PharmacologyTier 2Medical Sciences concept

CAR-T cell therapy - mechanism and effector cell

Core concept

The effector cell is the patient's own T lymphocyte (autologous), genetically engineered ex vivo

The receptor - a synthetic fusion

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Extracellular: scFv from a monoclonal antibody (e.g. anti-CD19)

Hinge + transmembrane

Intracellular:

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The whole point: antigen recognition is MHC-independent
- -> bypasses tumour downregulation of MHC class I, the commonest immune escape mechanism
- -> one construct works in any HLA type
Signal 1 + signal 2 in a single molecule -> full T-cell activation, proliferation and cytotoxicity without a professional antigen-presenting cell
Co-stimulatory domain shapes behaviour: CD28 -> rapid, intense expansion, more CRS; 4-1BB -> slower, more durable persistence

The process

Leukapheresis -> transduction (lentiviral/retroviral) -> ex vivo expansion (2-6 weeks) -> lymphodepleting fludarabine/cyclophosphamide -> infusion
- Lymphodepletion creates the cytokine "space" (IL-7, IL-15) the CAR-T cells need to expand - it is not chemotherapy for the tumour
- Bridging therapy is often needed to hold the disease during manufacture

Products and targets

Target	Products	Disease
CD19	tisagenlecleucel, axicabtagene ciloleucel, lisocabtagene maraleucel, brexucabtagene	DLBCL/HGBL, B-ALL, mantle cell, follicular
BCMA	idecabtagene vicleucel, ciltacabtagene autoleucel	Multiple myeloma

In Australia, CD19 CAR-T is funded through MSAC-approved centres for relapsed/refractory large B-cell lymphoma and paediatric/young adult B-ALL; cilta-cel is TGA-approved and MSAC-recommended for myeloma after four prior lines - late-line, not second-line

The two signature toxicities - ASTCT grading

Cytokine release syndrome (CRS) - 40-90%, onset 1-14 days

Driven by IL-6, IL-1 and IFN-gamma from activated macrophages, not the T cells themselves
Fever is grade 1 by definition; graded by hypotension and hypoxia requirement
-> Tocilizumab (anti-IL-6R) for grade >=2; corticosteroids added for refractory or severe disease
- Tocilizumab does not cross the blood-brain barrier and does not treat ICANS
Differential: infection, HLH/MAS, tumour lysis, disease progression

ICANS (immune effector cell-associated neurotoxicity) - 20-60%, typically days 4-10, after CRS

Expressive aphasia and dysgraphia first -> confusion, tremor, seizure, cerebral oedema
Graded by the ICE score, level of consciousness, seizures, motor findings and cerebral oedema
-> Corticosteroids (dexamethasone/methylprednisolone) are first-line - not tocilizumab
Anti-seizure prophylaxis (levetiracetam) per protocol

Other toxicities

Prolonged cytopenias (weeks-months), B-cell aplasia and hypogammaglobulinaemia -> IVIg, infection risk, antimicrobial prophylaxis
IEC-HS (HLH-like syndrome)
Secondary T-cell malignancy - a rare class warning added to CAR-T labelling; absolute risk very low, but lifelong follow-up is mandated
No driving for 8 weeks; stay near the treating centre for 4 weeks

The clinical registrar's job is recognition, not administration
- Any fever in a patient within weeks of CAR-T is CRS and neutropenic sepsis until proven otherwise - culture, antibiotics, and call the treating centre
- Any new confusion or word-finding difficulty is ICANS until proven otherwise
Relapse mechanisms: antigen loss/CD19-negative escape (~30%), poor CAR-T persistence, T-cell exhaustion, an immunosuppressive tumour microenvironment
Response rates: ~40-50% durable complete remission in relapsed/refractory large B-cell lymphoma - in a population with a previous median survival of ~6 months
Where it sits: ZUMA-7 and TRANSFORM established CD19 CAR-T over salvage chemotherapy + autologous transplant as second-line therapy for early-relapsing/refractory LBCL internationally - Australian funding criteria lag the evidence; check current MSAC criteria
Emerging: bispecific T-cell engagers (blinatumomab, glofitamab, teclistamab) - off-the-shelf, no manufacturing delay, same CRS/ICANS toxicities

Immune checkpoint inhibitor mechanism (a different way of restoring T-cell killing)
Monoclonal antibody targeted cancer therapy; antibody-drug conjugate mechanism
Haemophagocytic lymphohistiocytosis; tumour lysis syndrome
Immunosuppressant mechanisms of action (tocilizumab, corticosteroids)
Cytogenetic and molecular studies in lymphoma and myeloma (patient selection)
Principles of transfusion and bone marrow transplantation
Febrile neutropenia and infection prophylaxis in the immunocompromised host

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