Fas acts as a T cell co-stimulatory receptor via both death domain-independent and -dependent mechanisms.

T cell responses are regulated by co-stimulatory and inhibitory signalling pathways, driven by receptors and their ligands designated as immune checkpoints. These receptors belong either to the immunoglobulin superfamily or the tumour necrosis factor receptor (TNFR) superfamily. Despite their crucial role in enhancing T effector cell activity, the molecular mechanisms triggered by TNFR co-stimulatory receptors, particularly their integration into the T cell receptor (TCR) signalling network, remain incompletely characterised. The receptor Fas is classically recognised as the prototypical TNFR death receptor due to the presence of an intracellular death domain (DD). While best known for its ability to trigger cell death, Fas has also been involved in non-death functions, including T cell co-stimulation. Despite its key role in dictating T cell fate, the molecular mechanisms underlying its TCR co-stimulatory receptor function have not been fully elucidated. We show here that Fas constitutively concentrates in plasma membrane domains at the immune synapse where it can be activated by antigen-presenting cell (APC) membrane-anchored FasL. This proximity to the TCR signalling molecules allows Fas, in a DD-independent manner, to lower the TCR activation threshold thereby enhancing the number of activated cells. Additionally, Fas triggers a DD-dependent signalling cascade composed of FADD, caspase-8 and RIPK1, ultimately leading to TAK1 activation. TAK1 in turn initiates gene-activating signalling pathways essential for an optimal Fas co-stimulatory response. Moreover, we identified the catalytic activity of caspase-8 as a tipping point in Fas signalling during TCR co-stimulation. Indeed, inhibiting caspase 8 not only prevents cell death but also promotes T cell co-stimulation by favouring RIPK1-dependent signalling. Altogether, our findings reveal a complex mechanism by which Fas integrates at multiple levels of the TCR spatial organization and signalling network, synergising with TCR signalling to achieve full T cell activation.