Battle of arms: human cytomegalovirus manipulates monocyte survival for viral dissemination.

Abstract

Human cytomegalovirus (HCMV) is a highly prevalent pathogen with seropositivity rates reaching upwards of 90% in the United States. Most primary infections are asymptomatic in immunocompetent individuals, but HCMV poses a significant risk in immunocompromised and immunonaїve individuals including transplant patients and developing fetuses in utero. The key to systemic dissemination of HCMV relies upon the infection of monocytes, which function as non-permissive vehicles to deliver virus to end organ tissues. These primary infected monocytes also travel to the bone marrow, infecting CD34+ stem cells, leading to the establishment of a lifelong HCMV infection. HCMV can reactivate at any point throughout the host's lifetime, leading to HCMV-infected stem cells exiting the bone marrow as monocytes, disseminating to end-organ tissue, and perpetuating HCMV disease. In circulation, monocytes have a short life span of 48 hours that can be accelerated by the cellular death pathway, apoptosis, as a cellular defense mechanism against viral infection. Our lab has shown during primary infection, HCMV circumvents intrinsic apoptotic pathways, however, the mechanism by which HCMV blocks extrinsic apoptosis is unclear. The studies in this thesis reveal that HCMV induces cFLIP expression, inhibiting extrinsic apoptosis effector caspase 8. This effective inhibition of intrinsic and extrinsic apoptosis prompts trap-door death pathway necroptosis. However, the mechanism in which this pathway is activated and how HCMV modulates this pathway to promote cell survival is unknown. In these works, we identified TLR3 as the death receptor responsible for inducing necroptosis. To circumvent this activation, HCMV upregulates autophagy, a ubiquitous cellular recycling process. We saw the inhibition of autophagy altered nucleocytoplasmic shuttling and activation of executioner kinase, MLKL, culminating in necrotic cell death. This work highlights the delicate balance between pro-survival and pro-death elements in HCMV infected monocytes. However, investigating how HCMV modulates cellular death pathways in a primary infection monocyte model does not fully encapsulate the role of monocytes in HCMV dissemination. Once HCMV latency is established in CD34+ stem cells, this allows HCMV the ability to persist in the host for their entire life span as monocytes derived from latently infected stem cells that can re-seed HCMV to peripheral organs to establish a chronic lytic infection. To investigate this understudied secondary population of HCMV-infected monocytes, we developed a model in which primary HCMV-infected monocytes and infected monocytes derived from latently infected stem cells are on the same genetic background by differentiating a CD34+ myeloblastic cell line. Though preliminary, we believe that this model, combined with investigations of mechanisms in which HCMV promotes survival in primary infected monocytes, will allow for the development of novel therapies that specifically target HCMV-infected monocytes, thus preventing viral dissemination and the establishment of disease.