TPE for ICI Encephalitis: A Primer for the Overworked Fellow

The call comes in from oncology. Their patient — a fifty-something with metastatic melanoma, two months into pembrolizumab — has been confused for three days. Low-grade fever. Can’t tell you the year. Can’t tell you where they are. Infectious workup negative. LP unremarkable. MRI with some FLAIR signal in the mesial temporal lobes. They started high-dose IV methylprednisolone 48 hours ago and the patient isn’t better. Neurology thinks it’s ICI encephalitis. Oncology wants to know if you can do something.

You can. Here’s what you need to know.

What Is ICI Encephalitis

Immune checkpoint inhibitors are monoclonal antibodies. They work by blocking co-inhibitory receptors — CTLA-4, PD-1, PD-L1 — that normally keep T and B cells in check. Releasing those brakes is the whole point: you want the immune system to attack the tumor. The problem is that the same mechanism that kills cancer cells also breaks peripheral self-tolerance, and the collateral damage can affect virtually any organ system. These off-target autoimmune and inflammatory complications are collectively called immune-related adverse events, or irAEs.

The nervous system is a particularly vulnerable target. Neurological irAEs occur in roughly 3–12% of patients on ICI therapy, depending on the agent and whether it’s given as monotherapy or in combination. The spectrum is broad: encephalitis, meningitis, Guillain-Barré syndrome, myasthenia gravis, transverse myelitis, cranial neuropathies. ICI-associated encephalitis specifically — inflammation of the brain parenchyma — is rare but can be severe and difficult to treat, particularly once it has failed first-line steroids.

Why the Immune System Attacks the Brain

The pathophysiology of ICI encephalitis is not a single pathway. It is a convergence of mechanisms, and understanding them matters because the mechanism predicts who will respond to TPE.

The most common mechanism is amplification of pre-existing subclinical autoimmunity. Many cancer patients harbor latent B cells or low-titer neural autoantibodies before they ever start ICI therapy — a consequence of the immune system’s exposure to tumor antigens that cross-react with neuronal proteins. These pre-formed autoantibodies are usually held in check by co-inhibitory signaling. When checkpoint blockade removes those inhibitory signals, a subclinical autoimmune process that was already present gets amplified to clinical disease. This is supported by retrospective data showing that anti-Ma2 and anti-acetylcholine receptor antibodies were detectable in pre-treatment sera of patients who later developed ICI-related neurological syndromes. The drug didn’t create the autoimmunity from scratch. It unmasked it.

A second mechanism is tumor-driven antigen presentation. Many tumors ectopically express neuronal proteins that are normally sequestered behind the blood-brain barrier. When tumor cells die and release these antigens, dendritic cells take them up and present them to the immune system. Under normal conditions, co-inhibitory signals prevent a full response. Block those signals with a checkpoint inhibitor, and the immune system is primed against neuronal targets shared between the tumor and the brain.

The clinical distinction that follows from this is the one that determines your treatment strategy. Encephalitis associated with cell-surface antibodies — anti-NMDAR, anti-LGI1, anti-GABA-B, anti-GAD65, anti-AMPA — is directly antibody-mediated. These antibodies internalize receptors, block synaptic signaling, and activate complement. Remove the antibodies, and the pathogenic process is interrupted. This is why IVIG, TPE, and rituximab work in these cases. Encephalitis associated with intracellular or onconeuronal antibodies — anti-Hu, anti-Ma2, anti-Yo — is driven primarily by a cytotoxic T-cell response. The antibodies are biomarkers, not effectors, and removing them with PLEX is unlikely to change the course of disease. These cases carry substantially worse prognosis — mortality around 23–35% — and respond poorly to antibody-depleting therapies.

Why TPE — and Why It’s Different Here

When you’re called about a patient with steroid-refractory ICI encephalitis, or other irAE, TPE offers something that other immunosuppression doesn’t: two mechanisms operating simultaneously.

The first is the one you’d expect — removing the pathogenic autoantibodies driving the disease process. The second is less obvious and easy to overlook: you are also removing the checkpoint inhibitor itself. Remember that ICIs are monoclonal antibodies, and monoclonal antibodies have long half-lives. Pembrolizumab’s half-life is approximately 27 days. Nivolumab’s is around 27 days as well. Ipilimumab’s is about 14 days. Discontinuing the drug, which is always the first step, does not mean the drug is gone. The patient in your opening scenario stopped pembrolizumab when their symptoms started, but they still have weeks of circulating drug maintaining receptor occupancy on T cells and continuing to drive the inflammatory process injuring their brain. TPE accelerates clearance of the ICI in a way that waiting simply cannot. This dual mechanism — antibody removal plus drug clearance — is what makes TPE uniquely well-suited to the ICI setting.

TPE Beyond Encephalitis

ICI encephalitis is one indication for TPE. It is not the strongest one. The table below summarizes the irAEs for which PLEX appears in guideline-based management, the role it plays, and key clinical notes. The ICI half-life argument applies across all of them: regardless of the specific irAE, you are treating the immune injury and simultaneously clearing the drug that is sustaining it.

Disorder	Role of TPE	Notes
Myasthenia gravis / MG-like syndrome	First-line	Initiate PLEX or IVIG alongside IV methylprednisolone 1–2 mg/kg/day at grade 3–4; do not wait for steroid failure
Guillain-Barré syndrome	First-line	Start PLEX or IVIG at any grade above mild; steroids added in ICI-related GBS unlike idiopathic GBS
Encephalitis	Steroid-refractory escalation	Add PLEX or IVIG if severe or progressing after 24–48 hours on high-dose methylprednisolone; cell-surface antibody profile predicts better response than intracellular antibody profile
Demyelinating disease (optic neuritis, transverse myelitis, ADEM)	Steroid-refractory escalation	Consider PLEX or IVIG if no response or worsening after 48 hours of high-dose IV methylprednisolone
Myocarditis	Steroid-refractory escalation	Among additional options for hemodynamically unstable patients not improving within 24–48 hours on steroids
Triple M syndrome (myocarditis + myositis + MG)	First-line for MG component	IVIG and/or PLEX specifically indicated for MG-like presentations within the syndrome
ICI-induced TTP	First-line	Standard TTP protocol; single procedure simultaneously removes anti-ADAMTS13 antibody, the ICI itself, and replaces ADAMTS13 from donor plasma

What to Tell Oncology When They Call

Back to your patient, still confused on day two of methylprednisolone. You are not going to wait for an antibody panel — those are send-outs, and in most centers they aren’t available at all outside a research context. The clinical picture is enough: steroid-refractory ICI encephalitis in a patient who has been off pembrolizumab for days and still isn’t improving. That’s a patient with weeks of circulating drug still driving the process. TPE is a reasonable next step.

When oncology asks why TPE and not just more immunosuppression: you are doing two things at once. You are removing whatever antibodies are driving the encephalitis. And you are clearing weeks of circulating pembrolizumab that discontinuation alone cannot touch. That dual mechanism is what makes this worth doing — and what makes it a transfusion medicine problem, not just a neurology one.

Checkpoint inhibitors have transformed oncology. They have also created a new category of patient that transfusion medicine is increasingly being asked to manage — patients whose immune systems have been deliberately unleashed and are now doing damage that can’t be walked back with steroids alone. The irAE landscape is broad and still evolving, and the role of TPE within it is broader than most people outside the field realize. Understanding why it works here, and when to reach for it, is increasingly part of what it means to practice transfusion medicine.