Clinical significance of measuring serum cytokine levels as inflammatory biomarkers in adult and pediatric COVID-19 cases: A review

Abstract

Coronavirus disease 2019 (COVID-19) is a rapidly evolving infectious/inflammatory disorder which has turned into a global pandemic. With severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) as its etiologic agent, severe COVID-19 cases usually develop uncontrolled inflammatory responses and cytokine storm-like syndromes. Measuring serum levels of pro-inflammatory cytokines (e.g., IL-6 and others) as inflammatory biomarkers may have several potential applications in the management of COVID-19, including risk assessment, monitoring of disease progression, determination of prognosis, selection of therapy and prediction of response to treatment.This is especially true for pediatric patients with COVID-19 associated Kawasaki-like disease and similar syndromes. In this report, we review the current knowledge of COVID-19 associated cytokines, their roles in host immune and inflammatory responses, the clinical significance and utility of cytokine immunoassays in adult and pediatric COVID-19 patients, as well as the challenges and pitfalls in implementation and interpretation of cytokine immunoassays. Given that cytokines are implicated in different immunological disorders and diseases, it is challenging to interpret the multiplex cytokine data for COVID-19 patients. Also, it should be taken into consideration that biological and technical variables may affect the commutability of cytokine immunoassays and enhance complexity of cytokine immunoassay interpretation. It is recommended that the same method, platform and laboratory should be used when monitoring differences in cytokine levels between groups of individuals or for the same individual over time. It may be important to correlate cytokine profiling data with the SARS-CoV-2 nucleic acid amplification testing and imaging observations to make an accurate interpretation of the inflammatory status and disease progression in COVID-19 patients.

Keywords: COVID-19; Cytokine; Inflammatory biomarkers; Kawasaki disease; Multiplex cytokine analysis.

Fig. 1

The clinical course of COVID-19 and host immune and inflammatory responses to SARS-CoV-2 infection. (Top panel) The clinical course of COVID-19 can be categorized into an asymptomatic incubation period, disease onset with respiratory symptoms, disease progression and severe disease phase . Severe COVID-19 usually necessitates intensive care unit (ICU) admission and mechanical ventilation . (Middle panel) Concomitant with the basic phases of COVID-19, host immune and inflammatory responses to SARS-CoV-2 infection may be divided into an early local innate immune response (sensing viral infection and antiviral defense) phase in the lungs, a later local/systemic immune response phase, followed by uncontrolled inflammatory responses and cytokine storm syndromes , . Patients with severe cases of COVID-19 may develop acute respiratory distress syndrome (ARDS) and multi-organ failure . (Lower panel) Armed with multi-layered mechanisms, the innate immune system represents a front line of host defense against the viral invasion. Of these, pattern recognition receptors (PRRs) sense viral pathogen-associated molecular patterns (PAMPs), triggering the innate immune signaling pathway that culminate in the production of interferons (IFNs), IFN-stimulated gene (ISG) products and pro-inflammatory cytokines/chemokines . Type I IFNs and ISG products play important roles in early-phase antiviral defense while cytokines/chemokines are responsible for initiation of later inflammatory responses and cytokine storm . The pro-inflammatory cytokine release may also lead to a series of destructive effects on human tissue, ARDS and multi-organ failure , .

Fig. 2

Dynamic changes in cytokine test volumes at a national reference laboratory (ARUP Laboratories) in response to COVID-19 pandemic. Fold change of test volumes are shown for a cytokine panel (“Panel”; containing 12 cytokines IL-1β, IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-12, IL-13, IL-17, TNF-α, IFN-γ and the cytokine receptor IL-2R), the indicated individual cytokines, and the total volume of tests (“Total”) performed at ARUP Laboratories between November 2019 and October 2020. In response to the COVID-19 pandemic, the Panel testing volume surged in April 2020 (Peak 1), which was concurrent with the outbreak in New York City, NY . It then dropped gradually to the pre-COVID-19 levels in June 2020. Similarly, the test volumes for IFN-γ, TNF-α, IL-1β, IL-2, IL-8 and IL-10 peaked in July 2020 (Peak 2) and returned to pre-COVID-19 levels in October 2020. Strikingly, IL-6 and total testing volumes participated in both Peaks 1 and 2 and has continued to maintain a significantly higher level than pre-COVID-19 levels, e.g., 11.9-fold higher IL-6 tests in October 2020 than November 2019.

Fig. 3

Life cycle of human coronavirus (HCoV) and innate immune signaling pathways in response to HCoV infection. (Left and middle) HCoV invades and infects a lung cell after cell attachment and viral entry. Then viral replication occurs, followed by viral assembly, budding and release from the infected cell. (Right) Viral RNA fragments or replication intermediates as viral pathogen-associated molecular patterns (PAMPs) are sensed by pattern recognition receptors (PRRs), thereby triggering innate immune signaling pathways. PAMP-PRR interactions activate adaptor proteins and downstream kinases. The activated innate immune signaling pathways culminate in activation of two types of critical transcription factors - interferon regulatory factors (IRFs) and NF-κB. IRF3-mediated interferon (IFN) and IFN stimulated gene (ISG) products play important roles in antiviral defense. For example, Tripartite Motif Containing 56 (TRIM56), an antiviral ISG, inhibits HCoV-OC43 propagation by targeting a later step of the viral life cycle after RNA replication . Type I IFN secreted by the infected cells is responsible for establishing an “antiviral” state in neighboring cells. Whereas, NF-κB-mediated cytokines/chemokines (e.g., IL-6 and TNF-α) are indispensable for later inflammatory responses, cytokine storm and immunopathogenesis in different cell types and tissues.