Structure of human cytomegalovirus UL144, an HVEM orthologue, bound to the B and T cell Lymphocyte Attenuator

Human cytomegalovirus (HCMV) is a {beta}-herpesvirus that has co-evolved with the host immune system to establish lifelong persistence. HCMV encodes many immune-modulatory molecules, including the glycoprotein UL144. UL144 is a structural mimic of the TNFRSF member HVEM, which binds to various ligands LIGHT, LT, BTLA, CD160 and gD. However, in contrast to HVEM, UL144 selectively binds to only BTLA, inhibiting T cell activation. Here, we report the crystal structure of the UL144/BTLA complex, providing key insights into the molecular mechanisms underlying this virus-host protein interaction. Our structure reveals that UL144 utilizes residues from its N-terminal CRD1 to interact with BTLA in an orientation similar, but not exactly, to that of HVEM. The structural modifications at the CRD1 region of UL144 compared to HVEM have a significant impact on the fine-tuning of BTLA-binding. In addition, the N-terminal CRD2 loop of UL144 is shorter compared to the corresponding region of HVEM, altering the relative orientation of CRD2 with respect to CRD1. Employing structure-guided mutagenesis we have identified a mutant of BTLA (L123A) that interferes with binding to HVEM while preserving interaction towards UL144. Furthermore, our results illuminate structural differences between UL144 and HVEM that explain the inability of UL144 to bind to either LIGHT or CD160. In summary, the specific molecular differences that UL144 has evolved to exclusively target BTLA highlight it as a suitable scaffold for designing superior BTLA agonists that have high potential for potently inhibiting immune responses.\n\nImportanceThe co-evolution of HCMV with its host over millions of years has allowed the virus to develop an efficient and specific immune modulatory protein, UL144, that binds exclusively to an immune inhibitory receptor BTLA. The crystal structure of the UL144/BTLA complex presented in this manuscript provides key insights into the molecular mechanisms underlying the virus-host protein interaction. The structure guided mutagenesis revealed select structural hot spots of the UL144/BTLA interaction. The structural details of this viral protein that has evolved to target only BTLA helps in successful design of BTLA agonists to target various T and B cell mediated autoimmune diseases.