Current Research and Scholarly Interests
For more than two decades at Stanford, my group made novel discoveries in cytomegalovirus (CMV) biology, major contributions to understanding this opportunistic pathogen. After 2006 through 2021, my Emory laboratory focused on the infected cell response to infection, mainly the contribution of regulated cell death pathways to host defense against herpesviruses and poxviruses. Our mechanistic understanding of virus-encoded modulators of cell death brought forward discovery of alternate cell death pathways beyond apoptosis that contribute to host defense in mammals. Large mammalian DNA viruses encode multiple cell death suppressors that have block either mitochondrial steps or cytosolic cell death steps leading to cell death as well as inflammatory cytokine signaling. Direct studies into both human CMV and mouse CMV, both species restricted viruses provided information on the natural role these suppressors play in CMV pathogenesis. After identifying the CMV-encoded viral inhibitor of mitochondrial apoptosis (vMIA) and characterizing the caspase-independent cell death pathway that this suppressor blocked during infection, we described a second player, the CMV-encoded viral inhibitor of caspase-8 activation (vICA), capable of suppressing extrinsic apoptosis and necessary for either human or murine CMV replication in macrophage lineage cells. A third cell death suppressor, viral inhibitor of receptor-interacting protein (RIP) activation (vIRA) unveiled a second caspase-independent pathway, programmed necrosis (necroptosis). Suppression of caspase-8 activity by vICA unleashed this alternative necroptosis, a pathway that is triggered by the pathogen sensor, Z-nucleic acid binding protein (ZBP)1, mediated by receptor-interacting protein kinase (RIPK3) and executed by a protein called mixed lineage kinase-like (MLKL). The pathogen sensor ZBP1 (also called DAI) senses newly synthesized Z-form double stranded RNA, oligomerizes with RIPK3 via RIP homotypic interaction motif (RHIM)-mediated protein-protein binding. This step is targeted and blocked by murine CMV-encoded vIRA, a RHIM competitor that prevents necroptosis as well as inflammatory signaling. We have established that vICA suppression of caspase 8 activity is an essential part of this process in most cell types, and that, together, vICA and vIRA represent key modulators of potent mammalian host defense pathways. We were the first to demonstrate the importance of RIPK3-dependent necroptosis in innate host resistance to viral infection through the elaboration of necroptosis inhibitors encoded by CMV, herpes simplex virus and the poxvirus, vaccinia. This work has highlighted the importance of ZBP1 as a specialized pathogen sensor of double stranded Z-form RNA triggering RIPK3-dependent cell death and inflammatory pathways.
CMV is such a master manipulator of the host response to infection, other individual viral gene products have provided us with high impact observations, such as the virus-encoded chemokine whose function assures CMV-susceptible myeloid cells are recruited to sites of infection as vehicles for dissemination as well as to downmodulate the CD8 T cell response to infection.
Through investigations of cell death machinery in mammals, we discovered that developmental death of caspase-8-deficient and RIPK1-deficient mice results from dysregulated induction alternate cell death pathways controlled by RIPK3 that evolved to control intracellular pathogens such as viruses. We demonstrated that mice lacking caspase-8 and RIPK3 as well as mice deficient in RIPK1, caspase-8 and RIPK3 survive to be fertile adults capable of mounting a robust immune response that controls virus infection despite the absence of cell death machinery. Host control over viral infection is attributed to the ability to mount robust virus-specific T cell responses that develop independently of cell death pathways to control viral infection.
