• Cytokine Release Syndrome is a systemic inflammatory response characterized by the mass release of pro-inflammatory cytokines from immune cells.
• COVID-19 is thought to induce CRS in some cases of the disease.
• Treatment primarily focuses on reducing the levels of cytokines present or halting their downstream effects.

    Background

    Causes

    CRS in COVID-19

    Symptoms

    Pathogenesis

    Treatment

Cytokine release syndrome, also called cytokine storm, is a systemic inflammatory response characterised by a large and dysregulated release of pro-inflammatory cytokines.

Under homeostatic conditions, cytokines help to coordinate the immune system’s defense against pathogens. However, large quantities of cytokines can spur the recruitment of mass amounts of immune cells, causing widespread inflammation along with a decrease in the functionality of local organs affected.

Certain types of immunotherapy can cause CRS, with monoclonal antibodies and CAR-T cells the most common treatments to induce the syndrome. The immune cells affected by immunotherapy, as well as the therapeutics cells themselves, contribute to the rapid outpouring of cytokines. Markers and symptoms for CRS are discussed in detail below. Although most patients experience only a mild reaction, NCI and othr institutions state that the condition can be life-threatening.

Cytokines are small small protein messengers which aid in cell signalling. Generally, they are secreted by cells of the immune system and affect the behaviour of nearby cells which have corresponding receptors. They are defined in the 5th edition of Janeway’s Immunobiology as, “any protein secreted by immune cells that affects the behaviour of nearby cells bearing appropriate receptors.“ Cytokines are produced by a host of immune cells as well as other cell types, notably epithelial and endothelial cells. Ultimately, their primary function is to facilitate communication between cells and enact change in a locally or systemically.

Major cytokine groups are outlined in the table below.

Table 1. Cytokine groups

Another prominent cause of CRS is chimeric antigen receptor T-cell therapy (CART). These cells are created through autologous transfer from patients . CART is a promising application of immunotherapy in many respects, and has met with great success in cases of acute lymphoblastic leukemia (ALL) and other hematological cancers. CAR T-cells bind with their specific antigen to become activated. Their ability to travel through liquid media make them a strong treatment for blood cancers, but less successful in cases of solid tumours.

The infusion of large quantities of engineered, often robust T-cells can prompt the creation and release of cytokines, which in turn can recruit more immune cells to do the same. The addition of certain stimulating factors can spur their in vivo expansion as well. Once the cycle of activation and expansion begins, there are very few ways to regulate the release of cytokines. CRS symptoms have been reported to develop within 3-14 days after CAR-T therapy such as Kymriah® or Yescarta®.

Cytokine storm is thought to be associated with disease severity. At present, multiple therapeutics target the pathways which are affected by the pro-inflammatory cytokines of CRS. Blocking IL-6 in particular is an attractive treatment option which has received considerable pharmaceutical and media investment.

Fig 1. CRS cytokines and symptoms

CRS most commonly begins with a fever and flu-like symptoms, but can quickly worsen. Patients who develop the syndrome generally recover within two weeks; most do not suffer repercussions or long-term effects after CRS has been treated.

Common symptoms of CRS are outlined below. However, it should be noted that CRS is a heterogeneous condition which can present with different symptoms in varying levels of severity.

Table 2. Adapted from St. Jude’s Children’s Research Hospital patient information

The pathogenesis of CRS is not fully understood. However, certain key mediators of cytokine release syndrome have been identified. Chief among these is interleukin 6 (IL-6), an activator of the JAK/STAT signalling pathway which can bind to two forms of its receptor (soluble and transmembrane).

IL-6, along with IL-10 and IFNγ are the cytokines most consistently implicated in CRS. These three cytokines are usually elevated in the serum of patients with cytokine storm. TNF, IL-1, IL-8 and MCP-1 are more prominent early in CRS onset, with IL-6 produced later in the progression of the syndrome in higher quantities. IL-6 production is increased by TNF and IL-1, thus linking the signal of early-response cytokines to late onset profiles.

Elevated levels of IL-6 have been reported in patients with CRS and in murine studies. IL-6 is unique in that it signals through two different modalities: ‘trans’ signalling via soluble receptors and ‘classic’ membrane-bound receptor interactions. Both ultimately utilize the membrane-bound gp130 receptor subunit for signal transduction. After IL-6 binds to its soluble receptor, it can bind the gp130 present on cell types which do not express the membrane-bound IL-6 receptor. Thus, it can induce and initialise signalling in a wide range of cells. This feature may be part of what makes IL-6 such a pervasive mediator of CRS.

IL-6 is thought to contribute to some of the most common symptoms of cytokine release syndrome. The soluble receptor ‘trans’ signalling is key in this feature. Hallmarks of severe CRS like complement activation, vascular leakage, and disseminated intravascular coagulation are thought to be mediated by soluble receptor/IL-6 signalling paths. Trans signalling can also intensify the effects of IL-6 on target cells by enhancing TNF and IL-1 activity when concentrations of TNF and IL-1 soluble receptors are high.

IFNγ can cause many flu-like symptoms, including fever, headache, dizziness and fatigue. It can also induce the activation of different immune cells, including macrophages. Once active, these cells can produce massive quantities of many different cytokines, including IL-6, TNFα and IL-10. In turn, TNFα can generate further flu-like symptoms (fever, fatigue, etc.) and mediate vascular leakage, acute phase protein production, and lung injury.

Emerging markers of severe CRS include Ang-2 and von Willebrand factor, both signs of endothelial cell activation. These are often found to be elevated in CRS patients. Endothelial activation implies that these cells play a crucial role in the disease course of CRS, both as targets and as inducers of further inflammatory responses. These cells mediate the vascular dysfunction which contributes to the symptoms seen in severe cases, such as hypotension and vascular leakage. Endothelial cells are also thought to be another source of IL-6 in severe cases of cytokine storm.

The course of severity of CRS has been judged and reported with the use of differing scales over time. The American Society for Transplantation and Cellular Therapy (ASTCT) has put forth guidelines for grading CRS associated with CART therapy. In this system, hypoxia and hypotension are the core features of CRS, with fever a key concern as well. An abridged grading scale is below.

Table 3. CRS grading adapted from ASTCT guidelines

IL-6 has been implicated as one of the prominent driving cytokines in CRS. IL-1 is a prolific inflammatory cytokine which has also been targeted for treatment. Other emerging treatments are detailed below as well.

Table 4. Drugs used in the treatment of CRS or thought to have a potential effect in treating CRS