Boal-Carvalho, I. | bioRxiv Science

bioRxiv2025

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 complex (IKK) is a critical regulator of cell death and inflammatory signaling in multiple cell types. Phosphorylation of I{kappa}B proteins by IKK results in their degradation and consequent activation of NF-{kappa}B transcription factors. RIPK1, a critical cell death regulator, is also a direct target of IKK kinase activity, thereby repressing its cell death activity. In {beta} T cells, the RIPK1 kinase activity of IKK is critical for normal thymic development while mature {beta} T cells require IKK for both activation of NF-{kappa}B dependent survival programmes, and repression of RIPK1. {gamma}{delta} T cells play a unique and versatile role in host immunity with specific effector functions that enables 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 use mouse genetics to dissect the function of IKK and downstream pathways in the normal homeostasis of {gamma}{delta} T cells. We find that IKK expression is critical to establish a replete {gamma}{delta} T cell compartment, but that requires vary between different subsets. Type 1 {gamma}{delta} T cells require IKK dependent NF-{kappa}B activation for their generation, while IKK is redundant for development of adaptive {gamma}{delta} T cells. Instead, IKK dependent NF-{kappa}B activation is required for their longterm survival. We also find evidence that IKK repression of RIPK1 is required for survival of peripheral but not thymic {gamma}{delta} T cells. Ablation of CASPASE8 did not rescue {gamma}{delta} T cells in the absence of IKK but rather revealed a potent sensitivity of all {gamma}{delta} subsets to necroptosis, that was rescued by kinase dead RIPK1. Overall, we reveal critical requirements for IKK regulated inflammatory pathways by {gamma}{delta} T cells that contrast with those of {beta} T cells, and between different subsets, highlighting the complexity of the regulation of these pathways in the adaptive immune system.

immunology↗

bioRxiv2025

Evidence for a RIPK1-independent survival mechanism for CASPASE-8 in αβ T cells

CASPASE8 promotes both cell death and survival by acting as a trigger of apoptosis but also a repressor of necroptosis. In T cells, CASPASE8 is required for FAS induced apoptosis, but protects activated T cells from necroptosis. The broader role and mechanisms of CASPASE8 in the wider T cell compartment is less certain. Here, we analysed mice in which Casp8 was conditionally deleted in the lymphoid compartment by huCD2iCre. In the thymus, we found evidence of a modest impairment of early thymic progenitors and a striking absence of NKT cell development. Amongst mature peripheral T cells, there was a substantial and specific reduction in the CD8 T cell compartment, that included naive, central memory and virtual memory subsets. While life spans and turnover of CD8 T cells appeared largely normal, we did identify an acute requirement for continued CASPASE8 expression for survival of a fraction of CD8 T cells, since induced Casp8 deletion by tamoxifen induced CD8CreERT resulted in an acute loss of CD8 T cells. CASPASE8 deficient T cells were resistant to FAS or TNF induced cell death in vitro. Generating Casp8 deficient mice that express a kinase dead RIPK1 confirmed that necroptosis contributed to death of thymic progenitors and some peripheral CD8 T cell subsets in the absence of CASPASE8. However, kinase dead RIPK1 failed to restore NKT cell development and only partially rescued CD8 VM T cells, while analysing mixed bone marrow chimeras suggested that CASPASE8 deficient CD4 and CD8 T cells were less competitively fit than WT T cells, even in the absence of RIPK1 kinase activity. These latter observations suggest the existence of a novel mechanism by which CASPASE8 promotes T cell survival that is independent of its established role in repressing necroptosis.

immunology↗