PROJECT 5 PROJECT SUMMARY Human cytomegalovirus (HCMV) remains a significant cause of morbidity and mortality after Hematopoietic Stem Cell Transplantion (HSCT) and Solid Organ Transplantation (SOT). A commonly encountered clinical manifestation of HCMV infection in the HSCT or SOT recipient is myelosuppression. HCMV reactivation and use of the antiviral ganciclovir is associated with a number of cytopenias that increase the risk of secondary bacterial or fungal infections and require growth factor support or transfusion of blood products. Despite the clear clinical relevance of myelosuppression to the transplant recipient with HCMV infection, little is known about the mechanism(s) by which HCMV infection inhibits normal hematopoiesis. HCMV has been shown to inhibit hematopoiesis by the direct infection of hematopoietic progenitor cells (HPCs) and indirectly by the effect of infected HPCs on the microenvironment supporting hematopoiesis. We have recapitulated HCMV myelosuppression in vivo using a humanized mouse model. We further show in both in vitro and in vivo models that the addition of increasing numbers of infected CD34+ HPCs increases the degree of myelosuppression and that supernatant from infected CD34+ HPCs, as opposed to mock-infected HPCs, suppresses myelopoiesis. Further, Projects 1, 2, 3 and 4 have identified viral proteins and miRNAs that directly alter signaling and either promote or suppress hematopoiesis in vitro. Therefore, we hypothesize that HCMV infection reprograms signaling and cytokine secretion in infected cells resulting in a microenvironment that inhibits hematopoiesis and contributes to clinical myelosuppression and hematopoietic failure. To test our hypothesis, we propose a systems approach to define the changes in signaling and cytokine secretion using both in vitro and in vivo models. We propose the following specific aims. Specific Aim 1. How does HCMV infection of CD34+ HPCs in vitro regulate hematopoiesis? We hypothesize that HCMV suppresses CD34+ HPC differentiation by altering signaling and secretion in the infected cell. Specific Aim 2. How does HCMV regulate hematopoiesis in vivo? We hypothesize that HCMV infection alters the microenvironment for hematopoiesis in the host organism. Specific Aim 3. What are the molecular signatures associated with HCMV myelosuppression in HSCT and SOT patients? We hypothesize that HCMV-mediated changes in cytokine secretion impede hematopoietic reconstitution in HSCT and SOT patients. IMPACT: Taken together, our project provides a unique opportunity to make unprecedented advancements in our understanding of the mechanistic basis of HCMV-mediated myelosuppression. This advancement is driven by the development of the sta...