Structural basis for T cell costimulation
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Positive and negative signals delivered by a family of costimulatory receptors are critical for effective activation and regulation of T cells. CTLA-4 and PD-1 are the two members in the costimulatory receptor family that, upon engaging their respective ligands B7-1/B7-2 and PD-L1/PD-L2, negatively regulate the activity of T cells. In order to elucidate the structural basis for T cell regulation by CTLA-4, we have solved the structure of the complex between the disulfide-linked homodimer of human CTLA-4 and the receptor-binding domain of human B7-2 (hB7-2V). The structure shows that CTLA-4 binds B7-2 in an orthogonal mode with the majority of the interface formed by the conserved CDR3 (99MYPPPY104) loop of CTLA-4 and the front face of hB7-2V. Interestingly, the structure reveals that both CTLA-4 and B7-2 form bivalent dimers which promote the formation of a periodic receptor/ligand assembly that extends throughout the entirety of the crystal. This observation provides a model for the organization of these molecules within the immunological synapse and sheds light on the signal mechanism of the CTLA-4/B7 complex. PD-1 is a unique member in the family as it lacks the XXPPP(Y/F) ligand-binding motif and the interchain disulfide-forming Cys conserved in CTLA-4 and other costimulatory receptors, which are important for the formation of the periodic network observed for CTLA-4/B7-2. We have solved the structure of PD-1 which appears to be similar to that of the CTLA-4 monomer, with significant differences existing in the C" strand, etc. Importantly, PD-1 is shown to be exclusively monomeric, and consequently, may not support the formation of a periodic signaling complex. Mutagenesis studies revealed that residues in the F and G strands, but not those in the CDR3 loop, are critical for ligand binding of PD-1, suggesting that the PD-1/PD-L complex has a binding mode significantly different from that of CTLA-4/137. These studies on CTLA-4/B7 and PD-1/PD-L provide a framework for considering the structure features and signaling mechanisms of the whole costimulatory molecule family. Furthermore, ligand-induced ordered clustering of the receptor as observed for the CTLA-4/B7-2 complex may represent a distinct signaling mechanism available for cell surface receptors.