Nectins (nectin1-4) and Necls ] are Ig superfamily cell adhesion molecules that regulate cell differentiation and tissue morphogenesis. Adherens junction formation and subsequent cell-cell signaling is initiated by the assembly of higher-order receptor clusters of cognate molecules on juxtaposed cells. However, the structural and mechanistic details of signaling cluster formation remain unclear. Here, we report the crystal structure of poliovirus receptor (PVR)/Nectin-like-5/CD155) in complex with its cognate immunoreceptor ligand T-cell-Ig-and-ITIM-domain (TIGIT). The TIGIT/ PVR interface reveals a conserved specific "lock-and-key" interaction. Notably, two TIGIT/PVR dimers assemble into a heterotetramer with a core TIGIT/TIGIT cis-homodimer, each TIGIT molecule binding one PVR molecule. Structure-guided mutations that disrupt the TIGIT/ TIGIT interface limit both TIGIT/PVR-mediated cell adhesion and TIGIT-induced PVR phosphorylation in primary dendritic cells. Our data suggest a cis-trans receptor clustering mechanism for cell adhesion and signaling by the TIGIT/PVR complex and provide structural insights into how the PVR family of immunoregulators function.N ectins (nectin1-4) and nectin-like (Necl1-5) molecules are members of the large Ig superfamily (IgSF) of cell-surface receptors that play central roles in cell adhesion, cell movement, proliferation, and survival and contribute to the morphogenesis and differentiation of many cell and tissue types by inducing an intracellular signaling cascade (1-5). Nectins and Necls can function as both ligands and receptors and therefore are able to signal bidirectionally into juxtaposed cells (3,6). To mediate the formation of cell adherens junctions, a model suggests that the extracellular domains of these molecules form ligand-dependent homo-or heterodimers in trans (between molecules located on the same or opposite cell surfaces, respectively) and lateral homodimers in cis, creating a tight network of nectin zippers between juxtaposed cells (7,8). To date, structural and functional studies suggest a mechanism whereby the cis-homodimerization of a receptor on the same cell surface is followed by the formation of a trans-dimer between juxtaposed cells using identical protein interfaces. This assembly is noteworthy because it requires a rearrangement and breakup of the cis-homodimer followed by a transdimerization across the adherens junction. The cis-trans clustering is then initiated through another unknown protein interface, likely involving a different receptor domain.Several high-affinity homophilic trans-interactions have been described in detail for nectins/Necls and similar molecules (8-13). However, the structure and function of the presumably weaker lateral homophilic cis-dimers in cell adhesion and their role in intracellular signaling is not known. Because all structures solved to date are homodimers, it is unclear if they represent the cisor the trans-state. Thus, the question of how cis-trans heterodimerization drives cell adhesion and intracellular sign...