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

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.