Contrasting roles for IKK-regulated inflammatory signalling pathways for development and maintenance of type 1 and adaptive γδ T cells.

The inhibitor of kappa-B kinase (IKK) complex is a critical regulator of cell death and inflammatory signalling in multiple cell types. Phosphorylation of IκB proteins by IKK results in their degradation and consequent activation of NF-κB transcription factors. RIPK1, a critical cell death regulator, is also a direct target of IKK kinase activity, thereby repressing its cell death activity. In αβ T cells, the RIPK1 kinase activity of IKK is critical for normal thymic development while mature αβ T cells require IKK for both activation of NF-κB dependent survival programmes and repression of RIPK1. γδ T cells play a unique and versatile role in host immunity with specific effector functions that enable them to act as early responders in immune defence. The role of IKK-regulated pathways in their development and survival is not known. Here, we dissect the function of IKK and downstream pathways for normal γδ T cell homeostasis. We find that IKK is critical to establish replete γδ T cell populations, but that mechanism varys between different subsets. Type 1 γδ T cells require IKK-dependent NF-κB activation for their generation, while IKK is redundant for development of adaptive γδ T cells. Instead, IKK-dependent NF-κB activation is required for their long-term survival. We also find evidence that IKK repression of RIPK1 is required for survival of peripheral but not thymic γδ T cells. Ablation of CASPASE8 did not rescue γδ T cells in the absence of IKK but rather revealed a potent sensitivity of all γδ subsets to necroptosis, which was rescued by kinase-dead RIPK1. Overall, we reveal critical requirements for IKK-regulated inflammatory pathways by γδ T cells that contrast with those of αβ T cells, and between different subsets, highlighting the complexity of the regulation of these pathways in the adaptive immune system.