Interplay of TLR4 and SARS-CoV-2: Unveiling the Complex Mechanisms of Inflammation and Severity in COVID-19 Infections

Abstract

The late 2019 emergence of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, caused profound and unprecedented disruption to the global socio-economic structure, negatively affecting millions of lives worldwide. A typical hallmark of severe COVID-19 is hyper inflammation due to aberrant cytokine release (cytokine storm) by innate immune cells. Recent studies have revealed that SARS-CoV-2, through its spike (S) protein, can activate the body's innate immune cells via Toll-Like Receptors (TLRs), particularly TLR4. In silico studies have demonstrated that the S protein binds with high affinity to TLR4, triggering downstream signaling processes that result in pro-inflammatory cytokine release. Compared to other TLRs, such as TLR2, TLR4 plays a more significant role in initiating and sustaining the inflammatory response associated with severe COVID-19. Furthermore, interactions between the virus and target cells can enhance the cellular expression of TLR4, making cells more susceptible to viral interactions and subsequent inflammation. This increased expression of TLR4 upon viral entry creates a feedback loop, where heightened TLR4 levels lead to amplified inflammatory responses, contributing to the severity of the disease. Additionally, TLR4's potent activation of inflammatory pathways sets it apart from other TLRs, underscoring its pivotal role in the pathogenesis of COVID-19. In this review, we thoroughly explore the multitude of regulatory signaling pathways that SARS-CoV-2 employs to incite inflammation. We specifically focus on the critical impact of TLR4 activation compared to other TLRs, highlighting how TLR4's interactions with the viral S protein can exacerbate the severity of COVID-19. By delving into the mechanisms of TLR4-mediated inflammation, we aim to shed light on potential therapeutic targets that could mitigate the inflammatory damage caused by severe COVID-19. Understanding the unique role of TLR4 in the context of SARS-CoV-2 infection could pave the way for novel treatment strategies that specifically inhibit this receptor's activity, thereby reducing the overall disease burden and improving patient outcomes.

Keywords: ACE2 receptors; SARS-CoV-2; cytokine storm; hyperinflammation; innate immunity; toll-like receptor 4.

Figure 1

Schematic overview of possible downstream signaling events triggered upon engagement of the SARS-CoV-2 spike protein with the TLR4/MD2 complex. The engagement of SARS-CoV-2’s spike protein with the TLR4/MD2 complex on the surface of host cells triggers a critical immune reaction. This begins with a transformation in TLR4 that facilitates the attraction of MyD88 and TRIF proteins, initiating the NF-κB pathway which results in inflammation and an immune reaction through the activation of the IRAK4-IRAK1/IRAK2 complex and TRAF6. This process leads to the activation of TAK1, which in turn causes the degradation of IκBα, permitting NF-κB to enter the nucleus and initiate the transcription of genes for immune activation. In parallel, TAK1 stimulates the MAPK pathways (ERK, JNK, p38 MAPK), culminating in the activation of AP1, which is crucial for cell survival and the proliferation of immune cells. The endosomal pathway dependent on TRIF leads to the activation of TRAF3 and TRAF6, which then activates TBK1 and IKKε. These enzymes phosphorylate IRF3, and the phosphorylated IRF3 moves into the nucleus to induce the transcription of genes for type I interferon, essential for the antiviral response and for halting the spread of the virus. This concise schematic underscores the pivotal roles played by AP1, NF-κB, and IRF3 in the body’s defense mechanisms against viral infections, illustrating a complex yet systematic response that encompasses inflammation, cell proliferation, and antiviral activities. Created with BioRender.com.

Figure 2

Decrease in surfactant production due to SARS-CoV-2 infection of ATII cells leads to TLR4 upregulation. The exposition of TLR4 engenders its interaction with SARS-CoV-2 viral proteins and DAMPs released from dying cells leading the TLR4 activation and signaling pathways. TLR4 signaling via MyD88 leads to inflammation via NF-kB proinflammatory pathway while signaling via TRIF/TRAM leads to a partial anti-inflammatory pathway via IRF3. The IRF3 pathway fosters the upregulation of ISGs, including ACE2, which further binds to SARS-CoV-2, enhancing viral infection and replication. The increased viral replication then indirectly exacerbates the proinflammatory cascade, depleting surfactant more, and causing a hyperinflammatory state leading to potential cytokine storm. Created with BioRender.com.

Figure 3

The figure summarizes the downstream effect of activation of platelets by the SARS-CoV-2 spike protein via the ACE2 receptor. (A) Binding of SARS-CoV-2 virus to the ACE2 receptor activates downstream signaling cascades leading to the release of inflammatory mediators and TF which will go on to aid in converting prothrombin to thrombin. The release of TF can be indirect if the platelet previously interacted with monocytes or PMN leukocytes which transferred TF to the platelet for release. Alternatively, it is considered direct when the platelet releases TF upon activation without prior interactions with these cell types. (B) Binding of thrombin to the PAR1 or PAR4 receptor leads to intracellular calcium mobilization through the PLC pathway, the calcium-dependent protease calpain is activated and cleaves myosin-9 on the surface of α-granules containing TLR4. This disrupts myosin-9 interaction with Rab7b and the two no longer coordinate to negatively regulate the trafficking of these α-granules. (C) TLR4 expression is upregulated due to the intracellular events and the receptor is then accessible for further binding by additional SARS-CoV-2 viruses within the site of infection, which may lead to further inflammation. Created with BioRender.com.

Figure 4

The Effect of SARS-CoV-2 on the AngII/AT1-R Pathway and the Decrease of ACE2. This illustration presents the complex pathophysiological processes initiated by the abnormal activation of the Angiotensin II (AngII)/Angiotensin II Type 1 Receptor (AT1-R) pathway due to SARS-CoV-2 infection. The initial steps where the virus binds to and reduces ACE2 receptors, which are crucial for managing cardiovascular and inflammatory reactions. This reduction disturbs the equilibrium, enhancing the AngII/AT1-R interaction because of the decreased ACE2-mediated control (A and B).The effects of this imbalance highlight the increase in signaling pathways that cause inflammation and damage to tissues. The uncontrolled stimulation of the AngII/AT1-R pathway increases pro-inflammatory reactions, leading to a hyperinflammatory state characterized by significant cytokine production, termed a “cytokine storm”. This event is a key element in the progression of serious COVID-19 complications, such as acute respiratory distress syndrome (ARDS) and systemic multi-organ failure (C–E).This figure not only offers a graphical overview but also portrays a detailed summary of the vital interactions between SARS-CoV-2 and the AngII/AT1-R pathway, which are at the core of the severe symptoms of COVID-19 through the reduction of ACE2 and activation of subsequent signaling pathways. It highlights the potential profound effects of viral infection on the regulation of the cardiovascular and immune systems, enhancing our comprehension of the disease’s pathology and identifying potential areas for therapeutic intervention. Created with BioRender.com.