Cytokine release syndrome and associated neurotoxicity in cancer immunotherapy

Abstract

A paradigm shift has recently occurred in the field of cancer therapeutics. Traditional anticancer agents, such as chemotherapy, radiotherapy and small-molecule drugs targeting specific signalling pathways, have been joined by cellular immunotherapies based on T cell engineering. The rapid adoption of novel, patient-specific cellular therapies builds on scientific developments in tumour immunology, genetic engineering and cell manufacturing, best illustrated by the curative potential of chimeric antigen receptor (CAR) T cell therapy targeting CD19-expressing malignancies. However, the clinical benefit observed in many patients may come at a cost. In up to one-third of patients, significant toxicities occur that are directly associated with the induction of powerful immune effector responses. The most frequently observed immune-mediated toxicities are cytokine release syndrome and immune effector cell-associated neurotoxicity syndrome. This Review discusses our current understanding of their pathophysiology and clinical features, as well as the development of novel therapeutics for their prevention and/or management.

Fig. 1. Schematic diagram showing a relative timescale for the onset and duration of CRS and ICANS.

Also shown are the kinetics of chimeric antigen receptor (CAR) T cell proliferation and cytokine levels in peripheral blood. Conditioning chemotherapy, such as with cyclophosphamide and fludarabine, was given for a period of 3–5 days before infusion of CD19-directed CAR T cells (CD19CAR T cells; day 0) in patients with lymphoma or leukaemia. The onset and peak of cytokine release syndrome (CRS) generally precede those of immune effector cell-associated neurotoxicity syndrome (ICANS), with CRS generally occurring in the first week after CAR T cell infusion and ICANS occurring in the second week after infusion. CAR T cell numbers peak in the peripheral blood 1–2 weeks after infusion. Following activation-induced cell death of effector CAR T cells, some of the CAR T cells may persist as long-term memory T cells. Levels of homeostatic cytokines, such as IL-2, IL-7 and IL-15, may increase after conditioning therapy owing to lymphodepletion and the elimination of cytokine sinks, with a further increase in cytokine levels observed after CAR T cell infusion. These cytokines promote the survival and proliferation of CAR T cells. Levels of IL-1β and its natural antagonist IL-1 receptor agonist (IL-1Ra), as well as of granulocyte–macrophage colony-stimulating factor (GM-CSF), tend to peak earlier than other pro-inflammatory cytokines, which suggests that these cytokines might have a role in initiating the inflammatory cascade, together with IL-6. The pathophysiology of CRS can be divided into five main phases as indicated (see main text for details). IFNγ, interferon-γ; TNF, tumour necrosis factor.

Fig. 2. Working model of the pathophysiological mechanisms of CRS.

Upon target recognition, chimeric antigen receptor (CAR) T cells are activated to produce cytokines, such as interferon-γ (IFNγ), granulocyte–macrophage colony-stimulating factor (GM-CSF) and tumour necrosis factor (TNF), and soluble inflammatory mediators (for example, catecholamines) that can activate macrophages and surrounding tissues. In addition, damage-associated molecular patterns (DAMPs) released by pyroptotic tumour cells are recognized by pattern-recognition receptors (PRRs) on macrophages and can further amplify their activation. In a contact-dependent manner, macrophage-expressed CD40 can be engaged by CAR T cell-expressed CD40 ligand (CD40L) and promote macrophage activation. Activated macrophages secrete inflammatory mediators, among which IL-6, IL-1 and nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS) have been shown to be directly involved in the pathology of cytokine release syndrome (CRS). These cytokines may drive the systemic pathology of CRS owing to their ability to signal to a range of non-immune tissues such as the endothelium, which can result in vascular leakage, hypotension and further amplification of the inflammatory response by the secretion of cytokines and chemokines. It is hypothesized that locally produced chemokines attract circulating monocytes, which leads to an accumulation of activated macrophages at the site of interaction between CAR T cells and tumour cells, thus amplifying the inflammatory loop. Dashed lines denote hypothesized pathways that have not been experimentally confirmed in the context of CRS.

Fig. 3. Schematic representation of current and potential therapeutic interventions for CRS.

Blockade of the IL-6 receptor (IL-6R; using the monoclonal antibody tocilizumab) has shown clinical benefit in patients with cytokine release syndrome (CRS)^,,,. In vivo and in vitro models have proposed novel therapeutic interventions for CRS that directly target pro-inflammatory cytokines — such as IL-1 (the IL-1 receptor antagonist anakinra)^,,,, tumour necrosis factor (TNF; adalimumab or etanercept) and granulocyte–macrophage colony-stimulating factor (GM-CSF; lenzilumab)^, — or other pro-inflammatory mediators such as catecholamines (for example metirosine, which inhibits catecholamine synthesis). Furthermore, kinase inhibitors such as dasatinib can inhibit chimeric antigen receptor (CAR) T cell functionality, with subsequent reduction in effector cytokine secretion. Other broad-spectrum small-molecule inhibitors, such as ruxolitinib and ibrutinib (not shown here), that can broadly inhibit cytokine signalling and cytokine production across multiple cell types have been proposed for use in CRS.

Fig. 4. Pathophysiology of ICANS.

Similar to cytokine release syndrome (CRS), the pathophysiology of immune effector cell-associated neurotoxicity syndrome (ICANS) seems to start with the production of pro-inflammatory cytokines by chimeric antigen receptor (CAR) T cells and the activation of bystander immune cells such as macrophages in the tumour microenvironment. Inflammatory cytokines and chemokines produced by CAR T cells and myeloid cells in the tumour microenvironment — such as IL-1β, IL-6, IL-10, the chemokines CXCL8 and CCL2, interferon-γ, granulocyte–macrophage colony-stimulating factor and tumour necrosis factor — diffuse into the bloodstream and, eventually, result in disruption of the blood–brain barrier (BBB), with accumulation of cytokines and CAR T cells in the central nervous system (CNS) together with activation of resident microglial cells.