DR4 (TRAIL-R1) and DR5 (TRAIL-R2) are both functional TRAIL death receptors with intracellular death domains. They share ~58% extracellular domain homology. DR5 has two splice variants: DR5-Long and DR5-Short, differing by a 23-amino acid insertion. In many tumor types, DR5 is more highly expressed than DR4 and is the predominant mediator of TRAIL-induced apoptosis. Most clinical anti-TRAIL-R agonist programs target DR5 specifically, though some multivalent TRAIL mimetics engage both receptors.

Similar to OX40, DR5 is a TNFR superfamily member that requires receptor multimerization beyond simple bivalent cross-linking for efficient DISC formation and apoptosis. Standard IgG1 antibodies provide only bivalent binding, which is insufficient for robust receptor clustering. Early candidates (Conatumumab, Lexatumumab) failed to achieve sufficient tumor cell apoptosis at tolerable doses. Next-generation approaches use hexavalent or multimeric formats, Fc-engineered antibodies with enhanced FcγR-mediated cross-linking, or bispecific constructs that co-engage DR5 with a second tumor antigen for selective activation.

Seed TRAIL-sensitive tumor cells (Jurkat, COLO 205, MDA-MB-231 are commonly used) in 96-well plates. Add serial dilutions of recombinant TRAIL (typically 1-1000 ng/mL). Incubate 4-24 hours depending on readout. Measure apoptosis by: (1) CellTiter-Glo luminescence (cell viability), (2) Caspase-Glo 3/7 (caspase activation), or (3) Annexin V/PI flow cytometry (apoptotic vs necrotic cells). For cross-linking-dependent agonist antibodies, add anti-human IgG Fc secondary antibody or plate-coat the antibody to provide cross-linking.

DcR1 (TRAIL-R3) has a GPI anchor instead of an intracellular domain — it binds TRAIL but cannot transmit a death signal. DcR2 (TRAIL-R4) has a truncated death domain that prevents DISC formation. Both compete with DR4/DR5 for TRAIL binding, acting as molecular sinks. Normal cells often express high decoy receptor levels, providing protection from TRAIL-induced apoptosis. Tumor cells that overexpress decoy receptors show TRAIL resistance. Some DR5-selective agonist antibodies bypass decoy receptors because they bind DR5 specifically, not TRAIL's binding site.

Common TRAIL resistance mechanisms include: c-FLIP overexpression (blocks caspase-8 activation), Bcl-2/Bcl-xL overexpression (blocks mitochondrial amplification loop), low DR5 surface expression, and high decoy receptor expression. Sensitizing strategies include: BH3 mimetics (ABT-199, ABT-263) to block Bcl-2, proteasome inhibitors (Bortezomib) to upregulate DR5 and downregulate c-FLIP, HDAC inhibitors to increase DR5 transcription, and Smac mimetics (IAP antagonists) to promote caspase activation. TRAIL + sensitizer combinations frequently achieve synergistic apoptosis in resistant lines.