This project is directed at defining intercellular communication pathways critical for host defense to a persistent pathogen. The TNF related cytokines, Lymphotoxin-ap and LIGHT form an integrated communication network between lymphocytes, dendritic cells and stromal cells that is specifically targeted by herpesviruses. We uncovered a molecular pathway of mutual cooperation between these cytokines and the viral pathogen, human cytomegalovirus. Signaling by the Lymphotoxin (LT)-p receptor inhibited cytopathic effects and replication of human CMV that depended on cooperative induction of interferon-p (IFNp). In the mouse, the LTap-IFNp pathway is required for controlling mouse CMV in part by providing critical survival signals for T and B lymphocytes. However, the cell-to-cell networks that form the LTap-IFNp defense mechanism are not defined. We recently identified an inhibitory cosignaling pathway that regulates T cell proliferation. The herpesyirus entry mediator (HVEM) acts as a dual switch by propagating positive signals from LIGHT and activating inhibitory signals through the Ig-family member, B T lymphocyte attenuator (BTLA). BTLA contains an ITIM motif that when activated by HVEM inhibits T cell signaling. Human CMV may usurp the HVEM-BTLA inhibitory cosignaling pathway through the UL144 orf. UL144 is an ortholog of HVEM that binds BTLA, but not LIGHT, and thus may attenuate T cell activation. The experiments proposed for this project are designed to identify genetic and molecular mechanisms of how LTap/LIGHT-related cytokines coordinate cell-to-cell communication that orchestrate host defense to cytomegalovirus. Three aims are proposed to address these questions. In aim 1 the cellular source of LTap required to induce IFNp during MCMV infection will be addressed using mice with conditional deletion of LTp in T or B cells. The cells producing IFNp in response to LTap signaling will be defined using an IFNp knockin reporter mouse. The role of HVEM-BTLA system in resistance to MCMV will be tested in HVEM and BTLA deficient mice. The mouse CMV model provides genetic and physiological context in which to view intercellular interactions mediated by these cytokines. In aim 2 the role of UL144 as coreceptor for BTLA will be addressed by mutagenesis to define the binding interactions and determine if UL144 mimics or antagonizes BTLA cosignaling activity in human T cells. We developed a cervical tissue explant model in which to study HCMV pathogenesis that will be applied in Aim 3 to evaluate the LTap-IFNp and UL144-BTLA pathways in human CMV pathogenesis. Lay summary: This project investigates how lymphoid cells communicate with each other to defend against a persistent virus. We discovered several tricks used by herpesviruses that thwart the immune system that may prevent clearance of the virus from the body. The experiments here will provide a comprehensive investigation with mouse and humans that may reveal new approaches to altering immune responses to a persistent pathogen.