IRAK-2 participates in multiple toll-like receptor signaling pathways to NFκB via activation of TRAF6 ubiquitination

Toll-like receptor (TLR) signaling is known to involve interleukin-1 receptor-associated kinases (IRAKs), however the particular role of IRAK-2 has remained unclear. Further, although IRAK-1 was originally thought to be central for the TLR-NF kappa B signaling axis, recent data have shown that it is dispensable for NF kappa B activation for some TLRs and demonstrated an alternative role for it in interferon regulatory factor activation. Here we show that IRAK-2 is critical for the TLR-mediated NF kappa B activation pathway. The poxviral TLR antagonist A52 inhibited NF kappa B activation by TLR2, -3, -4, -5, -7, and -9 ligands, via its interaction with IRAK-2, while not affecting interferon regulatory factor activation. Knockdown of IRAK-2 expression by small interfering RNA suppressed TLR3, TLR4, and TLR8 signaling to NF kappa B in human cell lines, and importantly, TLR4-mediated chemokine production in primary human cells. IRAK-2 usage by different TLRs was distinct, because it acted downstream of the TLR adaptors MyD88 and Mal but upstream of TRIF. Expression of IRAK-2, but not IRAK-1, led to TRAF6 ubiquitination, an event critical for NF kappa B activation. Further, IRAK-2 loss-of-function mutants, which could not activate NF kappa B, were incapable of promoting TRAF6 ubiquitination. Thus we propose that IRAK-2 plays a more central role than IRAK-1 in TLR signaling to NF kappa B.