Objective IBD is characterised by dysregulated intestinal immune homeostasis and cytokine secretion. In the intestine, properly regulating pattern recognition receptor (PRR)-mediated signalling and cytokines is crucial given the ongoing host–microbial interactions. TPL2 (MAP3K8, COT) contributes to PRR-initiated pathways, yet the mechanisms for TPL2 signalling contributions in primary human myeloid cells are incompletely understood and its role in intestinal myeloid cells is poorly defined. Furthermore, functional consequences for the IBD-risk locus rs1042058 in TPL2 are unknown.

Methods We analysed protein, cytokine and RNA expression, and signalling in human monocyte-derived macrophages (MDMs) through western blot, ELISA, real-time PCR and flow cytometry.

Results PRR-induced cytokine secretion was increased in MDMs from rs1042058 TPL2 GG risk individuals. TPL2 activation by the Crohn's disease-associated PRR nucleotide-oligomerisation domain (NOD)2 required PKC, and IKKβ, IKKα and IKKγ signalling. TPL2, in turn, significantly enhanced NOD2-induced ERK, JNK and NFκB signalling. We found that another major mechanism for the TPL2 contribution to NOD2 signalling was through ERK-dependent and JNK-dependent caspase-1 and caspase-8 activation, which in turn, led to early autocrine interleukin (IL)-1β and IL-18 secretion and amplification of long-term cytokines. Importantly, Salmonella typhimurium-induced cytokines from human intestinal myeloid-derived cells required TPL2 as well as autocrine IL-1β and IL-18. Finally, rs1042058 GG risk carrier MDMs from healthy individuals and patients with Crohn's disease had increased TPL2 expression and NOD2-initiated TPL2 phosphorylation, ERK, JNK and NFκB activation, and early autocrine IL-1β and IL-18 secretion.

Conclusions Taken together, the rs1042058 GG IBD-risk polymorphism in TPL2 results in a gain-of-function by increasing TPL2 expression and signalling, thereby amplifying PRR-initiated outcomes.