
At the far left edge of the spectrum sits immunosuppression: it asks the most of the person through the downstream effects of suppressing their immune system, in addition to a lifelong medication regimen.
In the middle, there is modifying the local environment the cells live in: capsules, engineered materials, co-delivered signals. At the far right, you ask the least of the person receiving the cells, and instead change the cells themselves, editing them for immune protection and, at the far end, outright evasion.
Strategies That Modify the Person’s Immune Function
Broad immunosuppression
The tried-and-true method for protecting transplanted islets is to weaken the immune system via immunosuppressive drugs: broad immunosuppression. While the current standard of care, broad immunosuppression has that old-timey feel to it: heavy-handed, labor-intensive, and kind of dangerous. It means taking a combination of medications on a strict schedule throughout the day, drugs that weaken the whole immune system and not only the parts implicated in killing islets.
Central to this combination of medications are calcineurin inhibitors, which work by blocking the enzymes that T cells need to activate. Because T cells play such a central role in killing, this method works well enough that experienced islet transplant centers report insulin independence rates above 50% at five years. But this comes at a steep price: the immunosuppressive medications that comprise the standard of care are nephrotoxic, can cause encephalitis, increase risks for cancers and deadly opportunistic infections. And, in a particularly cruel twist, calcineurin inhibitors are toxic to beta cells themselves.
Targeted blockades
For a T cell to activate, it requires two distinct immune signals: antigen recognition and binding to the cell that presents the antigen. This is often thought of as a “handshake.” Newer drugs work as targeted blockades to interrupt the specific handshake a T cell needs before it can target cells, an approach called a costimulation blockade.
Examples include belatacept, which acts as a decoy that grabs the binding sites first, so that the T cell finds nothing to hold. Tegobrubart (Eledon, not yet clinically approved) interrupts the handshake that authorizes the T-cell attack. Costimulation blockades have tremendous potential: tegobrubart paired with deceased donor islets achieved 100% insulin independence in all participants in its investigator-led trial. No recipients have reported any side effects as of the time of this writing, with one even working in the front lines of healthcare.
While these drugs spare people and their islets from a certain level of toxicity, they do not spare the person a lifetime of medication and all of the potential side effects that come with it. (See: Tzield’s boxed warning from the FDA for life-threatening cases of viral reactivation, or efluzimab’s withdrawal from the market after being linked to cases of a rare fatal brain infection; more on this shortly). And even the most precise immunosuppressant is a resignation to a weaker immune system in a world where you probably really need a strong one.
Tolerance induction
A more ambitious idea than suppressing the immune system for life is to persuade it to accept the transplanted cells, an approach known as tolerance. There are several proposed mechanisms for achieving this.
The first is central tolerance, also known as mixed chimerism. It starts in the thymus, where developing T cells that react to “self vs. other” are deleted before reaching the bloodstream. This central tolerance can be recreated for a graft by transplanting a sliver of the donor’s bone marrow so the recipient’s thymus also learns to delete anti-donor T cells. But that demands harsh conditioning involving irradiation and chemotherapy. And it has not proven resilient; even more, in T1D specifically, a clinical trial investigating this approach to central tolerance failed to achieve it.
Another strategy is Treg therapy, or peripheral tolerance. It relies on regulatory T cells (Tregs), the cells responsible for dampening immune responses after an infection is beaten and reducing inflammation overall. This strategy includes Treg infusion, Treg expansion, tolerogenic dendritic cells, and slow drug withdrawal.
Tolerance’s Proof of Concept
As ambitious as tolerance may sound, it has achieved proof of concept in a human being, albeit accidentally.
This happened when Wisel, Stock, and colleagues tested calcineurin-sparing regimens built on belatacept or efalizumab in a cohort of ten people with T1D who received islet replacement therapy. Then, while the study was humming along, tragedy struck. Efalizumab was unexpectedly withdrawn from the market after reported side effects of a fatal brain virus. This meant the people taking efalizumab had to switch to another regimen. One person formerly on efalizumab, referred to in the study as EFA-4, could not tolerate the alternative immunosuppressants, developing life-threatening infections and side effects. Eventually nothing was left that she could safely take, and her physicians stopped all of it, expecting she would lose the islet transplant and insulin independence with it.
