Background/Purpose: Interleukin-1 receptor (IL-1R)-associated kinase 4 (IRAK4) is a key component of the Myddosome complex, which is essential for signalling downstream of IL-1R and most Toll-like receptors (TLRs) except TLR3. Aberrant TLR signalling has been implicated in the development of disease in both murine models of lupus and human systemic lupus erythematous (SLE). Herein we tested the hypothesis that inhibition of IRAK4 kinase activity can dampen the production of pro-inflammatory cytokines, type I interferon (IFN), and potentially autoantibodies.

Methods:  An IRAK4 tool inhibitor (compound 1) was used to treat PBMCs from SLE or lupus nephritis (LN) patients followed by TLR7 or IL-1β stimulation overnight. Cytokine (IL-6) production in the culture supernatant was then measured by AlphaLISA. Either fresh SLE/LN whole blood or healthy control PBMCs stimulated with 5% SLE/LN plasma were treated with either 5 µM of compound 1, 20 µM of Hydroxychloroquine (HCQ) or DMSO control overnight, and gene expression was then measured by NanoString with a representative gene panel. The effect of the IRAK4 inhibitor on human plasmablast differentiation driven by TLR7 activation was also examined using freshly isolated B cells activated with a selective TLR7 ligand for 7 days, followed by flow cytometry analysis of the CD38^hi CD19^low plasmablast population. Lastly, the IRAK4 inhibitor was dosed by chow in a Pristane-induced lupus mouse model. Arthritis score, plasma levels of autoantibodies (anti-dsDNA, anti-Histone, anti-SmRNP, anti-RiboP, and anti-Ro/SSA), and gene expression were followed to evaluate disease development.  

Results: Blockade of IRAK4 with a selective inhibitor (compound 1) led to concentration-dependent decrease of inflammatory cytokine (IL-6) production by TLR7 or IL-1β-stimulated SLE PBMCs. In addition, treatment with the IRAK4 inhibitor significantly reduced the expression of IRAK4-dependent genes, many of which were also observed to be elevated in whole blood from SLE and LN patients as well as in healthy control PBMCs stimulated with SLE plasma. Our data suggested that IRAK4 blockade produced unique but differential effects on type I IFN-inducible and other IRAK4-dependent genes compared to HCQ treatment. In contrast to its potent effects in suppressing inflammation/type I IFN-inducible genes, IRAK4 blockade with compound 1 only produced modest inhibition of plasmablast differentiation in vitro driven by TLR7, even at a saturating concentration. Lastly, we showed that dosing of the IRAK4-selective inhibitor in a Pristane-induced lupus model produced significant inhibition of joint inflammation measured by arthritis score, but no effect on autoantibody production.

Conclusion: IRAK4 kinase activity is essential for the induction of inflammatory cytokines and chemokines in myeloid cells, but less important for TLR-driven activation of B cells. Pharmacological inhibition of IRAK4 in SLE (and LN) could potentially relieve inflammation by dampening constitutively activated innate immunity and subsequently decrease end organ damage.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/critical-roles-of-irak4-kinase-activity-in-inflammation-but-not-b-cell-response-in-sle/