Immune Complex-Mediated Cell Activation from Systemic Lupus Erythematosus and Rheumatoid Arthritis Patients Elaborate Different Requirements for IRAK1/4 Kinase Activity across Human Cell Types

Abstract

IL-1R–associated kinases (IRAKs) are important mediators of MyD88-dependent signaling by the TLR/IL-1R superfamily and facilitate inflammatory responses. IRAK4 and IRAK1 function as active kinases and as scaffolds for protein–protein interactions. We report that although IRAK1/4 kinase activity is essential for human plasmacytoid dendritic cell (pDC) activation, it is dispensable in B, T, dendritic, and monocytic cells, which is in contrast with an essential active kinase role in comparable mouse cell types. An IRAK1/4 kinase inhibitor abrogated TLR7/9-induced IFN-α responses in both mouse and human pDCs, but other human immune cell populations activated via TLR7/9 or IL-1R were refractory to IRAK4 kinase inhibition. Gene ablation experiments using small interfering RNA demonstrated an essential scaffolding role for IRAK1 and IRAK4 in MyD88-dependent signaling. Finally, we demonstrate that autoimmune patient (systemic lupus erythematosus and rheumatoid arthritis) serum activates both pDC and B cells, but IRAK1/4 kinase inhibition affects only the pDC response, underscoring the differential IRAK1/4 functional requirements in human immune cells. These data reveal important species differences and elaborate cell type requirements for IRAK1/4 kinase activity.

Interleukin 1/Toll-like Receptor-induced Autophosphorylation Activates Interleukin 1 Receptor-associated Kinase 4 and Controls Cytokine Induction in a Cell Type-specific Manner

“…The inhibitor does not block IL-6 and TNF-␣ production in human dermal fibroblasts, whereas these cytokines are blocked in human monocytes, suggesting cell type-specific requirements for IRAK4 kinase activity in human cells. Our data are not consistent with that of Chiang et al (30), who showed no effect of an IRAK4/IRAK1 dual inhibitor on cytokine production in human monocytes. It is possible that the inhibitor used by Chiang et al (30) was not potent enough to inhibit IRAK4 kinase activity as there was no measurement of pIRAK4 to gauge the extent of inhibition in the monocytes.…”

“…Our data are not consistent with that of Chiang et al (30), who showed no effect of an IRAK4/IRAK1 dual inhibitor on cytokine production in human monocytes. It is possible that the inhibitor used by Chiang et al (30) was not potent enough to inhibit IRAK4 kinase activity as there was no measurement of pIRAK4 to gauge the extent of inhibition in the monocytes. Indeed, the potency of the inhibitor described by Chiang et al (30) was considerably less potent against IRAK4 and IRAK1 than the inhibitor used in this study, and we have been unable to document inhibition of pIRAK4 in either IL-1␤-stimulated dermal fibroblasts or R848-stimulated primary human monocytes at the concentrations described in that paper (data not shown).…”

“…However, in monocytes, the kinase activity is required for cytokine production. This suggests cell type-specific requirements for IRAK4 scaffolding versus kinase activity consistent with previous reports (24,30,31). More detailed studies need to be undertaken on cell type differences in the requirement for the IRAK4 kinase.…”

A Novel IRAK1–IKKε Signaling Axis Limits the Activation of TAK1–IKKβ Downstream of TLR3

“…Therefore, our data suggest that the kinase activity of IRAK1, rather than IRAK4, is required to mediate this negative regulation of the TLR3-NF-kB pathway. Recently, differences in the requirements for IRAK1/4 kinase activity in IL-1R signaling have emerged between human and mouse immune cells, indicating that the kinase activity is dispensable in IL-1R signaling in human cells (32). We found that blocking the kinase activity of IRAK1 and IRAK4 through the use of the dual IRAK1/4 kinase inhibitor limits the activation of the TLR3 pathway in human cells, because increased IL-6 and IFN-b levels were found in human microglial cells, which were pretreated with the IRAK1/4 inhibitor, following poly(I:C) stimulation.…”

Interleukin-1 Receptor-Associated Kinase 1/4 as a Novel Target for Inhibiting Neointimal Formation After Carotid Balloon Injury

“…Vascular smooth muscle cell (VSMC) proliferation and migration are critical to 1318 tion of IRAK1 and IRAK4 kinase activities would be predicted to leave some degree of host defense intact while still reducing inflammatory responses 18) . A novel cell-permeable benzimidazole compound can act as a potent and selective inhibitor of IRAK1 and IRAK4 and has been tested in plasmacytoid dendritic cells from systemic lupus erythematosus and rheumatoid arthritis patients 19) , human primary small airway epithelial cells in chronic obstructive pulmonary disease 20) , and rat brain hypoxia/ischemiainduced neuronal injury 21) ; however, it has not yet been employed in vascular research. To the best of our knowledge, there are no published studies on the role of IRAK1 and IRAK4 kinase activities in vascular restenosis through the TLR4-mediated NF B signaling pathway.…”