# Modulation of Inflammatory Cytokine Production in Human Monocytes by cGMP and IRAK3
## 1 Introduction  
Inflammation is part of the host defensive reaction generated in response to infection, trauma or toxic exposure. Inflammatory responses activated by cytokine and chemokine production are decisive in eliminating pathogenic agents and repairing tissues, leading to host recovery from infection or injury, thus forming a major component of innate immunity [1]. However, hyper-inflammatory processes cause dysregulated excessive cytokine and chemokine production, known as a cytokine storm, that is linked to multi-organ dysfunction or failure frequently seen in patients with severe conditions such as sepsis or septic shock and coronavirus disease (COVID-19) [1, 2]. Excessive cytokine and chemokine production is necessarily suppressed by checkpoint molecules responsible for maintaining homeostasis of inflammation [1, 2]. Interleukin-1 receptor-associated kinase-3 (IRAK3 also called IRAK-M) is one checkpoint molecule, mainly expressed in immune cells such as monocytes, macrophages, dendritic cells and epithelial cells [3, 4]. Similar to other members of this family (IRAK1, 2, and 4) IRAK3 is acytoplasmic regulator of Toll-like receptor or Interleukin-1 receptor (TLRs/IL-1R) signalling pathways [3; 5; 6]. Members of the IRAK family have a conserved domain architecture including an N-terminal death domain, kinase (in IRAK1 and 4) or pseudokinase (in IRAK2 and 3) domain, and C-terminal domain or tumour necrosis factor receptor-associated factor 6 (TRAF6) binding domain (IRAK4 lacks this domain) [7; 8; 9]. Whereas other IRAKs play roles as positive regulators of TLRs/IL-1R pathways, IRAK3 acts as a negative regulator, resulting in downregulation of inflammation in the innate immune system [3; 10]. The ligand engagement of TLRs or IL-1R activates the assembly of myddosome complexes composed of myeloid differentiation primary response 88 (MyD88), IRAK4, and IRAK1 or IRAK2 that interact via their death domains [8; 9]. IRAK1 or 2 then dissociate from the complex and interact with TRAF6 to mediate transforming growth factor beta kinase 1 (TAK1) dependent activation of nuclear factor kappa-light-chain enhancer of activated B cells (NF*B), eventually leading to the production of pro-inflammatory cytokines such as tumour necrosis factor-\(\alpha\) (TNF-\(\alpha\)) and interleukin-6 (IL-6) [8; 9]. IRAK3 downregulates the signal cascade by inhibiting the dissociation of IRAK-TRAF6 from the receptor-myddosome complex [3; 10]. The suppressive effect of IRAK3 on inflammatory responses via NF*B activation pathway (or other pathways such as inhibiting translational control of pro-inflammatory cytokines) subtly occur at different time points depending on cell type, mode and concentration of stimuli [10; 11; 12; 13]. IRAK3 also promotes expression of inflammatory mediators (NF*B and IL-8) upon stimulation with IL-1 or IL-1B [5; 7], and induces IL-6, IL-5 and IL-13 production in response to extracellular IL-33 stimulation [12]. Therefore, the regulatory mechanism of IRAK3 function in inflammation is not fully understood. IRAK3 contains a cryptic guanylate cyclase (GC) centre, capable of producing cyclic guanosine monophosphate (cGMP) from guanosine triphosphate, embedded within the pseudokinase domain and connected with its suppressive inflammatory function [14; 15]. Since cGMP also suppresses inflammatory responses [16; 17; 18; 19], it is possible that there is an association between cGMP and IRAK3 actions. cGMP synthesis by GCs is enhanced in response to various signals including nitric oxide (NO), peptide ligands or fluxes in intracellular calcium ions [20]. There are two well-known classes of GCs: soluble GCs and transmembrane GCs. Soluble GCs are stimulated by the binding of NO (a free radical gas synthesized by nitric oxide synthases) to the GC heme cofactor [21; 22]. Transmembrane GCs are integral plasma membrane receptors containing an intracellular GC domain and activated by the extracellular atrial or A-type, brain or B-type and C-type natriuretic peptides (ANP, BNP and CNP) to generate cGMP [23].