But she didn’t. At the time of the report, she had been off immunosuppression for nine years, with an islet transplant placed roughly fourteen years earlier, and remained insulin independent.
Notably, nothing about this was engineered. The study had no Treg product, no tolerogenic cells, meaning that whatever tolerance she reached, her body reached on its own. It happened completely by accident, which is both the most hopeful and the maddening part.
Strategies That Modify the Local Immune Environment
Encapsulation
The idea of encapsulation is to create a physical barrier between the immune system and the transplanted cells that isolates them from the immune system. The barrier would ideally have pores sized so that glucose, insulin, and oxygen could pass freely, while immune cells and the large proteins of immunity could not.
There are three scales of encapsulation, in the order of impacting the person to impacting the cells. In macroencapsulation, many islets are placed in one retrievable device, which is then surgically placed within a person. In microencapsulation, a handful of islets are placed in a capsule. In nanoencapsulation, a conformal coating is applied a few molecules thick around a single islet.
Encapsulation approaches face many challenges, both for the encapsulated cells and the person receiving them. Since its inception, it has been plagued by fibrosis (scarring) resulting from a foreign body response to capsules or a device. Barriers create problems with diffusion, carrying over one of the core problems with exogenous insulin. Additionally, encapsulated cells tend to suffocate: they struggle with hypoxia, or a lack of oxygen, from poor vascularization. Islets are quite metabolically demanding. They need some of the most oxygen and blood supply in the body; they cannot have either compromised by a barrier and survive. No encapsulation strategy has yet achieved durable, immunosuppression-free islet function that results in insulin independence in a human being.
Biomaterials and co-delivery
In response to hypoxia and fibrosis challenges, researchers have been developing biomaterial strategies to engineer a more protective local environment for transplanted islets. One example is building the chemokine CXCL12 into alginate microcapsules, which protects islets without immunosuppression by recruiting regulatory cells and reducing inflammation. Another is FasL on microgels, which induced acceptance of islets in nonhuman primates. Another promising strategy is co-delivering mesenchymal stem cells with islets, which has been shown to calm inflammation and help them vascularize. There is also, of course, combining these approaches.
Strategies That Modify the Cells
Gene Editing: Ghost Cells
On the far end of the spectrum, we have a strategy that changes only the cells: gene editing approaches to immune protection and evasion.
The idea of gene-edited cells for immune protection comes from how the body already runs immune-privileged sites — the testis, the eye, the maternal-fetal interface — where foreign tissue survives indefinitely: the body has designed the cells in these sites to help them evade recognition and stay protected.
Remove immune recognition
HLA is the foundation of immune recognition and evaluation. Gene editing approaches that modify cells to not express HLA class I or II have shown significant promise. They achieve this through two gene knockouts that determine HLA presentation. The first knockout is β2M, pronounced “beta-2 microglobulin,” which is required for HLA class I to be shown on the cell surface. The second knockout is removing CIITA, or Class II HLA Transactivator, which is, well, the activator for HLA class II.
Some programs target HLA-A/B/C (class I) directly rather than targeting their mechanisms of presentation. Alternatively, some target TAP, which prevents peptides from being loaded onto HLA for expression. Because every T-cell arm of the attack against transplanted islets depends on HLA presentation, preventing HLA expression blinds them all at once.
Add immune inhibitors
A cell showing no HLA, unfortunately, is its own alarm. Natural killer cells patrol for exactly that absence — missing-self recognition — on the logic that a cell hiding its identity usually has something to hide. One approach to address this is to add a countersignal that inhibits this response cascade. CD47 is the canonical one, a “don’t-eat-me” signal that many healthy, naturally occurring cells already display, engaging the SIRPα checkpoint to send macrophages and NK cells along their merry way. Other approaches include adding inhibitors via HLA-E or G, which also ward off NK cells, and PD-L1 and FasL, which act on T cells by exhausting or killing them if they get too close.
Taken together, these gene edits are referred to as immune evasion, a kind of invisibility cloak that allows these previously vulnerable cells to avoid being targeted by the immune system – or even seen by it at all. Islets that receive these edits are called hypoimmune islets, operating beneath the immune system’s awareness – ghost cells.
Gene editing islets for immune evasion has shown some pretty exciting proof of concept in the UPF 421 Carlsson case. Like the tolerance case above, this is also N=1. But in this study, it was done on purpose. This study involved deceased donor islets that were edited to remove immune recognition (the β2M and CIITA knockouts discussed above) and to add immune inhibitor CD47, injected into a man’s forearm muscle.
