CD95/Fas Apoptosis Signal Initiation Depends on the Ligand Oligomerization State and Formation of Small Ligand-Receptor Complexes

The death receptor cluster of differentiation 95 (CD95 / Fas) is an important inducer of apoptotic activity in tumor cells, but may exhibit differential cell responses (ranging from predominantly apoptosis up to proliferation) when stimulated by its CD95 ligand (CD95L/FasL). How ligand form and receptor-ligand stoichiometry shape these divergent outcomes remain unresolved. To define structural and functional determinants of CD95 signaling, we systematically compared the oligomerization and activity of native CD95L with CD95L variants, where ligand trimers were stabilized either by a FLAG-tag for subsequent monoclonal antibody (mAb) crosslinking or by genetically fused isoleucine zippers (IZ). Apoptotic activity varied markedly with both ligand concentration and ligand variant type. To quantify receptor-ligand stoichiometry at the cell membrane, we combined quantitative Stimulated Emission Depletion (qSTED) nanoscopy, simulations, and biochemical assays. Across conditions, signaling complexes consisted of small oligomeric assemblies, with up to three CD95 receptors engaging a trimeric ligand. Despite similar stoichiometry, biochemical measurements revealed substantial differences in ligand oligomerization and binding affinity, with IZ-CD95L exhibiting markedly higher affinity than FLAG-CD95L (KD of 0.81 nM and 18.4 nM, respectively). Dimeric/trimeric CD95 formed by CD95L complexation or bridged CD95 by mAb enabled flexible intracellular FADD linkage for enhanced signaling.These results indicate that apoptotic potency is governed by the proximity of CD95 receptors, achieved through receptor bridging as well as by a stabilized trimeric ligand, both of which enhance receptor recruitment and binding avidity. Our findings provide mechanistic insight into CD95 signaling and suggest strategies for optimized apoptosis-inducing therapeutics. Significance StatementThe CD95/Fas receptor-ligand system is a central regulator of programmed cell death, yet the earliest molecular events of CD95 activation remain unclear. We systematically compared different CD95L/FasL variants and show that ligand architecture critically shapes receptor engagement and death signaling. An engineered FasL with an isoleucine zipper exhibited larger trimeric lig- and fractions, stability, and receptor binding, resulting in stronger apoptotic activity. Super-resolution imaging and simulations indicate that CD95 is mainly monomeric at the membrane and forms small oligomers after ligand binding. The flexibility of intracellular FADD linkage trigerred by dimeric/trimeric or bridged CD95 enables more intense signaling. These findings clarify how ligand structure governs CD95 activation and provide design principles for death receptor-targeted therapeutics.