A matter of life and death: human cytomegalovirus induction of monocyte survival and differentiation into macrophages through manipulation of the PI3K/Akt pathway

Abstract

Human cytomegalovirus (HCMV) is a ubiquitous β-herpesvirus infecting up to 80% of the US population and reaching 100% seroprevalence in many parts of the world. In mostindividuals HCMV infection is usually asymptomatic. In contrast, in immunodeficient or immunonaive people, such as transplant recipients and the developing fetus, the virus is a major cause of morbidity and mortality. During a primary infection, HCMVcan spread very effectively in the body infecting many organ types and monocytes are believed to be the principal cell type responsible for HCMV dissemination throughout the body. Monocytes, however, are naturally programmed to undergo apoptosis after 48h in the circulation and are not permissive for viral replication. Our lab has shown that in order to combat these biological hurdles, HCMV promotes survival of these short-lived cells past their 48h “viability gate”. Besides inducing survival, the virus also mediates the differentiation of monocytes into macrophages skewed towards an M1 pro-inflammatory phenotype with select M2 anti-inflammatory features, which are long-lived cells, permissive for viral replication. However, the mechanisms used by HCMV to concomitantly induce survival and macrophage differentiation -two linked but separate processes, are not fully understood. The studies in this thesis reveal that upon binding and entry, HCMV initiates a survival program in monocytes by inducing a rapid and sustained activation of the PI3K/Akt pathway, which isdifferent from that induced by myeloid growth factors. Moreover, after inducing cellular survival across the 48-h viability gate, the virus also employsthe PI3K/Akt pathway to regulate caspase 3 activation which mediatesthe atypical M1/M2 polarization. Our work suggests that virus not only makes use of the PI3K/Akt pathway, but manipulates it at multiple levels toallow for viral-specific downstream functional changes.Deciphering how the virus uniquely maneuvers signaling pathways in monocytes to drive their survival and differentiation might allow us to develop new treatments targeting HCMV-infected monocytes and preventing viral spread and disease.