Investigators injected three types of cells: unedited ones, partially edited ones that received only the HLA knockouts, and ones that received both the HLA knockouts and overexpression-of-CD47 edits to become hypoimmune. Over fourteen months, the fully edited hypoimmune cells survived untouched. In the same forearm, at the same time, the unedited and partially edited cells in the product were rejected and killed right on schedule, with a strong T-cell attack peaking at day 7, and antibodies class-switching by week three. The fully edited cells sat right next to that carnage and were ignored, secreting glucose-responsive C-peptide the whole time.
Unfortunately, the hypoimmune strategy comes with serious risks. Gene editing can be dangerous. Each knockout, knock-in, and round of expansion represents another chance to miscopy three billion letters of DNA. The more edits, the better the odds that a slightly off cell gains a growth advantage or becomes malignant. A nickname making the rounds for heavily edited cells is Franken-cells, or Frankenstein cells, implying that the more edits you add, the more you lose control of your potentially dangerous creation.
The need for a kill switch
These risks are why a kill switch belongs in the design of any hypoimmune islet product. The hypoimmune platform in the UPF 421 study built one in: because the cells overexpress CD47, a systemic anti-CD47 antibody clears them selectively and spares the person’s own CD47-expressing cells. Other programs bolt on a kill switch, such as HSV-thymidine kinase (vulnerability to ganciclovir), or RapaCasp9 (cell death on demand). Both work but neither is complete, since reported kill switch systems clear about 95 percent of cells, and in a dividing population the surviving 5 percent is significant.
Tolerance vs. Evasion
Only two strategies have ever produced drug-free, device-free islet survival in a human being. Tolerance did it once, by accident, in the Wisel et al study where efalizumab was withdrawn. Evasion did it once, on purpose, in a study designed to do so. Everything else on the spectrum still needs drugs, a foreign body such as capsules or devices, or both.
Tolerance is an equilibrium, and equilibria tip. The immune system is always evolving. Out of the blue, a viral infection, or even a change in gut bacteria, might provoke a broad T-cell response and expand the immune memory pool. Infection simultaneously blunts Treg suppression and raises HLA, brightening the target exactly when the immune system is most agitated.
This means that tolerance would have to hold not against the current immune system but against every future version of it.
Evasion has one piece of evidence tolerance cannot match: edited cells survive inside a hostile environment of active rejection. In primates, and then in the UP421 recipient, they lived while unedited cells died a few centimeters away in the same body at the same time, over multiple versions of who-knows-what kind of immune system evolution.
Conclusion and the Path Forward: Why I Believe in Ghosts
Over the course of researching and writing this article, I’ve encountered countless comparison tables that score immune protection approaches on efficacy, safety, durability, cost, and scalability. I would like to add the attributes that dominate life with T1D: attention and labor. Physical, intellectual, emotional. Islet replacement is worth the risks not only because it reduces complications and physical suffering, but because it could give people back energy and agency.
If there were one word I would use to describe this state of no-days-off, it would be precarity. Evasion is the approach that leaves the least precarity intact. Not because it can’t fail (there is no system which cannot), but because it does not depend on a fragile détente with the immune system, officially the most haunted system in the human body.
So, the ghosts I believe in: immune evasion along with biomaterial microenvironmental engineering.
This ghost approach is a combined one: hypoimmmune cells produced by evasion edits, and biomaterial “ghosts” too. In local microenvironmental engineering, the most sophisticated materials disappear. The forefront of biomaterial engineering is moving toward resorption, an ECM scaffold that helps with vascularization and oxygenation through the vulnerable early window of islet engraftment, before disappearing without a trace.
Kim and colleagues put it best: “Neither genetic immune modulation nor extrinsic microenvironmental control alone is sufficient to ensure durable graft function.” While gene edits would protect the islets from attack, biomaterial engineering would make sure they were not just surviving but thriving. Taken together, this combinatorial ghost approach could create an environment where a haunted immune system, replacement islets, and the person they inhabit may rest in peace.
And besides, sometimes there is good in what you cannot see. If the immune system is reading this, somewhere, I would remind it: the more you stare into the abyss, the more the abyss stares back into you.
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